PHOTOPHYSICAL PROPERTIES OF POLYPYRIDYL CARBONYL-COMPLEXES OF RHENIUM(I)

The photophysical properties of the metal to ligand charge transfer (m.l.c.t.) excited states of the complexes [Re(4,4'-X2-bipy)(CO)3Cl] (X = NH2, NEt2, NHCOCH3, OCH3, CH3, H, Ph, Cl, CO2Et or NO2, bipy = bipyridine) vary systematically as the substituent X is varied. For the cases where m.l.c.t. states are lowest lying a quantitative correlation exists between In(k(nr) x 1 s)(k(nr) is the rate constant for nonradiative decay) and the Franck-Condon factor calculated from parameters obtained by emission spectral fitting. The solvent reorganizational energy for [Re(bipy)(CO)3Cl] has been determined to be 1100 cm-1 in EtOH-MeOH (4:1 v/v) and 650 cm-1 in 2-methyltetrahydrofuran by a temperature dependent bandwidth study. Based on a comparative analysis of properties with related polypyridyl complexes of Ru parallel-to and Os parallel-to it has been concluded that: (1) the extent of distortion at the 4,4'-X2-bipy acceptor ligand correlates with the energy gap between the excited and ground states; these results are in agreement with an earlier correlation found for polypyridyl complexes of Os parallel-to; (2) the unusually large Stokes shift and the broadening of the vibronic components in absorption and emission spectra arise from a combination of increased solvent reorganizational energies and greater distortions in the low-frequency modes between the excited and ground states; and (3) the relatively short lifetimes for the complexes of Re(I) have as a major contributing factor the participation of a nu-(CO) mode at ca. 2020-2040 cm-1 as an energy acceptor in non-radiative decay.