Interfacial electron-transfer dynamics on TiO2 and ZrO2 nanoparticle surface sensitized by new catechol derivatives of Os(II)-polypyridyl complexes: Monitoring by charge-transfer emission

摘要

We report synthesis of a new catechol derivative of 2,2'-bipyridyl (L-1) and two new Os(II)-polypyridyl complexes, I and II, with pendant catechol functionality (Scheme 1). Both complexes show two strong metal-to-ligand charge transfer (MLCT) absorption bands in the visible region that are attributed to (MLCT)-M-1 (spin allowed) and (MLCT)-M-3 (spin forbidden) transitions. We have recorded photoluminescence spectra for both complexes at room temperature and at 77 K. We have determined the lifetime for the excited triplet (3 MLCT) states using time-resolved emission spectroscopy. Optical absorption studies reveal that both the complexes I and II form charge transfer (CT) complex with TiO2 and ZrO2 (higher band gap) nanoparticles. Photoinduced electron injection takes place from the Os(II)-complexes to the conduction band of TiO2 and surface states of ZrO2 nanoparticles following excitation of the respective CT complexes. On recombination of these respective charge-separated complexes, CT emission has been detected in the above dyeanoparticle systems. Monitoring the CT emission, we could determine back electron transfer (BET) rate for the charge recombination process.