Tuning the photophysical and electrochemical properties of iridium(III) 2-aryl-1-phenylbenzimidazole complexes

A series of heteroleptic bis-cyclometalated iridium(III) complexes, [Ir(cpbi)2(H2dcbpy)][PF6] (1), [Ir(pbi)2(H2dcbpy)][PF6] (2), and [Ir(mpbi)2(H2dcbpy)][PF6] (3), where pbi = 1,2-diphenylbenzimidazole, cpbi = 2-(4-chlorophenyl)-1-phenylbenzimidazole, mpbi = 2-(3,4-dimethoxyphenyl)-1-phenylbenzimidazole, and H2dcbpy = 2,2′-bipyridine-4,4′-dicarboxylic acid has been synthesized and characterized by elemental analysis, 1H, 31P NMR, and high resolution mass-spectra. Molecular structure of complex 3 has been determined from single-crystal X-ray analysis. The complexes exhibit absorption up to 550 nm with molar absorptivities of 2500 M−1 cm−1. They have strong luminescence in broad yellow-to-red region in solutions at room temperature. While chloro-substituent (complex 1) causes a little hypsochromic shift of the absorption maxima compared to unsubstituted 2, introduction of two methoxy-groups (complex 3) gives rise to a bathochromic shift of about 100 nm. Alternating current voltammetry studies of the complexes indicates reversible oxidation and reduction potentials. Calculated excited state oxidation potentials for 1–3 are negative enough to efficiently inject electrons into the conduction band of TiO2 (EF ≈ −0.5 V). Thus, from the electrochemical point of view, all the complexes, especially 3, are good candidates for operation in DSSC as dyes.