Photoinduced Electron Transfer Across a Molecular Wall: Coumarin Dyes as Donors and Methyl viologen and TiO2 as Acceptors

Coumarins C-153, C-480, and C-1 formed 1:2 (guest:host) complexes with a water-soluble cavitand having eight carboxylic acid groups (OA) in aqueous borate buffer solution. The complexes were photoexcited in the presence of electron acceptors (methyl viologen, MV2+, or TiO2) to probe the possibility of electron transfer between a donor and an acceptor physically separated by a molecular wall. In solution at basic pH, the dication MV2+ was associated to the exterior of the complex C-153@OA(2), as suggested by diffusion constants (similar to 1.2 X 10(-6) cm(2)/S) determined by DOSY NMR. The fluorescence of C-153@OA(2) was quenched in the presence of increasing amounts of MV2+ and Stern-Volmer plots of I-o/I and tau(o)/tau vs [MV2+] indicated that the quenching was static. As per FT-IR-ATR spectra, the capsule C-153@OA(2) was bound to TiO2 nanoparticle films. Selective excitation (l(exc) = 420) of the above bound complex resulted in fluorescence quenching. When adsorbed on insulating ZrO2 nanoparticle films, excitation of the complex resulted in a broad fluorescence spectrum centered at 500 nm and consistent with C-153 being within the lipophilic capsule interior. Consistent with the above results, colloidal TiO2 quenched the emission while colloidal ZrO2 did not.