NONRADIATIVE DECAY IN RHENIUM(I) MONOMETALLIC COMPLEXES OF 2,3-DI(2-PYRIDYL)PYRAZINE AND 2,3-DI(2-PYRIDYL) QUINOXALINE

The photophysical properties of the low-lying, emissive metal to ligan d 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, trimethylp hosphine, acetonitrile, and substituted pyridines) have been investiga ted. These systems have been studied by emission spectroscopy, electro nic absorption spectroscopy, infrared absorption spectroscopy, excited state lifetime measurements, electrochemistry, and resonance Raman sp ectroscopy of the emissive states. The results show that the rate cons tant for nonradiative decay (k(nr)) is the dominant mode of excited st ate 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-mod e emission band-shape analysis and h omega(m). The h omega(m), values were determined from a quantitative analysis of resonance Raman data o f the emissive state. The h omega(m) values for both classes of comple xes include contributions from a C=O stretching acceptor mode at ca. 2 020-2040 cm(-1), which is observed in the resonance Raman of all compl exes. The results of this analysis show that the CO modes make a small contribution to the overall nonradiative decay rate. The greater degr ee of complex solvent interactions, as evidenced by large Delta upsilo n(1/2) values, is the major factor in the large values of k(m), relati ve to non-carbonyl-containing complexes.