Investigating the spectroscopic properties and interaction with DNA of new Ru^II-naphthalimide complexes

The solvothermal synthesis, TD-DFT calculations, and spectroscopic/electrochemical characterization of new ruthenium naphthalimide complexes, namely [Ru(bpy)₂(py-NI-Br)Cl]Cl (1), [Ru(bpy)₂(py-NI-NO₂)Cl]Cl (2), and [Ru(bpy)₂(py-NI-NH₂)Cl]Cl (3) (where py-NI-Br, py-NI-NO₂, and py-NI-NH₂ = 4-bromo-N-(4-pyridyl)-1,8-naphthalimide, 4-nitro-N-(4-pyridyl)-1,8-naphthalimide, and 4-amino-N-(4-pyridyl)-1,8-naphthalimide, respectively) are presented. Complexes 1-3 lose the chlorido ligand when a positive potential is applied, to yield the solvate complex in solution. The Ru-bpy chromophore dominates the photoluminescence profile because the MLCT (metal-to-ligand charge transfer) levels have lower energy than the NI (naphthalimide) IL (intraligand) levels. In aqueous solutions, the chlorido ligand in the ground state is slightly labile. On the other hand, photosubstitution reactions can be tuned by choosing the appropriate solvent. In coordinating solvents with relatively high dielectric constants, the chlorido ligand is substituted, and a charged photoproduct arises. In non-polar solvents, the NI ligand is photolabile, and neutral products emerge. All the complexes investigated herein are solvatochromic. Although the NI IL transitions have charge-transfer character, which is due to the substituent groups, the MLCT transitions of complexes 1-3 feature higher solvatochromic responses. The exception is free py-NI-NH₂, in which the amino group inductive effect produces Δv∼ = 1574 cm^-1. Complexes 1-3 intercalate into DNA better than the corresponding free NI ligands and display moderate K_sv constants (∼10⁴ M^-1). Complex 3 increases the DNA melting temperature by 10 ^oC and has the highest intercalation ability in the series.