Photoinduced Electron Transfer in Pentaammineruthenium(II) Complexes of 1-(4-Cyanophenyl)imidazole

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Abstract
A new bridging ligand, 1-(4-cyanophenyl)imidazole (CPI) has been prepared, as well as its N-methylated derivative 1-methyl-3-(4-cyanophenyl)imidazolium iodide (CPI-Me+I-). The mononuclear and binuclear complexes [(NH3)5Ru−CPI−Me]3+ and [(NH3)5Ru-CPI-Ru(NH3)5]4+ have been obtained. Free CPI is planar, according to theoretical calculations (MMX and MNDO), and its luminescence properties suggest the occurence of a twisted internal charge transfer (TICT) state. The comparison of the two ruthenium complexes reveals the spectral and electrochemical features of coordination by the cyanophenyl or by the imidazole groups. Controlled oxidation of the binuclear complex [(NH3)5Ru−CPI−Ru(NH3)5]4+ yields the mixed valence species [(NH3)5Ru−CPI−Ru(NH3)5]5+ in which the ruthenium coordinated to the cyanophenyl group is ruthenium(II) while the ruthenium linked to imidazole is ruthenium(III). An intervalence band is observed at 640 nm (ε = 188), from which the effective metal−metal coupling through the bridging ligand is determined as 0.032 eV. This value is satisfactorily reproduced by a theoretical calculation using the effective Hamiltonian theory. Finally the binuclear complex exhibits a weak luminescence when excited either on the ligand band near 260 nm or on the metal-to-ligand charge transfer band near 410 nm. The CPI ligand is the first example of a TICT-forming species with appreciable coupling between metallic sites and can be considered as a first step toward a molecular switch.

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