Photoinitiated Electron Transfer Dynamics of a Terthiophene Carboxylate on Monodispersed Zinc Oxide Nanocrystals

Abstract

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.

Effect of extending conjugation via thiophene-based oligomers on the excited state electron transfer rates to ZnO nanocrystals

“…The general shape of the Stern Volmer graphs observed for these dyes paralleled those reported in earlier studies, where the suggested causes were concentration quenching 21 or competition between dye binding sites that were or were not quenched. 3,4 The current work focused on the ultrafast quenching kinetics at a dye to ZnO NC of 2:1 in order to minimize any complications attributed to dye-dye interactions. 2T.…”

“…The lifetimes observed for 3T, 4T and 5T were similar to related terthiophene-based dyes that did not have cyanoacrylate anchoring groups. 3,5 The smallest dye, 2T, exhibited much shorter lifetimes. When ZnO NCs were combined with the dyes in an approximate 2:1 dye to NC ratio, a distinct transient absorption was observed that corresponded closely to the absorption observed from chemical oxidation of the dye using nitrosonium ion.…”

“…Monodisperse zinc oxide nanocrystals (NCs) have been shown to serve as a useful platform for studying light-induced charge separation as found in dye-sensitized solar cells. Using a variety of dyes, including porphyrins, 1 ruthenium tris(bipyridine) complexes 2 and terthiophenes, [3][4][5] our past work has addressed the impact of dye structure, excited state energy of the dye, anchoring group and ZnO NC diameter on the excited state electron transfer lifetime. Thiophene oligomers offer the ability to tune the dye absorption through systematic extension of the oligomer chain, and associated conjugation, with the potential to cover the entire visible spectrum within a chemically homologous series.…”

Tuning Electron Transfer Rates via Systematic Shifts in the Acceptor State Density Using Size-Selected ZnO Colloids

“…5 Compound Time-integrated emission quenching and dye binding constant in detail in another study. 9,10 Time-resolved measurements Time correlated single photon counting Figure S5: Unmodified TCSPC raw data (points) and the biexponential fits used to determine electron injection timescales (lines). The instrument response is shown as the dotted line.…”

Zinc oxide nanocrystal quenching of emission from electron-rich ruthenium-bipyridine complexes

“…16 In previous work we demonstrated that ZnO NCs quenched the excited state by electron transfer of two carboxylate-bound fluorophores, a terthiophene derivative and a zinc porphyrin. [17][18][19] Although capable of binding to the ZnO NCs, the ruthenium-polypyridyl complex, [Ru(bpy) 2 (dcbpy)](PF 6 ) 2 where bpy = 2,2′-bipyridine and dcbpy = 4,4′-dicarboxylic acid-2,2′-bipyridine, exhibited fluorescence that was not quenched. This was initially unexpected based on the reported fluorescence quenching of [Ru(bpy) 2 (dcbpy)] 2+ when it was bound to a colloidal ZnO film.…”