Proton-transfer Tautomer Research Articles

Anionic Hydrogen-Bonded Frameworks Showing Tautomerism and Colorful Luminescence for the Ultrasensitive Detection of Acetone.

Tautomers coexisting in an equilibrium system have significant potential for regulating luminescent properties because of their structural differences. However, separating and stabilizing tautomers at room temperature is a considerable challenge. In this study, it is found that hydrogen-bonded organic frameworks (HOFs) composed of Br- anions can effectively separate and stabilize two proton-transfer tautomers of triarylformamidinium bromide: namely, the nitrogen cation (BA-N) and carbon cation (BA-C). The BA-N crystal consisting of a dense anionic HOF and parallelly aligned organic cations exhibits green thermally activated delayed fluorescence and red room-temperature phosphorescence (RTP). The BA-C crystal contains acetone molecules that induce an antiparallel arrangement of the organic cations to form a loose HOF, producing blue prompt fluorescence and green RTP. Interestingly, switching of the HOFs between BA-N and BA-C can be achieved through the uptake and release of acetone, thereby dynamically adjusting multiple luminescent properties. Consequently, the HOF crystals can be used for the highly sensitive and specific sensing of acetone with a detection limit of 66.74 ppm. This study not only stabilizes tautomeric luminescent materials at room temperature, but also provides a new method for constructing smart HOFs with a sensitive response to a stimulus.

Five-membered vs. six-membered ESIPT in 3,5-dihydroxychromone

3,5-dihydroxychromone (3,5-DHC) possesses five-membered and six-membered ring hydrogen-bonding structures that would facilitate an excited-state intramolecular proton transfer upon photoexcitation to the dipole-allowed S1 (ππ∗) state. Our investigation based on surface hopping trajectory-based simulations show that proton transfer in 3,5-DHC happens efficiently via the six-membered ring structure with an average timescale of 29 fs. The proton transfer via this pathway is a barrierless process, whereas a barrier of ∼0.46 eV exists for the other pathway that proceeds via a five-membered ring structure. The proton-transferred tautomer occurring via the former pathway is more stable than the one originating from the latter. Hence, the former tautomer exhibits a relatively larger Stokes-shifted fluorescence emission than the latter one.

ESIPT and triplet generation dynamics of 5-sulfanylchromen-4-one

Ultrafast photophysical processes of normal and proton-transferred tautomers associated with the excited-state intramolecular proton transfer (ESIPT) in 5-sulfanylchromen-4-one (5-SC) are explored theoretically. The normal tautomer undergoes the proton transfer predominantly via S1, populating the proton-transferred tautomer in its lowest singlet excited state, i.e., S1′. Subsequently, the S1′-T2′ intersystem crossing pathway efficiently induces triplet formation in the latter tautomer. Our findings show that the energetics and dynamics associated with the triplet formation are similar to that of respective tautomers of positional isomer, i.e., of 3-sulfanylchromen-4-one (3-SC). However, the proton transfer might be more efficient in 5-SC than in 3-SC as this process takes place via a barrierless pathway in 5-SC.

Ultrafast ESIPT dynamics of 4-methyl-2,6-diformylphenol

We investigate the gas-phase photodynamics associated with the S1 (nπ*), S2 (ππ*) and S3 (nπ*) states of 4-methyl-2,6-diformylphenol using trajectory-based surface hopping dynamics simulations. A rapid internal conversion would populate S1 and S3 upon initiating the trajectories on “bright” S2. This nonadiabatic population behavior would lead to intramolecular proton-transfer tautomerization via three states, where the predominant tautomerization happens via S2. An average timescale of ∼ 44 fs is estimated for the tautomerization. The triplet formation can occur via energetically feasible S1-T3 (at the Franck-Condon geometry of normal tautomer) and S1-T2 (at the S1 equilibrium minimum of proton-transferred tautomer) pathways. Intersystem crossing and the forbidden nature of S1 (nπ*) makes the molecule exhibit weak fluorescence.

CASPT2//CASSCF studies on mechanistic photophysics of 3-hydroxyflavone

3-Hydroxyflavone (3-HF) as the backbone of flavonols has been extensively studied on the dual fluorescence and excited-state intramolecular proton transfer (ESIPT) after photoexcitation. However, its detailed excited-state relaxation mechanism is still unclear. Herein, the minimum-energy structures, conical intersections and singlet–triplet crossing points of 3-HF in the 1ππ∗, 1nπ∗, 3ππ∗, 3nπ∗, and S0 states in the gas phase were first optimized by the CASPT2//CASSCF method. Then, the minimum-energy proton transfer (MEPT) and excited-state decay (ESD) pathways were calculated as well. Based on the calculation results, we proposed several possible ESD pathways from the initially populated bright S2(ππ*) state. The major channel is singlet-involved and stretch-torsion coupled ESIPT pathway. 3-HF in normal (N) form first undergoes efficient 1ππ* ESIPT with a small barrier of 2.2 kcal/mol to yield the 1ππ* proton-transferred tautomer (T). Subsequently, the T tautomer approaches the nearby accessible S1/S0 conical intersection and then occurs internal conversion (IC) to fall back to the S0 state. Finally, the S0 species quickly transfers into the N conformer via the reverse ground state intramolecular proton transfer (GSIPT) to realize the ground-state recovery. In addition, the minor ESD pathways are involved less efficient intersystem crossing (ISC) processes. The present work could provide insights into understanding the photophysics of flavonols and their derivatives.

Triplet state generation followed by the excited-state intramolecular proton transfer in 3-sulfanylchromen-4-one

We theoretically study the triplet formation dynamics of tautomers associated with the excited-state intramolecular proton transfer of 3-sulfanylchromen-4-one (3-SC). The S1-T3 intersystem crossing pathway is expected to compete with the intramolecular thiol proton transfer upon photoexciting the normal tautomer (parent molecule) to its S1 state. The triplet formation via the S1′-T2′ intersystem crossing tends to be the dominant nonradiative process in the proton-transferred tautomer. The wavepacket transfer to the respective accessible receiver triplet state is more pronounced in the proton-transferred tautomer than its parent tautomer, implying an efficient triplet formation followed by the proton transfer via S1 in 3-SC.

Elaborating and regulating ESIPT associated with solvent polarity for the novel 2-(benzo[d]thiazol-2-yl)-4-(9H-diphenylamino-9-yl)phenol fluorophore

In light of the potential applications as full-colour organic emitting materials crossing the entire visible spectra, the novel diphenylamino submitted 2-(2’-hydroxyphenyl) benzothiazole derivative 2-(benzo[d]thiazol-2-yl)-4-(9H-diphenylamino-9-yl)phenol (pPhOH) is explored in this work about its excited state behaviours. Solvent-polarity-related photo-induced hydrogen bonding effects of pPhOH indicate nonpolar aprotic solvents largely enhance S1-state hydrogen bonding interactions. Charge reorganisation stemming from photoexcitation and polarity-dependent energy gap between HOMO and LUMO orbitals further reveals the excited state intramolecular proton transfer (ESIPT) tendency. Insights into potential energy curves (PECs) along ESIPT paths in solvents with different polarities and electronic spectral behaviours of pPhOH as well as its proton-transfer tautomer pPhOH-T, we present solvent polarity could harness the excited state behaviours for pPhOH system.

Understanding the photodynamics of 3-hydroxypyran-4-one using surface hopping simulations

Photoinduced relaxation mechanisms of 3-hydroxypyran-4-one (3HOX) are explored by performing surface hopping dynamics simulations. Multiple conical intersections promote an internal conversion from higher singlet states to S1 within 50 fs. On a similar timescale, the intramolecular proton transfer occurs via a barrierless channel on S2. The proton-transferred (keto) tautomer then decays to its ground electronic state through an energetically accessible S1/S0 conical intersection. Eventually, the keto tautomer returns to S0 of enol form by back proton transfer. As events of the proton transfer cycle happen in the ultrafast timescale, 3HOX is expected to be highly photostable.

Chapter Open for the Excited-State Intramolecular Thiol Proton Transfer in the Room-Temperature Solution.

We report here, for the first time, the experimental observation on the excited-state intramolecular proton transfer (ESIPT) reaction of the thiol proton in room-temperature solution. This phenomenon is demonstrated by a derivative of 3-thiolflavone (3TF), namely, 2-(4-(diethylamino)phenyl)-3-mercapto-4H-chromen-4-one (3NTF), which possesses an -S-H···O═ intramolecular H-bond (denoted by the dashed line) and has an S1 absorption at 383 nm. Upon photoexcitation, 3NTF exhibits a distinctly red emission maximized at 710 nm in cyclohexane with an anomalously large Stokes shift of 12 230 cm-1. Upon methylation on the thiol group, 3MeNTF, lacking the thiol proton, exhibits a normal Stokes-shifted emission at 472 nm. These, in combination with the computational approaches, lead to the conclusion of thiol-type ESIPT unambiguously. Further time-resolved study renders an unresolvable (<180 fs) ESIPT rate for 3NTF, followed by a tautomer emission lifetime of 120 ps. In sharp contrast to 3NTF, both 3TF and 3-mercapto-2-(4-(trifluoromethyl)phenyl)-4H-chromen-4-one (3FTF) are non-emissive. Detailed computational approaches indicate that all studied thiols undergo thermally favorable ESIPT. However, once forming the proton-transferred tautomer, the lone-pair electrons on the sulfur atom brings non-negligible nπ* contribution to the S1' state (prime indicates the proton-transferred tautomer), for which the relaxation is dominated by the non-radiative deactivation. For 3NTF, the extension of π-electron delocalization by the diethylamino electron-donating group endows the S1' state primarily in the ππ* configuration, exhibiting the prominent tautomer emission. The results open a new chapter in the field of ESIPT, covering the non-canonical sulfur intramolecular H-bond and its associated ESIPT at ambient temperature.

Correlation between Kinetics and Thermodynamics for Excited-State Intramolecular Proton Transfer Reactions.

Finding the relation between thermodynamics and kinetics for a reaction is of fundamental importance. Here, the thermodynamics and kinetics correlation of excited-state intramolecular proton transfer (ESIPT) was investigated by the TD-DFT calculation under the CAM-B3LYP/6-311+G** level. We choose the family 2-(2'-aminophyenyl)benzothiazole and its amino derivatives as paradigms, which all possess the NH-type intramolecular hydrogen bond (H-bond), and investigate the corresponding ESIPT reaction. The H-bond strength can be systematically tuned, so both activation energy ΔG‡ and free energy difference between proton transfer tautomer (T*, product) and normal species (N*, reactant) ΔGT*-N* can be varied. To minimize the environmental interference such as solvent external H-bond and polarity perturbation, a nonpolar solvent such as cyclohexane is chosen as a bath with a polarizable continuum solvation model for the calculation. As a result, the comprehensive computational approach reveals a linear relationship between ΔGT*-N* and ΔG‡, which can be expressed as ΔG‡ = ΔG0 + αΔGT*-N*. The fundamental insight is reminiscent of the Bell-Evans-Polanyi (BEP) principle where α represents the character of the position of the transition state along the proton motion coordinate. In other words, the more exergonic the ESIPT reaction is, the faster the proton transfer rate can be observed. To verify that such a correlation is not a sporadic event, another ESIPT family with an -OH proton, 1-hydroxy-11H-benzo[b]fluoren-11-one and its derivatives, was also investigated and proved to follow the BEP principle as well. Unlike the quantum mechanics description of proton transfer where either proton tunneling is dominant or solute/solvent is coupled in ESIPT, this work demonstrates that reaction kinetics and thermodynamics are strongly correlated within the same class of ESIPT molecules with an intrinsic barrier free from solvent perturbation, being faster with the more exergonic reaction.

Manifestation of the dynamics of ultrafast photoprocesses in the transient absorption spectra of diflavonoid solutions

Herein, using the femtosecond absorption spectroscopy method, the dynamics of the nonstationary induced absorption spectra of diflavonoid 3,7-dihydroxy-2,8-di(4-methoxyphenyl)-4H, 6H-pyrano[3,2-g]chromene-4,6-dione (DFV) in solvents of different polarities is studied. It is found that the rapid transformation of the transient absorption spectra of DFV in time is due to the processes of intramolecular protons transfer in excited singlet states. For a nonpolar solvent, two protons are transferred in two stages. Initially, during the sub-picosecond times, a form with a single transferred proton is formed from the Frank-Condon state. From this transition state, in a time range of about 9 ps, the second proton is transferred and the two proton transfer tautomer with a high quantum yield of fluorescence ~0.66 is formed, which has the gain band in the transient absorption spectra. For the polar solvent dimethylformamide only the short-lived form with a single proton transferred is formed also during the subpicosecond times practically the same ones as for the nonpolar solution and has a lifetime of about 20 ps. The polarity of the medium, which affects the formation of a set of the “closed” and “open” forms of DFV in the ground state, differing in relative positions in the space of hydroxyl and carbonyl groups, largely determines the mechanism of the intramolecular proton transfer process in the DFV molecule, which consists in the sequential transfer of two protons in a non-polar solvent to form a fluorescent long-lived tautomer and the transfer of one proton in polar solvents to form a short-lived non-fluorescent form.

The influence of base pair tautomerism on single point mutations in aqueous DNA.

The relationship between base pair hydrogen bond proton transfer and the rate of spontaneous single point mutations at ambient temperatures and pressures in aqueous DNA is investigated. By using an ensemble-based multiscale computational modelling method, statistically robust rates of proton transfer for the A:T and G:C base pairs within a solvated DNA dodecamer are calculated. Several different proton transfer pathways are observed within the same base pair. It is shown that, in G:C, the double proton transfer tautomer is preferred, while the single proton transfer process is favoured in A:T. The reported range of rate coefficients for double proton transfer is consistent with recent experimental data. Notwithstanding the approximately 1000 times more common presence of single proton transfer products from A:T, observationally there is bias towards G:C to A:T mutations in a wide range of living organisms. We infer that the double proton transfer reactions between G:C base pairs have a negligible contribution towards this bias for the following reasons: (i) the maximum half-life of the G*:C* tautomer is in the range of picoseconds, which is significantly smaller than the milliseconds it takes for DNA to unwind during replication, (ii) statistically, the majority of G*:C* tautomers revert back to their canonical forms through a barrierless process, and (iii) the thermodynamic instability of the tautomers with respect to the canonical base pairs. Through similar reasoning, we also deduce that proton transfer in the A:T base pair does not contribute to single point mutations in DNA.

Theoretical insights into the spectral properties of a salicylideneaniline-based fluorescence chemosensor (SB1) and its sensing mechanism for cyanide anion

The sensing mechanism of a salicylideneaniline-based fluorescence chemosensor (SB1) for the cyanide anion (CN−) has been investigated by nonadiabatic dynamics and transition state (TS) theory. The intramolecular hydrogen bonds in SB1 and the product SB1-CN are strengthened after photoexcitation, which induce the intramolecular proton transfer in 114 fs and 167 fs by the results of nonadiabatic dynamics. The fluorescence (626 nm) of SB1 emits from the proton-transfer tautomer rather than the excited state intramolecular charge transfer (ICT) state. The new appeared emission peak (523 nm) after the addition of CN− is attributed to the proton-transfer tautomer of SB1-CN, which has a large blue shift (103 nm) compared to that of SB1. The TS calculations demonstrate the occurrence of reaction from SB1to SB1-CN after overcoming an energy barrier of 10.52 kcal/mol and there is a large interaction energy (95.92 kJ/mol) between SB1 and CN−, both of which confirm that SB1 has high sensitivity and selectivity to CN−. The nucleophilic addition reaction of CN− interrupts the π-conjugation structure of SB1, resulting in an inefficient ICT, which in turn destroys the excited state intramolecular charge/proton transfer (ESICT-ESIPT) coupled system and causes the blue shift of the spectral peak. The work gives a complete explanation about ESICT-ESIPT coupling switching behavior and the sensing mechanism of SB1 for CN−.

Excited-state intramolecular proton transfer driven by conical intersection in hydroxychromones.

Nonradiative decay pathways associated with vibronically coupled S1 (ππ*)-S2 (nπ*) potential energy surfaces of 3- and 5-hydroxychromones are investigated by employing the linear vibronic coupling approach. The presence of a conical intersection close to the Franck-Condon point is identified based on the critical examination of computed energetics and structural parameters of stationary points. We show that very minimal displacements of relevant atoms of intramolecular proton transfer geometry are adequate to drive the molecule toward the conical intersection nuclear configuration. The evolving wavepacket on S1 (ππ*) bifurcates at the conical intersection: a part of the wavepacket moves to S2 (nπ*) within a few femtoseconds while the other decays to S1 minimum. Our findings indicate the possibility of forming the proton transfer tautomer product via S2 (nπ*), competing with the traditional pathway occurring on S1 (ππ*).

Excited-state proton-coupled electron transfer within ion pairs.

The use of light to drive proton-coupled electron transfer (PCET) reactions has received growing interest, with recent focus on the direct use of excited states in PCET reactions (ES-PCET). Electrostatic ion pairs provide a scaffold to reduce reaction orders and have facilitated many discoveries in electron-transfer chemistry. Their use, however, has not translated to PCET. Herein, we show that ion pairs, formed solely through electrostatic interactions, provide a general, facile means to study an ES-PCET mechanism. These ion pairs formed readily between salicylate anions and tetracationic ruthenium complexes in acetonitrile solution. Upon light excitation, quenching of the ruthenium excited state occurred through ES-PCET oxidation of salicylate within the ion pair. Transient absorption spectroscopy identified the reduced ruthenium complex and oxidized salicylate radical as the primary photoproducts of this reaction. The reduced reaction order due to ion pairing allowed the first-order PCET rate constants to be directly measured through nanosecond photoluminescence spectroscopy. These PCET rate constants saturated at larger driving forces consistent with approaching the Marcus barrierless region. Surprisingly, a proton-transfer tautomer of salicylate, with the proton localized on the carboxylate functional group, was present in acetonitrile. A pre-equilibrium model based on this tautomerization provided non-adiabatic electron-transfer rate constants that were well described by Marcus theory. Electrostatic ion pairs were critical to our ability to investigate this PCET mechanism without the need to covalently link the donor and acceptor or introduce specific hydrogen bonding sites that could compete in alternate PCET pathways.

A dynamic and electrostatic potential prediction of the prototropic tautomerism between imidazole 3-oxide and 1-hydroxyimidazole in external electric field.

In order to obtain an optimum scheme for separating the proton-transfer tautomer, a dynamic investigation into the effect of the external electric field on the proton-transfer tautomeric conversion in imidazole 3-oxide and 1-hydroxyimidazole was carried out at the M06-2X/6-311++G** and CCSD(T)/6-311++G(2d,p) level, accompanied by the analysis of the surface electrostatic potentials. The results show that, for both the forward reaction "imidazole 3-oxide → N-hydroxyimidazole free radical → 1-hydroxyimidazole" and its reverse reaction processes, the fields parallel to the N→O or N-OH bond axis affect the barrier heights and rate constants considerably more than those parallel to the other orientations. As the field strength is increased along the orientation from the O to N atom, the chemical equilibrium moves toward the direction for the formation of 1-hydroxyimidazole, while the amount of imidazole 3-oxide is increased with the increased field strength along the opposite orientation. In the fields along the orientation consistent with the dipole moment, the electrostatic potentials and their variances "abnormally" increase for the transition states with the N→O bond in comparison with those in no field (they decrease generally), which enhances the nucleophilicity of the coordination O atom and the electrophilicity of the activated H atom. The analyses of the AIM (atoms in molecules) and NICS (nucleus-independent chemical shift) were used to explain the above anomaly. Graphical Abstract Electrostatic potentials and their variances "abnormally" increase in the external electric field, which greatly affects tautomeric conversion.

Catalytic-Type Excited-State N-H Proton-Transfer Reaction in 7-Aminoquinoline and Its Derivatives.

7-Aminoquinoline (7AQ) and various amino derivatives thereof (-NHR) have been strategically designed and synthesized to study their excited-state proton-transfer (ESPT) properties. Due to the large separation between the proton donor -NHR and the acceptor -N- site, ESPT in 7AQ derivatives, if available, should proceed under protic solvent catalysis. ESPT is found to be influenced by the acidity of -NHR and the basicity of the proton-acceptor -N- in the quinoline moiety. The latter is varied by the resonance effect at the quinoline -N- site induced by the -NHR substituent. For those 7AQ derivatives undergoing ESPT, increased quinoline basicity results in a faster rate of ESPT, implying that proton donation from methanol to the quinoline moiety may serve as a key step in the process. Our studies also indicate the existence of an equilibrium between cis and trans arrangements of -NHR in terms of its hydrogen-bond (H-bond) configuration with methanol, whereby only the cis-H-bonded form undergoes methanol-assisted ESPT. With one exception, the interconversion between cis and trans configurations is much faster than the rate of ESPT, yielding amino-type (normal form) and imine-type (proton-transfer tautomer) emissions with distinct relaxation dynamics.

Femtosecond real-time probing of the excited-state intramolecular proton transfer reaction in methyl salicylate.

The excited-state intramolecular proton transfer (ESIPT) process and subsequent electronic relaxation dynamics in methyl salicylate have been investigated using femtosecond time-resolved ion yield spectroscopy combined with time-resolved photoelectron imaging. Excitation with a tunable pump pulse populates the keto tautomer in the first excited electronic state S1(ππ*). As a hydrogen atom transfers from the phenolic group to the carbonyl group within 100 fs, the molecular geometry changes gradually, leading to a variation in the electronic photoionization channel. By virtue of the accidental resonance with some intermediate Rydberg states, the time-dependent photoelectron spectra provide a direct mapping of the ESIPT reaction from the initially populated keto tautomer to the proton-transferred enol tautomer. Subsequently, the population around the enol configuration undergoes intramolecular vibrational redistribution on a subpicosecond time scale, followed by internal conversion to the ground state with a wavelength-dependent lifetime in the picosecond range. Furthermore, the excitation energies of several Rydberg states in methyl salicylate are determined experimentally.

Proton-Transfer-Regulated Magnetic Spin Couplings in Nitroxide-Functionalized Porphycene Diradicaloids

Porphycene has found many promising applications due to its unique structure including flexible proton-transfer tautomerization, while its large conjugated configuration can also assure it as a cou...

Synthesis of Some Pyrimidine, Pyrazole, and Pyridine Derivatives and Their Reactivity Descriptors

A series of novel pyrimidine (2, 3), pyrazole (4, 5), and pyridine (6) derivatives were synthesized using a chalcone-bearing thiophene nucleus (1). The target compounds were synthesized by reaction of compound (1) with urea, thiourea, malononitrile, hydrazine hydrate, and 2,4-dinitrophenyl hydrazine, respectively. Molecular electronic structures have been modeled within density functional theory framework (DFT). Reactivity indices and electrostatic surface potential maps (ESP maps) allow us to establish trends that enable making predictions about chemical characteristics of the newly synthesized molecules and their proton transfer tautomers. Proton transfer is generally more favored in solution than in the gas phase. In acetonitrile, keto-form tautomers and thione-form tautomers become more energetically stable than the corresponding enol or thiol tautomers due to solvent-induced enhancement in the molecular polarity identified by computed dipole moment.