Bridge-Dependent Interfacial Electron Transfer from Rhenium-Bipyridine Complexes to TiO2 Nanocrystalline Thin Films

We have measured the electron injection kinetics of four rhenium bipyridine complexes (Re1C, ReEC, Re1TC, and Re2TC) on TiO2 nanocrystalline films using transient infrared spectroscopy. The self-assembled monolayer formation of these complexes was characterized by UV-visible spectroscopy, infrared reflection absorption spectroscopy, and X-ray photoelectron spectroscopy. These complexes bind to the TiO2 surface through the formation of carboxylate groups, and these self-assembled layers are approximately a monolayer. The kinetics studies address the effect of insulating and conjugated spacers and the length of conjugation on the electron-transfer process. The insulating bridge leads to a slower injection rate and poorer injection yield compared with the conjugated spacers. The electron injection of Re2TC was found to be a fast, high-yielding, and multiple electron injector process. The ground and electronically excited states of the dye complexes were characterized using ground-state and time-dependent density functional theory. We present the role of electronic conjugation in modulating electron injection using a combination of computational and experimental work and find that these metal-based complexes adsorbed on a semiconductor surface can be used to read out the electron injection kinetics through tailored molecular bridges.