A 3.0 μs Room Temperature Excited State Lifetime of a Bistridentate Ru~Ⅱ-Polypyridine Complex for Rod-like Molecular Arrays

摘要

Light-induced electron and energy transfer in molecular arrays is being extensively studied in view of artificial photosynthesis and molecular electronics. The ideal photosensitizer for this purpose should have an excited state with high energy and sufficiently long lifetime to promote the desired electron and/or energy transfer processes. In this context, [Ru(bpy)_3]~(2+) derivatives (bpy = 2,2′-bipyridine) have been widely used due to the favorable properties of their lowest excited triplet metal-to-ligand charge transfer (~3MLCT) state, including room temperature ~3MLCT lifetimes up to 1 μs, high emission quantum yields, and strong oxidizing and reducing capability. In tris(diimine)-based donor—chromophore— acceptor assemblies, however, different isomers are formed in most cases, which precludes the desired vectorial electron transfer and complicates kinetic analyses. In contrast, the achiral [Ru(tpy)_2]~(2+) complex (tpy = 2,2′:6′,2″-terpyridine) gives rod-like donor—photosensitizer—acceptor assemblies when substituted at the 4′-position of the tpy ligands, but the complex is practically nonluminescent at room temperature and its short excited state lifetime (τ = 0.25 ns) limits its use as a photosensitizer. The short lifetime is due to deactivation via short-lived metal-centered (~3MC) states that are thermally populated from the ~3MLCT state. Recent strategies to prolong the excited state lifetime of bistridentate Ru~Ⅱ complexes include modified tpy ligands to increase the energy gap between the ~3MLCT and ~3MC states. However, this approach often results in a substantial decrease in ~3MLCT energy that makes these chromophores less useful in photosensitizer applications. An alternative strategy is to increase the ligand field, and thus the energy of the ~3MC states, by making the complex more octahedral. With the insertion of a methylene group between two pyridines in a tpy ligand, six-membered chelates may be formed. Following this strategy, we recently reported a Ru~Ⅱ—bistridentate complex with a much longer excited state lifetime (τ = 15 ns in CH_3CN) than that in [Ru(tpy)_2]~(2+) without any substantial decrease in excited state energy.