RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Serial No. 63/013,679, filed April 22, 2020, entitled “Heavy Atom- Functionalized Upconverters for Increasing Upconversion Thresholds for 3D Printing,” by Congreve, et al., and to U.S. Provisional Patent Application Serial No. 62/911,128, filed October 4, 2019, entitled “Heavy Atom-Functionalized Upconverters for Increasing Upconversion Thresholds for 3D Printing,” by Congreve, et al., which are each incorporated herein by reference in their entirety for all purposes.

TECHNICAL FIELD

Systems and methods for increasing the upconversion threshold via heavy atom- functionalized triplet exciton upconverters are generally described.

BACKGROUND

Additive manufacturing or “3D printing” finds uses in industries such as prototyping and manufacturing. Several methods of 3D printing are known, but none of these methods truly operate in three dimensions. Instead, these methods use some form of extrusion, either layer by layer in most cases, or continuous withdrawal methods, to photopolymerize a polymer at a liquid-solid interface. The main limitation with these approaches is the inability to truly 3D “print” a pattern because light absorption occurs not only at the desired location, but also at the interface, which leads to undesired, uncontrolled, or inadequate polymerization. Instead, a very slow interfacial process is used, limiting throughput, practicality, and cost efficiency.

Typical implementations of 3D printing involve a container of liquid and a solid stage where the solid stage is lowered until a short layer of liquid polymer covers the stage. A laser “writes” a pattern onto this thin layer which hardens upon exposure. The stage then lowers further to immerse this material in more liquid, and exposure repeats until the desired structure has been formed. Due to the ability to create arbitrary designs, as well as form shapes that would be difficult to achieve by standard machining techniques, this technique has garnered incredible interest on the market. However, as mentioned, one of the main challenges in this field is that the stepwise printing nature limits printing speed and introduces steps into the surface, as a single layer of material is printed at a time. Thus, improvements in 3D printing technologies are needed. SUMMARY

Systems and methods for increasing the upconversion threshold via heavy atom- functionalized upconverters are generally described. The subject matter of the present invention involves, in some cases, interrelated products, alternative solutions to a particular problem, and/or a plurality of different uses of one or more systems and/or articles.

In one aspect, a liquid is described, where the liquid comprises a sensitizer that may be configured to absorb a first energy to form a first triplet state, and an upconverter, wherein the upconverter may be configured to receive the first triplet state from the sensitizer to produce a second triplet state. The upconverter may be configured to upconvert the first energy upon interaction with a second upconverter to produce a second energy where the second energy is greater than the first energy. In some cases, the upconverter comprises at least one heavy atom with an atomic number of at least 17. The liquid may further comprise a polymerizable species configured to receive the second energy from the upconverter to cause polymerization of the polymerizable species to occur.

In another aspect, a liquid is described, where the liquid comprises a metal porphyrin having a formula (I):

wherein M is selected from the group consisting of platinum, palladium, manganese, and zinc, R ³ , R ⁶ , R ⁹ , R ¹² are independently selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted aryl, and optionally substituted alkenyl, and R ¹ , R ² , R ⁴ , R ⁵ , R ⁷ , R ⁸ , R ¹⁰ , and R ¹¹ are independently selected from the group consisting of hydrogen, optionally substituted alkyl, optionally substituted aryl, and optionally substituted alkenyl. The liquid, in some cases, also comprises a diphenyl anthracene having a formula (II): wherein R ^A and R ^B are independently selected from the group consisting of optionally substituted alkyl and optionally substituted aryl, and wherein the diphenyl anthracene having the formula (II) further comprises at least one heavy atom with an atomic number of at least 17.

In yet another aspect, a method for 3D printing a polymeric object is described.

In accordance with certain embodiments, the method comprises providing a liquid, the liquid comprising a polymerizable species, a sensitizer, and an upconverter. In some cases, the upconverter comprises at least one heavy atom with an atomic number of at least 17. In certain embodiments, the method also comprises focusing at least one laser beam on a focal region of the liquid. At least some of the laser beam with a first energy may be absorbed by the sensitizer, wherein the first energy can be transmitted from the sensitizer to the upconverter to produce a triplet state in the upconverter. In some cases, the triplet state decays via upconversion to produce light (e.g., a photon) of a second energy, where the second energy may be greater than the first energy. The second energy, in some embodiments, polymerizes the polymerizable species within the focal region to produce a polymeric object. In some cases, the method is performed where substantially no polymerization occurs outside of the focal region of the liquid due to the at least one laser beam. In some cases, the method also includes separating the polymeric object from the liquid.

Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures: FIG. 1 shows a schematic diagram of a liquid configured to increase the upconversion threshold using a triplet exciton acceptor functionalized with a heavy atom, according to one set of embodiments;

FIG. 2 shows example upconverter (e.g. emitter, annihilator) molecules functionalized with heavy atoms, according to one set of embodiments;

FIG. 3 shows a plot of upconverted photoluminescence at 476 nm vs input laser at 638 nm to illustrate the increase in upconversion threshold, according to one set of embodiments;

FIGS. 4A-4B are photographic images comparing linear and quadratic processes in a 1 cm cuvette of PdTPBP and Br-TIPS anthracene in oleic acid where in the linear process, blue light is generated throughout the vial, while in a quadratic process, blue light is generated only at the focal spot whereby the linear process results from directly exciting the annihilator at 365 nm, while the quadratic process results from triplet fusion upconversion excited at 637 nm, according to one set of embodiments;

FIG. 4C is a schematic diagram of the upconversion process where two low energy photons generate two singlet excitons on sensitizer molecules, which intersystem cross (1) to generate triplet excitons, and then these excitons triplet energy transfer to the annihilator (2) where they undergo triplet fusion (3) to generate a higher energy singlet, which can radiatively decay by emitting a high energy photon that couples to the photoinitiator, according to some embodiments;

FIG. 5A shows chemical structures of examples of upconversion molecules showing, from left to right, the chemical structures of the sensitizer PdTPBP, and the annihilators TIPS -anthracene, Cl-TIPS anthracene, and 2Cl-TIPS-anthracene, according to one set of embodiments;

FIG. 5B is a plot of the absorption of the sensitizer PdTPBP and the annihilators TIPS-anthracene, Cl-TIPS anthracene, and 2Cl-TIPS-anthracene, according to one set of embodiments;

FIG. 5C is a plot of the upconversion efficiency as a function of input power for the annihilators TIPS-anthracene, Cl-TIPS anthracene, and 2C1-TIPS -anthracene where a linear fit (dotted lines) to the quadratic regime gives the threshold of each material (circles), according to some embodiments;

FIG. 6A is a schematic overview of the UCNC synthesis, according to one embodiment; FIG. 6B is a photographic image of UCNCs diluted in acetone showing penetration depths where the capsules are excited at 635 nm and are imaged through a 600 nm shortpass filter, according to one set of embodiments;

FIG. 6C is an SEM image of the UCNCs showing the scale and uniformity of the synthesis, according to one embodiment;

FIG. 6D shows a TEM image of the UCNCs, according to one embodiment;

FIG. 6E shows an image of a dispersion of the initial micelles and final UCNCs in acetone under the same conditions showing the necessity of the silica shell to upconversion survival where the emission peak at -800 nm corresponds to phosphorescence from the sensitizer PdTPBP, according to one set of embodiments;

FIG. 7A is a photographic image of a printing setup that moves a laser spot in three dimensions, according to one embodiment;

FIG. 7B is a photographic image of a benchmark boat called “Benchy,” printed according to certain embodiments described herein, sitting on a dime for scale, according to one set of embodiments;

FIG. 7C is a photographic image of multiple Benchy prints showing the repeatability of the printing process using a resin, according to some embodiments;

FIGS. 7D-7E are schematic illustrations of side and top views of the Benchy STL file, respectively, according to one set of embodiments;

FIGS. 7F-7G are photographic images of side and top views of the final print that show the fidelity in reproduction of the main features, according to one set of embodiments;

FIG. 8 shows a plot of emission-absorption overlap between the upconverted emission and the titanocene photoinitiator, according to one set of embodiments;

FIG. 9 shows the absorption spectra of several annihilators, according to some embodiments;

FIG. 10 shows UCNCs and F127 micelles dispersed in various solvents where the UCNCs and F127 micelles were both synthesized in water and added at 1:30 ratio to the listed solvents in which they were then excited at 635 nm and imaged through a 550 nm shortpass filter, and the tap water sample was dispersed in water directly from the tap and left uncapped for 20 minutes before taking the image whereby acrylic acid and PEGDA were each used to assess capsule durability in acrylate -based monomers for printing resins, according to one set of embodiments; FIGS. 1 lA-1 IB show Formlab’s print of the same Benchy STL where the file was imported into the free software Preform 3.2.2 and simulated for printing on a Form 3B printer at 50 micron layer height and whereby the object was scaled to be 8.35 mm in height to match the dimensions of the Benchy printed using liquids and methods described herein, and whereby the object required 176 layers and less than 0.1 mL of resin utilizing the "one click print" function to generate the printable structure, which resulted in an object with 250 layers at 50 micron layer height, using 0.59 mL of resin, according to one set of embodiments;

FIG. 12A-12B are photographic images showing an overprinted boat with a lack of discernable features whereby the issue can be remedied by altering the print speed and irradiation power, according to one set of embodiments;

FIG. 12C is a photographic image of an underprinted boat that shows missing features and damage from the wash process whereby the issue can be remedied by altering the print speed and irradiation power, according to one set of embodiments;

FIGS. 13A-13B show the Harvard logo printed from the PETA-based resin and corresponding STL file used to generate the print, according to one set of embodiments; and

FIG. 14A-14B are photographs of a pyramid printed from PEGDA-based resin and FIG. 14C shows and the corresponding STL file used to generate the print, according to one set of embodiments.

DETAILED DESCRIPTION

Articles and methods for increasing the triplet upconversion threshold, e.g, by utilizing a heavy atom- functionalized triplet exciton upconverter, are generally described. Some embodiments, for example, are directed to articles and methods that use a triplet sensitizer and a heavy atom-functionalized upconverter to produce upconverted photons (e.g., light of a second energy). The light can be used to polymerize a polymerizable species. Other upconversion configurations can also be used in other embodiments. In some cases, this may allow true 3D printing to be achieved due to improved control of light absorption, e.g., without needing to “print” on a layer-by-layer basis.

Referring now to FIG. 1, this figure illustrates a non-limiting example of a liquid configured to produce photons via triplet upconversion. A liquid 100 comprises a sensitizer 110, which may form a triplet state upon photoexcitation (for example by laser 115 with first energy 120). The triplet state formed by sensitizer 110 may first start as a singlet state and convert to a triplet state via intersystem crossing (not pictured) within sensitizer 110. Sensitizer 110 may then transfer this triplet state to an upconverter 130, illustrated with arrow 129. Upconverter 130 may then interact with another upconverter 140 and undergo triplet-triplet annihilation to produce upconverted photons (i.e., photons of higher energy than the photons used to photoexcite the sensitizer). Although not pictured, either (or both) of upconverters 130 and 140 may further comprise a heavy atom, described in more detail below. An acceptor 160 may then receive a triplet state from upconverters 130 and 140 where acceptor 160 has a triplet exciton energy level lower than both the sensitizer 110 and upconverters 130 and 140. The higher energy photons may be used, for example, to cause polymerization of a polymerizable entity within the liquid, which can be used for 3D printing, or other applications as discussed herein.

Accordingly, in certain embodiments, triplet upconversion (or triplet-triplet annihilation, TTA) may be used to produce light of a higher energy relative to light used to photoexcite the sensitizer or the upconverter. In some cases, the inclusion of a heavy atom-functionalized upconverter or heavy atom-functionalized auxiliary molecule may slow or prevent two triplet-excited upconverters from undergoing TTA, thereby slowing the process of TTA and hence avoiding saturation of triplet upconverters. Without wishing to be bound by any theory, an upconverter functionalized with a heavy atom may help facilitate the relaxation of an excited state triplet on an upconverter. In this case, relaxation of the excited state triplet on the upconverter limits its ability to undergo upconversion with another upconverter. This may advantageously allow for an increase in laser power to result in an increase of the rate of TTA (i.e. the upconversion frequency) and thus can allow higher powered lasers to maintain a quadratic or other dependence on the photoluminescence of upconverter as a function of laser power (i.e., upconversion remains second order with respect to the input laser power or a higher order).

Thus, two photons absorbed by the sensitizer may be combined by the upconverter to produce upconverted photons (e.g., having higher energy) that can be used to cause polymerization of polymerizable entity. In some cases, based on the quadratic dependence of the upconversion process, lasers can be focused on polymerizable entities within a liquid to cause polymerization to occur due to the high number of higher-energy photons produced by the upconversion of the laser light, while elsewhere within the liquid, minimal or no upconversion of light occurs, and thus, no polymerization of the polymerizable entity can occur. This can be used, for example, to achieve true 3D-printing within the liquid, e.g., by focusing one or multiple lasers to illuminate appropriate locations within the liquid, without requiring layer-by-layer printing.

Functionalizing the upconverter with a heavy atom may act to enhance the overall above-described effect; that is, an upconverter (e.g., an emitter) functionalized with a heavy atom may further slow or prevent two triplet-excited upconverters from undergoing TTA. As used herein, a “heavy atom” is any atom with an atomic number greater than 17 (i.e., chlorine or heavier).

The selection of a heavy atom on an upconverter can advantageously be used to tune the upconversion threshold in some cases. For example, in some embodiments, an upconverter without a heavy atom can have a first upconversion threshold, while that same upconverter functionalized with a first heavy atom (e.g., chlorine) can have a second upconversion threshold higher than the first upconversion threshold.

Furthermore, that same upconverter functionalized with a second heavy atom (e.g., bromine) can have a third upconversion threshold higher than the second upconversion threshold and the first upconversion threshold. Those of ordinary skill in the art in view of the teachings of this disclosure will be capable of selecting the appropriate number, kind (e.g., atomic number of at least 17), and/or the number of heavy atoms on an upconverter in order to tune the upconversion threshold over a particular range of upconversion thresholds (e.g., at least 1.7 W/cm ² and no greater than 283 W/cm ² , or other ranges such as are discussed herein).

Without wishing to be bound by theory, an upconverter comprising at least one heavy atom may advantageously permit faster relaxation of a triplet state on an upconverter. This may be achieved by enhancing the ability of an upconverter to undergo intersystem crossing. It will be understood by those skilled in the art that a transition from a triplet state to a singlet state (or vice versa) is “forbidden” in a quantum mechanical sense; however, the probability of this process (i.e., intersystem crossing) is more favorable when spin/orbit interactions (i.e., spin-orbit coupling) increase by inclusion of atoms with a larger number of occupied atomic orbitals. Heavy atoms, such as bromine or iodine, increase spin-orbit coupling interactions and may facilitate faster relaxation times of the triplet state of an upconverter. In one instance, the heavy atom is chlorine. In another instance, the heavy atom is bromine. Still other examples include iodine, calcium, nickel, iron, platinum, palladium, manganese, or zinc, or the like. In some cases, more than one heavy atom is present. In cases where more than one heavy atom is present, these heavy atoms can be identical or distinct.

In some cases, the addition of a suitable heavy atom reduces triplet lifetimes of the upconverter, causing the relationship between the power of input laser 115 and the upconverted light 150 to remain quadratic at higher input powers than in the absence of the additional heavy atoms. This advantageously allows for upconversion to occur with high spatial resolution either at the intersection or focus of laser beams at higher powers. Because upconversion with high spatial resolution can be used to cause polymerization of the polymerizable entity, the use of heavy atoms to increase the power at which upconversion remains quadratic allows for high-spatial resolution 3D printing at higher input powers, and therefore faster 3D printing.

In some instances, an external heavy atom may be used. This effect may include a heavy atom facilitating spin-orbit coupling as described above, but when the heavy atom is attached to molecule present in the system that is not the upconverter, such as the sensitizer, an acceptor, or an auxiliary molecule, such as a solvent molecule, as non limiting examples. In some embodiments, a solvent molecule is optionally present and distinct from the sensitizer or the upconverter. Non-limiting examples of solvent molecules comprising a heavy atom include iodobenzene, ethyl iodide, 1,2- dibromoethane, 1,2-dichloroethane, dichloromethane, 2-bromopropane, and 1- bromopropane.

While the heavy atom effect described above may enhance the ability of a forbidden transition, any effect that creates this effect may be suitable. For example, other effects that facilitate or enhance intersystem crossing may also be used, such as the including of a paramagnetic species in the liquid.

As described above, certain embodiments comprise a liquid. The liquid may be a solvent, such as an organic solvent, that dissolves or otherwise contains the sensitizer, the upconverter, the acceptor, and/or the polymerizable species. These are discussed in more detail below. In some embodiments, a sensitizer is present, used interchangeably herein with “triplet sensitizer.” As understood by those skilled in the art, a sensitizer (or a triplet sensitizer) can readily intersystem cross to a triplet state following excitation to its singlet state (i.e., by a stimulus, such as light, heat, etc.). Without wishing to be bound by theory, the sensitizer may be excited (e.g., by a photon) to produce an excited state sensitizer comprising a singlet excited state, wherein the excited state singlet may rapidly produce an excited state triplet in the sensitizer via intersystem crossing. The sensitizer can then, for example, transfer an excited state triplet to an upconverter. In some embodiments, the sensitizer is a photosensitizer, which includes compounds that can be efficiently excited to an excited triplet excited state (e.g., a first triplet state, a second triplet state), e.g., using light or electromagnetic radiation. In some cases, the sensitizer absorbs low energy light (relative to the energy of the upconverted light) to produce a triplet state that is subsequently transferred to a triplet upconverter, which may then produce high energy light (relative to light incident to the sensitizer). In certain cases, the sensitizer may reach a triplet state upon excitation, e.g., without the need of an additional external stimulus.

In some embodiments, the sensitizer transfers its energy state, e.g., a triplet state (or its corresponding triplet state energy) to an upconverter. The upconverter may be configured to upconvert this energy, as further described below, in some instances. For some embodiments, sensitizers may excite at least two upconverters, such that the two upconverters may undergo triplet-triplet annihilation. And according to some embodiments, the sensitizer may transfer a triplet state and/or corresponding energy to an upconverter.

A variety of sensitizers may be used in various embodiments. For instance, in some embodiments, the sensitizer comprises a metal porphyrin having a formula (I): wherein M is selected from the group consisting of platinum, palladium, manganese, and zinc, wherein R ³ , R ⁶ , R ⁹ , R ¹² are independently selected from the group consisting of hydrogen, optionally-substituted alkyl, optionally-substituted aryl, and optionally-substituted alkenyl, and wherein R ¹ , R ² , R ⁴ , R ⁵ , R ⁷ , R ⁸ , R ¹⁰ , and R ¹¹ are independently selected from the group consisting of hydrogen, optionally-substituted alkyl, optionally-substituted aryl, and optionally-substituted alkenyl. In some embodiments, the sensitizer comprises formula (I). In some cases, the sensitizer comprises an optionally-substituted metal porphyrin. In certain embodiments, the sensitizer is palladium tetraphenyl porphyrin.

In certain embodiments, the sensitizer comprises a structure: In certain embodiments, the sensitizer is palladium tetraphenyl porphyrin. Other sensitizers are possible. Some non-limiting examples of other sensitizers include, but are not limited to, palladium octabutoxy phthalocyanine (PdOBuPc), platinum tetraphenyltetranaphthoporphyrin (PtTPTNP), palladium(II)-meso-tetraphenyl- tetrabenzoporphyrin (PdTPTBP), [Ru(dmb)3] ²⁺ (dmb is 4,4’ -dimethyl-2,2’ -bipyridine), 2,3-butanedione (biacetyl), palladium(II) tertraanthraporphyrin (PdTAP), platinum(II)tetraphenyltetrabenzoporphyrin (PtTPBP), palladium meso- tetraphenylltetrabenzoporphyrin (PdPh4TBP), palladium octaethylporphyrin (PdOEP), 11,15,18,22,25 octabutoxyphthalocyanine (PdPc(OBu)s), platinum octaethylporphyrin (PtOEP), zinc(II) meso-tetraphenylporphine (ZnTPP), [Ru(dmb)3] ²⁺ , palladium(II)tetraphenyltetrabenzoporphyrin (PdTPBP), palladium(II) meso-tetraphenyl- octamethoxidetetranaphtholporphyrin (PdPluOMegTNP), 2-methoxythioxanthone (2MeOTX), and Ir(ppy)3 (ppy = 2-phenylpyridine). Other examples may be possible.

As mentioned above, the sensitizer transfers a triplet state to an upconverter. As understood by those skilled in the art, the term “upconverter” may be used interchangeably with “emitter,” “triplet upconverter,” “annihilator,” and “triplet annihilator.” An upconverter may receive a triplet state and/or a triplet energy from the sensitizer to enter a first excited triplet state of the upconverter. The upconverter, in some embodiments, is configured to undergo upconversion (or triplet upconversion). As understood by those skilled in the art, an upconverter may undergo upconversion (i.e., “triplet upconversion,” “annihilation,” “triplet-triplet annihilation,” “fusion,” “triplet fusion,” etc.) when two upconverters in a triplet excited state collide or otherwise combine their energy to produce a higher energy excited state (relative to the individual energies of the excited upconverters), which may then emit a photon of higher energy than the original excitation photon. Two upconverters in an excited triplet state may undergo triplet-triplet annihilation such that one upconverter returns to the ground state (and can, thus, be re-excited by a sensitizer) and the other upconverter may enter a second excited state (e.g. a singlet excited state, SI) and subsequently relax to its ground state, for example, by emitting the upconverted photon (which can be used, for example, for polymerization, or other applications including those described herein). In some cases, a triplet acceptor with lower triplet energy than the sensitizer or upconverter may be included to remove triplets from the system and shorten the effective triplet lifetimes. In some cases, this emission is fluorescence. In some cases, this emission is blue-shifted relative to the excitation light. In some cases still, the fluorescent emission is an anti- Stokes emission.

A variety of upconverters are used in different embodiments. As examples, according to certain embodiments, the upconverter comprises a diphenyl anthracene or an optionally-substituted diphenyl anthracene. In certain embodiments, the upconverter comprise a diphenyl anthracene having a formula (II):

(II), wherein R ^A and R ^B are independently selected from the group consisting of optionally- substituted alkyl, and optionally-substituted aryl, and wherein the diphenyl anthracene having the formula (II) further comprises at least one heavy atom with an atomic number of at least 17.

According to certain embodiments, the upconverter may comprise an anthracene having a formula (IV): wherein Rw, Rx, R _Y , and Ry are independently hydrogen or any element with an atomic number of at least 17.

In certain embodiments, an upconverter (e.g., an emitter) comprises one or more structures, such as: In certain embodiments, the upconverter is a heavy atom- functionalized dihexyl diphenyl anthracene. Other examples are possible. Non-limiting examples of upconverters include heavy atom- functionalized derivatives of 9,10-diphenylanthracene (DPA), TIPS-tetracene (TIPS = triisoprop ylsilyl), tetra-tert-butylperylene, anthracene (An), 2,5-diphenyloxazole (PPO), rubrene, 2-chloro-bis-phenylethynylanthracene (2CBPEA), 9,10-bis(phenylethnyl)anthracene (BPEA), 9,10- bis(phenylethynyl)napthacene (BPEN), perylene, coumarin 343 (C343), 9,10- dimethylanthracene (DMA), pyrene, tert-butylpyrene, and iodophenyl-bearing boron dipyrromethene (BODIPY) derivatives BD-1 and BD-2.

Certain embodiments comprise an ethynyl anthracene having a formula (IV), wherein R ^c and R° are independently selected from the group consisting of optionally substituted alkyl and optionally substituted silyl. In some embodiments, an acceptor comprises formula (IV).

As used herein, the term “alkyl” includes straight, branched and cyclic alkyl groups. An analogous convention applies to other generic terms such as “alkenyl”, “alkynyl”, and the like. Furthermore, as used herein, the terms “alkyl”, “alkenyl”, “alkynyl”, and the like encompass both substituted and unsubstituted groups. In certain embodiments, as used herein, “alkyl” is used to indicate those aliphatic groups (cyclic, acyclic, substituted, unsubstituted, branched or unbranched) having 1-20 carbon atoms. Aliphatic group substituents include, but are not limited to, any of the substituents described herein, that result in the formation of a stable moiety ( e.g ., aliphatic, alkyl, alkenyl, alkynyl, hetero aliphatic, heterocyclic, aryl, heteroaryl, acyl, nitrido, imino, thionitrido, cyano, isocyano, amino, azido, nitro, hydroxyl, thiol, halo, aliphaticamino, heteroaliphaticamino, alkylamino, heteroalkylamino, arylamino, heteroarylamino, alkylaryl, arylalkyl, aliphaticoxy, heteroaliphaticoxy, alkyloxy, heteroalkyloxy, aryloxy, heteroaryloxy, aliphaticthioxy, heteroaliphaticthioxy, alkylthioxy, hetero alky lthioxy, arylthioxy, heteroarylthioxy, acyloxy, and the like, each of which may or may not be further substituted).

As used herein, the term “alkyl” is given its ordinary meaning in the art and refers to the radical of saturated aliphatic groups, including straight-chain alkyl groups, branched-chain alkyl groups, cycloalkyl (alicyclic) groups, alkyl substituted cycloalkyl groups, and cycloalkyl substituted alkyl groups. In some cases, the alkyl group may be a lower alkyl group, i.e., an alkyl group having 1 to 10 carbon atoms (e.g., methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, or decyl). In some embodiments, a straight chain or branched chain alkyl may have 30 or fewer carbon atoms in its backbone, and, in some cases, 20 or fewer. In some embodiments, a straight chain or branched chain alkyl may have 12 or fewer carbon atoms in its backbone (e.g., C1-C12 for straight chain, C3-C12 for branched chain), 6 or fewer, or 4 or fewer. Likewise, cycloalkyls may have from 3-10 carbon atoms in their ring structure, or 5, 6, or 7 carbons in the ring structure. Examples of alkyl groups include, but are not limited to, methyl, ethyl, propyl, isopropyl, cyclopropyl, butyl, isobutyl, /-butyl, cyclobutyl, hexyl, and cyclochexyl.

In certain other embodiments, the alkyl groups employed in the invention contain 1-10 aliphatic carbon atoms. In yet other embodiments, the alkyl, alkenyl, and alkynyl groups employed in the invention contain 1-8 aliphatic carbon atoms. In still other embodiments, the alkyl groups employed in the invention contain 1-6 aliphatic carbon atoms. In yet other embodiments, the alkyl groups employed in the invention contain 1- 4 carbon atoms. Illustrative aliphatic groups thus include, but are not limited to, for example, methyl, ethyl, n-propyl, isopropyl, allyl, n-butyl, sec-butyl, isobutyl, /-butyl, n- pentyl, sec-pentyl, isopentyl, /-pentyl, n-hexyl, sec -hexyl, moieties and the like, which again, may bear one or more substituents.

The term “cycloalkyl,” as used herein, refers specifically to groups having three to ten, preferably three to seven carbon atoms. Suitable cycloalkyls include, but are not limited to cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl and the like, which, as in the case of other aliphatic, heteroaliphatic, or hetercyclic moieties, may optionally be substituted with substituents including, but not limited to aliphatic; heteroaliphatic; aryl; heteroaryl; arylalkyl; heteroarylalkyl; alkoxy; aryloxy; heteroalkoxy; heteroaryloxy; alkylthio; arylthio; heteroalky lthio; heteroaryl thio; -F; -Cl; -Br; -I; -OH; -NO2; -CN; -CF ; -CH2CF3; -CHCI2; -CH2OH; -CH2CH2OH; -CH ₂ NH ₂ ; - CH2SO2CH3; -C(0)R _x ; -C0 ₂ (RX); -CON(R _x ) ₂ ; -OC(0)R _x ; -OC0 ₂ R _x ; -OCON(R _x ) ₂ ; - N(R _X )2; -S(0) ₂ R _x ; -NR _X (CO)R _x , wherein each occurrence of R _x independently includes, but is not limited to, aliphatic, heteroaliphatic, aryl, heteroaryl, arylalkyl, or heteroarylalkyl, wherein any of the aliphatic, heteroaliphatic, arylalkyl, or heteroarylalkyl substituents described above and herein may be substituted or unsubstituted, branched or unbranched, cyclic or acyclic, and wherein any of the aryl or heteroaryl substituents described above and herein may be substituted or unsubstituted. Additional examples of generally applicable substituents are illustrated by the specific embodiments shown in the Examples that are described herein.

The term “aryl” is given its ordinary meaning in the art and refers to aromatic carbocyclic groups, optionally substituted, having a single ring (e.g., phenyl), multiple rings (e.g., biphenyl), or multiple fused rings in which at least one is aromatic (e.g., 1,2,3,4-tetrahydronaphthyl, naphthyl, anthryl, or phenanthryl). That is, at least one ring may have a conjugated pi electron system, while other, adjoining rings can be cycloalkyls, cycloalkenyls, cycloalkynyls, aryls and/or heterocyclyls. The aryl group may be optionally substituted, as described herein. Substituents include, but are not limited to, any of the previously mentioned substituents, i.e., the substituents recited for aliphatic moieties, or for other moieties as disclosed herein, resulting in the formation of a stable compound. In some cases, an aryl group is a stable mono- or polycyclic unsaturated moiety having preferably 3-14 carbon atoms, each of which may be substituted or unsubstituted. “Carbocyclic aryl groups” refer to aryl groups wherein the ring atoms on the aromatic ring are carbon atoms. Carbocyclic aryl groups include monocyclic carbocyclic aryl groups and polycyclic or fused compounds (e.g., two or more adjacent ring atoms are common to two adjoining rings) such as naphthyl groups.

In some embodiments, the aryl group is a radical of a monocyclic or polycyclic (e.g., bicyclic or tricyclic) 4n+2 aromatic ring system (e.g., having 6, 10, or 14 p electrons shared in a cyclic array) having 6-14 ring carbon atoms and zero heteroatoms provided in the aromatic ring system (“C6 14 aryl”). In some embodiments, an aryl group has 6 ring carbon atoms (“Ce aryl”; e.g., phenyl). In some embodiments, an aryl group has 10 ring carbon atoms (“Cio aryl”; e.g., naphthyl such as 1-naphthyl and 2-naphthyl). In some embodiments, an aryl group has 14 ring carbon atoms (“Ci4 aryl”; e.g., anthracyl). “Aryl” also includes ring systems wherein the aryl ring, as defined above, is fused with one or more carbocyclyl or heterocyclyl groups wherein the radical or point of attachment is on the aryl ring, and in such instances, the number of carbon atoms continue to designate the number of carbon atoms in the aryl ring system.

As used herein, the term “silyl” is given its ordinary meaning in the art and refers to the radical of a silicon-containing saturated silane group (e.g., -S1H3) in which zero or one or more of hydrogens has been replaced with an organic group (e.g., alkyl, aryl), including straight-chain silyl groups, branched-chain silyl groups, cyclosilyl groups, alkyl substituted cyclosilyl groups, cycloalkyl substituted silyl groups, and aryl silyl groups. Examples of silyl groups include, but are not limited to, trimethylsilyl (-SiMe3, TMS), triethylsilyl (-SiEt3, TES), triisopropylsilyl (Si(iPr)3, TIPS), tert- butyldimethylsilyl (TBS or TBDMS), and / _<?/7 -butyldiphcnyl silyl (TBDPS).

It will be appreciated that the above groups and/or compounds, as described herein, may be optionally substituted with any number of substituents or functional moieties. That is, any of the above groups may be optionally substituted. As used herein, the term “substituted” is contemplated to include all permissible substituents of organic compounds, “permissible” being in the context of the chemical rules of valence known to those of ordinary skill in the art. In general, the term “substituted” whether preceded by the term “optionally” or not, and substituents contained in formulas of this invention, refer to the replacement of hydrogen radicals in a given structure with the radical of a specified substituent. When more than one position in any given structure may be substituted with more than one substituent selected from a specified group, the substituent may be either the same or different at every position. It will be understood that “substituted” also includes that the substitution results in a stable compound, e.g., which does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc. In some cases, “substituted” may generally refer to replacement of a hydrogen with a substituent as described herein. However, “substituted,” as used herein, does not encompass replacement and/or alteration of a key functional group by which a molecule is identified, e.g., such that the “substituted” functional group becomes, through substitution, a different functional group. For example, a “substituted phenyl group” must still comprise the phenyl moiety and cannot be modified by substitution, in this definition, to become, e.g., a pyridine ring. In a broad aspect, the permissible substituents include acyclic and cyclic, branched and unbranched, carbocyclic and heterocyclic, aromatic and nonaromatic substituents of organic compounds. Illustrative substituents include, for example, those described herein. The permissible substituents can be one or more and the same or different for appropriate organic compounds. For purposes of this invention, the heteroatoms such as nitrogen may have hydrogen substituents and/or any permissible substituents of organic compounds described herein which satisfy the valences of the heteroatoms.

Furthermore, this invention is not intended to be limited in any manner by the permissible substituents of organic compounds. The term “stable,” as used herein, preferably refers to compounds which possess stability sufficient to allow manufacture and which maintain the integrity of the compound for a sufficient period of time to be detected and preferably for a sufficient period of time to be useful for the purposes detailed herein.

Examples of substituents include, but are not limited to, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether, alkylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl, aromatic or heteroaromatic moieties, -CF ₃ , -CN, aryl, aryloxy, perhaloalkoxy, aralkoxy, heteroaryl, heteroaryloxy, heteroarylalkyl, heteroaralkoxy, azido, amino, halide, alkylthio, nitrido, acylalkyl, carboxy esters, -carboxamido, acyloxy, aminoalkyl, alkylaminoaryl, alkylaryl, alkylaminoalkyl, alkoxyaryl, arylamino,aralkylamino, alkylsulfonyl, -carboxamidoalkylaryl, -carboxamidoaryl, hydroxyalkyl, haloalkyl, alkylaminoalkylcarboxy-, aminocarboxamidoalkyl-, cyano, alkoxyalkyl, perhaloalkyl, arylalkyloxyalkyl, and the like.

Without wishing to be bound by any theory, it is believed that triplet upconversion (or triplet-triplet annihilation, TTA) may be used to produce light of a higher energy relative to light used to photoexcite the sensitizer or the upconverter. TTA refers to the energy transfer mechanism between two molecules (e.g., two upconverters) in their triplet state, and is related to the Dexter energy transfer mechanism. If TTA occurs between two molecules in their excited states, one molecule transfers its excited state energy to the second molecule, resulting in one molecule returning to its ground state and the second molecule being promoted to a higher excited singlet, triplet, or quintet state. Because TTA combines the energy of two triplet excited molecules onto one molecule to produce a higher excited state, it may be used to convert the energy of two photons each of a lower energy into one photon of higher energy (i.e., photon upconversion or triplet upconversion, as described herein). To achieve photon upconversion through triplet-triplet annihilation, two types of molecules may be combined: a sensitizer and an upconverter (i.e., annihilator). The sensitizer absorbs a low energy photon and populates its first excited triplet state (Tl) through intersystem crossing. The sensitizer then transfers the excitation energy to the upconverter, resulting in a triplet excited upconverter and a ground state sensitizer. Two triplet-excited upconverters may then undergo triplet-triplet annihilation, and if a singlet excited state (SI) of the upconverter is populated, fluorescence results in an upconverted photon. For certain embodiments, a heavy atom-functionalized upconverter may slow or prevent two triplet-excited upconverters from colliding and undergoing TTA, thereby slowing the process of TTA and hence avoiding saturation of triplet upconverters until higher powers than in the absence of the same upconverter without the heavy atom functionalization, thus increasing the upconversion threshold of the system. Thus, certain embodiments can include heavy atom-functionalized upconverters, such as those described herein.

The inclusion of a heavy atom-functionalized upconverter may advantageously allow for an increase in laser power to be used while maintaining a quadratic dependence on the photoluminescence of the upconverter as a function of laser power.

In certain embodiments, a heavy atom- functionalized upconverter may increase the upconversion threshold. The upconversion threshold may, in certain embodiments, refer to the point at which the amount of upconverted light ceases to increase quadratically with input light (e.g., laser light) and begins to increase linearly instead. As described above and without wishing to be bound by any theory, the a heavy atom- functionalized upconverter or a heavy atom-functionalized auxiliary molecule may act to reduce the number of excited upconverters such that the upconversion (or triplet-triplet annihilation) processes remains second order with respected to the upconverter and that incident light (i.e., photons) may increase the upconversion frequency. In this case, the upconversion threshold is the point where the process switches from second order to first order with respect to the upconverter, such that incident light (e.g., laser light) no longer increases the upconversion frequency. The upconversion threshold may be measured by plotting photoluminescence versus input power laser power, as illustrated by the Examples below. Other methods of measuring the upconversion threshold are possible.

According to certain embodiments, the acceptor comprises an optionally substituted ethynyl anthracene or diethynyl anthracene. In certain embodiments, the acceptor is bisphenyl ethynyl anthracene. Additional non-limiting examples of acceptors may include 9,10-diphenylanthracene (DPA), TIPS-tetracene, tetra-tert-butylperylene, anthracene (An), 2,5-diphenyloxazole (PPO), rubrene, 2-chloro-bis- phenylethynylanthracene (2CBPEA), 9,10-bis(phenylethnyl)anthracene (BPEA), 9,10- bis(phenylethynyl)napthacene (BPEN), perylene, coumarin 343 (C343), 9,10- dimethylanthracene (DMA), pyrene, tert-butylpyrene, and iodophenyl-bearing boron dipyrromethene (BODIPY) derivatives BD-1 and BD-2. Other acceptors may be acceptable as this disclosure is not so limiting.

The sensitizer, the upconverter, and/or the acceptor can be present at any suitable amount or concentration. In some embodiments, the concentration may be expressed as a molar ratio (and/or a mole fraction) of a sensitizer, upconverter, and/or an acceptor. For example, in some cases, the ratio of upconverter to sensitizer is 10:1. In some cases, the ratio of the upconverter to the sensitizer is no more than 100:1, no more than 75:1, no more than 50:1, no more than 25:1, no more than 10:1, no more than 5:1, no more than 3:1, or no more than 1:1. in some cases, the ratio of upconverter to sensitizer is 10:1. In some embodiments, the ratio of the upconverter to the sensitizer is at least 100:1, at least 75:1, at least 50:1, at least 25:1, at least 10:1, at least 5:1, at least 3:1, or at least 1:1. In addition more than one sensitizer, more than one upconverter, and/or more than one acceptor may be present in some embodiments.

In some embodiments, the concentration (of a sensitizer, of an upconverter, of an acceptor, etc.) may be expressed in terms of molar concentration or molarity (M). In some embodiments, the concentration of a sensitizer, an upconverter, and/or an acceptor is at least 5 M, at least 6 M, at least 7 M, at least 8 M, at least 9 M, or at least 10 M. In some embodiments, the concentration of a sensitizer, an upconverter, and/or an acceptor is no greater than 10 M, no greater than 9 M, no greater 8 M, no greater than 7 M, no greater than 6 M, or no greater than 5 M. Combinations of the above-reference ranges are also possible (e.g., at least 5 M and no greater than 6 M). Other ranges are possible. The sensitizer, the upconverter, and the acceptor can be present at any suitable amount or concentration, and it should be understood that the concentration of one of these (e.g., as discussed in this paragraph) can be independent of the concentration of the others. In certain embodiments, the sensitizer, the upconverter, and the acceptor are contained within a liquid, which also may comprise a polymerizable species. A polymerizable species describes a chemical entity capable of undergoing a chemical reaction to produce a polymer, such as plastics, resins, etc. The polymerizable species may be, for example, monomers or other entities that can be polymerized to form a polymer, such as oligomers or other partially-formed polymers. In some cases, light may be used to cause the polymerizable species to polymerize; that is, the polymerizable entities may be photopolymerizable. In some cases, the polymerizable entities may be polymerized to form a polymeric solid object. For certain embodiments, the polymerizable species is a precursor to a polymeric object produced by 3D printing.

In some embodiments, photons produced by the upconversion of two upconverters is used to caused polymerization of the polymerizable species. Referring now to FIG. 1, for example, upconverters 130 and 140 may interact and/or collide to produce upcoverted photon 150. Photon 150 may then cause the polymerization of polymerizable species 160. Although not pictured, either upconverters 130 or 140 (or both) may be in an excited state, excited by, for example, sensitizer 110. Optionally, a triplet acceptor which can receive triplets from either sensitizer or upconverter can also be included to increase upconversion threshold. According to one set of embodiments, the polymerizable species may comprise a resin, such as a 3D printing resin. Examples of 3D printing resins include, but are not limited to, thermoplastics and thermo- setting resins. Many of these are commercially available. Specific non-limiting examples include polyamides, polypropylene, ABS, PLA, PVA, PET, PETT, HIPS, nylon, etc. Additional examples of monomers include vinyl monomers, acrylates, styrenic monomers, and the like. In some cases, the monomer has a double bond, e.g., an alkene. A variety of monomers can be used, e.g., for 3D printing. For instance, examples of acrylates include, but are not limited to, methacrylate, methyl methacrylate, polyacrylates, or the like.

Still other examples of monomers include, but are not limited to, branched polyethylene glycol; linear polyethylene glycol; polyamides and polyamines such as nylon 6, nylon 6,6-poly(pyromellitic dianhydride-co-4,4'-oxydianiline); polyesters 5 such as poly(ethylene terephthalate, poly(4,4'-methylenebis(phenyl isocyanate)-alt-l,4- butanediol/di(propylene gl yco 1 )/po 1 ycapro 1 actonc) ; poly ethers such as Pluronic®F127, poly(2, 6-dimethyl- 1 ,4-phenylene oxide) ; poly(oxy- 1 ,4-phenylenesulfonyl- 1,4- phenylene); silicones such as poly(dimethylsiloxane); vinyl polymers such as HDPE, poly(acrylonitrile-co-butadiene) acrylonitrile, poly( 1 -(4-(3-carboxy-4- hydroxyphenylazo)benzenesulfonamido)-l,2-ethanediyl, sodium salt), polychloroprene, polyethylene, PMMA, polystyrene, poly(styrene-co-acrylonitrile), polystyrene-block- poly(ethylene-ran-butylene)-block-poly styrene, poly(vinyl acetate); poly(vinyl alcohol), polyvinylpyrrolidone; etc. Other monomers still are also possible.

The liquid containing components such as the sensitizer, the upconverter, and the acceptor may be any suitable liquid. For instance, the liquid may be a solvent, including benzene, toluene, iodobenzene, dichloromethane, acetonitrile, methanol, ethanol, as non limiting examples, or any organic solvent capable of dissolving or suspending the components of the liquid. The liquid may also be transparent in some cases, e.g., so as to allow light of a certain wavelength or a particular range of wavelengths to pass through the liquid in order to, for example, interact with the sensitizer or other component of the liquid.

Thus, in some embodiments, the liquid may help to facilitate polymerization of a polymerizable species. For instance, light or other electromagnetic radiation may be focused on specific regions within the liquid that can be upconverted as discussed herein to cause polymerization of a polymerizable species in the liquid in those regions to occur, e.g., while avoiding or minimizing polymerization in other regions of the liquid. Thus, in some cases, the liquid may be one that is optically transparent for a certain set of wavelengths. For example, in embodiments, the liquid is optically transparent to light of a wavelength of 450 nm. In some embodiments, the liquid is optically transparent to light of a wavelength of 1100 nm. In some cases still, the liquid is optically transparent to a wavelength between 450 nm and 1100 nm (e.g. 455 nm, 460 nm, 465 nm, ..., 1090 nm, 1095 nm). Other wavelengths outside of 450 nm to 1100 nm may also be possible.

Optical transparency may be determined, for example, by taking an absorption spectrum. The transmission of light, or the optical transparency, can be determined as absorbance = 2 - log(transmittance). The liquid may have any suitable viscosity. In some cases, the viscosity is relatively low (e.g., similar to water), although in other cases, the viscosity may be higher. For example, relatively high viscosities may be useful to allow relatively fast polymerization of the polymerizable species to form a polymeric object to occur within the liquid or other material, e.g., without the polymeric object being able to drift too far or too quickly away from its initial position, due to the viscosity of the liquid. Thus, in certain embodiments, the polymerizable species may be polymerized into a solid object while free-floating in a liquid. Thus the viscosity of the liquid may be at least about 1 cP, at least about 3 cP, at least about 5 cP, at least about 10 cP, at least about 30 cP, at least about 50 cP, at least about 100 cP, at least about 300 cP, at least about 500 cP, at least about 1,000 cP, at least about 3,000 cP, at least about 5,000 cP, at least about 10,000 cP, at least about 30,000 cP, at least about 50,000 cP, at least about 100,000 cP, etc. In some cases, the viscosity may be less than about 300,000 cP, less than about 100,000 cP, less than about 50,000 cP, less than about 30,000 cP, less than about 10,000 cP, less than about 5,000 cP, less than about 3,000 cP, less than about 1,000 cP, less than about 500 cP, less than about 300 cP, less than about 100 cP, less than about 50 cP, less than about 30 cP, less than about 10 cP, less than about 5 cP, less than about 3 cP, etc. Combinations of any of these ranges are also possible. For example, the viscosity of the liquid may be between 10,000 cP and 300,000 cP.

A variety of techniques or components may be used within the liquid to increase its viscosity. Examples of components that can be added include, but are not limited to, gelatin, xanthan gum or other macromolecules. In some cases, a polymer of the resin itself may be used to increase the viscosity of the liquid. For example, for a methacrylate monomer, a component such as polymethacrylate may be added to the liquid to increase its viscosity. In addition, in some cases, a combination of techniques and/or components may be used.

Thus, in some embodiments, methods of 3D printing a polymeric object is provided, e.g., as discussed above. In some cases, the method includes providing a liquid comprising a polymerizable species, a sensitizer, an upconverter, and an acceptor.

For example, polymerization of the polymerizable species may be facilitated using a laser, e.g., to cause upconversion and the production of higher-energy photons that can be used for polymerization. Thus, in certain embodiments, one or more lasers are present. An example of such a laser is illustrated by laser 115 in FIG. 1. In some cases, this laser is a part of a 3D printing device. The laser may be the source of photons, e.g., that can be used to cause photoexcitation of the sensitizer and/or the upconverter. The laser may be focused, and/or may intersect with other lasers to create regions of more and less intense laser light. The laser may have a particular excitation wavelength, e.g., as discussed below. In some cases, as mentioned, the light or photons produced by upconversion are higher in energy than the excitation wavelength (i.e., its corresponding excitation energy) of the laser. According to certain embodiments, two, three, four, or more lasers may be present, for example, controlled to focus on a location or region within a liquid.

In some cases, the light may be directed at the upconversion compositions, e.g., such that the resulting upconverted light is able to initiate polymerization. In some embodiments, as described above, a laser may be the source of the light. For example, the mixture or liquid within a container containing the upconversion materials may be irradiated with light (e.g., laser light) to initiate upconversion and/or to initiate polymerization of the polymerizable species. Suitable wavelengths include, for example, 400 nm to 800 nm, e.g., as the excitation wavelength. As a non-limiting example, upconverted light can be produced locally between 390-500 nm using 532 nm laser light, which is in the range of some common photopolymerization initiators. As another example, light can be applied having a range of between 600 nm and 700 nm, or between 600 nm and 650 nm, which can then be upconverted as discussed herein, e.g., producing shorter wavelengths (or equivalently, higher frequencies or energies). The light may be applied using any suitable light or electromagnetic radiation source, such as a laser or other coherent light source. For example, in one embodiment, the light source is a laser diode, such as those available commercially.

In some embodiments, a laser has a characteristic intensity or power density. This intensity or power density can be selected in some cases to match the upconversion threshold (i.e., the value where the upconversion threshold changes from quadratic to linear, from second order to first order). For instance, the intensity or power density of the applied electromagnetic radiation applied to the focal point or region to cause polymerization to occur may be less than 5,000 W/cm ² , less than 3,000 W/cm ² , less than 2,000 W/cm ² , less than 1,000 W/cm ² , less than 500 W/cm ² , less than 300 W/cm ² , less than 200 W/cm ² , less than 100 W/cm ² , less than 50 W/cm ² , less than 30 W/cm ² , less than 20 W/cm ² , less than 10 W/cm ² , less than 5 W/cm ² , less than 3 W/cm ² , less than 2 W/cm ² , less than 1 W/cm ² , less than 500 mW/cm ² , less than 300 mW/cm ² , less than 200 mW/cm ² , less than 100 mW/cm ² , etc.

According to certain embodiments, one, two, or more (i.e., three, four, etc.) laser beams may be focused in at least a portion of a container, e.g., containing a liquid and other components such as those discussed herein. In some cases, the focus of the laser beams may be altered or moved around within the container, which can be used to define an object, e.g., by causing polymerizable entity within the focus to polymerize to produce the object. It should be understood that the focus need not define a contiguous region. For instance, one or more lasers may be turned on and off as necessary to define two, three, four, or more objects within the container. In some embodiments, areas surrounding the focus of the lasers may also receive sufficient light to cause polymerization to occur, e.g., using upconversion as discussed herein. In some embodiments, the area of a spot created by at least one laser beam is at least 300 nm. In some embodiments, the area of a spot created by at least one laser beam is no greater than 1 mm. In some embodiments, the area of a spot created by at least one laser beam is between 300 nm and 1 mm.

Thus, in some embodiments, a method of 3D printing involves focusing at least one laser beam on at least a portion of the liquid, e.g., a focal region, wherein at least some of the laser beam with a first energy is absorbed by the sensitizer. As mentioned above, the sensitizer may absorb a photon. As a non-limiting example, a laser, such as laser 115 in FIG. 1 provides the laser beam of first energy 120 to sensitizer 110.

For certain embodiments, substantially no polymerization occurs outside of the focal region of the laser beam in the liquid, e.g., due to the quadratic dependence of the upconverter as a function of laser power. This may advantageously allow for formation of a polymeric object to occur in specified areas while preventing polymerization in other areas, in certain embodiments.

In addition, in some instances, the liquid may comprise additional components. Several of these additional components will be described below.

According to certain embodiments, the liquid may further comprise a micelle forming agent or micelle-forming molecule. In some embodiments, the micelle-forming agent is a surfactant. In certain embodiments, the micelle-forming agent is oleic acid. The micelle-forming agent may interact with other components comprising the liquid as to form a micelle to encapsulate the components. Non-limiting examples of micelle forming agents include Triton™ X100, Pluronic® F-127, sodium dodecyl sulfate, and bovine serum albumin.

Certain embodiments use a nanocapsule to encapsulate the liquid, e.g., one or more of the sensitizer, the upconverter, and/or the acceptor. The nanocapsule may, in some cases, include a vesicular system made of a membrane or a shell which encapsulates an inner liquid core at the nanoscale. In some embodiments, the shell is a silica-based shell (e.g., SiC ). A nanocapsule may contain upconversion materials or molecules (e.g., a sensitizer, an upconverter, an acceptor) that can be used to facilitate photon upconversion. The nanocapsules may be contained within a liquid or other within a container of a 3D printing device, which may also contain polymerizable species, cross-linking agents, photopolymerization initiators, or the like, e.g., as discussed herein. Light focused on the nanocapsules may be upconverted to produce wavelengths sufficient to cause polymerization to occur, e.g., as discussed herein. However, in contrast, although other regions within the liquid may receive some light, that light may not be sufficient to be upconverted, and thus, any polymerizable species in those regions would generally not polymerize.

The nanocapsules are typically approximately spherical and may have an average diameter of less than 1 micrometer, e.g., such that the nanocapsules have an average diameter on the order of nanometers. The nanocapsules, for example, may have an average diameter of less than about 1 micrometer, less than about 900 nm, less than about 800 nm, less than about 700 nm, less than about 600 nm, less than about 500 nm, less than about 400 nm, less than about 300 nm, less than about 200 nm, less than about 100 nm, less than about 90 nm, less than about 80 nm, less than about 70 nm, less than about 60 nm, less than about 50 nm, less than 40 nm, less than about 30 nm, less than about 20 nm, less than about 10 nm, less than about 5 nm, less than about 3 nm, etc. In addition, some cases, the nanocapsules may have an average diameter of at 20 least about 5 nm, at least about 10 nm, at least about 20 nm, at least about 30 nm, at least about 40 nm, at least about 50 nm, at least about 60 nm, at least about 70 nm, at least about 80 nm, at least about 90 nm, at least about 100 nm, at least about 200 nm, at least about 300 nm, at least about 400 nm, at least about 500 nm, at least about 600 nm, at least about 700 nm, at least about 800 nm, at least about 900 nm, etc. In some cases, combinations of any of these are also possible. For example, the nanocapsules may have a diameter between or equal to 30 and 40 nm between 50 nm and 100 nm, between 100 nm and 400 nm, or the like. In addition, it should be understood that in some embodiments, the nanocapsules may be present with a range of sizes or average diameters (i.e., the nanocapsules need not all have precisely the same dimensions), which may include any suitable combination of any of the above-described dimensions.

In some cases, the nanocapsules are smaller than the wavelength of visible light. Nanocapsules having smaller dimensions may be useful in certain embodiments, as they do not substantially interfere with the passage of visible light, thus allowing liquids containing such nanocapsules to appear optically transparent, or to allow visible light to pass without significant scatter.

As mentioned above, the nanocapsules may comprise a silica (S1O2) shell. This may, for instance, impart some rigidity to the nanocapsules. Such a shell may be formed, for example, upon reaction of a silane (e.g., 3-aminopropyl triethoxysilane) with a silicate (e.g., tetraethyl orthosilicate). The silica shell may also be crosslinked together in certain embodiments. In addition, in some cases, the silicate may comprise a hydrophilic portion (e.g., methoxy polyethylene glycol tetraethyl orthosilicate), such that upon formation of the silica shell, the nanocapsule comprises an outer portion that is relatively hydrophilic (e.g., comprising polyethylene glycol). Such a relatively hydrophilic outer portion may, for example, allow dispersion or dissolution of the nanocapsules in a number of different solvents or liquids. In addition, the relatively hydrophilic portions (e.g., comprising polyethylene glycol units) thus can be covalently linked to the silica shell.

The liquid may also optionally contain one or more photopolymerization initiators according to certain embodiments. The initiators may form free radicals or cations upon initiation. Examples of photopolymerization initiators, but are not limited to, isopropylthioxanthone, benzophenone, 2,2-azobisisobutyronitrile, camphorquinone, diphenyltrimethylbenzoylphosphine oxide (TPO), HCP (1- hydroxycyclohexylphenylketone), BAPO (phenyl bis-2,4,6-(trimethylbenzoyl)phosphine oxide). Other examples include Norrish Type-1 and Norrish Type-2 initiators.

In addition, in some cases, the liquid may also contain one or more cross-linking agents that are able to polymerize with the polymerizable species. Non-limiting examples of crosslinking agents include ethylene glycol dimethacrylate, trimethylolpropane triacrylate, divinylbenzene, N,N’-methylenebisacrylamide, etc.

In certain aspects, the liquid may be contained within a container, and the container may be transparent to light (or other suitable electromagnetic radiation) applied to the liquid. The light may be visible light, ultraviolet light, or other suitable forms of

As mentioned, it should be understood that the photon upconversion materials discussed herein are not limited to only 3D printing applications. Other applications, such as photoredox catalysis chemistry or anti-counterfeiting, are also contemplated as well. For instance, for photoredox catalysis chemistry, the nanocapsules may be used to control delivery of high energy light to a sample. For example, laser light may be applied to a sample that is of a relatively low intensity, long wavelength, etc., but due to the presence of the nanocapsules, that light may be upconverted to a shorter wavelength that can induce a photoredox reaction to occur. In this way, the amount of light applied to the sample may be controlled. This approach may be particularly useful in the event that shorter wavelength light is prone to scatter, either by the reaction medium, by biological tissue, or whatever medium the photoredox chemistry occurs in. In this case, upconversion may be useful in delivering upconverted short wavelength light further into a reaction than is possible by direct illumination at the same wavelength.

Similarly, for anti-counterfeiting, the nanocapsules may be contained within a suitable component (e.g., paper, a polymer, a metal, or the like), and the presence of upconversion may be used to determine whether the component is genuine or counterfeit. Thus, for instance, laser light may be applied to the component, and if the material produces emission of light at shorter wavelengths than the excitation wavelengths (for example, due to the presence of the nanocapsules), the component can be identified as being genuine.

The following are each incorporated herein by reference in their entireties: U.S. Pat. Apl. Ser. No. 62/771,996, filed on November 27, 2018, entitled “Photon Upconversion Micelles for 3d Printing and Other Applications”; U.S. Pat. Apl. Ser. No. 62/800,680, filed on February 4, 2019, entitled “Photon Upconversion Micelles for 3d Printing and Other Applications”; Int. Pat. Apl. Ser. No. PCT/US 19/63629, filed November 27, 2019, entitled “Photon Upconversion Micelles for 3d Printing and Other Applications”; and U.S. Pat. Apl. Ser. No. 62/864,595, filed on June 21, 2019, entitled “Triplet Exciton Acceptors for Increasing Upconversion Threshold 3d Printing.” In addition, U.S. Pat. Apl. Ser. No. 62/911,125, filed on October 4, 2019, entitled “Triplet Exciton Acceptors for Increasing Upconversion Threshold for 3D Printing” by Congreve, et al., and Int. Pat. Apl. Ser. No. PCT/US20/38057, filed June 17, 2020, entitled “Triplet Exciton Acceptors for Increasing Upconversion Threshold for 3D Printing” by Congreve, et al. are also incorporated herein by reference in its entirety.

U.S. Pat. Apl. Ser. No. 62/911,128, filed on October 4, 2019, entitled “Heavy Atom- Functionalized Upconverters for Increasing Upconversion Thresholds for 3D Printing” and U.S. Pat. Apl. Ser. No. 63/013,679, filed April 22, 2020, entitled “Heavy Atom- Functionalized Upconverters for Increasing Upconversion Thresholds for 3D Printing” are also each incorporated herein by reference in its entirety.

The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention.

EXAMPLE 1

Photoluminescence as a function of input power for continuous wave illumination was probed at a series of excitation powers to produce the plot shown in FIG. 3. While in the control experiment without any biphenyl ethynyl anthracene, the quadratic regime did not persist past 1 mW of input power, when a saturated solution of bisphenyl ethynyl anthracene is added up to 14 uL per mL, the plot of photoluminescence versus power remains quadratic up to -10 mW. In the context of 3D printing, this formulation may allow printing at ~10x higher powers without losing contrast between emission from the focused and unfocused parts of our laser beam.

EXAMPLE 2

The following example describes the preparation of several annhilators that can be tuned to adjust the linear vs. quadratic threshold. These annihilators are then used in a 3D-printing setup to print an object, “Benchy.”

Three-dimensional (3D) printing can be used in manufacturing, providing custom parts. In certain existing systems, stereolithography, where layers are patterned sequentially by two-dimensional photopolymerization, has been shown to be particularly successful due to its relatively high resolution and surface finish. The layer-by-layer nature of this process, however, introduces limitations that hinder resin choice, and shape gamut and material quality. Further, the need for support structures in these existing systems often necessitates significant postprocessing. By contrast, as described herein, triplet fusion upconversion is utilized to perform volumetric 3D printing by taking advantage of the quadratic nature of photon upconversion, which occurs predominately where light is most intense. By encapsulating upconverting materials inside a resin- dispersible nanocapsule, the ability to upconvert locally throughout the resin is imparted to the resin, allowing photopolymerization within the printing volume. As described herein, the upconversion threshold can be extensively tuned to suit different printing setups. The entire printing process can be performed air-free, opening up new material and geometric opportunities for 3D printing.

Three-dimensional (3D) printing, also known as additive manufacturing, has received considerable interest as technologies have opened up a variety of applications for the technique. One mode of 3D printing is stereolithography (SLA), where a photopolymer is patterned using a laser or light emitting diode. However, due to the linear absorption of the light, this technique requires photopolymerization to occur at the surface of the printing volume, necessitating layer-by-layer printing. While some technologies have attempted to circumvent this limitation, the interfacial nature of this printing technique imparts fundamental limitations on resin choice and shape gamut and often necessitates use of extensive support structures.

One promising way to circumvent this layer-by-layer paradigm is to move beyond linear absorption. The use of photon absorption (2PA) to print in a volumetric fashion, rather than at the interface of a surface is described below and elsewhere herein. Advantageously, the quadratic dependence of 2PA on incident light intensity allows for the generation of short wavelength photopolymerizing light only at the focal point of a laser beam, which can then be scanned in three dimensions to generate a 3D print. However, the laser power (and thus cost and complexity) required to drive this process has limited print size and speed, which has prevented widespread application. Other issues such as the use of pulsed lasers with defined repetition rates, dielectric breakdown, and resin boiling at intense light fields also introduce further complications. Thus, as discussed herein, systems and methods for volumetric 3D printing may advantageously use a quadratic process that occurs at low power, supports continuous wave excitation to allow for large scale printing, and has an inexpensive, simple implementation.

Triplet fusion upconversion is one approach, as previously discussed elsewhere herein, that can be applied to volumetric 3D printing, an example of which is illustrated in FIG. 4. This process takes advantage of excitonic states in molecules to generate blueshifted anti-Stokes emission, as shown in FIG. 4C. The final upconversion step requires molecular collisions of two excited annihilator triplets, which fuse to form one higher energy annihilator singlet which then emits anti-Stokes shifted light that can be used to drive photopolymerization. Therefore, this process has a quadratic nature due to the requirement for two triplets to meet, yet also has high absorption and requires relatively low light fluences due to the high extinction coefficient of the sensitizer. It is also tunable in both excitation and emission wavelength.

Triplet fusion upconversion can be applied to volumetric 3D printing. However, upconversion may exhibit a threshold behavior where the process crosses over from quadratic to linear above a certain fluence. Achieving control over this threshold value may be important in some cases to applying upconversion to different printing schemes. For example, a single voxel at a time was targeted with a focal point on the order of 100 W/cm ² . While this operating fluence is enormously smaller than the 10 ¹² W/cm ² required for 2PA, it is considerably higher than the threshold value for typical upconversion systems. In some cases, because triplet fusion upconversion relies on high concentrations of strongly absorbing molecules undergoing frequent collisions, direct addition of the sensitizer and annihilator to the resin poses severe restrictions; high concentrations of the molecules are needed to dissolve in the 3D printing resin, resulting in excessive attenuation of the input light and limited print volumes, as well as potentially affecting the properties of printed parts. Finally, the rate of collision varies with viscosity, and as printing occurs the resin viscosity changes, changing the upconversion threshold and efficiency and thus losing the selectivity of the print.

The following are examples to address these issues. TIPS -anthracene was selected as the annihilator and Pd(II) meso-tetraphenyl tetrabenzoporphine (PdTPBP) was selected as the sensitizer. This red-to-blue upconversion system can work with efficiencies of up to 30%. However, the TIPS -anthracene threshold of 1.7 W/cm ² . To tune the upconversion threshold to relatively higher powers, heavy atom-functionalized annihilators were synthesized (e.g., TIPS-anthracene derivatized with an atom with an atomic number of at least 17). Without wishing to be bound by any particular theory, the threshold value of a triplet fusion system can be related to the nonradiative triplet decay rate k _A and the rate of triplet fusion kn :

The triplet recombination rate was focused on in order to increase the threshold by adding heavy atoms to the molecule. By adapting acetylation chemistry, a series of molecules with heavy atom substitution was prepared, see FIG. 5A, in order to adjust the threshold. This substitution introduces only small differences in emission or absorbance other than a slight redshift, as shown in FIG. 5B and FIG. 9. In FIG. 5C, the relative upconversion efficiency of each of these emitted molecules is plotted against power density. This relative efficiency is the derivative of the upconverted light versus the input light and should be constant at high powers and linear at low powers. Indeed, this relationship was observed for all five molecules. Importantly, the measured thresholds range from 1.7 W/cm ² for unsubstituted TIPS-anthracene all the way up to 283 W/cm ² for the 2Br-TIPS-anthracene, spanning over two orders of magnitude. This can impart enormous flexibility when used in printer design: a monovoxel excitation source can take advantage of the high threshold of the bromide materials at a focused laser spot, while a large area parallel excitation printer, which normally works with lower excitation intensities, can use the chlorides or even the unsubstituted TIPS-anthracene. With the upconversion materials now in hand, the challenge of embedding these materials into resins was approached. While some upconverting micelles can be prepared using a block copolymer, these materials can be unstable in the resin environment and released their contents, resulting in a loss of upconversion. As another approach a nano encapsulation technique was employed that can be dispersed in 3D printing resins without scattering light. The upconversion materials were built using upconverting silica nanocapsules that were substantially durable in water. Due to aggregation of the nanocapsules, these materials sometimes scattered the input laser beam and were not dispersible in resins. To overcome these challenges, a nanocapsule synthesis that incorporated a long PEG chain as a solubilizing ligand on the exterior of the silica shell was used. Details of the optimized synthesis can be found in at least FIG. 6A and Example 3. Silane-terminated PEG, which can covalently graft to the nanocapsule, can prevent aggregation over time and allowed the nanocapsules to disperse without scatter in 3D printing resins. Electron microscopy of the resulting upconverting nanocapsules (UCNCs) (FIGS. 6C and 6D) showed uniform capsules approximately 50 nm in diameter. Of particular importance is the stability imparted by the silica shell; to demonstrate this, the nanocapsule solution was diluted from both the initial micelle formation after dropwise addition of (3-aminopropyl)triethoxysilane (APTES) and the final shelled UCNCs at 100:1 in acetone. In the former case, the micelles fall apart, leading to limited upconversion and significant PdTPBP phosphorescence at -800 nm, while the in latter case the bright upconversion from the UCNCs is preserved with minimal phosphorescence, FIG. 6E. The capsules were further able to be dispered in a number of common solvents while maintaining bright upconversion, see FIG. 10.

The capsules could be introduced to 3D printing resins. See, for example, Int. Pat. Apl. Pub. No. PCT/US 19/63629, incorporated herein by reference in its entirety. Acrylic acid was used to disperse the capsules in an acrylate-based monomer resin. Then PETA (pentaerythritol tetraacrylate) was used to introduce the capsules into a highly cross- linkable resins to obtain rigid prints. By overfilling a back aperture of a 0.5 NA objective with a collimated laser beam and mounting the objective onto a fused deposition modeling (FDM) printer, trace of a well-defined focal point in space could be drawn throughout the upconversion-enabled resin, as shown in FIG. 7A. Due to the quadratic power dependence of triplet fusion, the upconversion is active predominantly at the focal point of the laser where the light is most intense, resulting in a well-confined blue voxel that drives local photopolymerization. The addition of a radical inhibitor, such as (2, 2, 6, 6- Tetramethylpiperidin-l-yl)oxyl (TEMPO), improved the print resolution by preventing significant polymerization outside of the irradiated voxel.

A standard yet difficult test of 3D printing systems is the benchmark boat print (often referred to as “Benchy”), shown in FIG. 7. This Benchy print was able to be reproduced at small scales using the liquids and methods described herein. Advantageously, printing could be achieved without any support structures, simplifying post-processing and limiting surface blemishes. By contrast, compare with the same file printed on a commercial printer where >80% of the resin is used in the support structure, FIG. 11. The power and print speed were carefully optimized to prevent “underprinting” and “overprinting”, FIG. 12, to realize high fidelity, reproducible prints, FIG. 7C. Additional examples of the fine details are shown by printing a Harvard logo (FIG. 13). Finally, print parts could be produced in a poly ethylene glycol diacrylate (PEGDA)- based resin, opening up this technology towards hydrogel printing for biological applications (see FIG. 14).

It is contemplated that this technique can enable printing in resins that cannot be printed in a traditional SLA 3D printing setup, such as those with high viscosity, soft parts, or resins requiring air-free polymerizations techniques. Further, the liquids and methods described herein can print surfaces without steps between layers or support structures. These features suggest promise in printing channels, fine filigree features, and other shapes that may be challenging for traditional macroscale SLA approaches. The ability to tune the threshold behavior over orders of magnitude to facilitate tailoring of these UCNCs to a variety of printing excitation schemes beyond the monovoxel printer demonstrated here are also contemplated, such as projector-based printing approaches. It is also contemplated to combine this technique with recent technological developments in optical parallelization to greatly increase print speeds. Other approaches, such as lanthanide-based upconverting nanoparticles, can likely yield successful prints in a similar fashion, with a different set of application-based tradeoffs for their implementation. In general, the liquids and methods described herein show the strength in volumetric, upconversion-nanoparticle-driven 3D printing towards fourth generation manufacturing.

EXAMPLE 3 The following example describes the preparation of several upconversion materials (e.g., annihilators), their characterization, and their use in 3D printing systems.

Materials and Methods

Materials: (Triisopropylsilyl)acetylene and n-butyllithium (2.5 M in hexanes), Pentaerythritol tetraacrylate (PETA), 2,2,6,6-Tetramethyl-l-piperidinyloxy (TEMPO), poly(ethylene glycol) diacrylate (PEGDA) and acrylic acid were purchased from Sigma Aldrich and used as received. Bis (5-2,4-cylcopentadien-l-yl)-bis(2,6-difluoro-3-(lH- pyrrol-l-yl)- phenyl) titanium (Titanocene) was purchased from Gelest and used as received. 2,6-dichloroanthracenequinone was purchased from SRP Laboratories and used as received.

Annihilator Synthesis: TIPS -anthracene (9,10-bis((triisopropylsilyl)ethynyl)anthracene) was purchased commercially, and bromo TIPS -anthracene (((2-bromoanthracene-9,10- diyl)bis(ethyne-2,l-diyl))bis(triisopropylsilane)) and dibromo TIPS-anthracene (((2,6- dibromoanthracene-9,10-diyl)bis(ethyne-2,l-diyl))bis(triisop ropylsilane)) synthesis have been reported previously. The chloro TIPS-anthracene and dichloro TIPS-anthracene were synthesized according to the same protocol.

Chloro TIPS-anthracene (((2-chloroanthracene-9, 10-diyl)bis(ethyne-2, 1 - diyl))bis(triisopropylsilane)).

To a solution of 8.21 mL (36.6 mmol, 3.5 eq) of TIPS -acetylene in 10 mL of dry tetrahydrofuran in inert atmosphere was added at 0 °C 13.8 mL of 2.5 M n-butyllithium in hexanes (34.5 mmol, 3.3 eq). This solution was kept at 0 °C for 30 minutes, when 2.54 grams (10.4 mmol, 1 eq) of 2-chloro, 9,10- anthracenequinone was added. The solution was allowed to warm to room temperature over the course of an hour when 6.00 grams of SnCl ₂ *2H ₂ 0 (26.6 mmol, 2.6 eq) was added along with 1 mL of 10% aqueous hydrochloric acid and the reaction was stirred another hour. The resulting solution was extracted between hexanes and water three times, then solvent and residual TIPS- acetylene were removed in vacuo. Column chromatography on silica gel (hexanes and dichloromethane) yielded products as bright yellow solids. 1.83 grams (31% yield) ^l U NMR (600 MHz, CDCh, d ppm): 8.66 (d, 1H), 8.62 (m, 2H), 8.66-8.63 (m, 2H), 8.59 (d, 1H), 7.63 (m, 2H), 7.55 (m, 1H), 1.28 (m, 42H) ¹³ C NMR (125 MHz, CDCb, d ppm): 135.80, 135.47, 135.45, 135.07, 133.25, 131.72,

130.62, 130.04, 129.97, 129.90, 129.76, 128.60, 121.65, 120.55, 108.24, 108.11, 105.50, 105.43, 21.53, 14.17, 14.15

MS (ESI): Calculated m/z: 572.305; Observed m/z: 572.3066

Dichloro TIPS-anthracene (((2,6-dichloroanthracene-9,10-diyl)bis(ethyne-2,l- diyl))bis(triisopropylsilane)). To a solution of 8.21 mL (36.6 mmol, 3.5 eq) of TIPS -acetylene in 10 mL of dry tetrahydrofuran in inert atmosphere was added at 0 °C 13.8 mL of 2.5 M n-butyllithium in hexanes (34.5 mmol, 3.3 eq). This solution was kept at 0 °C for 30 minutes, when 2.90 grams (10.4 mmol) of 2,6-dichloro, 9,10-anthracenequinone was added. The solution was allowed to warm to room temperature over the course of an hour when 6.00 grams of SnCl ₂ *2H ₂ 0 (26.6 mmol, 2.6 eq) was added along with 1 mL of 10% aqueous hydrochloric acid and the reaction was stirred another hour. The resulting solution was extracted between hexanes and water three times, then solvent and residual TIPS acetylene were removed in vacuo. Column chromatography on silica gel (hexanes and dichloromethane) yielded products as bright yellow solids.

2.14 grams (34% yield)

NMR (600 MHz, CDCb, d ppm): 8.62 (d, 2H), 8.54 (d, 2H), 7.55 (m, 2H) 1.28 (m, 42H)

¹³ C NMR (125 MHz, CDCb, 5 ppm): 136.13, 135.44, 133.60, 131.66, 131.20, 128.67, 120.84, 108.87, 104.92, 21.49, 14.13 MS (ESI): Calculated m/z: 606.2666; Observed m/z: 606.2676

Capsule Synthesis: Milli-Q water (150 mL) was chilled over an ice bath (temperature ~5 °C) and then poured into to a Vitamix blender (received from Amazon.com) in an inert atmosphere. The stock solution containing sensitizer and annihilator (1.45 mL) was carefully dispensed into the water in one portion (stock solution: 1 mL of saturated PdTBTP (~5 mg/mL, Frontier Scientific) in 99% oleic acid 3 mL of saturated TIPS anthracene (-50 mg/mL) in 99% oleic acid, and 1 mL 99% oleic acid (Beantown Chemical)). The solution was blended for 60 s at the maximum speed, and the emulsion was transferred to a 500 mL flask and immediately stirred at high speed. (3- aminopropyl)triethoxysilane (0.75 mL, Acros Organics) was added until the mixture became transparent, and then 10K MPEG-Silane (4 g, Nanosoft Polymers) was immediately added to prevent capsule aggregation. After 10 minutes, tetraethyl orthosilicate (TEOS, 10 mL, Sigma Aldrich) was added in one portion. The flask was sealed with a septum and the solution stirred vigorously at room temperature. All the above process was performed under inert N2 environment in a glovebox. After 30 mins, the flask was sealed tight before taking out of the glovebox and heated to 65 °C under Ar environment. A second portion of TEOS (10 mL) was added after 48 hours. A third portion of TEOS (10 mL) and second portion of MPEG-Silane (4 g) were added after 96 hours and 102 hours respectively. After 6 days, the reaction crude was cooled to room temperature, poured into a centrifuge tube, and centrifuged at 7000 rpm in a JA10 rotor on a J2- MC Beckman Coulter centrifuge for one hour at room temperature (18-22 °C), after which the pellet was discarded. The supernatant was further centrifuged at 7000 rpm for 14 hours at room temperature. After the second centrifuge, the paste-like UCNCs (-10 g) were immediately transferred to the glovebox.

Acrylate Resin: 200 mg of titanocene was added to a 20 mL scintillation vial. 16 mL of pentaerythritol tetraacrylate (PET A) was added and the vial stirred at 90 °C until the titanocene was well dissolved. 6 mL of acrylic acid was added to 4 mL of capsule paste and stirred until well dissolved at room temperature. The PETA solution was cooled to room temperature and 4 mL of the acrylic acid/capsule solution and 140 pL of 1 mg/mL TEMPO in acrylic acid were added. This solution was stirred until well mixed then sparged with nitrogen for 15 minutes. Linally, the solution was mixed in a LlackTek SpeedMixer at 1000 rpm for 5 minutes to remove bubbles. The resin was then poured into 4 mL cuvettes, mixed in the SpeedMixer again to remove bubbles, and used immediately for printing.

Hydrogel Resin: 90 mg of titanocene was added to a 20 mL scintillation vial. 16 mL of poly(ethylene glycol) diacrylate (PEGDA) was added and the vial was stirred at 90 °C until the titanocene was well dissolved. The PEGDA solution was cooled to room temperature and 2 mL of the same acrylic acid/capsule solution and 40 uL of 1 mg/mL TEMPO in acrylic acid were added. This was stirred until well mixed then sparged with nitrogen for 15 minutes. Linally, the solution was mixed in a LlackTek SpeedMixer at 1000 rpm for 5 minutes to remove bubbles. The resin was then poured into 4 mL cuvettes, mixed in the SpeedMixer again to remove bubbles, and used immediately for printing. Printing: The printing is performed on a custom built, highly modified Kossel Delta configuration reprap printer, with many modifications gathered from Haydn Huntley (https://www.kosselplus.com/). The firmware run on the printer is a fork of Reprap: RepRapFirmware-dc42, available at https://github.com/dc42/RepRapFirmware, and the printing electronics are the Duet 2 Wifi controller (https://www.duet3d.com/DuetWifi). STL files are sliced in Simplify3D to generate gCode inputs to the printer. The printing is powered by a Thorlabs S4FC637 637 nm 70 mW fiber coupled laser. The laser is collimated with a 30 mm focal length lens then fed into a 50X Mitutoyo Plan Apochromat Objective where it is focused into the resin. The entire optical system is moved by the printer in three dimensions to generate the print. Power was adjusted as a function of height to maintain consistent voxel power across the height of the print. After printing, the final part was rinsed twice in propylene glycol diacetate to remove unreacted resin before a final cure with a 405 nm laser pointer. Control resins without upconversion nanocapsules showed no printing at all.

Measurements: Printing images were taken with a Kiralux™ 8.9 Megapixel Color CMOS Camera. FIG. 8 images are of a dilute stock solution of PdTPBP and Br- TIPS- anthracene. The quadratic voxel was generated with 637 nm light. The linear voxel was generated from a fiber coupled 365 nm LED coupled into the same optical path. The linear cuvette was diluted by a factor of 2 relative to the quadratic cuvette to better show the entire voxel. FIG. 10 images are taken with the same camera with the sample excited in free space by a 635 nm laser from the right through a 550 nm short pass filter. F127 micelles were made as discussed previously with the sole change that Br-TIPS- anthracene was used as the annihilator. Capsules and micelles were diluted 30: 1 in the specified resin. The tap water was taken directly from a sink and left uncapped for 20 minutes before the image was taken. All other cuvettes were mixed in the glovebox. Photoluminescence spectra were recorded with an Ocean Optics QE Pro. Absorption spectra were taken on a Cary-5000 UV-Vis spectrometer. For intensity dependence, incident laser intensity was measured with a calibrated Si photodetector from Newport and varied using ND filters. The emission intensity was measured with the QE Pro spectrometer and integrated. No variation in emission shape or time was observed throughout the measurement. The spot size was measured by moving a razor blade through the spot with a micrometer. SEM image was captured by using an in-lens (immersion lens) detector on Supra55VP Field Emission Scanning Electron Microscope (FESEM) at 10 keV. TEM image was captured by JEOL-2100 HR-TEM operated at 200 kV. The sample was drop casted on a polymer coated Cu grid. The image in Fig. 3D was taken with an EOS Rebel T6i through Thorlabs LG4 laser safety glasses that have a 400 - 600 nm bandpass. 1 g of capsules was dissolved in 2 mL of acrylic acid and then diluted with -800 mL of acetone. All images are unedited.

While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and/or methods, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present invention.

The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.”

The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc.

As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of’ or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e. “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law.

As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. Some embodiments may be embodied as a method, of which various examples have been described. The acts performed as part of the methods may be ordered in any suitable way. Accordingly, embodiments may be constructed in which acts are performed in an order different than illustrated, which may include different (e.g., more or less) acts than those that are described, and/or that may involve performing some acts simultaneously, even though the acts are shown as being performed sequentially in the embodiments specifically described above.

Use of ordinal terms such as “first,” “second,” “third,” etc., in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements.

In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of’ and “consisting essentially of’ shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.0