Effect of bridging units on the photophysical properties of 4-NEt2-appended salen-indium complexes

Abstract

A series of 4-NEt2-appended salen−indium complexes bridged by different diimine units (ethylene [Et, 1], phenylene [Ph, 2], and benzonitrile [PhCN, 3]) with different electronic effects were prepared in high yields by a one-pot synthetic procedure. Among them, the solid-state structure of 1 was identified by X-ray crystallography, featuring a square-pyramidal structure around the indium center. Ultraviolet–visible (UV/Vis) absorption and emission spectra of 1–3 showed typical ππ* electronic transitions centered on the salen ligands, which underwent gradual redshifts as the electron-withdrawing ability of the bridging units increased. Such bathochromic shifts resulted from lowering the energy levels of the lowest unoccupied molecular orbital (LUMO). In addition, the full width at half maxima for the emissions of 1–4 gradually decreased with increasing electron-accepting property of the bridging units. The observed photophysical properties were further supported by theoretical calculations.