Photophysical study of a family of [Ru(phen)2(Mendpq)]2+ complexes in different solvents and DNA: a specific water effect promoted by methyl substitution

Abstract

The synthesis and spectroscopic characterisation of a family of ruthenium complexes [Ru(phen)₂(Me_ndpq)]²⁺ (n = 0, 1, 2) are reported and the photophysical effect of the progressive methyl substitution on the dpq ligand ascertained in a range of solvents and in the presence of DNA: [Ru(phen)₂(dpq)]²⁺ (1), [Ru(phen)₂(Medpq)]²⁺ (2) and [Ru(phen)₂(Me₂dpq)]²⁺ (3) (where dpq is dipyrido[3,2-f:2′,3′-h]-quinoxaline; Medpq is 2-methyldipyrido[3,2-f:2′,3′-h]-quinoxaline; Me₂dpq is 2,3-dimethyldipyrido[3,2-f:2′,3′-h]-quinoxaline and phen is 1,10-phenanthroline). The increase in electron density following substitution renders the quinoxaline nitrogen atoms more basic in the ground state to yield pK_a values of −1.9, −2.3, and −2.7 for 3, 2, and 1, respectively. The methyl groups have a pronounced effect on the excited state photophysics of 1–3. In organic solvents, the non-radiative decay constant correlates well with the solvent polarity parameter π*, with the effect being more pronounced with increasing methyl substitution. On the contrary, in aqueous solution there is a ca. four-fold decrease in the non-radiative decay constant upon methyl substitution. This ‘specific water effect’ may be explained in terms of deactivation of the excited state by hydrogen bonding interactions between water and the quinoxaline nitrogen atoms, which is reduced on moving from 1 to 3. The excited state lifetimes and luminescence intensities for 1 substantially increase when bound to DNA in aerated aqueous solutions, with a six-fold increase, compared to a more moderate three-fold increase for 2 and 3.