Excited State Electron Transfer of All-Inorganic Heterobinuclear TiOMn2+ Chromophore Anchored on Silica Nanoparticle Surface

The oxo-bridged heterobinuclear chromophore TiOMn2+ has been covalently anchored onto a silica nanoparticle surface, allowing for the preparation of optically transparent solid state samples. Optical spectra of the material yield a broad metal-to-metal charge-transfer (MMCT) transition in the UV with a tail extending to 550 nm. Fitting the absorption band to a Gaussian curve gives a maximum at 44 610 (+/- 230) cm(-1) (587 (+/- 20) M-1 cm(-1)) with a full width at half-maximum of 17 320 (+/- 160) cm(-1), and yields an electronic coupling constant, H-ABopt, of 4200 cm(-1) by Hush analysis. Transient absorption measurements of transparent pressed pellets in vacuum yield a 2.43 (+/- 0.20) mu s lifetime at room temperature. The temperature dependence of the lifetime in the range of 10-40 degrees C gives an activation energy of 1.67 (+/- 0.36) kcal/mol and pre-exponential factor A of 7.3 (+/- 4.4) x 10(6) s(-1). This temperature dependence results in an electronic coupling constant H-ABkin, of less than 1 cm(-1). The disparity between the large electronic coupling indicated by fitting the absorption band and the very small coupling based on temperature dependent kinetic measurements suggests that the observed microsecond back electron transfer is from an electronic state that is distinct from the initially excited MMCT state. It is proposed that the initially excited high spin state (S = 5/2) branches on the ultrafast time scale between back electron transfer to the ground state (S = 5/2) and intersystem crossing to a low spin MMCT state (S = 3/2 or 1/2). With intersystem crossing to a lower spin state available as a competitive pathway, these robust chromophores are promising for driving multielectron catalysis in an integrated artificial photosynthesis system.