Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Mo2 Quadruple Bonds: Suppression of the Photochemical Enol-to-Keto Transformation

From the reactions between Mo2(TiPB)4, where TiPB is 2,4,6-triisopropylbenzoate, and 2 equiv of the acids 4-formylbenzoic acid, HBzald; 4-(3-oxo-3-phenylpropanoyl)­benzoic acid, HAvo; and 4-(2,2-difluoro-6-phenyl-2H-1λ3,3,2λ4-dioxaborinin-4-yl)­benzoic acid, HAvoBF2, the compounds Mo2(TiPB)2(Bzald)2, I; Mo2(TiPB)2(Avo)2, II; and Mo2(TiPB)2(AvoBF2)2, III, have been isolated. Compounds I and II are red, and compound III is blue. The new compounds have been characterized by 1H NMR, MALDI-TOF MS, steady-state absorption and emission spectroscopies, and femtosecond and nanosecond time-resolved transient absorption and infrared spectroscopies. Electronic structure calculations employing density functional theory and time-dependent density functional theory have been carried out to aid in the interpretation of these data. These compounds have strong metal-to-ligand charge transfer, MLCT, and transitions in the visible region of their spectra, and these comprise the S1 states having lifetimes ∼5–15 ps. The triplet states are Mo2δδ* with lifetimes in the microseconds. The spectroscopic properties of I and II are similar, whereas the planarity of the ligand in III greatly lowers the energy of the MLCT and enhances the intensity of the time-resolved spectra. The Mo2 unit shifts the ground state equilibrium entirely to the enol form and quenches the degradation pathways of the avobenzone moiety.