Excited-state Proton Transfer Research Articles

Fluorescence probes detecting O2•_ based on intramolecular charge transfer and excited-state intramolecular proton transfer mechanisms

Detection of O2•_ ions in living organisms plays an important role in the prevention of cardiomyopathy, autism, diabetes, cancer and neurodegenerative diseases. Recently, Wu et al. have successfully synthesized a fluorescence probe 2-(Benzo [d] thiazol-2-yl)-6-methoxyphenyltrifluoromethanesulfonate (HMBT-LW) based on excited intramolecular proton transfer (ESIPT) mechanism [L. Wu, et al. New J. Chem., 2019, 43, 2875–2877]. In experiment, HMBT-LW reacted with O2•_ ions in solution to generate the molecule 2-(Benzo [d] thiazol-2-yl) -6-methoxyphenol (HMBT) with a hydrogen bonding structure. Because only the single fluorescence phenomenon was detected during the experiment, we could not determine whether the fluorescence was emitted by HMBT or its isomer structure. We reported the theoretical mechanism of photochemistry and photophysics in the whole reaction process at the first time. The calculated hydrogen bonding parameters indicated that the intramolecular hydrogen bonding interaction was enhanced in the excited state. The analysis of the frontier molecular orbitals (FMOs) and Mulliken charge illustrated that the intramolecular charges would be redistributed during photoexcitation process, which fundamentally explained the enhanced hydrogen bonding provided the driving force for the ESIPT processes. Infrared spectra (IR) showed that the strength of the hydrogen bond interaction would be changed in different electronic states. Afterwards, the calculations of molecular potential energy curves quantitatively explained the feasibility of the ESIPT reactions. The calculated spectral values were consistent with the absorption and emission energies in the experiment, proving the scientificity of our calculation method. The isosurfaces of reduced density gradient (RDG) was used to visually distinguish the complex non-covalent bond interactions and showed us the mechanism of excited state proton transfer intuitively. Combining the above data, we finally came to a conclusion that the single fluorescence observed in the experiment was emitted by the isomer structure of HMBT.

Effects of solvent polarity on excited state behaviors for the two intramolecular proton-transfer-site 4,4’-(hydrazine-1,2-diylidene-bis(methanylylidene))-bis(3-hydroxybenzoic acid) compound

Excited state proton transfer (ESPT) behavior of organic fluorophores has been drawing continuously considerable interests, since it owns enormous potential in wide of application fields. Given the paramount importance of excited state relaxations, in this work, we focus on deciphering excited state behaviors for the novel two proton-transfer-site 4,4’-(hydrazine-1,2-diylidene-bis(methanylylidene))-bis(3-hydroxybenzoic acid) (HDBB) system. In aprotic solvents with different polarities, we confirm the dual hydrogen bonds of HDBB should be enhanced in S1 state. It is a remarkable fact that nonpolar solvents play a more vital role in enhancing dual hydrogen bonds in S1 state. Probing into photo-induced excitation, we verify charge reorganization around proton acceptor and donor triggers ESPT tendency. Comparing energy gaps of frontier molecular orbitals (MOs), we further predict the ESPT process of HDBB could be facilitated by nonpolar solvents. To clarify the specific excited state behaviors, potential energy surfaces (PESs) in different solvents are constructed along with dual hydrogen bonds to present the overall ESPT processes. Searching transition state (TS) along reaction path, we confirm the excited state intramolecular single proton transfer (ESISPT) mechanism for HDBB. Further, we successfully present the excited state regulation mechanism via solvent polarity, namely, nonpolar solvents contribute to ESISPT reaction for HDBB compound.

Excited State Proton Transfer of Quinone Cyanine 9: Implications on the Origin of Super‐Photoacidity

AbstractQuinone cyanine 9 (QCy9) is one of the strongest photoacids known to date. The origin of “super”‐photoacidity was investigated via time‐resolved fluorescence (TF) with high time resolution to accurately resolve the ultrafast dynamics of the excited‐state proton transfer (ESPT) to solvent. The ESPT of QCy9 in water is found to proceed fully by two ultrafast time constants of 40 fs (65 %) and 200 fs (35 %). More importantly, the initially created base form of QCy9 in the excited state undergoes a significant dynamic red shift of over 2700 cm−1 by solvation. We suggest that the entirely ultrafast reaction rate and the massive solvation of the conjugate base could make QCy9 a particularly strong photoacid. Quantum chemical calculations show that structural diversity leads to the nonexponential behavior of the ESPT dynamics.

Introduction to Femtochemistry: Excited-State Proton Transfer from Pyranine to Water Studied by Femtosecond Transient Absorption

In order to introduce students to the fascinating field of femtochemistry, we propose here a practical laboratory training course conceived for second-year master’s students in chemistry. We describe the use of a broadband femtosecond transient absorption (pump–probe) experiment for monitoring a fast light-triggered chemical reaction in solution. The experiments are performed on the pyranine photoacid, which upon photoexcitation at 390 nm undergoes a proton transfer to the solvent in about 90 ps. While this practical course involves advanced equipment and techniques, the measured transient absorption data allow easy analysis and interpretation. The transient absorption spectra at a few selected delay times can be analyzed qualitatively in terms of bleach, induced absorption, and stimulated emission. Likewise, the transient absorption signals at a few chosen wavelengths can be quantitatively analyzed and explained with simple kinetic models to determine the time constant of the proton-transfer reaction. This training aims at giving the students the opportunity to face some of the current challenges in contemporary chemistry by learning the basics of ultrafast spectroscopy.

Modeling Excited-State Proton Transfer to Solvent: A Dynamics Study of a Super Photoacid with a Hybrid Implicit/Explicit Solvent Model.

The rapid growth of time-resolved spectroscopies and the theoretical advances in ab initio molecular dynamics (AIMD) pave the way to look at the real-time molecular motion following the electronic excitation. Here, we exploited the capabilities of AIMD combined with a hybrid implicit/explicit model of solvation to investigate the ultrafast excited-state proton transfer (ESPT) reaction of a super photoacid, known as QCy9, in water solution. QCy9 transfers a proton to a water solvent molecule within 100 fs upon the electronic excitation in aqueous solution, and it is the strongest photoacid reported in the literature so far. Because of the ultrafast kinetics, it has been experimentally hypothesized that the ESPT escapes the solvent dynamics control (Huppert et al., J. Photochem. Photobiol. A2014,277, 90). The sampling of the solvent configuration space on the ground electronic state is the first key step toward the simulation of the ESPT event. Therefore, several configurations in the Franck–Condon region, describing an average solvation, were chosen as starting points for the excited-state dynamics. In all cases, the excited-state evolution spontaneously leads to the proton transfer event, whose rate is strongly dependent on the hydrogen bond network around the proton acceptor solvent molecule. Our study revealed that the explicit representation at least of three solvation shells around the proton acceptor molecule is necessary to stabilize the excess proton. Furthermore, the analysis of the solvent molecule motions in proximity of the reaction site suggested that even in the case of the strongest photoacid, the ESPT is actually assisted by the solvation dynamics of the first and second solvation shells of the water accepting molecule.

Could Chemical Reaction at the Molecular Level Show Distinction between Two Liquid-Water States? Study of the Excited-State Water-Catalyzed Proton Transfer Reaction Provides a Clue.

The two liquid-water states, which lead to some anomalies when temperature crosses over 50 ± 10 °C at the atmospheric pressure, have been continuously catching popular attention. In this study, using the excited-state proton transfer (ESPT) catalyzed by water molecules as a prototypical reaction, we demonstrate that the kinetics of ESPT indeed is influenced by the two liquid-water states. In the water-catalyzed ESPT of 3-cyano-7-azaindole (3CAI), a repetitive and comprehensive temperature-dependent study of ESPT in H2O from 0 to 90 °C shows anomalous behavior. The plot of the logarithm of ESPT rate constant as a function of inverse of absolute temperature deviates from a straight line. The convex-Arrhenius behavior manifests the activation free energy for water-assisted ESPT being dependent on temperature and hence the liquid water structure. To simplify the discussion, the plot is well fitted by using two straight lines that are crossed over in the vicinity of 40 °C. The free energy difference between water-solvated 3CAI and the 1:1 H2O:3CAI complex is deduced to be 2.29 ± 0.04 and 1.96 ± 0.04 kcal·mol-1 in the regions of 0-40 and 40-90 °C water, respectively, which also results in different frequency factors, i.e., the proton transfer/tunneling rates of (5.83 ± 0.36) × 1010 and (3.48 ± 0.27) × 1010 s-1, respectively. In a qualitative manner, the results are then rationalized by the different types of H-bonding configuration as proposed for two liquid-water phases, rendering experimental evidence to support the different water phases in ambient temperatures at 1 bar.

A Not Obvious Correlation Between the Structure of Green Fluorescent Protein Chromophore Pocket and Hydrogen Bond Dynamics: A Choreography From ab initio Molecular Dynamics.

The Green Fluorescent Protein (GFP) is a widely studied chemical system both for its large amount of applications and the complexity of the excited state proton transfer responsible of the change in the protonation state of the chromophore. A detailed investigation on the structure of the chromophore environment and the influence of chromophore form (either neutral or anionic) on it is of crucial importance to understand how these factors could potentially influence the protein function. In this study, we perform a detailed computational investigation based on the analysis of ab-initio molecular dynamics simulations, to disentangle the main structural quantities determining the fine balance in the chromophore environment. We found that specific hydrogen bonds interactions directly involving the chromophore (or not), are correlated to quantities, such as the volume of the cavity in which the chromophore is embedded and that it is importantly affected by the chromophore protonation state. The cross-correlation analysis performed on some of these hydrogen bonds and the cavity volume, demonstrates a direct correlation among them and we also identified the ones specifically involved in this correlation. We also found that specific interactions among residues far in the space are correlated, demonstrating the complexity of the chromophore environment and that many structural quantities have to be taken into account to properly describe and understand the main factors tuning the active site of the protein. From an overall evaluation of the results obtained in this work, it is shown that the residues which a priori are perceived to be spectators play instead an important role in both influencing the chromophore environment (cavity volume) and its dynamics (cross-correlations among spatially distant residues).

Recent Advances in Photoacid Catalysis for Organic Synthesis

AbstractPhotoacids are molecules that become more acidic upon the absorption of light. This short review highlights recent advances in the use of photoacids as catalysts for organic synthesis. Photoacid-catalyzed­ transformations discussed herein include: Protonation, glycosylation, acetalization, and arylation reactions.1 Introduction2 Protonation: Excited-State Proton Transfer (ESPT)3 Glycosylation4 Acetalization5 Friedel–Crafts Arylation6 Additional C–C and C–S Bond-Forming Reactions7 Conclusion

The direct evidence for ESPT route and ICT emission of N6-Methyladenine in aqueous solution

N6-Methyladenine (6MeAde) was regarded as an important epigenetic biomarker in gene regulation. Recently, Zhou et al. observed a dual peak fluorescence emission of 6MeAde in aqueous solution (Phys. Chem. Chem. Phys., 2019, 21, 6878), in which the long-wavelength fluorescence was speculated to the isomerized intramolecular charge transfer (ICT). Nevertheless, this speculation still lacked a direct evidence and the essential mechanism of how 6MeAde was regulated by water molecules remained ambiguous. In present work, we theoretically studied the effect of water molecules on excited-state proton transfer (ESPT) process of 6MeAde based on the time-dependent DFT (TDDFT) methods. 6MeAde possessed both hydrogen acceptor and donor that formed intermolecular hydrogen bonds with water molecules. Thus, the excited-state hydrogen bonding dynamics may occur in three scenarios. The constructed potential energy surfaces provided direct evidence that proton transfer cannot occur for 6MeAde monomer and 6MeAde + H2O, but could occur spontaneously for 6MeAde+2H2O due to the coplanar closed-loop hydrogen bond structure. This result pointed out that determining the energy barrier of a potential energy surface was the key to explore the real ESPT channel. Besides, our analyses of electron spectra and electronic structures confirmed that the long-wavelength fluorescence originated from the ICT emission.

4’-Nitroflavonol fluorescence: Excited state intramolecular proton transfer reaction from the non-emissive excited state

2-(4′-nitrophenyl)-3-hydroxy-chromone (4′-nitroflavonol, NO3 H F), which can be considered as non-typical representative of fluorescent dyes of flavonol series, was synthesized. Its spectral and photophysical properties were studied experimentally and modeled with several relevant quantum-chemical approaches. Single banded, long-wavelength highly Stokes shifted fluorescence phototautomer emission was observed for the title compound in solvents of different polarity. Absence of its short-wavelength “normal” emission band was explained by the effect of ultrafast intersystem crossing (ISC) with participation of triplet nπ* levels of 4′-nitro group. Such behavior, absence of fluorescence emission below ∼500 nm is generally expected for nitro-aromatics. Thus, a rare example of excited state intramolecular proton transfer reaction (ESIPT) originated from the non-emissive “dark” excited state at conditions of its concurrence with ultrafast ISC should be considered for the title compound. Excited state proton transfer reaction plays in this case the role of an emission regulating factor: the higher is its rate, the more intensive phototautomer fluorescence is observed. Relatively long fluorescence lifetimes of 4′-nitroflavonol in solvents of intermediate polarity (∼ 4 ns) were attributed to low efficiency of radiationless processes in its phototautomer form.

Synthesis and photophysical properties of methyl 2-hydroxy-4-(5-R-thiophen-2-yl)benzoate: Quantum yields and excited-state proton transfer (R=CH3O and CN)

Methyl 2-hydroxy-4-(5-methoxythiophen-2-yl)benzoate (MHmoTB) and methyl 2-hydroxy-4-(5-cyanothiophen-2-yl)benzoate (MHncTB) have been synthesized and their photophysical properties are investigated in various solvents. Introducing methoxy and cyano groups to 5 position of the thiophenyl moiety results in very unique characters in the luminescence properties depending on the substituted group. Compared with the unsubstituted MHTB, the methoxy group as electron donator enhances the quantum yield of the deep blue luminescence more than 9 times in methylene chloride, while do not present an apparent solvatochromic effect on the luminescence spectral feature. In contrast, the cyano group as electron withdrawer enhances the quantum yield slightly. However, the UV excitation (<360 nm) of MHncTB produces not only deep blue emission but also orange emission in dimethylformamide and dimethyl sulfoxide. The new low-energy emission is attributed to an excited-state proton transfer from the solute to the solvent. Quantum mechanical calculations on the structural geometries and the electronic structures have been conducted to reveal ESPT of MHncTB by using the time-dependent density functional theory.

Enabling Control over Mechanical Conformity and Luminescence in Molecular Crystals: Interaction Engineering in Action

Molecular crystals of π-conjugated molecules are of great interest as the highly ordered dense packing offers superior charge and exciton transport compared with its amorphous counterparts. However, integration into optoelectronic devices remains a major challenge owing to its inherently brittle nature. Herein, control over the mechanical conformity in single crystals of pyridine-appended thiazolothiazole derivatives is reported by modulating the molecular packing through interaction engineering. Two polymorphs were prepared by achieving control over the thermodynamic/kinetic factors of crystallization; one of the polymorphs exhibits elastic bending whereas the other is brittle. The control over the bending ability was achieved by forming co-crystals with hydrogen/halogen bond donors. A seamless extended crisscross pattern with respect to the bend plane through a ditopic hydrogen-bonding motif showed the highest compliance towards mechanical bending, whereas the co-crystals with a layered crisscross arrangement with segregated layers of co-formers exhibit slightly lower bending conformity. These results update the rationale behind the plastic/elastic bending in molecular crystals. The co-crystals of ditopic halogen bond co-assemblies are particularly appealing for waveguiding applications as the co-crystals blend high mechanical flexibility and luminescence properties. The hydrogen bonded co-crystals are non-emissive in nature owing to excited state proton transfer dynamics. The rationale behind the fluorescence properties of these materials was also established from DFT calculations in a quantum mechanics/molecular mechanics (QM/MM) framework.

Excited-state proton transfer mechanism of benzothiazole-coupled salicylaldehyde imine

The excited-state intramolecular proton transfer (ESIPT) processes of 2-(2′-hydroxyphenyl)-benzothiazole (HBT) and salicylaldehyde imine (SAI) have been extensively studied. However, the ESIPT mechanism of HBT-coupled SAI was seldom reported. Herein, we synthesized a dye THER that integrating HBT and SAI, and investigated its excited-state dynamics in combination with femtosecond transient absorption spectrum and DFT/TDDFT calculations. Results indicate two proton transfer pathways could proceed in THER. The ESIPT rate in the HBT part is slower about 10-fold than in SAI part because of a relative higher energy barrier. Besides, torsion tends to occur after the ESIPT in the SAI part.

Excited state proton transfer in reverse micelles: Effect of temperature and a possible interplay with solvation

Excited state proton transfer (ESPT) is a fundamental process of immense biophysical interest and considering the heterogeneity existing in real biological environments we investigate the process in a bio-mimicking reverse micellar (RM) systems. We herein report a detailed study on the ESPT process of a photo-acid d-luciferin at different temperatures in RMs composed of: anionic AOT, cationic DDAB, and neutral Igepal-520 using steady state and time resolved fluorescence measurements. We found that with increasing temperature both solvation as well as the ESPT rate accelerate, however, the extent of the increase is RM specific, and they even not complement each other. Our study clearly identifies the pivotal role of solvation, specially in micro-heterogeneous environments, to guide the ESPT process.

Novel Norrish type I flavonoid photoinitiator for safe LED light with high activity and low toxicity by inhibiting the ESIPT process

In this study, flavonoid with N,N-diphenylamino group, 2-(4-(diphenylamino)phenyl)-3-hydroxy-4H-chromen-4-one (3HF–OH) was designed and synthesized as low-toxicity photoinitiator for photopolymerization under safe visible LED sources, however, low photoinitiation activity of 3HF–OH was found due to the excited state proton transfer (ESIPT) process. Hence, by replacing the 3-position hydroxyl group of 3HF–OH with benzoyl chloride and benzenesulfonyl chloride, flavonol carboxylate (3HF–C) and flavonol sulfonate (3HF–S) were synthesized, respectively. 3HF–C and 3HF–S showed not only one-component photoinitiating activity but also two-component photoinitiating activity when they were used with triethanolamine (TEOA) and iodonium salt (ONI) under safe, long-wavelength LED soft light sources. Photopolymerization kinetics reveals that photoinitiator 3HFs at low concentration exhibits the high polymerization rate and conversion. Results also showed that 3HF–S has much higher photoinitiation activity during photopolymerization compared with 3HF–OH and 3HF–C. The high extinction coefficient and aggregation-induced emission (AIE) characteristics of 3HF–S were observed, which proved that reducing the photoinitiator usage can effectively progress the photopolymerization activity. At the same time, the low usage of 3HF–S further reduces the toxicity of the cured material. Through electron spin resonance (ESR) experiment, the photoinitiation and sensitization mechanisms of these photoinitiators were observed. The school badge structure of hydrogels could be successfully 3D printed with 3HF–S and the stiffness of the structure was sufficient for further processing without breaking the structure. Moreover, the use of the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide (MTT) method confirmed the lower toxicity for the final photopolymer provided.

4-OH coumarin based rotary switches: Tautomeric state and effect of the stator

Two new 4-OH coumarin based rotary switches, containing fixed carbonyl groups in the rotor, have been synthesized and their properties have been studied by combined use of DFT calculations and molecular spectroscopy (UV–Vis absorbance and emission, NMR). It was found that the structure of the stator (naphthyl in 2 or quinolyl moiety in 3) and solvents polarity do not influence their azo-hydrazone tautomerism. Both compounds exist as keto (hydrazone) tautomers. The NMR data indicate a mixture of E (major) and Z (minor) keto form isomers in solution. The results are in very good agreement with ground state DFT calculations. The keto tautomers exhibit different emission behavior depending on the structure of the stators. Comparing to 2, more intensive emission and higher lifetime was observed in 3, where the formation of intramolecular hydrogen bonding of the hydrazone NH with the N atom of quinoline restricts the rotation of the stator. According to the spectral data and the TDDFT analysis the observed emission originates from the excitation of the keto tautomers and does not include excited state proton transfer. It was shown that the protonation is a suitable stimulus for E/Z switching in 2, where a possible mechanism is sketched. In the case of 3, the addition of acid leads to the protonation of the quinolyl nitrogen atom, which slightly affects the E/Z isomerization ratio.

Solvent and pH-sensitive Fluorescence Response of Alloxazine

The fluorescence response of Alloxazine was found to be sensitive to solvent properties like polarity and H-bonding capacity, which also played a significant role in controlling the incidence of Alloxazine – Isoalloxazine photo-tautomerization. The latter was strongly promoted in polar, protic solvents like alcohols, via an N(1)→N(10) excited state proton transfer mechanism involving Alloxazine : solvent (1:2) intermolecular H-bonded complexes, similar to that proposed earlier for 7,8-Dimethylalloxazine. Even for Alloxazine in aqueous solutions over a wide range of acid/base strengths from pH = 2 to pH = 13, the fluorescence was found to be always dominated by two emissive species, which were identified as the Alloxazine – Isoalloxazine couple, irrespective of the pH of the solution, and of the nature of the ionic equilibrium prevailing at that pH. The results in aqueous pH solutions could be explained by a mechanism where the photo-tautomerization was resolved into two independent dynamics involving deprotonation and protonation of the N(1) and N(10) atoms of Alloxazine, respectively.

Dual Function Based on Switchable Colorimetric Luminescence for Water and Temperature Sensing in Two-Dimensional Metal-Organic Framework Nanosheets.

A simple, rapid, highly selective, and real-time determination of water is urgently required for preventing danger from water contamination in materials. Herein, the excited-state proton transfer (ESPT) concept-based luminescent sensor [Cd2(2,5-tpt)(4,5-idc)(H2O)4] (1) (2,5-tpt = 2,5-dihydroxyterephthalic acid and 4,5-idc = 4,5-imidazoledicarboxylic acid) has been designed for discriminative detection via enol-keto tautomerism. To improve the sensitivity, two-dimensional (2D) nanosheets of 1 have been synthesized by top-down liquid ultrasonic exfoliation technology for sensing water in dimethylformamide, which lead to fast detection (<30 s), high selectivity, broad-range detection (0-50% v/v), and a low detection limit value (0.25% v/v). This sensor can serve dual sensing mechanisms along with a luminescent color change via shifted emission (green→yellow) in low water content and a turn-off method in high water content. For ease of use, the test-strip paper-based 2D nanosheets of 1 have been prepared and applied for water detection with long-term stability, pH stability, and good reusability. On-site water detection in real time can be evaluated using a smartphone color-scanning application for quantitative scanometric assays coupled with test-strip paper-based 2D nanosheets of 1. Also, 1 can be utilized for a colorimetric luminescent thermometer in the ranges of physiological and high temperature with good linearity and recyclability.

Computation of electrical responsive properties and global reactivity descriptors along the proton transfer co-ordinate of donor–acceptor substituted pyrazole derivatives

Ground and excited-state proton transfer and electrical responsive properties along the proton transfer co-ordinate of four donor–acceptor substituted pyrazole derivatives have been investigated using the density function-based range separated hybrid GGA functional ωB97XD with 6-311G(d) basis. Both intrinsic reaction coordinate (IRC) and distinguished co-ordinate (variation of O–H distance of the enol tautomer) have been chosen as a proton transfer co-ordinate of the titled compounds for comparison. Our study reveals that the various electrical responsive parameters like average polarizability (α av), first hyperpolarizability (β av) and global reactivity descriptors e.g., chemical hardness (η), electrophilicity index (ω) along the proton transfer co-ordinate are in conformity with the respective optimum principles. Maximum value of β av is found in the case of nitro substituted pyrazole derivative. Variation of β av along the proton transfer co-ordinate for all compounds have been correlated with the variation of Δµf 0/(S 1-S 0)3 using two-level approximation and electronic special extent (<R 2>) along the proton transfer coordinate.

Effect of Moderate Hydrogen Bonding on Tautomer Formation via Excited-State Intermolecular Proton-Transfer Reactions in an Aromatic Urea Compound with a Steric Base.

1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU), which forms weak hydrogen bonds despite the high basicity caused by its hindered structure, was used to investigate tautomer formation via excited-state intermolecular proton-transfer (ESPT) reactions. The kinetics of the ESPT reactions of anthracen-2-yl-3-phenylurea (2PUA) in the presence of DBU were compared to that observed for the acetate anion (Ac) using time-resolved fluorescence measurement. Based on the association constants in the ground state, the intermolecular hydrogen bond between 2PUA and DBU was less stable than the bond between 2PUA and Ac due to steric hindrance and the geometry of the hydrogen bond. In the fluorescence spectra, 2PUA-DBU displayed prominent tautomeric emission in chloroform (CHCl3), whereas 2PUA-Ac exhibited distinct tautomeric emissions in dimethyl sulfoxide (DMSO). Kinetic analysis revealed that the rate constant of the ESPT reaction of 2PUA-DBU remarkably decreased when the proton-accepting ability of the solvent increased whereas the reaction of 2PUA-Ac was linked to the solvent polarity rather than proton-accepting ability. These results indicated that moderate hydrogen bonds due to steric hindrance were influenced by the type of solvent present, particularly if the solvents exhibited proton-accepting capabilities like DMSO. This, in turn, affected the rate constant of tautomer formation.