Luminescence of ESIPT-capable zinc(ii) complexes with a 1-hydroxy-1H-imidazole-based ligand: exploring the impact of substitution in the proton-donating moiety

• Published in: Dalton transactions : an international journal of inorganic chemistry 2023-06, Vol.52 (23), p.8114-8134
• Main Authors: Shekhovtsov, Nikita A, Nikolaenkova, Elena B, Vorobyova, Sofya N, Plyusnin, Victor F, Vinogradova, Katerina A, Sukhikh, Taisiya S, Alexsei Ya Tikhonov, Bushuev, Mark B
• Format: Article
• Language: English
• Subjects: Binding sites; Chelation; Conjugation; Coordination compounds; Destabilization; Emission; Energy gap; Excitation; Excitation spectra; Imidazole; Ligands; Photoluminescence; Protons; Zinc; Zinc compounds
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d3dt01190a

The rational design of ESIPT-capable metal complexes (ESIPT – Excited State Intramolecular Proton Transfer) requires two sites, namely, an ESIPT site and a metal binding site, to be spatially separated into the ligand core. Ligands featuring such sites are able to bind metal ions without being deprotonated upon their coordination. The use of ESIPT-capable ligands for the synthesis of metal complexes paves the way toward the exploration of ESIPT in the field of coordination chemistry. In this study, we present a new ESIPT-capable ligand on the base of 1-hydroxy-1H-imidazole, 1-hydroxy-5-methyl-4-[(2,2′-bipyridin)-6-yl]-2-(pyridin-2-yl)-1H-imidazole (HLb), and a series of ESIPT-capable zinc(ii) halido complexes, [Zn(HLb)X2] (X = Cl, Br, I). Due to the incorporation of a (2,2′-bipyridin)-6-yl group at position 4 of the imidazole cycle, HLb acts as an N,N,N-chelating ligand. In the solid state, HLb and [Zn(HLb)X2] emit in the yellow region of the spectrum with excited state lifetimes in the nanosecond domain. Chelation-induced emission enhancement (CHEF) effect in zinc(ii) complexes leads to an increase in the photoluminescence quantum yield (PLQY) for these compounds in comparison with free HLb ligand. The ESIPT process in HLb and [Zn(HLb)X2] is barrierless. The emission of [Zn(HLb)X2] is associated with the S1T → S0 transition in the tautomeric form (T-form). In contrast, due to (i) the dark nature of the S1 state and the bright nature of the S2 state and (ii) the large S1–S2 energy gap, HLb shows weak S2T → S0 fluorescence, in violation of Kasha's rule. Finally, the analysis of atomic charges in a series of ESIPT-capable 1-hydroxy-1H-imidazoles and their zinc(ii) complexes allowed us to reveal the influence of expanding π-conjugation in the proton-donating and proton-accepting moieties on the stabilization/destabilization of the T-form and on the position of the emission band.