Authors: Nikita A. Shekhovtsov | Елена Б. Николаенкова | Alexey S. Berezin | Victor F. Plyusnin | Katerina A. Vinogradova | Dmitry Yu. Naumov | Natalia V. Pervukhina | А. Ya. Tikhonov | Mark B. Bushuev
Source: Dalton Transactions (article, 2022-01-01)
Field: Materials Science — Materials Chemistry

Tuning ESIPT-coupled luminescence by expanding π-conjugation of a proton acceptor moiety in ESIPT-capable zinc(<scp>ii</scp>) complexes with 1-hydroxy-1<i>H</i>-imidazole-based ligands The emission of ESIPT-fluorophores is known to be sensitive to various external and internal stimuli and can be fine-tuned through substitution in the proton-donating and proton-accepting groups. The incorporation of metal ions in the molecules of ESIPT fluorophores without their deprotonation is an emerging area of research in coordination chemistry which provides chemists with a new factor affecting the ESIPT reaction and ESIPT-coupled luminescence. In this paper we present 1-hydroxy-5-methyl-4-(pyridin-2-yl)-2-(quinolin-2-yl)-1<i>H</i>-imidazole (HLq) as a new ESIPT-capable ligand. Due to the spatial separation of metal binding and ESIPT sites this ligand can coordinate metal ions without being deprotonated. The reactions of ZnHal<sub>2</sub> with HLq afford ESIPT-capable [Zn(HLq)Hal2] (Hal = Cl, Br, I) complexes. In the solid state HLq and [Zn(HLq)Hal2] luminesce in the orange region (<i>λ</i><sub>max</sub> = 600-650 nm). The coordination of HLq by Zn<sup>2+</sup> ions leads to the increase in the photoluminescence quantum yield due to the chelation-enhanced fluorescence effect. The ESIPT process is barrierless in the S<sub>1</sub> state, leading to the only possible fluorescence channel in the tautomeric form (T), S<sub>1</sub><sup>T</sup> → S<sub>0</sub><sup>T</sup>. The emission of [Zn(HLq)Hal2] in the solid state is blue-shifted as compared with HLq due to the stabilization of the ground state and destabilization of the excited state. In CH<sub>2</sub>Cl<sub>2</sub> solutions, the compounds demonstrate dual emission in the UV (<i>λ</i><sub>max</sub> = 358 nm) and green (<i>λ</i><sub>max</sub> = 530 nm) regions. This dual emission is associated with two radiative deactivation channels in the normal (N) and tautomeric (T) forms, S<sub>1</sub><sup>N</sup> → S<sub>0</sub><sup>N</sup> and S<sub>1</sub><sup>T</sup> → S<sub>0</sub><sup>T</sup>, originating from two minima on the excited state potential energy surfaces. High energy barriers for the GSIPT process allow the trapping of molecules in the minimum of the tautomeric form, S<sub>0</sub><sup>T</sup>, resulting in the possibility of the S<sub>0</sub><sup>T</sup> → S<sub>1</sub><sup>T</sup> photoexcitation and extraordinarily small Stokes shifts in the solid state. Finally, the π-system of quinolin-2-yl group facilitates the delocalization of the positive charge in the proton-accepting part of the molecule and promotes the ESIPT reaction. Photochemistry ; Chemistry ; Deprotonation ; Moiety ; Imidazole ; Quantum yield ; Luminescence ; Metal ions in aqueous solution ; Fluorescence ; Metal ; Ion ; Materials science ; Stereochemistry ; Organic chemistry Luminescence and Fluorescent Materials ; Lanthanide and Transition Metal Complexes ; Fluorine in Organic Chemistry