Luminescent indium complexes with ONN-donor Schiff bases: syntheses, structures, and DFT investigations

Abstract

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