Quadruply Bonded Dimetal Units Supported by 2,4,6-Triisopropylbenzoates MM(TiPB)<sub>4</sub> (MM = Mo<sub>2</sub>, MoW, and W<sub>2</sub>): Preparation and Photophysical Properties
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The preparation and characterization (elemental analysis, 1H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)4, where MM = MoW and W2 and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo2(TiPB)4, previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMδ to arylcarboxylate π* transitions, 1MLCT. Each compound also shows luminescence from two excited states, assigned as the 1MLCT and 3MMδδ* states. The energy of the emission from the 1MLCT state follows the energy ordering MM = Mo2 > MoW > W2, but the emission from the 3MMδδ* state follows the inverse order: MM = W2 > MoW > Mo2. Evidence is presented to support the view that the lower energy emission in each case arises from the 3MMδδ* state. Lifetimes of the 1MLCT states in these systems are ∼0.4−6 ps, whereas phosphorescence is dependent on the MM center: Mo2 ∼ 40 μs, MoW ∼ 30 μs, and W2 ∼ 1 μs.
