The investigation of excited state proton transfer mechanism in water-bridged 7-azaindole

The effect of the number of conjugated C=C bonds on the ESIPT and ICT reactions of SNCN derivatives

The electronic structure of the molecule is significantly influenced by the number of conjugated C=C bonds. In this work, the influence of the conjugated C=C bonds of the SNCN derivatives on the excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) properties are studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculation level is proved to be reasonable by calculating electronic spectra. The hydrogen bond parameters, infrared vibrational frequency (IR), reduction density gradient (RDG) isosurface, topological analysis and potential energy curves of SNCN derivatives in ground state (S0) and the first excited state (S1) are analyzed. According to theoretical research results, ESIPT reaction has a higher likelihood of occurring in the S1 state. Moreover, the ESIPT reaction becomes more challenging to occur with the number of conjugated C=C bonds rising. Finally, the analyses of the frontier molecular orbitals (FMOs), dipole moment and charge transfer transition confirm that the ICT effect is aided by the increased number of conjugated C=C bonds. This work indicates that the number of conjugated C=C bonds can regulate the ESIPT and ICT processes, which provides guidance for the study of fluorescent groups with similar characteristics.

Systematic theoretical investigation of two novel molecules BtyC-1 and BtyC-2 based on ESIPT mechanism

Inexperiment, Song et al. have successfully synthesizedtwo novel molecules BtyC-1 and BtyC-2 and observedasingle and dual fluorescence peaks in these two molecules respectively. (Song et al. Tetrahedron Lett. 2019, 60, 1696-1701) However, they still lack a detailed and reasonable theoretical explanation. Then we wonder why these two similar structures behave so much differently? In this work, we focus on explaining the photochemical and photophysical properties of BtyC-1 and BtyC-2 by studying the excited state intramolecular proton transfer (ESIPT) mechanisms. Based on the optimized geometric configurations, the calculated infrared spectra indicate the intramolecular hydrogen bonding interactions are heightened in their excited states. The frontier molecular orbitals reflect the charge redistribution in photoinduced process, which explains that the driving force of ESIPT process is provided by enhanced hydrogen bonding interactions. In the meantime, the calculations of potential energy curves vividly explain the principle of the experimental dual fluorescence phenomenon. The analysis of Mulliken charges deepens the discussion of molecular structures on the potential energy barriers. Calculated absorption spectra via using density functional theory and emission spectra via using time-dependent density functional theory are consistent with the experimental data, which confirms the correctness of our calculation methods. The reduced density gradient isosurfaces help us distinguish the complex non-covalent bonds. Base on the above analyses, we conclude that there is no stable structure for BtyC-1 in excited state, which make it occur the ESIPT reaction spontaneously. BtyC-2 exists a stable normal structure in excited state. Its dual fluorescence signals are emitted by its normal and isomer structures, respectively.

Solvent effect on the excited-state intramolecular double proton transfer of 1,3-bis(2-pyridylimino)-4,7-dihydroxyisoindole

Density functional theory (DFT) and time-dependent density functional theory (TDDFT) are used to study the solvatochromic effect and the excited-state intramolecular double proton transfer (ESIDPT) of 1,3-Bis(2-pyridylimino)-4,7-dihydroxyisoindole (BPI-OH) in different kinds of solvents. The hydrogen bonding parameters and IR spectra reveal that in the excited state, the strength of excited hydrogen bond increase with the decrease of solvent polarity. Furthermore, the reduction density gradient (RDG) analysis confirms the corresponding conclusion. Frontier molecular orbitals (FMOs) are analyzed, illuminating that the smaller the polarity of solvent, the smaller the energy gap between the HOMO and LUMO. The structures of BPI-OH (N) (normal), BPI-OH (T₁) (single), and BPI-OH (T₂) (double) were optimized. Previous reports found the double protons in BPI-OH molecule are transferred step-by-step process BPI-OH(N)→BPI-OH(T₁)→BPI-OH(T₂) in the ground state (S₀) and the first excited singlet state (S₁). Here, the potential energy curves of O₁-H₂ and O₄-H₅ in the S₀ and S₁ states were scanned in four kinds of solvents, respectively. It was found that in S₁ state, BPI-OH(N)→BPI-OH(T₁) was more prone to proton transfer than BPI-OH(T₁)→BPI-OH(T₂). In addition, by comparing the reaction energy barriers of the four kinds of solvents, it can be found that ESIPT is difficult to occur with the increase of solvent polarity. Meanwhile, it was also studied that MeOH as an explicit solvent was more likely to promote the ESIPT process than other implicit solvents.

Theoretical Insights into Excited-State Intermolecular Proton Transfers of 2,7-Diazaindole in Water Using a Microsolvation Approach

The detailed excited-state intermolecular proton transfer (ESInterPT) mechanism of 2,7-diazaindole with water wires consisting of either one or two shells [2,7-DAI(H₂O)_n; n = 1-5] has been theoretically explored by time-dependent density functional theory using microsolvation with an implicit solvent model. On the basis of the excited-state potential energy surfaces along the proton transfer (PT) coordinates, among all 2,7-DAI(H₂O)_n, the multiple ESInterPT of 2,7-DAI(H₂O)₂₊₃ through the first hydration shell (inner circuit) is the most easy process to occur with the lowest PT barrier and a highly exothermic reaction. The lowest PT barrier resulted from the outer three waters pushing the inner circuit waters to be much closer to 2,7-DAI, leading to the enhanced intermolecular hydrogen-bonding strength of the inner two waters. Moreover, on-the-fly dynamic simulations show that the multiple ESInterPT mechanism of 2,7-DAI(H₂O)₂₊₃ is the triple PT in a stepwise mechanism with the highest PT probability. This solvation effect using microsolvation and dynamic simulation is a cost-effect approach to reveal the solvent-assisted multiple proton relay of chromophores based on excited-state proton transfer.

Fluorescence deactivation mechanism for a new probe detecting phosgene based on ESIPT and TICT

The ESIPT-fluorescence deactivation is caused by ISC and phosphorescence.

The solvent effect on the excited-state intramolecular proton transfer of cyanine derivative molecules

Essential comprehension of the ESIPT mechanism in different solvents is helpful to design excellent fluorescent probes for lysosome organelles.

The effect of different environments on excited-state intramolecular proton transfer in 4′-methoxy-3-hydroxyflavone

Excited state intramolecular proton transfer reaction occurs with increasing difficulty in the solvents tested in the order toluene → ACN → DMF.

Revelation solvent effects: excited state hydrogen bond and proton transfer of 2-(benzo[d]thiazol-2-yl)-3-methoxynaphthalen-1-ol

ESIPT reaction of an MMT molecule is gradually inhibited with increasing solvent polarity.

The order of multiple excited state proton transfer in ternary complex of norharmane and acetic acids

Dolores Reyman et al. found the norharmane (9H-pyrido [3,4-b] indole) (NHM) and two acetic acid molecules can form the ternary complex (NHM-2A) in component solvent of dichloromethane and acetic acid via the hydrogen bond chain (J. Lumin. 2014, 148, 64). But the specific reaction details during this process were rarely reported. In this study, we will give an insight into the reasons which promote the occurrence of this reaction as well as its reaction order. The hydrogen bond enhancing behavior in first excited state (S₁) is verified through the analysis of geometric configurations, infrared spectra, frontier molecular orbitals and potential energy curves. The absorption and fluorescence spectra we calculated are well coincident with the experimental results. Meanwhile, it is obvious that the hydrogen bond intensity is gradually enhanced from N₁H₂⋯O₃, O₄H₅⋯O₆ to O₇H₈⋯N₉ by analyzing the reduced density gradient (RDG) isosurface. The hydrogen bond strengthening mechanism has been confirmed in which the hydrogen bond interaction acts as driving force for excited state proton transfer (ESPT) reaction. In order to provide a reliable description of the reaction energy profiles, we compare the barrier differences obtained by m062x and B3LYP methods. We might safely draw the conclusion that the multiple ESPT is a gradual process initiated by the proton transfer of O₇H₈⋯N₉. And we further proof the ESPT process can be completed via the NHM-2A → NHM-2AS → NHM-2AD → NHM-2AT in S₁ state. Theoretical research of NHM-2A has been carried out by density functional theory (DFT) and time-dependent density functional theory (TDDFT). It is worth noting that we predicted that the fluorescence at 400 nm observed in experiment is more likely to be emitted by NHM-2AS in S₁ state.

Effects of different substituents of methyl 5-R-salicylates on the excited state intramolecular proton transfer process

The proton transfer reaction in methyl 5-R-salicylate is found to be highly sensitive to the presence of specific substituents in resonance with the hydroxyl group, leading to different fluorescence behaviors of methyl 5-R-salicylate with different substituents.

The role played by ethanol in achieving the successive versus simultaneous mechanism of excited-state double proton transfer in dipyrido[2,3-a:3′,2′-i]carbazole

The excited-state double proton transfer (ESDPT) process of dipyrido[2,3-a:3′,2′-i]carbazole (DPC) in ethanol (EtOH) solvent is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT).

Theoretical study of excited-state proton transfer of 2,7-diazaindole·(H2O)2 cluster via hydrogen bonding dynamics

A new chromophore, 2,7-diazaindole (2,7-DAI), has been designed to surpass the limitation of 7-azaindole (7AI). It exhibits remarkable water catalyzed proton-transfer properties. Excited-state proton transfer (ESPT) has been investigated based on the time-dependent density functional theory method. The calculated vertical excitation energies in the S₀ and S₁ states agree well with the experimental values. Proton transfer couples with hydrogen-bonding dynamics between the 2,7-diazaindole and the surrounding water molecules. Hydrogen bond strengthening has been testified in the S₁ state based on a comparison of primary bond lengths and hydrogen bond energy that is involved in the intermolecular hydrogen bond between the S₀ and S₁ states. Frontier molecular further suggest that the electron density changes between the ground and excited states serve as basic driving forces for proton transfer. We determined the potential-energy curves of the S₀ and S₁ states to characterize the ESPT process. This work explains that the ESPT process for 2,7-DAI·(H₂O)₂ clusters at the molecular level, and highlights the importance of hydrogen bonding in ESPT.

The theoretical study of excited-state intramolecular proton transfer of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol

The symmetrical structures 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (BBTD) can take shape two intramolecular hydrogen bonds in chloroform. In order to research the molecular dynamic behavior of BBTD upon photo-induced process, we utilize density functional theory (DFT) and time-dependent density functional theory (TDDFT) to complete theoretical calculation. Through the comparison of bond length, bond angle, IR spectra, and frontier molecular orbitals between ground state (S₀) and first excited state (S₁), it clearly indicates that photoexcitation have slightly influence for intensity of hydrogen bond. For the sake of understanding the mechanism of excited state intramolecular proton transfer (ESIPT) of BBTD in chloroform, potential energy surfaces have been scanned along with the orientation of O₁-H₂ and O₄-H₅ in S₀ and S₁ state, respectively. A intrigued hydrogen bond dynamic phenomenon has been found that ESIPT of BBTD is not a synergetic double proton transfer process, but a stepwise single proton transfer process BBTD→BBTD-S→BBTD-D. Moreover, the proton transfer process of BBTD-S→BBTD-D is easier to occur than that of BBTD→BBTD-S in S₁ state.

The new competitive mechanism of hydrogen bonding interactions and transition process for the hydroxyphenyl imidazo [1, 2-a] pyridine in mixed liquid solution

The new competitive mechanism of intermolecular and intramolecular hydrogen bond can be proposed with an improved mixed model. Upon the photoinduced process, the twisting intramolecular charge transfer (TICT) structure of the hydroxyphenyl imidazo [1, 2-a] pyridine (HPIP) can be obtained. TICT character prompts the fluorescent inactivation via non-radiative decay process. For exploring the photochemical and photophysical properties, the electronic spectra and the infrared (IR) vibrational spectra of titled compounds have been detailedly investigated. In addition, the frontier molecular orbitals (MOs) analysis visually reveals that the unbalanced electron population can give rise to the torsion of molecular structure. To further give an attractive insight into the non-radiative decay process, the potential energy curves have been depicted on the ground state (S₀), the first excited state (S₁) and the triple excited state (T₁). Minimum energy crossing point (MECP) has been found in the S₁ and T₁ state. On the MECP, the intersystem crossing (ISC) might be dominant channel. The density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods have been throughout employed in the S₀ state, T₁ state and S₁ state, respectively. The theoretical results are consistent with experiment in mixed and PCM model.

Solvent-dependent dynamics of hydrogen bonding structure 5-(methylthio)-1, 3, 4-thiadiazole-2(3H)-thione as determined by Raman spectroscopy and theoretical calculation

The vibration spectra of 5-(methylthio)-1,3,4-thiadiazole-2(3H)-thione (MTTN) in acetonitrile (CH₃CN), methanol (CH₃OH) and water (H₂O) solvents were collected and evaluated via deuterium isotopic substitution Raman spectroscopic experiments. These experiments were combined with the quantum chemical theoretical calculations using the PCM solvent model and normal mode analysis. The results confirmed that the MTTN in CH₃CN, CH₃OH and H₂O have hydrogen bonding (H-bonding) MTTN(solvent)_n clusters that produce significantly different Raman intensity patterns in different solvents. Combined with the normal Raman assignment, most resonance Raman spectra were assigned to the vibration modes of the H-bonding MTTN(CH₃CN), MTTN(CH₃OH)₃ and MTTN(H₂O)₃ clusters in CH₃CN, CH₃OH and H₂O. The theoretically-predicted frequencies and intensities in different surrounding environments enabled reliable assignments of Raman bands. The intermolecular>NH⋯O and >NH⋯N H-bonding interactions are key constituents of stable thione structures in MTTN. This underlines the significant structural differences of MTTN in CH₃CN, CH₃OH and H₂O. H-bonding perturbation of MTTN reveal important insights about the intermolecular excited state proton transfer (ESPT) reaction mechanisms in the Franck-Condon region structural dynamics of the thione→thiol tautomer in CH₃OH and H₂O.

The discovery of the hydrogen bond from p-Nitrothiophenol by Raman spectroscopy: Guideline for the thioalcohol molecule recognition tool

Inter- and intra- molecular hydrogen bonding plays important role in determining molecular structure, physical and chemical properties, which may be easily ignored for molecules with a non-typical hydrogen bonding structure. We demonstrated in this paper that the hydrogen bonding is responsible for the different Raman spectra in solid and solution states of p-Nitrothiophenol (PNTP). The consistence of the theoretical calculation and experiment reveals that the intermolecular hydrogen bonding yields an octatomic ring structure (8) of PNTP in the solid state, confirmed by the characteristic S-H---O stretching vibration mode at 2550 cm^-1; when it comes to the solution state, the breakage of hydrogen bond of S-H---O induced the S-H stretching vibration at 2590 cm^-1. Our findings may provide a simple and fast method for identifying the intermolecular hydrogen bonding.

Theoretical Study of the ESIPT Process for a New Natural Product Quercetin

The investigation of excited-state intramolecular proton transfer (ESIPT) has been carried out via the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) method for natural product quercetin in dichloromethane (DCM) solvent. For distinguishing different types of intramolecular interaction, the reduced density gradient (RDG) function also has been used. In this study, we have clearly clarified the viewpoint that two kinds of tautomeric forms (K₁, K₂)originated from ESIPT processconsist inthe first electronic excited state (S₁). The phenomenon of hydrogen bonding interaction strengtheninghas been proved by comparing the changes of infrared (IR) vibrational spectra and bond parameters of the hydrogen bonding groups in the ground state with that in the first excited state. The frontier molecular orbitals (MOs)provided visual electron density redistribution have further verified the hydrogen bond strengthening mechanism. It should be noted that the ESIPT process of the K₂ form is easier to occur than that of the K₁ form via observing the potential energy profiles. Furthermore, the RDG isosurfaces has indicated that hydrogen bonding interaction of the K₂ form is stronger than that of the K₁ formin the S₁ state, which is also the reason why the ESIPT process of the K₂ form is easier to occur.

How was the proton transfer process in bis-3, 6-(2- benzoxazolyl)-pyrocatechol, single or double proton transfer?

A theoretical analysis of proton transfer process for the symmetric systems with two intramolecular hydrogen bonds, bis-3,6-(2-benzoxazolyl)-pyrocatechol(BBPC) in hexane solvent, has been researched. In this study, we utilized ωB97X-D/ 6-311 + g (d,p) and B3LYP/6-31 + G(d) two procedures calculating the foremost bond length and bond angle, respectively. Our calculations demonstrate the two intramolecular hydrogen bonds were strengthened in S1 state, thus the proton transfer reaction can be facilitated. Furthermore, the calculated IR vibrational spectra confirmed hydrogen bonds were enhanced in S1 state. We found three local minima A B and C from the potential energy surfaces (PESs) on the S1 state, and the energy of B point and C point are identical. A new ESIPT mechanism has been proposed that was not equal to the previous conclusions. The new ESIPT mechanism elucidates that single proton transfer more likely occurs in the symmetric BBPC molecule in comparison with the double proton transfer reaction. And the frontier molecular orbitals(MOs) further illustrate the trend of ESIPT reaction.