Synthesis, electrochemical potentials, static emission, and temperature-dependent excited-state lifetimes of several 4'-aryl-substituted terpyridyl complexes of ruthenium(II) are reported. Synthetic tuning is explored within three conceptual series of complexes. The first series explores the impact of introducing a strong σ-donating 4,4',4″-tri-tert-butyl-2,2':6',2″-terpyridine (tbtpy) opposite to an arylated terpyridine ligand 4'-(4-methylphenyl)-2,2':6',2″-terpyridine (ttpy). It is found that (3)MLCT (triplet metal-to-ligand charge-transfer state) stabilization concomitant with (3)MC (triplet metal-centered state) destabilization in the heteroleptic parent complex [Ru(ttpy)(tbtpy)](2+) leads to an extended excited-state lifetime relative to the structurally related bis-homoleptic species [Ru(ttpy)2](2+). The second series explores the impact of introducing a carboxylic acid or a methyl ester moiety at the para-position of the arylterpyridyl ligand (R1 = R2 = H) within heteroleptic complexes as a platform for future semiconductor attachment studies. This substitution leads to further lifetime enhancements, understood as arising from (3)MLCT stabilization. Such complexes are referred to as [Ru(1)(tbtpy)](2+) (for the acid at R3) and [Ru(1')(tbtpy)](2+) (for the ester at R3). In the final series, methyl substituents are sequentially added at the R1 and R2 positions for both the acid ([Ru(2)(tbtpy)](2+) and [Ru(3)(tbtpy)](2+)) and ester ([Ru(2')(tbtpy)](2+) and [Ru(3')(tbtpy)](2+)) analogues to eventually explore dynamical electron transfer coupling at dye/semiconductor interfaces. In these complexes, sequential addition of steric bulk decreases excited state lifetimes. This can be understood to arise primarily from the increase of the (3)MLCT level, as excited-state electron delocalization is limited by inter-ring twisting in the lower-energy arylated ligand. The introduction of a dimethylated sterically encumbered ligand lead to a notable 14-fold increase in knr from [Ru(1')(tbtpy)](2+) to [Ru(3')(tbtpy)](2+) (or [Ru(1)(tbtpy)](2+) to [Ru(3)(tbtpy)](2+)).

To explore the impact of dye structure on photoinduced interfacial electron-transfer (ET) processes, a series of systematically tuned 4′-aryl-substituted terpyridyl ruthenium­(II) complexes have been studied in TiO2 film and dye-sensitized solar cell (DSSC) device settings. Structural tuning is achieved by the introduction of methyl substituents at the ortho positions of a ligand aryl moiety. Solar power conversion efficiencies are measured, and these values are deconstructed to better understand the fundamental processes that control light-to-current conversion. Injection yields are identified as the primary factor limiting efficiencies, due in large part to significant nonradiative decay pathways in these bis-terpyridyl Ru­(II) systems. Encouragingly, the addition of methyl steric bulk is found to inhibit charge recombination, with measured recombination lifetimes increasing by over 12-fold across the series of structurally tuned complexes. If injection yields can be improved, the structural tuning of recombination rate constants may be an important design strategy for improving solar conversion efficiency in solar cells and water-splitting devices.

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