Photophysics and relaxation dynamics of Ru(4,4′dicarboxy-2,2′-bipyridine)2cis(NCS)2 in solution

The relaxation acid interligand electron-transfer dynamics of Ru(4.4 ' dicarboxy-2,2 ' -bipyridine)(2)cis(NCS)(2) the N3-dye, have been studied using femtosecond polarized transient absorption spectroscopy. These studies have been performed in room temperature acetonitrile, methanol, and ethanol solutions. The results indicate that the dynamics are strongly dependent on the solvent and the excitation wavelength; results following 520 and 650 nm excitation are herein reported. These results may be semiquantitatively understood in terms of a model in which the MLCT pi* electron is intrinsically localized on a single bipyridine ligand. In this model, the intersection of the MLCT states associated with each of the bipyridine ligands results in an avoided crossing on the MLCT surface, producing upper and lower MLCT states. 520 nm light primarily populates the upper surface, whereas 650 nm light primarily populates the lower surface. The rate of upper to lower surface relaxation is strongly solvent dependent, varying from about 24 ps in acetonitrile to about 1.1 ns in methanol. There is a large barrier to interligand electron transfer on the lower surface and this process occurs on the 1.5 ns time scale. The results are fit with an interligand electronic coupling of about 200 cm(-1) and a barrier height of 1050 cm(-1).