Unusual solvent dependent emission property of two new ruthenium(II) complexes

Two ruthenium(II) mixed ligand complexes [Ru(bpy)2(imiphen)](PF6)2, 1 and [Ru(bpy)2(phimiphen)](PF6)2, 2, where imiphen = 2-(1H-imidazol-2yl)-1H-imidazo[4,5-F][1,10]phenanthroline and phimiphen = 2-(1H-benzimidazol-2yl)-1H-imidazo[4,5-F][1,10]phenanthroline have been synthesized and characterized using various spectroscopic techniques like ESI mass, electronic spectroscopy and fluorescence spectroscopy. Both these complexes exhibit unusual fluorescence spectra. Complexes 1 and 2 show emission maxima at 620 and 617 nm when excited at their MLCT band at 450 nm. The emission intensity has been found to depend on the nature of the solvent. Emission intensity of the aqueous solution of complexes 1 and 2 has been found to be 75–143% greater than the emission intensity of acetonitrile solution of these two complexes. This increase in the emission intensity in aqueous solution is accompanied by an increase in the lifetime of the excited state of the complex in aqueous solution. Emission intensity increase observed for these complexes is due to the involvement of the H2O molecule in the hydrogen bonding with imidazole N–H of the ligand, making the ligand rigid. On the other hand, in acetonitrile medium because of lack of hydrogen bonding, the imidazole ring can have free rotation.