Establishing new mechanisms with triplet and singlet excited-state hydrogen bonding roles in photoinduced liquid dynamics

Theoretical uncovering of the chalcogen element regulated ESDPT behaviors for 2,5-bis(2-benzoxazolyl)-hydroquinone derivatives

Inspired by the captivating allure of exquisitely regulated characteristics exhibited by 2-(2-hydroxyphenyl)-benzoxazole and its derivatives in the realms of photochemistry and photophysics, our current endeavor primarily revolves around delving into the intricacies of photo-induced excited state reactions for derivatives of 2,5-bis(2-benzoxazolyl)-hydroquinone (BBHQ). Given the significant impact of chalcogen element doping, herein we predominantly focus on exploring the excited state behaviors of BBHQ-OO, BBHQ-SS, and BBHQ-SeSe fluorophores. Our simulations, resulting from variations in geometry and vertical excitation charge reorganization, reveal atomic-electronegativity-dependent hydrogen bonding interactions and charge recombination induced by photoexcitation that can significantly enhance the excited state intramolecular double proton transfer (ESDPT) reaction for BBHQ-OO, BBHQ-SS, and BBHQ-SeSe fluorophores. By constructing potential energy surfaces (PESs) and identifying transition states (TS), we unveil the ultrafast stepwise ESDPT mechanism due to the low potential barriers. Additionally, by employing heterosubstituted BBHQ-OS and BBHQ-OSe compounds, we rigorously validate the stepwise ESDPT mechanism regulated by chalcogen atomic electronegativity. We sincerely anticipate that the modulation of solvent polarity on excited state behaviors will pave the way for groundbreaking advancements in luminescent materials.

Theoretical Insights into the Effect of Different Numbers of Thiophene Groups on Hydrogen Bond Interaction and Excited-State Intramolecular Proton-Transfer Process for Flavonoid Derivatives

In this study, we systematically explored the impact of varying the number of thiophene groups on the hydrogen bond interaction and excited-state intramolecular proton-transfer (ESIPT) processes in flavonoid derivatives (STF, DTF, and TTF) using the density functional theory and time-dependent density functional theory methods. Initially, a thorough analysis of the optimized geometric structures revealed that the intramolecular hydrogen bond in the S1 state is enhanced and gradually weakened as the number of thiophene groups increases. To gain a deeper understanding of the hydrogen bond interaction, topological analysis, interaction region indicator scatter plots, and isosurface plots were employed. These images provide further insights that align with the structural analysis. Additionally, we observed a red-shift in the electronic spectra (absorption and fluorescence spectra), which is primarily attributed to the narrowing of the energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, as elucidated by the frontier molecular orbitals. Furthermore, a combined analysis between the hole-electron distribution and the transition density matrix heat map shows that electron excitation involves the unidirectional charge-transfer mechanism. In the end, by conducting relaxed potential energy curve scans, we found that an increase in the number of thiophene groups elevates the energy barrier for ESIPT, making it more challenging for the reaction. In summary, our study underscores the vital effect of thiophene-substituted numbers in modulating the ESIPT process, which is able to provide valuable insights for the design and synthesis of desired organic fluorescent probes in the future.

Study on ESIPT of salicylaldehyde derivative EQCN in DCM solvent

The fluorescence of salicylaldehyde derivative (EQCN) as an excitation-wavelength-dependent molecule with long-persistent luminescence has been investigated experimentally and theoretically. However, the excited-state intramolecular proton transfer (ESIPT) process mechanism and optical property associated with photochemical process of EQCN molecule in dichloromethane (DCM) solvent has not been discussed in detail. In this work, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were used to investigate ESIPT process of EQCN molecule in DCM solvent. By optimizing the geometry of the EQCN molecule, the hydrogen bond interaction of EQCN molecule of Enol structure in excited state (S₁ state) is strengthened. The calculated absorption peak and fluorescence peak agree well with the experimental values. Based on the optimized geometric structure, the frontier molecular orbital isosurface (FMOs) were drawn, and the redistribution of electron density in DCM solvent was depicted, which intuitively explain the changes in the photophysical properties of EQCN. Through the calculated potential energy curves (PECs) of EQCN in both DCM solvent and ethanol solvent, the ESIPT process of EQCN was found more likely to occur in ethanol solvents.

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.

Substituent Effects on Excited-State Intramolecular Proton Transfer Reaction of 2-Aryloxazoline Derivatives

Different substituents and benzene ring numbers had significant effects on the fluorescence phenomenon of 2-aryloxazoline derivatives as observed in an experiment. Here, we select five 2-aryloxazoline derivatives with different substituents and benzene ring numbers (2u, 2ad, 2af, 2ai, and 2ah) to analyze the effects on the fluorescence phenomena. For 2ad, 2ah, and 2ai, first, the geometric structures are optimized based on the density functional theory and time-dependent density functional theory methods. The analysis of the obtained bond parameters reveals the variation of hydrogen bond interactions from S₀ to S₁ states. Second, the calculated absorption and emission spectra are consistent with the experimental values, which proves that the theoretical method is feasible. Finally, through the analysis of the infrared vibrational spectrum, reduced density gradient isosurfaces, frontier molecular orbitals, and potential energy curves, the strengthening mechanism of the hydrogen bond interaction and the ability of the excited-state intramolecular proton transfer (ESIPT) reaction to occur are further explained. Since the proton transfer reactions of 2u and 2af occur spontaneously under photoexcitation, they have no stable structures in the S₁ state. In conclusion, due to the different substituents, 2u is more prone to the proton transfer reaction than 2ad. For 2af, 2ai, and 2ah with different benzene ring numbers, the ESIPT reaction is more difficult to occur as the number of benzene rings increases. The ability of the ESIPT reaction to occur follows the order 2af → 2ah → 2ai. For 2-aryloxazoline derivatives with different substituents or different benzene ring numbers, the hydrogen bond strengthening mechanism has been authenticated, which promotes the occurrence of the ESIPT reactions.

Substituent effect on ESIPT and hydrogen bond mechanism of N-(8-Quinolyl) salicylaldimine: A detailed theoretical exploration

The effects of substituent on excited-state intramolecular proton transfer (ESIPT) and hydrogen bonding of N-(8-Quinolyl) salicylaldimine (QS) have been studied by theoretical calculation with DFT and TDDFT. The representative electron-withdrawing nitryl and electron-donating methoxyl were selected to analyze the effects on geometries, intramolecular hydrogen bond interaction, absorption/fluorescence spectra, and the ESIPT process. The configurations of the three molecules (QS, QS-OMe and QS-NO₂) were optimized in the ground and excited states. The structure parameters, infrared spectra, hydrogen bond interactions, frontier molecular orbitals, absorption/fluorescence spectra, and potential curves have cross-validated the current results. The results show that the introduction of substituent results in a bathochromic-shift of the absorption and fluorescence spectra with large Stokes shift, and is more beneficial to the ESIPT process. The current work will be beneficial to the improvement of ESIPT properties and deepen understanding of the mechanism of ESIPT process.

Equilibrium Geometries, Adiabatic Excitation Energies and Intrinsic C=C/C–H Bond Strengths of Ethylene in Lowest Singlet Excited States Described by TDDFT

Seventeen singlet excited states of ethylene have been calculated via time-dependent density functional theory (TDDFT) with the CAM-B3LYP functional and the geometries of 11 excited states were optimized successfully. The local vibrational mode theory was employed to examine the intrinsic C=C/C–H bond strengths and their change upon excitation. The natural transition orbital (NTO) analysis was used to further analyze the C=C/C–H bond strength change in excited states versus the ground state. For the first time, three excited states including πy′ → 3s, πy′ → 3py and πy′ → 3pz were identified with stronger C=C ethylene double bonds than in the ground state.

Tautomers of homophthalic anhydride in the ground and excited electronic states: analysis through energy, hardness and vibrational signatures

The keto-enol tautomerisation in homophthalic anhydride (HA) is investigated in the ground (S₀) and excited (S₁) electronic states. The keto form with a dicarbonyl structure is found to be the most stable form in S₀ and enol form with a monocarbonyl structure in S₁ indicating an excited state intramolecular proton transfer (ESIPT) process. The computed results show consistency with the change in basis sets and methods of calculations. Apart from the two tautomers, transition states are also identified. The barrier to interconversion is found to reduce substantially in S₁. Internal reaction coordinate (IRC) calculations confirm the pathway of interconversion between the two forms in S₀ and S₁. The observed FT-IR spectra corroborate well with our computed spectra. The appearance of two strong lines around 1800 cm^-1 confirms the lowest energy structure to be the keto tautomer with a dicarbonyl form in S₀. Our computations corroborate well with the crystal structure data for an analogous molecule. Electron distribution in HOMO and LUMO indicate the excitation process as π → π^* in nature. The qualitative chemical concepts like hardness and electrophilicity are calculated to estimate the stability of the tautomers. The energy and hardness profiles with the variation of IRC are opposite to each other, verifying the principle of maximum hardness.

TD-DFT insights into the sensing potential of the luminescent covalent organic framework for indoor pollutant formaldehyde

This paper investigates the sensitivity of the luminescent thieno[2,3-b]thiophene-based covalent organic framework (TT-COF) towards the formaldehyde using the density functional theory and time-dependent method. The hydrogen bonding dynamics is explored by comparison of geometries, electronic transition energies, binding energies, UV-vis, and infrared spectra. Frontier molecular orbitals examination, natural population analysis, and plotted electron density difference map describe the quenching process explicitly via electron density distribution. The MOMAP program illuminates the quenching owing to TT-COF-HCHO complex radiative rate constant. Furthermore, the S1-T1 energy gap describes the facilitation of the luminescence quenching through the intersystem crossing. Above all results elaborate the TT-COF's potential to detect the formaldehyde.

The mechanism of the excited-state proton transfer of Salicylaldehyde azine and 2,2'-[1,4-Phenylenebis{(E)- nitrilomethylidyne}] bisphenol: Via single or double proton transfer

The Salicylaldehyde azine (H₂SA) and 2,2'-[1,4-Phenylenebis{(E)-nitrilomethylidyne}] bisphenol (H₂SPA) with double proton transfer characteristics were synthesized recently (Phys. Chem. Chem. Phys., 2018, 20, 23,762). However, the detailed theoretical interpretation of proton transfer (PT) mechanism is inadequate. In the present work, density functional theory (DFT) and time-density functional theory (TDDFT) are employed to study the proton transfer mechanism of H₂SA and H₂SPA in detail. Bond parameters, infrared (IR) spectra and frontier molecular orbitals (FMOs) calculated by PBE0/TZVP method indicate the strength of hydrogen bond is enhanced in S₁ state, which can be visualized by the reduced density gradient (RDG) analysis. The potential energy surfaces (PESs) of H₂SA and H₂SPA are also constructed. The small barriers indicate that both the single proton transfer and double proton transfer of H₂SA and H₂SPA are more likely to occur in the S₁ state. In addition, the properties of H₂SA and H₂SPA after chelation with Li⁺ have also been theoretically characterized. According to the calculated fluorescence spectra of compounds (H₂SA-Li⁺ and H₂SPA-Li⁺), it was found that only the planar structure of H₂SA-Li⁺ can form metallogel, which verified the experimental results.

Effect of solvent environment on excited state intramolecular proton transfer in 2-(4-(dimethylamino)phenyl)-3-hydroxy-6,7-dimethoxy-4h-chromen-4-one

The new ratiometric fluorescent probe 2-(4-(dimethylamino)phenyl)-3-hydroxy-6,7-dimethoxy-4h-chromen-4-one (HOF) monitoring of methanol in biodiesel was discovered experimentally (T. Y. Qin et al., Sens. Actuators, B, 2018, 277, 484-491). But the experimental study did not report the reaction mechanism in detail. In this study, density functional theory (DFT) and time-density functional theory (TDDFT) methods were used to theoretically study the excited-state intramolecular proton transfer (ESIPT) process of the HOF molecule. The molecular structure in the ground state and the excited state was optimized, and the infrared vibrational spectra, the frontier molecular orbitals, the charge transfer, the potential energy curves and the transition-state structures were calculated. The calculated results prove that the solvent polarity has a great influence on the ESIPT reaction of the HOF molecule. As the solvent polarity increased, the intensity of the intramolecular hydrogen bond decreased, and ESIPT was more difficult to occur. This work has studied the mechanism of the ESIPT reaction in more detail, and paved the way for future research on HOF molecules.

New perspective on the fluorescence and sensing mechanism of TNP chemosensor 2-(4,5-bis(4-chlorophenyl)-1H-imidazol-2-yl)-4-chlorolphenol

For TNP chemosensor 2-(4,5-Bis(4-Chlorophenyl)-1H-Imidazol-2-yl)-4-Chlorolphenol (HPICI), previous thought with no theoretical basis was that excited-state intramolecular proton transfer (ESIPT) process and the ground-state HPICI-TNP complex are mainly responsible for its fluorescence emission and the detection of TNP. However, this interpretation has been proved to be wrong by the present theoretical DFT/TDDFT explorations. Actually, the strong fluorescence of HPICI is mainly induced by the local excitation of the enol form HPICI(E) without ESIPT, and the fluorescence quenching by TNP is due to the photo-induced electron transfer (PET) process together with the cooperative effect of hydrogen-bonding interaction and π-π stacking interaction coexisting in the HPICI-TNP complex. The strengthened excited-state hydrogen bond promotes the PET process, thus facilitates the fluorescence quenching. This mechanism is proposed on the basis of the theoretical analyses on molecule geometry, binding energy, Gibbs free energy, electronic transitions, and frontier molecular orbitals (FMOs).

Excited state hydrogen bond and proton transfer mechanism for (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine: A theoretical investigation

A novel fluorescence molecule (2‑hydroxy‑4‑methoxyphenyl)(phenyl)‑methanone azine (HMPM) has been explored theoretically in this present work. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, we investigate the excited state hydrogen bonding behaviors and excite state intramolecular proton transfer (ESIPT) process for HMPM molecule. Via simulating the reduced density gradient (RDG) versus sign(λ₂)ρ, we firstly verify the double intramolecular hydrogen bonds (O1H2⋯N3 and O4H5⋯N6) for HMPM system. Comparing with the changes about these two hydrogen bonds (i.e., bond distances, bond angles and infrared (IR) vibrational spectra), we find that they should be enhanced in the first excited state upon the photo-excitation. The shortened hydrogen bonding distance of H2⋯N3 and H5⋯N6 provide the possibility for ESIPT reaction. Given the photo-excitation process, we confirm the charge redistribution around the hydrogen bonding moieties plays an important role as a driving force for the ESIPT process. Further, via constructing S₀-state and S₁-state potential energy surfaces (PESs), we confirm the excited state double proton transfer (ESDPT) is excludable since the high optimized energy and high potential energy barrier. While the low potential barrier for excited state single proton transfer path results in the ultrafast ESIPT reaction, which explains why the initial HMPM fluorescence peak cannot be detected in previous experimental phenomenon. This work not only clarifies the excited state dynamical behavior for HMPM system, but also explains previous experimental phenomenon and attributions about steady state spectra. We hope this work can facilitate novel applications based on the novel HMPM system in future.

A detecting Al3+ ion luminophor 2-(Anthracen-1-yliminomethyl)-phenol: Theoretical investigation on the fluorescence properties and ESIPT mechanism

2-(Anthracen-1-yliminomethyl)-phenol (AYP) had been synthesized recently and used as a chemosensor to detect Al³⁺ ion, while its fluorescent properties and excited-state intramolecular proton transfer (ESIPT) process were not investigated in detail. In this study, the molecular absorption and emission spectra were accurately reproduced by using TDDFT/CAM-B3LYP/6-31 + G(d,p) computational method. The ESIPT- chromophore photochemical behaviors and detecting Al³⁺ ion photophysical changes were explained for the first time at the molecular level. As driving force of ESIPT reaction, the bond parameters and vibrational frequencies of intramolecular hydrogen bond were analyzed by optimizing structures and calculating infrared spectra, analysis of frontier molecular orbitals and reduced density gradient isosurfaces. To further elucidate the proton transfer reactive paths, we scanned the potential energy curves of AYP chemosensor in different electronic states. By comparing potential barriers of the S₀ and S₁ states, the proton transfer is confirmed to occur in the S₁ state. In addition, the experimentally unpresented AYP-Enol fluorescence signal was assigned via analyzing molecular fluorescent properties. Moreover, the calculated fluorescence spectra were employed to explain carefully the mechanism of detection of Al³⁺ ion.

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

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

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.

Fluoride anion sensing mechanism of a BODIPY-linked hydrogen-bonding probe

The sensing mechanism of a fluoride-anion probe BODIPY-amidothiourea (1c) has been elucidated through the density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations. The theoretical study indicates that in the DMSO/water mixtures the fluorescent sensing has been regulated by the fluoride complex that formed between the probe 1c/two water molecules and the fluoride anion, and the excited-state intermolecular hydrogen bond (H-B) plays an important role in the fluoride sensing mechanism. In the first excited state, the H-Bs of the fluoride complex 1cFH₂ are overall strengthened, which induces the weak fluorescence emission. In addition, molecular orbital analysis demonstrates that 1cFH₂ has more obvious intramolecular charge transfer (ICT) character in the S₁ state than 1cH₂ formed between the probe 1c and two water molecules, which also gives reason to the weaker fluorescence intensity of 1cFH₂ . Further, our calculated UV-vis absorbance and fluorescence spectra are in accordance with the experimental measurements. © 2018 Wiley Periodicals, Inc.

Exploring and elaborating the novel excited state dynamical behavior of a bisflavonol system

In this work, we investigate the dual hydrogen bonded 1,4-bis-(3-hydroxy-4-oxo-4H-chromen-2-yl)-benzene (bisflavonol) system in detail.

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.