4734 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA J. zyxwvutsrq Phys. Chem. 1988, 92, zyxwvut 4734-4738 Effects of the Changes in the Properties of Silica Cage along the GeVXerogel Transition on the Photochromic Behavior of Trapped Spiropyrans zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJ David Levy and David Avnir* Department of Organic Chemistry and the F. Haber Research Center for Molecular Dynamics, The Hebrew University zyxwvuts of Jerusalem, Jerusalem 91904, Israel (Received: February zyxw 1, 1988) The photochromism of a series of spiropyrans trapped in a polymerizing silicon tetramethoxide system (the "sol/gel process") was studied as a function of the reaction time. The continuous environmental changes along the gel/xerogel transition are reflected in the gradual changes of the photochromic behavior of the trapped molecule. In particular, the photochromism changes at a certain point to reversed photochromism. Hammett analysis showed that the reversal point is sensitive to substituent effects. The spectra and photochromic changes are interpreted as reflecting a gradual shift in the equilibrium between the adsorbed and dissolved molecule within the cage, enriching the system with the former as the reaction proceeds. This shift is caused by the change in the composition of the cage-trapped solvent from methanol/water to water, by solvent evaporation, and by the gradual shrinkage in the cage size. We attribute the structure known as isomer "X" to the colored species at the reversal photochromism stage. 1. Introduction One of the more promising directions in modern materials science has been the low-temperature synthesis of inorganic glasses in a process generally known as the sol/gel method.' The basic idea has been to prepare inorganic matrices by polymerization of suitable monomers rather than by the classical high-temperature melting processes of oxides and For instance, the polym- erization we study in this report produces Si02 from the following hydrolysis/condensation reaction of tetramethyl orthosilicate (TMOS):3 ~zS~(OCH~)~ zyxwvutsrqp + mH20 - (0-Si-0)" + 4nCH30H + (m - 2n)H20 The many advantages of the sol/gel route to ceramic materials4 have zyxwvutsrqp led to a thorough study of the first stage of this process, which is the sequence of events leading from the monomer to the porous xerogel (porous silica gel, in our case).5 Interest in this process stems also from a different, more basic point of view, Le., the intensive recent activity in investigating the development and properties of irregular and disordered (fractal) systems.6 The sol/gel process, which is basically a polymerization-aggregation process, is an excellent experimental probe for such st~dies.~ Virtually all modern analytical techniques have been utilized to study the sol/gel process. These include spectroscopic methods (IR,8 NMR,9 absorption of visible lightlo), scattering of light," x-ray^,^!'^ and neutrons,I3 thermal analysi~,'~ and chromatogra- (1) (a) Sakka, S.; Kamiya, K. J. Non-Cryst. Solids 1980, 42, 403. (b) Mukherjee, S. P. J. Non-Cryst. Solids 1980, 42, 477. (c) Klein, L. C. Annu. Rev. Mater. Sci. 1985, 15, 227. (2) (a) Yamane, M.; Aso, S.; Sakaino, T. J. Mater. Sci. 1978,13, 865. (b) Yamane, M.; Aso, S.; Okano, S.; Sakaino, T. J. Mater. Sci. 1979, 14, 607. (c) Mukherjee, S. P.; Zarzycki, J.; Traverse, J. P. J. Mater. Sci. 1976, 11, 341. (d) Yoldas, B. E. J. Non-Cryst. Solids 1980, 38, 81. (e) Suzuki, H.; Saito, H.; Hayashi, T. J. J. Mater. Sci. 1984, 19, 396. (3). (a) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (b) Schmidt, H.; Scholze, H. NATO ASI Ser. (Glass) 1985, E92, 263. (c) Schmidt, H.; Scholze, H.; Kaiser, A. J. Non-Cryst. Solids 1984, 63, 1. (4) Dislich, H. D. J. Non-Cryst. Solids 1983, 57, 371. (5) Brinker, C. J.; Scherer, G. W. J. Non-Cryst. Solids 1985, 70, 301. (6) (a) Keefer, K. D.; Schaefer, D. W. Phys. Rev. Lett. 1986,56, 2376. (b) Pines-Rojanski, D.; Huppert, D.; Avnir, D. Chem. Phys. Lett. 1987, 139, 109. (7) Schaefer, D. W.; Keefer, K. D. Phys. Rev. Lett. 1984, 53, 1383. (8) Yoldas, B. E. J. Non-Cryst. Solids 1984, 63, 145. (9) (a) Assink, R. A,; Kay, B. D. Mater. Res. SOC. Symp. Proc. 1984, 32, 301. (b) Rosenbexger, H.; Scheler, G.; Burger, H.; Jakob, M. Colloids Surf. 1984, 12, 53. (c) Artaki, I.; Sinha, S.; Irwin, A. D.; Jonas, J. J. Non-Cryst. Solids 1985, 72, 1985. (d) Yamane, M.; Inoue, S.; Yasumori, A. J. Non- Cryst. Solids 1984, 63, 13. (10) Tewari, P. H.; Hunt, A. J.; Lieber, J. G.; Lofftus, K. Book of Ab- stracts of the First International Symposium on Aerogel, Wurzburg, FRG, Sept 1985. (1 1) Hunt, A. J.; Berdahl, P. Mater Res. SOC. Symp. Proc. 1984.32, 275. 0022-3654/88/2092-4734$0l.50/0 phy.2a,9d*'Za An excellent account of the state of the art is ref 15. Much of our activity in this field has been devoted to the study of photoprocesses of probe molecules added to the polymerizing sol/gel system.1621 Photochemistry and photophysics offer today a wide variety of probes which show selective high sensitivity to environmental parameters such as polarity, viscosity, porosity, and local geometry. Examples for previous studies in which photo- probes were utilized for investigating the sol/gel process include the use of Eu3+emission as a probe for local geometric ~ymmetry,'~ the use of rhodamine 6G absorption and emission to probe the silica cage polarity and the ability to isolate trapped molecules,16a the use of pyrene excimerization as a probe for the evolution of pore network during the polymerization,l* and the use of excit- ed-state strong acids to probe the rate of water consumption at the early stages of this pr~cess.'~ Of particular practical interest is the ability to trap organic photoactive molecules in the ceramic matrix at room tempera- t ~ r e . ~ ~ , ~ ~ ~ ~ It is perhaps not an exaggeration to describe the ability to dope oxide glasses with organic molecules as a revolution in glass technology: The classical, ancient high-temperature tech- niques excluded completely the possibility of preparation of such composite materials. We have demonstrated the feasibility of this idea first by trapping the laser dye rhodamine 6G in a monolithic block of SiO2Ih and then by trapping many other molecules,16b.M*21 and showed the advantages of this matrix over plastic ones. For other trapping studies see the works of Tani et al.,2zMcKenzey et al.,23 and Locher et al.24 (12) (a) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Assink, R. A.; Kay, B. D.; Ashley, C. S. J. Non-Cryst. Solids 1984, 63, 45. (b) Schaefer, D. W.; Martin, J. E. Phys. Rev. Lett. 1984, 52, 2371. (c) Strawbridge, I.; Craievich, A. F.; James, B. F. J. Non-Cryst. Solids 1985, 72, 139. (13) Kjems, J.; Freltoft, T. Adu. Solid State Phys. 1985, 25, 669. (14) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S. J. Non-Cryst. Solids 1982, 48, 47. (15) (a) Scherer, G. W. Yogvo Kyokai Shi 1987, 95, 21. (b) Chen, K. C.; Tsuchiya, T.; MacKenzie, J. D. J. Non-Cryst. Solids 1986, 81, 227. (16) (a) Avnir, D.; Levy, D.; Reisfeld, R. J. Phys. Chem. 1984.88, 5956. (b) Avnir, D.; Kaufman, V.; Reisfeld, R. J. Non-Cryst. Solids 1985, 74, 395. (c) Kaufman, V.; Avnir, D. In Better Ceramics Through Chemistry; Brinker, C. J.; Clarck, D. E.; Ulrich, D. R., Eds.; Materials Research Society Sym- posium Series 73; Materials Research Society: Pittsburgh, 1986; p 145. (17) Levy, D.; Reisfeld, R.; Avnir, D. Chem. Phys. Lett. 1984, 109, 593. (18) Kaufman, V.; Avnir, D. Langmuir 1986, 2, 717. (19) Kaufman, V.; Avnir, D.; Pines-Rojanski, D.; Huppert, D. J. Non- Cryst. Solids 1988, 99, 379. (20) (a) Levy, D.; Avnir, D. In Proceedings of the 2nd International Conference on Unconventional Photoactive Materials, Cleveland, Sept 1985; Scher, H., Ed.; Plenum, New York, 1988. (b) Kaufman, V.; Avnir, D. Ibid. (21) Kaufman, V.; Levy, D.; Avnir, D. J. Non-Cryst. Solids 1986.82, 103. (22) Pope, E. J. A,; MacKenzie, J. D. MRS Bull. 1987, March 17, 29. (23) (a) Tani, T.; Narnikawa, H.; Arai, K.; Makashirna, A. J. Appl. Phys. 1985, 58, 9. (b) Makishima, A,; Tani, T. J. Am. Ceram. zyx SOC. 1986, 69, (2-72. 0 1988 American Chemical Society Downloaded by HEBREW UNIV on August 20, 2009 Published on August 1, 1988 on http://pubs.acs.org | doi: 10.1021/j100327a035