A DFT/TDDFT study on the structures, trend in DNA-binding and spectral properties of molecular “light switch” complexes [Ru(phen)2(L)]2+(L = dppz, taptp, phehat)

Theoretical studies on a series of molecular “light switch” complexes [Ru(phen)2L]2+ (phen = 1,10-phenanthroline; L: dppz = dipyrido [3,2-a:2′,3′-c]phenazine; taptp = 4,5,9,18-tetraazaphenanthreno-[9,10-b]triphenylene; phehat = 1,10-phenanthrolino[5,6-b]1,4,5,8,9,12-hexaazatriphenylene) 1–3 have been carried out applying DFT/TDDFT (density functional theory and time-dependent DFT) methods. The geometric and electric structure-characteristics of these complexes have been revealed, and the trend in their DNA-binding constants (Kb), i.e., Kb (2) < Kb (3) < Kb (1), which closely relates to the luminescence properties of the complexes in DNA, has been reasonably explained. The results show that simply increasing the planar area of intercalative ligand may be ineffective on improvement of DNA-binding of resulting complex because of going with the increase in the LUMO (and LUMO + x) energy, but introducing some heteroatoms (e.g., N atom) with stronger electronegativity into the ring skeleton of intercalative ligand should be effective because of the decrease in the LUMO (and LUMO + x) energy to a certain extent. In addition, the spectra of this series of complexes in vacuo are also computed, simulated, and minutely discussed by the DFT/TDDFT methods, and it is interesting to find that the symmetries of the excited and accepting orbitals of the transition with the largest f value are the same.