Bistridentate ruthenium(II)polypyridyl-type complexes with microsecond ³MLCT state lifetimes: Sensitizers for rod-like molecular arrays

A series of bistridentate ruthenium(II) polypyridyl-type complexes based on the novel 2,6-di(quinolin-8-yl)pyridine (dqp) ligand have been synthesized and their photophysical properties have been studied. The complexes are amenable to substitution in the 4-position of the central pyridine with conserved quasi-C_2v symmetry, which allows for extension to isomer-free, rod-like molecular arrays for vectorial control of electron and energy transfer. DFT calculations performed on the parent [Ru(dqp)₂]²⁺ complex (1) predicted a more octahedral structure than in the typical bistridentate complex [Ru(tpy)₂]²⁺ (tpy is 2,2′:6′,2″-terpyridine) thanks to the larger ligand bite angle, which was confirmed by X-ray crystallography. A strong visible absorption band, with a maximum at 491 nm was assigned to a metal-to-ligand charge transfer (MLCT) transition, based on time-dependent DFT calculations. 1 shows room temperature emission (Φ = 0.02) from its lowest excited ( ³MLCT) state that has a very long lifetime (τ = 3 μs). The long lifetime is due to a stronger ligand field, because of the more octahedral structure, which makes the often dominant activated decay via short-lived metal-centered states insignificant also at elevated temperatures. A series of complexes based on dqp with electron donating and/or accepting substituents in the 4-position of the pyridine was prepared and the properties were compared to those of 1. An unprecedented ³MLCT state lifetime of 5.5 μs was demonstrated for the homoleptic complex based on dqpCO₂Et. The favorable photosensitizer properties of 1, such as a high extinction coefficient, high excited-state energy and long lifetime, and tunable redox potentials, are maintained upon substitution. In addition, the parent complex 1 is shown to be remarkably photostable and displays a high reactivity in light-induced electron and energy transfer reactions with typical energy and electron acceptors and donors: methylviologen, tetrathiofulvalene, and 9,10-diphenylanthracene. This new class of complexes constitutes a promising starting point for the construction of linear, rod-like molecular arrays for photosensitized reactions and applications in artificial photosynthesis and molecular electronics.