Swift photoswitching in a binuclear Zn(ii) metallacycle relative to a salen-type ligand

Abstract

The synthesis, characterization and photoswitching behavior of a new salen type Schiff base N,N'-bis(2-hydroxy-5-phenylazobenzilidene)-2,4,6-trimethylbenzene-1,3-diamine (H₂L) and a binuclear zinc(ii) metallacycle [{Zn(L)}2·2H2O] (1) have been described. Both H₂L and 1 have been characterized by satisfactory elemental analyses, spectral (FT-IR, (1)H, (13)C NMR, ESI-MS, electronic absorption, emission) and electrochemical studies. Crystal structures of both H₂L and 1 have been authenticated by single crystal X-ray diffraction analyses. These exhibit trans-cis photoisomerization upon exposure to UV light (365 nm) and get back to a more stable trans-form after withdrawal of the light. Electronic absorption, emission, (1)H NMR and cyclic voltammetric studies revealed that trans-cis isomerization in metallacycle 1 is rather rapid (~5.0 s) relative to H₂L (~25 s) which has been supported by theoretical studies (DFT). Relatively fast photoisomerization in 1 compared to H₂L is facilitated by a small energy gap between HOMO levels of the trans- and cis-isomers. The percentage trans-cis conversion ratio for both H₂L and 1 has been evaluated (55-45, H₂L; 60-40%, 1) by (1)H NMR studies.

Azobenzene-Appended Bis-Cyclometalated Iridium(III) Bipyridyl Complexes

“…Two different arrangements with 50% probability each have been detected (Figure S15, SI). The same type of crystallographic disorder was previously reported for other azobenzene-containing complexes . As expected, all the complexes presented the azobenzene fragments in the thermodynamically most stable E form.…”

Recent Developments in the Preparation and Chemistry of Metallacycles and Metallacages via Coordination

“…Photoswitching of SCCs containing functionally similar azobenezene-derived sites, inducing cis/trans-isomerization, has been explored on Zn 2 metallacycles by Pandey and co-workers. 187 Newkome and Wesdemiotis have explored 2,2′:6′,2″terpyridine (tpy)−metal coordination for SCC formation to great effect. 188,189 In 2012 they continued their work with this motif with the formation of rhomboidal and trigonal metallacycles using Zn(II) and Cd(II) ions.…”

“…Upon irradiation with shorter wavelengths, larger metallacycles form due to mixed E - and Z -ligands in solution. Photoswitching of SCCs containing functionally similar azobenezene-derived sites, inducing cis / trans -isomerization, has been explored on Zn 2 metallacycles by Pandey and co-workers …”

Structural Correlations between Luminescent Properties and Excited State Internal Proton Transfer in Some Zinc(II) N,N′-Bis(salicylidenes)

“…Organic electronics is a field that has attracted increasing attention due to their practical applications − as light emitting diodes, , solar cells, transistors, − super capacitors, sensors and actuators, − photoswitching, , etc. Many materials can be used as organic semiconductors, including coordination compounds, polymers, hybrids, and nanocomposites. − Aromatic molecules with extended π frameworks are particularly interesting for this aim .…”

“…Salicylidenes are aromatic compounds that have been the subject of extensive research for many years due to their applications in different fields such as catalysis, − molecular architectures, − magnetism, , materials chemistry, biomarkers, and optical chemosensors, among others. − These tetradentate chelating Schiff bases can coordinate transition metals, allowing for the tuning of their luminescent properties by structural modification in the ligand framework and by changing the metal in their coordination site. ,,,− Electroluminescent compounds can be obtained through the coordination of zinc(II) ions by salicylidenes. − Metal-to-ligand and intraligand charge transfer processes can be observed in the absorption spectrum of zinc(II) salicylidene compounds …”