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( i PrN)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.