Emission property and DFT calculation for the 3MLCT luminescence of Ru(bpy)2(L)2+ complex

Abstract

Electronic structures of ruthenium complexes [Ru(bpy)2(L)]2+ were studied by DFT calculations, where bpy is 2,2′-bipyridine and L's are its derivatives. They are parent (1) and five complexes with L = 4,4′-dimethyl-2,2′-bipyridine (2), 6,6′-dimethyl-2,2′-bipyridine (3), biquinoline (4), 5-nitro-1,10-phenanthroline (5) and two acetonitrile molecules (6). These complexes were characterized by ESI-MS, 1H NMR, UV–vis spectroscopy and elementary analysis. The crystal structures of 3·(PF6)2, 4·(PF6)2, 5·(PF6)2 and 6·(PF6)2 were also determined.Spin densities were analyzed to characterize the excited triplet states in those complexes. It was found that the complexes have three triplet (called here 3MLCT, 3MC1 and 3MC2) states. The 3MC1 and 3MC2 states have elongated RuN bonds and large spin densities on Ru2+. Those states are stable and would lead to the nonradiative relaxation to the ground state. In particular, those of 3, 5 and 6 are very stable, which results in their poor quantum yields (Φ < 0.01) of the emissions. In contrast, both 1 and 2 showed the 3MLCT states. Energy gaps between triplet and ground states were compared with energies corresponding to wavelengths of emission spectra of 1–5. Good agreement between the calculated and the experimental energies was found. Moreover, dual emission which depends on the excitation wavelength was observed in complex 4.