Design of\nNear-Infrared Emissive Molecular Switches\nBased on Stilbene-Appended Cyclometalated Bimetallic Ru(II)-Terpyridine\nComplexes

In the present study, we have synthesized and thoroughly\ncharacterized\ntwo Ru(II) dimers with compositions [(ttpy)Ru(tpvpt’)Ru(ttpy)](ClO<sub>4</sub>)<sub>3</sub> and [(ttpy)Ru(t’pvpvpt’)Ru(ttpy)](ClO<sub>4</sub>)<sub>2</sub> incorporating phenylene-vinylene-substituted\nterpyridine bridging ligands capable of coordinating in both an NNN-\nand cyclometalated NNC-fashion. The complexes display strong absorption\nacross the entire UV–vis spectral domain and exhibit luminescence\nin the NIR region (820–850 nm). The N atoms in the outer coordination\nsphere were employed for alteration of the photoredox behaviors of\nthe complexes via acid–base equilibria. Decoordination of the\nRu–C bonds in the presence of acid, followed by recoordination\nin the presence of base and heat, is also possible. Additionally,\nthe phenylene-vinylene units in the bridging ligands facilitate <i>trans</i> → <i>cis</i> and <i>trans–trans</i> → <i>trans–cis</i> isomerization under visible\nlight irradiation. The reverse isomerization (<i>cis</i> → <i>trans</i> and <i>trans–cis</i> → <i>trans–trans</i>) is also achieved upon\nUV light irradiation. Thus, “on–off” and “off–on”\nemission switching is facilitated through a judicious choice of external\nstimuli like acid, base, temperature, or light of particular wavelengths.\nInterestingly, the rate of photoswitching is significantly faster\nin the presence of acid compared to that in the absence of acid. DFT\nand TD-DFT calculations were also conducted to clearly visualize the\nelectronic environment around the complex backbone and also for accurate\nassignment of the absorption and emission bands.