Flash kinetic spectroscopic studies of dinuclear rhodium(I) complexes

Abstract

Excitation of concentrated acetonitrile solutions of [Rh(tol)₄]PF₆ (tol is p-methylphenyl isocyanide) at 562 nm, where strong absorption attributable to [Rh₂(tol₄)₈]²⁺ occurs, produces emission (λₘₐₓ 697 nm) with a quantum yield of 0.0065 and a lifetime of ⩽2 ns. Excitation of [Rh₂(bridge)₄](BPh₄)₂ (bridge is 1,3-diisocyanopropane) in acetonitrile solution at 553 nm gives emission at 656 nm with a quantum yield of 0.056 and a lifetime of ⩽2 ns. The emission is assigned to ¹A_(2u) → ¹A_(1g) (2a_(ig) → ¹a_(2u)) in both dinuclear Rh(I) complexes. Excitation also gives rise to long-lived transient absorptions attributable to the following dinuclear and trinuclear species: ~8 ms, Rh₂(bridge)₄](BPh₄)₂; 0.09 ms, [Rh₂(tol₄)₈]²⁺; and 0.14 ms, [Rh₃(tol)₁₂]³⁺. In the dinuclear complexes, this transient is most likely ³A_(2u), which is the triplet excited-state partner of ¹A_(2u). The results suggest that the spin-orbit components of ³A_(2u) possess very little singlet character.

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