Improved photocatalytic hydrogen evolution driven by chloro(terpyridine)platinum(ii) derivatives tethered to a single pendant viologen acceptor.

Three chloro(4'-(N-methylpyridinium)-2,2':6',2''-terpyridine)platinum(ii) (abbreviated as ) derivatives tethered to a single alkyl viologen unit (-(CH2)n-CH2-N(+)C5H4-C5H4N(+)-CH3; abbreviated as -, where n = 1, 3, and 4), i.e., , have been synthesized and investigated in detail. It is shown that the turnover number (TON) for the photocatalytic H2 evolution from water in the presence of a sacrificial electron donor EDTA (ethylenediaminetetraacetic acid disodium salt) is dramatically improved by the attachment of a single alkyl unit (TON = 21.5-25.2, 12 h). Spectrophotometric studies reveal that the photoirradiation of in the presence of EDTA initially leads to the formation of a 1-electron-reduced species, and then to a 2-electron-reduced species, where reductive quenching of a photoexcited species is a major path to the reduced photoproduct in each step. Electrochemical studies show that two consecutive 1-electron reductions at the unit are nearly overlapped with the corresponding reductions at the unit. The 1-electron-reduced species can be thus expressed as either or , while the 2-electron-reduced one as . Moreover, the latter products behave as stacked species involving three types of π-dimer sites, ()2, ()2, and ()(), and do not drive thermal H2 evolution according to the reaction: + 2H(+) → + H2. The H2 evolution from water photocatalyzed by has been found to occur via formation of 3-electron-reduced species; + EDTA + hν → (or ) + EDTA(ox), and (or ) + 2H(+) → (or ) + H2.