Photophysical Studies of trans-Bis(phenylethynyldiisopropylamidinato)bis(acetato)dimetal Complexes Involving MM Quadruple Bonds Where M = Mo or W

The title compounds trans-M₂(O₂CMe)₂[C(ⁱPrN)₂CCPh]₂, I (M = Mo) and II (M = W), show electronic absorptions in the visible region of the spectrum assignable to ¹MLCT [M₂δ to phenylethynylamidinate π*]. These compounds show dual emission from S₁ and T₁ states. For both I and II, S₁ is ¹MLCT, but for I the T₁ state is shown to be MMδδ* while for II T₁ is ³MLCT. The lifetimes of the S₁ and T₁ states have been determined by femtosecond and nanosecond transient absorption spectroscopy: for I S₁ ∼ 20 ps and T₁ ∼ 100 μs and for II S₁ ∼ 6 ps and T₁ ∼ 5 μs. From solvent dependence of the absorption and emission spectra, we suggest that the S₁ states are localized on one amidinate ligand though the initial absorption is to a delocalized state.