Photochromic thiazolothiazole composites and applications thereof

Abstract

Photochromic composites are described herein comprising pyridinium thiazolothiazole compounds disposed in various non-hydrogel polymeric matrix constructions. As described further herein, such composites can find employment in a number of applications, including oxygen sensing, NIR photonic and communications, and various technologies requiring light-induced color changes, such as non-electrochromic tinting.

Description

[0001] PHOTOCHROMIC THIAZOLOTHIAZOLE COMPOSITES AND APPLICATIONS THEREOF

[0002] RELATED APPLICATION DATA

[0003] The present application claims priority pursuant to 35 U.S.C. § 119(e) to United States Provisional Patent Application Serial Number 63 / 521,958 filed June 20, 2023 which is incorporated herein by reference in its entirety.

[0004] FIELD

[0005] The present invention relates to thi azol othi azole compounds and, in particular, to photochromic composite films comprising pyridinium thiazolothiazole compounds.

[0006] BACKGROUND

[0007] Photochromic materials are a popular research topic because of their potential applications in self-tinting smart windows, eyeglasses, displays, and glucose sensors. Some materials also exhibit photofluorochromism, where fluorescence intensity or wavelength changes with the light exposure, which can be used for displays, erasable memory devices, or sensors. Color changing oxygen sensing materials have a wide variety of uses for smart packaging, medical bandages, and wearable devices for confined spaces. Organic materials are advantageous because of their high contrast, flexibility, easy processing, and inexpensive starting materials.

[0008] There have been many materials used for photochromic and photofluorochromic devices and films. Organic dyes like Berry Red have been suspended in Paraloid B-72 and polyvinyl butyral polymeric films to yield high contrast photochromism that bleaches with heat. Ideally, the photochromic material would be organic, flexible, and cast out of water for fast roll-to-roll processing that is organic solvent-free, and environmentally-friendly. Many groups have pursued this, by coating indolinospirooxazine / ethylene-vinyl acetate copolymer in a two roll mill, spirooxazine or spiropyran / disentangled ultrahigh molecular weight polyethylene via two roll mill, and slot die coating spiropyran-based or spirooxazine-based photochromic compounds out of alcohols onto polyethylene terephthalate (PET) substrates.

[0009] The absorbance of near infrared (NIR) light is advantageous for applications like photonics and telecommunications, since fiber optics use 1310 and 1550 nm light. NIR absorbing materials have also been studied for organic photovoltaics, since 52% of solar energy is in the NIR. This is targeted for semi-transparent high efficiency window solar cells. Materials that absorb NIR light can also be used as window glazing that rivals current low-e coatings for more energy efficient buildings by reducing solar heat gain. Using molecular systems that absorb NIR light are useful for photothermal conversion and photothermal therapy, as NIR light, particularly 1000 - 1350 nm, can efficiently penetrate flesh.

[0010] Oxygen sensing is advantageous for smart packaging of perishable items or monitoring oxygen levels in confined work spaces. Food is commonly packaged under nitrogen or carbon dioxide to reduce oxygen content to 0.5 to 2%, which decreases spoilage. Oxygen induced spoilage occurs from aerobic microorganism growth, oxidation of oils or lipids, or enzymatic reactions that cause fruit / vegetable browning. Packaging under inert atmospheres is also important for electronics, medical equipment, and pharmaceuticals to prohibit oxidation. Decataldo et al. used agarose hydrogel based organic electrochemical sensors made of PEDOT:PSS for flexible, wearable sensing in the tight range of 13-21% oxygen levels. Quantitative color changes have been reported using platinum porphyrin and CdTe quantum dots. Viologen containing MOF’s have also been used for high contrast photochromism and oxygen sensing. Material that not only senses the presence of oxygen, but also indicates the direction or location of a leak is advantageous to eliminate leaks or failure points.

[0011] SUMMARY

[0012] In view of the foregoing, photochromic composites are described herein comprising pyridinium thi azol othi azole compounds disposed in various polymeric matrix constructions. As described further herein, such composites can find employment in a number of applications, including oxygen sensing, NIR photonics and communications, and various technologies requiring light-induced color changes, such as non-electrochromic tinting. In some embodiments, a photochromic composite comprises a polymeric matrix, and a pyridinium thi azol othi azole compound disposed in the polymeric matrix, wherein a first excited state reduction potential and / or a second excited state reduction potential of the pyridinium thi azol othi azole compound is greater than oxidation potential of the polymeric matrix permitting a photoinduced first electron transfer and photoinduced second electron transfer from the polymeric matrix to the pyridinium thi azol othi azole compound, wherein the first electron transfer provides a first color change of the pyridinium thiazolothiazole compound, and the second electron transfer provides a second color change of the pyridinium thiazolothiazole compound. The first and second color changes of the pyridinium thiazolothiazole compound can be reversed by exposure of the photochromic composite to molecular oxygen. Moreover, the first and / or second color change is stable under a non-oxygenated environment of the photochromic composite.

[0013] In another aspect, a photochromic composite comprises a polymeric matrix, a redox agent disposed in the polymeric matrix, and a pyridinium thiazolothiazole compound disposed in the polymeric matrix, wherein a first excited state reduction potential and / or a second excited state reduction potential of the pyridinium thiazolothiazole compound is greater than oxidation potential of the redox agent permitting a photoinduced first electron transfer and photoinduced second electron transfer from the redox agent to the pyridinium thiazolothiazole compound, wherein the first electron transfer provides a first color change of the pyridinium thiazolothiazole compound, and the second electron transfer provides a second color change of the pyridinium thiazolothiazole compound. In some embodiments, the first and second color changes of the pyridinium thiazolothiazole compound are only associated with electron transfer from the redox agent. Alternatively, the first and second color changes of the pyridinium thiazolothiazole compound can be associated with electron transfer from the redox agent and the polymeric matrix.

[0014] In another aspect, methods of detecting molecular oxygen are described herein. In some embodiments, a method of detecting molecular oxygen comprises providing a photochromic composite comprising a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change of the pyridinium thiazolothiazole compound due to a first photooxidation of the polymeric matrix by the pyridinium thiazolothiazole compound. Reversal of the first color change can be registered due to oxidation of the reduced pyridinium thiazolothiazole compound by the presence of molecular oxygen in the environment. Irradiating the photochromic composite further induces a second color change of the pyridinium thiazolothiazole compound due to a second photooxidation of the polymeric matrix by the pyridinium thiazolothiazole compound. Reversal of the second color change of the pyridinium thiazolothiazole compound can be registered by presence of molecular oxygen in the environment. In another aspect, a method of detecting molecular oxygen in an environment comprises providing a photochromic composite comprising a redox agent and a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change of the pyridinium thiazolothiazole compound due to a first photooxidation of the redox agent by the pyridinium thiazolothiazole compound. Reversal of the first color change can be registered due to oxidation of the reduced pyridinium thiazolothiazole compound by the presence of molecular oxygen in the environment. Irradiating the photochromic composite further induces a second color change of the pyridinium thiazolothiazole compound due to a second photooxidation of the redox agent by the pyridinium thiazolothiazole compound. Reversal of the second color change of the pyridinium thiazolothiazole compound can be registered by presence of molecular oxygen in the environment. In some embodiments, the pyridinium thiazolothiazole compound may also oxidize the polymeric matrix in addition to the redox agent for the first and / or second color change.

[0015] In another aspect, methods of photoactuation are described herein. In some embodiments, a method of photoactuation comprises providing a photochromic composite comprising a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change and / or second color change of the pyridinium thiazolothiazole compound, wherein the photochromic composite bends or deflects in response to the light exposure. In another aspect, a method of photoactuation comprises providing a photochromic composite comprising a redox agent and a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change and / or second color change of the pyridinium thiazolothiazole compound, wherein the photochromic composite bends or deflects in response to the light exposure.

[0016] In some embodiments of photoactuation described herein, the photochromic composite bends in a direction toward the incoming light. Photoactuation by photochromic composites described herein can permit the composites to be used in various mechanical applications, including switched and soft robotics.

[0017] These and other embodiments are further described in the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG. 1 provides various dipyridinium thi azol othi azole compounds that can be employed in photochromic composites described herein.

[0019] FIG. 2a illustrates the reversible photochromism of the TTz molecule.

[0020] FIG. 2b illustrate photochromic writing with a photochromic film using a 405 nm laser pointer, according to some embodiments.

[0021] FIG. 2c illustrates a free-standing photochromic flexible film exhibiting illumination dependent color contrast, according to some embodiments.

[0022] FIG. 2d illustrates photolithography performed with the photochromic film, according to some embodiments.

[0023] FIGS. 3a-3f: Photochromism of PVA / Borax films with different borax concentrations, 3a) 0% borax, 3b) 5% borax, 3c) 10% borax, 3d) 14% borax, 3e) visual representation of photochromism, 3f) photofluorochromism of 14% borax PVA / Borax film (420 nm excitation), with inset visual representation

[0024] FIG. 3g is a bandgap diagram of PVA, borax, PVA / borax mixture, and crosslinked PVA / borax.

[0025] FIGS. 4a-4h: Photochromism of PVA / Borax films with different NPrTTz concentrations, 4a) 0.4%, 4b) 1.7%, 4c) 3.4%, 4d) 5%, e) change in 710 nm absorbance over photochromism time, f) visual representation of photochromism, 4g) visible / NIR absorbance of 0.4% TTz film, 4h) visible / NIR absorbance of 5% TTz film.

[0026] FIGS. 5a and 5b illustrate absorbance and emission of 0.4% NPrTTz 14% Borax film before and after 30 min of illumination while in liquid nitrogen, respectively.

[0027] FIG. 5c illustrates photochromism of 0.4% NPrTTz 14% Borax film dried with normal conditions.

[0028] FIG. 5d illustrates photochromism of 0.4% NPrTTz 14% Borax film dried in vacuum oven for 72 hours.

[0029] FIGS. 5e and 5f illustrate photochromism and photofluorochromism of 0.5% NPrTTz agarose film, respectively.

[0030] FIGS. 5g and 5h illustrate absorbance and fluorescence of NPrTTz photodegradation in a PMMA film, respectively. FIG. 6a provides change in 710 nm absorbance of the photochromic film over time at low ppm O2 levels and ambient conditions showing oxygen sensitivity, with corresponding pictures inset.

[0031] FIG. 6b provides photochromic film absorbance change at 630 nm over 14 days in the glovebox ~100 ppm O2.

[0032] FIG. 6c are pictures of 0.4% TTz film in glovebox atmosphere.

[0033] FIG. 6d are pictures of 5% TTz film in glovebox atmosphere.

[0034] FIG. 6e are pictures of oxygen sensing TTz film in nitrogen flushed zipper closed bag.

[0035] FIG. 7 illustrates photoactuation of photochromic composites described herein according to some embodiments.

[0036] DETIALED DESCRIPTION

[0037] Embodiments described herein can be understood more readily by reference to the following detailed description and examples and their previous and following descriptions. Elements, apparatus and methods described herein, however, are not limited to the specific embodiments presented in the detailed description and examples. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations will be readily apparent to those of skill in the art without departing from the spirit and scope of the invention.

[0038] Definitions

[0039] “Alkyl,” as used herein, refers to a straight or branched chain hydrocarbon containing from 1 or 2 to 10 or 20 or more carbon atoms (e.g., C2, C3, C4, C5, C6, C7, C8, C9, CIO, Cl 1, C12, C13, C14, C15, etc.). In some embodiments the alkyl can be a lower alkyl. "Lower alkyl" refers to a straight or branched chain alkyl having from 1 to 3, or from 1 to 5, or from 1 to 8 carbon atoms. Representative examples of alkyl include, but are not limited to, methyl, ethyl, n- propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, n- hexyl, 3 -methylhexyl, 2,2-dimethylpentyl, 2,3-dimethylpentyl, n-heptyl, n-octyl, n-nonyl, n- decyl, and the like. In some embodiments, alkyl groups as described herein are optionally substituted (e.g., from 1 to 3 or 4 times) with independently selected halo, hydroxy, acyl, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycle, aryl, heteroaryl, alkoxy, amino, amide, thiol, sulfide, sulfone, sulfoxide, carbonyl, or carboxy. Representative examples of halo substituted alkyls include, but are not limited to, fluoromethyl, difluoromethyl and trifluoromethyl.

[0040] “Alkenyl,” as used herein, refers to a straight or branched chain hydrocarbon containing from 2 to 10 or 20 or more carbons, and containing at least one carbon-carbon double bond, formed structurally, for example, by the replacement of two hydrogens. Representative examples of “alkenyl” include, but are not limited to, ethenyl, 2-propenyl, 2-methyl-2-propenyl, 3-butenyl, 4-pentenyl, 5-hexenyl, 2-heptenyl, 2-methyl-l -heptenyl, 3 -decenyl and the like. In some embodiments, alkenyl groups as described herein are optionally substituted (e.g., from 1 to 3 or 4 times) with independently selected halo, hydroxy, acyl, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycle, aryl, heteroaryl, alkoxy, amino, amide, thiol, sulfide, sulfone, sulfoxide, carbonyl, or carboxy.

[0041] “Cycloalkyl,” as used herein, refers to a saturated cyclic hydrocarbon group containing from 3 to 8 carbons or more. Representative examples of cycloalkyl include, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl. In some embodiments, cycloalkyl groups as described herein are optionally substituted (e.g., from 1 to 3 or 4 times) with independently selected halo, hydroxy, acyl, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycle, aryl, heteroaryl, alkoxy, amino, amide, thiol, sulfide, sulfone, sulfoxide, carbonyl, or carboxy. A representative example of a substituted cycloalkyl include epoxide.

[0042] “Heterocycle,” as used herein, refers to a monocyclic, bicyclic, or tricyclic ring system comprising at least one heteroatom. Monocyclic heterocycle ring systems are exemplified by any 4-, 5-, 6- or 7-member ring containing 1, 2, 3, or 4 heteroatoms independently selected from the group consisting of: O, N, and S. The 4-member ring has 0 to 1 double bond, the 5-member ring has from 0 to 2 double bonds, and the 6 and 7 member rings have from 0 to 3 double bonds.

[0043] “Aryl” as used herein refers to a ring system having one or more aromatic rings. Representative examples of aryl include azulenyl, indanyl, indenyl, naphthyl, phenyl, tetrahydronaphthyl, and the like. The aryl groups of this invention can be substituted with 1, 2, 3, 4, or 5 substituents independently selected from alkenyl, alkenyloxy, alkoxy, alkoxyalkoxy, alkoxycarbonyl, alkyl, alkylcarbonyl, alkylcarbonyloxy, alkylsulfinyl, alkyl sulfonyl, alkylthio, alkynyl, aryl, aryloxy, azido, arylalkoxy, arylalkyl, aryloxy, carboxy, cyano, formyl, halogen, haloalkyl, haloalkoxy, hydroxy, hydroxyalkyl, mercapto, nitro, sulfamyl, sulfo, sulfonate, -NR’R” (wherein, R’ and R” are independently selected from hydrogen, alkyl, alkylcarbonyl, aryl, arylalkyl and formyl), and -C(O)NR’R” (wherein R’ and R” are independently selected from hydrogen, alkyl, alkylcarbonyl, aryl, arylalkyl, and formyl). In some embodiments, aryl groups as described herein are optionally substituted (e.g., from 1 to 3 or 4 times) with independently selected halo, hydroxy, acyl, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycle, aryl, heteroaryl, alkoxy, amino, amide, thiol, sulfide, sulfone, sulfoxide, carbonyl, or carboxy.

[0044] “Heteroaryl” means a cyclic, aromatic hydrocarbon in which one or more carbon atoms have been replaced with heteroatoms. If the heteroaryl group contains more than one heteroatom, the heteroatoms may be the same or different. Examples of heteroaryl groups include pyridyl, pyrimidinyl, imidazolyl, thienyl, furyl, pyrazinyl, pyrrolyl, benzofuranyl, isobenzofuranyl, chromenyl, xanthenyl, indolyl, isoindolyl, indolizinyl, triazolyl, pyridazinyl, indazolyl, purinyl, quinolizinyl, isoquinolyl, quinolyl, phthalazinyl, naphthyridinyl, quinoxalinyl, isothiazolyl, and benzo[b]thienyl. Preferred heteroaryl groups are five and six membered rings and contain from one to three heteroatoms independently selected from the group consisting of: O, N, and S. The heteroaryl group, including each heteroatom, can be unsubstituted or substituted with from 1 to 4 suitable substituents, as chemically feasible. For example, the heteroatom S may be substituted with one or two oxo groups, which may be shown as =0. In some embodiments, heteroaryl groups as described herein are optionally substituted (e.g., from 1 to 3 or 4 times) with independently selected halo, hydroxy, acyl, alkyl, alkenyl, alkynyl, cycloalkyl, heterocycle, aryl, heteroaryl, alkoxy, amino, amide, thiol, sulfide, sulfone, sulfoxide, carbonyl, or carboxy.

[0045] I. Photochromic Composites

[0046] Photochromic composites are described herein comprising pyridinium thiazolothiazole compounds disposed in various polymeric matrix constructions. In some embodiments, a photochromic composite comprises a polymeric matrix, and a pyridinium thiazolothiazole compound disposed in the polymeric matrix, wherein a first excited state reduction potential and a second excited state reduction potential of the pyridinium thiazolothiazole compound is greater than oxidation potential of the polymeric matrix, permitting a photoinduced first electron transfer and photoinduced second electron transfer from the polymeric matrix to the pyridinium thiazolothiazole compound, wherein the first electron transfer provides a first color change of the pyridinium thiazolothiazole compound, and the second electron transfer provides a second color change of the pyridinium thi azol othi azole compound. The first and second color changes of the pyridinium thi azol othi azole compound can be reversed by exposure of the photochromic composite to molecular oxygen. Moreover, the first and / or second color change is stable under a non-oxygenated environment of the photochromic composite. The first and second color change, in some embodiments, occurs in the visible region of the electromagnetic spectrum. Moreover, the photochromic composite can have one or more absorption peaks in the near-infrared region of the electromagnetic spectrum.

[0047] Turning now to specific components, the polymeric matrix can comprise any polymer or mixture of polymers conforming to the reduction potential requirements described herein. Polymer forming the polymeric matrix can comprise oxidizable functional groups including alcohols, amines, thiols, and / or conjugated moieties. In some embodiments, inorganic or organometallic functionalities may also be employed, such as ferrocene derivatives. Such functional groups can be associated with the polymer backbone, in some embodiments.

[0048] Polymer of the polymeric matrix can exhibit an oxidation potential less than 2.16 V vs. SCE for the first electron transfer to the pyridinium thi azol othi azole compound, and an oxidation potential less than 1.19 V vs. SCE for the second electron transfer to the pyridinium thi azol othi azole compound, in some embodiments. The polymeric matrix, in some embodiments, comprises polyvinylalcohol (PVA), polyvinylalcohol copolymer, agarose, carboxymethylcellulose, polysaccharides, polypyrrole, polyaniline, polythiophene, poly(3,4- ethylenedioxythiophene (PEDOT), polyamines, polyacetylene, polyamines, thiol-containing polymers, or mixtures thereof.

[0049] In some embodiments, the polymeric matrix is not a hydrogel or does not exhibit hydrogel morphology and / or properties. For example, photochromic composites having composition and properties described herein are in a dry or substantially dry state, in contrast to hydrogels. Moreover, photochromic composite films described herein are operable for detecting molecular oxygen in the environment whereby molecular oxygen oxidizes the reduced pyridinium thi azol othi azole compound to reverse the first and / or second color changes of the pyridinium thi azol othi azole compound. Hydrogels comprising pyridinium thi azol othi azole compounds cannot perform oxygen sensing since the hydrogels must be sealed to work and exhibit oxygen transport restrictions precluding oxygen sensing. Additionally, hydrogels comprising pyridinium thiazolothiazole compounds require application of current to cycle the pyridinium thiazolothiazole compounds between colorless and colored states.

[0050] The polymeric matrix, in some embodiments, is crosslinked. The crosslinking agent can be organic or inorganic in nature. In some embodiments, the crosslinking agent is an inorganic oxide. The crosslinking agent, for example, can be borax. Crosslinking the polymeric matrix can lower the oxidation potential of the polymeric matrix, thereby facilitating the two electron reduction of the pyridinium thiazolothiazole compound. Crosslinking agent can be present in the polymeric matrix in any amount consistent with the technical objectives described herein. Crosslinking agent can be present in an amount of 0.1 weight percent to 20 weight percent or 0.5 weight percent to 15 weight percent of the photochromic composite, in some embodiments.

[0051] The photochromic composite comprises pyridinium thiazolothiazole compounds conforming to the reduction potential requirements described herein. The pyridinium thiazolothiazole compound can be a monopyridinium or a dipyrudinium. The pyridinium thiazolothiazole compound can be symmetric or asymmetric. In some embodiments, the pyridinium thiazolothiazole compound is a dipyridinium thiazolo[5,4 7]-thiazole. The dipyridinium thiazolothiazole compound, in some embodiments, is of formula: wherein Ri and R2 are independently selected from the group consisting of alkyl, alkenyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, quaternary amine, Rs-C(O)OR4, and Rs-SCh" , wherein Ri and R5 are independently alkyl or alkenyl, and R4 is hydrogen or alkyl. FIG. 1 provides various dipyridinium thiazolothiazole compounds that can be employed in photochromic composites described herein. In some embodiments, the identity of Ri and / or R2 can be chosen according to the specific identity of the polymeric matrix and / or processing conditions employed to make photochromic composites described herein.

[0052] The pyridinium thiazolothiazole compound can be present in the composite in any desired amount consistent with the technical objectives described herein. In some embodiments, the pyridinium thiazolothiazole compound is present in an amount of 0.25 weight percent to 10 weight percent of the photochromic composite. As described further herein, lower amounts of the pyridinium thiazolothiazole compound can produce faster and / or more responsive color changes in the photochromic composite.

[0053] In another aspect, a photochromic composite comprises a polymeric matrix, a redox agent disposed in the polymeric matrix, and a pyridinium thiazolothiazole compound disposed in the polymeric matrix, wherein a first excited state reduction potential and a second excited state reduction potential of the pyridinium thiazolothiazole compound is greater than the oxidation potential of the redox agent, permitting a photoinduced first electron transfer and a photoinduced second electron transfer from the redox agent to the pyridinium thiazolothiazole compound, wherein the first electron transfer provides a first color change of the pyridinium thiazolothiazole compound, and the second electron transfer provides a second color change of the pyridinium thiazolothiazole compound. In some embodiments, the first and second color changes of the pyridinium thiazolothiazole compound are only associated with electron transfer from the redox agent. Alternatively, the first and second color changes of the pyridinium thiazolothiazole compound can be associated with electron transfer from the redox agent and the polymeric matrix.

[0054] The pyridinium thiazolothiazole compound can have an identity described hereinabove. In contrast, the polymeric matrix may not exhibit oxidations potentials permitting one electron and / or two electron reduction of the pyridinium thiazolothiazole compound. The polymeric matrix, for example, may be an acrylic polymer or acrylic copolymer. In such embodiments, the redox agent disposed or dispersed in the polymeric matrix exhibits suitable electronic structure of two electron reduction of the pyridinium thiazolothiazole compound. In some embodiments, the polymeric matrix is not a hydrogel or does not exhibit hydrogel morphology and / or properties.

[0055] In some embodiments, the redox agent comprises oxidizable groups, as described above. Suitable redox agent can be a saccharide, oligosaccharide, or polysaccharide. Redox agent, for example, can be agarose. Additional redox agents can include ferrocene, ferrocyanide / ferricyanide, TEMPO, and other stable radical species. Organometallic complexes containing metals with low oxidation potentials (2.16V / 1.19V discussed above), including complexes of Cu, Fe, V, and Co, can be used as the redox agent / Metal organic frameworks (MOFs) and covalent organic frameworks (COFs) can also meet the reduction potential requirements of the pyridinium thiazolothiazole compounds described herein. Employment of redox agent can greatly expand the compositional range of the polymeric matrix, with the general requirement the polymeric matrix have limited absorption over part of or the entirely of the visible region of the electromagnetic spectrum.

[0056] In some embodiments, one or more non-photochromic dyes disposed in the polymeric matrix. Such dyes can be employed to change the color characteristics of the photochromic composites as the pyridinium thi azol othi azole compounds undergo the electron transfer process described herein.

[0057] Photochromic composites described herein can exhibit a fdm morphology. In some embodiments, photochromic composite films have a thickness of 10 pm to 100 pm or greater than 100 pm. Photochromic composite films described herein can be flexible and / or foldable. As described further herein and illustrated in FIG. 7, the polymeric matrix, in some embodiments, contracts upon exposure to light. The polymeric matrix, for example, can contract in a direction toward the light. In such embodiments, the photochromic composite films are photo-actuated and can be used in various mechanical applications, including switches and soft robotics.

[0058] II. Methods of Molecular Oxygen Detection

[0059] In another aspect, methods of detecting molecular oxygen are described herein. In some embodiments, a method of detecting molecular oxygen comprises providing a photochromic composite comprising a pyridinium thi azol othi azole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change of the pyridinium thi azol othi azole compound due to a first photooxidation of the polymeric matrix by the pyridinium thi azol othi azole compound. Reversal of the first color change can be registered due to oxidation of the reduced pyridinium thiazolothiazole compound by the presence of molecular oxygen in the environment. Irradiating the photochromic composite further induces a second color change of the pyridinium thiazolothiazole compound due to a second photooxidation of the polymeric matrix by the pyridinium thiazolothiazole compound. Reversal of the second color change of the pyridinium thiazolothiazole compound can be registered by presence of molecular oxygen in the environment.

[0060] In another aspect, a method of detecting molecular oxygen in an environment comprises providing a photochromic composite comprising a redox agent and a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change of the pyridinium thi azol othi azole compound due to a first photooxidation of the redox agent by the pyridinium thiazolothiazole compound. Reversal of the first color change can be registered due to oxidation of the reduced pyridinium thiazolothiazole compound by the presence of molecular oxygen in the environment. Irradiating the photochromic composite further induces a second color change of the pyridinium thiazolothiazole compound due to a second photooxidation of the redox agent by the pyridinium thiazolothiazole compound.

[0061] Reversal of the second color change of the pyridinium thiazolothiazole compound can be registered by presence of molecular oxygen in the environment. In some embodiments, the pyridinium thiazolothiazole compound may also oxidize the polymeric matrix in addition to the redox agent for the first and / or second color change.

[0062] Components of the photochromic composites can have any identity, architecture, and / or properties described in Section I above. Additionally, the photochromic composites can be employed in any desired environment for oxygen detection. In some embodiments, the environment is sealed food packaging or an inert chemical manufacturing or storage environment.

[0063] III. Methods of Photoactuation

[0064] In another aspect, methods of photoactuation are described herein. In some embodiments, a method of photoactuation comprises providing a photochromic composite comprising a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change and / or second color change of the pyridinium thiazolothiazole compound, wherein the photochromic composite bends or deflects in response to the light exposure. In another aspect, a method of photoactuation providing a photochromic composite comprising a redox agent and a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change and / or second color change of the pyridinium thiazolothiazole compound, wherein the photochromic composite bends or deflects in response to the light exposure. Components of the photochromic composites can have any identity, architecture, and / or properties described in Section I above. FIG. 7 illustrates photoactuation of photochromic composites described herein according to some embodiments. These and other embodiments are further illustrated in the following non-limiting examples.

[0065] EXMAPLE 1 - Photochromic Composite

[0066] In the present example, dipyridinium TTz’s and their photochromic, photofluorochromic, and oxygen sensing properties in PVA / Borax, agarose, and PMMA polymer films are reported and demonstrate the group’s first TTz viologen fluorescing in solid state. FIG. 2a illustrates the reversible photochromism of the TTz molecule. When the yellow TTz2is excited by blue light, it photooxidizes the crosslinked PVA / borax and reduces to TTz‘+and reduces a second time via photooxidation of the PVA / borax to TTz0. The TTz0can be oxidized back to TTz2+when exposed to oxygen in the dark. The flexible photochromic film is activated by blue light, changing from yellow to blue. FIGS. 2b to 2d illustrate photochromic writing with a 405 nm laser pointer, a free-standing photochromic flexible film exhibiting illumination dependent color contrast, and photolithography performed with the photochromic film, respectively.

[0067] Borax Crosslinking Dependence

[0068] When coating the PVA / Borax film, it was noted that without borax, the solution was watery and thinned out on the plastic backing, whereas inclusion of the borax thickened the solution and yielded better adhesion to the plastic backing. When comparing the PVA only film to the 14% borax film, the absorbance of the PVA only film showed much less reduced TTz0concentration than the 14% borax film. Two other borax concentration films were tested to determine if the 710 nm absorbance intensity was borax concentration dependent. With increasing borax concentration, the intensity of the 710 nm absorbance increases when comparing the max absorbance after 30 min of illumination and the rate of color change also increases, meaning the borax helps reduce more of the TTz and reduces the TTz faster (FIGS. 3a-3e). Cyclic voltammetry of the PVA and borax shows the oxidation of the polymer becomes easier once crosslinked with the borate. The onset potential for PVA alone is 1.18 V vs SCE, but decreases to 1.04 V vs SCE when borax is introduced and 1.00 V vs SCE upon crosslinking. This is shown and compared to the reduction potentials of TTz in FIG. 3g. This decrease in oxidation level makes the photo-induced electron transfer more favorable and accelerated. Although the TTz2+state is highly fluorescent, the TTz*+and TTz° are non-emissive, which causes the photofluorochromism. The photofluor ochromism occurs quickly, starting with just 5 s of light exposure. After 1 min of illumination, the fluorescence drops 88%, 89%, 90%, and 94% for the 0%, 5%, 10% and 14% borax concentrations, respectively. The addition of borax does not shift the excitation or emission wavelengths, as they stay around 420 nm and 465 nm, respectively. Representative fluorescence spectra and inset pictures are shown in FIG. 3f.

[0069] Effect of TTz Concentration

[0070] In the PVA / Borax films with 14% borax, different TTz concentrations were tested. The 0.4% TTz film showed immediate double reduction to the TTz0state, whereas the higher TTz concentration films show slower and stepwise reductions (FIGS. 4a-4d). The 3.4% and 5% TTz films show the TTz2+(400 nm absorbance) reduction to TTz‘+(610 nm absorbance) happens before reduction to the TTz0state (710 nm). The rate of TTz reduction is shown in FIG. 4e, which compares onset speed the 710 nm absorbance (TTz0). The 0.4% TTz film takes only 5 min for full reduction whereas 5% TTz takes 30 min. As expected, the higher concentrations yield much darker films when reduced (FIG. 4f).

[0071] When in the yellow TTz2+state, the only absorbance is at 400 nm, with no other absorbance from 500 to 2500 nm. When the photochromism occurs and the TTz*+is formed, in addition to the 610 nm absorbance, absorbances in the NIR at 1150 and 1350 nm occur. When analyzing the 0.4% TTz film (FIG. 4g), both the first (610 nm) and second (710 nm) reductions occur quickly and the NIR absorbance (1150 and 1350 nm) increase steadily. However, after the 1350 nm peak maximizes at 2 min of illumination, the absorbance steadily decreases with further illumination, as the TTz0710 nm peak continues to increase until 10 min of light exposure. With the higher concentration 5% TTz film, the 1350 nm peak increased at a similar rate as the 610 nm TTz1absorbance, before substantial 710 nm absorbance TTz0was formed. With continued illumination, all absorbances increased. This may suggest that the NIR absorbances at 1150 and 1350 nm are caused by the radical cation TTz’+. The NIR absorbances overlap with the 1310 nm light used for fiber optic communications. TTz Film Interactions

[0072] To get a better understanding of the mechanism behind the color change, a PVA / Borax film was submerged in liquid nitrogen and illuminated for 30 min. The absorbance and fluorescence spectra in FIGS 5a-5b show the reduction of the TTz is much slower in the liquid nitrogen, taking 30 min of illumination to achieve the same photofluorochromism that takes 5 s at room temperature. The reduction may be slower because the films components (TTz, PVA, Borax) cannot move freely enough to interact and cause the photoreduction. The trend continued when a film was further dried under vacuum, as TTz reduction was also slower in the drier film, as TTz could not move as much as with a slightly hydrated film (FIGS. 5c-5d).

[0073] In addition to PVA / borax, films were made using agarose and poly(methyl methacrylate) (PMMA). Agarose, like PVA / borax, is a hydrogel and contains many alcohol groups that could get oxidized by the photo-activated TTz. Figure 4 e-f shows how the agarose film is photochromic, showing the TTz'+state at 600 nm and the TTz0state at 710 nm, although the intensities are not as high as PVA / Borax films. The photofluorochromism however was nearly immediate, turning off fluorescence 95% within 5 s. The PMMA film that was cast out of di chloromethane, shown in figure 4 g-h, does not exhibit photochromism or photofluorochromism. The absorbance does not show any TTz'+or TTz0at the 610 nm or 710 nm regions, respectively. Instead, the TTz degrades with prolonged illumination, which is indicated by the loss of 372 nm absorbance and emission intensity.

[0074] When the PVA / Borax or agarose TTz films are exposed to light, they show photomechanochromism as they curl while they change from yellow TTz2+to blue TTz0. As films are illuminated, they curl towards the light, regardless of how they were originally coated. Films with increased TTz content curl faster, more drastically, and are more sensitive to light. As expected, the photomechanochromism is thickness dependent, where thinner films (approx. 20 - 30 pm) curled faster than thicker films (53 pm). The photomechanochromism is reversible, because the film uncurls and goes flat when light is taken away. This photomechanochromism is dependent upon TTz and illumination because films without TTz do not curl and curling does not occur with the 2 °C heat increase from the blue light. Oxygen Sensing

[0075] The PVA / Borax films return to the yellow TTz2+state from the blue TTz° via interaction with oxygen, giving the films the ability to sense oxygen and give a visual indication. FIG. 6a shows initial high absorbance of 710 nm light after 1 min of illumination and how the film’s absorbance changes depending on its environment. When in an open container, the film returns to yellow within 12 hours, meanwhile if the film is put in a low (sub 100 ppm) oxygen sealed cuvette, it takes over 72 hours to return to yellow. If the cuvette is purged with nitrogen gas to ensure minimal oxygen, the film stays consistently blue, which suggests that testing in the sealed cuvette, oxygen was slowly leaking in. Pictures of the film in the cuvette indicate that the film nearest to the cap was yellow while the bottom was still blue, suggesting the TTz film also indicates the direction of oxygen leakage. To verify the sensitivity, a similar experiment was conducted, monitoring the absorbance change from TTz0to TTz2+while in a -100 ppm O2 nitrogen atmosphere glovebox (FIG. 6b). The absorbance did not decrease to show the return to TTz21, instead the overall absorbance at 630 nm increased over the 14 days. Although the measurement was shielded from light, small amounts of ambient light may have further reduced the TTz film. To visually monitor longer term color change, two films were kept in the glovebox atmosphere for 6 weeks and show little visible color change since being activated (FIG. 6c and 6d).

[0076] In the food packaging industry, a vast number of products are sealed under nitrogen. To mimic this, a film was placed in a nitrogen flushed, zipper-closed food storage bag to show long term oxygen leakage (FIG. 6e). Not only did it indicate the presence of oxygen after 2 weeks, but also showed what direction the leak was coming from, in this case the zipper corner of the bag. These results show the TTz films are sensitive to oxygen exposure and yield clear, high contrast visual indication which can be used for smart packaging and other oxygen susceptible applications.

[0077] In the present example, water soluble dipyridinium thi azol othi azole compounds incorporated into inexpensive PVA / borax films and exhibit fast and high contrast photochromism, photofluorochromism, and oxygen sensing. When exposed to light, the films change color from yellow TTz2+to purple TTz'+, then blue TTz0. The contrast and speed of the photochromism is dependent upon the polymer matrix and how easily it can be oxidized and the concentration of photoactive TTz. In addition to visible light absorbance, the films also absorb near infrared. The blue film returns to yellow via oxidation of the TTz when exposed to O2, making the films light activated oxygen sensors that can also sense leak direction for smart packaging. These films show potential to be used in self-tinting smart windows, eyeglasses, displays, erasable memory devices, fiber optic communication, and oxygen sensing. Additional applications for photochromic composites described herein can include amine / alcohol sensing, CO2 conversion, laser sight calibration, damage detection for vehicles, coolant leak sensors, additive manufacturing, fingerprint chemical sensors, methane sensors, CO sensors, nitrogen sensors, biometric probes for cardio cells, remote sensing, and clothing applications.

[0078] Materials and Instrumentation

[0079] Dithiooxamide, 4-pyridinecarboxaldehyde, (3-bromopropyl)-trimethylammonium bromide, poly(vinyl alcohol) (PVA) Mw 11000 - 31000, sodium tetraborate decahydrate (Borax), Poly(methyl methacrylate-co-methacrylic acid) (PMMA) Mw 34,000, methyl p- tosylate, hexanes, and dimethyl formamide (DMF) were all purchased from Sigma-Aldrich and Baker Scientific.1H-NMR measurements were taken using a JEOL 500 MHz NMR and a JEOL 300 MHz NMR. Mass spectrometry measurements were obtained with a Perceptive Biosystems Voyager MALDI-TOF mass spectrometer.

[0080] A Varian Cary 50 Bio was used for UV-vis measurements and a Shimadzu RF-5301PC was used for fluorescence measurements. NIR measurements were collected with a Varian Cary 5000.

[0081] A Gamry Reference 600 was used for cyclic voltammetry with a saturated calomel electrode reference, platinum foil counter electrode, and platinum button working electrode.

[0082] The 14 day low oxygen studies were conducted in an MBraun MB-20G glovebox, using an Ocean Optics QE65000 spectrophotometer and ecosmart 800 lumen multicolor LED light bulb selected to 630 nm red light.

[0083] Film preparation

[0084] PVA / Borax films were made by dissolving NPrTTz in 4% PVA solution, then adding the appropriate amount of 4% borax solution. Depending on borax concentration, additional water was added for a thinner consistency for coating. The solutions were made as follows: 5% Borax: 60 mL 4% PVA solution, 10.4 mg NPrTTz, 3 mL 4% Borax solution, 60 pm coater gap; 10% borax: 60 mL 4% PVA solution, 10.5 mg NPrTTz, 7 mL 4% Borax solution, 5 mL DI water, 60 pm coater gap; 14% borax: 60 mL 4% PVA solution, 11.0 mg NPrTTz, 10 mL 4% Borax solution, 10 mL DI water, 60 pm coater gap. PVA films were made by dissolving 10.0 mg NPrTTz in 60 mL 4% PVA solution and coated at 60 pm. Agarose films were made by dissolving 1.0148 g agarose and 5.3 mg NPrTTz in 15 mL water, then coated (80 pm gap) while warm. The PMMA film was made by mixing 5.0003 g PMMA, 25 mg Me2TTz2+2Tos', and 10 mL di chloromethane and coating with a 50 pm coater gap. A LianDu six-inch adjustable film coating applicator was used to coat the films in a doctor blade like fashion. The films were coated onto mylar sheets (0.1 mm, 4 mil PET). Film thickness was measured with a digital micrometer, 20 - 30 pm film thickness.

[0085] For differing TTz concentrations, the solutions were made as follows: 0.4% NPrTTz: 60 mL 4% PVA solution, 11.0 mg NPrTTz, 10 mL 4% Borax solution, 10 mL DI water, 60 pm coater gap; 1.7% NPrTTz: 60 mL 4% PVA solution, 51.8 mg NPrTTz, 10 mL 4% Borax solution, 8 mL DI water, 60 pm coater gap; 3.4% NPrTTz: 60 mL 4% PVA solution, 102.6 mg NPrTTz, 10 mL 4% Borax solution, 13 mL DI water, 60 pm coater gap; 5% NPrTTz: 60 mL 4% PVA solution, 150 mg NPrTTz, 10 mL 4% Borax solution, 13 mL DI water, 60 pm coater gap.

[0086] The uvBeast V3 flashlight was held 11 cm from the film when conducting photochromic, photofluorochromic, and photomechanochromic testing. At this distance, the flashlight irradiates the film with 0.54 mW / cm2of 394 nm light.

[0087] Synthesis

[0088] Synthesis of 2, 5-di(pyridin-4-yl)thiazolo[5,4-d]thiazole (PyfTTz). Dithiooxamide (1.9916 g, 16.6 mmol) and 4-pyridinecarboxaldehyde (4.4 mL, 46.7 mmol) were refluxed in 60 mL of DMF at 153 °C for 8 h in an aerated environment. The reaction mixture was cooled to room temperature, and the obtained tan precipitate was filtered via vacuum. The solid was then washed with water and dried under vacuum to give a tan solid (3.732 g, 75.9% yield). Molecular characterization data quantitatively matched previously reported values.22'23’32'H-NMR. (500 MHz, CDCh), 8.78 (dd, J = 1.6, 4.6 Hz, 4H), 7.88 (dd, J = 1.6, 4.6 Hz, 4H) ppm. MS [MALDI-TOF]: m / z calculated for C14H8N4S2 296.376, found 298.66. Synthesis of N,N ’-di( trime thylaminopropyl) -2,5-Bis( 4-pyridinium ) thiazolo[5, 4-d] thiazole [((NPr)2TTz+4)Br4]. Py2TTz (2.9906 g, 10.1 mmol) was heated with (3 -bromopropyl) trimethylammonium bromide (6.5995 g, 25.3 mmol) in 35 mL of DMF under nitrogen at 100 °C for 72 h. The precipitate obtained was vacuum filtered and rinsed with DMF and acetonitrile, then dried in the vacuum oven to give a yellow solid (7.2742 g, 87.8% yield). Molecular characterization data quantitatively matched previously reported values.23, 32'H-NMR. (500 MHz, D2O): 2.55 (m, 4H), 3.08 (s, 18H), 3.46 (t, J = 8.0 Hz, 4H), 4.67 (t, J = 6.5Hz, 4H), 8.59 (d, J = 5.5 Hz, 4H), 8.95 ( d, J = 5.5 Hz, 4H) ppm.

[0089] Synthesis of N,N’-Dimethyl 2,5-Bis(4-pyridininm)thiazolo[5,4-d]thiazole ditosylate [(MezTTz2+)Tos2]. Py2TTz (0.2891 g, 0.98 mmol) and was warmed to 30 °C for 48 h in 10 mL of methyl p-tosylate. The precipitate was collected, washed with hexanes, and dried under vacuum to yield 0.5804 g (89% yield) of a brownish yellow solid.22'23'H-NMR (300 MHz, CD3CN): 8.74 (d, J = 6.87 Hz, 4H), 8.50 (d, J = 6.87 Hz, 4H), 7.57 (d, J = 7.98 Hz, 4H), 7.12 (d, J = 7.98 Hz, 4H), 4.32 (s, 6H), 3.24 (s, 3H) ppm.

[0090] Various embodiments of the invention have been described in fulfillment of the various objectives of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention.

Claims

CLAIMS1. A photochromic composite comprising: a polymeric matrix; and a pyridinium thi azol othi azole compound disposed in the polymeric matrix, wherein a first excited state reduction potential and a second excited state reduction potential of the pyridinium thi azol othi azole compound is greater than oxidation potential of the polymeric matrix permitting a photoinduced first electron transfer and second electron transfer from the polymeric matrix to the pyridinium thiazolothiazole compound, wherein the first electron transfer provides a first color change of the pyridinium thiazolothiazole compound, and the second electron transfer provides a second color change of the pyridinium thiazolothiazole compound.

2. The photochromic composite of claim 1, wherein the first and second color changes occur in the visible region of the electromagnetic spectrum.

3. The photochromic composite of claim 1 having adsorption in the near-infrared region of the electromagnetic spectrum.

4. The photochromic composite of claim 1, wherein the oxidation potential of the polymeric matrix is less than 1.2 V.

5. The photochromic composite of claim 1, wherein the first and second color changes of the pyridinium thiazolothiazole compound is reversed by exposure of the photochromic composite to molecular oxygen.

6. The photochromic composite of claim 5, wherein the second color change is stable under a non-oxygenated environment of the photochromic composite.

7. The photochromic composite of claim 1, wherein the polymeric matrix comprises oxidizable functional groups.

8. The photochromic composite of claim 7, wherein the oxidizable functional groups are associated with polymer backbone.

9. The photochromic composite of claim 1, wherein the polymeric matrix is crosslinked.

10. The photochromic composite of claim 9, wherein the polymeric matrix comprises an inorganic crosslinking agent.

11. The photochromic composite of claim 10, wherein the inorganic crosslinking agent is an inorganic oxide.

12. The photochromic composite of claim 9, wherein the crosslinked polymeric matrix has a lower oxidation potential than the un-crosslinked polymeric matrix.

13. The photochromic composite of claim 1, wherein the polymeric matrix comprises a homopolymer.

14. The photochromic composite of claim 1, wherein the polymeric matrix comprises a copolymer.

15. The photochromic composite of claim 1, wherein the polymeric matrix comprises polyvinylalcohol (PVA).

16. The photochromic composite of claim 1, wherein the polymeric matrix comprises a polyvinylalcohol copolymer.

17. The photochromic composite of claim 1, wherein the photochromic composite is a fdm.

18. The photochromic composite of claiml7, wherein the film is self-supporting.

19. The photochromic composite of claim 1 further comprising at least one non- photochromic dye disposed in the polymeric matrix.

20. The photochromic composite of claim 1, wherein the polymeric matrix contracts upon exposure to light.

21. The photochromic composite of claim 20, wherein the polymeric matrix contracts in a direction toward the light.

22. The photochromic composite of claim 1, wherein the pyridinium thi azol othi azole compound is present in an amount of 0.25 weight percent to 10 weight percent of the photochromic composite.

23. The photochromic composite of claim 1, wherein the pyridinium thi azol othi azole compound is a dipyridinium thiazolothiazole compound.

24. The photochromic composite of claim 1, wherein the pyridinium thiazolothiazole compound is symmetric.

25. The photochromic composite of claim 1, wherein the pyridinium thiazolothiazole compound is asymmetric.

26. The photochromic composite of claim 23, wherein the dipyridinium thiazolothiazole compound is a dipyridinium thiazolo[5,4- ]-thiazole.

27. The photochromic composite of claim 26, wherein the dipyridinium thiazolothiazole compound is of formula:wherein Ri and R2 are independently selected from the group consisting of alkyl, alkenyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, quaternary amine, Ra-C(0)0R4, and Rs-SCh' , wherein R3 and R5 are independently alkyl or alkenyl, and R4 is hydrogen or alkyl.

28. A photochromic composite comprising: a polymeric matrix; a redox agent disposed in the polymeric matrix; and a pyridinium thiazolothiazole compound disposed in the polymeric matrix, wherein a first excited state reduction potential and a second excited state reduction potential of the pyridinium thiazolothiazole compound is greater than oxidation potential of the redox agent permitting a photoinduced first electron transfer and second electron transfer from the redox agent to the pyridinium thiazolothiazole compound, wherein the first electron transfer provides a first color change of the pyridinium thiazolothiazole compound, and the second electron transfer provides a second color change of the pyridinium thiazolothiazole compound.

29. The photochromic composite of claim 28, wherein the first and second color changes occur in the visible region of the electromagnetic spectrum.

30. The photochromic composite of claim 28 having adsorption in the near-infrared region of the electromagnetic spectrum.

31. The photochromic composite of claim 28, wherein the oxidation potential of the polymeric matrix is less than 1.2 V.

32. The photochromic composite of claim 28, wherein the first and second color changes of the pyridinium thiazolothiazole compound is reversed by exposure of the photochromic composite to molecular oxygen.

33. The photochromic composite of claim 32, wherein the second color change is stable under a non-oxygenated environment of the photochromic composite.

34. The photochromic composite of claim 28, wherein the redox agent comprises oxidizable functional groups.

35. A method of detecting molecular oxygen comprising in an environment comprising: providing a photochromic composite comprising a pyridinium thi azol othi azole compound disposed in the polymeric matrix; irradiating the photochromic composite to induce a first color change of the pyridinium thi azol othi azole compound due to a first photooxidation of the polymeric matrix by the pyridinium thi azol othi azole compound; and registering reversal of the first color change due to oxidation of the reduced pyridinium thi azol othi azole compound by presence of molecular oxygen in the environment.

36. The method of claim 35, wherein the irradiating the photochromic composite further induces a second color change of the pyridinium thiazolothiazole compound due to a second photooxidation of the polymeric matrix by the pyridinium thiazolothiazole compound.

37. The method of claim 36 further comprising registering reversal of the second color change of the reduced pyridinium thiazolothiazole compound by presence of molecular oxygen in the environment.

38. The method of claim 36, wherein the first and second color changes occur in the visible region of the electromagnetic spectrum.

39. The method of claim 36, wherein the oxidation potential of the polymeric matrix is less than 1.2 V.

40. The method of claim 36, wherein a first excited state reduction potential and a second excited state reduction potential of the pyridinium thiazolothiazole compound is greater than oxidation potential of the polymeric matrix41. The method of claim 36, wherein the polymeric matrix is crosslinked.

42. The method of claim 41, wherein the crosslinked polymeric matrix has a lower oxidation potential than the un-crosslinked polymeric matrix.

43. The method of claim 36, wherein the pyridinium thiazolothiazole compound is present in an amount of 0.25 weight percent to 10 weight percent of the photochromic composite.

44. The method of claim 36, wherein the pyridinium thiazolothiazole compound is a dipyridinium thiazolothiazole compound.

45. The method of claim 36, wherein the pyridinium thiazolothiazole compound is symmetric.

46. The method of claim 36, wherein the pyridinium thiazolothiazole compound is asymmetric.

47. The method of claim 36, wherein the dipyridinium thiazolothiazole compound is a dipyridinium thiazolo[5,4- ]-thiazole.

48. The method of claim 47, wherein the dipyridinium thiazolothiazole compound is of formula:wherein Ri and R2 are independently selected from the group consisting of alkyl, alkenyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, quaternary amine, R3-C(O)OR4, and Rs-SCh" , wherein Rj and R5 are independently alkyl or alkenyl, and R4 is hydrogen or alkyl.

49. The method of any of claims 35 to 48, wherein the environment is sealed food packaging.

50. The method of any of claims 35 to 48, wherein the environment is an inert chemical storage environment.

51. A method of detecting molecular oxygen comprising in an environment comprising: providing a photochromic composite comprising a redox agent and a pyridinium thi azol othi azole compound disposed in the polymeric matrix; irradiating the photochromic composite to induce a first color change of the pyridinium thi azol othi azole compound due to a first photooxidation of the redox agent by the pyridinium thi azol othi azole compound; and registering reversal of the first color change due to oxidation of the reduced pyridinium thiazolothiazole compound by molecular oxygen in the environment.

52. The method of claim 51, wherein the irradiating the photochromic composite further induces a second color change of the pyridinium thiazolothiazole compound due to a second photooxidation of the redox agent by the pyridinium thiazolothiazole compound.

53. The method of claim 52 further comprising registering reversal of the second color change of the reduced pyridinium thiazolothiazole compound by molecular oxygen in the environment.

54. The method of claim 52, wherein the first and second color changes occur in the visible region of the electromagnetic spectrum.

55. The method of claim 52, wherein the pyridinium thiazolothiazole compound is present in an amount of 0.25 weight percent to 10 weight percent of the photochromic composite.

56. The method of claim 52, wherein the pyridinium thiazolothiazole compound is a dipyridinium thiazolothiazole compound.

57. The method of claim 52, wherein the pyridinium thiazolothiazole compound is symmetric.

58. The method of claim 52, wherein the pyridinium thiazolothiazole compound is asymmetric.

59. The method of claim 52, wherein the dipyridinium thiazolothiazole compound is a dipyridinium thiazolo[5,4- ]-thiazole.

60. The method of claim 59, wherein the dipyridinium thiazolothiazole compound is of formula:wherein Ri and R2 are independently selected from the group consisting of alkyl, alkenyl, cycloalkyl, heterocyclyl, aryl, heteroaryl, quaternary amine, R.3-C(O)OR4, and Rs-SCh' , wherein R3 and R5 are independently alkyl or alkenyl, and R4 is hydrogen or alkyl.

61. The method of claim 52, wherein the redox agent is a saccharide, oligosaccharide, or polysaccharide.

62. The method of any of claims 51 to 61, wherein the environment is sealed food packaging.

63. The method of any of claims 51 to 61, wherein the environment is an inert chemical storage environment.

64. A method of photoactuation comprising: providing a photochromic composite comprising a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change and / or second color change of the pyridinium thiazolothiazole compound, wherein the photochromic composite bends or deflects in response to the light exposure.

65. A method of photoactuation comprising: providing a photochromic composite comprising a redox agent and a pyridinium thiazolothiazole compound disposed in a polymeric matrix, and irradiating the photochromic composite to induce a first color change and / or second color change of the pyridiniumthi azol othi azole compound, wherein the photochromic composite bends or deflects in response to the light exposure.

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