The Nature of the Lowest Excited State in Tricarbonylchloro-1,10-phenanthrolinerhenium(I) and Related Complexes

Complexes of the general formula ClRe(CO)₃X (X = 1,10-phenanthroline and related ligands) are found to undergo radiative decay from their lowest electronically excited state both in 298 °K fluid solution and at 77 °K in glassy media. Emission lifetimes are near 10^-5 sec at 77 °K in EPA and at least 1 order of magnitude shorter at 298°K in deoxygenated solutions. Luminescence quantum yields at 77°K are of the order of 10^-1. Variations in the lowest absorption maximum with X and solvent polarity are consistent with a M → X π^∗ chargetransfer (CT) transition. The sensitivity of the emission spectrum to its environment and X point to an association with the M → X π^∗; CT absorption. Both the emission lifetimes and preliminary energy transfer experiments are consistent with an emitting state which has substantial triplet character. The room temperature fluid solution luminescence observed in these complexes constitutes the first direct observation of excited state decay in metal carbonyls under conditions where photochemistry is normally performed. These complexes also exhibit a unique effect referred to as luminescence rigidochromism (spectral distribution of luminescence in fluid media and rigid media is vastly different). In these ClRe(CO)₃X complexes emission maxima shift up to ∼ 1500 cm^-1 depending on the environmental rigidity, yielding red-orange luminescence in fluid solution and yellow-green luminescence in rigid media.