Off–on–off pH luminescence switching and DNA binding properties of a free terpyridine-appended ruthenium complex

Abstract

A ruthenium(II) complex [Ru(bpy)2(HL1)](ClO4)2·3H2O {bpy=2,2′-bipyridine and HL1=2-(2,6-di(pyridin-2-yl)pyridin-4-yl)-1H-imidazo[4,5-f][1,10]phenanthroline}, denoted as RuHL1, was synthesized and characterized by elemental analysis, proton nuclear magnetic resonance spectroscopy, and mass spectrometry. The pH effects on the UV–visible (UV–vis) absorption and emission spectra and the luminescence lifetimes of RuHL1 have been studied, and their ground- and excited-state acid ionization constants were derived. The complex was found to exhibit pH-induced “off–on–off” luminescence switching properties via protonation/deprotonation of the grafted imidazole and terpyridyl groups. A pH-induced emission on–off intensity ratio of 166 was observed as pH was increased from 2.00 to 5.44, which is one of the largest values observed among imidazole-containing Ru(II) complex-based acidic-pH-induced pH luminescence switches. RuHL1 also strongly binds to calf thymus DNA in mixed binding modes involving classic intercalation and partial intercalation, as determined by UV–vis absorption and emission spectrophotometric DNA titration, ethidium bromide displacement, DNA denaturation and DNA viscosity measurements.