STUDIES OF THE EMISSION BEHAVIOR OF QUADRUPLY BONDED BINUCLEAR COMPLEXES IN SOLUTION - EVIDENCE FOR D4D DISTORTIONS

The sterically hindered complex Mo2Cl4(PBu3)4 (Bu=n-butyl) is highly emissive in fluid solution (2-methylpentane) at room temperature, with λmax(emission)=665 nm and φ=0.013. The emission at 77 K(φ=0.33) is vibronically structured (Δv=340 cm-1, corresponding to v(MoMo)) and is a mirror image of the δ→ δ* absorption band at this temperature. Nanosecond laser flash experiments indicate the emission lifetimes to be 16 ns at room temperature and 165 ns at 77 K; the recovery of bleaching of ground-state absorption has the same lifetime, with no longer lived transients being discernible, and the magnitude of bleaching is consistent with a unit quantum yield for formation of the transient. These results are interpreted in terms of a very large splitting between the triplet and singlet δδ* excited states, the emission being assigned to δδ* (singlet) → δ2. Studies of the unhindered D4h complexes Re2X82- (X=Cl, Br) in fluid solution (CH3CN, CH2C12) reveal very weak (φ=10-4— 10-5) emissions. The emission bands in these cases are broad and the emissive states are relatively long lived (τ=50-140 ns) at room temperature. Transient ground-state bleaching again indicates near unit quantum yields. The radiative rate constants for the Re2X82-transients are all estimated to be 5.5 × 102 s-1. We propose that these transients possess an equilibrium geometry near D4d, with the emission being assigned to δδ* (singlet) (D4 or D4d) →1 A1g (D4h). © 1979, American Chemical Society. All rights reserved.