Synthesis, Structural Characterization and Rigidity Dependent Dual Luminescence Behavior of High-Energy Phosphorescent Zinc(II) Cyclic (Alkyl)(Amino)carbene Complexes

Abstract

The study explores the synthesis, structural characterization, and photophysical properties of cyclic (alkyl)(amino)carbene (cAAC) zinc(II) complexes, focusing on their unique rigidity-dependent dual phosphorescence behavior. Starting from previously reported dimeric [Zn2X2(μ-X)2(cAAC)2] (X = Cl, Br), halide abstraction with Et3Si+ gives dicationic tetramers with an adamantyl-like Zn4X6 core, while the reaction with Na(acac) (acac = acetylacetonate) or KCz (Cz = carbazolate) leads to ligand exchange and isolation of [ZnCl(acac)(cAAC)] (11) and [ZnCl(Cz)(THF)(cAAC)] (12), respectively. While 12 displays orange phosphorescence from 3LLCT (LLCT = ligand-to-ligand charge transfer) states both at room temperature and at 77 K, complex 11 and the tetrameric compounds exhibit excitation wavelength-dependent unusual high-energy UV-blue phosphorescence and green-yellow luminescence. The experimental data suggest that high-energy excitation leads to emission primarily from high-energy states due to strong Franck-Condon overlaps, while low-energy excitation enables direct triplet state population and geometry-driven transitions. This behavior, influenced by the molecular rigidity, appears to be the result of an equilibrium between different excited states and emphasize the impact of the molecular structure on the emission profiles.