J . Phys. Chem. 1989, 93, 6423-6428
6423
Photochemistry on Surfaces. 3. Spectral and Photophysical Properties of Monomeric and Dimeric Anthracenesulfonates Adsorbed to Colloidal Alumina-Coated Silica Particles William E. Ford*” and Prashant V. Kamat* Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received: September 12. 1988; In Final Form: February 6, 1989) Anthracene-2-sulfonate(2AS) molecules dimerized upon adsorption from aqueous solution onto positively charged alumina-coated silica particles (25-nm diameter). This ground-state dimerization was accompanied by an intensification and red-shift (from 370 to 401 nm) of the 0-0 absorption band and a large decrease in fluorescence quantum yield. Anthracene-l,5-disulfonate (1,5AS) and anthracene-1-sulfonatemolecules also adsorbed to the particles but did not dimerize. Triplet states of the adsorbed molecules were characterized by laser flash photolysis. Direct photoexcitation of the 2AS dimer failed to produce an observable triplet state, but photosensitization yielded a triplet state of the dimer resembling that of the monomer. The triplet state of the adsorbed 1,5AS molecule was obtained by either direct or sensitized excitation. Binding capacities of the particles for 2AS and 1,5AS indicated that at maximal coverage the 2AS molecules were tilted with respect to the surface while the 1,5AS molecules were coplanar. Dimerization of 2AS occurred even when fewer than 1% of available adsorption sites were occupied. provided to us that were synthesized according to literature Introduction procedures.6a,8 The sodium salt of 2AS was purchased from Our continuing interest1 in the photochemistry of molecules Molecular Probes (Eugene, OR) and recrystallized from water adsorbed onto the surfaces of submicron-sized inorganic particles before use. The molar extinction coefficient of 2AS in aqueous has led us to examine colloids containing derivatives of anthracene. 1 X lo4 M HCI was determined to be 4690 100 M-‘ cm-I at Anthracene and its derivatives undergo a wide variety of photochemical reactions (e.g., energy transfer,1a,2electron t r a n ~ f e r ? ~ , ~ , ~ 359 nm, which is substantially lower than the value reported by Rohatgi and Singh6”but somewhat greater than the value reported ph~tooxygenation,~ and ph~todimerization~), so the development by Tamaki.sc The corresponding value for 1,SAS was found to of a well-characterized colloid system with surface-adsorbed anbe 7600 f 200 M-’ cm-l at 366 nm, which is close to the value thracene molecules could prove to be useful for studying a wide reported by Rohatgi and Singh.6a Tris(2,2’-bipyridine)ruthenivariety of surface photochemical reactions involving the same um(I1) ( R ~ ( b p y ) ~ ~ dichloride ’) was purchased frm Aldrich chromophore. (Milwaukee, WI). An aqueous colloidal suspension (pH 4.2, This paper examines how the spectral and photophysical specific gravity 1.23) of positively charged alumina-coated silica properties of sulfonated anthracenes change when the molecules particles containing 26% S i 0 2 and 4% A1203,with chloride (
