The environmental issues and impending energy crisis arising from the rapid consumption of traditional fossil fuels has led to an upsurgence in research aiming at developing clean and renewable energy resources. One simple but effective way of obtaining clean energy is to harvest sunlight for the photocatalytic reduction of H2O to H2. Photocatalysts for water reduction studied to date are typically multicomponent systems that incorporate a redox active electron mediator couple to transfer the electrons from the excited photosensitizer to the photocatalyst where the reduction of H+ into H2 occurs and a sacrificial electron donor to regenerate the oxidized photosensitizer. One of the major drawbacks, however, is poor charge transfer efficiency between the photosensitizer and catalyst. This talk will focus on the design of dinuclear metal complexes that can act as a dual photosensitizer and photocatalyst which serves to eliminate the additional charge transfer process, an approach that can lead to higher efficiencies. The families of dirhodium complexes cis-[Rh2(DTolF)2(L)2(CH3CN)][BF4]2 and cis-[Rh2(F-form)2(L)2(CH3CN)2][BF4]2 (DTolF = p-ditolylformamidinate, F-Form = p-difluorobenzylformamidinate; L = the chelating diimine ligands dpq (dipyrido[3,2-f:2',3'-h]-quinoxaline), dppz (dipyrido[3,2-a:2',3'-c]phenazine) and dppn (benzo<i>dipyrido[3,2-a:2',3'-h]quinoxaline) were synthesized and characterized by a variety of techniques including X-ray crystallography and 1H-NMR, infrared and electronic absorption spectroscopies. Aided by computational studies, the excited states of these compounds were investigated by transient absorption spectroscopy, the results of which indicate a short-lived initial singlet excited state arising from ligand to ligand charge transition (1LLCT), followed by an intersystem crossing into a relative longer-lived triplet excited state corresponding to a metal centered (3MC) excited state. Investigation of cis-[Rh2(DTolF)2(dpq)2(CH3CN)][BF4]2 in the presence of H+ confirmed its ability to generate H2 in the absence of external photosensitizers in a mixture of THF/H2O upon irradiation (?> 450 nm). Further efforts are directed at improving the stability and water solubility.

Directional charge transfer and highly reducing excited states of new dirhodium(II,II) complexes: Potential applications in solar energy conversion

247th National Meeting (2014)

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