Electron transfer kinetics and excited state mechanism of oxo-bridged heterobinuclear chromophores for artificial photosynthesis

Oral, Beth Anne McClure, presentation 006
Publication date: 16th April 2014

For the development of an integrated artificial photosynthesis system it is imperative to understand not only the thermodynamics of the constituent parts, but also the kinetics of electron transfer between them. A series of robust all inorganic oxo-bridged heterobinuclear chromophores have been developed comprised of Ti or Zr as an acceptor metal and various transition metals as the donor metal covalently anchored to a silica surface. These chromophores exhibit broad absorbance in the visible region due to a metal-to-metal charge-transfer (MMCT) transition and have previously been shown to drive electron transfer for hydrocarbon oxidation,^1,2 reduction of carbon dioxide to carbon monoxide^3,4 and water oxidation when coupled to an iridium oxide nanoparticle catalyst.^5,6 Adaptation of the syntheses to use silica nanoparticles has allowed for the preparation of porous transparent (non-scattering) pressed pellet samples. Spectroscopic analysis of the TiOMn heterobinuclear unit reveals a broad transition centered at 44,610 cm^-1 (587 M^-1 cm^-1, 17,360 cm^-1 full-width at half maximum) and yields a large electronic coupling constant of 4200 cm^-1 by Hush analysis. Kinetic analysis by transient absorption spectroscopy reveals a 2.43 μs lifetime in vacuum. Temperature dependence shows a modest activation energy of 1.67 kcal/mol and a pre-exponential factor of 7.3 x 10⁶ s^-1. The electronic coupling constant calculated from the temperature dependent kinetic data is less than 1 cm^-1, suggesting very weak coupling between the excited MMCT state and the ground state. The disparity between the electronic coupling constants calculated by spectral analysis and kinetic temperature dependence suggests that the observed microsecond back electron transfer occurs from an electronic state that differs from the initially formed MMCT state. An excited state mechanism is proposed in which the initially excited high spin MMCT state rapidly branches between relaxation to the ground state and intersystem crossing to a lower spin MMCT state, from which relaxation back to the ground state is spin forbidden, which is consistent with the observed long lived excited state. The quantum yield for intersystem crossing is estimated to be >20%, which combined with the long life time makes this class of chromophores ideal for driving the multielectron catalysis reactions in an artificial photosynthetic system.⁷  [1]Wu, X. et al. Dalton Trans. 2009, 10114; [2]Nakamura, R. et al. J. Am. Chem. Soc. 2007, 129, 9596; [3]Lin, W.; Frei, H. J. Am. Chem. Soc. 2005, 129, 1610; [4]Macnaughtan, M. L. et al. J. Phys. Chem. C, submitted.; [5]Han, H.; Frei, H. J. Phys. Chem. C. 2008, 112, 16156; [6]Kim, W. Y. et al. to be submitted.; [7]McClure, B. A.; Frei, H., to be submitted.

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