ESIPT-based benzazole-pyromellitic diimide derivatives. A thermal, electrochemical, and photochemical investigation

This study describes the synthesis of new pyromellitic diimide (PMDI) derivatives obtained in good yields from the reaction between pyromellitic dianhydride and aminobenzazoles reactive to proton-transfer in the excited state (ESIPT). In this investigation, a non-ESIPT PMDI was also prepared for comparison. These compounds presented absorption maxima in the ultraviolet region attributed to the allowed ¹π-π* electronic transitions. Redshifted absorptions were observed for the ESIPT compounds (3b-3c) due to their π-extended conjugation if compared to the non-ESIPT dye (3a). The compounds presented fluorescence emissions between 300 and 600 nm, dependent on the solvent polarity and their chemical structures. While compound 3a presents a single emission, a dual fluorescence could be observed for compounds 3b-3c. As expected for ESIPT compounds, the emission at higher energies could be related to the excited enol conformer (E*), and the emission with a large Stokes shift was attributed to the keto tautomer (K*). All compounds presented fluorescence emission in the solid state, whereas the ESIPT derivatives presented redshifted emissions with a large Stokes shift, as expected. Cyclic voltammetry was employed to investigate the electrochemical properties of these compounds. The HOMO and LUMO energy levels were estimated at -5.40 to -5.00 eV and -2.84 to -2.62 eV, and good thermal stability (T_d > 150 °C) was observed. Quantum chemical calculationsusingTD-DFT and DFT were performed to investigate the electronic and photophysical features of the molecules.

Interaction Study between ESIPT Fluorescent Lipophile-Based Benzazoles and BSA

In this study, the interactions of ESIPT fluorescent lipophile-based benzazoles with bovine serum albumin (BSA) were studied and their binding affinity was evaluated. In phosphate-buffered saline (PBS) solution these compounds produce absorption maxima in the UV region and a main fluorescence emission with a large Stokes shift in the blue–green regions due to a proton transfer process in the excited state. The interactions of the benzazoles with BSA were studied using UV-Vis absorption and steady-state fluorescence spectroscopy. The observed spectral quenching of BSA indicates that these compounds could bind to BSA through a strong binding affinity afforded by a static quenching mechanism (K_q~10¹² L·mol⁻¹·s⁻¹). The docking simulations indicate that compounds 13 and 16 bind closely to Trp134 in domain I, adopting similar binding poses and interactions. On the other hand, compounds 12, 14, 15, and 17 were bound between domains I and III and did not directly interact with Trp134.

Substituent effect on ESIPT mechanisms and photophysical properties of HBT derivatives

Substituent effects on excited-state intramolecular proton transfer (ESIPT) and photophysical properties of 2-(2-Hydroxyphenyl) benzothiazole (HBT) derivatives have been theoretically unveiled via the density functional theory (DFT) and time-dependent DFT (TDDFT). The optimized geometrical configurations and normal mode analyses confirm that the proton transfer processes are more reactive in excited state. Through calculating the activation energies and rate constants of ESIPT processes, finding that the processes are increasingly inactive when substituent group changes from -CN, -CO₂Me, -Cl, -Me, -NMe₂ to -NO₂. In addition, the photophysical properties analyses indicate the vertical transition energies are in good agreement with those observed in experiment. Note that all the absorption and emission maxima of enol and keto forms have the significant red-shift. In order to clarify the substituent effect on ESIPT and photophysical properties, we draw the frontier molecular orbitals (FMOs) isosurfaces and calculate the distances of electrons and holes and atomic charges. It follows that the intramolecular charge transfer (ICT) degrees are increasingly enlarged as substituting from -CN, -CO₂Me, -Cl, -Me, -NMe₂ to -NO₂ groups, which not only causes the red-shift of absorption and emission of enol and keto forms, but also affects the charge distribution of proton donor and acceptor, inhibiting the occurrence of ESIPT processes.

Integrating amino acid oxidase with photoresponsive probe: A fast quantitative readout platform of amino acid enantiomers

Low-cost, high-throughput, broadly useful photoresponsive enantiomeric excess (ee) sensing of amino acids remains challenging to date. Herein, based on the selective oxidation reaction of amino acid oxidase (AAO) to amino acid enantiomers (D/L-AA) and the oxidation reaction of substrate (H₂O₂) with aromatic boronic ester, we put forward a photoresponsive strategy for the determination of D/L-AA at a certain concentration. In this scheme, the substrate H₂O₂ produced by the enzyme-catalyzed reaction was determined by sensitive fluorescent and colorimetric response of ethyl-3-(3-(benzothiazol-2-yl)-5-methyl-2-((4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)benzyl)oxy)phenyl)-2-cyanoacrylate (HBT-PB) to reflect the enantiomeric content at a certain concentration. The photoresponsive probe HBT-PB was readily available and inexpensive with sensitive long-wavelength red fluorescence and colorimetric light response to H₂O₂, the detection limit (LOD) was estimated as 2.91 μM. The operation of the sensing method was simple and data collection and processing are straightforward. The practicability of the scheme was favorably confirmed by accurate and scientific analysis of methionine and Dopa samples. As a result, the scheme was not only suitable for high-throughput screening but also adaptable to low-cost and sensitive RGB colorimetric analysis platform (LOD of methionine and Dopa was calculated as 9.23 μM and 8.34 μM respectively) with modern plate readers, and possessed extremely high enantioselectivity and wide applicability which benefited from the specificity and efficiency of enzyme catalytic reaction.

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

Benzothiazole derivatives were used as models to study the excited-state intramolecular proton transfer (ESIPT) from an experimental and theoretical point of view. The experimental electronic and vibrational results were compared with a comprehensive selection of state-of-the-art computational methods in a workflow approach. The latter were performed based on modern techniques, such as DLPNO-CCSD(T), which gives the reference energies and current methodologies for ESIPT analysis, such as molecular dynamics and charge density difference testing. The theoretical vibrational results were focused on the stretch vibrational-mode of the hydroxyl group, which indicated a large increase in the intramolecular hydrogen bond strength, which facilitates the ESIPT process. Theoretically, the optimization of a large number of molecules shows that π-stacking plays a fundamental role in benzothiazole stabilization, with a remarkably strong intramolecular hydrogen bond. The potential energy surface of the ESIPT reactive benzothiazole (4HBS) has a clear transition state where ESIPT is easily observed with a large difference in energy between the enol and keto tautomer. Additionally, molecular dynamics showed that the ESIPT process occurs very fast. The tautomer appears around 8.7 fs and the enolic form is regenerated in just 24 fs, closing the Förster cycle. The calculated Stokes shift could be related to the ESIPT process and the experimental solid-state emission spectrum matched almost perfectly with the theoretical one. In contrast, for the non-ESIPT benzothiazole (4HBSN), the agreement between theory and experiment was limited, probably due to intermolecular interaction effects that are not considered in these calculations.

Experimental and theoretical investigation of long-wavelength fluorescence emission in push–pull benzazoles: intramolecular proton transfer or charge transfer in the excited state?

ESIPT is disfavoured and charge-transfer emission, prior to ESIPT, seems to be responsible for long-emission wavelengths.

Proton transfer in fluorescent secondary amines: synthesis, photophysics, theoretical calculation and preparation of photoactive phosphatidylcholine-based liposomes

New proton-transfer lipophilic based benzazoles.

The effects of different heterocycles and solvents on the ESIPT mechanisms of three novel photoactive mono-formylated benzoxazole derivatives

The detailed effects of different heterocycles and solvents on the dynamical ESIPT mechanisms of three novel mono-formylated benzoxazole derivatives A–C in two different surroundings have been expounded by the TDDFT method at the B3LYP/TZVP level.

Synthesis and photophysical study of new fluorescent proton transfer dihydropyrimidinone hybrids as potential candidates for molecular probes

New photoactive hybrid dihydropyrimidinones were achieved by the Biginelli multicomponent reaction.

2-Phenylbenzoxazole derivatives: a family of robust emitters of solid-state fluorescence

In addition to thermal, chemical and photochemical stability, the 2-phenylbenzoxazole fragment exhibits attractive emission properties in the solid state, thus leading to highly photoluminescent materials and sensors.