Unusually Long Lifetime of Excited Charge-Transfer State of All-Inorganic Binuclear TiOMn^II Unit Anchored on Silica Nanopore Surface

The lifetime and back electron transfer kinetics of an all-inorganic, oxo-bridged heterobinuclear Ti IV OMn II group covalently anchored on a silica nanopore surface was investigated by transient optical absorption spectroscopy. Mesoporous silica particles of type SBA-15 loaded with TiOMn sites (1 wt %) were suspended in an index matching liquid for performing spectroscopy in transmission mode. Upon excitation of the Ti IV OMn II → Ti III OMn III metal-to-metal charge-transfer transition (MMCT) by a visible laser pulse of 8 ns duration, a transient bleach was observed whose intensity versus pump wavelength dependence agreed with the MMCT absorption profile. The decay kinetics is well described by a superposition of first-order rates with a mean time constant of 1.8 ± 0.3 μs (room temperature). The dispersion of γ = 2 ± 0.2 (Albery model) is attributed to variations in the local silica coordination environment reflecting the disordered, amorphous nature of the silica nanopore surface. The result constitutes the first observation of the electron transfer kinetics of an all-inorganic heterobinuclear group. It is proposed that the microsecond lifetime, unusually long for such as small charge transfer chromophore, originates from strong polarization of the local and remote silica environment upon light-triggered electron transfer from Mn to Ti. This results in a substantial reorganization barrier for back electron transfer. The long lifetime makes oxo-bridged heterobinuclear units anchored on silica surfaces efficient visible light photocatalysts and suitable as charge-transfer chromophores for driving multielectron catalysts in artificial photosynthetic systems.