Solar-driven water reduction, to produce hydrogen gas, is an attractive method to store solar energy in the form of chemical bonds. A typical three-component, homogeneous photosystem for studying water reduction consists of a photosensitizer to absorb light, a water reduction catalyst for accumulating electrons and producing hydrogen, and a sacrificial electron donor to regenerate the catalytic cycle. Our group focuses on the development of Earth-abundant, visible-light-absorbing Cu(I) photosensitizers, in conjunction with highly active Rh(III) water reduction catalysts, to photocatalytically produce hydrogen. While much has been done to understand the excited state dynamics of these new heteroleptic Cu(I) photosensitizers, their long term photostability remains an issue. In this work, a series of heteroleptic Cu(I) photosensitizers containing an alkylated diphosphine backbone were synthesized and characterized to suppress possible decomposition pathways. In previous works, significant decomposition was observed for the parent [Cu(Nixantphos)(biq)]+ complex upon irradiation, which was attributed to photooxidation, excited state deprotonation and/or dimerization of phenoxazine. Alkylation at the phenoxazine backbone not only removes possible excited state deactivation pathways leading to the longer excited state lifetime, but also limits photochemical decomposition pathways such as excited state deprotonation. The results highlight that alkylated Nixantphos Cu(I) PSs maintain favorable properties for visible-light-induced electron transfer; however, long term photostability remains an issue to prevent formation of the inactive, homoleptic [Cu(biq)2]+ species.

Improving photostability of heteroleptic Cu(I) photosensitizers for solar H2 production

ACS Fall 2021

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