1870
Chem. Mater. 2008, 20, 1870–1876
Photoresponsive Hybrid Materials: Synthesis and Characterization of Coumarin-Dimer-Bridged Polysilsesquioxanes Lihua Zhao,† Matthias Vaupel,§ Douglas A. Loy,‡ and Kenneth J. Shea*,† Department of Chemistry, UniVersity of California, IrVine, California 92696; Department of Material Science and Engineering and Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721; and Nanofilm Technologie, Goettingen, Germany ReceiVed September 28, 2007. ReVised Manuscript ReceiVed December 18, 2007
A chemically stable photodimer of 7-allyloxycoumarin (1) has been identified. An efficient synthesis of the syn-ht photodimer (2) was developed, and the compound was subsequently elaborated to the bis(triethoxysilyl) derivative (7). Sol–gel polymerization produced a robust hybrid glass (7X) that could be fabricated as monoliths or cast as transparent colorless thin films. Photopatterning of thin films with UV light (254 nm) produces fluorescent and refractive index patterns which are revealed by fluorescence microscopy (λex ) 330 nm, λem ) 392 nm) and imaging ellipsometry. At short irradiation times the photoinduced cleavage produces a slight collapse of the silsesquioxane network (∼3%) that is attributed to rupture of the coumarin photodimer cross-links. The topological patterns are also observed by imaging ellipsometry. This material provides a new robust matrix for producing “hidden” fluorescent images, refractive index gratings, and topographical features in dense, hybrid glassy materials. Prolonged irradiation at short wavelengths results in a hard f soft transformation that deforms and “melts” the hard brittle thermoset.
Introduction Photoresponsive materials play essential roles in microelectronics, medical, and optoelectronic devices.1–3 These materials have mechanical, electrical, and/or optical properties that can be modulated by light.3–5 Examples include photopatternable materials that have applications for secure recognition,6 as photoresists,1 for low-power, high-resolution optical data storage elements,7 and for fabricating waveguides and optical interference filters.8 Practical photoresponsive materials must meet a combination of criteria, which can lead to a trade-off between photoresponsive efficiency, material processability, and long-term stability. To minimize these trade-offs, the design and development of new multifunctional materials is an important objective in materials synthesis. Both organic polymers and inorganics are being employed; however, each have their limitations.9–11 We have * Corresponding author. E-mail:
[email protected]. † University of California, Irvine. ‡ University of Arizona. § Nanofilm Technologie.
(1) Balaji, R.; Grande, D.; Nanjundan, S. Polymer 2004, 45, 1089–1099. (2) Szczubialka, K.; Jankowska, M.; Nowakowska, M. J. Mater. Sci.: Mater. Med. 2003, 14, 699–703. (3) Irie, M.; Ikeda, T. Photoresponsive Polymers. In Functional Monomers and Polymers; 2nd ed.; Takemoto, K., Ottenbrite, R. M., Kamachi, M., Eds.; Marcel Dekker: New York, 1997; pp 65–116. (4) Pieroni, O.; Ciardelli, F. Trends Polym. Sci. (Cambridge, U.K.) 1995, 3, 282–287. (5) Irie, M. Pure Appl. Chem. 1990, 62, 1495–1502. (6) Kishimura, A.; Yamashita, T.; Yamaguchi, K.; Aida, T. Nat. Mater. 2005, 4, 546–549. (7) Kinoshita, T. J. Photochem. Photobiol., B 1998, 42, 12–19. (8) Hornak, L. A. Polymers for LightwaVe and Integrated Optics: Technology and Applications; M. Dekker: New York, 1992. (9) Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. D. Science 1999, 286, 945–947. (10) Innocenzi, P.; Lebeau, B. J. Mater. Chem. 2005, 15, 3821–3831.
Figure 1. Concept of a photoresponsive hybrid material incorporating a photocleavable motif.
focused our attention on the development of thermally robust, optically transparent photoresponsive hybrid materials. The class of materials we have identified, bridged polysilsesquioxanes, has tunable properties,12–16 achieved by covalently incorporating photodimers as the photoresonsive linkage into the hybrid network (Figure 1). The photoresponsive linkage can be cleaved by a specific wavelength of light (hυ1), which produces fluorescent (hυ2) structural units in the hybrid system. Rupture of organic bridging groups may result in a reorganization of the amorphous network structure. Several photodimer systems were considered including coumarin,17 cinnamic esters,18 anthracene,19,20 and thymine.21 Coumarin and its derivatives are readily available, and their (11) Huang, G. T. Technol. ReV. 2005, 108, 64–67. (12) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. J. Mater. Chem. 2005, 15, 3559–3592. (13) Shea, K. J.; Loy, D. A. MRS Bull. 2001, 26, 368–376. (14) Sanchez, C.; Lebeau, B.; Chaput, F.; Boilot, J. P. AdV. Mater. 2003, 15, 1969–1994. (15) Gomez-Romero, P.; Sanchez, C. Functional Hybrid Materials; WileyVCH: Weinheim, 2004. (16) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Angew. Chem., Int. Ed. 2006, 45, 3216–3251. (17) Trenor, S. R.; Shultz, A. R.; Love, B. J.; Long, T. E. Chem. ReV. 2004, 104, 3059–3077. (18) Bassani, D. M. CRC Handbook of Organic Photochemistry and Photobiology, 2nd ed.; CRC Press: Boca Raton, FL, 2004; pp 20/1– 20/20. (19) Bouas-Laurent, H.; Desvergne, J.-P.; Castellan, A.; Lapouyade, R. Chem. Soc. ReV. 2000, 29, 43–55.
10.1021/cm702804r CCC: $40.75 2008 American Chemical Society Published on Web 02/01/2008
PhotoresponsiVe Hybrid Materials
photochemistry is well documented. We reasoned therefore that the photodimers of coumarin offered the greatest potential. Coumarin photodimerization has been studied in homogeneous solution, in organized media, and in polymers.17 When covalently bound in polymers or inorganic materials, photoinduced dimerization can result in cross-linking or aggregation.22–25 On the other hand, the photodimers of coumarin can be photocleaved upon irradiation with wavelengths shorter than 300 nm. They have also been cleaved via a two-photon excitation using 532 nm light.26 If coumarin photodimers are incorporated in polymers and network materials as structural units, photoinduced bond cleavage could be used to alter the physical, optical, and mechanical properties of the material. Photolysis could be used to decrease moduli, lower molecular weight, alter the compliance and porosity of a network material, and produce a pair of intensely fluorescent coumarin monomers. Despite the potential utility of the photocleavage reaction, there are few examples of the coumarin photodimer used as building blocks or precursors for polymers or materials.27–29 Coumarin photodimerization can give rise to four isomers. As a result, it is challenging to prepare useful quantities of coumarin dimers in pure form.30–38 In addition, there are reports that some coumarin photodimers have low chemical stability.28,39 This chemical instability can further limit their utility. Here we report the synthesis of isomeric photodimers of 7-alloxycoumarin. An evaluation of their stability and photochemical behavior has led to the identification of a chemically stable isomer, the syn-ht derivative. This isomer has been elaborated to a bis(triethoxysilyl) monomer and polymerized under sol–gel conditions to a bridged polysilsesquioxane. The following sections describe the fabrication (20) Bouas-Laurent, H.; Desvergne, J.-P.; Castellan, A.; Lapouyade, R. Chem. Soc. ReV. 2001, 30, 248–263. (21) Inaki, Y. CRC Handbook of Organic Photochemistry and Photobiology, 2nd ed.; CRC Press: Boca Raton, FL, 2004; pp 104/1– 104/34. (22) Mal, N. K.; Fujiwara, M.; Tanaka, Y. Nature (London) 2003, 421, 350–353. (23) Fujiwara, M.; Shiokawa, K.; Kawasaki, N.; Tanaka, Y. AdV. Funct. Mater. 2003, 13, 371–376. (24) Chen, Y.; Hong, R. T. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2999–3008. (25) Schadt, M.; Seiberle, H.; Schuster, A. Nature (London) 1996, 381, 212–215. (26) Kim, H. C.; Kreiling, S.; Greiner, A.; Hampp, N. Chem. Phy. Lett. 2003, 372, 899–903. (27) Chen, Y.; Chen, K. H. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 613–624. (28) Saigo, K.; Nakamura, M.; Suzuki, Y.; Fang, L.; Hasegawa, M. Macromolecules 1990, 23, 3722–3729. (29) Saigo, K.; Shiwaku, T.; Hayashi, K.; Fujioka, K.; Sukegawa, M.; Chen, Y.; Yonezawa, N.; Hasegawa, M.; Hashimoto, T. Macromolecules 1990, 23, 2830–2836. (30) Hammond, G. S.; Stout, C. A.; Lamola, A. A. J. Am. Chem. Soc. 1964, 86, 3103. (31) Morrison, H.; Curtis, H.; Mcdowell, T. J. Am. Chem. Soc. 1966, 88, 5415. (32) Morrison, H.; Hoffman, R. Chem. Commun. 1968, 1453. (33) Hoffman, R.; Morrison, H. Abstr. Pap. Am. Chem. Soc. 1970, 95. (34) Hoffman, R.; Wells, P.; Morrison, H. J. Org. Chem. 1971, 36, 102. (35) Lewis, F. D.; Howard, D. K.; Oxman, J. D. J. Am. Chem. Soc. 1983, 105, 3344–3345. (36) Gnanaguru, K.; Murthy, G. S.; Venkatesan, K.; Ramamurthy, V. Chem. Phys. Lett. 1984, 109, 255–258. (37) Lewis, F. D.; Barancyk, S. V. J. Am. Chem. Soc. 1989, 111, 8653– 8661. (38) Gnanaguru, K.; Ramasubbu, N.; Venkatesan, K.; Ramamurthy, V. J. Org. Chem. 1985, 50, 2337–46. (39) Yu, X.; Scheller, D.; Rademacher, O.; Wolff, T. J. Org. Chem. 2003, 68, 7386–7399.
Chem. Mater., Vol. 20, No. 5, 2008 1871 Scheme 1. Direct Photodimerization of 7-Allyloxycoumarin
a
a The yields were determined by 1H NMR and are based on 40% total conversion.
and photopatterning of thin films of this monomer and the characterization of their optical and physical properties. Results and Discussion Synthesis and Photochemistry of the Key Precursor a 7-Allyloxycoumarin Photodimer. I. Examination of Photodimerization of 7-Allyoxycoumarin. UV irradiation (>300 nm) of dilute CH3CN solutions of 7-allyloxycoumarin (1), which is readily synthesized following a literature procedure,40 produced dimers 2, 3, 4, and 5 in a low total conversion (e40% calculated by 1H NMR) (Scheme 1). The assignments of these isomers were based upon the distinctive signals of cyclobutane ring of each diastereomer in the 1H NMR (see Supporting Information for the spectrum). When the mixture was subjected to SiO2 chromatography, dimer 2 (syn-ht isomer) was the only product that could be isolated in pure form. Analysis of the residue from efforts to purify the photolysis reaction mixture indicated that the most common decomposition pathway was cleavage of one or both of the lactone rings. Lactone cleavage was evidenced by replacement of carbonyl absorption at 1760 cm-1 by carbonyl absorptions at 1702 cm-1. Chemical instability of some of the coumarin photodimers limits their utility as a monomer and/or building blocks for incorporation into materials such as bridged polysilsesquioxanes. The ability to isolate dimer 2 indicated stability to column chromatography. Indeed, dimer 2 can be fully recovered after stirring with 1 N HCl aqueous solution overnight. Despite this encouraging result, the yield of photodimer 2 was quite low due to the low photochemical conversion and the subsequent difficulties of purification and isolation. Since there were no previous synthetic entries into this material, it was necessary to develop modified photodimerization conditions to produce useful quantities of the syn-ht derivative 2. II. Synthesis and PhotocleaVage of syn-Head-to-Tail-7Allyloxycoumarin Dimer 2. Synthesis. Lewis and co-workers reported the influence of Lewis acids on the photodimerization of coumarin.35,37,41 In contrast to direct photolysis, photoirradiation of coumarin 1 in the presence of stoichiometric BF3 · OEt2 resulted in predominant formation of the syn-ht dimer 2 (Scheme 2). Pure bis(allyloxy) dimer 2 could be isolated by column chromatography. Optimized conditions (40) Clarke, D. J.; Robinson, R. S. Tetrahedron 2002, 58, 2831–2837. (41) Shim, S. C.; Ilkim, E.; Lee, K. T. Bull. Korean Chem. Soc. 1987, 8, 140–144.
1872
Chem. Mater., Vol. 20, No. 5, 2008
Scheme 2. Lewis Acid-Catalyzed Photodimerization of 7-Allyloxycoumarin (1) and Ring-Closing Metathesis of syn-ht-7-Allyloxycoumarin Dimer (6)
provided the dimer 2 in a yield of 82% based on 64% total conversion. The structure of the photodimer 2 was unequivocally established by X-ray crystallography of the ring-closed olefin metathesis product 6. PhotocleaVage. The absorption spectra of coumarin 1 and dimer 2 are shown in Figure 2a. Dimer 2 shows an absorption band at ∼280 nm; the corresponding monomer 1 has a strong absorption band at 322 nm. Irradiation of a dilute CH2Cl2 solution of dimer 2 at 254 nm produced a decrease in absorption intensity at wavelengths shorter than 268 nm and an increase in absorbance at λmax ) 322 nm (Figure 2b) consistent with a retro-[2 + 2] photoreaction of dimer 2 (Scheme 3). The isosbestic point at 270 nm is evidence for a single photocleavage pathway. 1H NMR spectra confirmed
Figure 2. (a) UV–vis absorption spectra of dimer 2 and coumarin 1 in CH2Cl2. (b) Change of UV–vis spectra upon irradiation of dimer 2 with UV light at 254 nm in CH2Cl2.
Zhao et al. Scheme 3. Photocleavage of syn-ht-7-Allyloxycoumarin Dimer Monitored by 1H NMR in CDC
that the photocleavage leads exclusively to the formation of coumarin 1 in quantitative yield. Upon excitation at 330 nm, coumarin 1 exhibits a strong fluorescence emission at 388 nm (Figure 3a). syn-ht-Dimer 2 is not fluorescent at this wavelength. Figure 3b shows the change in intensity of fluorescence emission (λex ) 330 nm) at 392 nm during irradiation of dimer 2 due to the formation of coumarin 1. Film Formation and Photopatterning of Coumarin-DimerBridged Polysilsesquioxane 7X. Synthesis and Characterization of syn-ht-Coumarin-Dimer-Bridged Polysilsesquioxane 7X. syn-ht-Dimer 2 can be transformed into polymerizable derivatives. For example, platinum-catalyzed hydrosilylation of dimer 2 with triethoxysilane provided the bis(trieth-
Figure 3. (a) Excitation and fluorescence emission spectra of coumarin 1 in CH2Cl2. (b) Buildup of fluorescence intensity during irradiation of coumarin dimer 2 with 254 nm UV light in CH2Cl2. Excitation at 330 nm was used for the fluorescence measurements.
PhotoresponsiVe Hybrid Materials
Chem. Mater., Vol. 20, No. 5, 2008 1873
Scheme 4. Synthesis of syn-ht-Coumarin-Dimer-Bridged Silsesquioxane Monomer 7 and Polysilsesquioxane 7X
oxysilyl) monomer 7 in good yield (Scheme 4a). Sol–gel precursors containing two or more triethoxysilyl groups have been polymerized to produce monolithic gels, fibers, and/or transparent thin films.42 Many similar hexafunctional precursors gel in minutes, even at monomer concentrations as low as 0.2 M.43,44 Monomer 7, however, required 48 h to gel at a concentration of 1 M in THF (aqueous HCl catalyst) (Scheme 4b). This high concentration of monomer 7 and the relatively prolonged gelation time may be attributed to intramolecular cyclization of the monomer, a phenomena which was also observed with certain short alkylene (1–4 carbons) bridged monomers.45,46 Following gelation, the gel was aged and slowly air-dried to produce a transparent monolithic xerogel 7X as a hard brittle glass. Thermogravametric analysis (TGA) (N2, 10 °C/min; see Supporting Information for the TGA curve) revealed the onset of weight loss does not occur until temperatures approaching 300 °C. Thermal robustness makes this material compatible with a number of annealing and processing steps in microfabrication. FT-IR analysis of xerogel 7X reveals the bis(lactone) substructure (lactone diagnostic: ∼1750 cm-1; see Supporting Information for spectra) remains intact in the dried xerogel; the photolabile bridging dimer is an integral structural unit of the xerogel. Nitrogen absorption studies revealed an absence of porosity in the bulk xerogel, which implies a relatively compliant network that collapses upon drying to give a dense material. The morphology of sections of the monolith 7X was investigated by high-resolution transmission electron microscopy (TEM) (see Supporting Information for images). The absence of features in the diffraction pattern recorded from TEM indicates that the material 7X is composed of an amorphous but dense structure with no evidence for phase separation and/or aggregation between the organic and inorganic components. Interestingly aged bulk samples were found to be comprised of a matrix of spherical particles in the 50–100 nm size range.47 Sol–gel polymerization conditions and subsequent processing for thin film fabrication in this work differ significantly from preparation of bulk (42) (43) (44) (45)
Loy, D. A.; Shea, K. J. Chem. ReV. 1995, 95, 1431–1442. Shea, K. J.; Loy, D. A. Chem. Mater. 2001, 13, 3306–3319. Shea, K. J.; Loy, D. A. Acc. Chem. Res. 2001, 34, 707–716. Loy, D. A.; Carpenter, J. P.; Myers, S. A.; Assink, R. A.; Small, J. H.; Greaves, J.; Shea, K. J. J. Am. Chem. Soc. 1996, 118, 8501–8502. (46) Loy, D. A.; Carpenter, J. P.; Alam, T. M.; Shaltout, R.; Dorhout, P. K.; Greaves, J.; Small, J. H.; Shea, K. J. J. Am. Chem. Soc. 1999, 121, 5413–5425. (47) Zhao, L.; Loy, D. A.; Shea, K. J. J. Am. Chem. Soc. 2006, 128, 14250– 14251.
Figure 4. Changes in absorption (a and b) and appearance of fluorescence emission (c and d, λex ) 351 nm) in thin films of 7X before (a, c) and after (b, d) irradiation with
