Stepwise Excited-state Double Proton Transfer and Fluorescence Decay Analysis

This work considers excited state intramolecular proton transfers (ESIPT) occurred in multiple hydroxyl-containing compounds with one proton transfer site in the normal form. If several hydroxyl groups are located close to each other in a molecule, then the ESIPT process can lead to the next one. A proton donor site in the first ESIPT will be a proton acceptor during the second reaction. Therefore, a number of consecutive excited state proton transfers can occur. This work deals with the case of two successive proton transfers occurred in the molecular system. Such process is called as a stepwise excited state intramolecular double proton transfer (stepwise ESIDPT). It leads to the formation of two molecular tautomers. Therefore, fluorescence of such compounds can contain different emission bands correspond to emission of normal form and two tautomers. In this work, a rigorous analysis of fluorescence decay kinetics has been made using the model with three species, including a normal molecular form and two tautomers. The work presents theoretical framework of fluorescence decay analysis of ESIDPT process taking into account three species emission. Theoretically, the stepwise proton transfers can be consisted of more than two ESIPT reactions. It depends on molecular structure and number of involved hydroxyl groups. Here, a formal analysis of fluorescence decay kinetics has been made in the case of a stepwise process consisting of two proton transfers. Moreover, the quantum-chemical calculations have been performed in the case of scutellarein. It is a multiple hydroxyl-containing flavone and, therefore, it can be applied as a model molecule to study stepwise intramolecular proton transfers. The hypothetical scheme of ESIDPT has been proposed for this compound.

Fluorescence Lifetimes of NIR-Emitting Molecules with Excited-State Intramolecular Proton Transfer

Molecular probes based on the excited-state intramolecular proton-transfer (ESIPT) mechanism have emerged to be attractive candidates for various applications. Although the steady-state fluorescence mechanisms of these ESIPT-based probes have been reported extensively, less information is available about the fluorescence lifetime characteristics of newly developed NIR-emitting dyes. In this study, four NIR-emitting ESIPT dyes with different cyanine terminal groups were investigated to evaluate their fluorescence lifetime characteristics in a polar aprotic solvent such as CH₂Cl₂. By using the time-correlated single-photon counting (TCSPC) method, these ESIPT-based dyes revealed a two-component exponential decay (τ₁ and τ₂) in about 2-4 nanoseconds (ns). These two components could be related to the excited keto tautomers. With the aid of model compounds (5 and 6) and low-temperature fluorescence spectroscopy (at -189 ℃), this study identified the intramolecular charge transfer (ICT) as one of the major factors that influenced the τ values. The results of this study also revealed that both fluorescence lifetimes and fractional contributions of each component were significantly affected by the probe structures.

Unveiling the sensing mechanism and luminescence property of a new ESIPT-based fluorescent sensor for detecting Zn2

Recently, based on the mechanism of excited-state intramolecular proton transfer (ESIPT), a new fluorescent probe named 3-(benzo[d]thiazol-2-yl)-5-bromosalicylaldehyde-⁴N-phenyl thiosemicarbazone (BTT) was successfully synthesized [Analyst 146 (2021) 4348-4356.]. However, the importance of ESIPT processes of BTT probe and the mechanism of detecting Zn²⁺ ions have not been studied in detail. In this study, the photochemical behavior of ESIPT-chromophore and the photophysical changes of detecting Zn²⁺ ions were explained at the molecular level for the first time. The calculated spectral values were in agreement with the experiment. We not only confirmed the excited state hydrogen-bond strengthening by interaction region indicator (IRI), but also scanned the potential energy curves of BTT molecule in different electronic states, which confirmed that the hydrogen proton is easier to transfer in the first excited state. In addition, we had given the reasonable structure of the BTT-Zn²⁺ complex (L1) by comparing the binding free energies. The hole-electron distribution and interfragment charge transfer (IFCT) methods proved the excitation type of intraligand charge transfer (ILCT). Finally, the photophysical phenomenon of BTT for detecting Zn²⁺ ions is explained by calculating the electronic spectra and the energy gap (E_gap) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).

Theoretical Investigations on the Sensing Mechanism of Phenanthroimidazole Fluorescent Probes for the Detection of Selenocysteine

The level of selenocysteine (Sec) in the human body is closely related to a variety of pathophysiological states, so it is important to study its fluorescence sensing mechanism for designing efficient fluorescent probes. Herein, we used time-dependent density functional theory to investigate the fluorescence sensing mechanism of phenanthroimidazole derivates A4 and B4 for the detection of Sec, which are proposed to be designed based on excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) mechanisms. The calculation results show that the fluorescence quenching mechanism of A4 and B4 is due to the photo-induced electron transfer (PET) process with the sulfonate group acts as the electron acceptor. Subsequently, A4 and B4 react with Sec, the sulfonate group is substituted by hydroxyl groups, PET is turned off, and significant fluorescence enhancement of the formed A3 and B3 is observed. The theoretical results suggest that the fluorescence enhancement mechanism of B3 is not based on ICT mechanism, and the charge transfer phenomenon was not observed by calculating the frontier molecular orbitals, and proved to be a local excitation mode. The reason for the fluorescence enhancement of A3 based on ESIPT is also explained by the calculated potential energy curves.

Surprising Solid-State ESIPT Emission from Apparently Ordinary Salicyliden Glycinates Schiff Bases

Excited-State Intramolecular Photon Transfer (ESIPT) is known for the geometry-related phenolic and imine groups. The Schiff bases formed upon condensation of salicyl aldehyde and glycine led to the formation of ESIPT models. A series of alkali metal salicyliden glycinates were analyzed by X-ray diffraction of their monocrystals and spectroscopy measurements. The X-ray analysis revealed varied hydration levels between the salts. They adapted trans geometry on the imine groups and mostly anticlinal conformation with the neighboring atoms, which is different from the other structurally-related compounds in literature. Fluorescence of these compounds was found for the crystalline forms only. Protonation of the imine nitrogen atom and further proton distribution was consistent with the ESIPT theory, which also explained the observed fluorescence with the highest Stokes shift of 10,181 cm^-1 and 10.1% of fluorescence quantum yield for the sodium salt.

Long-Range Proton Transfer in 7-Hydroxy-Quinoline-Based Azomethine Dyes: A Hidden Reason for the Low Efficiency

In the tautomeric Schiff bases, derived from 7-hydroxyquinoline, two competitive channels are possible upon excitation of the enol tautomer, namely proton transfer (PT) through intramolecular hydrogen bonding to the corresponding keto form and trans-cis isomerization around the azomethine double bond. The former leads to switching, based on twist-assisted excited state intramolecular PT, where the long-range proton transfer can occur as a targeted process. The latter, determined by the flexibility of the crane part, reduces the efficiency of the main targeted process. In previously studied molecular switches based on the 7-hydroxyquinoline skeleton, only the intramolecular PT photo-process undergoing from the excited enol form towards the keto tautomer, which is in most cases barrierless, has been discussed. Therefore, in the current study, the ground state PT properties and isomerization of (E)-8-((phenylimino)methyl)quinolin-7-ol and (E)-8-(((pentafluorophenyl)imino)methyl)quinolin-7-ol are investigated in depth using the MP2 methodology, while the excited state energy profiles are calculated with the ADC(2) method. The obtained results are discussed in light of the existing experimental data.

A Reversible Fluorescent Probe Based on a Redox-Switchable Excited-State Intramolecular Proton-Transfer Active Metal-Organic Framework for Detection and Imaging of Highly Reactive Oxygen Species in Live Cells

Detection and imaging of highly reactive oxygen species (hROS) in biological systems using fluorescent probes are critical for the study of physiological and pathological processes induced by hROS. Herein, we report a redox-active luminescent metal-organic framework (MOF), which incorporates a hydroquinone moiety that can undergo a reversible transformation from the hydroquinone to the quinone by hROS like •OH and ClO^-. Moreover, the intrinsic fluorescence originating from the excited-state intramolecular proton transfer (ESIPT) property of the organic linker can be finely regulated during this redox-switchable process. A reversible fluorescent probe for hROS is thus developed. The presented probe shows a sensitive, selective, and reversible response to hROS due to the integration of excellent structural characteristics and unique spectral properties of the MOF. The detection limits of •OH and ClO^- are 0.22 and 0.18 μM, respectively. Furthermore, with good photostability and super biocompatibility, this simple yet efficient fluorescent probe has been successfully applied to dynamic monitoring of endogenous and exogenous •OH and ClO^- in live cells.

A Reversible Optical Sensor Film for Mercury Ions Discrimination Based on Isoxazolidine Derivative and Exhibiting pH Sensing

We developed a new optical sensor for tracing Hg(II) ions. The detection affinity examines within a concentration range of 0-4.0 µM Hg(II). The sensor film is based on Methyl 2-hydroxy-3-(((2S,2'R,3a'S,5R)-2-isopropyl-5,5'-dimethyl-4'-oxotetrahydro-2'H-spiro[cy-clohexane-1,6'-im-idazo[1,5-b]isoxazol]-2'-yl)methyl)-5-methylbenzoate (IXZD). The novel synthesized compound could be utilized as an optical turn-on chemosensor for pH. The emission intensity is highly enhanced for the deprotonated form concerning the protonated form. IXZD probe has a characteristic fluorescence peak at 481 nm under excitation of 351 nm with large Stocks shift of approximately 130 nm. In addition, the binding process of IXZD:Hg(II) presents a 1:1 molar ratio which is proved by the large quench of the 481 nm emission peak of IXZD and the growth of a new emission peak at 399 nm (blue shift). The binding configurations with one Hg(II) cation and its electronic characteristics were investigated by applying the Density Functional Theory (DFT) and the time-dependent DFT (TDDFT) calculations. Density functional theory (DFT) and the time-dependent DFT (TDDFT) theoretical results were provided to examine Hg(II)-IXZD structures and their electronic properties in solution. The developed chemical sensor was offered based on the intramolecular charge transfer (ICT) mechanism. The sensor film has a significantly low limit of detection (LOD) for Hg(II) of 0.025 μM in pH 7.4, with a relative standard deviation RSD_r (1%, n = 3). Lastly, the IXZD shows effective binding affinity to mercury ions, and the binding constant K_b was estimated to be 5.80 × 10⁵ M^-1. Hence, this developed optical sensor film has a significant efficiency for tracing mercury ions based on IXZD molecule-doped sensor film.

Rapid and ratiometric fluorescent detection of phosgene by a red-emissive ESIPT-based-benzoquinolone probe

Phosgene is a highly toxic gas that poses a serious threat to human health and public safety. Therefore, it is of great importance to develop an available detection method enabling on-the-spot measurement of phosgene. In this paper, we report a novel ESIPT fluorescent probe for phosgene detection based on quinolone fluorophore. This probe exhibits rapid response (in 10 s), stable signal output (last for 10 min), high sensitivity (LOD ∼ 6.7 nM), and distinct emission color change (red to green) towards phosgene. The sensing mechanism was investigated by using ¹H NMR, HRMS and fluorescence lifetime techniques, confirming that the amidation reaction between phosgene and quinolone effectively suppressed the ESIPT process of probe. Eventually, this probe was fabricated into polymer nanofibers by electrospinning and successfully employed to monitor gaseous phosgene with high specificity. This work provided a promising analytical tool for rapid and ratiometric detection of phosgene both in solution and in the gas phase.

Host-guest interaction aided Zinc carry and delivery by ESIPT active 2-(2'-hydroxyphenyl)benzoxazole

The effect of solvents and supramolecular hosts on the binding of metal ion with an excited state intramolecular proton transfer (ESIPT) active fluorophore 2-(2'-hydroxyphenyl)benzoxazole (HPBO) are investigated to scrutinize a possible metal ion carry and delivery system. The fluorophore forms strong fluorescent complex with Zn²⁺ ion. In aqueous medium, β-cyclodextrin (β-CD) breaks the HPBO-Zn²⁺ complex and encapsulate the freed fluorophore. Hence, the initially blocked ESIPT process is restored by forming an inclusion complex with the host molecules. However, in dimethyl sulphoxide (DMSO), β-CD does not break the complex. But cucurbit[7]uril (CB-7) breaks the complex in both DMSO and water. The tuned emission characteristics are considered for constructing different molecular logic gates. BUFFER, NOT, PASS, IMPLICATION and INHIBIT logic operations are substantiated based on Zn²⁺, CB-7 and β-CD response.

Tracking HOCl by an incredibly simple fluorescent probe with AIE plus ESIPT in vitro and in vivo

Hypochlorous acid is an important active substance involved in a variety of physiological processes in living organisms, while abnormal concentrations of HOCl are strongly associated with a variety of diseases such as cancer, inflammation, atherosclerosis, and Alzheimer's disease. As a result, it's crucial to establish a reliable method for tracking HOCl in vivo in order to investigate its physiological consequences. In this work, we developed a fluorescent probe DFSN with both AIE and ESIPT for imaging HOCl in vivo. DFSN not only has a basic structure and is easy to synthesize, but also has superior performance. The probe responds to HOCl in less than 10 s and has good selectivity and sensitivity to HOCl (DL = 6.3 nM), with a 110-fold increase in fluorescence intensity following response. In addition, DFSN can realize the rapid detection of hypochlorous acid with naked eyes. Moreover, DFSN can be used for the detection of exogenous and endogenous HOCl in RAW264.7 cells, and additionally enables the tracking of HOCl in cancer cells (Hela cells and HepG2 cells). More notably, it has been utilized to image hypochlorous acid in zebrafish with great success. The probe DFSN will be useful in determining the physiological significance of HOCl.

A novel ESIPT-based fluorescent probe with dual recognition sites for the detection of hydrazine in the environmental water samples and in-vivo bioimaging

Hydrazine (N₂H₄), an important chemical intermediate, has been widely used in industrial production and agricultural life, but it has also caused environmental pollution. A novel ESIPT-based fluorescent probe with dual recognition sites, 2-(benzothiazole-2-yl)-1,4-imphenyl bis 4-bromobutyric acid (BRBA), was developed to selectively detect N₂H₄ under complex conditions. BRBA exhibits accurate detection for N₂H₄ with a good linear relationship ranging from 0 to 150 μM, and the LOD can reach 0.1 μM. Importantly, taking advantage of low cytotoxicity and a large Stokes shift, BRBA can be utilized to monitor environmental water samples and successfully applied to imaging HeLa cells and zebrafish.

Insights into the photophysical properties of 2-(2'-hydroxyphenyl) benzazoles derivatives: Application of ESIPT mechanism on UV absorbers

In this present work, four novel molecules (BPN, BPNS, BPS, and BPSN), possessing excited-state intramolecular proton transfer (ESIPT) characteristics, were designed to quantify the impacts of substituent effects on their photophysical properties. By exploring the primary geometrical parameters concerning hydrogen bonds, it should be noticed that the intramolecular hydrogen bonds (IHBs) of the studied molecules have been strengthened at S₁ state. Infrared vibrational spectra analysis illustrates that adding electron-donating group thiophene to the proton donor side can weaken the IHBs in comparison to the electron-withdrawing group pyridine. Through investigating the absorption and fluorescence spectra, it can be clearly found that the maximum absorption peaks of the studied molecules are all located in the UVA region, and their regions of fluorescence peaks are harmless to human skin. Furthermore, considering the light intensity factor, it can be concluded that BPNS is the most potential to be used as UV absorbers in the studied molecules. This work investigates the effects of the positions and types of substituent groups on photophysical properties of 2-(2'-hydroxyphenyl) benzazoles derivatives, which can help design and exploit novel UV absorbers.

Aryl-Phenanthro[9,10-d]imidazole: A Versatile Scaffold for the Design of Optical-Based Sensors

Fluorescent and colorimetric sensors are important tools for investigating the chemical compositions of different matrices, including foods, environmental samples, and water. The high sensitivity, low interference, and low detection limits of these sensors have inspired scientists to investigate this class of sensing molecules for ion and molecule detection. Several examples of fluorescent and colorimetric sensors have been described in the literature; this Review focuses particularly on phenanthro[9,10-d]imidazoles. Different strategies have been developed for obtaining phenanthro[9,10-d]imidazoles, which enable modification of their optical properties upon interaction with specific analytes. These sensing responses usually involve changes in the fluorescence intensity and/or color arising from processes like photoinduced electron transfer, intramolecular charge transfer, intramolecular proton transfer in the excited state, and Förster resonance energy transfer. In this Review, we categorized these sensors into two different groups: those bearing formyl groups and their derivatives and those based on other molecular groups. The different optical responses of phenanthro[9,10-d]imidazole-based sensors upon interaction with specific analytes are discussed.

Substituent derivatives of benzothiazole-based fluorescence probes for hydrazine with conspicuous luminescence properties: A theoretical study

In the present work, four probe molecules for detecting hydrazine have been designed based on the 2-(4-Acetoxy-3-benzothiazole-2-yl-phenyl)-4-methyl-thiazole- 5-carboxylic acid ethyl ester (HP1) to investigate the influence of the amino and cyano groups on the excited-state intramolecular proton transfer (ESIPT) behavior and photophysical properties. The changes in hydrogen bond strength indicate that the intramolecular hydrogen bond of all probe products is enhanced upon photoexcitation. Frontier molecular orbitals (FMOs) and natural bond orbital (NBO) reveal the driving force of ESIPT. In addition, the potential energy curves and transition state theory explain the reason for the single fluorescence phenomenon in the experiment. The simulated absorption and fluorescence spectra of HP1 and its product (HPP1) are completely consistent with the experimental results, which also verify the viewpoint. Meanwhile the cyano derivative HPP4 exhibits a larger Stokes-shift (201 nm) than that of HPP1 (145 nm) and has the same low energy barrier as HPP1. These excellent properties allow HPP4 to be a fluorescent probe with superior performance than the original molecule. In conclusion, this work can provide a theoretical basis for the design and synthesis of more sensitive fluorescent probes for the detection of hydrazine.

An ESIPT-based fluorescent turn-on probe with isothiocyanate for detecting hydrogen sulfide in environmental and biological systems

A probe with an isothiocyanate group was synthesized and evaluated for its H₂S sensing ability. Upon addition of H₂S, the probe exhibited ratiometric properties during absorption with a red-shift. The probe exhibited fluorescent off-on responses towards H₂S via the ESIPT process, due to the conversion of isocyanate into amine. UV-vis, fluorescence, and ¹H NMR spectroscopic analyses were performed to investigate the sensing mechanism. The probe has a large Stokes shift, short response time, and low detection limit. It can be used to estimate H₂S levels within the range of 0-36 nM. The practical applicability of the probe was demonstrated using water samples and living cells.

Novel Approach for Detecting Vapors of Acids and Bases with Proton-Transfer Luminescent Dyes Encapsulated within Metal-Organic Frameworks

Luminescent metal-organic frameworks (LMOFs) are one of the most promising materials for being implemented as active layers in the fabrication of photonic devices such as luminescent sensors of harmful chemicals. It is highly desirable that these materials undergo quantifiable spectroscopic (absorption or emission) changes in the presence of vapors of those analytes, as in many industrial processes, these toxic compounds are in the gas phase. Although great progresses have been achieved in the field, in most of the examples reported hitherto, the detection of chemicals by LMOFs is attained in solution. Herein, we present a novel approach consisting of the encapsulation of proton transfer dyes (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt, HPTS, and 3-hydroxyflavone, 3-HF) within the pores of two distinct MOFs. The trapped proton transfer dyes (PT-dyes) may exist as different structures (enol, anion, or zwitterion), each of these exhibiting unique optical properties. Indeed, our findings reveal that the dyes can be encapsulated as anionic or enol species. Remarkably, the PT-dye@MOF composites exhibit a high luminescence quantum yield (up to 30%), which is sensitive (showing shifting in the emission wavelengths with a concomitant quenching/enhancement of the intensity) in the presence of vapors of an acid (HCl) and a base (triethylamine). These results open a novel avenue for the development of smarter vapoluminescent MOF-based materials.

A photochromic salicylaldehyde hydrazone derivative based on CN isomerization and ESIPT mechanisms and its detection of Al3+ in aqueous solution

A simple photochromic Schiff base was successfully prepared by the condensation of salicylaldehyde and benzoyl hydrazine. This compound has reversible photochromic properties based on isomerization and ESIPT mechanisms. In organic solvents, after irradiation with 365 nm UV light for 2 min, the absorption peak at 367 nm of the compound showed a significant decrease, while a double absorption peak appeared at 418 nm and 438 nm, accompanied by a significant change of the solution color from colorless to yellow. The compound can also complex with Al³⁺ at the molar ratio of 2:1 in the water solution (acetonitrile/water, v/v, 1:99), resulting in significantly enhanced fluorescence of the compound, so as to achieve fluorescence detection of Al³⁺ in living cells and water samples.

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.

Excited-State Intramolecular Proton Transfer in 2-(2'-Hydroxyphenyl)pyrimidines: Synthesis, Optical Properties, and Theoretical Studies

The development of fluorescence materials with switched on/off emission has attracted great attention owing to the potential application of these materials in chemical sensing. In this work, the photophysical properties of a series of original 2-(2'-hydroxyphenyl)pyrimidines were thoroughly studied. The compounds were prepared by following well-established and straightforward methodologies and showed very little or null photoluminescence both in solution and in the solid state. This absence of emission can be explained by a fast proton transfer from the OH group to the nitrogen atoms of the pyrimidine ring to yield an excited tautomer that deactivates through a nonradiative pathway. The key role of the OH group in the emission quenching was demonstrated by the preparation of 2'-unsubstituted derivatives, all of which exhibited violet or blue luminescence. Single crystals of some compounds suitable for an X-ray diffraction analysis could be obtained, which permitted us to investigate inter- and intramolecular interactions and molecular packing structures. The protonation of the pyrimidine ring by an addition of trifluoroacetic acid inhibited the excited-state intramolecular proton transfer (ESIPT) process, causing a reversible switch on fluorescence response detectable by the naked eye. This acidochromic behavior allows 2-(2'-hydroxyphenyl)pyrimidines to be used as solid-state acid-base vapor sensors and anticounterfeiting agents. Extensive density functional theory and its time-dependent counterpart calculations at the M06-2X/6-31+G** level of theory were performed to rationalize all the experimental results and understand the impact of protonation on the different optical transitions.

A ratiometric fluorescence probe for imaging endoplasmic reticulum (ER) hypochlorous acid in living cells undergoing excited state intramolecular proton transfer

Hypochlorous acid (HOCl), one of the most important ROS in living organisms, appears to serve an important role in the immune system in vivo. Endoplasmic reticulum (ER), the largest organelle in cells, manages many biological processes connected to vital activities. To better obtain insight into the relationship of ER stress and HOCl level, a ratiometric fluorescent probe RHE, based on rhodamine combined with HBT and ER-targeting group, was designed and synthesized for HOCl detection in the ER. Probe RHE shows a large stokes shift about 155 nm, which is derived to ESIPT principle. In addition, probe RHE exhibited excellent properties such as fast response (<80 s), high sensitivity with a low detection limit (40 nM), high selectivity and anti-interference. Moreover, probe RHE displayed an excellent ER-targeting ability and had been successfully applied for detection of exogenous and endogenous HOCl in HepG2 cells.

Exploring the effect of nitrile substituent position on fluorescence quantum yield of ESIPT-based oxazoline derivatives: A TDDFT investigation

We explore the mechanism specifically on quantum yields difference of 2-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxybenzonitrile (1-CN) and 4-(4,4-Dimethyl-4,5-dihydrooxazol-2-yl)-3-hydroxybenzonitrile (3-CN) by density functional theory and time-dependent density functional theory within the Tamm-Dancoff approximation. The structures optimization and the potential energy curves scanning of singlet excited state directly prove that the excited state intramolecular proton transfer (ESIPT) can take place in 1-CN and 3-CN molecules. The calculated spectra show that the fluorescence peaks of two molecules come from the emission of keto* configuration. The non-covalent interaction and the atomic dipole moment corrected Hirshfeld charge are also analyzed. Through the comparison of emission oscillator strength between 1-CN and 3-CN molecules suggests that the radiative transition process is not the main reason for the difference on quantum yields. Internal conversion process is also excluded on account of the large energy gap between S₀ and S₁. Considering the interaction between singlet and triplet states, both molecules can undergo intersystem crossing. The prominent difference is that, compared with 3-CN, the larger spin-orbit coupling constant and smaller energy level difference promote the intersystem crossing process of 1-CN. This provides direct evidence for the fluorescence quantum yield of 1-CN is lower than that of 3-CN. We envision that the present work can provide help for the synthesis and application of ESIPT compounds with high quantum yields.

Modifying the proton transfer of 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole by water, confinement and confined water

The effect of water, confinement and confined water on the proton transfer of 3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazole (bis-HPTA) was investigated. Water alters the proton transfer process. At higher pH, an anion is formed in water and it undergoes intermolecular proton transfer and forms a keto tautomer. Confinement of molecule in β-cyclodextrin affects the intramolecular proton transfer. It also prevents the intermolecular proton transfer of the anionic form. In reverse micelle, the molecule resides in the interfacial region and interacts with bound water. The intermolecular hydrogen bond of the surfactants opens the intramolecular hydrogen bond in the weaker β-ring of bis-HPTA. It led to single tautomer emission from bis-HPTA. An increase in water amount enhances the relative amount of trans-enol, but predominantly tautomer emission is observed.

A novel flavone-based ESIPT ratiometric fluorescent probe for selective sensing and imaging of hydrogen polysulfides

Hydrogen polysulfides (H₂S_n) as an important member of reactive sulfur species is closely relevant to many physiological functions in redox homeostasis and metabolism. Dual-channel monitor the changes of H₂S_n level in vivo is highly desired. Herein we design a simple ratiometric fluorescent probe based on flavone skeleton for highly selective detection of H₂S_n. The probe HF-NA-MC bearing 2-fluoro-5-nitrobenzoic acid group inhibited the intramolecular ESIPT process, which show the blue fluorescence of adjacent naphthalene unit. In the presence of H₂S_n, the enol form of probe is converted to conjugated keto form, resulted in a 90 nm red-shift of fluorescence emission from 450 nm to 540 nm. The ratiometric intensity (I₅₄₀/I₄₅₀) of the probe exhibits a good linear relationship toward H₂S_n in the range of 0-120 μM, and the detection limit is estimated to be 0.63 μM. The ratiometric fluorescent probe shows high specificity and anti-interference ability for H₂S_n over other related reactive sulfur species. The probe HF-NA-MC shows promising outlook and could be applied to the confocal imaging of H₂S_n by dual emission channels in Hela cells.

Theoretical approach to modeling the early nonadiabatic events of ESIPT originating from three-state conical intersection in quinophthalone

We explore the excited-state intramolecular proton transfer process of quinophthalone theoretically. This molecule possesses three low-lying singlet excited states ([Formula: see text] and [Formula: see text]) in a narrow energy gap of less than the N-H stretching frequency. Dynamics simulations show nonadiabatic wavepacket transfer to [Formula: see text] and [Formula: see text] upon initiating the wavepacket on [Formula: see text]. Multiple accessible conical intersections that lie in the Franck-Condon region facilitate the nonadiabatic wavepacket transfer. Nuclear densities associated with the proton transfer promoting vibrations would start accumulating on [Formula: see text] and [Formula: see text] within a few tens of femtoseconds, validating the involvement of these vibrations in the nonadiabatic events that occur before the proton transfer process. Our findings emphasize the necessity of refined kinetic models for assigning the time constants of ultrafast transient spectroscopy measurements due to the simultaneous evolution of nonadiabatic events and proton transfer kinetics in quinophthalone.