Multi-azobenzene moieties on rigid cyclotriphosphazene core: Synthesis, structural characterization, electrochemistry, and photoisomerization study

Abstract

Multi-azobenzene derivatives capable of showing multi-stimuli responsive material can exhibit superior performance in biomedicine, the dye industry, cosmetics, and photonics. Here, a nucleophilic substitution reaction was employed to assemble two (Az2CPZ(biph)2) or four (Az4CPZ(biph)) or six (Az6CPZ) units of azobenzene moieties on the cyclotriphosphazene (CPZ) core. We have observed that the color of each compound varied by changing the number of azobenzene moieties on the CPZ core. These compounds were characterized by elemental analysis, 1H, 13C{1H}, and 31P{1H} NMR techniques. Single crystal X-ray diffraction techniques ensure the trans conformation in the azobenzene for Az2CPZ(biph)2 and Az4CPZ(biph). Electronic spectroscopy forAz2CPZ(biph)2,Az4CPZ(biph), andAz6CPZhaving π-π* transition around 325 nm show a hypsochromic shift with ∼25 nm compared to the parent molecule, 4-hydroxyazobenzene AzH. As a result of the electronic spectroscopy, the cis isomer formation enhanced upon increasing the concentration ofAz2CPZ(biph)2/Az4CPZ(biph)/Az6CPZ in DCM/DMF. The photoisomerization study confirms that these molecules having at least one azo moiety can switch from trans to cis conformation upon exposure to 365 nm light. Interestingly, the thin-layer chromatography (TLC) corroborated the switch of trans to cis conformation occurs at room temperature in a solution state even without UV irradiation. The proton NMR integration providesthe generationofthe cis-azobenzene moiety is about 12–20 % inAz2CPZ(biph)2upon exposure to 365 nm light for 15 to 60 mins. Additionally, TLC verifies the generation of multiple isomers that confirm multi-functional switch behavior when the sample with UV exposure time increases. Owing to these molecules undergoing a isomerization process (trans-azo⇌cis-azo) even without UV, their fluorescence gets quenched in the solution state. Cyclic voltammetry confirms the quasi-reversible type redox behavior of all compounds in dry dichloromethane. We believe the results presented in this work will be significant for dye and biomedical applications.