Tunability of triplet excited states and photophysical behaviour of bis-cyclometalated iridium(iii) complexes with imidazo[4,5-f][1,10]phenanthroline

Abstract

This is a comprehensive study of the photophysical behaviour of heteroleptic iridium(III) complexes with imidazo[4,5-f][1,10]phenanthroline (imphen) as an ancillary ligand, represented by the general formula [Ir(N∩C)₂(imphen)]PF₆. As cyclometalating ligands, 2-phenylpyridine (Hppy), 2-phenylquinoline (Hpquin), 2-phenylbenzothiazole (Hpbztz), and 2-(2-pyridyl)benzothiophene (pybzthH) were used. The impact of structural modifications of cyclometalating ligands was widely explored by a combination of steady-state and time-resolved optical techniques accompanied by theoretical calculations. We evidenced that the cyclometalating ligands induce essential changes in the nature of the emissive excited state and the emission characteristics of [Ir(N∩C)₂(imphen)]PF₆. While the complex [Ir(ppy)₂(imphen)]PF₆ (1) is a typical ³MLLCT emitter, the lowest triplet states of [Ir(pquin)₂(imphen)]PF₆ (2), [Ir(pbztz)₂(imphen)]PF₆ (3) and [Ir(pybzth)₂(imphen)]PF₆ (4) have a predominant ³LC_N∩C character. The phosphorescence colour of the investigated Ir(III) complexes changes from greenish-yellow to red, their quantum yields vary from 56 to 2%, and their triplet excited-state lifetimes fall in the 743–3840 ns range. The highest photoluminescence quantum yield was revealed for 2 in CH₂Cl₂, while complex 3 in MeCN shows the most pronounced increase in the lifetime. Both complexes 2 and 3 show an increased efficiency of singlet oxygen generation. The herein discussed structure–property relationships are of high significance for controlling photoinduced processes in heteroleptic iridium(III) complexes with the imphen-based ancillary ligand, and making further progress in effectively tuning the emission energies, quantum yields and excited-state lifetimes of these systems by structural modifications of cyclometalating ligands, especially the π-conjugation, the position of the N-donor and the presence of sulfur heteroatoms.