Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Ditungsten Quadruple Bonds

From the reactions between W2(T i PB)4, where T i PB is 2,4,6-triisopropylbenzoate, and 2 equiv of acid, 4-formylbenzoic acid, HBzald, 4-(3-oxo-3-phenylpropanoyl)­benzoic acid, HAvo, or 4-(2,2-difluoro-6-phenyl-2H-1λ3,3,2λ4-dioxaborinin-4-yl)­benzoic acid, HAvoBF2, three new compounds W2(T i PB)2(Bzald)2, I, W2(T i PB)2(Avo)2, II, and W2(T i PB)2(AvoBF2)2, III, have been prepared. As solid compounds I and II are blue while compound III is green. Characterization of these compounds has been carried out by means of 1H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond transient absorption and time-resolved infrared spectroscopies. Compounds I and II have strong metal to ligand charge transfer, MLCT, transitions in the visible region of their spectra while compound III exhibits MLCT absorption in the near-infrared (λmax = 1017 nm). All three have S1 states that have corresponding lifetimes of ∼3–30 ps and are 1MLCT in character. The triplet states are 3MLCT with lifetimes in the range 3–10 ns. Density functional theory and time-dependent density functional theory were employed to perform electronic structure calculations in order to aid in the interpretation of these data. The spectroscopic properties of I and II are similar while the planarity of the ligand in III greatly lowers the energy of the MLCT state. The W2 unit enables direct observation of intersystem crossing from the 1MLCT state to 3MLCT state via the use of ultrafast spectroscopy.