Excited-state Intramolecular Proton Transfer Research Articles

Computational Insights into Excited State Intramolecular Double Proton Transfer Behavior Associated with Atomic Electronegativity for Bis(2'-benzothiazolyl)hydroquinone.

Inspired by the distinguished regulated photochemical and photophysical properties of 2-(2'-hydroxyphenyl)benzazole derivatives, in this work, the novel bis(2'-benzothiazolyl)hydroquinone (BBTHQ) fluorophore is explored, looking at its photo-induced behaviors associated with different substituted atomic electronegativities, i.e., BBTHQ-SO, BBTHQ-SS and BBTHQ-Se compounds. From the structural changes, infrared (IR) vibrational variations and simulated core-valence bifurcation (CVB) indexes for the dual hydrogen bonds for the three BBTHQ derivatives, we see that low atomic electronegativity could be conducive to enhancing hydrogen bonding effects in the S1 state. Particularly, the O4-H5⋯N6 of BBTHQ-SO and the O1-H2⋯N3 of BBTHQ-SSe could be strengthened to be more intensive in the S1 state, respectively. Looking into the charge recombination induced by photoexcitation, we confirm a favorable ESDPT trend deriving from the charge reorganization of the dual hydrogen bonding regions. By constructing the potential energy surfaces (PESs) along with the ESDPT paths for the BBTHQ-SO, BBTHQ-SS and BBTHQ-Se compounds, we not only unveil stepwise ESDPT behaviors, but also present an atomic electronegativity-regulated ESDPT mechanism.

Achieving the cooperative and stepwise regulation of ESIDPT process in AFBD by introducing different electronic groups

In this paper, the effect of different electronic group substituents on the excited state intramolecular double proton transfer (ESIDPT) in AFBD is comprehensively researched by using quantum chemistry methods. Two electronic groups with comparable capabilities, including the electron-withdrawing group (-Br) and the electron-donating group (–OCH3), are chose to accomplish this study. The obtained geometric parameters, reduced density gradient and infrared spectra calculations have demonstrated that AFBD and its two derivatives experience the ESIDPT process. Subsequently, the conclusion of the potential energy surface and Bonn-Oppenheimer molecular dynamics further unveil that AFBD undergoes a cooperative ESIDPT process at ∼15 fs. Nevertheless, it should be noted that the introduction of -Br and –OCH3 transforms the proton transfer path. That is: "the cooperative ESIDPT is converted into the stepwise ESIDPT". Besides, we also confirm that the substitution of the electronic groups OCH3→H→Br can accelerate the ESIDPT process. Our work not only regulates the path of ESIDPT process, but also provides a time scale for this process, which is conducive to developing new intelligent fluorophores based on ESIDPT behavior.

ESIPT‐Active Pyrene‐imidazole Fluorophores: GSIPT, Dual Solid‐ and Solution‐State Emission plus Counter‐Intuitive Crystal Packing**

AbstractWe report a new excited‐state intramolecular proton transfer (ESIPT)‐inspired hydroxyl (OH)‐substituted pyrene‐imidazole that promote unprecedented intramolecular proton transfer in the ground state (GSIPT). The enol and keto isomers are in equilibrium, with a high keto‐isomer component in the solid state. In conjunction with DFT calculations, the photophysical studies and crystal structure analysis shred substantial evidence for the ground state transformation. The ESIPT‐active compounds also show the rare feature of high dual solution (φf=70 %) and solid state (φf=41 %) emission characteristics and counter‐intuitive stronger intermolecular interactions in the solid state versus the ESIPT‐inactive counterparts.

Tetraphenylethene derivatives with AIE-ESIPT characteristics: Synthesis, crystal structures, photophysical properties and application for ClO− detection and pH sensing

The fluorescent probe with both aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) shows significant advantages, such as no self-quenching at high concentration, large Stokes shift, high selectivity towards analyte etc. Based on tetraphenylethene and ESIPT structural modules, three AIE-ESIPT molecules containing one different atom (S, O, N) have been constructed in this paper. The effects of three atoms on the optical properties of fluorescent molecules have been investigated by fluorescence spectroscopy and X-ray crystallographic analyses. Interestingly, the cyclic detection of HCl and NH3 gases can be achieved with the BTY-TPE-based portable test paper. In addition, BOY-TPE and HBIY-TPE can detect ClO− by naked eye with low detection limit, good biocompatibility.

Turn on the luminescence channel by the suppression of conical intersection process for the sulfur-containing hydrogen-bonded molecules

The sulfur-containing hydrogen bonds exhibit notable potential in gene and protein engineering. As an important research direction of sulfur-containing hydrogen bonding dynamics, the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton has garnered significant interest. Recently, Chou et al. synthesized a series of sulfur-containing H-bonded molecules, inclusive of 3-thiolflavone (3 TF), its trifluoromethyl derivative (3FTF), and its diethylamino derivative (3NTF), thereby inaugurating a new chapter in the ESIPT reaction of the thiol proton. Nevertheless, many open issues cannot be answered solely by experimental means and call for a detailed theoretical investigation, especially the relationship between the molecular excited-state properties and different luminescence properties of the three molecules. Herein, we validate that the molecules undergo the ESIPT reaction of the thiol proton and identify the time scale through the time-dependent density functional theory (TDDFT) calculation. Moreover, employing the spin-flip TDDFT approach, the analysis of the non-radiative transition process reveals that difference in Gibbs free energy of activation (ΔG⧧) for the transition from Frank-Condon point to minimum energy conical intersection (MECI) point primarily accounts for the different luminescence properties of the three molecules. Besides, we also reveal the reason for the abnormal blue-shifted fluorescence of the 3NTF in the aggregated state by the ONIOM (TDDFT: UFF) strategy. These findings are favorable to understanding the luminescence mechanism of sulfur-containing H-bonded molecules and provide a reference for the synthesis and design of the luminescent materials based on the molecules with thiol proton transfer properties.

One/Two-Photon-Excited ESIPT-Attributed Coordination Polymers with Wide Temperature Range and Color-Tunable Long Persistent Luminescence.

The multiple metastable excited states provided by excited-state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature-dependent and color-tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D-π-A structure to obtain both one/two-photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM-106) from ESIPT ligand to achieve the above goals. By comparing LIFM-106 with the counterparts, we established a temperature-regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM-106 shows outstanding one/two-photon excited LPL performance with wide temperature range (100-380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti-counterfeiting systems.

One/Two‐Photon‐Excited ESIPT‐Attributed Coordination Polymers with Wide Temperature Range and Color‐Tunable Long Persistent Luminescence

AbstractThe multiple metastable excited states provided by excited‐state intramolecular proton transfer (ESIPT) molecules are beneficial to bring temperature‐dependent and color‐tunable long persistent luminescence (LPL). Meanwhile, ESIPT molecules are intrinsically suitable to be modulated as D‐π‐A structure to obtain both one/two‐photon excitation and LPL emission simultaneously. Herein, we report the rational design of a dynamic CdII coordination polymer (LIFM‐106) from ESIPT ligand to achieve the above goals. By comparing LIFM‐106 with the counterparts, we established a temperature‐regulated competitive relationship between singlet excimer and triplet LPL emission. The optimization of ligand aggregation mode effectively boost the competitiveness of the latter. In result, LIFM‐106 shows outstanding one/two‐photon excited LPL performance with wide temperature range (100–380 K) and tunable color (green to red). The multichannel radiation process was further elucidated by transient absorption and theoretical calculations, benefiting for the application in anti‐counterfeiting systems.

A butterfly-shaped AIEgen with excited-state intramolecular proton transfer effect for colorimetric and fluorescent detection of copper and zinc ions

Multipurpose aggregation-induced emission (AIE) fluorescence probes for metal ion detection have drawn extensive interest in recent years. Here, a butterfly-shaped AIE luminogen (TOPD) was synthesized from 2-hydroxy-5-(1,2,2-triphenylethenyl)-benzaldehyde and o-phenylenediamine via a one-step Schiff-base condensation for flexibly detecting copper and zinc ions. The aggregated state of TOPD revealed a large Stokes shift of 263 nm due to the excited-state intramolecular proton transfer (ESIPT) effect benefiting from the planar cis-configuration and enol-keto tautomerization of TOPD molecules confirmed by the modeling investigation. TOPD showed remarkable colorimetric and high selective fluorescence turn-off response to Cu2+ in phosphate buffer saline/tetrahydrofuran (PBS/THF, 9:1, v/v) with a low detection limit (0.14 μM) and quick response time (<30 s). TOPD realized the Zn2+ sensing via turn-on and blue-shifted fluorescence in PBS/THF (8:2, v/v) with a detection limit of 0.93 μM. The significant color changes can be directly discriminated by the “naked eye”. Experiments and DFT theoretical calculations strongly suggested that sensing mechanisms were via 1:1 TOPD-metal complexes. The strategy of tunable AIE fluorescence could provide new ideas for potential applications in the detection of multiple ions in environmental fields.

Theoretical insights into ultrafast excited state proton transfer coupled with twisted intramolecular charge transfer mechanism in 7-(2ʹ-pyridyl)indole: Effect of hydrogen bonding

The ultrafast proton transfer (PT) process in the excited state of 7-(2′-pyridyl)indole (abbreviated as 7PyIn) has been elucidated by on-the-fly dynamics simulations using the algebraic-diagrammatic construction scheme of second order (ADC(2)) method under the resolution of identity (RI) approximation. The tautomers of 7PyIn alone and associated with an acetonitrile molecule in the 7PyIn-CH3CN complex are most likely found after the excited state intramolecular proton transfer (ESIntraPT) within an ultrafast time scale between 40 and 60 fs. While a methanol molecule in the complex of 7PyIn-CH3OH could establish both ESIntraPT and excited-state intermolecular proton transfer (ESInterPT). ESInterPT of 7PyIn-CH3OH possibly occurs through a concerted process via the intermolecular hydrogen bonding bridge of the methanol molecule almost twice faster than that of ESIntraPT. This slower circumstance of the ESIntraPT process found in 7PyIn-CH3OH may be caused by the competition between formations of the intramolecular and intermolecular hydrogen bonds between 7PyIn and methanol. In addition, after the accomplishment of excited state proton transfer (ESPT), the initiation of conical intersection could be taking place by the structural twist of 7PyIn. The geometries, energies, proton probabilities, and provided mechanisms via the dynamics simulations of 7PyIn and its solvent clusters exhibiting ESIntraPT and ESInterPT are explored to provide the dynamic pictures of the ESPT processes at the molecular level.

Tunable ESIPT pathways in 3-hydroxychromone derivatives

The excited-state intramolecular proton transfer (ESIPT) chromophores garner tremendous research interest owing to their peculiar optical properties. Here, we explore the influence of structural modification through π-conjugation elongation on the optical properties of 3-hydroxychromone derivatives, 3-hydroxy-2-(thiophen-2-yl)chromen-4-one (3-HTC) and 5-(3-hydroxy-4-oxo-4H-chromen-2-yl)thiophen-2-carbaldehyde (3-HTCA). Upon photoexcitation to S1, the intramolecular proton transfer is expected to occur via this state in 3-HTC. The energetically accessible S1-S2 conical intersection would promote the proton transfer via both S1 and S2 states in 3-HTCA. Our study suggests that compared to 3-HTC, 3-HTCA exhibits a red-shifted dual emission with a larger Stokes shift and covers a broader range of the visible spectrum.

The case for an oxidopyrylium intermediate in the mechanism of quercetin dioxygenases

The quercetin dioxygenases (QDOs) are unusual metalloenzymes in that they display ring-opening dioxygenase activity with several different first-row transition metal ions which do not undergo redox changes during turnover. The QDOs are also unique in that the substrate binds as an η1-flavonolate rather than the η2 -bidentate mode seen in all reported model complexes. The flavonol substrates were early examples of excited state intramolecular proton transfer (ESIPT) phenomena, in which photoexcitation causes an H-atom exchange between the adjacent hydroxyl and ketone, generating an oxidopyrylium emissive state. These oxidopyryliums undergo ring-opening dioxygenations analogous to the enzymatic reactions. Our hypothesis is that lability of the divalent metal ion may allow access to a reactive oxidopyrylium intermediate via coordination switching from the oxy to ketone position, which allows reaction with O2. In this report, we use a straight-forward methylation strategy to generate a panel of flavonol and thioflavonol derivatives modeling several η1- and η2−coordination modes. Methylation of 3-hydroxythioflavone generates an air stable η1 hydroxopyrylium salt, which undergoes rapid ring-opening dioxygenation by deprotonation or photoexcitation. By comparison, the η1-methoxyflavonol does not react with O2 under any condition. We find that any of the studied flavonol derivatives, η1 or η2, which demonstrates ESIPT-like oxidopyrylium emissions undergo QDO-like ring-opening reactions with dioxygen. The implications of these results concerning the mechanism of QDOs and related dioxygenases is discussed.

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.

Regulating excited‐state intramolecular proton transfer behavior and fluorescent property of 3‐thiolflavone by electron‐donating ability: A time‐dependent density functional theory study

AbstractBackgroundSince the excited‐state intramolecular proton transfer (ESIPT) process involving sulfur is extremely fast, its mechanism is still unclear, and its derivatives are rarely studied, we have performed a theoretical study the ESIPT process of 3‐thiolflavone (3TF) and its derivatives.ObjectiveThe ESIPT mechanism and photophysicalproperties of 3‐thiolflavone (3TF) and its derivatives 3NTF, 3ATF and 3OTF modified by ‐N(C2H5)2, ‐NH2 and ‐OCH3 groups were studied theoretically.MethodsDensity functional theory (DFT) and time‐dependent density functional theory (TD‐DFT) with PBE0 functional and 6‐31G(d, p) basis set.ResultsFor 3TF, 3NTF, 3ATF and 3OTF, the ESIPT processes are barrier‐less, and the barrier of ground‐state intramolecular proton transfer (GSIPT) reduces in the order of 3TF &gt; 3OTF &gt; 3ATF &gt; 3NTF, as the electron‐donating ability of the substituent increases. When the electron‐donating groups are introduced, the absorption and fluorescence wavelengths of 3TF red‐shift and blue‐shift, respectively.ConclusionSubstituents with electron‐donating ability can regulate the ground‐state intramolecular proton transfer process and photophysical properties of 3TF.

The Synergistic Mechanisms of AIE, ESIPT and ICT in the α-cyanostilbene-based Derivative: A Red-fluorescence Probe With a Large Stokes' Shift for Copper (II) Ion Determination and Reversible Response to Amine/acid Vapor.

Herein, α-cyanostilbene-based luminogen with an electron donor-π-electron acceptor (D-π-A) architecture was formylated into the salicylaldehyde-analogue luminogen, followed by the Schiff base reaction with phenylamine, a red-emitting luminogen was elaborately designed and successfully synthesized in a high yield of 89%. Its well-defined structure was confirmed by FT-IR, MALDI-TOF-MS, HR-MS and 1H/13C NMR technologies. Based on the synergistic mechanisms of aggregation-induced emission (AIE), excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT), it enjoyed a red-fluorescence emission at 627 nm in THF/water mixtures (fw = 95%) and was used as a probe. Moreover, the TLC-based test strips loaded with the probe not only exhibited the reversible fluorescence response to amine/acid vapor but also showed sensitive and selective fluorescence response towards Cu2+. Furthermore, the fluorescence titration experiment between the probe and Cu2+ in THF/water mixtures (fw = 95%, pH = 7.4) revealed that the detection limit was 1.18 × 10-7 M and the binding constant was 1.59 × 105. Job's plot experiment and HR-MS analysis revealed the 2:1 binding stoichiometry of the probe with Cu2+. The method enabled real-time assessment for Cu2+ in real water samples. This study could offer insightful opinions on the development of long-wavelength emissive luminogens based on α-cyanostilbene.

Improving Copper(II) Sensitivity by Combined use of AIEE Active and Inactive Schiff Bases.

An aggregation-induced emission enhancement (AIEE) active Schiff base PNN was synthesized by condensing benzidine with 2-hydroxynaphthaldehyde. The green-fluorescent PNN (λem = 510nm) in DMF turned to yellow-fluorescent PNN (λem = 557nm) upon increasing the fractions of HEPES buffer (10 mM, pH 7.4) above 40%. The DLS study supports the self-aggregation of PNN that restricts the intramolecular rotation and activates the excited-state intramolecular proton transfer (ESIPT) process. The fluorescence emission of AIEE active PNN was quenched by Cu2+ with an estimated detection limit of 2.1 µM. Interestingly, the detection limit of PNN towards Cu2+ was improved in the presence of an AIEE inactive Schiff base PBPM obtained by reacting 1,4-diaminobenzene with pyridine-4-carbaldehyde. The mixed PNN-PBPM showed a detection limit of 0.49 µM. The practical utility of PNN-PBPM was validated by quantifying Cu2+ ions in real environmental water samples and green tea.

An isonicotinohydrazide based fluorescence sensor for detection of Zn2+ in biological systems: Experimental and theoretical studies along with cell imagine

Zinc ions (Zn2+) play a significant role in our daily lives and production, which is related to human health and environment protection. Therefore, it is crucial in this field to develop sensors with improved analyte selectivity and sensitivity. We prepared a fluorescent sensor, DIH, through the reaction of 5-(tert-butyl)-2-hydroxyisophthalaldehyde with isonicotinohydrazide. DIH, in free form, can undergo excited state intramolecular proton transfer (ESIPT) process upon excitation. Among a wide range of metal ions tested, only Zn2+ inhibited this ESIPT process, resulting in an increase in fluorescence intensity (13.3-fold) with chelation enhancement fluorescence process (CHEF). The complex (DIH/Zn2+) stoichiometry was determined to be 1:1 and the binding constant of the complex was estimated as 3.18 × 107 M−1. DIH could also be applied to detect zinc ions down to a low concentration level of 11.2 nanomolar with fluorescence spectroscopy. Moreover, the cytotoxicity evaluation of DIH against human colon adenocarcinoma cells lines demonstrated that the sensor was nontoxic and could be used successfully in cellular imaging.

Spectroscopic studies on the supramolecular interactions of methyl benzoate derivatives with p-sulfocalix[6]arene macrocycles

This paper is a continuation of our previous research and aims to further investigate and elucidate the nature and mechanisms of noncovalent supramolecular interactions between four methyl benzoate derivatives (I-IV), which are capable of exhibiting Twisted Intramolecular Charge Transfer (TICT) and/or Excited State Intramolecular Proton Transfer (ESIPT)‐type behavior, and chemical and biological nanocavities. Photophysical and photochemical properties of molecules I-IV in aqueous solution in the presence of well-recognized macrocyclic host p-sulfocalix[6]arenes (SCA[6]) have been studied using steady-state, time-resolved and 1H NMR spectroscopic techniques. The changes in the ground- and excited-state spectroscopic characteristics (absorption and fluorescence spectra, time-resolved fluorescence spectra, fluorescence decay times and 1H NMR spectra) undergo significant modifications upon encapsulation of the investigated methyl benzoate derivative in the macromolecular cavity. For the two compounds (I and II), the interactions with the macrocycles with a hydrophobic SCA[6] cavity lead to the formation of stable inclusion complexes with 1:1 stoichiometry, both in the ground and excited state, while the stoichiometry of the III-SCA[6] and IV-SCA[6] complexes in the ground and excited states is 1:2. The values of the equilibrium constants have been determined from the spectroscopic data using Benesi-Hildebrand and nonlinear regression procedures. The location of the organic molecule inside the SCA[6] has been investigated by 1H NMR experiments. The changes in macrocyclic compound-induced NMR chemical shifts clearly indicate that the chemical structure of inclusion complexes is very different for methyl benzoate derivative-SCA[6] and methyl benzoate derivative-CB[7] systems. Finally, we have shown, using time-dependent fluorescence Stokes shift, that very fast solvation dynamics of pure water is markedly different from that of the confined water molecule in SCA[6] system.

Substituent effect on the intramolecular hydrogen bond and the proton transfer process in pyrimidine azo dye : A computional study

This study provides a complete analysis of the electronic and photophysical properties of, the derivative of uracil, IsoOrotic (IOA) azo dyes. The ability of the dye to work as an excited state intramolecular proton transfer (ESIPT) was investigated by using Density Functional Theory (DFT) and Time Dependent Density Functional Theory (TD-DFT) methods. The effect of electron-donating substituents (CH3 and NMe2) and an electron-withdrawing one (NO2) was examined. In addition, the effect of the solvent polarity on the ESIPT process is studied. All the geometrical structures in the singlet ground (S0) and excited (S1) states, were optimized using B3LYP/6–311 + G** level of theory. The intramolecular hydrogen bond parameters (IHBs), and the Infra-Red vibrational analysis of the O-H bond show that the IHBs are enhanced in the S1 state. Furthermore, the absorption and emission spectra were simulated and the values of stokes shifts were observed. The PAIOA derivative with an electron withdrawing group shows large stokes shift compared with those having electron-donating ones. Therefore, we can safely conclude that the substituent groups and the different solvents are extremely impactful on the ESIPT process.

Time-resolved fluorescence spectroscopy for studying stepwise excited-state intramolecular double proton transfer

Nowadays, single excited-state intramolecular proton transfer (ESIPT) is well studied and widely reported. However, excited-state multiple intramolecular proton transfer has not been thoroughly researched until the past decade. And now there is an increasing scientific interest not only in single but also in multiple ESIPT processes occurred in various molecules, because it offers great opportunities to apply such compounds in chemical sensors, molecular switches, NIR emitting devices and liquid crystals with high fluorescence. A stepwise excited-state intramolecular double proton transfer (ESIDPT) is among such multiple ESIPT processes. Similar to the single ESIPT, stepwise ESIDPT also occurs via an intramolecular hydrogen bonding way. But in the stepwise ESIDPT, two proton transfers occur sequentially in a molecule, because there is only one proton transfer site in the initial molecular structure. Then, after the first ESIPT, an additional proton transfer site appears, and the second proton transfer will occur.In this work, an emission of stepwise ESIDPT-active molecules was studied from the point of view of a time-resolved fluorescence spectroscopy. The theoretical model for emission decay was discussed on the basis of a six states model. This model consists of three pairs of ground and excited states for normal molecular form and two tautomers. It leads to three-exponential equations for emission decay functions. It was shown that a thorough analysis of fluorescence decay curves allows to estimate the total decay rate constants for each excited molecular form. The proposed model has been validated for a numerically simulated case. The experimental data have also been discussed from the point of view of the proposed model.

Theoretical Insights into Excited‐State Stepwise Double Proton Transfer Associated with Solvent Polarity for 2‐bis(benzothia‐zolyl)naphthalene‐Diol Compound

AbstractInspired by the excellent photochemical properties of 2‐(2′‐hydroxyphenyl)benzazole and its derivatives, in this work, the novel 2‐bis(benzothia‐zolyl)naphthalene‐diol (2‐BBTND) fluorophore is explored about its photo‐induced behaviors. The intramolecular double hydrogen bonding interaction and the excited state intramolecular double proton transfer (ESDPT) behavior of 2‐BBTND fluorophore are investigated in combination with different polar solvent environments. From the structural changes and charge recombination induced by photoexcitation, we can affirm that strong polar solvent environment promotes the excited state dynamical reaction for 2‐BBTND compound. By constructing potential energy surfaces (PESs) in S0 and S1 states, we clarify that 2‐BBTND fluorophore should undergo the stepwise ESDPT reaction after photoexcitation. Combined with the size of potential energy barriers along with reaction paths in different solvents, we finally propose a new solvent‐polarity‐dependent stepwise ESDPT for 2‐BBTND fluorophore.