Double Proton Transfer Process Research Articles

Effect of external electric fields on the ESDPT process and photophysical properties of 1,8-dihydroxy-2-naphthaldehyde.

In this work, by capitalizing on the density functional theory (DFT) and the time-dependent density functional theory (TD-DFT) methods, it has been systematically studied that the excited state double intramolecular proton transfer (ESDPT) process and the photophysical properties of 1,8-dihydroxy-2-naphthaldehyde (DHNA) are affected by the distinct external electric fields (EEFs). The obtained intramolecular hydrogen bond (IHB) parameters containing bond lengths and angles, as well as infrared (IR) vibrational spectra demonstrate that IHB strength changes in the distinct EEFs. Moreover, not only do the potential energy surfaces (PESs) indicate that the ESDPT process of DHNA is stepwise, but also increasing the positive EEF results in a decrease in the energy barrier accordingly, while vice versa. The absorption and fluorescence spectra also undergo a corresponding red or blue shift in the EEF; for instance, when the EEF changes from +10 × 10-4 a.u. to +20 × 10-4 a.u., the fluorescence peak undergoes a blue shift from 602 nm to 513 nm in the keto2 form. In a nutshell, the ESDPT process of DHNA can be influenced by the EEF, which will serve as a reference in regulating and controlling proton transfer that causes luminescence.

Excited state intramolecular double-proton transfer dynamics of [2,2′-bipyridyl]-3,3′-diol inside mesoporous silica nanochannels

The modulation of kinetics and pathways in the ESIPT process of proton transfer probes holds significant potential for advancing applications in bio-imaging, drug delivery, and OLEDs. One effective approach for achieving this modulation is altering the H-bonding donating capability of the surrounding medium. To investigate this, we conducted a comprehensive study on the excited state intramolecular double proton transfer process of [2,2′-bipyridyl]-3,3′-diol (BP(OH)2) within the confined spaces of silica nanochannels, namely, MCM-41. MCM-41, known for its versatile properties, has emerged as a promising host in various fields, such as drug delivery and heterogeneous catalysis. Upon encapsulation within the MCM-41, the double proton transfer process of BP(OH)2 is significantly modulated, which is reflected in both steady-state and time-resolved photophysical experiments. We have observed an almost 100 times increment in emission intensity and a 30 nm blue-shift in the emission maxima when the probe gets encapsulated inside the silica nanopores. Most importantly, the femtosecond up-conversion profile exhibits an interesting feature. The rise component of 10 ps, which was attributed to MK→DK conversion in bulk acetonitrile (MeCN), is not observed when the probe resides inside the MCM-41, suggesting the proton transfer is concerted rather than sequential, like in the case of bulk MeCN. This anomalous proton transfer mechanism inside the nanochannel was attributed to the weak H-bonding donating ability of the silanol groups, which could not stabilize the MK form, and thus favoured the concerted pathway over sequential. Moreover, DFT calculations corroborate the concerted pathway observed in the MCM-41 with the gas-phase calculations and the sequential mechanism observed in bulk MeCN with the solution-phase calculations.

Accelerating the concerted double proton transfer process of 2,2′-bipyridine-3,3′-diol-5,5′-dicarboxylate acid ethyl ester (BPDC) molecule by centrosymmetric dual intermolecular hydrogen bonds

This study has conducted a systematic investigation into the excited state double proton transfer (ESDPT) process of 2,2′-bipyridine-3,3′-diol-5,5′-dicarboxylate acid ethyl ester (BPDC) in cyclohexane (CYH) and water (H2O) by density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. The calculated geometrical parameters, reduced density gradient and infrared vibration spectra confirm that BPDC undergoes the ESDPT process in both CYH and H2O. In comparison to the CYH solvent, the intramolecular hydrogen bonds (IAHBs) are strengthened in the S0 state by the formation of centrosymmetric intermolecular hydrogen bonds (IEHBs) between H2O and the OH group of BPDC molecules. Furthermore, the constructed potential energy surfaces show that BPDC molecules undergo a concerted ESDPT process without potential barriers in both solvents. Time evolution analysis confirms that the ESDPT process of BPDC molecules in H2O solvent (19 fs) is faster than that in CYH solvent (62 fs). The results indicate that the formation of IEHBs causes zero disruption to the original I AHBs, but it can effectively promote the ESDPT process of BPDC molecules. Overall, this work serves as important theoretical guidance for the development of high-performance fluorescence materials.

Theoretical insights into the excited-state single and double proton transfer processes of DEASH in water

Recently, a new fluorescent chromophore 6,6′-((1E,1′E)-hydrazine-1,2-diylidenebis(methanylylidene))bis(3-(diethylamino)phenol) (DEASH) with double intramolecular hydrogen bonds (IHBs) was synthesized and could be used as an effective indicator of hydrazine. The experimental results showed that DEASH could emit bright green fluorescence due to the excited state intramolecular proton transfer (ESIPT) induced by IHBs. [Dyes Pigments, 2022, 197, 109880.] However, the underlying mechanism of the ESIPT process for DEASH is still unclear. In this paper, we investigate excited-state intramolecular single (ESISPT) and double (ESIDPT) proton transfer processes of DEASH by density functional theory (DFT) and time-dependent density functional theory (TD-DFT) methods. Through the theoretical analyses for the constructed potential energy surfaces (PESs), excited hydrogen bond strengthening and photophysical properties of the molecule, we finally come to the conclusion that ESIDPT process of the DEASH in water solution is inclined to a stepwise process.

Tunable fluorescence of Imidazo[1,2‐a]pyridine derivatives with additional proton transfer sites Harnessing excited-state intramolecular double proton transfer: Theoretical insight

Electronic properties and excited-state intramolecular proton transfers of a new designed series of doubly intramolecular hydrogen bonding molecules having imidazo[1,2-a]pyridines as proton acceptors with different types of proton donors (hydroxy-type, amino-type, and mixing-type) and their derivatives have been theoretically evaluated for fluorescent probes. Overall, all designed molecules are photoexcited at ∼420 nm and give fluorescence in the NIR region with large Stokes shifts. The fluorescence of NH- and Mixed-type molecules are notably longer than that of OH-type molecules, and those of the derivatives with a π-conjugated spacer are slightly longer than those of their analogues. Based on the kinetic and thermodynamic information, all designed molecules could not exhibit the double proton transfer (DPT) process but only single proton transfer (SPT) is preferred. Moreover, the dynamic simulations on the excited-state confirm that only SPT is allowed but not DPT in accordance with the predicted PT barriers and reaction energies, leading only NT* fluorescence but their NT* fluorescence maxima are still in the NIR region (600–900 nm) with large Stokes shifts (∼300 nm). The NIR fluorescence is caused by an additional PT unit and a π-conjugated spacer induce the ICT character evidenced by the EDD maps. Therefore, these designed molecules exhibiting NIR tautomer emission with large Stokes shifts could be potential candidates for NIR fluorescent probes with avoiding self-absorption and less photo-damage.

Concerted Electron-Nuclear Motion in Proton-Coupled Electron Transfer-Driven Grotthuss-Type Proton Translocation.

Photoinduced proton-coupled electron transfer and long-range two-proton transport via a Grotthuss-type mechanism are investigated in a biomimetic construct. The ultrafast, nonequilibrium dynamics are assessed via two-dimensional electronic vibrational spectroscopy, in concert with electrochemical and computational techniques. A low-frequency mode is identified experimentally and found to promote double proton and electron transfer, supported by recent theoretical simulations of a similar but abbreviated (non-photoactive) system. Excitation frequency peak evolution and center line slope dynamics show direct evidence of strongly coupled nuclear and electronic degrees of freedom, from which we can conclude that the double proton and electron transfer processes are concerted (up to an uncertainty of 24 fs). The nonequilibrium pathway from the photoexcited Franck–Condon region to the E2PT state is characterized by an ∼110 fs time scale. This study and the tools presented herein constitute a new window into hot charge transfer processes involving an electron and multiple protons.

Theoretical Investigation on the ESIPT Process and Detection Mechanism for Dual-Proton Type Fluorescent Probe.

Recently, a new fluorescent probe AE-Phoswas reported to detect the activity of alkaline phosphatases (ALP) in different living cell lines. Here, we present an in-depth computational analysis of the mechanism and source of the fluorescence of the AE-Phos probe. There is an intermediate product (AE-OH-Phos) in the experiment as well as a different configuration of products that may emit fluorescence. It is essential to investigate the origin of fluorescence and the detection mechanism of the probe, which could help us eliminate the interference of other substances (including an intermediate product and possible isomers) on fluorescence during the experiment. According to the change of geometric parameters and Infrared spectra, we deduce that the dual intramolecular hydrogen bonds of salicylaldehyde azine (SA) were enhanced at the excited state, while AE-OH-Phos was attenuated. Considering the complex ESIPT behavior of the dual proton-type probe, the potential energy surfaces were further discussed. It can be concluded that the single proton transfer structure of SA (SA-SPT) is the most stable form. Both the concerted double proton transfer process and stepwise single proton transfer process of SA were forbidden. The fluorescence for SA was 438 nm, while that of SA-SPT was 521 nm, which agrees with the experimentally measured fluorescence wavelength (536 nm). The conclusion that single proton transfer occurs in SA is once again verified. In addition, the distribution of electron-hole and relative index was analyzed to investigate the intrinsic mechanism for the fluorescence quenching of the probe and the intermediate product. The identification of the origin of fluorescence sheds light on the design and use of dual-proton type fluorescent probes in the future.

Discriminating between the dissociative photoionization and thermal decomposition products of ethylene glycol by synchrotron VUV photoionization mass spectrometry and theoretical calculations.

Understanding the combustion chemistry of biofuel compounds is of great importance in the intelligent selection of next-generation alternative fuels. Ethylene glycol (C6H10O2) is a prototypical representative of potential biofuels. In this work, the thermal decompositions along with the dissociative ionization of ethylene glycol are studied by synchrotron VUV photoionization mass spectrometry. As a part of the dissociative ionization study, the appearance energies of seven fragments are measured. Using the theoretical calculation results, the possible formation channels of these fragments are proposed. In particular, the productions of CH3OH+ and CH3OH2+ are suggested to be from the isomerization/dissociation process, where double proton transfer processes are highlighted. Using a tunable VUV source, the high-temperature pyrolysis products of ethylene glycol are differentiated from the dissociative ionization products. Specifically, two isomeric products vinyl alcohol and acetaldehyde by H2O elimination are obtained. Formaldehyde and methanol from direct C-C bond cleavage are identified. The fragmentations of fragile radicals such as hydroxymethyl, methoxy and ethoxy are used to explain the missing products from the direct C-C and C-O bond dissociation reactions. There is no experimental evidence for the occurrence of the H and H2 elimination reactions which may have not been accessed under the present temperature conditions.

Effect of solvent polarity on excited-state double proton transfer process of 1,5-dihydroxyanthraquinone

The excited-state double proton transfer (ES-DPT) properties of 1,5-dihydroxyanthraquinone (1,5-DHAQ) in various solvents were investigated using femtosecond transient absorption spectroscopy and the DFT/TDDFT method. The steady-state fluorescence spectra in toluene, tetrahydrofuran (THF) and acetonitrile (ACN) solvents presented that the solvent polarity has an effect on the position of the ESDPT fluorescence emission peak for the 1,5-DHAQ system. Transient absorption spectra show that the increasing polarity of the solvent accelerates the rate of excited state dynamics. Calculated potential energy curves analysis further verified the experimental results. The ESDPT barrier decreases gradually with the increase of solvent polarity from toluene, THF to ACN solvent. It is convinced that the increase of solvent polarity can promote the occurrence of the ESDPT dynamic processes for the 1,5-DHAQ system. This work clarifies the mechanism of the influence of solvent polarity on the ESDPT process of 1,5-DHAQ, which provides novel ideas for design and synthesis of new hydroxyanthraquinone derivatives.

How proton transfer affects the helical parameters in DNA:DNA microhelices

Proton transfer reactions are a widespread phenomenon in many areas of the life sciences and it is one of the origins of the spontaneous point mutations during DNA replication. Because of its importance, many studies have been reported on these reactions. However, the present work is the first one focused on the structural geometrical changes by double proton transfer (DPT). Thus, different Watson–Crick (WC) pairs were optimized first in a simple model with one nucleoside base pair, and in a microhelix form with three nucleoside base pairs. The canonical and few tautomeric forms were considered in DNA:DNA microhelices with A-type and B-type helical forms. The stability of these structures and how the DPT process affects the main geometrical parameters was analyzed, in particular the deformation of the helical parameters. The M06-2X DFT method was used for this purpose. The purine/pyrimidine ring in the keto form appears easier to be deformed than when it is in the enol form. The weaker WC base pair formed with mixed microhelices than with nucleobases alone and the significant deformation of the helical and backbone parameters with the DPT appears to complicate this process in microhelices. Communicated by Ramaswamy H. Sarma

Theoretical study on the mechanism for the excited-state double proton transfer process of an asymmetric Schiff base ligand

Excited-state double proton transfer (ESDPT) in the 1-[(2-hydroxy-3-methoxy-benzylidene)-hydrazonomethyl]-naphthalen-2-ol (HYDRAVH2) ligand was studied by the density functional theory and time-dependent density functional theory method. The analysis of frontier molecular orbitals, infrared spectra, and non-covalent interactions have cross-validated that the asymmetric structure has an influence on the proton transfer, which makes the proton transfer ability of the two hydrogen protons different. The potential energy surfaces in both S0 and S1 states were scanned with varying O–H bond lengths. The results of potential energy surface analysis adequately proved that the HYDRAVH2 can undergo the ESDPT process in the S1 state and the double proton transfer process is a stepwise proton transfer mechanism. Our work can pave the way towards the design and synthesis of new molecules.

A theoretical study on water-assisted excited state double proton transfer process in substituted 2,7-diazaindole-H2O complex

The substituted effect on the first excited-state proton transfer (ESPT) process in 2,7-diazaindole-H2O (2,7-DAI-H2O) complex in water was studied in detail at the TD-M06-2X/6–311 + G(d, p) level. The frontier molecular orbital, geometries, reaction mechanism and energies of ESPT process with different substituent have been analyzed. ESPT process in the title complex occurred concertedly but highly asynchronously no matter of the electronic nature of substituent. The absorption and fluorescence peaks, H-bond distances, asynchronicity of ESPT and barrier height were affected by the substituent. The Hammett’s substituent constant had linear correlation with the difference between the sum of N1−O11 and O11−N7 distances in the reactant and that in the TS and Mulliken charge of H3O+.

How the atomic electron-accepting ability affect the double ESIPT process of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol?

By adopting the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods, the effects of atomic electron-accepting ability on the double excited state intramolecular proton transfer (ESIPT) processes of the molecules (BBO, BBT and BBSe) were systematically investigated. The obtained bond lengths, bond angles and IR vibration analysis prove that the strength of the hydrogen bonds in the three molecules gradually increases from BBO to BBT and BBSe. Moreover, the potential energy surfaces and topology analysis indicate that all the three molecules prefer to the ultrafast stepwise double ESIPT reaction in the S1 state. And the excited state proton transfer occurs more easily in the studied molecular systems along with the weakened electron-accepting ability of atom. It is concluded that the decreasing electron-accepting ability of atom can promote the molecular ESIPT behavior, which provides the theoretical basis for the synthesis of the new molecular systems with double ESIPT properties.

Theoretical study of the excited state intramolecular double proton transfer and spectral behaviors of 7-hydroxyquinoline-8-carboxylic acid

The excited-state molecular dynamics of 7-hydroxyquinoline-8-carboxylic acid (HCA) were studied to observe its fluorescent properties and proton transfer process. The two intramolecular hydrogen bonds (O-H⋯O and O-H⋯N) of the Enol form were both strengthened after photoexcitation to the first excited (S1) state. The experimentally observed fluorescence emission (465 nm) was attributed to the theoretical Keto form (470 nm), which implies the occurrence of an excited-state intramolecular double proton transfer (ESIDPT) process. The nonadiabatic dynamics results demonstrate that a single proton transfer from the carboxylic to the nitrogen atom (process-A) occurs in 55 fs, which excludes the ESIDPT concert pathway. The potential energy surface results indicate that process-A (0.08 kcal/mol) induces the occurrence of a second proton transfer from the phenol to the oxygen atom (process-B). Compared to the stepwise ESIDPT reaction that begins with process-B (1.53 kcal/mol), process-A is energy favorable in the S1 state. Therefore, we propose a reaction path of the following: Enol in the ground state (S0) → Enol in the S1 state → proton transferred Keto in the S1 state (stepwise pathway begins with process-A) → Keto in the S0 state (fluorescence emission at 470 nm) → Enol in the S0 state (reversed proton transfer). This process confirms what was predicted by Chou (J. Phys. Chem. Lett. 2011, 2, 3063).

A distinct excited-state proton transfer mechanism for 4-(N-Substituted-amino)-1H-pyrrolo[2,3-b]pyridines in aprotic and protic solvents.

Time-dependent density functional theory (TDDFT) method was used to study the different excited states proton transfer mechanism of DPP in cyclohexane (CHE) and Methanol (MeOH). The results indicate that the concerted mechanism and the stepwise mechanism coexist in the double proton transfer process of DPP dimer in the aprotic solvent CHE, the stepwise mechanism predominates. The stepwise mechanism can only carry out single proton transfer (DPP-SPT), the second proton cannot be transferred because it is hindered by high energy barriers. The concerted mechanism produces a double proton transfer (DPP-DPT). The potential energy surface of the DPP dimer was calculated and the double fluorescence phenomenon of DPP dimer observed by Chou et al. (P.T. Chou, Y.I. Liu, H.W. Liu, W.S. Yu, Dual Excitation behavior of double proton transfer versus Charge Transfer in 4-(N-Substituted Amino)-1H-pyrrolo[2,3-b]pyridines tuned by dielectric and hydrogen-bonding perturbation, J. Am. Chem. Soc., 123 (2001) 12119-12120) was explained. In view of the protonic solvent effect of methanol, the potential energy curve of the DPP/MeOH cluster was constructed. The fluorescence quenching process of DPP/MeOH clusters was elucidated. The proton transfer pathways of DPP/MeOH clusters are revealed in two different concerted ways (Type A: protons transfer from DPP molecules to MeOH solvent molecules; Type B: protons transfer from MeOH solvent to DPP molecules). The ESPT process of DPP molecules in the protic solvent MeOH was found to be more prone to Type B. The results can help to better understand the intermolecular hydrogen bonding mechanism of DPP molecules.

Evidence for stepwise excited state double proton transfer process in quinolone–pyrazole triggered by methanol solvent

Basing on the density functional theory and time-dependent density functional theory, we present a systematic theoretical investigation on the excited state double proton transfer process (ESDPT) of quinolone–pyrazole (QP) in methanol (MeOH). The theoretical results provide convincing evidence that ESDPT process takes place in QP-MeOH hydrogen bond (HB) complex. The analysis of the optimized geometry structures and non-covalent interactions adequately demonstrates that the strengthened intermolecular HBs make remarkable effect on the ESDPT process. Meanwhile, the frontier molecular orbitals have been delineated to understand the driving force of the ESDPT process. The changes of characteristic peaks in the infrared spectra further prove the occurrence of the ESDPT process. Combining with the analysis of constructed potential energy curves, the ESDPT mechanism for QP-MeOH complex is clearly clarified. Our studies not only shed light on the nature of ESDPT process of QP-MeOH, but also be helpful to design novel promising photoelectric materials.

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method.

The single deuterium substitution in porphycene leads to an asymmetric molecular geometry, which may affect the double proton transfer process in the porphycene molecule. In this study, we applied an enhanced QM/MM method called SITS-QM/MM to investigate hydrogen/deuterium (H/D) isotope effects on the double proton transfer in porphycene. Distance changes in SITS-QM/MM molecular dynamics simulations suggested that the deuterium substituted porphycene adopted the stepwise double proton transfer mechanism. The structural analysis and the free energy shifts of double proton transfer process indicated that the asymmetric isotopic substitution subtly compressed the covalent hydrogen bonds and may alter the original transition state location.

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method

Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method

A novel formulation of an approximate valence bond model (AVB2) and its application to the tautomeric forms of porphyrin and porphycene

This study presents a novel formulation of the approximate valence bond method, which can be applied as a very fast generator of the molecular potential energy function. The AVB2 model was formulated and parameterized for porphyrin and porphycene using results of quantum mechanical computations at the B3LYP/6-31G (d, p) level. The DFT potential energy, its gradients, and the Hessian-matrix elements, as well as effective atomic charges at local energy minima and transition states, were used for the parameterization of the AVB2 Hamiltonian matrix. The AVB2 method, and in particular its anharmonic version, very well reproduce the potential energy maps for all representative geometries of the studied systems, including harmonic frequencies, and possible proton translocations. For validation of the method, we performed molecular dynamics simulations for isolated molecules accounting for internal double proton transfer processes, which are strongly correlated with changes of the electronic charge density. The simulated power spectra were compared with the experimental infrared spectra. More precise simulations of IR spectra at the classical and quantum dynamics levels, as well as extensions of the AVB2 parameterization to electronic excited states, are the subject of further research.

New insights into 3M3M1B: the role of water in ˙OH-initiated degradation and aerosol formation in the presence of NOX (X = 1, 2) and an alkali.

The oxidation mechanisms and dynamics of 3-methoxy-3-methyl-1-butanol (3M3M1B) initiated by ˙OH radicals were assessed by the density functional theory and canonical variational transition state theory. The effects of ubiquitous water on the title reactions were analyzed by utilizing an implicit solvation model in the present system. The results suggested that aqueous water played a negative role in the ˙OH-initiated degradation of 3M3M1B with an increase in the Gibbs free barriers. Meanwhile, the barriers were almost independent when explicit water molecules were involved in the gaseous phase, which could reduce the rate constant by approximately 3 orders of magnitude. The kinetic calculations showed that the rate constants were smaller by about 15, 9, 8, and 8 orders of magnitude for hydroxyl-, ammonia-, formic acid-, and sulfur acid-participating reactions, respectively, than that from an unassisted reaction. The results indicated that water, hydroxyl, ammonia, formic acid, or sulfur acid could not facilitate the title reaction when performed in the atmosphere. The investigations of the subsequent oxidation processes of the alkyl radical CH3OC(CH3)2CH2C·HOH indicated that CH3OC(CH3)2CH2CHO was the most favorable product by eliminating an HO2˙ radical. Additionally, the HO2˙ radical could serve as a self-catalyst to affect the above reaction through a double proton transfer process. With the introduction of NO, CH3OC(CH3)2CH2COOH and HNO2 were found to be the main products, which may be regarded as the new source of atmospheric nitrous acid. In the NO2-rich environment, the peroxynitrate of CH3OC(CH3)2CH2CH(OONO2)OH could be formed via the reaction of the CH3OC(CH3)2CH2CH(OO˙)OH radical with NO2. The degradation mechanism of CH3OC(CH3)2CH2CH(OONO2)OH in the presence of water, ammonia, and methylamine was demonstrated, and it was shown that water, ammonia, and methylamine could promote the formation of nitric hydrate and nitrate aerosol. The main species detected in the experiment were confirmed by a theoretical study. The atmospheric lifetimes of 3M3M1B in the temperature range of 217-298 K and altitude of 0-12 km were within the range of 6.83-8.64 h. This study provides insights into the transformation of 3M3M1B in a complex environment.