ESIPT-Capable 4-(2-Hydroxyphenyl)-2-(Pyridin-2-yl)-1H-Imidazoles with Single and Double Proton Transfer: Synthesis, Selective Reduction of the Imidazolic OH Group and Luminescence

Azaindolizine proton cranes attached to 7-hydroxyquinoline and 3-hydroxypyridine: a comparative theoretical study

Theoretical design of several proton cranes, based on 7-hydroxyquinoline and 3-hydroxypyridine as proton-transfer frames, has been attempted using ground and excited-state density functional theory (DFT) calculations in various environments. Imidazo[1,2-a]pyridine, pyrazolo[1,5-a]pyridine and benzimidazole were considered as proton crane units. The proton crane action requires the existence of a single enol-like form in the ground state, which under excitation goes to the end keto-like one through a series of consecutive excited-state intramolecular proton transfers (ESIPT) and twisting steps with the participation of a crane unit, resulting in a long-range intramolecular proton transfer. The results suggest that 3-hydroxypyridine is not suitable for a proton-transfer frame and 8-(imidazo[1,2-a]pyridin-2-yl)quinolin-7-ol and 8-(pyrazolo[1,5-a]pyridin-2-yl)quinolin-7-ol behave as non-conjugated proton cranes, instead of tautomeric re-arrangement in the latter, which was thought to be possible.

Complexes on the Base of a Proton Transfer Capable Pyrimidine Derivative: How Protonation and Deprotonation Switch Emission Mechanisms

A rare example of pyrimidine-based ESIPT-capable compounds, 2-(2-hydroxyphenyl)-4-(1H-pyrazol-1-yl)-6-methylpyrimidine (HLH), was synthesized (ESIPT─excited state intramolecular proton transfer). Its reactions with zinc(II) salts under basic or acidic conditions afforded a dinuclear [Zn2LH2Cl2] complex and an ionic (H2LH)4[ZnCl4]2·3H2O solid. Another ionic solid, (H2LH)Br, was obtained from the solution of HLH acidified with HBr. In both ionic solids, the H+ ion protonates the same pyrimidinic N atom that accepts the O-H···N intramolecular hydrogen bond in the structure of free HLH, which breaks this hydrogen bond and switches off ESIPT in these compounds. This series of compounds which includes neutral HLH molecules and ionic (LH)- and (H2LH)+ species allowed us to elucidate the impact of protonation and coordination coupled deprotonation of HLH on the photoluminescence response and on altering the emission mechanism. The neutral HLH compound exhibits yellow emission as a result of the coexistence of two radiative decay channels: (i) T1 → S0 phosphorescence of the enol form and (ii) anti-Kasha S2 → S0 fluorescence of the keto form, which if feasible due to the large S2-S1 energy gap. However, owing to the efficient nonradiative decay through an energetically favorable conical intersection, the photoluminescence quantum yield of HLH is low. Protonation or deprotonation of the HLH ligand results in the significant blue-shift of the emission bands by more than 100 nm and boosts the quantum efficiency up to ca. 20% in the case of [Zn2LH2Cl2] and (H2LH)4[ZnCl4]2·3H2O. Despite both (H2LH)4[ZnCl4]2·3H2O and (H2LH)Br have the same (H2LH)+ cation in the structures, their emission properties differ significantly, whereas (H2LH)Br shows dual emission associated with two radiative decay channels: (i) S1 → S0 fluorescence and (ii) T1 → S0 phosphorescence, (H2LH)4[ZnCl4]2·3H2O exhibits only fluorescence. This difference in the emission properties can be associated with the external heavy atom effect in (H2LH)Br, which leads to faster intersystem crossing in this compound. Finally, a huge increase in the intensity of the phosphorescence of (H2LH)Br on cooling leads to pronounced luminescence thermochromism (violet emission at 300 K, sky-blue emission at 77 K).

Computational Insights into Excited State Intramolecular Double Proton Transfer Behavior Associated with Atomic Electronegativity for Bis(2'-benzothiazolyl)hydroquinone

Inspired by the distinguished regulated photochemical and photophysical properties of 2-(2'-hydroxyphenyl)benzazole derivatives, in this work, the novel bis(2'-benzothiazolyl)hydroquinone (BBTHQ) fluorophore is explored, looking at its photo-induced behaviors associated with different substituted atomic electronegativities, i.e., BBTHQ-SO, BBTHQ-SS and BBTHQ-Se compounds. From the structural changes, infrared (IR) vibrational variations and simulated core-valence bifurcation (CVB) indexes for the dual hydrogen bonds for the three BBTHQ derivatives, we see that low atomic electronegativity could be conducive to enhancing hydrogen bonding effects in the S1 state. Particularly, the O4-H5⋯N6 of BBTHQ-SO and the O1-H2⋯N3 of BBTHQ-SSe could be strengthened to be more intensive in the S1 state, respectively. Looking into the charge recombination induced by photoexcitation, we confirm a favorable ESDPT trend deriving from the charge reorganization of the dual hydrogen bonding regions. By constructing the potential energy surfaces (PESs) along with the ESDPT paths for the BBTHQ-SO, BBTHQ-SS and BBTHQ-Se compounds, we not only unveil stepwise ESDPT behaviors, but also present an atomic electronegativity-regulated ESDPT mechanism.