Photoinitiated electron transfer dynamics of a terthiophene carboxylate on monodispersed zinc oxide nanocrystals

Photophysical data for solution phase mixtures of a new terthiophene based organic dye, 3′,4′-dibutyl-2-phenyl-2,2″:5′,2″- terthiophene-5″-carboxylic acid, and size selected, well-dispersed zinc oxide nanocrystals are reported. Time-resolved fluorescence and time- and frequency-resolved pump-probe spectroscopy confirm and characterize electron injection from the dye to the semiconductor nanocrystals (NCs) in room temperature ethanol dispersions at a series of dye:ZnO NC concentration ratios. The spectrum of the oxidized dye was determined by spectroelectrochemistry. The singlet excited state of the dye (190 ps lifetime in ethanol) is quenched almost exclusively by electron transfer to the ZnO NC, and the electron transfer dynamics exhibit a single time scale of 3.5 ± 0.5 ps at all concentration ratios. In the measured transient responses at different dye:ZnO NC ratios, gain in the amplitude of the electron injection component is anticorrelated with loss of amplitude from unperturbed excited state dye molecules. The dependence of this amplitude on dye:ZnO NC ratio deviates significantly from the prediction of a standard Stern-Volmer model. This observation is in agreement with the static quenching studies presented in the companion manuscript (DOI: 10.1021/jp1080143). By identifying electron transfer as the quenching mechanism at all ratios, the work presented here helps to exclude concentration quenching as the basis for the complicated quenching results, and supports the model proposed in the companion work that incorporates competitive binding between ZnO NC s and free Zn²⁺ cations in solution.