The photophysical properties of the low-lying, emissive metal to ligand charge transfer (MLCT) excited states of two series of complexes of the type [Re(BL)(CO)(3)L](+) (BL = 2,3-di(2-pyridyl)pyrazine (dpp) and 2,3-di(2-pyridyl)quinoxaline (dpq); L = N-methylimidazole, trimethylphosphine, acetonitrile, and substituted pyridines) have been investigated. These systems have been studied by emission spectroscopy, electronic absorption spectroscopy, infrared absorption spectroscopy, excited state lifetime measurements, electrochemistry, and resonance Raman spectroscopy of the emissive states. The results show that the rate constant for nonradiative decay (k(nr)) is the dominant mode of excited state relaxation and that the "energy gap law" for radiationless decay in the weak coupling limit is obeyed by these two series of complexes. The relative nonradiative decay rates have been evaluated utilizing the parameters E(0), S-m, and Delta upsilon(1/2) Obtained from one-mode emission band-shape analysis and h omega(m). The h omega(m), values were determined from a quantitative analysis of resonance Raman data of the emissive state. The h omega(m) values for both classes of complexes include contributions from a C=O stretching acceptor mode at ca. 2020-2040 cm(-1), which is observed in the resonance Raman of all complexes. The results of this analysis show that the CO modes make a small contribution to the overall nonradiative decay rate. The greater degree of complex solvent interactions, as evidenced by large Delta upsilon(1/2) values, is the major factor in the large values of k(m), relative to non-carbonyl-containing complexes.