Quadruply bonded dimetal units supported by 2,4,6-triisopropylbenzoates MM(TiPB) ₄ (MM = Mo ₂, MoW, and W ₂): Preparation and photophysical properties

The preparation and characterization (elemental analysis, ¹H NMR, and cyclic voltammetry) of the new compounds MM(TiPB) ₄, where MM = MoW and W ₂ and TIPB = 2,4,6-trilsopropylbenzoate, are reported. Together with Mo ₂(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 MMo 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 = Mo ₂ > MoW > W ₂, but the emission from the 3MMδδ* state follows the Inverse order: MM = W ₂ > MoW > Mo ₂. 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: Mo ₂ ∼ 40 fis, MoW ∼ 30 μs, and W ₂ ∼ 1 μs.