Experimental and theoretical exploration of photophysics and trans-cis photoisomerization of styrylbenzene conjugated terpyridine complexes of Ru(II): Strong effect of deprotonation from second coordination sphere

Abstract

We report herein synthesis, characterization, photophysics, and photo-isomerization behaviors of a new class of heteroleptic Ru(II) complexes incorporating styrylbenzene conjugated terpyridine and 2,6–bis(benzimidazole–2–yl)pyridine ligands. All the complexes display moderately strong luminescence at room temperature with appreciably long lifetime varying in the range between 7.2 and 62.2 ns, depending upon the electronic nature of substituent as well as the solvent. The styrylbenzene moiety induces trans to cis isomerization resulting in remarkable changes in absorption and both steady state and time-resolved luminescence spectral characteristics upon irradiating with UV light. Reversal of cis to trans isomerization were also achieved on interacting with visible light as well as UV light of specific wavelength. Removal of two benzimidazole NH protons from the secondary coordination sphere of the complexes were made possible upon treatment with strong base accompanying with substantial modulation of their absorption and emission spectral properties. Interestingly, rate constant as well as quantum yield of photoisomerization dramatically increases upon deprotonation of the imidazole NH protons. Computational studies employing density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were also executed for understanding the electronic charge distribution within the complex backbone and appropriate assignment of the observed spectral bands. In addition, observed spectral trend upon NH deprotonation as well as deprotonation-induced photoisomerization processes were also compared with the calculated results.