Excited State and Injection Dynamics of Triphenylamine Sensitizers Containing a Benzothiazole Electron-Accepting Group on TiO2 and Al2O3 Thin Films

The excited state and electron injection dynamics of three new organic sensitizers, comprising a triphenylamine moiety connected by an ethenylene (C-C double-bond) or ethynylene (C-C triple-bond) p-spacer to an electron-withdrawing benzothiazole bearing a cyanoacrylic acid anchoring group, have been studied using a combination of steady-state and femtosecond-resolved spectroscopies. The measurements were carried out for the three dyes in predominantly neutral and completely deprotonated forms in liquid solutions and bound on nanocrystalline TiO2 and Al2O3 thin films. In addition, quantum-chemical calculations were performed to predict absorption spectra of the sensitizers and their corresponding cation radicals. Time-resolved fluorescence (TRF) measurements on TiO2 indicate that electron injection takes place on a <0.2 ps time scale. Transient electronic absorption (TA) measurements provide evidence for the formation of radical cations not only in dye-sensitized TiO2 films but also in Al2O3 ones. The cation lifetime in Al2O3 is significantly shorter compared to TiO2, indicating a faster recombination of injected electrons with the dye cations. In addition, the ground-state bleach band in dye-sensitized TiO2 films experiences a gradual red-shift, which is indicative of a transient Stark effect. Finally, femtosecond transient absorption measurements in the IR region point to an ultrafast generation of injected electrons for all dyes. A faster recombination of the injected electrons with the dye cations is observed for the sensitizer decorated with auxiliary electron-donating methoxy groups on the triphenylamine moiety.