J . Phys. Chem. 1989, 93, 373-376
373
Temperature Dependence of the Photoinduced Disproportionation of Ru( bpy):' Porous Vycor Glass
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Jianwei Fan, Wei Shi, Steven Tysoe, Thomas C. Strekas,* and Harry D. Gafney* Department of Chemistry, City University of New York, Queens College, Flushing, New York I 1 367 (Received: March 18, 1988; In Final Form: May 27, 1988)
The photophysical and photochemicalproperties of R~(bpy),~+ cation exchanged onto porous Vycor glass have been determined as a function of temperature. In the 5-95 OC range, the spectroscopic properties of the adsorbed complex are equivalent to aqueous solution spectra at the same temperature. The emission lifetime of the adsorbed complex declines with increasing temperature, but the emission polarization ratio is independent of temperature and equivalent to that measured in hydrocarbon glasses at 77 K. Photolysis of the adsorbed complex leads to disproportionation,and the quantum yield of the reaction increases with increasing temperature. The latter is interpreted within a surface conduction model where an Arrhenius plot of the quantum yield data indicates that the barrier to electron transport on the glass surface is 6.87 0.1 1 kcal/mol.
photolysis data confirm the presence of a photodetached electron, Introduction and quantum yield measurements indicate that disproportionation The use of heterogeneous media to control the photoredox depends on the mean separation between the redox partners.lO*ll chemistry of Ru(bpy)?+ has received considerable These data led to the postulation of a surface conduction modello There is now a growing body of evidence that hydroxylated silicas where biphotonic excitation of R~(bpy),~+(ads) (ads designates and cellulose films promote charge separation. Thomas and co-workers, for example, report that incorporating R ~ ( b p y ) ~ ~ + an adsorbed species) leads to ionization into silica colloids enhances electron transfer between the complex R~(bpy),~+(ads) + 2hv R~(bpy),~+(ads) + e- (1) and methylviologen, MV2+,*while photolysis of Ru(bpy)?+ adsorbed onto "dry" cellulose leads to disproporti~nation.~We find Surface conduction of the photodetached e- is thought to involve that 450-nm photolysis of R ~ ( b p y ) , ~adsorbed + onto Corning's the population of intermediate surface acceptor sites, S code 7930 porous Vycor glass, PVG, leads to disproportionation10 e- + S S(2) and, with coadsorbed MVZ+,a net formation MV+ in the absence of external electron donors." PVG is a porous, transparent glass These are thought to be shallow energy wells from which the that, like silica gel, possesses a hydroxylated ~ u r f a c e . ' ~ ~ ' ~The -'~ photodetached electron can be thermally activated but, neverband gap in S i 0 2 is 6.9 eV15 while the excitation energies used theless, present an energy barrier, albeit slight, that prevents in the above experiments are I 5 eV.lo-ll Therefore, unlike reimmediate recombination.1° Net electron transfer occurs when actions on semiconductive metal oxides, where charge separation the redox partners are within the electron migration distance, 50 occurs via population of a conduction band, lbZ2 the reactions on f 10 A, and when this distance exceeds that for the thermal hydroxylated silica surfaces must involve a different mechanism back-reaction, I 1 3 A in the disproportionation reaction, the redox of charge separation.1° products are stable.I0 R ~ ( b p y ) ~cation ~ + exchanges onto the glass surface, and Surface conduction implies that the quantum yield of electron emission polarization measurements indicate that the reactions transfer on the glass surface will exhibit an activation energy that occur between a fixed array of immobilized a d s ~ r b a t e s . ~Flash ~.~~ reflects the average depth of proposed surface acceptor sites. In this paper, we describe the results of a study of the temperature 111 Thomas. J. K. Chem. Reu. 1980.80. 283-299. dependence of the photoinduced disproportionation of Ru(2j Matsuo,'T.; Takuma, K.; Nishizima,'T.; Tsutsui, Y. J. Coord. Chem. (bpy)p(ads). To interpret these data, the effect of temperature 1980, 10, 195-198. on the mobility of the adsorbed complex and its photophysical (3) Rodgers, M. A. J.; Becker, J. C. J . Phys. Chem. 1980,84,2762-2766. properties have also been measured a t temperatures between 5 (41 Atik. S.S.: Thomas. J. K. J. Am. Chem. SOC.1981.103.7403-7406, 43671431 1. and 90 O C .
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( 5 ) Willner, I.; Otvos, J. W.; Calvin, M. J . Am. Chem. SOC.1981, 103, 3203-3205. (6) Willner, I.; Yang, J. M.; Laane, C.; Otvos, J. W.; Calvin, M. J . Phys. Chem. 1981, 85, 3277-3282. (7) Willner, I.; Degani, Y. J . Am. Chem. SOC.1983, 105, 6228-6233. (8) Wheeler, J.; Thomas, J. K. J . Phys. Chem. 1982, 86, 4540-4544. (9) Milosavijevic, B. H.; Thomas, J. K. J. Phys. Chem. 1983,87,616-621. (10) Kennelly, T.; Gafney, H. D.; Braun, M. J. Am. Chem. Soc. 1985,107, 443 1-4440. (11) Shi, W.; Gafney, H. D. J . Am. Chem. SOC.1987, 109, 1582-1583. (12) Cant, N. W.; Little, L. H. Can. J . Chem. 1964, 42, 802-809. (13) Elmer, T. H.; Chapman, I. D.; Nordkrg, M. E. J. Phys. Chem. 1962, 66, 1517-1519. (14) Darsillo, M. S.;Gafney, H. D.; Paquette, M. S. J . Am. Chem. SOC. 1987, 109, 3275-3286. (15) Kajiwara, T.; Hasimoto, K.; Kawal, T.; Sakata, T. J . Phys. Chem. 1982, 86, 4516-4522. (16) Gratzel, M. Acc. Chem. Res. 1981, 14, 316-384. (17) Borgarello, E.; Kiwi, J.; Pelizzetti, E.; Visca, M.; Gratzel, M. Nurure (London)1981, 289, 158-160. (18) Kawai, T.; Sakata, T. N o w . J . Chim. 1981, 5, 279. (19) Sato, S.; White, J. M. J. Phys. Chem. 1981, 85, 592-594. (20) Lehn, J. M.: Sauvage, J. P.; Ziessel, R. Nouu. J. Chim. 1980, 4, 623-627. (21) Novak, A. J. Appl. Phys. Len. 1977, 30, 567. (22) Clark, W. D.; Sutin, N. J . Am. Chem. SOC.1977, 99, 4676-4682. (23) Shi, W.; Wolfgang, S.; Strekas, T. C.; Gafney, H. D. J. Phys. Chem. 1985, 89, 914-918.
0022-3654/89/2093-0373%01 SO10 , , I
Experimental Section Materials. [Ru(bpy),] C12 was prepared according to the method of Palmer and Piper24 and twice recrystallized from distilled water as the chloride salt. Absorption, emission, and resonance Raman spectra of the recrystallized complex agreed with published s p e ~ t r a . ' ~Aqueous J~ solutions of the complex were prepared with water distilled in a Corning distillation unit. Code 7930 porous Vycor glass containing 70 f 21 A diameter cavities was obtained from the Corning Glass Works. The 25-mm X 25-mm X 4-mm pieces were extracted and calcined according to previously described procedures and stored at 550 "C until needed.I0J4 R ~ ( b p y ) , ~impregnation + was done by previously described procedure^.'^^^^^^^ All experiments were performed with "dry" samples where 199.97% of the water incorporated during impregnation was removed under vacuum a t room temperature.10 to The samples used in these experiments contain from 2 X 1.4 X mol of R~(bpy),~+(ads)/g. (24) Palmer, R. A.; Piper, T. S. Inorg. Chem. 1966, 5, 864-878. (25) Wolfgang, S.; Gafney, H. D. J. Phys. Chem. 1983,87, 5395-5401.
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374 The Journal of Physical Chemistry, Vol. 93, No. 1 , 1989
Fan et al.
Photolysis Procedures. Impregnated samples were mounted in previously described rectangular cellsI0 and evacuated to a was irradiated with pressure of HP0:- > HzP04-. Organic anions such as benzoate can also be ion exchanged, and the vibrational spectroscopy indicates that they occupy sites both parallel and perpendicular to the lithium aluminate layer. Finally, ion exchange with organometallic anions Fe(CN),& and nickel(I1) phthalocyaninetetrasulfonatehas been examined.
Introduction Layered claylike materials that exhibit anion-exchange properties are far less common that the ubiquitous cationic clays.' The natural minerals belonging to this class comprise of the pyroaurite sjogrenite group.2 Hydrotalcite, a member of this group, is represented by [Mg3A1(OH)8]+Cl- and has been one of the most extensively These materials can be thought of as
comprised of brucite-like (Mg(OH),) layers, in which each Mg2+ is surrounded by six hydroxyl groups in an octahedral arrangement. These octahedra share edges to form long sheets. The Mg2+can be replaced by an A13+, with the result that the layer gains a positive charge and needs to be neutralized by intercalated anions, which can be readily ion exchanged.,-* (3) Gastuche, M . C.; Brown, G.;Mortland, M.
(1) Grim, R. E. Clay Minerulogy, 2nd ed.; McGraw-Hill: New York, 1968. (2) Taylor, H. F. W. Mineral. Mag. 1973, 39, 371.
0022-3654/89/2093-0376$01 .50/0
177.
M. Clay Miner. 1967, 7,
(4) Reichle, W. T.; Kang, S. Y . ;Everhardt, D. S. J . Calal. 1986,101, 352.
(5) Allmann, R. Acta Crystallogr., Sect. B 1968, 24, 972. (6) Miyata, S. Clays Clay Mineral. 1983, 31, 305.
0 1989 American Chemical Society
