Hydrogen Isotope Effects on the Excited-State Decay Properties of Polynuclear Metal Carbonyl Hydrides. Emission Properties of H4Re4(CO)12 and [µz-Bu4N]2[H6Re4(CO)12]

The two pairs of complexes H4Re4(CO) 12,D4Re4(CO)!2 and [n-Bu4N]2[H6Re4(CO)12],[n-Bu4N]2[D8Re4(CO)12] have been compared with respect to excited-state decay properties. The H4Re4(CO)12 complex exhibits emission from the lowest excited state at 298 K in hydrocarbon solution or as a solid and also at 77 K in glassy hydrocarbon solution or as a pure solid. The emission maximum is between 13700 and 14600 cm”1 depending on solvent conditions, and the lifetimes range from <µ0.02 ms at 298 K in 3-methylpentane solution (Φ= 0.003) to 16 ms at 77 K in 3-methylpentane glass (Φ= 0.25). The effect of replacing by 2H is to lengthen emission lifetime by 20-30% with an equal percent increase in emission quantum yield. The lowest (emissive) excited state can be rapidly quenched by anthracene (triplet energy Φ= 42 kcal/mol) but not by trcns-stilbene (triplet energy = 50 kcal/mol), consistent with the high-energy onset (~46 kcal/mol) of the structureless emission spectrum of H4Re4(CO)12 attributable to a lowest-lying triplet excited state associated with a Re4 localized electronic transition. [n-Bu4N]2[H6Re4(CO)12] does not delectably emit in solution or as the solid at 298 K, but emission is detectable at 77 K from the solid or glassy solutions of the salt. The emission is at ~ 18 000 cm”1 with a lifetime of ~2 ms and a quantum yield of -0.01. The effect of replacement by 2H is to increase lifetime and quantum yield by ~50% depending on the medium. Under a given set of conditions for either H4Re4(CO)12 or [H6Re4(CO)12]2’, the effect of replacing by 2H is to lower the rate constant for nonradiative decay; there is no effect on the radiative decay constant.