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

Abstract

We report synthesis of a new catechol derivative of 2,2'-bipyridyl (L1) 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 ¹MLCT (spin allowed) and ³MLCT (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 (³MLCT) 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 dye/nanoparticle systems. Monitoring the CT emission, we could determine back electron transfer (BET) rate for the charge recombination process.