# Patent

## Title
Multi(azobenzene-amine) photoactive crosslinkers and methods of making the same

## Abstract

Photo-active cross-linkers derived from a tris(azobenzene)-containing compound with the following generic structure:




wherein for meta-azo substitution, R is equal to H, and for para-azo substitution, R is selected from the group consisting of H, F, Cl, CF3, and CH3.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Patent Application No. 61/636,134, filed on Apr. 20, 2012, by inventor Loon-Seng Tan, et al., entitled “Multi(Azobenzene-Amine) Photo-Active Crosslinkers,” and U.S. Provisional Patent Application No. 61/636,170, filed Apr. 20, 2012, by inventor Loon-Seng Tan, et al., entitled “Azobenzene-Containing Glassy Polyimides Capable of Photo-Induced Large-Angle Bending,” both of which are incorporated herein by reference in their entirety.


RIGHTS OF THE GOVERNMENT
The invention described herein may be manufactured and used by or for the Government of the United States for all governmental purposes without the payment of any royalty.


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a new multi(azobenzene-amine) cross-linker and its derivatives that are used to crosslink imide-based, photomechanical polymers derived from an aromatic amine and an aromatic dianhydride.
2. Description of the Related Art
Photomechanical polymers are a special class of smart (stimuli-responsive) polymers that are responsive to light and are capable of generating photo-directed motions and dimensional or shape alteration at the macro-scale level. These motions and/or alterations are driven by the collective molecular-volume change brought about by the chromophoric units upon appropriate irradiation. The chromophoric units in photoresponsive polymers are photochromic and have the unique ability to reversibly interconvert between two structural isomers (each with distinctly different optical and physical properties) under appropriate excitation conditions. Examples of chromophoric units can be found in photo-isomerizable molecules such as azobenzenes, spiropyrans, spirooxazines, diarylethylenes, and fulgides. However, with respect to photomechanical polymers, azobenzene is the most commonly used molecularly-actuating moiety because of its excellent thermal stability and resolved isomeric forms, as well as its ability to form surface gradient reliefs when subjected to conventional or polarization holography.
The resulting photomechanical output of a polymeric material depends not only on its optical properties (absorption wavelength, wavelength of exposure, polarization of exposure), but also on its molecular architecture and morphology (amorphous, crystalline, liquid crystalline). In addition, the photomechanical output is dependent on the polymeric material's thermomechanical properties, as well as the geometrical properties of the device, e.g. thickness of a cantilever.
Polyimides (PIs) represent an important class of heat-resistant polymers that have found utility in a wide spectrum of applications, ranging from structural components to electronic and photonic devices. The widespread application of polyimides is mainly the result of their excellent combination of physical properties, thermal stability, and processability. For example, polyimides containing azobenzene in the backbone or side-chain have been investigated for photo-induced alignment in liquid crystal display (LCD) as well as nonlinear optical applications. More recently, an azobenzene-containing poly(amic acid) (a PI precursor) was crosslinked by a triamine in N,N-dimethylformamide (DMF) and the resulting sol-gels showed a two-fold increase in the storage modulus after irradiation with 405 nm light. Another closely related aromatic poly(amic acid) was reported to be photomechanically active.
SUMMARY OF THE INVENTION
In accordance with the present invention, there are provided photo-active, multi-(azobenzene-amine) cross-linkers derived from a tris(azobenzene)-containing compound. The presently disclosed cross-linkers exhibit the following general structure in which, for meta-azo substitution (with reference to ether-oxygen), R=H, and for para-azo substitution, R=H, F, Cl, CF3, CH3:




The present invention further includes a method for making the above shown tris(azobenzene-amine) cross-linker comprising the steps of: providing 1,1,1,-tris(4-hydroxyphenyl)ethane; treating the 1,1,1,-tris(4-hydroxyphenyl)ethane with a nitro-activated aryl halide in presence of potassium carbonate to produce a tris[(nitrophenoxy)phenyl]ethane compound; reducing the tris[(nitrophenoxy)phenyl]ethane compound to a tris[(acetamidophenoxy)phenyl]ethane compound by catalytic hydrogenation; treating the tris[(acetamidophenoxy)phenyl]ethane compound with 4-nitrosoacetanilide in acetic acid to yield a tris(azobenzene-amide) compound; and converting the amide to amine in the tris(azobenzene-amide) compound via an alkaline deacetylation reaction.
In one embodiment of the method, the nitro-activated aryl halide is 1-fluoro-4-nitrobenzene and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane. In another embodiment, the nitro-activated aryl halide is 1,2-difluoro-4-nitrobenzene and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane. In another embodiment, the nitro-activated aryl halide is 2-chloro-1-fluoro-4-nitrobenzene, and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane. In an alternative embodiment of the method, the nitro-activated aryl halide is 1-fluoro-4-nitro-2-(trifluoromethyl)benzene, and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane. In a further embodiment, the nitro-activated aryl halide is 1-fluoro-2-methyl-4-nitrobenzene, and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an UV-vis spectra of model tris(azobenzene-N-phthalimide)/THF solution before and after the irradiation of UV lamp (366 nm, 10 min), in accordance with an embodiment of the invention.
FIG. 2 is an UV-vis spectra of model tris(azobenzene-N-phthalimide)/THF solution under the room light and in the dark intermittently in accordance with an embodiment of the invention.



DETAILED DESCRIPTION
It has been estimated that the mechanical work per polymer chain based on the azobenzene isomerization process is only 10% efficient, meaning that only 10% of the incident photon energy (4×10−19 J) is converted to mechanical work ( ̃4.5×10−20 J). As a way to improve the transduction efficiency via increasing the effective number density of azobenzene-actuating units in a polyimide (PI) network structure, the presently disclosed crosslinker is designed to contain three azobenzenes per molecule, via a trifunctional monomer, which can promote polyimide network formation under appropriate fabricating and curing conditions.
A photo-active, multi-(azobenzene-amine) crosslinker derived from a tris(azobenzene)-containing compound according to the present invention exhibits the following generic structure in which, for meta-azo substitution (with reference to ether-oxygen), R=H, and for para-azo substitution, R=H, F, Cl, CF3, CH3:




The present invention further includes a method for making the above shown tris(azobenzene-amine) cross-linker comprising the steps of: providing 1,1,1,-tris(4-hydroxyphenyl)ethane; treating the 1,1,1,-tris(4-hydroxyphenyl)ethane with a nitro-activated aryl halide such as 1-fluoro-4-nitrobenzene, 1,2-difluoro-4-nitrobenzene, 2-chloro-1-fluoro-4-nitrobenzene, 1-fluoro-4-nitro-2(trifluromethyl)benzene or 1-fluoro-2-methyl-4-nitrobenzene) in presence of potassium carbonate to produce a tris[(nitrophenoxy)phenyl]ethane compound; reducing the tris[(nitrophenoxy)phenyl]ethane compound to a tris[(acetamidophenoxy)phenyl]ethane compound by catalytic hydrogenation; treating the tris[(acetamidophenoxy)phenyl]ethane compound with 4-nitrosoacetanilide in acetic acid to yield a tris(azobenzene-amide) compound; and converting the amide to amine in the tris(azobenzene-amide) compound via an alkaline deacetylation reaction.
In various embodiments of the method, the nitro-activated aryl halide may comprise 1-fluoro-4-nitrobenzene; 1,2-difluoro-4-nitrobenzene; 2-chloro-1-fluoro-4-nitrobenzene; 1-fluoro-4-nitro-2-(trifluoromethyl)benzene; or 1-fluoro-2-methyl-4-nitrobenzene. In another embodiment of the method, the tris[(nitrophenoxy)phenyl]ethane compound may comprise 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane.
As an example, a representative synthesis of a photo-active, multi-(azobenzene-amine) cross-linker according to the present invention (compound 6) is shown in Scheme 1 below.







In Scheme 1, 1,1,1,-tris(4-hydroxyphenyl)ethane (compound 1) was treated with 1-fluoro-4-nitrobenzene in presence of potassium carbonate to make 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane (compound 2), which was reduced to 1,1,1-tris[4-(4-aminophenoxy)phenyl]ethane (compound 3) by catalytic hydrogenation. The reaction of compound 3 and 4-nitrosoacetanilide (compound 4) in acetic acid yielded the tris(azobenzene-amide) as compound 5. The tris(azobenzene-amine) monomer (compound 6) was generated after the deprotection of compound 5 via alkaline deacetylation reaction. By end-capping compound 6 with phthalic anhydride, a tris(azobenzene-N-phthalimide) model compound (compound 7) was obtained (see Scheme 1 above), which was useful in validating and semi-quantifying the kinetics of photoisomerization process in the cross-linked azo-polyimide system.
The following examples and methods are presented as illustrative of the present invention or methods of making or carrying out the invention and are not restrictive or limiting of the scope of the invention in any manner. The tris(azobenzene-amine) crosslinkers and the crosslinked, azobenzene-containing polyimides according to the present invention may be synthesized following the procedures given in the following examples.
EXAMPLE 1
Synthesis of 1,1,1-Tris[4-(4-nitrophenoxy)phenyl]ethane
Into a 250 mL three-necked flask equipped with a magnetic stir bar and nitrogen inlet and outlet were placed 1,1,1,-tris(4-hydroxyphenyl)ethane (10.0 g, 33.0 mmol), 1-fluoro-4-nitrobenzene (15.4 g, 109 mmol), potassium carbonate (15.1 g, 109 mmol) and DMF (100 mL). The mixture was stirred at room temperature for 24 hours and filtered. The filtrate was diluted with ethyl acetate (400 mL) and the organic layer was separated. The organic layer was washed with water 3 times. It was dried over anhydrous magnesium sulfate and filtered. The filtrate was concentrated to 75 mL on a rotary evaporator, and then stored in refrigerator for several days to afford 11.2 g (51%) of off-white crystals, m.p. 98-99° C. MS (m/e): 669 (M+). Anal. Calcd. for C38H27N3O9: C, 68.18%; H, 4.06%; N, 6.27%; O, 21.50%. Found: C, 67.69%; H, 4.26%; N, 6.21%; O, 21.22%. FT-IR (KBr, cm−1): 3076, 2979, 1586, 1513, 1486, 1344, 1248, 1165, 1107, 874, 846. 1H-NMR (DMSO-d6, 5 in ppm): 2.26 (s, 3H, CH3), 7.17-7.27 (m, 18H, Ar—H), 8.28-8.31 (d, 6H, Ar—H).
EXAMPLE 2
Synthesis of 1,1,1-Tris[4-(4-aminophenoxy)phenyl]ethane
1,1,1-Tris[4-(4-nitrophenoxy)phenyl]ethane (Example 1; 3; 5.0 g, 7.5 mmol), THF (50 mL) and 5% palladium on activated carbon (0.50 g) were added to a hydrogenation bottle. The bottle was secured on a Parr hydrogenation apparatus, flushed three times with hydrogen, and then pressurized to 55 psi. After the mixture had been agitated at room temperature for 24 hours under the hydrogen pressure of 55 psi, it was filtered through Celite. The filter cake was washed with THF, and then the filtrate was evaporated to dryness on a rotary evaporator to afford a 4.25 g (98%) of yellow crystal, which was used without further purification, m.p. 220-221° C. MS (m/e): 579 (M+). Anal. Calcd. for C38H33N3O3: C, 78.73%; H, 5.74%; N, 7.25%. Found: C, 78.17%; H, 5.78%; N, 7.04%. FT-IR (KBr, cm−1): 3441, 3361 (NH2), 3035, 2970, 1617, 1581, 1497, 1384, 1232, 1173, 1117, 1010, 871, 842. 1H-NMR (DMSO-d6, δ in ppm): 2.02 (s, 3H, CH3), 4.99 (s, 6H, NH2), 6.53-6.58 (d, 6H, Ar—H), 6.68-6.74 (m, 12H, Ar—H), 6.88-6.93 (d, 6H, Ar—H).
EXAMPLE 3
Synthesis of N-(4-nitrosophenyl)acetamide
After oxone (potassium peroxymonosulfate, from Sigma-Aldrich®; 12.28 g, 20.0 mmol) had dissolved completely in water (100 mL), potassium carbonate (4.14 g, 30 mmol) was added slowly to neutralize the solution to a weak acidity. The solution was added quickly into a solution of 4′-aminoacetanilide (from Sigma-Aldrich®; 1.50 g, 10.0 mmol) in water (150 mL). The mixture immediately turned green, and greenish-gray solids started to fall out of solution. After the mixture had been stirred for another 10 minutes, the precipitates were collected on a filter funnel, air dried, and recrystallized from hot ethanol. Two batches of products were collected to give a total yield of 1.06 g (65%) greenish crystals, mp 179-180° C. (lit. 179-180° C.).
EXAMPLE 4
Synthesis of 1,1,1-tris{4-[4-(4-acetamidophenyldiazenyl) phenoxy]phenyl}ethane
1,1,1-Tris(4-(4-aminophenoxy)phenyl)ethane (Example 2; 0.580 g, 1.00 mmol), N-(4-nitrosophenyl)acetamide (Example 3; 0.985 g, 6 mmol) and acetic acid (20 mL) were charged into a 150 mL round-bottomed flask equipped with a magnetic stir bar. The mixture was stirred at room temperature for 48 hours. The mixture was at first turned into a greenish solution, and then yellow particles started to precipitate out of the solution. The mixture was diluted by deionized water (100 mL). Solids were collected and washed with water (500 mL), followed by ethanol (200 mL) to remove most of the unreacted nitroso reagent. The solid was then extracted by hot acetone (100 mL) four times. The acetone extract was concentrated on a rotary evaporator to give yellow solids as raw product. The raw product was slurried in hot ethanol (50 mL) and filtered twice after cooling to room temperature in between to give 0.62 g (61%) of yellow solids, mp 220° C. (dec.). 1H-NMR (d6-DMSO, δ in ppm): 2.09 (s, 9H, COCH3), 2.15 (s, 3H, CCH3) 7.03-7.15 (m, 18H), 7.79-7.87 (m, 18H) 10.28 (s, 3H, NHCO). 13C-NMR (d6-DMSO, δ in ppm): 24.16, 30.28, 51.00, 118.50, 118.86, 119.18, 123.42, 124.35, 129.99, 142.09, 144.50, 147.38, 147.77, 153.78, 159.07, 168.06.
EXAMPLE 5
Synthesis of 1,1,1-tris{4-[4-(4-aminophenyldiazenyl) phenoxy]phenyl}ethane
To a 100 mL round-bottomed flask with a stir-bar and a condenser, 1,1,1-tris[4-(4-(4-acetamidophenyldiazenyl)phenoxy)phenyl]ethane (Example 4; 0.50 g, 0.49 mmol), 6 M HCl (20 mL) and 95% ethanol (20 mL) were charged and heated to 80° C. The mixture was stirred at 80° C. for 3 days. After the mixture was allowed to cool to room temperature, water (60 mL) added. The resulting red solid was collected by filtration and washed with dilute sodium bicarbonate solution, followed by deionized water (300 mL). After being air dried, the crude product was purified by column chromatography (silica gel, ethyl acetate as eluent). The solvent was removed by a rotary evaporator to afford 0.23 g (52.5%) of orange red solid, mp >300° C. MS (m/e): 892 (M+). Anal. Calcd. for C56H45N9O3: C, 75.40%; H, 5.08%; N, 14.13%. Found: C, 75.44%; H, 5.08%; N, 13.98%. 1H-NMR (d6-DMSO, δ in ppm): 2.12 (s, 3H, CCH3), 6.03 (s, 6H, NH2), 6.67-6.70 (d, 6H, J=8.7 Hz), 6.99-7.02 (d, 6H, J=8.7 Hz), 7.09-7.13 (m, 12H), 7.62-7.66 (d, 6H, J=8.7 Hz), 7.75-7.78 (d, 6H, J=8.7 Hz). 13C-NMR (d6-DMSO, δ in ppm): 30.34, 51.91, 113.41, 118.44, 118.69, 123.52, 124.96, 129.93, 142.92, 144.25, 148.41, 152.56, 154.21, 157.75.
EXAMPLE 6
Synthesis of 1,1,1-Tris{4-[4-(4-phthalimidophenyldiazenyl) phenoxy]phenyl}ethane (Model Compound) 
Into a 250 mL three-necked flask equipped with a magnetic stir bar and nitrogen inlet and outlet were placed 1,1,1-tris{4-[4-(4-aminophenyldiazenyl)phenoxy]phenyl}ethane (Example 5; 0.892 g, 1.00 mmol), phthalic anhydride (0,444 g, 3.00 mmol) and acetic acid (20 mL). The mixture was stirred under refluxing for 14 hours and allowed to cool to room temperature. The precipitate was collected by filtration and dried in an oven to afford 0.96 g (75%) of orange powder, mp >300° C. MS (m/e): 1282 (M+). Anal. Calcd. for C80H51N9O9: C, 74.93%; H, 4.01%; N, 9.83%. Found: C, 75.00%; H, 4.11%; N, 9.44%. 1H-NMR (d6-DMSO, δ in ppm): 2.21 (s, 3H, CCH3), 7.08-7.13 (d, 6H, Ar—H), 7.19-7.21 (dd, 12H, Ar—H), 7.67-7.71 (d, 6H, Ar—H), 7.87-8.03 (m, 24H, Ar—H).
EXAMPLE 7
Representative Procedure for the Synthesis of Azobenzene-Containing Polyimides (20 Mol %)
1,3-Bis(3-aminophenoxy)benzene (APB; 0.4093 g, 1.400 mmol) and DMAc (8 mL) were added to a 50 mL 3-necked flask equipped with a magnetic stirrer and a nitrogen inlet and outlet and stirred under dry nitrogen at room temperature for 30 minutes. 2,2-Bis(phthalic anhydride)-1,1,1,3,3,3-hexafluoroisopropane (6FDA; 0.8885, 2.000 mmol) was then introduced to the resulting solution. The light yellow solution was agitated at room temperature for 24 hours to afford a poly(amic acid) solution. Then, the tris(azobenzene-amine) cross-linker (Example 5; 0.3568 g, 0.400 mmol) was added to this solution. After the crosslinker had completely dissolved in DMAc, the mixture was poured into a glass petri dish, followed by vacuum evaporation of DMAc at 50° C., and heat-treated according to following schedule: 100° C./2 hrs, 150° C./2 hrs, 175° C./1 hr, 200° C./2 hrs, 250° C./1 hr and 300° C./1 hr to form polyimide films. The film thickness was approximately 20 μm. To prepare a polyimide with (ax100) mol % of the crosslinker, this procedure was followed with variation in the mole ratio of APB:6FDA:Crosslinker=(1−3a/2):1:a, where a can be varied from 0.05 to 0.50.
EXAMPLE 8
UV-Vis Spectroscopic Study of Photo-Isomerization of Model Compound 
The reversible trans-cis photoisomerization of the model compound (i.e compound 7 in Scheme 1; also see Example 6) was investigated in THF solution. After irradiation with a 366 nm UV-lamp for 10 minutes, the compound's UV absorption band at 383 nm disappeared, indicating that all the trans-isomers were converted into cis-isomers. At the same time, the absorbance at 443 nm, which corresponds to cis-isomers, increased to a maximum. After the UV-irradiation and having been placed on a lab bench, the cis-isomers underwent thermally activated cis-trans isomerization and the absorption peak of trans-isomers were totally recovered in 2 days (FIG. 1).
Interestingly, even the room light can convert trans-azobenzene units in the model compound to cis-conformations partially in 1 hour (FIG. 2). After its THF solution was stored in the dark for 2 days, the cis-azobenzenes fully reverted back to the trans-conformations. The trans-cis-trans cycles could be repeated several times at room temperature, indicating relatively low-energy barrier for the equilibration of isomers.
Although specific embodiments have been described in detail in the foregoing description and illustrated in the drawings, various other embodiments, changes, and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the spirit and scope of the appended claims.

## Claims

1. A tris(azobenzene-amine) cross-linker with the following structure:



wherein for meta-azo substitution, R is equal to H, and for para-azo substitution, R is selected from the group consisting of H, F, Cl, CF3, and CH3.

2. A method for making the tris(azobenzene-amine) cross-linker of claim 1 comprising the steps:
providing 1,1,1,-tris(4-hydroxyphenyl)ethane;
treating the 1,1,1,-tris(4-hydroxyphenyl)ethane with a nitro-activated aryl halide in presence of potassium carbonate to produce a tris[(nitrophenoxy)phenyl]ethane compound;
reducing the tris[(nitrophenoxy)phenyl]ethane compound to a tris[(acetamidophenoxy)phenyl]ethane compound by catalytic hydrogenation;
treating the tris[(acetamidophenoxy)phenyl]ethane compound with 4-nitrosoacetanilide in acetic acid to yield a tris(azobenzene-amide) compound; and
converting amides in the tris(azobenzene-amide) compound to amines via an alkaline deacetylation reaction to form the tris(azobenzene-amine) cross-linker.

3. The method for making the tris(azobenzene-amine) cross-linker of claim 2 wherein the nitro-activated aryl halide is 1-fluoro-4-nitrobenzene and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane.

4. The method for making the tris(azobenzene-amine) cross-linker of claim 2 wherein the nitro-activated aryl halide is 1,2-difluoro-4-nitrobenzene and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane.

5. The method for making the tris(azobenzene-amine) cross-linker of claim 2 wherein the nitro-activated aryl halide is 2-chloro-1-fluoro-4-nitrobenzene, and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane.

6. The method for making the tris(azobenzene-amine) cross-linker of claim 2 wherein the nitro-activated aryl halide is 1-fluoro-4-nitro-2-(trifluoromethyl)benzene, and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane.

7. The method for making the tris(azobenzene-amine) cross-linker of claim 2 wherein the nitro-activated aryl halide is 1-fluoro-2-methyl-4-nitrobenzene, and the tris[(nitrophenoxy)phenyl]ethane compound is 1,1,1-tris[4-(4-nitrophenoxy)phenyl]ethane.