Computational studies on amino-type excited-state intramolecular proton transfer and subsequent cis–trans isomerisation reactions of three 2-(2'-aminophenyl)benzothiazole derivatives

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).

Molecular design of amino-type hydrogen-bonding molecules for excited-state intramolecular proton transfer (ESIPT)-based fluorescent probe using the TD-DFT approach

A molecular screening of a new series of NH-type molecules for ESIPT-based fluorescent probes has been carried out using time-dependent density. functional theory.

Investigation of the intramolecular hydrogen bonding interactions and excited state proton transfer mechanism for both Br-BTN and CN-BTN systems

In the present work, two novel Br-BTN and CN-BTN compounds have been investigated theoretically. We in-depth explore the excited state hydrogen bonding interactions and the excited state intramolecular proton transfer (ESIPT) behaviors for the Br-BTN and CN-BTN system. We firstly verify the formation of hydrogen bond effects of O-H⋯N based on reduced density gradient (RDG) versus sign(λ 2)ρ. The simulated primary bond lengths and bond angles as well as infrared (IR) vibrational spectra reveal that the hydrogen bond O-H⋯N should be strengthened in the excited state. Combining the frontier molecular orbital (MO) investigations, we infer that the charge transfer phenomenon (from HOMO to LUMO) around hydrogen bonding moieties reveals the tendency of ESIPT reaction. Particularly, the increased electronic densities around proton acceptor atoms facilitate attracting a hydrogen proton, which plays a decisive role in opening the ESIPT reaction. Via constructing potential energy curves in both S0 and S1 states, the ultrafast ESIPT process can be verified which explains previous experimental characteristics. Furthermore, via searching the transition state (TS) structure and constructing the intrinsic reaction coordinate (IRC) reaction path, we check and confirm the ESIPT mechanism for both Br-BTN and CN-BTN systems. We sincerely hope that our theoretical work could guide novel applications based on Br-BTN and CN-BTN compounds in future.

Effects of different proton donor and acceptor groups on excited-state intramolecular proton transfers of amino-type and hydroxy-type hydrogen-bonding molecules: theoretical insights

The effect of proton donors (NH and OH) on ESIPT was investigated using DFT and TD-DFT.