Impacts of terminal modification of [Ru(phen)2dppz](2+) on the luminescence properties: a theoretical study

[Ru(phen)2dppz](2+) and other closely related ruthenium(II) complexes containing π-extended ligands were found to be non or weakly emissive in water, while exhibiting significant luminescence intensity growth when bound to DNA, however, a satisfactory interpretation has not been provided on this "light switch" mechanism. In the present study, we investigated the vertical transitions and triplet excited states of [Ru(phen)2dppz](2+) (1), [Ru(phen)2dppzi](2+) (2) and [Ru(phen)2dppz-idzo](2+) (3) in the gas phase and aqueous solution, through time dependent-density functional theory (TDDFT). Based on the optimized (3)MLCT and (3)LLCT structures and energies, we found that the (3)MLCT state might be responsible for the emissions of the complexes. Interesting connections between the singlet vertical transitions and the luminescence properties were noticed. Through ZORA-TDDFT calculation with perturbative SOC, we evaluated the intersystem crossing between the lowest singlet excited state, and both (3)MLCT state and (3)LLCT state, which gave a reasonable explanation for the luminescence properties of these complexes.