Photoinduced Electron Transfer from Conjugated Polymers to TiO₂

Photoinduced electron transfer from π-conjugated polymers, in particular derivatives of poly(p-phenylenevinylene)s (PPVs) and polythiophenes (PTs) (as a donor), to nanocrystalline TiO2 (as an acceptor) is studied using near-steady-state photoinduced absorption (PIA) spectroscopy and photoinduced electron spin resonance (ESR). The PIA spectra of the polymers adsorbed on nc-TiO2 recorded at 80 K reveal two subgap transitions in the near-IR and visible range of the spectrum, which are characteristic of the formation of cation radicals (polarons) on the polymer chains, as a result of a photoinduced electron-transfer reaction. When the PPVs are in contact with nc-ZrO2, which has a conduction band 1 eV higher in energy than nc-TiO2, the photoinduced electron-transfer reaction becomes energetically unfavorable and long-lived triplet photoexcitations are formed instead. The PIA experiments give direct spectral evidence that nc-TiO2 acts as an efficient electron acceptor toward the polymers in the excited state. In combination with nc-TiO2, partial quenching of the photoluminescence of the polymers occurs. The photoinduced electron transfer to nc-TiO2 is confirmed by ESR spectroscopy at 130 K, which reveals the reversible formation of polymer cation radicals upon excitation of the PPV in combination with nc-TiO2 but not with nc-ZrO2. Both PIA and ESR spectroscopies show that the recombination of charges is characterized by a distribution of lifetimes at cryogenic temperatures ranging from milliseconds to tenths of seconds.