Vectorial electron transfer in donor-photosensitizer-acceptor triads based on novel bis-tridentate ruthenium polypyridyl complexes

The first examples of rodlike donor-photosensitizer-acceptor arrays based on bis-2,6-di(quinolin-8-yl)pyridine Ru^II complexes 1a and 3a for photoinduced electron transfer have been synthesized and investigated. The complexes are synthesized in a convergent manner and are isolated as linear, single isomers. Time-resolved absorption spectroscopy reveals long-lived, photoinduced charge-separated states (τ_CSS (1a)=140 ns, τ_CSS (3a)=200 ns) formed by stepwise electron transfer. The overall yields of charge separation (≥50% for complex 1a and ≥95% for complex 3a) are unprecedented for bis-tridentate Ru^II polypyridyl complexes. This is attributed to the long-lived excited state of the [Ru(dqp)₂]²⁺ complex combined with fast electron transfer from the donor moiety following the initial charge separation. The rodlike arrangement of donor and acceptor gives controlled, vectorial electron transfer, free from the complications of stereoisomeric diversity. Thus, such arrays provide an excellent system for the study of photoinduced electron transfer and, ultimately, the harvesting of solar energy.