An inorganic chromophore developed by scientists in the US could be used in solar fuel generation. 

Chromophores are used in photocatalytic systems to absorb light which can then be converted to chemical energy. High redox potentials are required to achieve efficient conversion. Heinz Frei at the University of California, Berkeley and colleagues have used a manganese(III) center linked to titanium(IV), stabilised on a silica support to create an all inorganic binuclear chromophore. The chromophore unit absorbs has donor and acceptor redox potentials and absorbs light from the UV across the visibile into the red spectral region. Frei says he hopes this will form 'the basis for the design of new photocatalytic systems.' 

The excitation of the chromophore unit results in a complete electron transfer from the donor manganese(II) to the titanium(II) to produce the redox active Ti(III)OMn(III) centre. Frei used the metal-to-metal charge-transfer to produce formate and water from methanol and oxygen. 'The primary rationale for developing the new types of visible light photocatalysts is to explore an inorganic molecular approach to photocatalysis,' says Frei. 'Our flexible synthetic methods allow the use of a wide variety of donor or acceptor metal centres covering a considerable span of redox potentials, as well as being environmentally friendly, earth-abundant metals.' Kazuhito Hashimoto, an expert on photocatalysts at the University of Tokyo, Japan, states 'the binuclear charge-transfer chromophore is a very elegant photocatalytic reaction center, because its optical absorption as well as redox property can be tuned precisely by presetting the combination of two hetero-metal ions.' In the future Frei plans to couple this technology with multi-electron transfer catalysts for demanding chemical reactions such as artificial photosynthesis. 

Ian Coates 

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