Luminescent indium complexes with ONN-donor Schiff bases: synthesis, structure, and DFT investigation

Abstract

A series of dimethylindium complexes of general formula (R-ONN)InMe2 (R = H (1), Me (2), Cl (3), NO2 (4)) have been synthesized by the reaction of Me3In with an appropriate 4-R-2-((pyridin-2-ylmethylene)amino)phenol ((R-ONN)H) and characterized in detail. All complexes obtained are highly stable in air. According to single crystal X-ray analysis, in the crystal state 1-4 form dimers via coordination of the bridging oxygen atom of the phenolate to another indium atom. UV-vis absorption spectra of 1-4, recorded in different organic solvents, show a negative solvatochromic effect: the intense absorption band in the visible region, assigned to the HOMO-LUMO transition, is red-shifted as the complexes are dissolved in successively less polar solvents. This band position also depends on the nature of the R group in the ligand: acceptor Cl and NO2 substituents shift it to the short wavelength region, whereas donor methyl group shifts it in the opposite direction. The same trend is observed by CV: the HOMO-LUMO energy gap narrows as the electron donor properties of the R group increase (from 2.19 eV in 4 to 1.99 eV in 2). All compounds were found to be fluorescent in both dichloromethane (DCM) solution and solid state with broad emission bands between 600 and 700 nm. The fluorescence quantum yields reach 8.6 % in solution and 7.8 % in solid while the lifetimes of 1-4 lie between 2.2 and 2.9 ns. The results obtained were supported by theoretical calculations.