Anion- and solvent induced modulation of photophysical properties of a luminescent bimetallic Ru(II) complex: Experimental and TD-DFT study

Abstract

Anion- and solvent induced modulation of phtophysical behavior of a binuclear Ru(II) complex of the type [(bpy)2Ru(H2L)Ru(bpy)2]4+, where H2L = 2,5-bis[1,10]phenanthroline[4,5-f]-imidazol-2-yl)thiophene} and bpy = 2,2′-bipyridine was thoroughly investigated in this work. The complex shows strong room temperature emission with excited state lifetime varying between 122 and 1089 ns, depending upon the nature of the solvent. The complex possesses two imidazole NH protons in its second coordination sphere. These two acidic NH protons were subjected to successive deprotonation upon increasing the pH of the solution and pKa values were determined in both water and MeCN. Taking advantage of these acidic NH protons, anion sensing behavior of the complex was thoroughly investigated in both acetonitrile and water through multiple optical channels. The complex behaves as sensor for F−, CN−, AcO-and H2PO4- in acetonitrile without much selectivity, while selectivity dramatically increases in water where it exclusively sense CN−. In presence of excess anions, deprotonation of the imidazole polarized N–H fragments takes place which is also signaled by its change of color from yellow to orange visible to the naked eye. Absorption andemission titration data were used to calculate the binding/equilibrium constants of the interaction between the receptor and the anions as well as the detection limit of the receptor towards selected anions. Interestingly, substantial modulation of the lifetime also makes the complex a lifetime-based sensor for selective anions.In conjunction with the experimental investigations, detailed density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations were also performed and a good correlation between the experimental and the TD-DFT calculated results allowed us to provide a detailed assignment of the main absorption and emission spectral features of the complex.