Excitation Dependent Multiemissive Behaviour of Triimidazole Carboxylic Acid anti‐Kasha Luminophore and its Mononuclear Metal Complexes

Materials based on small molecules characterized by multi-component short and long-lived emissive behaviour are extremely desirable for various applications. The derivative of cyclic triimidazole (TT) functionalized with a carboxylic group, namely TT-COOH, is isolated, investigated and used as coordinative ligand to obtain a series of isomorphous mononuclear complexes of general formula [M(TT-COO)2(H2O)2] (M=Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)). Crystals of TT-COOH display aggregation induced enhanced emission (AIEE) comprising dual fluorescence and dual phosphorescence of molecular origin together with an aggregation induced long lived, low energy phosphorescence. Emissive features are retained in crystals of the Zn(II) and Cd(II) complexes, where the metal plays the role of an external heavy atom perturber, but strongly quenched in the others. The mechanisms involved in the photophysics of TT-COOH and its metal complexes have been disclosed thanks to combined structural, spectroscopic and computational investigations which reveal the presence of anti-Kasha emissions.