Structural and Electronic Tuning of Luminescent ZnII Complexes Based on an o-Terphenyl Ligand Motif

Abstract

The structural and photophysical properties of five chiral Zn complexes incorporating a carbazolate (Cz) donor that is electronically decoupled from a pyridyl acceptor by an ortho-connection to a bridging phenylene group are presented. The bidentate ligand in the unsubstituted bis-ligated parent complex was methylated at key positions to constrain the torsional freedom of the donor/acceptor moieties, resulting in three structurally modified bis-ligated derivatives, all exhibiting energy gaps between the singlet and triplet excited states (ΔE_ST) between 22 and 27 meV. Methylation improves the photoluminescence quantum yield (up to 30% in solution), while the low ΔE_ST of these complexes allows for dual-emission properties in all of the bis-ligated derivatives. Structural modification of the Cz/pyridyl ligand was also investigated by linking the unsubstituted bidentate ligand to generate a tetradentate, tetrapodal ligand. The solution-state structure of the tetradentate ligand is similar in its free and ligated forms, featuring a binding site reminiscent of enzymes and metal-sequestering ligands. The resulting tetradentate complex [Zn(N₂R₂)] shows enhanced through-bond conjugation, increasing the ΔE_ST to 89 meV, thereby eliminating the dual-emission characteristics of the bis-ligated complexes. Furthermore, this complex shows a 50-fold improvement in hydrolytic stability in organic solution relative to the parent complex. These compounds and their analyses are intended to enrich the understanding of compounds exhibiting through-space charge transfer and guide the search for earth-abundant metal complexes for applications in photosensitization and luminescence.