Excited-state Intramolecular Proton Transfer Process Research Articles

Controlling ESIPT-based AIE effects for designing optical materials with single-component white-light emission

Aggregation-induced emission (AIE) molecules have gained significant importance in various fields such as biological imaging and organic light-emitting diodes. Here, we focused on exploring the process of the induction and control of AIE effects by modifying intramolecular hydrogen bonds. Firstly, inspired by the AIE characteristics of our reported amino-type excited-state intramolecular proton transfer (ESIPT)-based molecules, qualitative and quantitative relationships between the acidity of the amino group and the ESIPT process were investigated. They were successfully revealed through artificial intelligence modeling and quantum chemical calculations. Based on this, eight compounds, (2-(2′-aminophenyl) benzothiazole (1) and its derivatives), were synthesized. Then, their mechanism of structural and optical phenomena (AIE or aggregation-caused quenching (ACQ)) was further verified by quantum chemical calculations and experiments. It has been proved that multistage photochemical reactions could lead to the AIE phenomenon. Finally, the eight compounds were applied to biological imaging and white light material applications, benefiting from tunable dual-emission spectroscopic properties of amino compounds with ESIPT properties. This study provided extensive guidance on developing molecules with AIE properties and opened up new avenues for AIE-based principles, as well as single-component white-light emission optical materials.

Theoretical Investigation on the "ON-OFF" Mechanism of a Fluorescent Probe for Thiophenols: Photoinduced Electron Transfer and Intramolecular Charge Transfer.

In this study, the sensing mechanism of (2E,4E)-5-(4-(dimethylamino)phenyl)-1-(2-(2,4dinitrophenoxy)phenyl)penta-2,4-dien-1-one (DAPH-DNP) towards thiophenols was investigated by density functional theory (DFT) and time-dependent DFT (TD-DFT). The DNP group plays an important role in charge transfer excitation. Due to the typical donor-excited photo-induced electron transfer (d-PET) process, DAPH-DNP has fluorescence quenching behavior. After the thiolysis reaction between DAPH-DNP and thiophenol, the hydroxyl group is released, and DAPH is generated with the reaction showing strong fluorescence. The fluorescence enhancement of DAPH is not caused by an excited-state intramolecular proton transfer (ESIPT) process. The potential energy curves (PECs) show that DAPH-keto is less stable than DAPH-enol. The frontier molecular orbitals (FMOs) of DAPH show that the excitation process is accompanied by intramolecular charger transfer (ICT), and the corresponding character of DAPH was further confirmed by hole-electron and interfragment charge transfer (IFCT) analysis methods. Above all, the sensing mechanism of the turn-on type probe DAPH-DNP towards thiophenol is based on the PET mechanism.

Regulating fluorescent properties and ESIPT behavior of novel flavone-based fluorophore by replacing oxygen atom in proton donor/acceptor with sulfur atom: A TD-DFT study

Recently, the fluorescent phenomena of a new excited state intramolecular proton transfer (ESIPT) based fluorophore 3-hydroxy-2-(naphthalen-2-yl)-4H-chromen-4-one (HFN) has been reported experimentally. However, the first-hand dynamic information of its ESIPT process is still lacking theoretically. Based on density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, the luminescent features and ESIPT processes of HFN and its two derivatives were studied in detail. Two HFN derivatives, 3-mercapto-2-(naphthalen-2-yl)-4H-chromen-4-one (TFN1) and 3-hydroxy-2-(naphthalen-2-yl)-4H-chromene-4-thione (TFN2) were obtained by replacing the oxygen atom of proton donor and proton acceptor in HFN with sulfur atom, respectively. The changes and effects of fluorescent properties and ESIPT process generated by different proton donors and proton acceptors are demonstrated. The simulated maximum absorption and emission peaks of HFN are well consistent with the experimental data. After replacing the oxygen atoms in proton donor and proton acceptor with sulfur atoms, the absorption wavelength of HFN slightly blue-shifts and red-shifts, respectively. And the fluorescence wavelengths of HFN obviously red-shift. The S0→S1 transitions of HFN and TFN1 are dominated by ππ* charge transfer transition, whereas the S0→S1/S0→S2 transition of TFN2 is a mixed charge transfer transition of nπ* and ππ*. The substitution of oxygen atom by sulfur atom in the proton donor or proton acceptor of HFN obviously decreases or increases the ESIPT barrier, due to its stronger proton-donating ability of S–H group or weaker proton-accepting ability of sulfur atom, respectively.

Paying Comprehensive Attention to the Temperature-Dependent Dual-Channel Excited-State Intramolecular Proton Transfer Mechanism of Fluorescence Ratio Probe BZ-DAM.

The mechanism of fluorescence detection of diethyl chlorophosphate (DCP) based on 2-substituted benzothiazole (BZ-DAM) was studied by a theoretical calculation method. It should not be ignored that both the BZ-DAM and the detection product BZ-CHO have two excited-state intramolecular proton transfer (ESIPT) channels. Density functional theory (DFT) and time-dependent DFT (TDDFT) theory were used to study the photophysical mechanism of two compounds in two channels in (acetonitrile) ACN solvent, and the temperature dependence of the two channels was given. Channel 1 is more likely to exist at low temperatures and channel 2 is more likely to exist at high temperatures. By theoretical analysis of the constructed potential energy curve, the hydrogen bond energy and electron-hole analysis, we confirmed that both molecules undergo ESIPT and intramolecular charge transfer (ICT) processes in channel 1 and ESIPT and twisted intramolecular charge transfer (TICT) coupling processes in channel 2. The formation of product BZ-CHO molecules led to a significant fluorescence blue-shift phenomenon and inhibited the ICT process, which confirmed that BZ-DAM could be used as a fluorescence probe for fluorescence detection. We sincerely hope that this work will not only help to clarify the excited-state dynamics behavior of the BZ-DAM probe but also provide a new idea for designing and optimizing a new chemical dosimeter.

Janus-Type ESIPT Chromophores with Distinctive Intramolecular Hydrogen-bonding Selectivity.

Excited-state intramolecular proton transfer (ESIPT)-based solid luminescent materials with multiple hydrogen bond acceptors (HBAs) remain unexplored. Herein, we introduced a family of Janus-type ESIPT chromophores featuring distinctive hydrogen bond (H-bond) selectivity between competitive HBAs in a single molecule. Our investigations showed that the central hydroxyl group preferentially forms intramolecular H-bonds with imines in imine-modified 2-hydroxyphenyl benzothiazole (HBT) chromophores but tethers the benzothiazole moiety in hydrazone-modified HBT chromophores. Imine-derived HBTs generally exhibit higher fluorescence efficiency, while hydrazone-derived HBTs show a reduced overlap between the absorption and fluorescence bands. Quantum chemical calculations unveiled the molecular origins of the biased intramolecular H-bonds and their impact on the ESIPT process. This Janus-type ESIPT chromophore skeleton provides new opportunities for the design of solid luminescent materials.

Proton coupled isomerization in double-hydrogen-bonding-center salicylaldehyde azine

The proton transfer dynamics of Schiff bases are of great importance due to the application potential. The excited state intramolecular proton transfer (ESIPT) of photochromic salicylaldehyde azine (SAA) in solutions was investigated by experimental and computational methods. Two distinguished spectral bands are observed in transient absorption spectra upon the photoexcitation with 400 nm. From the delayed stimulated emission signals, the ESIPT process is determined to be within <200 fs. Subsequently, the photoprotonated products undergo the vibrational relaxation with several picoseconds and the following isomerization with tens of picoseconds. These processes are significantly affected by the polarity of the solvents. The longest component with nanosecond scale can be explained to the relaxation to the enol structure. According to our observations, the SAA molecules with symmetric double-hydrogen-bonding-centers undergo single proton transfer rather than double proton transfer and subsequent intramolecular isomerization.

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.

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.

Reconsideration of the ESIPT off mechanism for fluorescent probe MNC in aqueous solution

Fluorescent probes with excited state intramolecular proton transfer (ESIPT) properties play a significant role in the research of life science and material science. Guo et al. designed 3-hydroxy-2-(6-Methoxynaphthalen-2-yl)-4H-chromen-4-one (MNC) as a control to achieve the dual-color fluorescence imaging of lipid droplets and endoplasmic reticulum (ER). They deemed that the ESIPT process would be turned off in ER with high water content [J. Am. Chem. Soc. 2021, 143, 3169–3179]. However, contrary to the conventional ESIPT off case, the enol* state fluorescence intensity that should have been enhanced was severely quenched in water. Here, combined with ultrafast spectrum, steady-state fluorescence spectrum and potential energy surface, the mechanism of ESIPT process of MNC turned off in water is revised. Furthermore, the formation of aggregated states in water is responsible for the quenching of MNC fluorescence. This work is expected to provide broader ideas for the design of hydrophobic fluorescent probes.

Study on ESIPT of salicylaldehyde derivative EQCN in DCM solvent

The fluorescence of salicylaldehyde derivative (EQCN) as an excitation-wavelength-dependent molecule with long-persistent luminescence has been investigated experimentally and theoretically. However, the excited-state intramolecular proton transfer (ESIPT) process mechanism and optical property associated with photochemical process of EQCN molecule in dichloromethane (DCM) solvent has not been discussed in detail. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to investigate ESIPT process of EQCN molecule in DCM solvent. By optimizing the geometry of the EQCN molecule, the hydrogen bond interaction of EQCN molecule of Enol structure in excited state (S1 state) is strengthened. The calculated absorption peak and fluorescence peak agree well with the experimental values. Based on the optimized geometric structure, the frontier molecular orbital isosurface (FMOs) were drawn, and the redistribution of electron density in DCM solvent was depicted, which intuitively explain the changes in the photophysical properties of EQCN. Through the calculated potential energy curves (PECs) of EQCN in both DCM solvent and ethanol solvent, the ESIPT process of EQCN was found more likely to occur in ethanol solvents.

Regulating the ESIPT processes and photophysical properties of indole derivates by introducing the appropriate functional groups

In this work, the substitution effect of different electron-donating and electron-withdrawing groups on the electron spectra and excited state intramolecular proton transfer (ESIPT) properties of the chromophore 2-(2-hydroxy-5-methylphenyl)benzo[d]oxazole-5-sulfonic acid (BOM-1) has been studied via the density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. All the compounds exhibit the dual-fluorescence emission, and the fluorescence spectra of the derivatives show redshift emission compared with that of the original molecule, resulting in the larger Stokes shift as expected. The results were cross-verified by the molecular geometry, frontier molecular orbitals, infrared spectroscopy and other auxiliary calculations. These results show that the electron-withdrawing group greatly promote the occurrence of the forward ESIPT process and increase the Stokes shift. What is more special is that the Stokes shift and the energy barrier of the forward ESIPT are significantly enlarged by introducing both electron-withdrawing and electron-donating groups. The energy barrier, absorption and fluorescence spectra of luminescent molecules can be adjusted effectively by introducing appropriate functional groups. In addition, the emission spectra from the ultraviolet region (359 nm) to the orange region (601 nm) are obtained. This work would contribute to the understanding of the mechanism of ESIPT reactions and improves the luminescence properties of ESIPT molecules.

Synthesis and Photochemical Properties of Fluorescent Metabolites Generated from Fluorinated Benzoylmenadiones in Living Cells.

This work describes the reactivity and properties of fluorinated derivatives (F-PD and F-PDO) of plasmodione (PD) and its metabolite, the plasmodione oxide (PDO). Introduction of a fluorine atom on the 2-methyl group markedly alters the redox properties of the 1,4-naphthoquinone electrophore, making the compound highly oxidizing and particularly photoreactive. A fruitful set of analytical methods (electrochemistry, absorption and emission spectrophotometry, and HRMS-ESI) have been used to highlight the products resulting from UV photoirradiation in the absence or presence of selected nucleophiles. With F-PDO and in the absence of nucleophile, photoreduction generates a highly reactive ortho-quinone methide (o-QM) capable of leading to the formation of a homodimer. In the presence of thiol nucleophiles such as β-mercaptoethanol, which was used as a model, o-QMs are continuously regenerated in sequential photoredox reactions generating mono- or disulfanylation products as well as various unreported sulfanyl products. Besides, these photoreduced adducts derived from F-PDO are characterized by a bright yellowish emission due to an excited-state intramolecular proton transfer (ESIPT) process between the dihydronapthoquinone and benzoyl units. In order to evidence the possibility of an intramolecular coupling of the o-QM intermediate, a synthetic route to the corresponding anthrones is described. Tautomerization of the targeted anthrones occurs and affords highly fluorescent stable hydroxyl-anthraquinones. Although probable to explain the intense visible fluorescence emission also observed in tobacco BY-2 cells used as a cellular model, these coupling products have never been observed during the photochemical reactions performed in this study. Our data suggest that the observed ESIPT-induced fluorescence most likely corresponds to the generation of alkylated products through reduction species, as demonstrated with the β-mercaptoethanol model. In conclusion, F-PDO thus acts as a novel (pro)-fluorescent probe for monitoring redox processes and protein alkylation in living cells.

Turning enol* emission of SBOH via restricting twisted intramolecular charge transfer behavior by pressure.

Stimuli-responsive luminogens with aggregation-induced emission and excited state intramolecular proton transfer (ESIPT) properties have applications in storage devices, anti-counterfeiting, imaging, and sensors. Nevertheless, group rotation appears in twisted intramolecular charge transfer (TICT) state, resulting in decreased fluorescence intensity. Inhibiting TICT remains a challenge based on their intrinsic molecular configuration. Herein, we present a simple facile pressure-induced method to restrict the TICT behavior. Steady-state spectroscopy measurement shows that fluorescence enhancement and color shifts can be achieved under high pressure. Combined with in situ high-pressure ultrafast spectroscopy and theoretical calculations, the TICT behavior was restricted in two aspects. The ESIPT process was damaged, hence more particles stored in the E* state, and transferred to the TICT state hardly. Also, the rotation of (E)-dimethyl5-((4-(diethylamino)-2-hydroxybenzylidene)amino)isophthalate (SBOH) was restricted, significantly increasing the fluorescence intensity. This approach provides a new strategy for the development of stimulus-responsive materials.

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.

Molecular Principles of the Excited-State Intramolecular Thiol Proton Transfers in 3-thiolflavone Derivatives.

The excited-state intramolecular proton transfer (ESIPT) effect has attracted considerable attention due to its potential applications in photoluminescent materials. However, only a few theoretical reports have investigated the ESIPT process involving sulfur-hydrogen bonds. Herein, we systematically investigated the ESIPT effect of three 3-thiolflavone derivatives containing sulfur-hydrogen bonds with M06-2X functional combined Def2-TZVP basis set. The intramolecular sulfur-hydrogen bonds were confirmed in the ground and excited states via analyzing the bond lengths, interaction energies, and infrared vibrational spectra. Besides, we demonstrated that the electron-withdrawing group led to a more stable tautomer compared to the electron-donating group. Conversely, the electron-donating group played a crucial role in reducing the energy barrier of the ESIPT reaction due to the strengthening of the hydrogen bond in the excited state. Interestingly, the substituent group can determine the excited-state electronic properties of keto tautomers. Specifically, the electron-withdrawing group caused significant improvement in the nπ* transition configuration, significantly reducing the radiation rate. The electron-donating group increased the proportion of the ππ* transition configuration, thus, the keto tautomer had bright emission. We expect these findings to open new avenues for designing potential luminescent materials.

Interface modification of perovskite solar cell for synergistic effect of surface defect passivation and excited state property enhancement

The efficiency of perovskite solar cell (PSC) is significantly affected by the charge transport and recombination behaviors at the interfaces between the perovskite photoactive layer and charge transport intermediate layers. Especially, the defects, existing in those interfaces, induce non-radiative charge carrier recombination, reducing the stability of the device along with its performance. In this work, we newly design an imidazole- and methoxy-functionalized triphenylamine based hole-transport material (HTM), which has defect-passivating functionality together with proper carrier-transportation property, for the p-i-n PSC devices. Additionally, it is also designed to preserve its enhanced molecular dipole moment at the excited-state with longer lifetime through excited-state intramolecular proton transfer (ESIPT) process, consequently beneficial to improve the Vbi of the device. The designed HTM molecule, having a synergistic effect of defect-passivation and photo-enhanced dipole characteristic, is confirmed to be advantageous to produce PSCs with higher performance and stability.

A simple AIE-active salicylideneaniline towards bimodal encryption-decryption with unique ESIPT-inhibited amorphous state

Multimodal encryption-decryption with photochromism has been proved to ensure information security with great feasibility and operability. However, most of the photochromic materials are applied in single-mode encryption-decryption and highly dependent on crystallization, requiring long manufacturing time and lacking of controllability. Hence, amorphous materials with simple preparation and bimodal encryption-decryption are of good alternates. Herein, we have succeeded in developing an aggregation-induced emission-active and photochromic ATPA-SAB with crystalline form via introduction of the propeller-like and bulky triphenylamine (TPA). Expectedly, the amorphous materials can be obtained by fast precipitation. Examining its amorphous structures, the intramolecular N‧‧‧H distances of around 4.000 Å is largely over the normal intramolecular hydrogen bond of circa 1.800 Å, impeding the intrinsic excited-state intramolecular proton transfer process (ESIPT) for the first time, hence prohibiting photochromism. This work not only provides a strategy to construct amorphous materials and inhibit ESIPT process with TPA, but also applies both crystalline and amorphous materials for bimodal encryption-decryption to upgrade information security.

Liquid-solid phase regulating excited-state intramolecular proton transfer process of HBT-d-NO2: A QM/MM study

The excited-state intramolecular proton transfer process of 2-(1,3-benzothiazol-2-yl)-4-[2-(4-nitrophenyl)ethynyl]phenol (HBT-d-NO2) in the different surrounding environment is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The optimized molecular structure provides convincing evidence that the intramolecular hydrogen bond is strengthened in the first excited (S1) state. The frontier molecular orbitals observed the HBT-d-NO2 exists obvious intramolecular charge translate phenomenon. The results of the potential energy curve show that HBT-d-NO2 is difficult to undergo proton transfer in the ground (S0) state due to the high energy barrier, while it becomes easier in the S1 state in both liquid and solid phases. By comparison, the energy barrier of ESIPT in the solid phase is higher than that in the liquid phase. We can conclude that the solid phase effectively hinders the ESIPT process compared with that the liquid phase. In this work, we illustrate the influence of liquid and solid phases on the intramolecular proton transfer process, which could promote further developments in biomedical and fluorophore applications.