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

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journal contribution
posted on 21.02.2012, 00:00 by Mintu Porel, Agnieszka Klimczak, Marina Freitag, Elena Galoppini, V. Ramamurthy
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@OA2, as suggested by diffusion constants (∼1.2 × 10–6 cm2/s) determined by DOSY NMR. The fluorescence of C-153@OA2 was quenched in the presence of increasing amounts of MV2+ and Stern–Volmer plots of Io/I and τo/τ vs [MV2+] indicated that the quenching was static. As per FT-IR-ATR spectra, the capsule C-153@OA2 was bound to TiO2 nanoparticle films. Selective excitation (λexc = 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.