Ultrafast electron transfer from Ru polypyridyl complexes to Nb2O5 nanoporous thin films

Electron transfer (ET) dynamics from Ru(dcbpy)(2)(X)(2) (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine; X-2 = 2SCN(-), 2CN(-), and dcbpy; referenced as RuN3, Ru505, and Ru470) to Nb2O5 nanoporous thin films are investigated by using ultrafast IR spectroscopy. For all three dyes, the injection dynamics following 400 nm excitation are shown to be biphasic, consisting of a significant ultrafast component (<100 fs) and slower component(s) ranging from 300 ps to about 1 ns. Comparing RuN3 to Ru505 to Ru470 (in order of less negative excited-state oxidation potentials), the amplitude of the fast component diminishes (from similar to37% in RuN3 to 25% in Ru470) and the rate of slow injection decreases. The dependence of injection kinetics on the crystallinity of Nb2O5 films is also investigated. The fast component amplitude is found to be similar for amorphous and crystalline films while the slow injection rate is an order of magnitude slower for the crystalline film. The injection dynamics to Nb2O5 are compared with earlier investigations of injection to TiO2, SnO2, and ZnO.