New insights on the ESIPT process based on solid-state data and state-of-the-art computational methods

Review on Synthesis of 2-(2-Hydroxyaryl) Benzothiazoles (HBT) for Excited-State Intra-molecular Proton Transfer (ESIPT)-Based Detection of Ions and Biomolecules

In this review, we present a systematic and comprehensive summary of the recent developments in the synthetic strategies of 2-(2-hydroxyarylsubstituted)-benzothiazole (HBT) framework along with incorporation of various substituents on phenolic and benzothiazole rings which affect the emission process. The literature, spanning the years 2015-2024, on excited-state intramolecular proton transfer (ESIPT)-based studies of HBT derivatives comprising the effects of solvent polarity, substituents, and extended conjugation on fluorophores has been searched. ESIPT, intramolecular charge transfer, and aggregation-induced emissions enable these fluorescent probes to specifically interact with analytes, thereby altering their luminescence characteristics to achieve analyte detection. These fluorescent probes exhibit large Stokes shifts, high quantum yields, and excellent color transitions. Finally, the applications of HBTs as ESIPT-based fluorescent probes for the detection of cations, anions, and biomolecules have been summarized. We anticipate that this review will provide a comprehensive overview of the current state of research in this field and encourage researchers to develop novel ESIPT-based fluorophores with new applications.

The photodynamic approach to the molecular-level origin of metal-guided photochromism and ultrafast absorption spectroscopy

Metal-guided photochromic material (photochromic complex) is one of the latest versions of photo-responsive materials due to their smart behaviour and promising real-world applications. The present work explores the molecular-level origin of metal-guided photochromism using a photodynamic approach and ultrafast absorption spectroscopy, to address all existing lacunas. Here, rhodamine B (RhB) dye containing the Schiff base zinc complex is considered a representative photochromic complex for both theoretical treatment and experimental observations. Detailed theoretical studies, including geometry optimization, frontier molecular orbital (FMO) analysis, transition state (TS) identification, and natural bond orbital (NBO) analysis, along with spectral studies, are employed to investigate the photodynamic equilibrium (enol-form keto-form). This equilibrium is regulated by the interplay of intrinsic factors (push-pull effect) and extrinsic factors (such as UV-light, the phenolic-OH group, metal ions, and solvents). The potential energy surface (PES) of the photo-conversion (enol →enol*→keto*→ meta-stable keto) is evaluated. While, the PES of the reversion (meta-stable keto →enol) is constructed based on the studies of thermo-reversion and photo-reversion. Finally, the theoretical findings related to the photodynamic equilibrium are validated by direct experimental evidence obtained through femtosecond transient absorption (fs-TA) spectroscopy.

Control of the fluorescence molecule 2-(2'-hydroxyphenyl) benzothiazole derivatives by introducing electron-donating and withdrawing substituents groups

Using density functional theory (DFT) and time-dependent density functional theory (TDDFT), we investigate the fluorescence mechanism of (E)-4-(3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylstyryl)-1-methylpyridin-1-ium (HBTMY) and the excited-state intramolecular proton transfer process (ESIPT) of hydroxyphenyl. Herein, we introduce two electron-donating (amino and methoxy) and two electron-withdrawing (hydrogen and cyano) groups into HBTMY to study their effects on the fluorescence and the ESIPT process. Structural parameters, infrared vibration frequency, vertical excitation and emission energies as well as frontier molecular orbitals show that the substituents have different impacts on intramolecular hydrogen bonding behavior. The result shows that the fluorescence wavelength of molecules with the amino group could reach the near-infrared area, which favors using this fluorescence in the living cell. As the ability of electron-absorbing groups increases, the forward energy barrier in the potential energy curves decreases sharply making the ESIPT process more familiar to take place. Thus, this work offers a guide for cell imaging and provides strategies to adjust and control fluorescence by introducing substituents.

Revealing the incorporation of an NH2 group into the edge of carbon dots for H2O2 sensing via the C-N⋯H hydrogen bond interaction

We investigated hydrogen peroxide (H₂O₂) sensing on NH₂-functionalized carbon dots (Cdots) for three different -NH₂ positions, and the N atom was found to be the active site using a quantum computational approach. B3LYP and 6-31G(d,p) were used for density functional theory (DFT) ground state calculations, whereas CAM-B3LYP and the same basis set were used in time-dependent density functional theory (TD-DFT) excited state calculations. Structural optimization showed that the H₂O₂ is chemisorbed on 1-sim via a C-N⋯H hydrogen bond interaction with an adsorption energy of -10.61 kcal mol^-1. Mulliken atomic charge distributions and electrostatic potential (ESP) analysis were both used to determine reactivity of the molecules at the atomic level. For in-depth analysis of the ground states, we utilized Frontier molecular orbital (FMO) theory, quantum theory of atoms in molecules (QTAIM), and non-covalent interaction (NCI) index analysis. In addition, we also present UV-vis absorption spectra and charge transfer lengths to understand the mechanism of H₂O₂ sensing in excited states. Based on the molecular and electronic properties of the NH₂-Cdots, it was shown that 1-sim is a potential candidate for use as an electrochemical sensor for H₂O₂ sensing. Whereas 3-sim is believed to be a potential candidate for use as an optical sensor of H₂O₂ based on the UV-vis characteristics via photoinduced charge transfer.

Unveiling the influence of atomic electronegativity on the double ESIPT processes of uralenol: A theoretical study

In this work, the effects of atomic electronegativity (O, S, and Se atoms) on the competitive double excited-state intramolecular proton transfer (ESIPT) reactions and photophysical characteristics of uralenol (URA) were systematically explored by using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated hydrogen bond parameters, infrared (IR) vibrational spectra, reduced density gradient (RDG) scatter plots, interaction region indicator (IRI) isosurface and topology parameters have confirmed the six-membered intramolecular hydrogen bond (IHB) O4H5^…O3 is the stronger one in all the three studied compounds. Subsequently, frontier molecular orbitals (FMOs) and natural bond orbital (NBO) population analysis essentially uncover that the electron redistribution has induced the ESIPT process. Besides, the constructed potential energy curves (PECs) have indicated that the ESIPT process prefers to occur along the O4H5^…O3 rather than the O1H2^…O3 and the proton-transfer energy barrier is gradually decreased with the weakening of atomic electronegativity from URA to URA-S and URA-Se. In a conclusion, the attenuating of atomic electronegativity has enhanced the IHBs of URA and thereby promoting the ESIPT reaction, which is helpful for further developing novel fluorophores based on ESIPT behavior in the future.

Modern Theoretical Approaches to Modeling the Excited-State Intramolecular Proton Transfer: An Overview

The excited-state intramolecular proton transfer (ESIPT) phenomenon is nowadays widely acknowledged to play a crucial role in many photobiological and photochemical processes. It is an extremely fast transformation, often taking place at sub-100 fs timescales. While its experimental characterization can be highly challenging, a rich manifold of theoretical approaches at different levels is nowadays available to support and guide experimental investigations. In this perspective, we summarize the state-of-the-art quantum-chemical methods, as well as molecular- and quantum-dynamics tools successfully applied in ESIPT process studies, focusing on a critical comparison of their specific properties.

Imidazoles and Oxazoles from Lapachones and Phenanthrene-9,10-dione: A Journey through their Synthesis, Biological Studies, and Optical Applications

Diverse structural frameworks are found in natural compounds and are well known for their chemical and biological properties; such compounds include the imidazoles and oxazoles. Researchers worldwide are continually working on the development of methods for synthesizing new molecules bearing these basic moiety and evaluating their properties and applications. To expand the knowledge related to azoles, this review summarizes important examples of imidazole and oxazole derivatives from 1,2-dicarbonyl compounds, such as lapachones and phenanthrene-9,10-diones, not only regarding their synthesis and biological applications but also their photophysical properties and uses. The data concerning the latter are particularly scarce in the literature, which leads to underestimation of the potential applications that can be envisaged for these compounds.

Perturbating Intramolecular Hydrogen Bonds through Substituent Effects or Non-Covalent Interactions

An analysis of the effects induced by F, Cl, and Br-substituents at the α-position of both, the hydroxyl or the amino group for a series of amino-alcohols, HOCH2(CH2)nCH2NH2 (n = 0-5) on the strength and characteristics of their OH···N or NH···O intramolecular hydrogen bonds (IMHBs) was carried out through the use of high-level G4 ab initio calculations. For the parent unsubstituted amino-alcohols, it is found that the strength of the OH···N IMHB goes through a maximum for n = 2, as revealed by the use of appropriate isodesmic reactions, natural bond orbital (NBO) analysis and atoms in molecules (AIM), and non-covalent interaction (NCI) procedures. The corresponding infrared (IR) spectra also reflect the same trends. When the α-position to the hydroxyl group is substituted by halogen atoms, the OH···N IMHB significantly reinforces following the trend H < F < Cl < Br. Conversely, when the substitution takes place at the α-position with respect to the amino group, the result is a weakening of the OH···N IMHB. A totally different scenario is found when the amino-alcohols HOCH2(CH2)nCH2NH2 (n = 0-3) interact with BeF2. Although the presence of the beryllium derivative dramatically increases the strength of the IMHBs, the possibility for the beryllium atom to interact simultaneously with the O and the N atoms of the amino-alcohol leads to the global minimum of the potential energy surface, with the result that the IMHBs are replaced by two beryllium bonds.