A new turn-on fluorescent probe with ultra-large fluorescence enhancement for detection of hydrogen polysulfides based on dual quenching strategy

Based on dual quenching strategy (ESIPT inhibited quenching and PET quenching), we have developed a new turn-on fluorescent probe 1. Combining 3-(benzo[d]thiazol-2-yl)-10-butyl-10H-phenothiazin-2-ol (dye 2) as the fluorophore and 2-fluoro-5-nitro-benzoic as the recognition moiety, probe 1 had feature of notable large Stokes shift, highly sensitivity and selective for monitoring H₂S_n with remarkable fluorescence enhancement (328-fold) response at 534 nm. Probe 1 exhibited excellent performance in the quantitative detection of H₂S_n with a 137 nm Stokes shift and a low detection limit of 26 nM in solution. Finally, probe 1 was successfully utilized to image H₂S_n in living A549 cells and zebrafish.

Development of three ways molecular logic gate based on water soluble phenazine fluorescent 'selective ion' sensor

New hydrophilic fluorescent selective ion sensor based on phenazine and phthalazine moieties, 1,1'-(phenazine-2,3-diyl)-bis(3-(1,4-dihydroxyphthalazin-6-yl)urea) (1), has been designed, synthesized and characterized. Interestingly, sensor 1 exhibits prominent "turn-on" and "turn-off" fluorogenic signaling at 580 nm towards Fe²⁺ & AcO^- and Sr²⁺ & Cu²⁺, respectively. The fluorescence titration experiments shed light on the nature of the interaction between 1 and guest molecules (Fe²⁺, Sr²⁺, Cu²⁺ and AcO^-), which divulge that 1 is flexible enough to orient itself according to the size of the guest molecule. Water mediated excited-state intramolecular proton transfer (ESIPT) and photo-induced electron transfer (PET) mechanisms are responsible for the dual behavior of 1, which binds with guest molecules in 1:1 stoichiometry. Based on the significant duplex fluorescence response of 1, a molecular logic gate keypad lock with sixteen "on" passwords for a storage system has been developed.

Theoretical insights into excited-state intramolecular and multiple intermolecular hydrogen bonds in 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone

The photophysical properties and photochemistry reactions of 2-(2-Hydroxy-phenyl)-4(3H)-quinazolinone (HPQ) system in different solutions are studied by using density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. Our theoretical investigation explores that an ultrafast barrier-free excited state intramolecular proton transfer (ESIPT) process occurs and the configuration twisting is found in the electronic excited state. In the polar protic methanol solution, the hydrogen-bonded complex composed by HPQ and two methanol molecules (HPQ-2M) could exist stably in the ground state. Upon photoexcitation the isolated HPQ is initially excited to the first excited state, while the HPQ-2M system is firstly excited to the S₃ state and undergoes internal conversion (IC) to the S₁ state. The intermolecular hydrogen bonds are strengthened in the excited state. The simulated electronic spectra agree well with the experimental results. The strengthening of the intermolecular hydrogen bonds is also confirmed by the calculated vibrational spectra. In addition, the intramolecular charge transfer happens in both HPQ and HPQ-2M systems from the frontier molecular orbital analysis.

Selective Sensing of Iron by Pyrrolo[2,3-c]Quinolines

This paper reports development of an iron sensor, 2-(3H-pyrrolo[2,3-c]quinolin-4-yl)aniline (APQ). The fluorophore facilitates micromolar detection of Fe³⁺/Fe²⁺ in the presence of various cations, including well-known interfering cations Co²⁺and Cu²⁺ by the process of fluorescence quenching. Graphical Abstract.

Excited-State Intramolecular Proton Transfer-Based Multifunctional Solid-State Emitter: A Fluorescent Platform with "Write-Erase-Write" Function

The excited-state intramolecular proton transfer (ESIPT)-based molecular probes have drawn significant attention owing to their environment-sensitive fluorescence properties, large Stokes shift, and emerged as building blocks for the development of molecular sensors and switches. However, most of the ESIPT-based fluorophores exhibit weak emission in the solid state limiting the scope of real-time applications. Addressing such issues, herein, we presented a C₃ symmetric-like molecular architecture employing a simple one-step Schiff base condensation between triaminoguanidinium chloride and 3,5-di- tert-butyl-2-hydroxybenzaldehyde (TGHB). The temperature-dependent fluorescence studies including at 77 K indicated the strong emission from the keto tautomer compared to that of the enol tautomer. The facile ESIPT in TGHB in the solid-state led to a remarkable enhancement of fluorescence quantum yield of 1600 times compared to that of the solution (λ_em = 545 nm) by restricting the intramolecular rotation and subsequently suppressing the nonradiative deactivation. The excited-state processes were further elucidated through time-resolved fluorescence measurements. TGHB exhibited turn on-off fluorescence upon exposure to acid/base vapor in the form of a powder as well as a transparent, free-standing thin film. A rewritable and erasable fluorescent platform was demonstrated using TGHB as molecular ink, which offers a potential testbed for performing "write-erase-write" cycles multiple times. In addition, TGHB, possessing multiple binding sites (O and N donors) involving the central core of the triaminoguanidinium cation displayed selective turn-on fluorescence with Zn²⁺. The structure-property relationship revealed in the present study provides insight into the development of novel cost-effective multifunctional materials, which are promising for stimuli-responsive molecular switches.

A lysosome targetable fluorescent probe for palladium species detection base on an ESIPT phthalimide derivative

A novel lysosome-targetable phthalimide fluorescent probe was designed for detecting palladium based on ESIPT for signal transduction. The fluorescent probe conjugating with allylcarbamate displayed weak fluorescent due to the ESIPT process hinder by allylcarbamate. But with the addition of palladium, the ESIPT emission was recovery though the palladium-catalyzed deallylation reaction and the fluorescence intensity exhibited 40-fold enhancement at 511 nm. In addition, the probe showed excellent selectivity, high sensitivity, fast responds and low limit detection for palladium with a larger Stoke-shift. Moreover, the targetable probe was also successfully applied for detecting palladium in lysosomes of living cells. Hence, the probe though ESIPT modulation is a promising for monitoring palladium in practical samples.

Spectroscopic methods for the study of energetic characteristics of the normal and photoproduct forms of 3-hydroxyflavones

We report spectroscopic properties of 3-hydroxyflavone (3-HF) and 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF) in acetonitrile and ethyl acetate at different temperatures in the range from 10 °C to about 67 °C. These compounds are characterized by excited-state intramolecular proton transfer (ESIPT) which leads to occurrence of two forms of these molecules. For this reason their fluorescence spectra have two bands which correspond to emission of normal and photoproduct (tautomer) forms. The correlation between ratio of integrated intensity of these two bands and inverse absolute temperature (the Arrhenius plot) have been applied to estimate energetic properties, such as difference between energy levels of excited states as well ground states for normal and tautomer forms for each molecule.

Intramolecular Proton and Charge Transfer of Pyrene-based trans-Stilbene Salicylic Acids Applied to Detection of Aggregated Proteins

Two analogues to the fluorescent amyloid probe 2,5-bis(4'-hydroxy-3'-carboxy-styryl)benzene (X-34) were synthesized based on the trans-stilbene pyrene scaffold (Py1SA and Py2SA). The compounds show strikingly different emission spectra when bound to preformed Aβ1-42 fibrils. This remarkable emission difference is retained when bound to amyloid fibrils of four distinct proteins, suggesting a common binding configuration for each molecule. Density functional theory calculations show that Py1SA is twisted, while Py2SA is more planar. Still, an analysis of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the two compounds indicates that the degree of electronic coupling between the pyrene and salicylic acid (SA) moieties is larger in Py1SA than in Py2SA. Excited state intramolecular proton transfer (ESIPT) coupled-charge transfer (ICT) was observed for the anionic form in polar solvents. We conclude that ICT properties of trans-stilbene derivatives can be utilized for amyloid probe design with large changes in emission spectra and decay times from analogous chemical structures depending on the detailed physical nature of the binding site.

Density Functional Theory Applied to Excited State Intramolecular Proton Transfer in Imidazole-, Oxazole-, and Thiazole-Based Systems

Excited state intramolecular proton transfer (ESIPT) is a photoinduced process strongly associated to hydrogen bonding within a molecular framework. In this manuscript, we computed potential energy data using Time Dependent Density Functional Theory (TDDFT) for triphenyl-substituted heterocycles, which evidenced an energetically favorable proton transfer on the excited state (i.e., ESIPT) but not on the ground state. Moreover, we describe how changes on heterocyclic functionalities, based on imidazole, oxazole, and thiazole systems, affect the ESIPT process that converts an enolic species to a ketonic one through photon-induced proton transfer. Structural and photophysical data were obtained theoretically by means of density functional theory (DFT) calculations and contrasted for the three heterocyclics. Different functionals were used, but B3LYP was the one that adequately predicted absorption data. It was observed that the intramolecular hydrogen bond is strengthened in the excited state, supporting the occurrence of ESIPT. Finally, it was observed that, with the formation of the excited state, there is a decrease in electronic density at the oxygen atom that acts as proton donor, while there is a substantial increase in the corresponding density at the nitrogen atom that serves as proton acceptor, thus, indicating that proton transfer is indeed favored after photon absorption.

Fine tuning the emission wavelengths of the 7-hydroxy-1-indanone based nano-structure dyes: Near-infrared (NIR) dual emission generation with large stokes shifts

Near-infrared (NIR) fluorescent dyes have recently gained special attention due to their applications to use as molecular probes for imaging of biological targets and sensitive determination. In this study, photophysical properties of the 7-hydroxy-1-indanone based fluorophors A1, A2, A3, B1, B2 and 3R-B2 (R=CF₃, NH₂, NO₂ and OMe) in the gas and three solution phases were probed using TD-DFT method at PBE0/6-311++G(d,p) and M06-2X/6-311++G(d,p) levels of theory. In addition to structural and photophysical properties as well as ESIPT mechanism of all mentioned molecules, the FC and relaxed potential energy surfaces of B2 and 3R-B2 (R=CF₃ and NH₂) molecules were explored in gas phase and acetonitrile, cyclohexane and water solvents. It is predicted that the A1, A3 and 3R-B2 chromophores afford normal (615-670nm) and NIR fluorescence emissions (770-940nm; biological window) with the large Stokes shifts of >160 and >300nm, respectively. A good aggrement was found between theoretical and experimental results. In sum, these new types of dyes may render the new approaches for the development of the most efficient NIR fluorescent probes for enhanced image contrast and optimal apparent brightness in biological applications.

Ratiometric Fluorescent Sensor Array as a Versatile Tool for Bacterial Pathogen Identification and Analysis

Rapid and reliable identification of pathogenic microorganisms is of great importance for human and animal health. Most conventional approaches are time-consuming and require expensive reagents, sophisticated equipment, trained personnel, and special storage and handling conditions. Sensor arrays based on small molecules offer a chemically stable and cost-effective alternative. Here we present a ratiometric fluorescent sensor array based on the derivatives of 2-(4'- N, N-dimethylamino)-3-hydroxyflavone and investigate its ability to provide a dual-channel ratiometric response. We demonstrate that, by using discriminant analysis of the sensor array responses, it is possible to effectively distinguish between eight bacterial species and recognize their Gram status. Thus, multiple parameters can be derived from the same data set. Moreover, the predictive potential of this sensor array is discussed, and its ability to analyze unknown samples beyond the list of species used for the training matrix is demonstrated. The proposed sensor array and analysis strategies open new avenues for the development of advanced ratiometric sensors for multiparametric analysis.

The excited-state intramolecular proton transfer in NH-type dye molecules with a seven-membered-ring intramolecular hydrogen bond: A theoretical insight

Excited-state intramolecular proton transfer (ESIPT) reactions of a series of N(R)H⋯N-type seven-membered-ring hydrogen-bonding compounds were explored by employing density functional theory/time-dependent density functional theory calculations with the PBE0 functional. Our results indicate that the absorption and emission spectra predicted theoretically match very well the experimental findings. Additionally, as the electron-withdrawing strength of R increases, the intramolecular H-bond of the NS₁ form gradually enhances, and the forward energy barrier along the ESIPT reaction gradually decreases. For compound 4, its ESIPT reaction is found to be a barrierless process due to the involvement of a strong electron-withdrawing COCF₃ group. It is therefore a reasonable presumption that the ESIPT efficiency of these N(R)H⋯N-type seven-membered-ring H-bonding systems can be improved when a strong electron-withdrawing group in R is introduced.

A simple fluorescent probe for the fast sequential detection of copper and biothiols based on a benzothiazole derivative

A simple benzothiazole fluorescent chemosensor was developed for the fast sequential detection of Cu²⁺ and biothiols through modulating the excited-state intramolecular proton transfer (ESIPT) process. The compound 1 exhibits highly selective and sensitive fluorescence "on-off" recognition to Cu²⁺ with a 1:1 binding stoichiometry by ESIPT hinder. The in situ generated 1-Cu²⁺ complex can serve as an "on-off" fluorescent probe for high selectivity toward biothiols via Cu²⁺ displacement approach, which exerts ESIPT recovery. It is worth pointing out that the 1-Cu²⁺ complex shows faster for cysteins (within 1min) than other biothiols such as homocysteine (25min) and glutathione (25min). Moreover, the compound 1 displays 160nm Stoke-shift for reversibly monitoring Cu²⁺ and biothiols. In addition, the probe is successfully used for fluorescent cellular imaging. This strategy via modulation the ESIPT state has been used for determination of Cu²⁺ and Cys with satisfactory results, which further demonstrates its value of practical applications.

A new fluorene-based Schiff-base as fluorescent chemosensor for selective detection of Cr3+ and Al3

2-((9H-fluoren-2-ylimino) methyl)phenol (F3) was synthesized by condensation reaction of 9H-fluoren-2-amine and 2-hydroxybenzaldehyde in EtOH and characterized by its melting point, ¹H-,¹³C NMR and molecular mass. F3 exhibits a high selectivity for detection of Cr³⁺ and Al³⁺ ions as a fluorescent chemosensor and showed a single emission band at 536nm upon excitation at 333nm according to fluorescence emission studies. The addition of Cr³⁺ and Al³⁺ make a significant increase in fluorescent intensity at 536nm in CH₃CN, while other metal ions have almost no influence on the fluorescence. The fluorescence enhancement was attributed to the inhibited CN isomerization and the obstructed excited state intra-molecular proton transfer (ESIPT) of compound F3. Job's plot and DFT calculations data showed that the binding stoichiometries of F3 with Cr³⁺ and Al³⁺ are 2:1. The association constants (K_a) for Cr³⁺ and Al³⁺ were calculated and found to be 8.33×10⁴M^-1 and 5.44×10⁴M^-1, respectively. The detection limits were also calculated for Cr³⁺ and Al³⁺ and found to be 2.5×10^-7mol/L and 3.1×10^-7mol/L, respectively.

A new simple phthalimide-based fluorescent probe for highly selective cysteine and bioimaging for living cells

A new turn-on phthalimide fluorescent probe has designed and synthesized for sensing cysteine (Cys) based on excited state intramolecular proton transfer (ESIPT) process. It is consisted of a 3-hydroxyphthalimide derivative moiety as the fluorophore and an acrylic ester group as a recognition receptor. The acrylic ester acts as an ESIPT blocking agent. Upon addition of cystein, intermolecular nucleophilic attack of cysteine on acrylic ester releases the fluorescent 3-hydroxyphthalimide derivative, thereby enabling the ESIPT process and leading to enhancement of fluorescence. The probe displays high sensitivity, excellent selectivity and with large Stokes shift toward cysteine. The linear interval range of the fluorescence titration ranged from 0 to 1.0×10^-5M and detection limit is low (6×10^-8M). In addition, the probe could be used for bio-imaging in living cells.

A quantum-chemical insight into the tunable fluorescence color and distinct photoisomerization mechanisms between a novel ESIPT fluorophore and its protonated form

Enol-keto proton tautomerization and cis-trans isomerization reactions of a novel excited-state intramolecular proton transfer (ESIPT) fluorophore of BTImP and its protonated form (BTImP⁺) were explored using density functional theory/time-dependent density functional theory (DFT/TD-DFT) computational methods with a B3LYP hybrid functional and the 6-31+G(d,p) basis set. In addition, the absorption and fluorescence spectra were calculated at the TD-B3LYP/6-31+G(d,p) level of theory. Our results reveal that both BTImP and BTImP⁺ can undergo an ultrafast ESIPT reaction, giving rise to the single fluorescence emission with different fluorescence colors, which are nicely consistent with the experimental findings. Calculations also show that following the ultrafast ESIPT, BTImP and BTImP⁺ can experience the distinctly different cis-trans isomerization processes. The intersystem crossing between the first excited singlet S₁ state and triplet T₁ state is found to play an important role in the photoisomerization process of BTImP⁺. In addition, the energy barrier of the trans-keto→cis-keto isomerization in the ground state of BTImP⁺ is calculated to be 10.49kcalmol^-1, which implies that there may exist a long-lived trans-keto species in the ground state for BTImP⁺.

Spectral studies on anthracene based dual sensor for Hg2+ and Al3+ ions with two distinct output modes of detection

A simple and easily synthesized colorimetric and fluorescent chemosensor 1, based on aromatic OH and imine moieties as a binding and anthracene as signaling unit, has been synthesized in a one-step procedure. The chemosensor 1 is developed as a dual chemosensor for detection of Hg²⁺ and Al³⁺ ions in CH₃OH, which exhibited a color change from light yellow to dark yellow with Hg²⁺ ions, enabling 1 a suitable "bare eye" indicator for Hg²⁺ ions. On the other hand, fluorescent enhancement with blue shift along with brilliant cyan fluorescence was observed upon binding with Al³⁺ ions. A possible sensing mechanism has been proposed by means of Job's plot and ¹H NMR titration.

A photochemical flow reactor for large scale syntheses of aglain and rocaglate natural product analogues

Herein, we report the development of continuous flow photoreactors for large scale ESIPT-mediated [3+2]-photocycloaddition of 2-(p-methoxyphenyl)-3-hydroxyflavone and cinnamate-derived dipolarophiles. These reactors can be efficiently numbered up to increase throughput two orders of magnitude greater than the corresponding batch reactions.

An excited state intramolecular proton transfer dye based fluorescence turn-on probe for fast detection of thiols and its applications in bioimaging

In this study, a new fluorescent probe 2-(2'-hydroxy-5'-N-maleimide phenyl)-benzothiazole (probe 1), was designed and synthesized by linking the excited state intramolecular proton transfer (ESIPT) fluorophore to the maleimide group for selective detection of thiols in aqueous solution. The fluorescence of probe 1 is strongly quenched by maleimide group through the photo-induced electron transfer (PET) mechanism, but after reaction with thiol, the fluorescence of ESIPT fluorophore is restored, affording a large Stokes shifts. Upon addition of cysteine (Cys), probe 1 exhibited a fast response time (complete within 30s) and a high signal-to-noise ratio (up to 23-fold). It showed a high selectivity and excellent sensitivity to thiols over other relevant biological species, with a detection limit of 3.78×10^-8M (S/N=3). Moreover, the probe was successfully applied to the imaging of thiols in living cells.

Theoretical insight into the excited-state intramolecular proton transfer mechanisms of three amino-type hydrogen-bonding molecules

Excited-state intramolecular proton transfer (ESIPT) dynamics of the amino-type hydrogen-bonding compound 2-(2'-aminophenyl)benzothiazole (PBT-NH₂) as well as its two derivatives 2-(5'-cyano-2'-aminophenyl)benzothiazole (CN-PBT-NH₂) and 2-(5'-cyano-2'-tosylaminophenyl)benzothiazole (CN-PBT-NHTs) were studied by the time-dependent density functional theory (TD-DFT) approach with the B3LYP density functional, and their absorption and emission spectra were also explored at the same level of theory. A good agreement is observed between the theoretical simulations and experimental spectra, indicating that the present calculations are reasonably reliable. In addition, it is also found that the energy barriers of the first excited singlet state of the three targeted molecules along the ESIPT reaction are computed to be 0.38, 0.34 and 0.12eV, respectively, showing the trend of gradual decrease, which implies that the introduction of the electron-withdrawing cyano or tosyl group can facilitate the occurrence of the ESIPT reaction of these amino-type H-bonding systems. Following the ESIPT, both CN-PBT-NH₂ and CN-PBT-NHTs dye molecules can undergo the cis-trans isomerization reactions in the ground-state and excited-state potential energy curves along the C2-C3 bond between benzothiazole and phenyl moieties, where the energy barriers of the trans-tautomer→cis-tautomer isomerizations in the ground states are calculated to be 0.83 and 0.34eV, respectively. According to our calculations, it is plausible that there may exist the long-lived trans-tautomer species in the ground states of CN-PBT-NH₂ and CN-PBT-NHTs.

A new 2-(2'-hydroxyphenyl)quinazolin-4(3H)-one derived acylhydrazone for fluorescence recognition of Al3

A new 2-(2'-hydroxyphenyl)quinazolin-4(3H)-one derived acylhydrazone (QP) was designed and synthesized as a fluorescent sensor. In Tris∙HCl buffer (10mM, pH7.4)/ethanol (1/9, v/v) solution, QP exhibits a highly selective fluorescence response to Al³⁺ over other metal ions with a significant blue-shifted and enhanced emission at 473nm. QP interacts with Al³⁺ reversibly through a 1:2 binding ratio with a detection limit of 4.79×10^-8M. Potential applicability of QP for Al³⁺ detection in tap and lake water samples were also examined by 'proof-of-concept' experiments.

Benzophenone based fluorophore for selective detection of Sn2+ ion: Experimental and theoretical study

Synthesis of novel benzophenone-based chemosensor is presented for the selective sensing of Sn²⁺ ion. Screening of competitive metal ions was performed by competitive experiments. The specific cation recognition ability of chemosensor towards Sn²⁺ was investigated by experimental (UV-visible, fluorescence spectroscopy, ¹H NMR, ¹³C NMR, FTIR and HRMS) methods and further supported by Density Functional Theory study. The stoichiometric binding ratio and binding constant (K_a) for complex is found to be 1:1 and 1.50×10⁴, respectively. The detection limit of Sn²⁺ towards chemosensor was found to be 0.3898ppb. Specific selectivity and superiority of chemosensor over another recently reported chemosensor is presented.

A new hydroxynaphthyl benzothiazole derived fluorescent probe for highly selective and sensitive Cu(2+) detection

A new reactive probe, 1-(benzo[d]thiazol-2-yl)naphthalen-2-yl-picolinate (BTNP), was designed and synthesized. BTNP acts as a highly selective probe to Cu(2+) in DMSO/H2O (7/3, v/v, Tris-HCl 10mM, pH=7.4) solution based on Cu(2+) catalyzed hydrolysis of the picolinate ester moiety in BTNP, which leads to the formation of an ESIPT active product with dual wavelength emission enhancement. The probe also possesses the advantages of simple synthesis, rapid response and high sensitivity. The pseudo-first-order reaction rate constant was calculated to be 0.205min(-1). Moreover, application of BTNP to Cu(2+) detection in living cells and real water samples was also explored.

ESIPT-Based Photoactivatable Fluorescent Probe for Ratiometric Spatiotemporal Bioimaging

Photoactivatable fluorophores have become an important technique for the high spatiotemporal resolution of biological imaging. Here, we developed a novel photoactivatable probe (PHBT), which is based on 2-(2-hydroxyphenyl)benzothiazole (HBT), a small organic fluorophore known for its classic luminescence mechanism through excited-state intramolecular proton transfer (ESIPT) with the keto form and the enol form. After photocleavage, PHBT released a ratiometric fluorophore HBT, which showed dual emission bands with more than 73-fold fluorescence enhancement at 512 nm in buffer and more than 69-fold enhancement at 452 nm in bovine serum. The probe displayed a high ratiometric imaging resolution and is believed to have a wide application in biological imaging.

Comprehensive DFT and TD-DFT Studies on the Photophysical Properties of 5,6-Dichloro-1,3-Bis(2-Pyridylimino)-4,7-Dihydroxyisoindole: A New Class of ESIPT Fluorophore

Hanson et al. [Org. Lett., 2011] reported the absorption and emission spectrum of 5,6-dichloro-1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole but the excited-state intramolecular proton transfer (ESIPT) process was not investigated. The photo-physical behaviour of 5,6-dichloro-1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole was studied using the density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The functional used was B3LYP and 6-31G(d) was the basis set for all the atoms. All the ten tautomers were studied for the absorption and emission properties. It is found that the tautomer where hydroxyl groups are syn to nitrogen of isoindoline ring is most stable and thus, responsible for the ESIPT process. The computed absorption and emission values of tautomers using TD-DFT are in good agreement with those obtained experimentally.