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Title:
THERMALLY STABLE NIR DYES FOR LASER WELDING OF PLASTIC MATERIALS
Document Type and Number:
WIPO Patent Application WO/2019/154974
Kind Code:
A1
Abstract:
The present invention discloses thermally stable free thiophene croconaine dyes of formula (I), where R1-R4 are 1-20C alkyl or alkenyl groups, and their even more thermally stable complexes with anthracenyl-tetralactam macro molecules, wherein the thiophene croconaine dyes are permanently interlocked. The dyes and the complexes are suitable for use in methods of NIR laser welding of plastic materials.

Inventors:
JEPPESEN, Anne (Kemisk Institut, Universitetsparken 5, 2100 Copenhagen Ø, 2100, DK)
NIELSEN, Bjarne E. (Kemisk Institut, Universitetsparken 5, 2100 Copenhagen Ø, 2100, DK)
BROCK-NANNESTAD, Theis (Kemisk Institut, Universitetsparken 5, 2100 Copenhagen Ø, 2100, DK)
PITTELKOW, Michael (Kemisk Institut, Universitetsparken 5, 2100 Copenhagen Ø, 2100, DK)
Application Number:
EP2019/053107
Publication Date:
August 15, 2019
Filing Date:
February 08, 2019
Export Citation:
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Assignee:
KØBENHAVNS UNIVERSITET (Nørregade 10, 1165 Copenhagen K, 1165, DK)
International Classes:
C07D333/20; B29C65/16; C07D257/10
Domestic Patent References:
WO2014166506A12014-10-16
WO2014166506A12014-10-16
WO1997008692A11997-03-06
Foreign References:
DD294961A51991-10-17
US20150005501A12015-01-01
US20150005501A12015-01-01
US20170285232A12017-10-05
Other References:
SPENCE G T ET AL: "Activated photothermal heating using croconaine dyes", CHEMICAL SCIENCE, vol. 4, no. 11, 20 August 2013 (2013-08-20), pages 4240 - 4244, XP055460758, ISSN: 2041-6520, DOI: 10.1039/c3sc51978c
SPENCE G T ET AL: "Near-Infrared Croconaine Rotaxanes and Doped Nanoparticles for Enhanced Aqueous Photothermal Heating", CHEMISTRY - A EUROPEAN JOURNAL, vol. 20, no. 39, 22 September 2014 (2014-09-22), pages 12628 - 12635, XP055460761, ISSN: 0947-6539, DOI: 10.1002/chem.201403315
Attorney, Agent or Firm:
AWA DENMARK A/S (Strandgade 56, 1401 Copenhagen K, 1401, DK)
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Claims:
Claims:

1. A thiophene croconaine dye of the formula I:

Formula I

wherein the functional groups R1, R2, R3, and R4 independently represent an alkyl or alkenyl, linear or branched, each having 1 to 20 carbon atoms.

2. The thiophene croconaine dye, wherein the functional groups R1, R2, R3, and R4 independently represent an alkyl or alkenyl, linear or branched, each having 4 to 20 carbon atoms.

3. The thiophene croconaine dye of claims 1 or 2, wherein the functional groups R1 = R3 and R2 = R4, or wherein R1, R2, R3, and R4 are identical. 4. The thiophene croconaine dye of claim 3, wherein the functional groups R1, R2, R3, and R4 are identical and each having 4 carbon atoms.

5. The thiophene croconaine dye of claim 4, wherein the functional groups are «-butyl.

6. A complex of an anthracenyl-tetralactam macro molecule of formula M and the thiophene croconaine dye as defined in any one of claims 2 to 5, Formula M wherein the functional groups R5 and R6 independently represent H, or a linear or branched alkyl each having 1 to 10 carbon atoms wherein said thiophene croconaine is permanently interlocked in M.

7. The complex of claim 6, wherein the thiophene croconaine dye is the dye of claim 5. 8. The complex of claim 7, wherein the functional groups R5 and R6 are /-butyl.

9. The use of the thiophene croconaine dyes of any one of claims 1 to 5 or the complexes of any one of claims 6 to 8 as near-infra red (NIR) absorbing compounds in the laser welding of plastic materials.

10. The use according to claim 9, wherein the NIR absorbing compound or compounds are admixed with or applied to the plastic materials prior to welding.

11. The use according to claims 9 or 10, wherein the NIR absorbing compound is dispersed in a plastic material imbued with a dye having its major absorbance in the visible part of the spectrum, thus imparting laser -weldability on said plastic material, without significantly changing its visual appearance.

12. A method of laser welding of plastics comprising the steps of:

a) providing a first plastic material to be welded comprising a compound capable of absorbing NIR light,

b) positioning a second plastic material in intimate contact with the first plastic material, and

c) activating a laser source emitting a wave length in the NIR spectrum to obtain attachments of the first and the second plastic material, optionally wherein external mechanical pressure is provided;

wherein the compound is one or more of the thiophene croconaine dyes of any one of claims 1 to 5 or the complex of any one of claims 6 to 8.

13. The method according to claim 12, wherein the first and the second plastic material have overlapping melting temperature intervals.

14. The method according to claim 13, wherein the first plastic material has a melting temperature above the melting temperature of the second plastic material.

15. A plastic product comprising a first plastic material welded to a second plastic material and a thiophene croconaine dye according to any one of claims 1 to 5 or a complex according to any one of claims 6 to 8, or which plastic product is obtainable by the method of any one of claims 12 to 14.

Description:
Thermally stable NIR dyes for laser welding of plastic materials

Introduction

The present invention relates to thiophene croconaine dyes alone and encapsuled by an an anthracenyl-tetralactam macrocycle, and their use in NIR laser welding of plastic materials; a method for laser welding of plastics; and a NIR welded plastic product obtainable by said method. The dye has improved stability towards heat, pressure, laser light typically used in NIR laser welding.

Background

Laser welding of plastic is an important emerging industrial process that utilizes near- infrared (NIR) lasers to weld pieces of plastic together.

For use in industrial laser welding processes NIR pigments such as Carbon black, various metallated phthalocyanines and more recently differently substituted perylenes have been used. These classes of compounds are well-studied, chemically inert, heat resistant, and often relatively cheap to produce. However, they suffer from the disadvantage, that they are colored in the visible region leaving the plastic colored after the welding has been performed.

Croconaine dyes have a very low light absorption in the visible region, and the prior art discloses a few examples of such dyes for NIR welding of plastic materials or biological tissue.

Specific examples of prior art publications include WO 2014/166506, which discloses a method for laser welding of plastics by use of a croconic acid derivative.

US 2015/0005501 discloses thiophene croconaine compounds for use in NIR welding of biological tissue in vivo.

Spence, G.T. et al. (2013) disclose a complex consisting of a thiophene croconaine dye dissociably encapsulated by an anthracene -based tetralactam When exposed to laser irradiation an organic solution of the complex has a higher temperature increase compared to a solution of the thiophene croconaine dye alone.

Spence, G.T. et al. (2014) disclose a complex consisting of a thiophene croconaine dye permanently encapsulated by an anthracene -based thiophene croconaine dye.

Unlike the dissociable encapsulated thiophene croconaine dye of Spence et al. 2013, the encapsulated thiophene croconaine dye is permanently interlocked.

However, both Spence et al. (2013) and Spence et al. (2014) are silent about NIR laser welding of plastic materials.

WO 97/08692 relates to an optical recording medium comprising a dichroic dye However, WO 97/08692 is silent about laser welding.

US 2017/0285232 relates to specific dithiolene metal complexes as colorless IR absorbers for use in laser welding of plastics. The dithiolene metal complexes may also be in the form of mixtures with further known IR absorbers, especially mixtures with polymethines including croconaines.

However, US 2017/0285232 is silent about NIR laser welding, where croconaines are used alone or as the primary NIR absorber.

Laser absorber dyes suitable for such as purpose should fulfill certain requirements:

1. The dye must absorb in the NIR region, where the laser in question radiates;

2. the dye must be colorless and not decompose to colored degradation products;

3. the dye should show no or little fluorescence;

4. the dye should be evenly distributable in the plastic material to be welded;

5. the dye must show stability towards the manufacturing conditions: heat, pressure, laser light; and

6. the dye should be easy and inexpensive to prepare and synthesize.

However, the prior art does not address all these requirements. Hence, there is a need for identifying improved croconaine dyes that are suitable for NIR laser welding of plastic materials and fulfilling all the requirements. Especially, there is a need for laser absorbing dyes with higher stability. Summary of the invention

In a first aspect the present invention relates to a thiophene croconaine dye of the formula I:

Formula I

where the functional groups R 1 , R 2 , R 3 , and R 4 independently represent an alkyl or alkenyl, linear, branched, or cyclic, each having 1 to 20 carbon atoms.

The specific choice of the functionalized groups R '-R 4 depends on the wavelength, solubility and/or thermal stability for which the croconaine dye is optimized. It will be appreciated, that the dye can be functionalized to impart specific properties, such as compatibility with a specific polymer towards the dye. This can be done through the use of specific groups where the functional groups R 1 , R 2 , R 3 and R 4 are alkyl or alkenyl, linear or branched, each having 1 to 20 carbon.

In a certain embodiment the functional groups R 1 = R 3 and R 2 = R 4 , or R 1 , R 2 , R 3 , and R 4 are identical. For example, R 1 , R 2 , R 3 , and R 4 are identical and each having 4 carbon atoms, such as «-butyl. Thus, the dye may be (£ ' )-2-(5-(dibutylamino)thiophcn-2-yl)- 5-(5-(dibutyliminio)thiophcn-2(5//)-ylidcnc)-3,4-dioxocyclop cnt- 1 -cn- 1 -olatc (also denoted 3.3).

The thiophene croconaine dyes exhibit an increase in thermal stability and extension in shelf life storage compared to the croconaine dyes of the prior art (see the examples 3 and 4 below and the figures referred to by the examples).

In another aspect the present invention relates to a complex of an anthracenyl- tetralactam macro molecule of formula M and any of the thiophene croconaine dyes according to formula I, where R 1 , R 2 , R 3 , and R 4 independently represent an alkyl or alkenyl, linear or branched, each having 4 to 20 carbon atoms, and where the functional groups R 5 and R 6 of said macromoleule independently represent H, or a linear or branched alkyl each having 1 to 10 carbon atoms.

Formula M and where said thiophene croconaine is permanently interlocked in M. The complex of the anthracenyl-tetralactam macro molecule of formula M and thiophene croconaine dye of Formula I is illustrated as Complex MI

Complex MI

The choice of the functional groups R 5 and R 6 depends on the desired solubility properties of the complex. For example, alkyls with lower numbers of carbon atoms are less soluble in organic solvents than alkyls with higher numbers of carbon atoms. In a prefered embodiment R 5 and R 6 are /-butyl.

By permanently interlocking the thiophene croconaine dye in a tetralactam macromolecule as defined above (as Complex MI), a decrease in fluorescence from the dye is obtained, as well as an increase in thermal stability and extension in shelf life storage. Without being bound by theory, the present inventors contemplate that these properties are attributed to the shielding effect the anthracenyl moieties of the macromolecule have on the encapsulated dye. The permanently interlocking is preferably obtained by the“clipping” method (see the examples below), where the macromolecule is synthesized around the dye.

The increase in thermal stability and extension in shelf life storage result in croconaine dyes and macromolecule encapsulated croconaine dyes more suitable for NIR laser welding of plastic materials than the croconaine dyes and macromolecule encapsulated croconaine dyes of the prior art, as a lower content the inventive dyes dispersed/dissolved in the plastic materials is necessary for obtaining a sufficiently durable weld than for the dyes of the prior art. The lower content also results in none to only a weak visible color contribution in the welded plastic material, whereas the color contribution is higher for the prior art.

In another aspect the present invention relates to the use of the thiophene croconaine dyes of the invention, or the complexes of the invention, i.e. with the macromolecule, as near-infra red (NIR) absorbing compounds in laser welding of plastic materials. Suitably, said thiophene croconaine dyes, or said complexes, are admixed with the plastic materials. Alternatively, the thiophene croconaine dyes or the complexes are applied to a surface of a plastic material to be welded.

The NIR absorbing compounds (e.g. the thiophene croconaine dyes or their complexes with the macromolecule) may be dispersed in a plastic material imbued with a dye having its major absorbance in the visible part of the spectrum, thus imparting laser- weldability on said plastic material, without significantly changing its visual appearance.

In another aspect the invention relates to a method of laser welding of plastics, comprising the steps of:

a) providing a first plastic material to be welded comprising a compound capable of absorbing NIR light,

b) positioning a second plastic material in intimate contact with the first plastic material, and

c) activating a laser source emitting a wave length in the NIR spectrum to obtain attachments of the first and the second plastic material; wherein the compound is one or more of the thiophene croconaine dyes or their complexes with the macromolecule as disclosed above.

The increase in thermal stability and extension in shelf life storage result in croconaine dyes and macromolecule encapsulated croconaine dyes of the invention have also an influence on the parameters for the method of laser welding of the invention as, at a given amount of croconaine dyes and macromolecule encapsulated comprised in the first plastic material, the NIR laser light source at a given wave length can operate at a lower energy output, and/or the welding speed can be increased relative to the amount of croconaine dyes and macromolecule encapsulated croconaine dyes present.

The efficiency of transduction of heat to the laser transparent plastic depends on a sufficient contact in the interface, because heat is best conducted where there is contact. Therefore, the present invention may favorably be practiced by providing external mechanical pressure in step c). If the contact is poor it is necessary to heat the laser absorbing plastic to a higher temperature to obtain a weld, increasing the risk of decomposition.

After absorption of the NIR laser light by the first plastic material the energy is converted into heat and the heat is subsequently dispersed into the second plastic material. The energy is absorbed either as surface or volume absorption and melts the plastic. The heat of the first plastic material is transferred to the second plastic material by heat transduction.

In accordance with the present invention, the first and the second plastic material may be of the same type and grade or a different type and grade dependent on desired end- product. In an embodiment of the present invention the first and the second plastic material have overlapping melting temperature intervals. Several factors influence how weldable two plastics are, one factor being the type of plastics. Generally, two plastics can be welded if they have overlapping melting intervals and are miscible. Plastics contain a polymer and optionally also several different additives that influence mechanical and aesthetic properties. The melting temperature intervals of a plastic material may be changed by adjusting the type and amount of additives. In another aspect the present invention relates to a plastic product, e.g. a NIR welded plastic product, obtainable by the method disclosed above. The plastic product may thus comprise a first plastic material welded to a second plastic material and a thiophene croconaine dye or a complex according to any embodiment of the invention. In the context of the invention, compounds and complexes are referred to by numbering, and thiophene croconaine dyes of the prior art are denoted“3.1” or“3.2”, respectively; further details may also be employed so that“3.1” may be referred to as “dye 3.1”,“croconaine dye 3.1”,“free dye 3.1”, and“3.2” may be referred to as“dye 3.2”,“croconaine dye 3.2” and“free dye 3.2”, respectively.

In an embodiment of the invention, the thiophene croconaine dyes of the invention is the dye interchangeably denoted “(£')-2-(5-(dibutylamino)thiophen-2-yl)-5-(5- (dibutyliminio)thiophen-2(5/ )-ylidene)-3 ,4-dioxocyclopent- 1 -en- 1 -olate” and“3.3”, “dye 3.3”,“croconaine dye 3.3” and“free dye 3.3”.

In an embodiment, the complex of the invention, i.e. the complexes (Complex MI) of thiophene croconaine dyes (of Formula I) and anthracenyl-tetralactam macro molecules (Formula M), where the given dye is permanently interlocked in the macro molecule, is denoted “3.6”, and interchangeably referred to as “complex 3.6”, “rotaxane 3.6” and“rotaxane dye 3.6”. Complexes of thiophene croconaine dyes from the prior art and anthracenyl-tetralactam macro molecules, where the given dye is dissociable interlocked in the macro molecule, are interchangeably denoted“3.4”, “complex 3.4”, pseudorotaxane 3.4”,“pseudorotaxane dye 3.4” and“3.5”,“complex 3.5”,“pseudorotaxane 3.5”,“pseudorotaxane dye 3.5”, respectively.

Brief description of the drawings

Figure 1 compares UV/Vis absorption spectra of croconaine dyes of the invention and the prior art;

Figure 2 shows photophysical studies of a dye and a complex of the invention;

Figure 3 shows decomposition studies of croconaine dyes of the invention and the prior art and a complex of the invention;

Figure 4 shows decomposition studies of croconaine dyes of the invention and the prior art and a complex of the invention; Figure 5 shows decomposition studies of croconaine dyes of the invention and the prior art and a complex of the invention;

Figure 6 shows solid phase decomposition studies of croconaine dyes of the invention and the prior art and a complex of the invention.

Detailed description of the invention

Thiophene croconaine dyes have a strong and sharp absorption in the near-infrared (NIR) region. Compared to other dyes with a sharp absorption in NIR the present inventors have found that the dyes can be synthesized in few steps. Furthermore, the crocanaine dyes are easy to adapt to the specific plastic material used because changing their functional groups disclosed above modify the solubility of the dyes. While the selection of aromatic or hetero-aromatic group can tune the overall absorption profile of the dye, the choice of their functional groups can provided physical chemical properties needed for efficient use in laser welding applications. Futhermore, the thiophene croconaine dyes of the invention may be permanently encapsulated by a macromolecule, for example by a anthracenyl-tetralactam macrocycle, in order to further adapt its solubility, lower its fluorescence and decomposing rate, both at elevated temperatures relevant for NIR welding of plastic materials and at storage temperature for prolonged shelf-life.

Laser welding of plastic is typically achieved by incorporating a (NIR) absorber into one of the two pieces of plastic one wish to weld together, or by applying the NIR absorber to a surface of the materials to be welded. The two pieces of plastic are placed on top of each other and laser irradiation using a laser with a wavelength that corresponds to the absorption of the NIR absorber is performed. Laser welding provides a convenient way to make very narrow welding zones as compared to other types of welding procedures.

According to the present invention the thiophene croconaine dye or its complex with the anthracenyl-tetralactam macro molecule are preferably present admixed with the plastic material. In certain applications it may however, be advantageous to have the thiophene croconaine dye or its complex with the anthracenyl-tetralactam macro molecule deposited on the surface between the parts to be joined by laser welding. It is desirable to have the NIR absorber evenly distributed in the plastic to ensure a smooth welding zone. The even distribution may be achieved by preparing NIR absorbers that are soluble in the type of plastic one wishes to weld. One strategy to achieve high solubility of organic absorbers in very non-polar solvents, and also to very non-polar polymers such as polyethylene and polypropylene, is to append long alkyl chains to the absorber.

Several factors influence how weldable two plastics are, one factor is the type of plastics. Examples of suitable plastic materials for the present invention includes ABS (acrylonitrile-butadiene-styrene-copolymer), ASA (acrylonitrile-styrene-acrylate- copolymer), MABS (methyl methacrylate-acrylonitrile-butadiene-styrene- copolymer), PA-6 (polyamide-6), PA-6.6 (polyamide-6.6), PA-12 (polyamide- 12), PBT (polybutylene terephthalate), PBT/ASA (polybutylene terephthalate/ acrylonitrile-styrene-acrylate-blends), PC (polycarbonate), PC/ABS (polycarbonate/ acrylonitrile-butadiene-styrene-blends), LDPE (low density-polyethylene), HDPE (high density-polyethylene), PEEK (poly ether ketone), PES (polyethersulfone), PET (polyethylene terephthalate), PMMA (poly(methyl methacrylate)), POM (polyoxy methylene), PP (polypropylene), PPS (poly(p-phenylene sulfide), PS (polystyrene), PSU (polysulfone), PVC (polyvinylchloride), SAN (styrene-acrylonitrile-copolymer). Generally, two plastics can be welded if they are miscible and have overlapping melting temperature intervals. Plastics contain a polymer and possibly also several different additives that influence mechanical and aesthetic properties. The additives also influence other characteristics of the plastic, such as the transmittance. When preparing plastics different additives may be added to the polymer to obtain the desired characteristics of the plastic. An even distribution of the different additives is advantageous to obtain the required properties. This is particularly important for additives in low concentrations, like laser absorbers. The additives are typically mixed with the polymer in an extruder; the polymer is melted and is mixed with the additives. The melted polymer admixed with additive may be used directly in the formation of the first plastic material used in the present invention or plastic is divided into small pellets ready for further processing. The increased heat stability of the inventive dyes and complexes is particularly advantageous when the dye or complex is admixed with a polymer that is molten, e.g. in an extruder, before forming the plastic material to be welded. As a consequence of the increased heat stability, lower amounts of NIR absorber, e.g. the dye or complex of the invention, are required to provide a sufficiently strong weld, and therefore the dye or complex can be invisible in the plastic product of the invention.

Different laser welding techniques can be used for welding. The most common is through-transmission laser welding (TTLW) another method is Butt-Joint welding. The basic principle in TTLW is to use laser light to join a laser transparent and a laser absorbing plastic. The laser light is transmitted through the laser-transparent plastic to the laser-absorbing plastic. The absorbing plastic converts the laser energy into heat and the polymers melt. In the contact surface between the absorbing and transparent plastic heat is transferred to the transparent plastic by heat conduction and the transparent plastic melts. If the absorber is deposited on the surface, by surface coating the energy will be transferred to both plastics.

In Butt-Joint welding the principle is joining of parallel surfaces of two components. The laser light interacts with the surface of the two plastics parallel to the contact interface. Depending on the choice of laser and the optical properties of the material the energy is absorbed into the surface, if the entire laser light is absorbed close to the surface. Alternatively, the laser light is absorbed by volume absorption, if the light transmits further into the plastic.

Absorption of laser light and generation of heat can be divided into two different interaction processes, direct absorption and indirect absorption. In direct absorption the macromolecules of the plastic materials absorb the energy from the laser light, resulting in oscillations in the macromolecules. Direct absorption can be obtained with laser light with wavelengths above 1200 nm, corresponding to frequencies below 8300 cm 1 . Plastics are typically transparent to laser light between 800 and 1200 nm. With addition of laser absorbers to the plastic indirect absorption is obtained. The laser absorber converts the energy from the laser light into heat that is transferred into the polymer. By heating with laser light in the absorbing plastic and heat flow into the transparent plastic, the contact interface of both plastics exceeds the melting temperature. The plastics change from a solid into the molten state, in which the mobility of the polymers are increased. With increasing temperature, the specific volume of the plastic is increased. It will generate an increased internal pressure between both molten interfaces. By applying external mechanical pressure to fix the plastics, macromolecules at each interface will penetrate into the other and will form the weld joint after re-solidification.

Different effects appear to contribute to the generation of the weld. The first effect is inter-diffusion process according to which segments of the macromolecular chain below the melting temperature can move caused by Brown’s micro mobility, but the mobility is restricted in a“cage” consisting of the other macromolecules. By exceeding the melting temperature segments of the macromolecular chain can leave the“cage” induced by Brown’s micro mobility. At the interface between two molten layers, macromolecular chain segments can step out of the surface of the layer and diffuse into the second layer. The extent of diffusion is a function of temperature.

The melting process is accompanied by an increase in volume, and with applied external pressure the molten areas will begin to squeeze into the free spaces between both layers. Macromolecules with parallel orientation to the interface surface will be stretched in the molten state along the squeeze flow direction, parallel to the interface. In the beginning of the re-solidification, the stretched macromolecules are subjected to relaxation process and they try to return to their original structure. Semi-crystalline plastics can, beside the two already mentioned effects for amorphous plastics, generate mixed crystalline phases in the re-solidification process.

TTLW is the most common applied process for industrial application of laser welding of plastics. The process is basically used in three different modifications as contour welding, quasi-simultaneous welding and simultaneous welding. Contour welding is characterized with a single laser beam is moved along the joint path on the plastic. The line energy, Es, quantifies the energy applied to the plastic in the laser welding process. The laser power and welding speed can be changed, if the line energy is identical, will the weld have similar quality.

The laser absorber is suitably in the polymer with the highest melting temperature if two different types of plastic are welded because the highest temperature is obtained in the plastic with the laser absorber. The laser transparent plastic is heated by transduction of heat from the laser absorbing plastic.

When welding two plastic materials they have to be compatible with each other. The materials properties, which have influence on the compatibility, include:

Chemical structure - polymeric molecules have to be miscible between each other. Thermal properties - the melt temperature ranges should overlap, heat expansion coefficients should be of the same order.

Surface energetic properties - the polar surface energy should be of the same order. Rheological properties - the melt-flow index should be of the same order.

If the plastic materials used in the first and the second plastic material are different from each other the compatibility needs to be examined. The table below, prepared R. Klein, Laser Welding of Plastics, Wiley-VCH, 2012, illustrates compatibilities between plastic materials. The table illustrates tested combinations of plastics and their weldability, in which the dark grey marks the strong welds, the pale grey marks combinations, which are weldable but not strong welding, whereas the blank marks incompatible combinations under the test conditions used. It should be kept in mind however, that the solubility of the various polymer materials may be changed by addition of additives. Therefore, a blank combination should not necessarily be understood as indicating that adherence is impossible in general.

Table 1. ABS (acrylonitrile-butadiene-styrene-copolymer), ASA (acrylonitrile- styrene-acrylate-copolymer), MABS (methyl methacrylate-acrylonitrile-butadiene- styrene-copolymer), PA-6 (polyamide-6), PA-6.6 (polyamide-6.6), PA-12 (polyamide- 12), PBT (polybutylene terephthalate), PBT/ASA (polybutylene terephthalate/ acrylonitrile-styrene-acrylate-blends), PC (polycarbonate), PC/ABS (polycarbonate/ acrylonitrile-butadiene-styrene -blends), LDPE (low density- polyethylene), HDPE (high density-polyethylene), PEEK (poly ether ketone), PES (polyethersulfone), PET (polyethylene terephthalate), PMMA (poly(methyl meth- acrylate)), POM (polyoxy methylene), PP (polypropylene), PPS (poly(p-phenylene sulfide), PS (polystyrene), PSU (polysulfone), PVC (polyvinylchloride), SAN (styrene-acrylonitrile-copolymer) .

A good weld is often characterized when breakage occurs in the plastic before the weld is broken in a pull-test. Strong welds are as a general seen between plastics with similar characteristics. Laser absorbers are added in low concentrations, typically 1% for inorganic absorbers and 0.01% for organic absorbers. Inorganic additives like carbon black are dispersed in the plastic and typical have particle sizes between 0.5 and 1 pm. Organic dyes, like azo or perylene dyes are dissolved, with a typical size between 0.01 and 0.1 pm. Both inorganic and organic dyes can absorb the laser radiation and transfer the laser energy into process-heat. Because organic dyes may be more homogeneously distributed and the direct contact to the macromolecular chains is better, the heat transfer is more effective compared to inorganic dyes. This is also the reason for the smaller amount organic dye necessary.

As stated above, dyes must have some special characteristics for being suitable as laser absorber for NIR welding of plastic materials. The thiophene croconaine dyes and their complexes, obtained by interlocking the thiophene croconaine dye in a tetralactam macromolecule, have all of these characteristics. Thus, they have their absorption maxima at about 800 nm or higher suitable for the relative inexpensive 808 nm diode lasers. The croconaine dyes may also be fine-tuned so that other lasers at even longer wavelength may be used. The molar absorbance coefficient is in the order 1-3 x 10 5 M '-cm 1 , a high molar absorbance coefficient is important to keep the concentration in the polymer low. The solubility can be modified by changing the substituents on the nitrogen atoms of the anilins, in order to optimize the solubility in different polymers, like PE and PP. The croconaine dyes, and their complexes, of the invention have little absorption in the visible area and can be used in plastic transparent in the NIR region of the spectrum. For colorless polymers, only imbued with croconaine dye, one can bleach the plastic after welding. Exposure to light from e.g. a xenon or mercury arc lamp leads to colorless decomposition products of the croconaine dye.

Croconaine dyes can be synthesized in few steps with reasonably good yields. This is important for a commercial perspective to produce the laser absorbers at low costs. Toxicity is another important criterion for some applications, as diffusion in the plastic will bring the croconaine dye in contact with the consumer, goods, or foodstuff. There are no reports with analysis of the toxicity of neither croconaine dyes nor its decomposition products. Compared to other NIR absorbers that contain large aromatic systems, the expected risk of toxicity is low.

While the chemical compounds of the present invention have been described in a single resonance form, it will be understood by the person skilled in the art that the thiophene croconaine dyes of the invention may be present in various resonance forms.

Examples

Examples 1 and 2 demonstrate the synthesis of the thiophene croconaine dyes, and their complexes with anthracenyl-tetralactam macro molecules, given as non-limiting examples. Thus, there may be alternatives to, and variations of, the reaction schemes disclosed below, which also results in the compounds of the invention. The examples 3 and 4 demonstrate the photophysical and Stability properties, respectively, of the dyes and complexes of the invention compared to relevant compounds of the prior art. Examples 3 and 4 refer where relevant to figures 1-6. In brief, the figures illustates: Figure 1. Overview of the UV/Vis absorption spectra of the croconaine dyes 3.1; 3.3 and the complexes 3.4; 3.5 and 3.6. The vertical black dotted line showing the wavelength of excitation by the laser (808 nm).

Figure 2. Photophysical studies of rotaxane 3.6 and the dye 3.3 in chloroform. UV/Vis absorption spectra. Black spectrum: 3.6 (2.5 x 10 6 M). Grey spectrum: 3.3 (2.3 x 10 6 M). b) Emission spectra. Black spectrum: 3.6 (2.2 x 10 7 M). Grey spectrum: 3.3 (1.5 x 10 7 M).

Figure 3. Decomposition studies in 1 ,2-dichlorobenzene at 150 °C monitored by decrease in UV/Vis absorption a) Free dyes 3.1 and 3.3 and pseudorotaxanes 3.4 and 3.5. b) Rotaxane 3.6.

Figure 4. Decompostion studies in nitrobenzene at 200 °C, monitored by decrease in UV/Vis absorption.

Figure 5. Decomposition studies by light exposure from a Xenon arc lamp, monitored by decrease in UV/Vis absorption.

Figure 6. Solid phase decomposition studies by heat exposure at 200°C. a) Dye 3.1. b) Pseudorotaxane 3.4, and rotaxane 3.6. Example 1: Synthesis of (ii)-2-(5-(dibutylamino)thiophen-2-yl)-5-(5-

(dibutyliminio)thiophen-2(5//)-ylidene)-3,4-dioxocyclopen t-l-en-l-olate (3.3) Step 1:

Synthesis of the reactant /V,/V-Dibutylthiophcn-2-aminc (2.17):

2-Bromothiophene (2.14, commercially available, 7.0 g, 4.2 ml, 43 mmol), cupper iodide (409 mg, 2.2 mmol), cupper powder (1.1 g, 17 mmol), tripotassium phosphate monohydrate (19.8 g, 86 mmol) was suspended in N,N-d imcthylcthanolaminc as solvent (10 ml) and dibutylamine (22.2 g, 29 ml, 172 mmol) was added. The flask was fitted with a Vigreux column and sealed with a septum. The reaction mixture was heated to 80°C for 72 h under a nitrogen atmosphere. The mixture was allowed to cool to room temperature, water (20ml) was added and the mixture was extracted with diethyl ether (3 X lOOml). The combined organic phases were washed with brine (50ml), dried over magnesium sulfate, filtered and evaporated in vacuo. The product was further purified by vacuum fractional distillation (bp. l30°C, 0.82 mbar) to obtain a mixture of the product and a byproduct. The mixture was dissolved in dichloromethane and evaporated on Celite and filtered through a column pretreated with heptane containing triethylamine (1 %). The product 2.17 was obtained as a clear oil (160 mg, 2 %). NMR (500 MHz, Chloroform-;/) d 6.74 (dd, J = 5.4, 3.7 Hz, 1H), 6.40 (dd, J= 5.4, l .4 Hz, 1H), 5.83 (dd, J= 3.7, l .4 Hz, 1H), 3.22 - 3.15 (m, 4H), 1.63 - 1.53 (m, 4H), 1.38 - 1.31 (m, 4H), 0.94 (t, J= 7.4 Hz, 6H). 13 C NMR (126 MHz, Chloroform-;/) d 158.26, 126.63, 108.70, 101.72, 53.92, 29.32, 20.46, 14.10. GC-MS (El): m/z =211.2 [M]

Step 2:

Synthesis of (E)-2-(5 -(dibutylamino)thiophen-2-yl)-5 -(5 -(dibutyliminio)thiophen- 2(5//)-ylidcnc)-3,4-dioxocyclopcnt- 1 -cn- 1 -olatc (3.3):

/ V, / V- D i b u t y 11 h i o p h c n - 2 - a m i n c (2.17, 160 mg, 0.8 mmol) obtained in step 1 and croconic acid (55 mg, 0.4 mmol) was refluxed in to luene/l -butanol (1 : 1, 10 ml) for a few minutes. The reaction mixture was evaporated to dryness. The crude product was purified by gradient column chromatography (heptane to 50 % ethylacetate in heptane) affording 3.3 as a dark solid (159 mg, 78 %). 'H NMR (500 MHz, Chloroform-;/) d 8.85 - 8.71 (m, 2H), 6.50 - 6.43 (m, 2H), 3.56 - 3.52 (m, 8H), 1.77 - 1.71 (m, 8H), 1.45 - 1.38 (m, 8H), 1.00 - 0.97 (m, 12H). 13 C NMR (126 MHz, Chloroform-;/) d 186.30, 185.58, 183.15, 173.49, 172.98, 172.90, 141.61, 140.93, 140.66, 137.21, 136.28, 136.17, 123.74, 123.68, 123.49, 112.73, 112.71, 112.66, 112.57, 54.58, 29.57,

20.19, 20.17, 13.81, 13.79. MALDI-HRMS: m/z = 529.2555 [M+H + ] (Calculated 529.2553). kmax (CHCh) = 798 nm, e (CHCh) 3.0 10 5 L mol 1 cm 1 , t (CHCh) = 0.06.

Example 2: Formation of the complex (rotaxane 3.6) of an anthracenyl- tetralactam macro molecule and (£)-2-(5-(dibutylamino)thiophen-2-yl)-5-(5- (dibutyliminio)thiophen-2(5//)-ylidene)-3,4-dioxocyclopent-l -en-l-olate

Step 1:

Synthesis of the reactant 5-(/e/7-butyl)isophthaloyl dichloride (2.19):

5-(tert-Butyl)isophthalic acid (2.18, 1.0 g, 4.5 mmol) was dissolved in thionyl chloride (11.5 ml, 160 mmol), and dimethylformamide (few drops) was added. The resulting reaction mixture was refluxed for five hours, then cooled to room temperature and evaporated in vacuo to yield the desired product 2.19 (1.14 g, 98 %), mp. 42 - 43°C.

NMR (500 MHz, Chloroform-;/) d 8.71 (t, J = 1.6 Hz, 1H), 8.40 (d, J = 1.6 Hz, 2H), 1.41 (s, 9H). 13 C NMR (126 MHz, Chloroform-;/) d 167.83, 154.10, 134.41, 134.21, 131.82, 35.48, 31.17. GC-MS (El): m/z =258.1 [M] Elemental analysis (%) calculated for C12H12CI2O2: C 55.62, H 4.67; Found: C 55.75, H 4.67.

Step 2:

Synthesis of the reactant anthracene-9, lO-diyldimethanamine (2.22):

First 9,lO-Bis(bromomethyl)anthracene (2.21) was synthesized:

2.20 2.21

Anthracene (2.20, 5.0 g, 28 mmol) was suspended in HBr in acetic acid (33 wt %, 100 ml), trioxane (5.05 g, 56 mmol) and tetradecyltrimethylammonium bromide (0.2 g, 0.6 mmol) was added to the suspension. The reaction mixture was stirred at 80°C over night. The reaction mixture was allowed to cool to room temperature, the precipitate was filtered off, washed with water (50 ml) and ethanol (100 ml) yielding 2.21 as a yellow solid (9.4 g, 92 %), mp. decompose. 1 H NMR (500 MHz, Chloroform-;/) d 8.39 - 8.37 (AA’XX’ system, 4H), 7.69 - 7.67 (AA’XX’ system, 4H), 5.52 (s, 4H). 13 C NMR (126 MHz, Chloroform-;/) d 130.43, 129.76, 126.86, 124.53, 26.76. Elemental analysis (%) calculated for Ci 6 Hi2Br2: C 52.78, H 3.3; Found: C 52.89, H 3.39.

There after the reactant anthracene-9, lO-diyldimethanamine (2.22) was synthesized:

2.21 2.22

9,lO-Bis(bromomethyl)anthracene (2.21, 2.0 g, 5.5 mmol) and hexamethylenetetraamine (1.7 g, 12.1 mmol) was suspended in chloroform (200 ml) and refluxed 20 hours. The reaction mixture was allowed to cool to room temperature, the precipitate was filtered off and washed with chloroform (30 ml). The solid was then suspended in a mixture of ethanol (96 %, 200 ml) and hydrochloric acid (cone., 25 ml) and refluxed for 48 hours. The reaction mixture was cooled to 0°C in an ice bath, the precipitate was filtered off and washed with water (20 ml) and ethanol (20 ml). The solid was dried with N 2 flow, and then dispersed into a sodium carbonate solution (10 %, 180 ml). The suspension was stirred for a few minutes, hereafter, chloroform (30 ml) was added, and the phases were separated and the aqueous phase extracted with chloroform (4 x 50 ml), dried over sodium sulfate, filtered and evaporated to obtain 2.22 as a yellow solid (576 mg, 44 %). mp. 227 - 228°C. 'H NMR (500 MHz, Chloroform-;/) d 8.43 - 8.41 (AA’XX’ system, 4H), 7.58 - 7.56 (AA’XX’ system, 4H), 4.85 (s, 4H). 13 C NMR (126 MHz, Chloroform-;/) d 134.91, 129.50, 125.95, 124.81, 38.61.

Step 3:

Finally the complex was formed by the“clipping” method, where the anthracenyl- tetralactam macromolecule is synthesized around the dye, whereby the dye become permanently interlocked in the macro molecule:

Dye (3.3 from example 1, 70 mg, 0.13 mmol) was dissolved in dry chloroform (20 ml) in a flask and triethylamine (one drop) was added. The flask was fitted with a rubber septum and a balloon filled with nitrogen. 2.22 (20 mg, 0.08 mmol) was dissolved in dry chloroform (2 ml), and 2.19 (22 mg, 0.08 mmol) was dissolved in dry chloroform (2 ml). The amine 2.22 and acid chloride 2.19 solutions were simultaneous added using a syringepump over 4 hours to the solution containing the dye 3.3. The reaction mixture was stirred for 12 hours. Subsequently 2.22 (20 mg, 0.08 mmol) was dissolved in dry chloroform (2 ml), and 2.19 (22 mg, 0.08 mmol) was dissolved in dry chloroform (2 ml). The amine and acid chloride solutions were simultaneous added with a syringepump over 1 hour to the reaction mixture. The resulting mixture was evaporated to dryness and purified by gradient column chromatography (dichloromethane to 10 % acetonitrile in dichloromethane). The product was further purified by recrystallisation from chloroform/heptane (24 mg, 13 %), mp. decompose. l H NMR (500 MHz, Chloroform-;/) d 9.78 - 9.25 (m, 2H), 8.57 - 8.45 (m, 4H), 8.24 - 7.48 (m, 14H), 7.11 - 6.74 (m, 8H), 5.78 - 5.68 (m, 2H), 5.30 - 5.17 (m, 8H), 3.35 - 3.17 (m, 8H), 1.65 - 1.59 (m, 8H), 1.54 - 1.50 (m, 18H), 1.31 - 1.26 (m, 8H), 1.04 -

0.88 (m, 12H). 13 C NMR (126 MHz, Chloroform-;/) d 184.50, 182.57, 172.24, 172.00,

167.34, 166.66, 152.87, 152.56, 139.47, 133.34, 133.18, 133.14, 133.05, 132.58,

130.67, 130.63, 130.58, 130.54, 129.88, 129.59, 129.47, 129.32, 129.30, 129.24,

129.15, 128.53, 126.47, 126.24, 125.78, 124.79, 124.12, 123.13, 123.07, 121.08, 112.28, 38.15, 37.97, 37.67, 37.49, 35.50, 32.03, 31.53, 29.85, 29.67, 29.60, 29.51,

29.17, 20.11, 13.90. LC-HRMS: m/z = 1373.6540 [M+H + ] (Calculated 1673.6542). kmax (CHCh) = 831 nm, e (CHCh) 1.6 x 10 5 L mol i, <D f (CHCh) = 0.02.

The photophysical and stability properties of the dye 3.3 and the complex 3.6 (also referred to as rotaxan) relevant for their use as NIR absorbers in welding of plastic materdials will be described in the following examples, where 3.3 and 3.6 of the invention are compared to the dye 3.1 from the prior art (WO 97/08692 and US 2017/0285232) and to the anthracenyl-tetralactam macro molecule dissociable interlocking the dyes 3.1 and 3.2 from the prior art (US 2017/0285232) denoted 3.4 and 3.5, respectively, and also referred to as pseudorotaxanes.

Example 3: Photophysical properties

The photophysical properties of the synthesized dyes and their complexes are important in the context of laser welding. As mentioned earlier, the dyes must absorb light at a wavelength where the laser in question radiates, and they should show little or no fluorescence. For this purpose, the dyes and a commercially available dye for laser welding were investigated, Lumogen765. This will enable comparison with a dye that is currently being used for laser welding. The exact structure of Lumogen765 is not available, therefore it was not possible to determine the concentration for this sample.

Absorption and emission properties

An overview of the croconaine dye structures (3.1; 3.3) and the complexes (3.4; 3.5; 3.6) are given in figure 1 along with the UV/Vis absorption spectra of the studied croconaine dyes and Lumogen765.The UV/V is absorption spectra of the croconaine dyes and the corresponding inclusion complexes unveil that they all display absorption in the NIR region around 800 nm. Hence, the diode laser emitting at 808 nm (the verical dotted line in the figure) will be able to lead to absorption for the croconaine dyes. The commercially available Lumogen765 show a blueshifted absorption with a maximum wavelength of 765 nm, compared to the croconaine dyes and the complexes investigated here (i.e. 3.1; 3.3; 3,4; 3.5 and 3.6). It appears to be the tailing of the maximum absorption of Lumogen765 into the region around 800 nm that enables Lumogen765 to be used as a dye for laser welding with the lasers emitting at 808 nm. Thus the croconaine dyes appear to overlap to a greater extend with the emission properties of the lasers that radiates at 808 nm.

The absorption spectra of the rotaxane 3.6 and the free dye 3.3 as illustrted by figure 2a reveal a redshift of approximately 30 nm upon encapsulation of the dye, from 798 nm of the free dye to 831 nm for the rotaxane. In addition, encapsulation of the dye leads to a reduction in the extinction coefficient. Fluorescence spectra of the free dye 3.3 and the rotaxane 3.6 show that the rotaxane lead to less fluorescence than the dye (figure 2b).

An overview of the absorption data for the investigated dyes is outlined in table 2. In general, the free dyes absorb at 798 nm, whereas formation of the inclusion complex with the macrocycle leads to a redshift of 20 to 30 nm. Furthermore, encapsulation of the dye leads to a reduction in the extinction coefficient.

Table 2. Photophysical properties of the synthesised dyes and Lumogen765 in CHCb.

Ί Ϊ 798 2.8 x 10 5

3.3 798 3.0 x 10 5

3.4 822 1.4 x 10 5

3.5 819 1.2 x 10 5

3.6 831 1.6 x 10 5

Lumogen765 [b] 765 _ - _

Determined by calibration curves.

M The molecular absorptivity coefficient (e) was not determined, as the concentration was unknown. Fluorescence quantum yields

The fluorescence quantum yields were determined in chloroform, as this is a measure of the amount of fluorescence emitted. The concentrations of croconaine dyes and the quantum yield standard, indocyanine green, was adjusted to the same absorbance at 750 nm, and the absorbance was less than 0.1 a.u. Emission spectra were then obtained for the dye and indocyanine green with excitation of both solutions at 750 nm. The areas of the emission spectra were determined and used to calculate the quantum yield with the following equation:

Fs fst Rs

F = F st

f Fst is n st 2

Where Fi is the fluorescence quantum yield, F the integrated fluorescence intensity, f the absorption factor and n is the refractive index of the solvent s denotes the sample, and st denotes the standard. Note that for solutions with identical absorbance values at the wavelength of excitation, the term for absorbance factors cancel out. The obtained quantum yields are outlined in table 3.

The quantum yields are in general low, meaning that the compounds show little fluorescence. The fluorescence quantum yield of the rotaxane 3.6 is lower than for the corresponding free dye 3.3, indicating that when the sample is excited at 750 nm, the macrocycle cause for even less fluorescence than the free dye, making the rotaxane a favorable dye for laser welding.

Table 3. Emission properties of the synthesised dyes in CHCb.

Dye _ max (nm) _ Of

3.1 810 0.05

3.3 810 0.06

3.6 833 0.02

Example 4: Stability studies

As stated above, one of the criteria for the for being suitable for NIR laser welding of plastic materials is (as steted above) that the dyes are able to sustain the excess heating in the process where the dye is mixed with the plastic polymer. No general protocol exist for testing if the dyes are prone to decompose in this compounding process, therefore different experiments were performed to examine the stability of the dyes. These will be described in the following sections.

Heat exposure

To evaluate the stability of the dyes at elevated temperatures, solutions of the dyes were prepared in 1 ,2-dichlorobenzene and heated to l50°C. The rate of decomposition was monitored by obtaining UV/Vis absorbance spectra over time. The absorption of the dyes was observed at the respective maximum wavelength absorption.

The results of the free dyes and pseudorotaxanes are shown in figure 3 a. The least stable dye under these conditions is the free dye 3.1, which does not show any observable absorption after 450 minutes. The two pseudorotaxanes 3.4 and 3.5 appear less prone to decomposition, as these dyes are not fully decomposed after close to 500 minutes. The dye 3.3 was even more stable, where significant absorption is still observed after more than 500 minutes. The rotaxane dye 3.6 showed a remarkably stability compared to the other dyes (figure 3b). Under these conditions, the rotaxane still shows significant absorption after approximately 100 hours. This preliminary study indicates that the pseudorotaxanes are more stable than the free dyes, but that the dye 3.3 possessing alkyl chains are more stable than the dye 3.1 with an aliphatic ring substituted on the thiophene. In addition, the rotaxane 3.6 is far more stable than any of the other investigated dyes.

To further investigate the stability of the dyes in solution, samples of the dyes were prepared in nitrobenzene and heated to 200°C. Again, the rate of decomposition was observed at the maximum wavelength absorption by obtaining UV/Vis absorption spectra over time. Unfortunately, the pseudorotaxanes were found to dethread in nitrobenzene, therefore only the rotaxane 3.6 and the free dyes 3.1 and 3.3 were examined. The results of the investigations are shown in figure 4. At 200°C, the free dyes are most prone to decomposition. Dye 3.1 is showing no significant absorption after approximately 100 minutes. The dye 3.3 shows no significant absorption after approximately 200 minutes, suggesting that dye 3.3 is the most stable of the free dyes. The rotaxane 3.6 is the far most stable dye, where no significant absorption is observed after approximately 600 minutes.

These results indicate that the rotaxane is the most stable dye in a heat exposed solution. Dye 3.3 was less prone to undergo decomposition compared to the other free dyes. Importantly, the dyes were not observed to degrade into colored decomposition products, as was observed in the UV/Vis absorption spectra. This suggests that any degradation of dye in the manufacturing process will most likely not lead to discoloration of the plastic items.

Light exposure

Verifying the decomposition rate of the dyes towards light exposure is important, as this is relevant for storage stability. Fading of the dye, i.e. loss of absorption intensity due to light exposure will make the dye less favorable, as the shelf life of the dye might decrease. A 200 Watt Xenon arc lamp was used for this experiment, as a Xenon lamp has a spectral range close to natural sunlight. A sample of the dye in question was dissolved in chlorobenzene. The decomposition was monitored for the maximum wavelength absorption of the dye by UV/Vis absorption spectra over time (figure 5). From the obtained data, a faster decomposition is observed for the free dyes 3.1 and 3.3. The pseudorotaxanes shows a higher stability than the free dyes, and the rotaxane 3.6 shows the highest stability of the tested dyes, indicating that the macrocycle provides further stability to the dyes, and that a rotaxane with a dye substituted with alkyl chains is favoured to a pseudorotaxane.

Heat exposure in the solid phase

Measurements of absorption at elevated temperature in the solid state are not limited to boiling points of solvents or solubility of the dyes in a given solvent. In addition, undesired solvent properties are excluded when the dye is heated neat.

In this study, a known amount of dye was heated to 200°C for a certain period of time, and the decomposition was monitored by dissolving the dye and obtaining UV/Vis absorption spectra. The decrease in absorption at the maximum wavelength according to time for the dyes 3.1, 3.4 and 3.6 is presented in figure 6. The data reveals that the free dye 3.1 decomposes rapidly in approximately 20 minutes, whereas the pseudorotaxane 3.4 decomposes at a slower rate, where the absorption has not totally ceased after 500 minutes. The rotaxane 3.6 exceeds the stability of free dye 3.1 and the pseudorotaxanes, as the absorption for his compound has not vanished after more than 1000 minutes, corresponding to more than 16 hours.

Conclusion examples 3 and 4

The photophysical studies presented in examples 3 and 4 is reveal that the dyes and complexes of the invention as exemplified by dye 3.3 and rotaxane 3.6 absorb intensely in the near-infrared region, and that they exhibit low fluorescence quantum yields. The permanently interlocked structures as exemplified by rotaxane 3.3 show lower quantum yields than the free dyes. Decomposition studies of the dyes by heat exposure in solution and in the solid phase, along with light exposure, unveil the stability of the dyes of the invention as exemplified by dye 3.3. The dye 3.3 and in particularly the rotaxane 3.6 were observed to be more stable, showing a slower rate of decomposition, compared to the other free dyes and pseudorotaxanes. These findings manifests that the dyes of the invention and their permanently interlocked complexes are suitable as dyes for NIR laser welding of plastic materials, as they full fill all the criteria of points 1.-6 as outlined above.




 
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