Synthesis, Electrochemical Characterization, and Photophysical Studies of Structurally Tuned Aryl-Substituted Terpyridyl Ruthenium(II) Complexes

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 s-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 (MLCT)-M-3 (triplet metal-to-ligand charge-transfer state) stabilization concomitant with (MC)-M-3 (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 (R-1 = R-2 = H) within heteroleptic complexes as a platform for future semiconductor attachment studies. This substitution leads to further lifetime enhancements, understood as arising from (MLCT)-M-3 stabilization. Such complexes are referred to as [Ru(1)(tbtpy)](2+) (for the acid at R-3) and [Ru(1')(tbtpy)](2+) (for the ester at R-3). 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 (MLCT)-M-3 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+).