Study of Photoinduced Electron Transfer Process in Ruthenium Complex Modified Zinc Oxide Nanoparticles by Ultrafast Time-Resolved Transient Absorption Spectroscopy

We report the preparation of ruthenium complex functionalized zinc oxide nanoparticles and the studies of the photophysical properties by ultrafast time-resolved transient absorption spectroscopy. Diazonium group was used as the anchoring unit on ZnO surface and covalent linkage was formed between the ruthenium complex and ZnO upon UV irradiation. The morphological and electron transfer properties of the ruthenium complex modified ZnO were studied. XPS and EDX results confirmed the presence of covalent linkage. The charge generation and the transport dynamics of this light harvesting system were probed by ultrafast transient absorption (TA) spectroscopy. No positive TA absorption band was observed, which suggested an ultrafast direct electron injection from the singlet 1 MLCT excited state of the ruthenium complex to the conduction band of ZnO without going via the triplet 3 MLCT excited state. The time constants for the ground states bleaching of the complex modified ZnO are 8.1 and 167 ps, both of which are shorter than those of the pure ruthenium complex (21.7 and 360 ps). This further suggests a strong electronic coupling between the ruthenium complex and ZnO.