N-Hydroxy-N-oxide photoinduced tautomerization and excitation wavelength dependent luminescence of ESIPT-capable zinc(II) complexes with a rationally designed 1-hydroxy-2,4-di(pyridin-2-yl)-1H-imidazole ESIPT-ligand

The ability of 1-hydroxy-1H-imidazoles to undergo proton transfer processes and to exist in N-hydroxy and N-oxide tautomeric forms can be used in coordination chemistry for the design of ESIPT-capable complexes. A series of ESIPT-capable zinc(II) complexes [Zn(HL)Hal2] (Hal = Cl, Br, I) with a rationally designed ESIPT-ligand 1-hydroxy-5-methyl-2,4-di(pyridin-2-yl)-1H-imidazole (HL) featuring spatially separated metal binding and ESIPT sites have been synthesized and characterized. Crystals of these compounds consist of a mixture of two isomers of [Zn(HL)Hal2]. Only a major isomer has a short intramolecular hydrogen bond O-H⋯N as a pre-requisite for ESIPT. In the solid state, the complexes [Zn(HL)Hal2] demonstrate temperature- and excitation wavelength dependent fluorescence in the cyan region due to the interplay of two intraligand fluorescence channels with excited state lifetimes spanning from 0.2 to 4.3 ns. The coordination of HL by Zn²⁺ ions results in an increase in the photoluminescence efficiency, and the photoluminescence quantum yields (PLQYs) of the complexes reach 12% at λ_ex = 300 nm and 27% at λ_ex = 400 nm in comparison with the PLQY of free HL of ca. 2%. Quantum chemical calculations indicate that N-hydroxy-N-oxide phototautomerization is both thermodynamically and kinetically favourable in the S₁ state for [Zn(HL)Hal2]. The proton transfer induces considerable geometrical reorganizations and therefore results in large Stokes shifts of ca. 230 nm. In contrast, auxiliary ESIPT-incapable complexes [ZnL₂][Zn(OAc)₂]₂·2H₂O and [ZnL₂][ZnCl₂]₂·4H₂O with the deprotonated ligand exhibit excitation wavelength independent emission in the violet region with the Stokes shift reduced to ca. 130 nm.

Unveiling the effect of solvent polarity on the excited state intramolecular proton transfer mechanism of new 3-hydroxy-4-pyridylisoquinoline compound

The new 3-hydroxy-4-pyridylisoquinoline compound is attractive and promising lead structure in drug discovery. The pronounced sensitivity of its emission property toward solvent polarity effect was presented in experiment (J. Org. Chem, 2019, 84, 3011). Nevertheless, the experiment was lack of solvent polarity effect on the excited state intramolecular proton transfer (ESIPT) mechanism in detail. In this study, the ESIPT process of this molecule in different polarity solvents were comprehensively expounded by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In order to ensure the accuracy of the experiment and roundly explore in theoretical level, two ESIPT pathways (1 and 2) based on the N1 and N2 forms of studied molecule were proposed, among which the ESIPT pathway 1 was derived from experiment. The calculated electronic spectrum of both N1 and N2 forms were rather comparable with the experiment. The calculated intramolecular hydrogen bond (IHB) parameters and infrared (IR) vibration spectra determined the enhancement of IHBs at the S₁ state under different solvents for both N1 and N2 forms. The frontier molecular orbitals (FMOs) analysis proved that the intramolecular charge transfer (ICT) taken place during photoexcitation. The potential energy curves (PECs) at the S₀ and S₁ states were constructed to illustrate the solvent polarity effect on ESIPT mechanism. According to potential energy barriers (PEBs) on the PECs at S₁ state, it is concluded that the ESIPT pathway 1 was forbidden with exceedingly high PEBs (24.585-25.322 kcal/mol), while the ESIPT pathway 2 was feasible with enough low PEBs (0.100-0.510 kcal/mol), which suggested the inconsequence of the experiment. Based on the PEBs of ESIPT pathway 2 in different solvent, the effect of solvent polarity on ESIPT mechanism was depicted. The results are as follows: the S₁ state IHB intensity was enhanced with increasing solvent polarity; the extent of ICT was decreased with the increment of solvent polarity; the S₁ state PEB was decreased as the solvent polarity increased. Indeed in short, the ESIPT reaction became more and more likely as the solvent polarity enhanced. We believe that this investigation will be useful to the utilization and development of property for such photochemical substances.

Theoretical research on excited-state intramolecular proton coupled charge transfer modulated by molecular structure

At the TD-B3LYP/TZVP/IEFPCM theory level, we have theoretically studied the excited-state intramolecular proton coupled charge transfer (ESIPCCT) process for both 4′-N,N-diethylamino-3-hydroxyflavone (3HFN) and 2-{[2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl]methylene}malononitrile (diCN-HBO) molecules. Our calculated hydrogen bond lengths and angles sufficiently confirm that the intramolecular hydrogen bonds O₁–H₁⋯O₂ and O₁–H₁⋯N₁ formed at the S₀ states of 3HFN and diCN-HBO should be significantly strengthened in the S₁ state, which is further supported by the results obtained based on the analyses of infrared spectra shifts, molecular orbitals and charge density differences maps. The significant strengthening of intramolecular hydrogen bonds O₁–H₁⋯O₂ and O₁–H₁⋯N₁ upon photoexcitation should facilitate the ESIPCCT process of the two title molecules. The scanned potential energy curves and confirmed excited-state transition states for both 3HFN and diCN-HBO show that the proton can be easily transferred from O₁ to O₂ (N₁ for diCN-HBO) through the strengthened intramolecular hydrogen bonds upon photoexcitation to the S₁ state.

Charge transfer from O₁ to O₂ of 3HFN results in proton H₁ transfer from O₁ to O₂ in S₁ state, while small energy barrier facilitates proton H₁ transfer from O₁ to N₁ in S₁ state of diCN-HBO, which results in charge transfer from O₁ to di-cyano.