Regulating and controlling the stepwise ESDPT channel of BP(OH)2DCEt2 using the strategy of solvent polarity and external electric field

Excited state double proton transfer (ESDPT) has attracted great scientific interest because of its excellent luminescent properties. However, the complex process of ESDPT has plagued theoretical and experimental scientists for a long time and has become a hot issue. In this work, the ESDPT process of 2,2'-bipyridine-3,3'-diol-5,5'-dicarboxylic acid ethyl ester (BP(OH)2DCEt2) is systematically studied and the regulation of the ESDPT process is further realized. The potential energy curves indicate that BP(OH)2DCEt2 shows the characteristics of stepwise ESDPT in different polar solvents. The increase in solvent polarity will be beneficial to the stepwise ESDPT reaction. Regrettably, it is not possible to distinguish the specific stepwise transfer path of the BP(OH)2DCEt2 molecule due to the symmetry of the potential energy surface along the diagonal. On this basis, we proposed a method to control and regulate the stepwise ESDPT path using an external electric field. The results show that the increase of external electric field intensity is favorable to stepwise ESDPT. It is interesting to note that applying an external electric field in a specific direction will effectively distinguish stepwise ESDPT reaction paths. Therefore, this work not only helps to understand the mechanism of ESDPT, but also contributes to regulation and design of new luminescent materials with excellent luminescent properties.

Interplay of Dual-Proton Transfer Relay to Achieve Full-Color Panel Luminescence in Excited-State Intramolecular Proton Transfer (ESIPT) Fluorophores

A new design was applied for the facile synthesis of pure organic photoluminescent molecules with dual excited-state intramolecular proton transfer (ESIPT) sites. In this novel class of emitters, full-color panel emission from blue, green, and yellow to red, including white light, can be achieved in different solvents as modulated by the enol-keto(1st)-keto(2nd) tautomer emissions. A comprehensive transient photophysical study verifies that keto(1st) and keto(2nd) have a precursor (<0.8 ps)-successor (∼20 ps)-relayed absorbance relationship, and then a fast equilibrium between the two is established, resulting in dual emissions in the nanosecond scale (∼1900 ps). Through the research on copper ions' selective PL response, the dual-ESIPT mechanism was further verified; in addition, the study of solid-state PL changes upon the stimulus of organic vapor manifests the potential application sensitivity of the molecules as dual-ESIPT sensors. Theoretical results including reaction potential energy surface analyses manifest the fact that dual-proton transfer goes along a sequential route with a smaller energy barrier, firmly supporting the experimental results. An intrinsic system that undergoes intramolecular double proton relayed transfer is thus established for the achievement of much broadened optical responses and full-color display, providing reference for the design and application of advanced dual-ESIPT optical materials.

Effects and Influence of External Electric Fields on the Equilibrium Properties of Tautomeric Molecules

In this review, we have attempted to briefly summarize the influence of an external electric field on an assembly of tautomeric molecules and to what experimentally observable effects this interaction can lead to. We have focused more extensively on the influence of an oriented external electric field (OEEF) on excited-state intramolecular proton transfer (ESIPT) from the studies available to date. The possibilities provided by OEEF for regulating several processes and studying physicochemical processes in tautomers have turned this direction into an attractive area of research due to its numerous applications.

Theoretical study on the optical properties of an ESIPT-based fluorescent probe for phosgene

A fluorescent probe Pi with the excited-state intramolecular proton transfer (ESIPT) properties was synthesized and used to detect the phosgene in solution and gas phases. However, the detection mechanism of the fluorescent probe needs to be further studied. Herein, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods were adopted to explore the molecular structures and electronic spectra properties of probe and its product Pio after reacting with phosgene. Through analysis for molecular structure parameters and infrared vibrations accompanied with the hydrogen bond of Pi, it is confirmed that the intramolecular hydrogen bond of Pi is enhanced under light excitation, which illustrates the occurrence of ESIPT reaction combined with the scanned potential energy curves. It can be seen from the simulated spectra that Pi shows double fluorescence through ESIPT process, while the fluorescent product Pio exhibits the single fluorescence due to the disappearance of intramolecular hydrogen bond. Through the study on the structure and optical properties of Pi and Pio, it can be helpful to deeply understand the intrinsic mechanism of the detection of phosgene by the Pi molecule probe, which also supplies a reference to the further study about the fluorescence probe.

Fluorescent detection mechanism of CO-releasing molecule-3: Competition of inter-/intra-molecular hydrogen bonds

The fluorescent detection mechanism of 2-(4-nitro-1,3-dioxoisoindolin-2-yl) acetic acid (CORM3-green) on CO-Releasing Molecule-3 (CORM-3) is theoretically studied. Upon reaction with CORM-3, the non-fluorescent CORM3-green is transferred to the keto form of 2-(4-amino-1,3-dioxoisoindolin-2-yl)acetic acid (PTI) to produce strong fluorescence peak located at 423 nm. This peak red-shifts to 489 nm, which is induced by the strengthening of intermolecular hydrogen bond (HB) between PTI and water molecules and attributed to the experimentally observed fluorescence emission at 503 nm. This result is dramatically different from previous reports that the experimental fluorescence corresponds to the proton transferred enol form of PTI. To illustrate this confusion, the calculated fluorescence peak of PTI-Enol is located at 689 nm, which is much larger than that of experimental result. This result excludes the occurrence of excited state intramolecular proton transfer (ESIPT). It is concluded that intermolecular HBs hinders the formation of intramolecular HB and the ESIPT of the keto form of PTI. This conclusion confirms that experimental Stokes shift of 113 nm is mainly caused by the intermolecular hydrogen bonding rather than by ESIPT process. This work proposes a reasonable explanation for the detection mechanism of CORM3-green and experimental fluorescence phenomenon.

Substituent effect on ESIPT and hydrogen bond mechanism of N-(8-Quinolyl) salicylaldimine: A detailed theoretical exploration

The effects of substituent on excited-state intramolecular proton transfer (ESIPT) and hydrogen bonding of N-(8-Quinolyl) salicylaldimine (QS) have been studied by theoretical calculation with DFT and TDDFT. The representative electron-withdrawing nitryl and electron-donating methoxyl were selected to analyze the effects on geometries, intramolecular hydrogen bond interaction, absorption/fluorescence spectra, and the ESIPT process. The configurations of the three molecules (QS, QS-OMe and QS-NO₂) were optimized in the ground and excited states. The structure parameters, infrared spectra, hydrogen bond interactions, frontier molecular orbitals, absorption/fluorescence spectra, and potential curves have cross-validated the current results. The results show that the introduction of substituent results in a bathochromic-shift of the absorption and fluorescence spectra with large Stokes shift, and is more beneficial to the ESIPT process. The current work will be beneficial to the improvement of ESIPT properties and deepen understanding of the mechanism of ESIPT process.

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.

Sensing mechanism of a fluorescent probe for thiophenols: Invalidity of excited-state intramolecular proton transfer mechanism

Simple and effective detection of thiophenols has attracted great attention. A fluorescent probe 1 with high selectivity and sensitivity is designed and synthesized based on the excited-state intramolecular proton transfer (ESIPT) in experiment. However, we conclude that the ESIPT process fails to happen actually based on the calculation results. In the present work, the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods are employed to investigate the real sensing mechanism. The calculated absorption and emission spectra agree well with the experimental results. By comparing the energy of enol and keto configurations and the constructed potential energy surfaces (PESs) in the ground (S₀) and excited (S₁) states of 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2), the ESIPT process is confirmed impossible because of the relatively high keto form energy and potential energy barrier. Besides, the transition state of dye 2 is optimized to offer the accurate potential energy barrier. The results of calculated frontier molecular orbitals (FMOs) and spectra indicate that it is the photoinduced electron transfer (PET) process that results in the fluorescence quenching of probe 1. After adding thiophenols, the thiolysis of 2,4-dinitrophenyl ether bond is triggered and dye 2, which emits strong fluorescence because of the absence of PET process, is obtained. Consequently, our study has demonstrated that probe 1 can act as a fluorescent probe to detect thiophenols through the off-on fluorescence variation based on the PET mechanism but not the ESIPT process.

Through bond energy transfer (TBET)-operated fluoride ion sensing via spirolactam ring opening of a coumarin–fluorescein bichromophoric dyad

The detection of fluoride ions in a competitive environment often poses several challenges. In this work, we have designed and synthesized a coumarin functionalized fluorescein dyad (R3) which represents an ideal through bond energy transfer (TBET) fluorophore with the coumarin unit as donor and fluorescein unit as acceptor. The bichromophoric dyad demonstrates the detection of fluoride ions in the parts per billion (ppb) concentration level (22.8 ppb) with high selectivity via a TBET emission signal at 548 nm with a diagnostic bright yellow colour fluorescence output. Based on UV-visible, fluorescence, ¹H NMR and DFT studies, it is shown that the fluoride ion induces the opening of the spirolactam ring of the fluorescein moiety and provides a π-conjugation link between the donor and acceptor units enabling a TBET phenomenon with a larger pseudo-Stokes shift of 172 nm. To the best of our knowledge, this is the first report where the fluoride ion is detected via a TBET signal between the coumarin and fluorescein units in a bichromophoric dyad.

A coumarin–fluorescein based bichromophoric dyad detects fluoride ions in parts per billion concentration level via a TBET emission signal at 548 nm with a diagnostic bright yellow fluorescence.

Is the origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals single or double proton transfer?

The origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals (BCLCs) is not understood clearly.

Determinant of ESIPT Mechanism by the Structure Designed for Symmetrical and Unsymmetrical Molecules

In this work, we present that different structures lead to different excited state properties based on the investigations of the systems with symmetrical and unsymmetrical properties. Our work shows that the symmetrical and unsymmetrical compounds with a modified structure play key roles in regulating the excited state intramolecular proton transfer (PT) (ESIPT) process. For N,N'-bis(salicylidene)-p-phenyle-nediamine (BSP) and N,N'-disalicylidene-1,6-pyrenediamine (BSD), when they are excited, the torsion angels result in the rematch of the spectra. By analyzing the potential energy surfaces (PESs) of the torsion angle and PT process, we conclude that the size of the π stack largely affects the molecular properties in the excited states. For 2'-hydroxychalcone derivatives, which have important applications in biotransformation reactions, investigating the molecules of M1 and M2 could promote innovation in bioengineering. The results of molecular electrostatic potential (MEP) and the real space intramolecular interactions show the state and position of the hydrogen bond (HB) in both S₀ and S₁ states. The corresponding PT PESs show that the ESIPT reaction is much easier for the M1 system due to the lack of the side chain hydroxyl compared with the M2 system. This work is not only consistent with experimental results and explains its mechanism but also presents that symmetric and nonsymmetric structures modify their own potential regulation and controlling effects for ESIPT behaviors.

Fluorescence deactivation mechanism for a new probe detecting phosgene based on ESIPT and TICT

The ESIPT-fluorescence deactivation is caused by ISC and phosphorescence.

Dual-mode recognition of biogenic amine tryptamine and fluoride ions by a naphthyl hydrazone platform: application in fluorescence imaging of HeLa cells and zebrafish embryos

A new type of selective and sensitive naphthyl hydrazone anchored Schiff base derivative (PYNA) was synthesized and applied to the fluorogenic and chromogenic recognition of both tryptamine (TryptA) and F⁻ions, further it has good cell permeability and biocompatibility in zebrafish embryos and HeLa cells.

A macrocyclic tetranuclear ZnII complex as a receptor for selective dual fluorescence sensing of F− and AcO−: effect of a macrocyclic ligand

Fluorimetric detection of F⁻/AcO⁻ with an additional ‘INHIBIT’ logic gate response for F⁻/H⁺ and AcO⁻/H⁺ in an aqueous-methanol medium by a 48-membered macrocyclic Zn^II-complex DAS has been explored by combined experimental and theoretical study.

Tunable ESIPT reaction and antioxidant activities of 3-hydroxyflavone and its derivatives by altering atomic electronegativity

3-HTF easiest to carry out ESIPT reaction would exhibit the highest efficient antioxidant activity among the three compounds.