In developing an integrated artificial photosynthesis system it is crucial to understand not only the thermodynamics of the components, but also the kinetics of electron transfer between them. A series of oxo-bridged, all-inorganic heterobinuclear chromophores anchored on silica surfaces have been developed comprised of Ti or Zr as an acceptor metal and various transition metals as the donor metal and exhibit broad absorbance in the visible region due to metal-to-metal charge-transfer. Spectroscopic analysis of the TiOMn chromophore yields a large electronic coupling constant of 4200 cm-1. Kinetic analysis by transient absorption spectroscopy reveals a 2.43 �s lifetime in vacuum. Temperature dependence shows a modest activation energy of 1.67 kcal/mol and a pre-exponential factor of 7.3x106 s-1, resulting in an electronic coupling constant of less than 1 cm-1. This suggests very weak coupling between the MMCT excited state and the ground state. An excited state mechanism is proposed in which the initially formed high spin MMCT state rapidly undergoes intersystem crossing to a lower spin MMCT state, from which relaxation back to the ground state is spin forbidden and yields a long lived excited state.

Excited state electron transfer mechanism of heterobinuclear metal-to-metal charge-transfer chromophores

248th National Meeting (2014)

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