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

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