Excited-state Intramolecular Proton Transfer Mechanism Research Articles

Developing an ESIPT-based ratiometric fluorescent probe with fast-response for cellular imaging of hydrogen sulfide

Hydrogen sulfide (H2S) plays a crucial role in various physiological processes and is implicated in the pathogenesis of several diseases. The fluorescent probe (E)-2-(3-(3-(benzo[d]thiazol-2-yl)-2-hydroxy-5-methylstyryl)-5,5-dimethylcyclohex-2-en-1-ylidene) malononitrile (BTCMN) has been designed to image H2S in living cells using the excited-state intramolecular proton transfer (ESIPT) mechanism. This probe is developed by coupling 2-(2′-hydroxyphenyl) benzothiazole (HBT) with dicyanoisophorone through a H2S-responsive CC bond. Upon interaction with H2S, BTCMN exhibits a remarkable fluorescent change from red to green. BTCMN, which has shown exceptional performance in detecting hydrogen sulfide (H2S) with super-fast response times, large Stokes shifts (252 nm), specific selection towards H2S, and low detection limits. Importantly, BTCMN has been successfully used for visualizing endogenous and exogenous H2S in HepG2 cells with low cytotoxicity. These features collectively contribute to its effectiveness as a fluorescent probe for detecting H2S in living cells.

Synthesis and application of the AIE-ESIPT fluorescent probe for Cystiene detection: Crystal structure analysis and DFT calculation

Accurate detection of Cysteine (Cys) in cells is critical for clinical diagnosis and pathogen detection. It was of practical significance to develop fluorescent probes with specific recognition and rapid detection of Cys. Therefore, based on the nucleophile reaction and intramolecular cyclization reaction between biothiols and acrylates (AAS), a fluorescence probe, BTY-TPE-AAS, was designed and synthesized by combining the advantages of the Aggregation-Induced Emission (AIE) and Excited-State Intramolecular Proton Transfer (ESIPT) mechanisms. BTY-TPE-AAS could specifically detect Cys with a short detection time and strong anti-interference, which had important application value in the monitoring of Cys in the biomedical field. It provided some reference values for the design of a specific fluorescent probe for Cys detection.

Computational Investigation about the Effects of Solvent Polarity and Chalcogen Element Electronegativity on ESIPT Behaviors for the Et2N-Substituted Flavonoid.

Inspired by the outstanding nature of flavonoid derivatives in the fields of chemistry and medicine, in this work we mainly focus on exploring the photo-induced properties of the novel Et2N-substituted flavonoid (ENF) fluorophore theoretically. Considering the potential photo-induced properties in different solvents and the chalcogen atomic electronegativity-associated photoexcitation, by time-dependent density functional theory (TDDFT) methods we primarily explore the intramolecular hydrogen bonding interactions and photo-induced charge redistribution behaviors. Via comparing geometrical data and the infrared (IR) spectral shifts-associated hydroxy moiety of ENF, we confirm that the intramolecular hydrogen bond O-H···O should be enhanced with facilitating an excited-state intramolecular proton-transfer (ESIPT) reaction. Particularly, the charge reorganization around hydrogen bonding moieties further reveals the tendency of ESIPT behavior. Combined with the construction of the potential energy surface and the search for reaction transition states, we finally confirmed the solvent-polarity-regulated behaviors as well as the chalcogen elements' electronegativity-dependent ESIPT mechanisms for the ENF fluorophore. We sincerely wish our work could accelerate the further development and applications of flavonoid derivatives.

A near-infrared fluorescent probe with thiadiazole unit as key skeleton for ICT and ESIPT mechanism and effective detection of Cu2+

Fluorescent probe L-I was synthesized to demonstrate that 1,3,4-thiadiazole is an attractive moiety and could be utilized as positive hydrogen bond acceptor for excited state intramolecular proton transfer (ESIPT) processes, guider of electrons movement for intramolecular charge transfer (ICT) process and identify group for mental ions. Furthermore, dicyanoisophorone framework was employed to improve the fluorescence characteristics and near-infrared (NIR) fluorescent emission at 695 nm accompanied by a Stoke's shift as large as 260 nm was obtained. L-I could selectively detect Cu2+ over other analytes taking advantages of high sensitivity, fast response within 30 s and low detection limit (0.026 μM). More important, L-I exhibited good performance for detection of Cu2+ in actual water samples, food products, traditional Chinese medicine and for cell imaging which demonstrates practical significance in the fields of environmental monitor, food safety and biotechnology.

Exploring electron donor and acceptor effects: DFT analysis of ESIPT/GSIPT in 2-(oxazolinyl)-phenols for photophysical and luminophore enhancement.

Understanding excited-state intramolecular proton transfer (ESIPT) is essential for designing organic molecules to enhance photophysical and luminophore properties in the development of optoelectronic devices. In this context, an attempt has been made to understand the impact of substituents on the ESIPT process of 2-(oxazolinyl)-phenol. Electron donating (EDG: -NH2, -OCH3, and -CH3) and electron withdrawing (EWG: -Cl, -Br, -COOH, -CF3, -CN, and -NO2) substitutions have been computationally designed and screened through density functional theory (DFT) and time-dependent density-functional theory (TDDFT) calculations. Furthermore, the ground state intramolecular proton transfer and ESIPT mechanisms of these designed luminophores are explored using the transition state theory. The results reveal that molecules with EDG show higher absorption and emission peaks than molecules with EWG and also indicate that the mobility of charge carriers in 2-(oxazolinyl)-phenol derivatives is significantly influenced by substituents. We found that the EWGs decrease the reorganization energy and increase the vertical ionization potential and electron affinity values, as well as the highest occupied molecular orbital-lowest unoccupied molecular orbital gap, compared to the EDG substituted molecules. Significantly, the excited state (S1) of the keto emission (K) form shows notably larger values for the EDG substitutions. The intersystem crossing pathway efficiency weakens with reduced spin-orbit coupling matrix element in the enol form with electron-donating substituents and vice versa in the keto form during S1-T3 transitions. Our research links intramolecular proton transfers and triplet generation, making these substituted molecules appealing for optoelectronic devices. Introducing EDGs, such as -NH2, boosts the ESIPT reaction in 2-(oxazolinyl)-phenol. This study guides designing ESIPT emitters with unique photophysical properties.

Concerning for the solvent-polarity-dependent conformational equilibrium and ESIPT mechanism in Pz3HC system: A novel insight

In this report, we propose a new insight into the interaction between the solvent-polarity-dependent conformational equilibrium and excited state intramolecular proton transfer (ESIPT) behavior of Pz3HC system in four different polar solvents (polarity order: ACN > THF > TOL > CYC). Using quantum chemistry method, we first announce a coexistence mechanism between Pz3HC-1 and Pz3HC-3 in the ground state in four solvents based on the Boltzmann distribution. In particular, Pz3HC-1 is the principal configuration in non-polar solvent, but Pz3HC-3 is the principal configuration in polar solvent. In addition, the simulated fluorescence spectra interprets the negative solvatochromism effect of Pz3HC-1 and Pz3HC-3 in four solvents. The evidence from intramolecular hydrogen bonding (IHB) parameters and electronic perspective collectively confirms the light-induced IHB enhancement and intramolecular charge transfer (ICT) properties in Pz3HC-1 and Pz3HC-3, which raises the likelihood of the ESIPT process. Combining the calculation of potential energy curve (PEC) and intrinsic reaction coordinate (IRC), we demonstrate that the ESIPT ease of Pz3HC-1 in different polar solvents obeys the order of CYC > TOL > THF > ACN, while the order of ESIPT ease in Pz3HC-3 is opposite. Notably, the ESIPT process of Pz3HC-3 in CYC solvent is accompanied by the twisted intramolecular charge transfer (TICT) process. In addition, we also reveal that the enol* and keto* fluorescence peaks of Pz3HC-3 in CYC solvent are quenched by ISC and TICT process, respectively. Our work not only provides a satisfactory explanation of the novel dynamics mechanism for Pz3HC system, but also brings light to the design and application of new sensing molecules in the future.

Mechanistic Insights into the Excited-State Intramolecular Proton Transfer (ESIPT) Process of 2-(2-Aminophenyl)naphthalene.

The 2-(2-aminophenyl)naphthalene molecule attracted much attention due to excited-state intramolecular proton transfer (ESIPT) from an amino NH2 group to a carbon atom of an adjacent aromatic ring. The ESIPT mechanisms of 2-(2-aminophenyl)naphthalene are still unclear. Herein, the decay pathways of this molecule in vacuum were investigated by combining static electronic structure calculations and nonadiabatic dynamics simulations. The calculations indicated the existence of two stable structures (S0-1 and S0-2) in the S0 and S1 states. For the S0-1 isomer, upon excitation to the Franck-Condon point, the system relaxed to the S1 minimum quickly, and then there exist four decay pathways (two ESIPT ones and two decay channels with C atom pyramidalization). In the ESIPT decay pathways, the system encounters the S1S0-PT-1 or S1S0-PT-2 conical intersection, which funnels the system rapidly to the S0 state. In the other two pathways, the system de-excited from the S1 to the S0 state via the S1S0-1 or S1S0-2 conical intersection. For the S0-2 structure, the decay pathways were similar to those of S0-1. The dynamics simulations showed that 75 and 69% of trajectories experienced the two ESIPT conical intersections for the S0-1 and S0-2 structures, respectively. Our simulations showed that the lifetime of the S1 state of S0-1 (S0-2) is estimated to be 358 (400) fs. Notably, we not only found the detailed reaction mechanism of the system but also found that the different ground-state configurations of this system have little effect on the reaction mechanism in vacuum.

Modulating the ESIPT Mechanism and Luminescence Characteristics of Two Reversible Fluorescent Probes by Solvent Polarity: A Novel Perspective.

As reversible fluorescent probes, HTP-1 and HTP-2 have favourable applications for the detection of Zn2+ and H2S. Herein, the impact of solvent on the excited-state intramolecular proton transfer (ESIPT) of HTP-1 and HTP-2 was comprehensively investigated. The obtained geometric parameters and infrared (IR) vibrational analysis associated with the intramolecular hydrogen bond (IHB) indicated that the strength of IHB for HTP-1 was weakened in the excited state. Moreover, structural torsion and almost no ICT behaviour indicated that the ESIPT process did not occur in HTP-1. Nevertheless, when the 7-nitro-1,2,3-benzoxadiazole (NBD) group replaced the H atom, the IHB strength of HTP-2 was enhanced after photoexcitation, which inhibited the twisting of tetraphenylethylene, thereby opening the ESIPT channel. Notably, hole-electron analysis and frontier molecular orbitals revealed that the charge decoupling effect was the reason for the fluorescence quenching of HTP-2. Furthermore, the potential energy curves (PECs) revealed that HTP-2 was more inclined to the ESIPT process in polar solvents than in nonpolar solvents. With a decrease in solvent polarity, it was more conducive to the ESIPT process. Our study systematically presents the ESIPT process and different detection mechanisms of the two reversible probe molecules regulated by solvent polarity, providing new insights into the design and development of novel fluorescent probes.

Effects of substitution and conjugation on photophysical properties of ESIPT-based fluorophores with the core of 4-aminophthalimide

4-Aminophthalimide is a highly fluorescent signaling unit with excellent photophysical properties and wide application foregrounds. Based on this, a range of theoretical investigations are conducted on the fluorescent probe (E)-5-((2-hydroxybenzylidene) amino) isoindoline-1, 3-dione (HID) with the core of 4-aminophthalimide using density functional theory (DFT) and time-containing density functional theory (TD-DFT) methods in this paper. The optimized configurations, vertical excitation and emission energies, electronic characteristics and excited-state intramolecular proton transfer (ESIPT) behaviors of the probe HID are discussed in detail. Furthermore, to enhance the luminescent properties of HID, five novel compounds have been designed based on the structure of HID by introducing amino, methoxy and naphthalene groups (–NH2, –OMe and C10H8). Our work thoroughly explores how the property and position of substituents and conjugation affect photophysical characteristics and ESIPT processes. We find that the ESIPT dynamics can be modulated by the substitution and conjugation effects. Specifically, the introduction of amino and methoxy groups at the ortho-position and the introduction of the naphthalene group promote the ESIPT behavior of HID1, whereas the introduction of amino and methoxy groups at the meta-position exhibits the contrary impact. Therefore, we boldly infer that the introduction of electron-donating groups in the ortho-position and the introduction of the conjugated group make the ESIPT process more effortless to occur, whereas the introduction of substituents with opposing natures in the meta-position makes the ESIPT process more difficult to occur. In addition, the ionization potentials (IP), electron affinities (EA) and reorganization energies (λh and λe) of molecules are calculated to assess their potential as luminescent materials. Our work not only reveals the luminescence and ESIPT mechanism of the probe HID1, but also proposes to modulate the ESIPT process through the substitution and conjugation effects. In particular, the designed molecules have better photoelectric properties as a result of their red-shifted absorption and fluorescence spectra, smaller energy gaps, larger transferred charges and greater charge transferred distances, which offers some valuable ideas for the experimental development of more efficient organic luminescent materials with ESIPT properties.

Incorporation of Photo- and Thermoresponsive N-Salicylidene Aniline Derivatives into Cobalt and Zinc Layered Hydroxides.

In search of new multifunctional hybrid materials and in order to investigate the influence of chemical modification on the possible synergy between properties, the carboxylate and sulfonate derivatives of photo- and thermochromic N-salicylidene aniline were successfully inserted into Co(II)- and Zn(II)-based layered simple hydroxides, resulting in four novel hybrids: Co-N-Sali-COO, Co-N-Sali-SO3, Zn-N-Sali-COO, and Zn-N-Sali-SO3. All synthesized hybrids adopt a double organic layered configuration, which prevents the cis-trans photoisomerization ability of N-Sali-R molecules in the hybrids. However, the Zn hybrids exhibit fluorescence upon exposure to UV light due to the excited-state intramolecular proton transfer (ESIPT) mechanism. The thermally stimulated keto-enol tautomerization of N-salicylidene aniline in the hybrids was related with the changes in interlamellar spacings observed by temperature-dependent PXRD. This tautomerization process was prominently evident in the Co-N-Sali-SO3 hybrid (about 11% increase in d-spacing upon decreasing the temperature to -180 °C). Finally, the Co-N-Sali-R hybrids exhibit the typical magnetic behavior associated with Co(II)-based LSHs (ferrimagnetic ordering at TN = 6.8 and 7.7 K for Co-N-Sali-COO and Co-N-Sali-SO3, respectively). This work offers insights into isomerization in LSHs and the ESIPT mechanism's potential in new luminescent materials and prospects for designing new multifunctional materials.

Effects of substituted chalcogen atoms on excited state reaction for 1-(benzo[d]thiazol-2-yl)-6-substituted-naphthalen-2-ol derivatives

Inspired by the brilliant photophysical properties of novel organic molecules containing chalcogenide substitution, in this work, effects of atomic electronegativity of chalcogen (O, S and Se) on hydrogen bond interactions and proton transfer (PT) reaction are focused. We present the characteristic CN-BTN is the main objective to explore the influence of atomic electronegativity on the hydrogen bond interaction and excited state intramolecular proton transfer (ESIPT) behaviour by photoexcitation. By comparing the structural changes and infrared (IR) vibrational spectra of the CN-BTN derivatives (CN-BTN-O, CN-BTN-S and CN-BTN-Se) fluorophores in S0 and S1 states, combined with the preliminary detection of hydrogen bond interaction by core-valence bifurcation (CVB) index, we can conclude that hydrogen bond is strengthened in S1 state, which is favourable for the occurrence of ESIPT reaction. The charge recombination behaviour of hydrogen bond induced by photoexcitation also further illustrates this point. Via constructing potential energy curves (PECs) based on restrictive optimisation and searching transition state (TS) configuration, we finally clarify the ESIPT mechanism for CN-BTN derivatives. Specially, we confirm change of atomic electronegativity has a regulatory effect on the ESIPT behaviour of CN-BTN derivatives, that is, the lower the atomic electronegativity is more conducive to the ESIPT reaction.

Effect of push–Pull functional group on the ESIPT behaviour and photophysical property of thiolflavone-based compound: a TD-DFT investigation

ABSTRACT The excited-state intramolecular proton transfer (ESIPT) mechanism and photophysical properties of three –CN-substituted (2-(4-(diethylamino)phenyl)- 3-mercapto-4H-chromen-4-one (3-NTF) compounds (3NTF-1, 3NTF-2, 3NTF-3) have been studied systematically based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. We analysed the structural parameters, infrared frequencies and electron densities of the studied compounds, and found that the intramolecular hydrogen bonds (IHBs) at tautomer form are weakened in the excited (S1) state. The weaker IHB at tautomer form is beneficial to ESIPT process. The potential energy curves (PECs) were established and showed that the ESIPT processes of 3NTF-1 and 3NTF-2 are barrierless. The ESIPT process of 3NTF-3 needs to overcome a very low energy barrier. The electron-withdrawing group (–CN) increases the energy barrier of ground state intramolecular proton transfer (GSIPT) process in 3NTF. The order of GSIPT barrier is consistent with the order of their IHB strengths of normal forms. When the electron-withdrawing group (–CN) was introduced, the absorption and fluorescence peaks red-shifted. Two –CN groups resulted in the fluorescence peak of 3NTF red-shift more than one –CN group did.

A fluorescent probe based on the ESIPT (excited state intramolecular proton transfer) mechanism for rapid detection of endogenous and exogenous H2O2 (hydrogen peroxide) in cells

Hydrogen peroxide (H2O2) is one of the important reactive oxygen species in the body and can be used as a marker of some diseases such as cancer and neurodegenerative diseases. Therefore, it is of great significance to develop fluorescent probes that can detect H2O2 in living organisms for early diagnosis of diseases. However, slow response time and low fluorescence quantum yield limit the application of many probes. In this study, using 2-(2-hydroxyphenyl) benzothiazole (HBT) as the fluorophore, the introduction of weakly absorbing bromine atoms can accelerate the speed of electron transfer during the recognition process. Three ESIPT (excited state intramolecular proton transfer) fluorescent probes JLO/JLM/JLP were designed and synthesized. The detection of H2O2 can be achieved with all three probes, and we screened a probe JLO with the fastest response time (30 min) and highest fluorescence quantum yield (Ф = 0.731). The probe also has a large Stokes shift, which can reduce fluorescence self-absorption and background interference, and also has a high sensitivity, which is designed to accurately detect endogenous and exogenous H2O2 in living cells, which has great potential for biological applications.

A TD‐DFT/DFT study on the ESIPT and photophysical properties of symmetrical 2‐hydroxybenzilidene1,3‐diamines derivatives

AbstractDensity functional theory (DFT) and Time dependent density functional theory (TD‐DFT) methods are used to simulate the photoexcitation and emission of the symmetrical 2‐hydroxybenzilidene1,3‐diamine (HBDA) Schiff base and some of its derivatives in the gas phase and solution. Our aim here is to explore the details of Excited‐State Intramolecular Proton Transfer (ESIPT) which underlies the activity of HBDA molecules as fluorescent probes. The structures of HBDA in S0 and the S1 states are optimized utilizing the DFT and TD‐DFT methods, respectively. Geometric configurations, electronic spectra, frontier molecular orbitals, and potential energy surfaces have all been computed and analyzed for the purpose of interpreting the mechanism of ESIPT. The geometries and relative stabilities of 15 tautomeric forms and the corresponding isomers of the studied molecules have been identified and their relative stabilities are investigated. The potential energy profiles and intrinsic reaction coordinates (IRC) calculations along the proton transfer coordinates both in the ground and in the excited state are monitored as well. The impact of the donating (OMe) and withdrawing (NO2) groups on the single and double proton transfers are investigated both in the gas phase and solution.

Dual action of a distinctive colloidal 3-(Benzimidazole-2-yl)-2-hydroxybenzaldehyde grafted chitosan support for the continual detection ability for Hg (II) in aqueous environ and inherent emission-induced live cell imaging

In this research work, an interesting sugar based fluorescent materials has been successfully developed to detect metal ions (Hg2+) in aqueous solution. The prepared colloid, 3-(benzimidazole-2-yl)-2-hydroxy benzaldehyde/chitosan (ChI) has significant characteristics as greener and eco-friendly material. ChI has found for switching of proton transfer between enol-imine and keto-enamine forms of ChI through excited-state intramolecular proton transfer (ESIPT) mechanism. The internal chemical-bonding and functionality of ChI are confirmed through FTIR, UV–Vis, fluorescence, SEM, TEM, DLS etc. ChI shows its fluorescence-quenching, selectivity, scavenging and chelating capabilities for Hg2+ ions in the presence of other cations in aqueous solution. Spectral and XRD analysis have established the remarkable continual and consistent detection of Hg2+ in aqueous system with a limit of detection of 0.204 nM. The strong chelating agent EDTA is used to regain of fluorescence of ChI in support of labile chelation ability of ChI and to show reversibility for continual detection of Hg2+ due to the strong EDTA/Hg2+ complexation. Scavenging ability is investigated using Na2S solution followed by the XRD analysis of precipitate appeared. Also, ChI shows its efficiency for the fluorescence cell imaging in live HADF cells with negligible cytotoxicity. Convincingly, ChI has processed as materials with very significant addition and advancement for its multimodal and eco-friendly behavior in current material development research.

ESIPT mechanism of triple emission with hydroxy-oxadiazole compound in DMSO: A theoretical reconsideration

The compound in solvents with triple fluorescence feature of excited state intramolecular proton transfer (ESIPT) has a broad prospect in fluorescent probes, dye sensors and molecular synthesis of photosensitive dyes. An ESIPT molecule hydroxy-bis-2,5-disubstituted-1,3,4-oxadiazoles (compound 1a) emits two fluorescence peaks in dichloromethane (DCM) and three fluorescence peaks in dimethyl sulfoxide (DMSO). [Dyes and Pigments 197 (2022) 109927]. Two longer peaks were attributed to enol and keto emission in both solvents and the shortest third peak in DMSO was just attributed simply. However, there is a significant difference in proton affinity between DCM and DMSO solvents which has influence on the position of emission peaks. Therefore, the correctness of this conclusion needs to be further verified. In this research, density functional theory and time-dependent density functional theory method are used to explore ESIPT process. Optimized structures indicate ESIPT occurs through molecular bridge assisted by DMSO. The calculated fluorescence spectra demonstrate two peaks indeed originated from enol and keto in DCM, while interestingly three peaks are originated from enol, keto and intermediate in DMSO. Infrared spectrum, electrostatic potential and potential energy curves further prove existence of three structures. We reveal the mechanisms that compound 1a molecule occurs ESIPT in DCM solvent and undergoes an ESIPT through assisted by DMSO molecular bridge. Additionally, three fluorescence peaks in DMSO are reattributed. Our work is expected to provide an insight for understanding intra- and intermolecular interactions and synthesis of efficient organic lighting-emitting molecule.

Computational insights of excited state intramolecular proton transfer (ESIPT) based fluorescent detection and imaging of γ-glutamytranspeptidase activity

γ-Glutamytranspeptidase (GGT) is an important tumor biomarker that widely appears in the tumor cells. Therefore, accurate imaging and detection of GGT activity in live cells, serum and pathological cells grasp great importance for the diagnosis, management, and treatment of cancer. Herein, 2-(2-hydroxyl-phenyl)-6-chloro-4-(3H)-quinazolinone (HPQ) is considered as the fluorophore probe for the detection of GGT activity, which is known for the typical mechanism of excited-state intramolecular proton transfer (ESIPT). All the simulations adopted to evaluate the sensing mechanism were carried out via DFT and TDDFT calculations at CAM-B3LYP/TZVP level of theory. The emission properties of HPQ and HPQ-TD are thoroughly studied to understand the photoinduced electron transfer (PET) and excited state intramolecular proton transfer (ESIPT) process. The results reveal that the fluorescence quenching of HPQ (enol form) is assigned to the PET process, whereas the large Stokes shift in fluorescence emission of HPQ (keto form) is related with ESIPT mechanism. The obtained results are further cross validated by frontier molecular orbital (FMO) analysis, geometric analysis, and potential energy curve (PEC) scanning. Our calculations provide powerful evidence for the ESIPT based sensing mechanism of HPQ (keto-enol form) for GGT activity.

Theoretical insights into atomic-electronegativity-regulated ESIPT behavior for B-bph-fla-OH fluorophore

Inspired by the potential applications for detecting cysteine and the novel phototriggered CO release molecule (photoCORM) property, B-bph-fla-OH is explored in this work about its excited state behaviors. In view of significance of atomic electronegativity in photochemical reactions, we mainly focus on decoding photo-induced hydrogen bonding interactions and atomic-electronegativity-dependent excited state intramolecular proton transfer (ESIPT) behavior for Bbph-O, Bbph-S and Bbph-Se derivatives. Combining optimized geometrical changes and infrared (IR) stretching vibration related to hydrogen bond Oa-Hb···Oc, the photo-induced hydrogen bonding strengthening can be confirmed. Via predicting bond energy (EHB), we find low atomic electronegativity is in favor of enhancing hydrogen bonding interactions in both S0 and S1 states. Charge reorganization stemming from photoexcitation and atomic-electronegativity energy gap between HOMO and LUMO orbitals further reveals the ESIPT tendency. Insights into potential energy surfaces (PESs) and constructing energy profiles via searching transition state (TS) for Bbph-O, Bbph-S and Bbph-Se, we present the atomic-electronegativity-regulated ESIPT mechanism and spectral behaviors. We sincerely wish the specific and controlled excited state dynamical behavior could pave the ways for designing and developing novel fluorescent sensors based on B-bph-fla-OH derivatives in future.

A theoretical study of the ESIPT mechanism for the 2-butyl-4-hydroxyisoindoline-1, 3-dione probe

For the synthesis of two-photon excited probes, fluorophores based on excited-state intramolecular proton transfer (ESIPT) become critical. The ESIPT process of 2-butyl-4-hydroxyisoindoline-1, 3-dione is analyzed in this paper by DFT and TDDFT methods. We choose three non-protonic solvents to study their solvation effect. By examining infrared vibration spectrum data, optimized geometric structures and reduced density gradient, the hydrogen bond strengthened after photoexcitation and increased gradually with the decrease of solvent polarity. The proton transfer in the excited state is a low-barrier ultrafast process. The observed fluorescence peak in the experiment is derived from the emission of the tautomer form. Compared with the reaction barriers on the potential energy curves for different non-protonic solvents, a weaker polar solvent is more favorable to the ESIPT process.

A Selective Excited-State Intramolecular-Proton-Transfer (ESIPT) Sensor for Copper(II) Based on Chelation-Enhanced Quenching and "Off-On" Detection of Amino Acids.

We report the synthesis of 2-(4,5-diphenyl-1H-imidazole-2-yl)phenol (TPI-9) as an interesting fluorescent molecule displaying Excited-State Intramolecular-Proton-Transfer (ESIPT) with stokes shift of 120 nm. Phenolic compounds with the ability to form intramolecular hydrogen bonds and subsequent proton transfer are known as ESIPT fluorophores. Proton accepting ability can increase significantly by tailoring electron-donating groups. With the assistance of an environment-friendly organocatalyst, 10-camphor sulfonic acid (10-CSA), TPI-9 was synthesized to introduce substituents with electron-donating abilities to develop an efficient ESIPT mechanism. Factors influencing the emission, such as solvent, pH, and metal ions, are investigated. Quenching of fluorescence by Cu2+ through chelation enhancement quenching effect with a high selectivity allowed the establishment of a Cu2+ sensor with an LoD of 0.57 ppm and a ratiometric estimation with an LoD of 0.73 ppm. Metal binding (2 : 1) stoichiometry and quenching constant (0.0072 mol-1 s-1 ) are calculated from Job's and Stern-Volmer plots. Density functional theory (DFT) calculations are in accordance with the experimental results. Competitive replacement of TPI-9 by amino acids restores ESIPT, consequently, the fluorescence. Thus, an "off-on" fluorescence sensor for amino acid estimation is developed under 1 minute incubation. A linear relationship between amino acid concentration and fluorescence intensity is in 0-20 μg/mL range, and the LoD is less than 2.2 μg/mL.