ELECTRONICS AND OPTOELECTRONICS

RELATED APPLICATIONS

This application claims priority to U.S. provisional serial number 61/582,143 filed December 30, 2011 and which is hereby incorporated by reference in its entirety for all purposes.

FEDERAL FUNDING STATEMENT

The inventions were made with United States Government support under Grant No. DMR-0805259 of the National Science Foundation. The Government has certain rights in the inventions.

BACKGROUND

Organic semiconductors have been of long interest for various applications in organic

1-8

electronics including organic photovoltaics (OPVs), ^" organic light-emitting diodes

(OLEDs), ^9"15 organic memories, ¹⁶ ' ¹⁷ and organic field-effect transistors (OFETs). ^18"20 Organic semiconductors' various advantages of light weight, low cost, mechanical flexibility, large- area device processing, and easy solution processability brought their extensive development over inorganic semiconductors. OFETs have shown rising interest steadily with development of the novel organic materials including small molecules, oligomers, and polymers which

21-23

shows performance as high as those of amorphous silicon device. ^" For many organic semiconductors, only p-channel performance is dominant. Recently, new solution processable organic semiconductors based on the donor-acceptor (D-A) approach are being investigated

94-28

for achieving n-channel performance or ambipolar performance from OFETs.

38-51

Though several thienothiadiazole-based materials were known in the art, ^" they generally lack good thermal or oxidative stability, or the practical processability

characteristics needed in order to make commercially practical electronic devices. Therefore, there exists an unsatisfied need for new donor-acceptor materials, based on thienothiadiazole or related molecular structures, and/or solid materials or compositions derived therefrom that can provide the needed properties for electron or hole transport, as well as improved processability, performance, cost, and thermal and oxidative stability for use in organic electronic devices, especially transistors and solar cells. SUMMARY

Provided herein are new solution processable thienothiadiazole-based oligomers (OTTDs). Examples of the molecular structures of the new OTTDs are given in Figure 1. Characteristics of OTTDs can include broad absorption bands, narrow band gap, and potential for ambipolar charge transport. OTTDs also can have several unexpected advantages derived from their novel molecular architecture. First, OTTDs can be solution processable (spin coating, printing, etc) and improve the film quality at the same time, which is closely related to performance in the electronic and optoelectronic devices, compared to small molecules. Second, OTTDs can have reproducibility and high purity compared to polymer organic semiconductors. In addition, the novel molecular architecture of OTTDs can allow one to tune the optical, electrochemical properties by varying the interconnecting X moieties (see Figure 1), including electron rich or electron deficient moieties, in the center of the oligomer's backbone. The charge carrier mobility may also be tuned.

Embodiments provided herein include compositions, devices, and articles, as well as methods of making and methods of using the compositions, devices, and articles.

For example, provided here is a composition comprising at least one oligomer, wherein the oligomer is represented by:

wherein: a) each X is independently O, S, Se, Te or NR', wherein R' is hydrogen or a C1-C30 linear, branched or cyclic alkyl group; b) each X' is independently S, Se, Te; c) each Y and Y' is N or CR", wherein R" is hydrogen, fluorine, cyano, or a C1-C30 linear, branched or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group; d) each a is independently 0, 1, 2, 3 or 4, each b is independently 0 or 1; e) L is a linker unit which is a single bond or an optionally substituted linear, branched, or cyclic C2-C30 conjugated organic group; and f) each EG is independently an end group which is hydrogen, halogen, cyano, or an optionally substituted linear, branched, or cyclic C1-C30 organic group.

In one embodiment, each X and X' is S.

In one embodiment, each Y and Y' is CR". In another embodiment, each Y is CH, each Y' is CR" with R" being an optionally substituted C1-C30 alkyl, alkoxy, or thioalkyl. In a further embodiment, each Y is CH, each Y' is CR" with R" being a branched alkyl. In an additional embodiment, Y and Y' together form a ring.

In one embodiment, each a and b is 1.

In one embodiment, the two subunits linked via L are structurally the same. In another embodiment, the two subunits linked via L are structurally different.

In one embodiment, L is represented by:

R

In one embodiment, L is ^" V R - , wherein R is hydrogen, fluorine, cyano, or a C ₁ -C30 linear, branched or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group. In

another embodiment, L is Y Y . In a further embodiment, L is . In an additional embodiment, L is . In et a another embodiment, L

is . In yet a further embodiment, L is . In yet an

additional embodiment, L i

In one embodiment, each EG is independently represented by:

In one embodiment, the oligomer has an ionization potential of 4.5 eV or higher. In one embodiment, the oligomer has an optical band gap of 1.2 eV or smaller. In one embodiment, the oligomer has an electrochemical band gap of 1.7 eV or smaller.

Also provided here are devices comprising the oligomers described above. In one embodiment, the device is a transistor. In another embodiment, the device is a photodetector. In a further embodiment, the device is a photovoltaic device. In an additional embodiment, the device is a light-emitting device.

In one embodiment, the device is a field-effect transistor. In another embodiment, the device is a field-effect transistor comprising a thin-film of the oligomer. In a further embodiment, the device is a field-effect transistor comprising a thin-film of the oligomer annealed at a temperature of 150°C or more.

In one embodiment, the field-effect transistor has a hole mobility of 1 x 10 ^~4 or higher. In another embodiment, the field-effect transistor has an electron mobility of 1 ^χ 10 ^~4 or higher. In a further embodiment, the field-effect transistor has a hole mobility of 1 x 10 ^~4 cm 2 /Vs or higher and an electron mobility of 1 x 10 - ^" 4 cm 2 /Vs or higher. In an additional embodiment, the field-effect transistor has a on/off current ratio of 10-10 ⁴ .

Also provided here is a thin- film field-effect transistor comprising at least one oligomer, wherein the oligomer comprises at least one donor moeity and at least one acceptor moeity, and wherein the acceptor moeity is an optionally substituted thieno[3,4- c] [ 1 ,2,5]thiadiazole.

DESCRIPTION OF THE FIGURES

Figure 1 shows examples of embodiments of molecular structures of thieno[3,4- c][l,2,5]thiadiazole based oligomers described herein.

Figures 2- A and 2-B illustrate synthetic routes to OTTDs.

Figure 3 shows voltammograms of OTTV and OTTP as thin films in 0.1 M Bu4NPF6 solution in acetonitrile at a scan rate of 40 mV/s. oxidation scans (A) and reduction scans (B).

Figure 4 shows voltammograms of OTTTh and OTTTt as thin films in 0.1 M

Bu4NPF6 solution in acetonitrile at a scan rate of 40 mV/s. oxidation scans (A, C) and reduction scans (B, D).

Figure 5 shows optical absorption spectra of OTTDs in dilute chloroform solution (A) and as thin films on glass substrates (B).

Figure 6 shows output (a) and transfer (b) characteristics of a thin film transistor based on OTTV (l,4-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3-ethylhexyl -2- thienyl)thieno[3,4-c][l,2,5]thiadiazole)vinylene). Devices were fabricated on OTS8-treated substrates, and Gate voltages (V _g ) in output curves (a) were set to be from 0 V to -80 V with step of -20 V. Source-drain voltage (Vd _s ) was -80 V for transfer curves (b).

Figure 7 shows output (a) and transfer (b) characteristics of a thin film transistor based on OTTP (l,4-Bis(5-(4'-trif uoromethylpyridine)-4,6-bis(3-ethylhexyl-2- thienyl)thieno[3,4-c][l,2,5]thiadiazole)phenylene). Devices were fabricated on BCB-treated substrates, and Gate voltages (V _g ) in output curves (a) were set to be from 0 V to -80 V with step of -20 V. Source-drain voltage (Vd _s ) was -80 V for transfer curves (b).

Figure 8 shows optical absorption spectra of EHT-TFPTT (A) and EHT-TFPyTT (B) in dilute chloroform solution and as thin films on glass subtrates.

Figure 9 shows cyclic voltammograms of TTD-based small molecules thin films in 0.1 M Bu4NPF6 solution in acetonitrile at a scan rate of 40 mV/s: EHT-TFPTT (A, B) and EHT-TFPyTT (C, D). DETAILED DESCRIPTION

INTRODUCTION

All references described herein are hereby incorporated by reference in their entirety. Priority U.S. provisional serial number 61/582,143 filed December 30, 2011 is hereby incorporated by reference in its entirety for all purposes.

Various terms are further described herein below:

"A", "an", and "the" refers to "at least one" or "one or more" unless specified otherwise.

"Optionally substituted" groups refers to, for example, functional groups that may be substituted or unsubstituted by additional functional groups. For example, when a group is unsubstituted by an additional group it can be referred to as the group name, for example alkyl or aryl. When a group is substituted with additional functional groups it may more generically be referred to as substituted alkyl or substituted aryl.

"Alkyl" refers to, for example, linear, branched or cyclic monovalent alkyl groups having from 1 to 24 carbon atoms. This term is exemplified by groups such as for example methyl, ethyl, n-propyl, iso-propyl, n-butyl, t-butyl, n-pentyl, ethylhexyl, dodecyl, isopentyl, and the like.

"Aryl" refers to, for example, a monovalent aromatic carbocyclic group of from 6 to 14 carbon atoms having a single ring (e.g., phenyl) or multiple condensed rings (e.g., naphthyl or anthryl) which condensed rings may or may not be aromatic provided that the point of attachment is at an aromatic carbon atom. Preferred aryls include phenyl, naphthyl, and the like.

"Heteroalkyl" refers to, for example, an alkyl group wherein one or more carbon atom is substituted with a heteroatom. The heteroatom can be, for example, O, S, N, Se, Te, Ge, etc.

"Heteroaryl" refers to, for example, an aryl group wherein one or more carbon atom is substituted with a heteroatom. The heteroatom can be, for example, O, S, N, Se, Te, Ge, etc.

"Alkoxy" refers to, for example, the group "alkyl-O-" which includes, by way of example, methoxy, ethoxy, n-propyloxy, iso-propyloxy, n-butyloxy, t-butyloxy, n-pentyloxy, 1-ethylhex-l-yloxy, dodecyloxy, isopentyloxy, and the like. "Aryloxy" refers, for example, to the group "aryl-O-" which includes, by way of example, phenoxy, naphthoxy, and the like.

"Thioalkyl" refers to, for example, the group "alkyl-S-" which includes, by way of example, thiomethyl, thioethyl, and the like.

"Thioaryl" refers, for example, to the group "aryl-S-" which includes, by way of example, thiophenyl, thionaphthyl, and the like.

"Alkylene" refers to, for example, linear, branched or cyclic divalent alkyl groups having from 1 to 20 carbon atoms.

"Arylene" refers to, for example, a divalent aromatic carbocyclic group of from 6 to 14 carbon atoms having a single ring (e.g., phenylene) or multiple condensed rings (e.g., naphthylene or anthrylene) which condensed rings may or may not be aromatic provided that the point of attachment is at an aromatic carbon atom.

"Heteroarylene" refers to, for example, an arylene group wherein one or more carbon atom is substituted with a heteroatom. The heteroatom can be, for example, O, S, N, etc.

"Alkenylene" refers to, for example, linear, branched or cyclic divalent alkene groups having from 1 to 20 carbon atoms. Alkenylene comprises at least one unsaturated carbon- carbon double bond.

"Alkynylene" refers to, for example, linear, branched or cyclic divalent alkyne groups having from 1 to 20 carbon atoms. Alkynylene comprises at least one unsaturated carbon- carbon triple bond.

"Salt" refers to, for example, derived from a variety of organic and inorganic counter ions well known in the art and include, by way of example only, sodium, potassium, calcium, magnesium, ammonium, tetraalkylammonium, and the like; and when the molecule contains a basic functionality, salts of organic or inorganic acids, such as hydrochloride, hydrobromide, tartrate, mesylate, acetate, maleate, oxalate and the like.

OLIGOMER/SMALL MOLECULES

Oligomers and small molecules are known in the art and include small molecules and compounds having molecular weights of, for example, about 2,500 g/mol or less, or about 2,000 g/mol or less, or about 1,500 g/mole or less. The oligomer does not necessarily need to have a repeat unit although it may have a repeat unit. In many cases, the oligomer will have different building blocks linked together. For example, a donor moiety can be linked to an acceptor moiety. Many embodiments described herein relate to an oligomer represented by:

is a linker unit, and each EG is an end group.

The oligomer described herein encompass (1) one oligomer subunit capped by EG through L being a single bond; and (2) two or more oligomer subunit linked via L and capped by EG, wherein the oligomer subunit is represented by:

wherein each a is 0, 1 , 2, 3 or 4 and each b is 0 or 1.

Each X and X' can be, for example, independently a heteroatom. Each X can be, for example, independently O, S, Se, Te or NR', wherein R' is hydrogen or a C ₁ -C30 linear, branched or cyclic alkyl group. Each X can be different or the same. In one embodiment, each X is S.

Each X' can be, for example, independently S, Se, or Te. Each X' can be different or the same. In one embodiment, each X' is S.

Each Y and Y' can be, for example, N or CR", wherein R" is hydrogen, fluorine, cyano, or a C ₁ -C30 linear, branched or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group. Each Y and Y' can be, for example, CR". Each Y and Y' can be different or the same. In one embodiment, each Y is CH, and each Y' is CR" with R" being an optionally substituted C ₁ -C30 alkyl, alkoxy, or thioalkyl. In another embodiment, each Y is CH, and each Y' is CR' ' with R' ' being a branched alkyl. Each Y and Y' can be CR", which together form a ring.

Each a can be different or the same. For example, each a can be 0, or 1, or 2, or 3, or 4. Each b can be different or the same. For example, each b can be 0 or 1. Each Υ' Y can be, for exam le, independently selected from:

In some embodiments, the two subunits linked via L are structurally the same. In other embodiments, the two subunits linked via L are structurally different.

In one embodiment the two subunits are both represented by:

wherein each Rl is independently a linear or branched alkyl, alkoxy, thioalkyl or polyether group. In a particular embodiment, each Rl is a branched alkyl group such as 2-ethylhexyl.

The oligomer can be selected from, for example, the following:

wherein R _{l s} R ₂ , R3, R4, R ₅ , R _5' , R ₆ are each independently hydrogen, halogen, cyano, or a Ci- C30 linear, branched, or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group.

LINKER UNIT (L)

Many embodiments described herein relate to a oligomer comprising at least one linker unit (L), wherein the linker unit can be, for example, a single bond or an optionally substituted divalent linear, branched, or cyclic C ₂ -C30 conjugated organic group. The linker unit can be an electron donor moiety or an electron acceptor moiety. The linker unit can comprise, for example, an optionally substituted arylene, an optionally substituted

heteroarylene, an optionally substituted alkenylene, or an optionally substituted alkynylene. In one embodiment, the linker unit comprises at least one cyano substitution group.

Said optionally substituted C ₂ -C30 conjugated organic group can be, for example, an optionally substituted alkyl, an optionally substituted aryl, an optionally substituted heteroaryl, or an optionally substituted heteroaryl. An optionally substituted alkyl can be, for example, a perfluoroalkyl or an aryl-substituted alkyl group. An optionally substituted aryl can be, for example, a perfluoroaryl or an alkyl- substituted aryl group. An optionally substituted heteroalkyl can be, for example, an alkoxy, a perfluoroalkoxy, a thioalkyl, or a perfluorothioalkyl. An optionally substituted heteroaryl can be, for example, an aryloxy, a perfluoroaryloxy, a thioaryl, or a perfluorothioaryl. The C ₂ -C30 organic group can comprise linear, branched, or cyclic functional groups.

Examples of the optionally substituted C ₂ -C30 organic group also include alkyl sulfoxide, perfluoroalkyl sulfoxide, alkyl sulfone, perfluoroalkyl sulfone, pyridyl, thiophene, furan, pyrrole, diazole, triazole, oxadiazole, carbonyl alkyl/aryl (e.g., "-C(0)-alkyl/aryl"), carboxyl alkyl/aryl (e.g., "-0-C(0)-alkyl/aryl"), ether (e.g., "-alkylene/arylene-O-alkyl/aryl"), ester (e.g., "-alkyl ene/arylene-0-C(0)-alkyl/aryl"), ketone (e.g., "-alkylene/arylene-C(0)- alkyl/aryl"), and cyano. One or more hydrogen atoms and/or carbon atoms of said Ci-Ci ₈ organic group can be further substituted with known chemical groups.

In some embodiments, the linker unit is represented by, for example, the following:

wherein each X is independently O, S, Se, Te or NR', wherein R' is hydrogen or a Ci C30 linear, branched or cyclic alkyl group; wherein each X' is independently S, Se, Te; and wherein each Y is independently N or CR", wherein R' ' is hydrogen, fluorine, cyano, or a C ₁ -C30 linear, branched or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group

In some embodiments the linker unit is selected from

wherein Ri is an alkyl such as, for example, 2-ethylhexyl and R ₂ is an alkyl such as, for example, 2-hexyldecyl.

In one embodiment, the linker unit does not comprise and is not thiophene. In another embodiment, the linker unit does not comprise and is not thieno[3,4-c][l,2,5]thiadiazole.

END GROUP (EG)

Many embodiments described herein relate to a oligomer comprising at least two end groups (EG), wherein the end groups can be, for example, hydrogen, halogen, cyano, or an optionally substituted linear, branched, or cyclic C1-C30 organic group. The end groups can be an electron donor moeity or an electron acceptor moiety. The end groups can comprise, for example, an optionally substituted aryl or an optionally substituted heteroaryl. The aryl and heteroaryl can be optionally substituted with, for example, one or more halogens such as fluorides, or one or more optionally substituted alkyl or heteroalkyl groups such as perfluoroalkyl and/or perfluorohetero alkyl.

In some embodiments, each end groups (EG1 and EG2) is independently represented

by, for example, the following or v■ v■ ^Y ; wherein each X is

independently O, S, Se, Te or NR', wherein R' is hydrogen or a C1-C30 linear, branched or cyclic alkyl group; and wherein each Y is independently N or CR", wherein R' ' is hydrogen, fluorine, cyano, or a C1-C30 linear, branched or cyclic alkyl, perfluoroalkyl, alkoxy, thioalkyl, or thioalkoxy group.

In one embodiment, the end groups are independently represented by, for example,

the following:

wherein R ₇ is hydrogen, halogen, or an optionally substituted linear, branched, or cyclic C\- C30 organic group described above, c is 1, 2, 3 or 4.

In some embodiments, the two end groups are structurally the same. In other embodiments, the two end groups are structurally different.

PROPERTIES OF THE OLIGOMER The ionization potential (IP) of the oligomer can be, for example, 4.5 eV or more, 4.6 eV or more, or 4.7 eV or more, or 4.8 eV or more, or 4.9 eV or more, or 5.0 eV or more, or 5.1 eV or more, or 5.2 eV or more, or 5.3 eV or more, or 5.4 eV or more.

The optical band gap of the oligomer can be, for example, 1.5 eV or lower, or 1.4 eV or lower, 1.3 eV or lower, or 1.2 eV or lower, or 1.1 eV or lower, or 1.0 eV or lower, or 0.9 eV or lower. The electrochemical band gap of the copolymer can be, for example, 1.8 eV or lower, or 1.7 eV or lower, or 1.6 eV or lower, or 1.5 eV or lower, or 1.4 eV or lower, 1.3 eV or lower, or 1.2 eV or lower.

The absorption maximum (λ _^ ) of the higher energy band due to π-π* transition of the oligomer in solution can be, for example, in the range of 350-650 nm. The absorption maximum (λ _^ ) of the higher energy band due to π-π* transition of the oligomer in thin film can be, for example, in the range of 400-700 nm.

The absorption maximum (λ _^ ) of the intramolecular charge transfer (ICT) band of the oligomer in solution can be, for example, about 600-1000 nm. The absorption maximum ( _π , _κ ) of the ICT band of the oligomer in thin film can be, for example, about 700-1500 nm.

EXAMPLES

Examples of the oligomer described herein include, but are not limited to, the following:

Ri, R ₂ , R3, R ₄ , R5 and R6 are each an alkyl such as, for example, 2-ethylhexyl, and R5 ' is is an alkyl such as, for example, 2-hexyldecyl.

SYNTHESIS OF OLIGOMER Methods for synthesizing thienothiadiazole groups are known in the art and described in cited references [38-51], all of which are incorporated herein by reference in their entireties. Methods for synthesizing oligomers comprising thienothiadiazole groups are described in Figures 2- A and 2-B and Examples 1-5 and 14-16.

In one embodiments, l,2-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3-ethylhexyl- 2- thienyl)thieno[3,4-c][l,2,5]thiadiazole)vinylene (OTTV) is synthesized by Stille coupling reaction of 4-(5 -bromo-3 -ethylhexyl-2-thienyl)-6-(5 (2-trifluoromethylpyridine)-3 -ethylhexyl- 2-thienyl))thieno[3,4-c][1.2.5]thiadiazole with trans- l,2-bis(tri-n-butylstannyl)ethylene.

In another embodiment, l,4-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3-ethylhexyl- 2-thienyl)thieno[3,4-c][l,2,5]thiadiazole)phenylene (OTTP) is synthesized by Stille coupling reaction of 4-(5 -bromo-3 -ethylhexyl -2 -thienyl)-6-(5 (2-trifluoromethylpyridine)-3 -ethylhexyl- 2-thienyl))thieno[3,4-c][1.2.5]thiadiazole with l,4-bis(tributylstannyl)benzene.

In a further embodiment, 2,5-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3-ethylhexyl- 2-thienyl)thieno[3,4-c][l,2,5]thiadiazole)thiophene (OTTTh) is synthesized by Stille coupling reaction of 4-(5-bromo-3-ethylhexyl-2-thienyl)-6-(5(2-trifluoromethylpyr idine)-3- ethylhexyl-2-thienyl))thieno[3,4-c][l .2.5]thiadiazole with 2,5-bis(tributylstannyl)thiophene.

In an additional embodiment, 2,5-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3- ethylhexyl-2-thienyl)thieno[3,4-c][l,2,5]thiadiazole)thieno[ 3,2-b]thiophene (OTTTt) is synthesized by Stille coupling reaction of 4-(5-bromo-3-ethylhexyl-2-thienyl)-6-(5(2- trifluoromethylpyridine)-3-ethylhexyl-2-thienyl))thieno[3,4- c][l .2.5]thiadiazole with 2,5- bis(trimethylstannyl)thieno [3 ,2-b]thiophene .

In yet another embodiment, 4,6-bis(5(2-trifluoromethylphenyl)-3-ethylhexyl-2- thienyl)thieno[3,4-c][l,2,5]thiadiazole (EHT-TFPTT) is synthesized by Stille coupling reaction of 4,6-bis(5-bromo-3-ethylhexyl-2-thienyl)thieno[3,4-c][l,2,5]t hiadiazole with 5- Trimethylstannyl-2-trifluoromethylphenyl.

In an additional embodiment, 4,6-bis(5(2-trifluoromethylpyridine)-3-ethylhexyl-2- thienyl)thieno[3,4-c][l,2,5]thiadiazole (EHT-TFPyTT) is synthesized by Stille coupling reaction of 4,6-bis(5-bromo-3-ethylhexyl-2-thienyl)thieno[3,4-c][l,2,5]t hiadiazole with 5- Tributylstannyl-2-trifluoromethylpyridine.

Additional literature describing Stille coupling reactions includes, for example, WO 2011/051292, which is incorporated herein by reference in its entirety.

DEVICES COMPRISING OLIGOMER DESCRIBED HEREIN Many embodiments described herein relate to novel organic electronic devices comprising the oligomer described herein, including transistors including field effect transistors, photodetectors, photovoltaic devices and photo luminescence devices. Each of these applications typically comprises the formation of a film of the oligomer described herein on a substrate. Organic films of the oligomer described herein can be prepared by known methods such as spin coating methods, casting methods, dip coating methods, inkjet methods, doctor blade coating methods, screen printing methods, and spray coating methods. By using such methods, organic films having good properties such as mechanical strength, toughness, and durability can be prepared without forming cracks in the films. Therefore, the organic films can be preferably used for organic electronic devices such as organic field- effect tansisors (OFETs), photodetectors, solar cells and organic light-emitting diodes (OLEDs).

Films of the oligomer described herein are prepared by coating a oligomer solution, which is prepared by dissolving the oligomer in a solvent such as dichloromethane, tetrahydrofuran, chloroform, toluene, chlorobenzene, dichlorobenzene, or xylene, on a substrate. Specific examples of the coating methods include spray coating methods, spin coating methods, blade coating methods, dip coating methods, cast coating methods, roll coating methods, bar coating methods, die coating methods, inkjet methods, dispense methods, etc. In this regard, methods and solvents are selected in consideration of the properties of the oligomer used.

Suitable materials for use as the substrate on which a film of the oligomer described herein is formed include inorganic substrates such as glass plates, silicon plates, indium tin oxide (ITO) plates, FTO plates, ITO-coated glass plates, and FTO-coated glass plates, and organic substrates such as plastic plates (e.g., PET films, polyimide films, and polystyrene films) and ITO or FTO coated plastic plates, which can be optionally subjected to a surface treatment. It is preferable that the substrate has a smooth surface.

The thicknesses of the organic film and the organic semiconductor layer of the organic thin film transistor described herein are not particularly limited. However, the thickness is determined such that the resultant film or layer is a uniform thin layer (i.e., the film or layer does not include gaps or holes adversely affecting the carrier transport property thereof). The thickness of the organic semiconductor layer is generally not greater than 1 micron, and preferably from 5 to 200 nm.

TRANSISTORS In some embodiments, the devices described herein comprise a field-effect transistor comprising at least one oligomer, wherein the oligomer comprises at least one donor moeity and at least one acceptor moeity, and wherein the acceptor moeity is an optionally substituted thieno[3 ,4-c] [ 1 ,2,5]thiadiazole.

In one embodiment, the field-effect transistor comprises a thin-film of the oligomer described herein. The thin film can be deposited from a solution of the oligomer. The thin- film can be fabricated by spin coating. The thin-film can be fabricated by vacuum vapor deposition. The thin- film can be annealed at a temperature of, for example, 150 °C or higher, or 170 °C or higher, or 190 °C or higher, or 210 °C or higher, or 230 °C or higher, or 250 °C or higher.

The hole mobility of the field-effect transistor can be, for example, l x lO ^"6 cm ² /Vs or higher, or 1 x 10 - ^" 5 cm 2 /Vs or higher, or 1 x 10 - ^" 4 cm 2 /Vs or higher, or 5 ^x 10 - ^" 4 cm 2 /Vs or higher, or 1 x 10 - ^" 3 cm 2 /Vs or higher, or 2.5 ^x 10 - ^" 3 cm 2 /Vs or higher, or 5 ^x 10 - ^" 3 cm 2 /Vs or higher, or

7.5 x 10 - ^" 3 cm 2 /Vs or higher, or 1 x 10 - ^" 2 cm 2 /Vs or higher, or 2.5 ^x 10 - ^" 2 cm 2 /Vs or higher, or

5 x 10 - ^" 2 cm 2 /Vs or higher, or 7.5 ^x 10 - ^" 2 cm 2 /Vs or higher, or 1 x 10 - ^" 1 cm 2 /Vs or higher.

The electron mobility of the field-effect transistor can be, for example, l x lO ^"6 cm ² /Vs or higher, or 1 x 10 - ^" 5 cm 2 /Vs or higher, or 1 x 10 - ^" 4 cm 2 /Vs or higher, or 2.5 ^x 10 - ^" 4 cm 2 /Vs or higher, or 5 ^x 10 - ^" 4 cm 2 /Vs or higher, or 7.5 ^x 10 - ^" 4 cm 2 /Vs or higher, or 1 x 10 - ^" 3 cm 2 /Vs or higher, or 2.5 x 10 - ^" 3 cm 2 /Vs or higher, or 5 ^x 10 - ^" 3 cm 2 /Vs or higher, or 7.5 ^x 10 - ^" 3 cm 2 /Vs or higher, or 1 x 10 ^"2 cm ² /Vs or higher.

The on/off current ratio of the field-effect transistor can be, for example, about 10-10 ⁶ , or about 10-10 ² , or about 10 ² -10 ³ , or about 10 ³ -10 ⁴ , or about 10 ⁴ -10 ⁵ , or about 10 ⁵ -10 ⁶ .

The organic thin film transistors described herein typically have a configuration such that an organic semiconductor layer including the oligomer described herein is formed therein while also contacting the source electrode, drain electrode and insulating dielectric layer of the transistor.

The organic thin film transistor prepared above is typically thermally annealed.

Annealing is performed while the film is set on a substrate, and is believed (without wishing to be bound by theory) to allow for at least partial self-ordering and/or π-stacking of the oligomer to occur in the solid state. The annealing temperature is determined depending on the property of the oligomer, but is preferably from room temperature to 300 °C, or from 50 to 300 °C. In many embodiments, thermal annealing is carried out at 150 °C or more, or at 170 ° C or more, or at 200 ° C or more. When the annealing temperature is too low, the organic solvent remaining in the organic film cannot be well removed therefrom. In contrast, when the annealing temperature is too high, the organic film can be thermally decomposed.

Annealing is preferably performed in a vacuum, or under nitrogen, argon or air atmosphere. In some embodiments annealing is performed in an atmosphere including a vapor of an organic solvent capable of dissolving the oligomer so that the molecular motion of the oligomer is accelerated, and thereby a good organic thin film can be prepared. The annealing time is properly determined depending on the aggregation speed of the oligomer.

An insulating (dielectric) layer is used in the organic thin film transistors comprising the oligomer described herein, situated between the gate electrode and the organic thin film comprising the oligomer. Various insulating materials can be used for the insulating layer. Specific examples of the insulating materials include inorganic insulating materials such as silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, titanium oxide, tantalum oxide, tin oxide, vanadium oxide, barium strontium titanate, barium zirconate titanate, lead zirconium titanate, lead lanthanum titanate, strontium titanate, barium titanate, barium magnesium fluoride, bismuth tantalate niobate, hafnium oxide, and trioxide yttrium; organic insulating materials such as polymer materials, e.g., polyimide, polyvinyl alcohol, polyvinyl phenol, polystyrene, polyester, polyethylene, polyphenylene sulfide, unsubstituted or halogen-atom substituted polyparaxylylene, polyacrylonitrile, and cyanoethylpullulan; etc. These materials can be used alone or in combination. Among these materials, materials having a high dielectric constant and a low conductivity are preferably used.

Suitable methods for forming such an insulating layer include dry processes such as CVD methods, plasma CVD methods, plasma polymerization methods, and vapor deposition methods; wet processes such as spray coating methods, spin coating methods, dip coating methods, inkjet coating methods, cast coating methods, blade coating methods, and bar coating methods; etc.

In order to improve the adhesion between the insulating layer and organic

semiconductor layer, to promote charge transport, and to reduce the gate voltage and leak current, an organic thin film (intermediate layer) can be employed between the insulating layer and organic semiconductor layer. The materials for use in the intermediate layer are not particularly limited as long as the materials do not chemically affect the properties of the organic semiconductor layer, and for example, molecular films of organic materials, and thin films of polymers can be used therefore. Specific examples of the materials for use in preparing the molecular films include coupling agents such as octadecyltrichlorosilane, octyltrichlorosilane, octyltrimethoxysilane, hexamethyldisilazane (HMDS), and octadecylphosphonic acid. Specific examples of the polymers for use in preparing the polymer films include the polymers mentioned above for use in the insulating layer. Such polymer films can serve as the insulating layer as well as the intermediate layer.

The materials of the electrodes (such as gate electrodes, source electrodes and drain electrodes) of the organic thin film transistor described herein are not particularly limited as long as the materials are electrically conductive. Specific examples of the materials include metals such as platinum, gold, silver, nickel, chromium, copper, iron, tin, antimony, lead, tantalum, indium, aluminum, zinc, tungsten, titanium, calcium, and magnesium; alloys of these metals; electrically conductive metal oxides such as indium tin oxide (ITO); inorganic or organic semiconductors, whose electroconductivity is improved by doping or the like, such as silicon single crystal, polysilicon, amorphous silicon, germanium, graphite, carbon nanotube, polyacetylene, polyparaphenylene, polythiophene, polypyrrole, polyaniline, polythienylenevinylene, polyparaphenylenevinylene, and complexes of

polyethylenedioxythiophene (PEDOT) and polystyrene sulfonic acid.

PHOTOVOLTAIC AND SOLAR CELLS

Solar cells described herein can be fabricated by first spin-coating a PEDOT buffer layer on top of ITO-coated glass substrates at 1500 rpm for 60 s and dried at 150°C for 10 min under vacuum. The thickness of PEDOT was around 40 nm.

The active layer of the solar cells comprising the oligomer described here normally comprise a mixed "heterojunction" active layer that is a phase separated blend of the oligomer described above and an electron acceptor material. The electron acceptor material can comprise a variety of organic materials (small molecules, oligomers, polymers, or copolymers) that have a LUMO energy level that is at least about 0.2 to 0.6 eV more negative than the LUMO energy level of the copolymers described herein, and a HOMO energy level that is more negative than the HOMO energy level of the copolymers described herein. In many embodiments, the electron acceptor material can be a fullerene or a modified fullerene (e.g., C ₆ i-phenyl-butyric acid methyl ester, PC ₆ iBM, or C ₇ i-phenyl-butyric acid methyl ester, PC ₇₁ BM). In other embodiments, the electron acceptor material can be an electron accepting semiconducting organic small molecule, oligomer, or polymer having appropriate LUMO and HOMO energies (at least about 0.2-0.6 eV more negative than the LUMO energy level and a more negative HOMO energy level than the HOMO energy level of the copolymers described herein). Examples of such electron acceptor materials can include small molecules, oligomers, polymers, or copolymers having highly electron deficient functional groups, such as for example napthalene diimide, perylene diimide, rylene, phalimide, and related derivatives comprising electron accepting groups.

In many embodiments of the solar cells, a composition comprising a solution or dispersion of one or more of the oligomers described herein and one or more acceptor materials (for example fullerene derivatives) is spin-coated on top of the PEDOT layer, for example at a speed of 1000 rpm for 30 seconds, to form a layer comprising the one or more oligomers and one or more electron accepting materials. In some embodiments, the solution or dispersion is applied using a hot solvent, and dried under vacuum immediately after the deposition the oligomer.

The coated device precursor can then be annealed, for example on a hot plate at 130 ± 10 °C for 10 min in a glove box, to form the active layer. The active layer can also be spin- coated in air and dried in a vacuum oven without thermal annealing. The solvents used for dissolving the mixture of the oligomer described herein and the electron acceptors can be chloroform, chlorobenzene, 1 ,2-dichlorbenzene, etc. The solvents for the oligomer blend can be a single solvent such as chloroform, chlorobenzene, 1,2-dichlorbenzene or a mixture of two or three different solvents, the second (third) solvent can be 1,8-diiodooctane, 1,8- dibromoctane, 1,8-octanedithiol, etc.

Optionally, the solvents can be heated so as to increase the solubility of the oligomer and/or electron acceptor, as an aid to film formation.

Thermal annealing is believed to induce at least partial phase separation between the oligomer described herein and the electron acceptors, forming the "heterojunctions" on the nanometer scale that are believed to be the site of light-induced charge separation.

In particular embodiments, after cooling down, the solar cell precursors comprising the oligomer-coated substrates can be taken out of the glove box and loaded in a thermal evaporator for the deposition of the cathode. The cathode can consist of 1.0 nm LiF and 80 nm aluminum layers sequentially deposited through a shadow mask on top of the active layers in a vacuum of 8x 10—7 torr. Each substrate can contain, for example, 5 solar cells with an active area of 4 mm.

WORKING EXAMPLES

Materials. 2,5-Bromo-3,4-dinitrothiophene and trans- 1 ,2-bis(tri-n-butyl- stannyl)ethylene were purchased from Fisher Scientific Inc. 5-Tributylstannyl-2- trifluoromethylpyridine was purchased from Synthonix. All other chemicals were purchased from Sigma-Aldrich. 4,6-Bis(3-ethylhexyl-2-thienyl)thieno[3,4-c][1.2.5]thiadiazo le and 5- Trimethylstannyl-2-trifluoromethylphenyl were synthesized according to a known procedure. ²⁹ ' ⁵⁵ ' ⁵⁶

Example 1 - Synthesis of 4-(3-ethylhexyl-2-thienyl)-6-(5-bromo-3-ethylhexyl-2- thienyl)thieno[3,4-c] [1.2.5]thiadiazole (1)

4,6-Bis(3-ethylhexyl-2-thienyl)thieno[3,4-c][1.2.5]thiadi azole (1.3294 g, 2.504 mmol) was dissolved into 65 mL pyridine, and N-bromosuccinimide (NBS) (0.446 g, 2.504 mmol) was added into the reaction solution at 0 °C in several portions under the absence of light. Reaction was monitored by TLC, and pyridine was evaporated under vacuum. A crude product was chromato graphed on silica gel with hexane and chloroform as eluents.

Compound 1 was collected as a blue solid, and subsequently used in next step without further purification (742.2 mg; yield = 48.6 %). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 7.4 (1H), 6.99 (2H), 2.9-2.82 (4H), 1.74 (2H), 1.4-0.8 (28 H).

Example 2 -Synthesis of 4-(3-ethylhexyl-2-thienyl)-6-(5(2-trifluoromethylpyridine)-3 - ethylhexyl-2-thienyl))thieno[3,4-c] [1.2.5]thiadiazole (2)

Compound 1 (742.2 mg, 1.22 mmol), 5-tributylstannyl-2-trifluoromethylpyridine (531mg, 1.22 mmol) and Pd(PPh ₃ ) ₄ (70 mg, 0.06 mmol) were dissolved into 70 mL toluene, and refluxed under argon overnight. Toluene was evaporated under vacuum. A crude solution was chromatographed on silica gel with hexane and chloroform as eluents. Compound 2 was collected as a greenish blue solid, and subsequently used in next step without further purification (588.7 mg; yield = 71.5 %). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 9.02 (1H), 8.08 (1H), 7.72 (1H), 7.43 (1H), 7.35 (1H), 7.03 (1H), 2.96-2.91 (4H), 1.81 (2H), 1.46-0.8 (28 H).

Example 3 - Synthesis of 4-(5-bromo-3-ethylhexyl-2-thienyl)-6-(5(2- trifluoromethylpyridine)-3-ethylhexyl-2-thienyl))thieno[3,4- c] [1.2.5]thiadiazole (3)

Compound 2 (588.7 mg, 0.87 mmol) was dissolved into 40 mL pyridine, and N- bromosuccinimide (NBS) (186 mg, 1.04 mmol) was added at 0 °C in several portions under the absence of light. Reaction was monitored by TLC, and pyridine was evaporated under vacuum. A crude solution was chromatographed on silica gel with hexane and chloroform as eluents. Compound 3 was collected as a greenish blue solid, and subsequently used in next step without further purification (360.6 mg; yield = 55 %). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 9.025 (1H), 8.08 (1H), 7.73 (1H), 7.35 (1H), 6.97 (1H), 2.94-2.84 (4H), 1.83 (2H), 1.4- 0.8 (28 H).

Example 4 - Synthesis of l,2-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3-ethylhexyl- 2- thienyl)thieno[3,4-c] [l,2,5]thiadiazole)vinylene (OTTV)

Compound 3 (423.14 mg, 0.56 mmol), trans- l,2-bis(tri-n-butylstannyl)ethylene (161.8 mg, 0.267 mmol), and Pd(PPh ₃ ) ₄ (31.5 mg, 0.027 mmol) were dissolved into 34 mL toluene and refluxed under argon overnight. Toluene was evaporated under vacuum, and a crude solution was chromatographed on silica gel with chloroform as an eluent. OTTV was collected as a green solid (200 mg; yield = 54%). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 9.027 (2H), 8.08 (2H), 7.72 (2H), 7.35 (2H), 6.95 (2H), 6.82 (2H), 2.97-2.88 (8H), 1.85 (4H), 1.46-0.88 (56 H).

Example 5 - Synthesis of l,4-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3-ethylhexyl- 2- thienyl)thieno[3,4-c] [l,2,5]thiadiazole)phenylene (OTTP)

Compound 3 (411 mg, 0.54 mmol), 1 ,4-bis(tributylstannyl)benzene (170.63 mg, 0.26 mmol), and Pd(PPh ₃ ) ₄ (31.17 mg, 0.027 mmol) were dissolved into 33 mL toluene and refluxed under argon overnight. Toluene was evaporated under vacuum, and a crude solution was chromatographed on silica gel with chloroform as an eluent. OTTP was collected as a green solid (250 mg; yield = 65%). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 9.03 (2H), 8.09 (2H), 7.71 (6H), 7.35 (2H), 2.97-2.95 (8H), 1.85 (4H), 1.46-0.89 (56 H).

Example 6 - Synthesis of 2,5-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3-ethylhexyl- 2- thienyl)thieno[3,4-c] [l,2,5]thiadiazole)thiophene (OTTTh)

Compound 3 (120 mg, 0.16 mmol), 2,5-bis(tributylstannyl)thiophene (50 mg, 0.075 mmol), and Pd(PPh ₃ ) ₄ (10 mg, 0.0087 mmol) were dissolved into 12 mL toluene and refluxed under argon overnight. Toluene was evaporated under vacuum, and a crude solution was chromatographed on silica gel with chloroform as an eluent. OTTTh was collected as a green solid (85 mg; yield = 36%). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 9.03 (2H), 8.09 (2H), 7.72 (2H), 7.35 (2H), 7.24 (2H), 7.10 (2H), 2.97-2.93 (8H), 1.87 (4H), 1.46-0.94 (56 H).

Example 7 - Synthesis of 2,5-Bis(5-(4'-trifluoromethylpyridine)-4,6-bis(3-ethylhexyl- 2- thienyl)thieno[3,4-c] [l,2,5]thiadiazole)thieno[3,2-b]thiophene (OTTTt)

Compound 3 (200 mg, 0.26 mmol), 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (60.8 mg, 0.13 mmol), and Pd(PPh ₃ ) ₄ (17 mg, 0.015 mmol) were dissolved into 17 mL toluene and refiuxed under argon overnight. Toluene was evaporated under vacuum, and a crude solution was chromatographed on silica gel with chloroform as an eluent. OTTTt was collected as a green solid ( 100 mg; yield = 26%). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 9.01 (2H), 8.06 (2H), 7.72 (2H), 7.42-7.13 (6H), 2.95-2.92 (8H), 1.87 (4H), 1.46-0.92 (56 H).

Example 8 - Characterization of oligothienothiadiazoles

To verify the molecular and physical properties of the new oligomers, 1H NMR and fast atom bombardment (FAB) mass spectrometry were performed. 1H NMR spectra at 300 MHz were recorded on a Bruker-AF300 spectrometer. Absorption spectra of oligomers were measured on a Perkin-Elmer model Lambda 900 UV/Vis/near-IR spectrophotometer.

Solution and solid state absorption spectra were obtained from oligomer solutions in chloroform and as thin films on glass substrates, respectively. Cyclic voltammetry (CV) experiments were done on an EG&G Princeton Applied Research potentiostat/galvanostat (model 273 A) in an electrolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate (Bu ₄ NPF ₆ ) in acetonitrile. A three-electrode cell was used in this analysis. Platinum wires were used as counter and working electrodes, and Ag/Ag ⁺ (Ag in 0.1 M AgN0 ₃ solution, Bioanalytical System, Inc.) was used as a reference electrode. Ferrocene/ferrocenium was used as an internal standard by running CV at the end, and this data was used for converting the potential to saturated calomel electrode (SCE) scale. The films of the oligomers were coated onto the Pt wires by dipping the wires into 1 wt % chloroform oligomer solutions.

Example 9 - Fabrication and characterization of field-effect transistors

Organic field-effect transistors (OTFTs) were made on top of n-doped silicon with thermally grown oxide (200 nm) substrates. The surface of the oxide was treated with

octyltrichlorosilane (OTS8) or BCB. OTS8-treated substrates were used for bottom-contact devices that had predefined gold source-drain electrodes (W=800-1000 μιη, L=20-100 μιη), whereas BCB-treated substrates were for top-contact devices with gold electrodes (W=1000 μιη, L=100 μιη). Thin films were deposited from solutions in 1 ,2-dichlorobenzene, chloroform, or chlorobenzene. Oligomer semiconductor films were annealed at 200 °C for 10 min under inert conditions. Devices were tested in nitrogen- filled dry box. Electrical parameters were calculated by using the standard equation for metal-oxide-semiconductor field-effect transistors in the saturation region similar to previous reports. 30 ' 31 Example 10 - Results and Discussion - Synthesis and Characterization The synthetic route to the final monomer 3 is presented in the scheme of Figure 2 -A. Compound 3 was synthesized from 4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4- c][1.2.5]thiadiazole by mono-bromination with NBS gave compound 1. Stille coupling reaction of compound 1 in the present of Pd(PPh ₃ )4 gave compound 2, and one more bromination of compound 2 was followed to obtain final monomer 3. 4,6-Bis(3-ethylhexyl-

29

2-thienyl)thieno[3,4-c][1.2.5]thiadiazole was synthesized according to a known procedure. The mono-bromination of 4,6-bis(3-ethylhexyl-2-thienyl)thieno[3,4-c][1.2.5]thiadiazo le was the most tricky reaction in the entire synthetic route to obtain the compound 1 because the bromination gave inevitable static compound and di-brominated compound at the same time with mono-brominated compound 1 whereas other reactions were straightforward. The mono-bromination reaction was monitored by TLC while adding NBS in several portions and the reaction was stopped when the mono-bromide product was the major spot on TLC. In this way, higher yield could be achieved. The final monomer 3 was collected as a greenish blue solid and verified by 1H NMR.

Oligothienothiadiazoles (OTTDs) were synthesized by Stille coupling reaction of compound 3 with various linking moieties including vinylene, phenylene, thiophene and thienothiophene. The color of the oligomerization solutions changed from greenish blue to green and collected as a green solid. The molecular structures of the new OTTDs were verified primarily by 1H NMR and fast atom bombardment (FAB) mass spectrometry, which were in good agreement with the proposed structures of the oligomers. OTTDs are readily soluble in common organic solvents (e.g. chloroform, chlorobenzene) at room temperature.

Example 11 - Electrochemical Properties

The redox properties and electronic structures of TTD-based oligomers were investigated by cyclic voltammetry (CV) of thin films on the platinum (Pt) electrodes. The films of the oligomers were coated onto Pt wires by dipping the wires into coloroform oligomer solutions. OTTDs showed a much larger oxidation current (Figure 3A, Figure 4A, C) than reduction current (Figure 3B, Figure 4B, D). Reduction waves were quasi-reversible whereas oxidation waves were all irreversible for all OTTDs.

The ionization potential (IP)/electron affinity (EA) and their associated

HOMO/LUMO energy levels were estimated from the onset redox potentials extracted from the cyclic voltammograms (IP = eE _ox ^onset + 4.4 eV, EA = eE _red ^onset +4.4 eV). ³² The IP value or HOMO level of the oligomers varied from 4.8 eV for OTTTt to 5.2 eV for OTTP. The EA or LUMO level of the oligomers also slightly varied from 3.5 eV for OTTTh and OTTTt to 3.7 eV for OTTV. The electrochemical energy band gap is 0.3-0.5 eV larger than the optical band gap (1.1 eV) and this can in part be explained by the strongly bound excitons in the

33 34

materials. ' The low lying LUMO energy levels brought to OTTDs by enhanced π- conjugation derived from the intensive D-A intramolecular charge transfer (ICT) between highly electron-accepting moieties, thienothiadiazole and trifluoromethylpyridine, and electron-donating moieties. This result implies that OTTDs have a great potential to have the field-effect charge carrier mobility of electrons.

In four oligomers, OTTV, OTTP, OTTTh, and OTTTt, the interconnecting moieties, vinylene, phenylene, thiophene, and thienothiophene, did not bring the significant difference on electronic structures. However, from the novel molecular architecture of oligomers, the electrochemical properties can be tuned by choosing the various interconnecting moieties of strong electron withdrawing groups, tetrafluorobenzene and naphthalene diimide (NDI), or strong electron donating group, dithienopyrrole, which are presented on Figure 1.

Example 12 - Optical Properties

Optical absorption spectra of OTTDs were recorded in dilute (10 ^~6 M ) chloroform solution and as spin-coated thin films on glass substrates. Normalized optical absorption spectra of OTTDs are shown in Figure 5. OTTDs characterized by broadened and red-shifted absorption bands owing to the strong intramolecular charge transfer (ICT) between electron- accepting thienothiadiazole and trifluoromethylpiridine moieties and electron-donating moieties along the oligomer backbone. All four OTTDs (OTTV, OTTP, OTTTh, OTTTt) showed two distinct absorption bands, which can be assigned to a π-π* transition and an intramolecular charge transfer (ICT) interaction for the higher energy absorption band and the lower energy absorption band, respectively. 35

In dilute solution, the absorption maximum (λ _^ ) of the higher energy band varied from 431 nm in OTTP to 454 nm in OTTV. The of the ICT absorption band varied from 692 nm in OTTP to 750 nm in OTTV. Compared to the solution spectra, the absorption spectra in thin films are significantly red-shifted. The absorption maximum (λ _^ ) of the higher energy band varied from 479 nm in OTTP to 496 nm in OTTTt. The of the ICT absorption band varied from 786 nm in OTTP to 810 nm in OTTTh in thin films. The ICT absorption maximum showed large redshift in the range of 46-94 nm whereas the high energy absorption maximum showed small redshift in the range of 27-48 nm. The shift of the absorption spectra in thin films compared to solution spectra can be explained by increased electronic derealization length of the oligomer in the solid state.

Example 13 - Field-Effect Transistors

Performance of organic thin film transistors (OTFTs) strongly depends on their film quality. OTTV with a vinylene linker had poor film forming tendency on both OTS8- and

BCB-treated surfaces, resulting in the low charge-carrier mobility less than 10 - ^" 4 cm 2 /(Vs) (Figure 6). OTTP with phenylene linkage, on the other hand, showed higher hole mobility of

-3 2

the order of 10 ^" cm /(Vs). When devices were made on BCB-treated substrate, electron transport appeared in addition to the increase of hole mobility (Figure 7). This is considered to be an effect of removal of silol groups that are known to act as trap sites for charge transport, especially electron transport. ³⁶ ' ³⁷ The electrical parameters of the oligomer transistors are summarized in Table 2.

In summary, we have synthesized new solution-processable thienothiadiazole-based oligomers (OTTDs) and could demonstrated high field-effect mobility in OFETs. OTTDs have broadened and red-shifted optical absorption bands with small energy band gap of 1.1 eV and low-lying LUMO levels at 3.5-3.7 eV as a result of strong ICT between strong electron-accepting thienothiadiazole moiety and electron-donating moieties. This result provides the possibility of OTTDs to show electron mobility as well as hole mobility in OFETs. OTTP showed electron and hole mobility as high as 0.0085 cm /(Vs) and 0.001 cm /(Vs), respectively. Still there is a big potential to tune the electrochemical properties and improve the charge carrier mobility by adjusting interconnecting X moieties of the oligomers.

Table 1. Optical and Electrochemical Properties of OTTDs.

EA ^a ΙΡ ^ϋ p el ^d p opt

Oligomer (eV) (eV) (eV) (nm) (nm) (eV)

OTTV 3.7 5.1 1 .4 454, 750 481 , 796 1 .1

OTTP 3.6 5.2 1 .6 431 , 692 479, 786 1 .1

O l 1 I h 3.5 4.9 1 .4 450, 722 491 , 810 1 .1

O l 1 I t 3.5 4.8 1 .3 452, 719 496, 803 1 .1 ^a Electron affinity was obtained based on EA = eE _re dox° ^nset + 4.4 eV.

Ionization potential was obtained based on IP = eE _ox ^onset + 4.4 eV.

c The absorption maximum in dilute solution.

d The thin film absorption maximum. Table 2. Field-effect Charge Transport Properties of OTTDs.

Device μ _η I on/ 1 off ½

Oligomer

Configuration ⁹ (cm ² /Vs) (cm ² /Vs) (V)

7.3x 10 ^"6 -1 .4x 10 ^"

OTTV OTS8/BC - 4

10 ² -10 ³ -4.9-1 .8

OTTP BCB/TC 1 .0 1 0 ^"3 8.5x10 ^"3 10 ² -6.0 ^a OTS8: octyltrichlorosilane-treated substrate; BCB: BCB-treated substrate; BC: bottom- contact; TC: top-contact.

Example 14 - Synthetic of 4,6-bis(5-bromo-3-ethylhexyl-2-thienyl)thieno[3,4- c] [l,2,5]thiadiazole.

EHT-TFPyTT SCHEME 1

Thienothiadiazole-based small molecules (EHT-TFPTT, EHT-TFPyTT) synthesized by Stille coupling reaction of 4,6-bis(5-bromo-3-ethylhexyl-2-thienyl)thieno[3,4- c][l,2,5]thiadiazole (5) with 5-Trimethylstannyl-2-trifluoromethylphenyl or 5- Tributylstannyl-2-trifluoromethylpyridine in the presence of Pd(PPh ₃ ) ₄ using toluene as the solvent (Scheme 1). The synthetic route to the dibromide monomer 5 is presented in Scheme 1. The final monomer 5 was obtained in three steps from dinitroterthiophene (8). Reduction of compound 8 with hydrochloric acid and tin powder gave the compound 7, and following ring closing reactions of compound 7 with N-thionylaniline and chlorotrimethylsilane in pyridine the thienothiadiazole compound 6 was obtained. Finally, boromination of compound 6 with N-bromosuccinimide (NBS) gave the monomer 5. Monomer 5 was obtained as a blue solid. Both EHT-TFPTT and EHT-TFPyTT obtained as a green solid and its molecular structures were verified by 1H NMR and LC mass spectrometry. These small molecules are readily soluble in common organic solvents (e.g. chloroform, chlorobenzene) at room temperature.

Example 15 - Synthesis of 4,6-bis(5(2-trifluoromethylphenyl)-3-ethylhexyl-2- thienyl)thieno[3,4-c] [l,2,5]thiadiazole (EHT-TFPTT).

Compound 5 (50 mg, 0.073 mmol), 5-Trimethylstannyl-2-trifluoromethylphenyl (83 mg, 0.22 mmol), and Pd(PPh ₃ ) ₄ (4.2 mg, 0.00365 mmol) were dissolved into 5 mL toluene and refluxed under argon overnight. Toluene was evaporated under vacuum, and a crude solution was chromatographed on silica gel with chloroform as an eluent. EHT-TFPTT was collected as a green solid (30 mg; yield = 50.2 %). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 7.73 (4H), 7.65 (4H), 7.29 (2H), 2.97 (4H), 1.85 (2H), 1.46-0.89 (28 H).

Example 16 - Synthesis of 4,6-bis(5(2-trifluoromethylpyridine)-3-ethylhexyl-2- thienyl)thieno[3,4-c] [l,2,5]thiadiazole (EHT-TFPyTT).

Compound 5 (70 mg, 0.1 mmol), 5-Tributylstannyl-2-trifluoromethylpyridine (150 mg, 0.34 mmol), and Pd(PPh ₃ ) ₄ (5.5 mg, 0.005 mmol) were dissolved into 7 mL toluene and refluxed under argon overnight. Toluene was evaporated under vacuum, and a crude solution was chromatographed on silica gel with chloroform as an eluent. EHT-TFPyTT was collected as a green solid (43 mg; yield = 52.4 %). 1H NMR (CDC1 ₃ , 300 MHz): δ (ppm) 9.02 (2H), 8.19 (2H), 7.72 (2H), 7.3 (2H), 2.97 (4H), 1.85 (2H), 1.46-0.89 (28 H). Example 17 - Optical and Electrochemical Properties of EHT-TFPTT and EHT- TFPyTT

The normalized optical absorption spectra of the TTD-based small molecules (EHT- TFPTT, EHT-TFPyTT) in dilute (10 ^~6 M) chloroform solutions and as spin-coated thin films on glass substrates are shown in Figure 8. Both small molecules show two distinct absorption bands which can be assigned as a π-π* transition band and an intramolecular charge transfer (ICT) band. EHT-TFPTT and EHT-TFPyTT have the λ of the π-π* transition band in thin films at 497 nm and 443 nm and the λ of ICT band in thin films at 728 nm and 696 nm with the optical band gap of 1.3 eV and 1.35 eV, respectively.

The redox properties and electronic structures of the new TTD-based small molecules were investigated by cyclic voltammetry (CV) of thin films on platinum (Pt) electrodes. The oxidation and reduction cyclic voltammograms of small molecules are shown in Figure 9. The cyclic voltammograms (CVs) of both small molecules showed quasi-reversible reduction and oxidation waves. The ionization potential (IP)/electron affinity (EA) and their associated HOMO/LUMO energy levels were estimated from the onset redox potentials extracted from the cyclic voltammograms (IP = eE _ox ^onset + 4.4 eV, EA = eE _red ^onset + 4.4 eV). ³² The IP value or HOMO level of EHT-TFPTT and EHT-TFPyTT were similar which are 5.36 eV and 5.4 eV, respectively. The small molecules have relatively same EA value or LUMO level of 3.8 eV. This low lying LUMO level provides possibility to EHT-TFPTT and EHT-TFPyTT for being used as electron transport material in the organic electronics and optoelectronics. The optical and electrochemical property values of TTD-based small molecules are summarized in Table

3.

Table 3. Optical and Electrical Properties of TTD-based Small molecules.

HOMO LUMO

m _ax (nm) ^a E _g ^opt (eV)

(eV) * (eV) ^c

EHT-TFPTT 497, 728 1.3 -5.36 -3.8

EHT-TFPyTT 443, 696 1.35 -5.4 -3.8

a The thin film absorption maximum.

Ionization potential was obtained based on IP = eE _ox ^onset + 4.4 eV.

c Electron affinity was obtained based on EA = eE _re do _X ^onset + 4.4 eV.

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