Synthesis, Photophysics, and Switchable Luminescence Properties of a New Class of Ruthenium(II)–Terpyridine Complexes Containing Photoisomerizable Styrylbenzene Units

[Image: see text] We report here the synthesis and structural characterization of a new class of homoleptic terpyridine complexes of Ru(II) containing styrylbenzene moieties to improve room-temperature luminescence properties. Solid-state structure determination of 2 was done through single-crystal X-ray diffraction. Tuning of photophysical properties was done by incorporating both electron-donating and electron-withdrawing substituents in the ligand. The complexes exhibit strong emission having lifetimes in the range of 10.0–158.5 ns, dependent on the substituent and the solvent. Good correlations were also observed between Hammett σ(p) parameters with the lifetimes of the complexes. Styrylbenzene moieties in the complexes induce trans–trans to trans–cis isomerization accompanied by huge alteration of their spectral profiles upon treating with UV light. Reversal of trans–cis to trans–trans forms was also achieved on interacting with visible light. Change from trans–trans to the corresponding trans–cis form leads to emission quenching, whereas trans–cis to the corresponding trans–trans form leads to restoration of emission. In essence, “on–off” and “off–on” photoswitching of luminescence was observed. Calculations involving density functional theory (DFT) and time-dependent-DFT methods were performed to understand the electronic structures as well as for appropriate assignment of the absorption and emission bands.