The photophysical properties of the metal to ligand charge transfer (m.1.c.t.) excited states of the complexes [Re(4,4'-X2-bipy)(CO),CI] (X = NH, , NEt,, NHCOCH,, OCH,, CH, , H, Ph, CI, C0,Et or NO,; bipy = bipyridine) vary systematically as the substituent X is varied. For the cases where m.1.c.t. states are lowest lying a quantitative correlation exists between In(k,, x 1 s)(k,, 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),CI] has been determined to be 1100 cm-l in EtOH-MeOH (4: 1 v/v) and 650 cm-' in 2-methyltetrahydrofuran by a temperature dependent bandwidth study. Based on a comparative analysis of properties with related polypyridyl complexes of Ru" and 0 s " it has been concluded that: (1) the extent of distortion at the 4,4'-X,-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 0s"; (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' have as a major contributing factor the participation of a v(C0) mode at ca. 2020-2040 cm-l as an energy acceptor in non-radiative decay.For the metal to ligand charge transfer (m.1.c.t.) excited states of polypyridyl complexes of the d6 metal ions Ru" and 0s" the results of photophysical and spectroscopic studies have led to a relatively clear insight into excited state structure and reactivity.' At the molecular level the factors that govern nonradiative and radiative decay rate constants, the configurations of the lower lying excited states, the structural differences between the ground and excited states, and the vibrational modes that participate in excited state decay are all becoming more a~p a r e n t .~-~ Polypyridyl complexes of Re' are playing an increasingly important role in studies involving photochemical electron and energy transfers.'-'Since the initial investigations by Wrighton and Morse,'= the results of a number of studies have appeared based on complexes of the type [Re(bipy)(CO),L]" + (bipy = 2,2'-bipyridine). These complexes have been of value in studying medium in exploring the fundamental photophysics of m.1.c.t. excited states,15 and as components of more complex excited state systems including chromophorequencher complexes.''~'6~'7 An important advantage for the complexes of Re' lies in the availability of a useful synthetic chemistry.8-'0~'5~'8 Typically, these complexes possess m.1.c.t. excited states which emit and can be monitored by standard emission and lifetime techniques. In this paper we report on the preparation and properties of the series fuc-[Re(4,4'-X,-bipy)(CO),Cl] (X...
