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

The relaxation acid interligand electron-transfer dynamics of Ru(4.4 ' dica rboxy-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 sol utions. The results indicate that the dynamics are strongly dependent on th e solvent and the excitation wavelength; results following 520 and 650 nm e xcitation are herein reported. These results may be semiquantitatively unde rstood 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 sta tes. 520 nm light primarily populates the upper surface, whereas 650 nm lig ht primarily populates the lower surface. The rate of upper to lower surfac e relaxation is strongly solvent dependent, varying from about 24 ps in ace tonitrile to about 1.1 ns in methanol. There is a large barrier to interlig and 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 coupli ng of about 200 cm(-1) and a barrier height of 1050 cm(-1).