Interfacial electron transfer dynamics of two newly synthesized catecholate bound Ru^II polypyridyl-based sensitizers on TiO₂ nanoparticle surface - a femtosecond pump probe spectroscopic study

Abstract

Two new catecholate-bound Ru^II-polypyridine based sensitizers, (2,2'-bipyridine){ethyl 3-(4-hydroxyphenyl)-2-[(4'-methyl-2,2'-bipyridinyl-4-carbonyl)amino]propionate}{4-[2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)}ruthenium(II) hexafluorophosphate (5) and [(2,2'-bipyridine)-(4-2,2'-bipyridinyl-4-yl-phenol)-(4-{2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl}benzene-1,2-diol)]ruthenium(II) hexafluorophosphate (6) with secondary electron-donating groups (tyrosine and phenol, respectively) were synthesized and characterized. Steady-state optical absorption and emission studies confirm strong coupling between the sensitizers and TiO₂ nanoparticles. Femtosecond visible transient absorption spectroscopy has been employed to study interfacial electron transfer (IET) dynamics in the dye-nanoparticle systems to explore the influence of the secondary electron-donating groups on IET dynamics. Electron injection into the conduction band of nanoparticulate TiO₂ has been confirmed by detection of the conduction band electrons in TiO₂ ([e^-]_TiO2^CB) and radical cation of the adsorbed dye (D^·+) in real time monitored by transient absorption spectroscopy. A single exponential and pulse-width limited (< 100 fs) electron injection has been observed. Back electron transfer (BET) dynamics have been studied by monitoring the decay kinetics of the injected electron in the conduction band of TiO₂ and by the recovery of the ground state bleach. BET dynamics in dye-TiO₂ systems for complexes 5 and 6 have been compared with those of [bis(2,2'-bpy)-(4-{2-(4'-methyl-2,2'-bipyridinyl-4-yl)vinyl}benzene-1,2-diol)]ruthenium(II) hexafluorophosphate (7), which does not have a secondary electron-donating group.