Intercalative binding, light-switch behavior, and theoretical analysis of Ru(bpy)2dppz-BTDZ]2+: A thiadiazole-extended ruthenium(II) complex.

The new ruthenium(II) polypyridyl complex [Ru(bpy)₂dppz-BTDZ]2+, featuring a thiadiazole-substituted dppz ligand, was synthesized and characterized to investigate its DNA-binding and photophysical properties. Its interaction with calf thymus DNA (Ct-DNA) was studied through UV-visible and emission spectroscopy, competitive binding assays with ethidium bromide (EB), anionic quenching with K4[Fe(CN)6], viscosity and molecular modeling. The results indicate that the complex intercalates into DNA, with a binding constant (Kb) of 6.57 ± 0.9 × 106 M-1 and has a 7 % enhancement in the light-switch effect compared to [Ru(bpy)₂dppz]2+. DFT calculations characterized the excited states, which were compared to those of [Ru(bpy)₂dppz]2+. The presence of the thiadiazole group promotes mixing of MLCT states. Franck-Condon analysis of emission spectra in MeCN, water, and DNA allowed for the estimation of the excited-state free energy. In water, the emissive bright state lies approximately 1300 cm-1 higher than in DNA and approximately 1100 cm-1 higher than in MeCN. This energy trend is consistent with the light-switch behavior observed in this class of complexes. Moreover, the bright-state energies correlated very well with all the observed lifetimes. Docking and molecular dynamics simulations revealed stable intercalative binding in the DNA minor groove, with the Δ-isomer showing enhanced stability.