Study of a carbazole–bromobenzothiadiazole derived fluorescent molecular rotor: crystal structure, redox activity, and solvatofluorochromic effects

Abstract

This work investigated the properties of a four-component molecular ensemble, in which a carbazole (CBz) donor and a benzothiadiazole (BTD) acceptor are connected through a phenylthiophene (PT) bridge. The multiple rotational freedoms at the C–C single bonds that interconnect the different arene units render this compound a molecular rotor that shows interesting intramolecular N⋯S contact and various intermolecular non-covalent interactions in the solid state. Detailed structural and crystal packing properties were elucidated by single crystal X-ray diffraction analysis in conjunction with Hirshfeld surface analysis and density functional theory (DFT) calculations. Cyclic voltammetric (CV) analysis indicated that this donor–acceptor ensemble exhibits electrochemical amphoteric redox behavior with a relatively narrow electronic bandgap (E_g) of 1.95 eV. Moreover, this CBz–PT–BTD molecular rotor was found to emit orange-to-red luminescence in different organic solvents. The solvatochromic behavior was due to twisted intramolecular charge-transfer (TICT) emission and agreed well with the Lippert–Magata relationship and solid-state fluorescence data. Moreover, the fluorescence efficiency of this molecular rotor showed dependence on solvent viscosity as well as other solvent parameters.