Photoinduced ultrafast dynamics of the triphenylamine-based organic sensitizer D35 on TiO2, ZrO2 and in acetonitrile

Abstract

The relaxation dynamics of the dye D35 has been characterized by transient absorption spectroscopy in acetonitrile and on TiO(2) and ZrO(2) thin films. In acetonitrile, upon photoexcitation of the dye via the S(0) → S(1) transition, we observed ultrafast solvation dynamics with subpicosecond time constants. Subsequent decay of the S(1) excited state absorption (ESA) band with a 7.1 ps time constant is tentatively assigned to structural relaxation in the excited state, and a spectral decay with 203 ps time constant results from internal conversion (IC) back to S(0). On TiO(2), we observed fast (<90 fs) electron injection from the S(1) state of D35 into the TiO(2) conduction band, followed by a biphasic dynamics arising from changes in a transient Stark field at the interface, with time constants of 0.8 and 12 ps, resulting in a characteristic blue-shift of the S(0) → S(1) absorption band. Several processes can contribute to this spectral shift: (i) photoexcitation induces immediate formation of D35˙(+) radical cations, which initially form electron-cation complexes; (ii) dissociation of these complexes generates mobile electrons, and when they start diffusing in the mesoporous TiO(2), the local electrostatic field may change; (iii) this may trigger the reorientation of D35 molecules in the changing electric field. A slower spectral decay on a nanosecond timescale is interpreted as a reduction of the local Stark field, as mobile electrons move deeper into TiO(2) and are progressively screened. Multiexponential electron-cation recombination occurs on much longer timescales, with time constants of 30 μs, 170 μs and 1.4 ms. For D35 adsorbed on ZrO(2), there is no clear evidence for a transient Stark shift, which suggests that initially formed cation-electron (trap state) complexes do not dissociate to form mobile conduction band electrons. Multiexponential decay with time constants of 4, 35, and 550 ps is assigned to recombination between cations and trapped electrons, and also to a fraction of D35 molecules in S(1) which decay by IC to S(0). Differential steady-state absorption spectra of D35˙(+) in acetonitrile and dichloromethane provide access to the complete D(0) → D(1) band. The absorption spectra of D35 and D35˙(+) are well described by TDDFT calculations employing the MPW1K functional.