A study on the redox, spectroscopic, and photophysical characteristics of a series of octahedral hexamolybdenum(II) clusters: [{Mo6X8}Y6]2- (X, Y = Cl, Br, or I)

We report a systematic study on the redox, spectroscopic, and photophysical properties of a series of [{Mo6X8}Y-6](2-) (X, Y = Cl, Br, or I. 1-9). All of the [{Mo6X8}Y-6](2-) clusters show intense and long-lived phosphorescence in both CH3CN and crystalline phases at 298 K. We found that the emission quantum yields (Phi(em)) of 1-9 increase in the sequences X = Cl < Br < I and Y = I < Br < Cl for given Y and X, respectively. The emission lifetimes (tau(em)) of the clusters also increase in the sequence Y = I < Br < Cl for given {Mo6X8}(4+) -core clusters. The present data demonstrate that arbitrary combinations of X and Y in [{Mo6X8}Y-6](2-) could tune tau(em) and Phi(em) in the ranges of 85-300 mu s and 0.09-0.47, respectively. Both capping (X) and terminal ligand (Y) effects on the photophysical properties of the clusters are discussed on the basis of the energy gap (i.e., emission energy) dependence of the nonradiative decay rate constant.