Electronic and Spectroscopic Properties of Avobenzone Derivatives Attached to Mo_2 Quadruple Bonds: Suppression of the Photochemical Enol-to-Keto Transformation

摘要

From the reactions between Mo_2(T~iPB)_4, where T~iPB 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-1λ~3,3,2λ~4-dioxaborinin-4-yl)benzoic acid, HAvoBF_2, the compounds Mo_2(T~iPB)_2(Bzald)_2, Ⅰ; Mo_2(T~iPB)_2(Avo)_2, Ⅱ; and Mo_2(T~iPB)_2(AvoBF_2)_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 S_1 states having lifetimes ～5-15 ps. The triplet states are Mo_2δδ~* 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 Mo_2 unit shifts the ground state equilibrium entirely to the enol form and quenches the degradation pathways of the avobenzone moiety.