Design of Near-Infrared Emissive Molecular Switches Based on Stilbene-Appended Cyclometalated Bimetallic Ru(II)-Terpyridine Complexes

Abstract

In the present study, we have synthesized and thoroughly characterized two Ru(II) dimers with compositions [(ttpy)Ru(tpvpt')Ru(ttpy)](ClO 4 ) 3 and [(ttpy)-Ru(t'pvpvpt')Ru(ttpy)](ClO 4 ) 2 incorporating phenylene-vinylene-substituted terpyridine bridging ligands capable of coordinating in both an NNN-and cyclometalated NNC-fashion. The complexes display strong absorption across the entire UV−vis spectral domain and exhibit luminescence in the NIR region (820−850 nm). The N atoms in the outer coordination sphere were employed for alteration of the photoredox behaviors of the complexes via acid−base equilibria. Decoordination of the Ru−C bonds in the presence of acid, followed by recoordination in the presence of base and heat, is also possible. Additionally, the phenylene-vinylene units in the bridging ligands facilitate trans → cis and trans−trans → trans−cis isomerization under visible light irradiation. The reverse isomerization (cis → trans and trans−cis → trans−trans) is also achieved upon UV light irradiation. Thus, "on−off" and "off−on" emission switching is facilitated through a judicious choice of external stimuli like acid, base, temperature, or light of particular wavelengths. Interestingly, the rate of photoswitching is significantly faster in the presence of acid compared to that in the absence of acid. DFT and TD-DFT calculations were also conducted to clearly visualize the electronic environment around the complex backbone and also for accurate assignment of the absorption and emission bands.