Three platinum(II) terpyridylacetylide charge-transfer complexes possessing a lone ancillary ligand systematically varied in phenylacetylide pi-conjugation length, [Pt((t)Bu(3)tpy)([C CC6H4](n)H)]ClO4 (n = 1-3), are evaluated as photosensitizers (PSs) for visible-light-driven (lambda > 420 nm) hydrogen production in the presence of a cobaloxime catalyst and the sacrificial electron donor triethanolamine (TEOA). Excited-state reductive quenching of the PS by TEOA produces PS- (k(q) scales with the driving force as 1 > 2 > 3), enabling thermal electron transfer to the cobalt catalyst. The initial H-2 evolution is directly proportional to the incident photon flux and visible-light harvesting capacity of the sensitizer, 3 > 2 > 1. The combined data suggest that PSs exhibiting attenuated bimolecular reductive quenching constants with respect to the diffusion limit can overcome this deficiency through improved light absorption in homogeneous H-2-evolving compositions.