Competitive excited-state single or double proton transfer mechanisms for bis-2,5-(2-benzoxazolyl)-hydroquinone and its derivatives

Computational explorations about the solvent-polarity-associated excited state proton transfer behaviors for the novel F-BSD compound

CONTEXT

Inspired by the excellent potential application prospects from the precisely controlled attributes displayed by fluorine-substituted-bis(salicylidene)-1,5-diaminonaphthalene (F-BSD) and its derivatives in the domains of photochemistry and photophysics, our present undertaking predominantly focuses on exploring the complexities of photo-induced excited state reactions for F-BSD fluorophores dissolved in solvents with diverse levels of polarity. Our simulations reveal that the excited state intramolecular double proton transfer (ESIDPT) reaction for F-BSD chemosensor can be significantly regulated by solvent polarity-dependent hydrogen bonding interactions and charge recombination induced by photoexcitation, which result from variations in geometries and vertical excitation charge reorganizations. By constructing potential energy surfaces (PESs), we also demonstrate that the stepwise ESIDPT reaction of F-BSD occurs with alternative dual intramolecular hydrogen bonds (O1-H2···N3 or O4-H5···N6). Interestingly, we affirm polar solvents should be conducive to the first-step of ESIDPT process, while nonpolar solvents are in favor of the second step. We sincerely hope solvent polarity-dependent ESIDPT behavior will pave the way for future design of novel luminescent materials.

METHODS

The molecular geometries were optimized by DFT//TDDFT D3-B3LYP/TZVP theoretical level with IEFPCM solvent model in S0 and S1 states, respectively. This work also explores the energy level of target molecules with the computational vertical absorption spectra by TDDFT. All the simulations were carried out based on Gaussian 16 software. The core-valence bifurcation (CVB) indexes were obtained by using Multiwfn 3.8. Potential energy surfaces were constructed by the DFT//TDDFT D3-B3LYP/TZVP level based on restricted optimization, also the transition state (TS) forms were searched using the same level.

Screening the Optimal Probe by Expounding the ESIPT Mechanism and Photophysical Properties in Bis-HBX with Multimodal Substitutions

DFT and TD-DFT were used in this article to investigate the effects of different substitutions at multiple sites on the photophysical mechanism of bis-HBX in the gas phase. Four different substitution modes were selected, denoted as A1 (X=Me, Y=S), A2 (X=OMe, Y=S), B1 (X=Me, Y=NH), and C1 (X=Me, Y=O). The geometric parameters proved that the IHBs enhanced after photoexcitation, which was conducive to promote the ESIPT process. Combining the analysis of the PECs, it was revealed that the bis-HBX molecule underwent the ESIPT process, and the ease of the ESIPT process was in the order of A1 > A2> B1 > C1. In particular, the TICT process in A1 and B1 promoted the occurrence of the ESIPT process. In addition, the IC process was identified, particularly in C1. Meanwhile, the calculation of fluorescence lifetime and fluorescence rate further confirmed that A1 was the most effective fluorescent probe molecule. This theoretical research provides an innovative theoretical reference for regulating ESIPT reactions and optimizing fluorescent probe molecules.

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.

Theoretical study based on the excited state dynamical of an oxadiazole derivative: A novel fluorescence mechanism

Excited intramolecular proton transfer (ESIPT) has been widely studied as a model system for proton transfer. In recent years, materials and biological systems containing two proton transfers have received special attention from researchers. In this work, the excited state intramolecular double-proton-transfer (ESIDPT) mechanism of a fluorescent compound based on an oxadiazole derivative, 2,5-bis-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-benzene-1,4-diol (DOX), has been comprehensively investigated through theoretical calculations. The potential energy surface curve of the reaction shows that ESIDPT can occur in the first excited state. This work proposes a new and reasonable fluorescence mechanism based on previous experiments, which has theoretical significance for the future research of DOX compounds in biomedicine and optoelectronics.

Computational Insights into Excited State Intramolecular Double Proton Transfer Behavior Associated with Atomic Electronegativity for Bis(2'-benzothiazolyl)hydroquinone

Inspired by the distinguished regulated photochemical and photophysical properties of 2-(2'-hydroxyphenyl)benzazole derivatives, in this work, the novel bis(2'-benzothiazolyl)hydroquinone (BBTHQ) fluorophore is explored, looking at its photo-induced behaviors associated with different substituted atomic electronegativities, i.e., BBTHQ-SO, BBTHQ-SS and BBTHQ-Se compounds. From the structural changes, infrared (IR) vibrational variations and simulated core-valence bifurcation (CVB) indexes for the dual hydrogen bonds for the three BBTHQ derivatives, we see that low atomic electronegativity could be conducive to enhancing hydrogen bonding effects in the S1 state. Particularly, the O4-H5⋯N6 of BBTHQ-SO and the O1-H2⋯N3 of BBTHQ-SSe could be strengthened to be more intensive in the S1 state, respectively. Looking into the charge recombination induced by photoexcitation, we confirm a favorable ESDPT trend deriving from the charge reorganization of the dual hydrogen bonding regions. By constructing the potential energy surfaces (PESs) along with the ESDPT paths for the BBTHQ-SO, BBTHQ-SS and BBTHQ-Se compounds, we not only unveil stepwise ESDPT behaviors, but also present an atomic electronegativity-regulated ESDPT mechanism.

Effects of solvent polarity on the novel excited-state intramolecular thiol proton transfer and photophysical property compared with the oxygen proton transfer

Recently, the dual-fluorescent phenomena of excited state intramolecular thiol proton transfer (ESIPT) for 3-thiolflavone derivative (3NTF) were reported by Chou and coworkers for the first time [J. Am. Chem. Soc. 143 (2021) 12715-12724], which opened a new chapter in the field of ESIPT. Based on density functional theory (DFT) and time-dependent density functional theory (TDDFT), the proton transfer processes of 3NTF in toluene, dichloromethane and acetonitrile were studied. By optimizing the structure of the ground (S0) state and first excited (S1) state of 3NTF in different solvents, the hydrogen-bond parameters and proton-transfer potential energy curves were calculated. It was shown that although photo-excitation enhanced the intramolecular hydrogen bonding strength and thus promoted the occurrence of ESIPT, the solvent polarities inhibited the enhancement of the hydrogen bond of S1 state, which was not conducive to ESIPT. The electron spectra analyses were consistent with experimental data, which confirmed the rationality of molecular configurations. The time-evolved excited state dynamics simulation was performed based on the optimized structure of 3NTF, indicating that the ESIPT was an ultrafast photochemical reaction less than 180 fs. Moreover, we compared the potential energy surfaces of ESIPT, electronic structures based on natural transition orbitals (NTOs) method and electron-hole isosurfaces for the 3NTF and the traditional flavone molecule (3NHF), concluded that the unusually large Stokes shift fluorescence of 3NTF was mainly caused by the coupling of ESIPT and twisting intramolecular charge transfer (TICT), and the 3NTF isomer had the more nπ* character in the electron transition process. The nπ* ICT significantly increased with the decrease of solvent polarities, affecting the molecular photophysical properties, this made it more widely used in biomedical, photochemical, materials science and other fields.

Unveiling and regulating the solvent-polarity-associated excited state intramolecular double proton transfer behavior for 1-bis(benzothiazolyl)naphthalene-diol fluorophore

Inspired by the regulatory luminescence properties of HBT derivatives, in this work, we mainly conduct a detailed theoretical exploration on the photoinduced excitation behavior of a novel di-proton-transfer type HBT derivative 1-bis(benzothiazolyl)naphthalene-diol (1-BBTND). The intramolecular double hydrogen bonding interaction and the excited state intramolecular double proton transfer (ESDPT) behavior of 1-BBTND fluorophore are investigated in combination with different polar solvent environments. From the structural changes and charge recombination induced by photoexcitation, we can conclude that strong polar solvent environment promotes the excited state dynamical reaction for 1-BBTND compound. By constructing potential energy surfaces (PESs) in S₀ and S₁ states, we clarify that 1-BBTND fluorophore should undergo a stepwise ESDPT reaction after photoexcitation. Combined with the size of potential energy barriers along with reaction paths in different solvents, we finally propose a new solvent-polarity-dependent stepwise ESDPT for 1-BBTND fluorophore.

Liquid-solid phase regulating excited-state intramolecular proton transfer process of HBT-d-NO2: A QM/MM study

The excited-state intramolecular proton transfer process of 2-(1,3-benzothiazol-2-yl)-4-[2-(4-nitrophenyl)ethynyl]phenol (HBT-d-NO₂) in the different surrounding environment is investigated using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The optimized molecular structure provides convincing evidence that the intramolecular hydrogen bond is strengthened in the first excited (S₁) state. The frontier molecular orbitals observed the HBT-d-NO₂ exists obvious intramolecular charge translate phenomenon. The results of the potential energy curve show that HBT-d-NO₂ is difficult to undergo proton transfer in the ground (S₀) state due to the high energy barrier, while it becomes easier in the S₁ state in both liquid and solid phases. By comparison, the energy barrier of ESIPT in the solid phase is higher than that in the liquid phase. We can conclude that the solid phase effectively hinders the ESIPT process compared with that the liquid phase. In this work, we illustrate the influence of liquid and solid phases on the intramolecular proton transfer process, which could promote further developments in biomedical and fluorophore applications.

The Role of H-Bonds in the Excited-State Properties of Multichromophoric Systems: Static and Dynamic Aspects

Given their importance, hydrogen bonds (H-bonds) have been the subject of intense investigation since their discovery. Indeed, H-bonds play a fundamental role in determining the structure, the electronic properties, and the dynamics of complex systems, including biologically relevant materials such as DNA and proteins. While H-bonds have been largely investigated for systems in their electronic ground state, fewer studies have focused on how the presence of H-bonds could affect the static and dynamic properties of electronic excited states. This review presents an overview of the more relevant progress in studying the role of H-bond interactions in modulating excited-state features in multichromophoric biomimetic complex systems. The most promising spectroscopic techniques that can be used for investigating the H-bond effects in excited states and for characterizing the ultrafast processes associated with their dynamics are briefly summarized. Then, experimental insights into the modulation of the electronic properties resulting from the presence of H-bond interactions are provided, and the role of the H-bond in tuning the excited-state dynamics and the related photophysical processes is discussed.

The two-pronged approach of heteroatoms and substituents to achieve a synergistic regulation of the ESIPT process in amino 2-(2'-hydroxyphenyl)benzoxazole derivatives

Amino 2-(2'-hydroxyphenyl)benzazole derivatives are a class of molecules with excellent photophysical properties. Most of them can be applied as a fluorescent probe via the excited-state intramolecular proton transfer (ESIPT) process. In this work, we focus on the effects of heteroatoms (O, S) and substituents (acetylacetone, hydrogen) in the derivatives. Using DFT/TDDFT methods with the B3LYP-D3BJ functionals, the absorption and emission peaks are in good agreement with the experimental data. Results of optimized structures, infrared vibrational spectra, and reduced density gradient present the existence of the ESIPT process in the S₁ state in these molecules, it also indirectly shows that the heteroatom S is more than O, and the substituent acetylacetone is more than hydrogen has stronger hydrogen bonds. The proton transfer (PT) potential energy curves (PECs) qualitatively show that it is easier for the heteroatom S to induce ESIPT than that of O. The same for the substituent acetylacetone than that of hydrogen. Under the joint influence of the simultaneous stacking of heteroatom S and acetylacetone substituent, the energy barrier of the PT process can be effectively lowered, realizing a synergistic strategy, which can provide some guidance for the design of fluorescent materials.

Regulating the photophysical properties of ESIPT-based fluorescent probes by functional group substitution: a DFT/TDDFT study

CONTEXT

In recent years, fluorescent probe technology has received more and more attention. However, the photophysical and photochemical properties of probe molecules still need to be further explored. This paper presents the excited state intramolecular proton transfer (ESIPT) processes and photophysical properties of the probe molecule 4-bromo-2-((E)-((Z)-((5-bromo-1H-indol-2-yl) methylene) hydrazono) methyl) phenol (BHPL) and its four derivatives (BHPL2, BHPL3, BHPL4, and BHPL5). Infrared spectra and geometric structure analyses revealed that introducing the -NH₂ group on the benzene ring with the hydroxyl group will enhance the intramolecular hydrogen bond, which benefits the ESIPT process. Combining their absorption and fluorescence spectra, it can be concluded that BHPL2 and BHPL4 are both excellent probe candidates due to their large Stokes shift. The hole and electron and root mean square displacement analyses manifest that the fluorescence quenching of BHPL4 may be due to the intramolecular charge transfer process. Potential energy curves of BHPL and its four derivatives noted that ESIPT process of the BHPL2 is the most favorable to occur. The frontier molecular orbital and NBO analyses indicated that besides introducing electron-donating groups to reduce the energy gap and enhance fluorescence emission, introducing double electron-withdrawing groups can also achieve this effect, explaining why the energy barrier of ESIPT process for BHPL2 is lower than BHPL5. This work would provide the theoretical basis for designing novel fluorescence probes with more prominent properties.

METHODS

The ground (S₀) and excited (S₁) state structures of all compounds were optimized by density functional theory (DFT) and time-dependent (TDDFT) method, with B3LYP/6-311+G(d,p) level, respectively. The infrared spectra and potential energy curves were simulated at the same theoretical level. The reduced density gradient scatter plots and interaction region indicator isosurfaces were drawn using Multiwfn and VMD programs. The absorption and fluorescence spectra were simulated by the TDDFT/B3PW91/6-311+G(d,p) method. All the calculations in this work are carried out in Gaussian 16 program package.

Unraveling photo-induced proton transfer mechanism and proposing solvent regulation manner for the two intramolecular proton-transfer-site BH-BA fluorophore

To expound specific excited state processes of the novel excitation wavelength dependent emission BH-BA fluorophore for better subsequent applications, this wok mainly focus on exploring photo-induced hydrogen bonding geometrical changes, excited state intramolecular proton transfer (ESIPT) mechanism and related regulated behavior via solvent polarity. The differences of structural parameters, infrared (IR) vibrational spectra, core-valence bifurcation (CVB) index as well as electronic densities ρ(r) between S₀ and S₁ states related to dual hydrogen bonds (O₁-H₂···N₃ and O₄-H₅···N₆) reveal S₁-state hydrogen bonding strength facilitate ESIPT behaviors for BH-BA system. Of particular note, O₄-H₅···N₆ plays a more dominant role. Photo-induced intramolecular charge transfer (ICT) and variations of Hirshfled and NPA charges over atoms related to hydrogen bonding moieties promote the ESIPT tendency for BH-BA. Combined potential energy surfaces (PESs), transition state (TS) and intrinsic reaction coordinate (IRC) paths, we illustrate the excited state intramolecular single proton transfer (ESISPT) mechanism of BH-BA should occur along with O₄-H₅···N₆ hydrogen bonding wire, which could be adjusted by surrounding solvent polarity.

Insights from computational analysis: Excited-state hydrogen-bonding interactions and ESIPT processes in phenothiazine derivatives

Organic materials with Mechanofluorochromism (MFC) properties have potential application value. Phenothiazine derivatives are a class of substances with MFC properties that have been synthesized and reported in experiments (Dyes and Pigments 172 (2020) 107835). Dual fluorescence of a series of phenothiazine derivatives is observed in the experiment, which proved that the ESIPT process is carried out. In this work, we choose phenothiazine derivatives (C2PAHN, C4PAHN, C8PAHN) as models to theoretically analyze the influence of different alkyl chain lengths on the excited state intramolecular proton transfer (ESIPT). In addition, the shift value of fluorescence spectrum is related to the length of alkyl chain. The fluorescence shift of C2PAHN is the largest (6.31 nm), and that of C8PAHN is the smallest (2.40 nm). The theory of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) are adopted to simulate the molecular dynamics in the ground state and excited state. The analysis of the optimized molecular geometry parameters and infrared vibrational spectroscopy (IR) illustrate the stronger hydrogen bonding of the excited state molecules, which is favorable for the progress of ESIPT. Fluorescence spectroscopy reveals that the appropriate increase or decrease of alkyl chains would change the photophysical properties of the molecules. Frontier molecular orbitals (FMOs) indicate that the rearrangement of electron density from electronic level to is the driving force of the ESIPT process. Reduction density gradient (RDG) surfaces and Natural Population Analysis (NPA) tentatively lead to the conclusion that alkyl chain length is inversely proportional to hydrogen bond strength. Finally, the data are qualitatively analyzed by scanning the potential energy curves, and it is concluded that the longer the alkyl chain, the weaker the hydrogen bonding effect and the more unfavorable the ESIPT process.

Stepwise Excited-state Double Proton Transfer and Fluorescence Decay Analysis

This work considers excited state intramolecular proton transfers (ESIPT) occurred in multiple hydroxyl-containing compounds with one proton transfer site in the normal form. If several hydroxyl groups are located close to each other in a molecule, then the ESIPT process can lead to the next one. A proton donor site in the first ESIPT will be a proton acceptor during the second reaction. Therefore, a number of consecutive excited state proton transfers can occur. This work deals with the case of two successive proton transfers occurred in the molecular system. Such process is called as a stepwise excited state intramolecular double proton transfer (stepwise ESIDPT). It leads to the formation of two molecular tautomers. Therefore, fluorescence of such compounds can contain different emission bands correspond to emission of normal form and two tautomers. In this work, a rigorous analysis of fluorescence decay kinetics has been made using the model with three species, including a normal molecular form and two tautomers. The work presents theoretical framework of fluorescence decay analysis of ESIDPT process taking into account three species emission. Theoretically, the stepwise proton transfers can be consisted of more than two ESIPT reactions. It depends on molecular structure and number of involved hydroxyl groups. Here, a formal analysis of fluorescence decay kinetics has been made in the case of a stepwise process consisting of two proton transfers. Moreover, the quantum-chemical calculations have been performed in the case of scutellarein. It is a multiple hydroxyl-containing flavone and, therefore, it can be applied as a model molecule to study stepwise intramolecular proton transfers. The hypothetical scheme of ESIDPT has been proposed for this compound.

Theoretical study on an intriguing excited-state proton transfer process induced by weakened intramolecular hydrogen bonds

In the present work, we have systematically investigated the dual hydrogen-bonded system 2Z,2'Z-3,3'-(4,4'-methylenebis(4,1-phenylene)bis(azanediyl)bis(1,3-diphenylprop-2-en-1-one)) (abbreviated as L) utilizing quantum chemistry methods, in which the excited-state intramolecular proton transfer (ESIPT) does not conform to the usual stereotype but proceeds along the weakened intramolecular hydrogen bonds (IHBs). Two primary configurations were confirmed to coexist in the ground state (i.e., anti-L and syn-L) by calculating the Boltzmann distribution in three different solvents. Based on the cardinal geometrical parameters involved in IHBs and the interaction region indicator (IRI) isosurface, it can be revealed that the dual IHBs of L were both weakened upon photoexcitation, not least the N₁-H₂⋯O₃ IHB was utterly destroyed in the excited state. The proton-transfer process of anti and syn in three solvents with different polarities has been analyzed by constructing S₀- and S₁-state potential energy surfaces (PESs). It can be concluded that only the single proton transfer behavior along N₁-H₂⋯O₃ occurs in the S₁ state, and the corresponding energy barrier is gradually enlarged with increasing solvent polarity. To further expound the weakened IHB-induced ESIPT mechanism, the scanned PESs connecting the transition state (TS) structures and the initial forms indicate that the ESIPT process is infeasible without the appropriate structural torsion. Our work not only unveils the extraordinary ESIPT process of L, but also complements the results obtained from previous experiments.

Unveiling the influence of atomic electronegativity on the double ESIPT processes of uralenol: A theoretical study

In this work, the effects of atomic electronegativity (O, S, and Se atoms) on the competitive double excited-state intramolecular proton transfer (ESIPT) reactions and photophysical characteristics of uralenol (URA) were systematically explored by using the density functional theory (DFT) and time-dependent DFT (TD-DFT) methods. The calculated hydrogen bond parameters, infrared (IR) vibrational spectra, reduced density gradient (RDG) scatter plots, interaction region indicator (IRI) isosurface and topology parameters have confirmed the six-membered intramolecular hydrogen bond (IHB) O4H5^…O3 is the stronger one in all the three studied compounds. Subsequently, frontier molecular orbitals (FMOs) and natural bond orbital (NBO) population analysis essentially uncover that the electron redistribution has induced the ESIPT process. Besides, the constructed potential energy curves (PECs) have indicated that the ESIPT process prefers to occur along the O4H5^…O3 rather than the O1H2^…O3 and the proton-transfer energy barrier is gradually decreased with the weakening of atomic electronegativity from URA to URA-S and URA-Se. In a conclusion, the attenuating of atomic electronegativity has enhanced the IHBs of URA and thereby promoting the ESIPT reaction, which is helpful for further developing novel fluorophores based on ESIPT behavior in the future.

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.

Solvent polarity dependent ESIPT behavior for the novel flavonoid-based solvatofluorochromic chemosensors

In this work, we explore the excited-state intramolecular proton transfer (ESIPT) mechanisms and relative solvent effects for three novel 3-hydroxylflavone derivatives (i.e., HOF, SHOF, and NSHOF) in acetonitrile, dichloromethane, and toluene solvents. Through calculations, we optimize the structures of HOF, SHOF, and NSHOF. Through the analysis of a series of structural parameters related to hydrogen bonding interactions, it could be found that the hydrogen bonds of the three derivatives are all enhanced in the S₁ state, and more importantly, the excited-state hydrogen bonds of HOF are stronger than those of SHOF and NSHOF. In order to explore the effects of solvent polarity, we analyze the core-valence bifurcation (CVB) index, infrared (IR) vibration spectrum, and the potential energy curves. We find that for HOF, SHOF, and NSHOF, the strength of the excited-state hydrogen bonds increases as the solvent polarity decreases. The solvent polarity dependent ESIPT mechanisms pave the way for further designing novel flavonoid-based solvatofluorochromic probes in future.

Effects of azole rings with different chalcogen atoms on ESIPT behavior for benzochalcogenazolyl-substituted hydroxyfluorenes

ESIPT behavior has attracted a lot of eyes of researchers in recent years because of its unique optical properties. Due to its large Stokes shift and double emission fluorescence, white light can be generated in the fluorophore based on the excited state intramolecular proton transfer (ESIPT) principle. The excited state proton transfer behavior of hydroxylated benzoxazole (BO-OH), benzothiazole (BS-OH) and benzoselenazole (BSe-OH) have been investigated in heptane, chloroform and DMF solvents. By comparing the infrared vibration spectra and the variation of bond parameters from the S₀ to S₁ states, and analyzing the frontier molecular orbitals, the influence of hydrogen bond dynamics, the solvent polarity, charge redistribution and the effects of different proton acceptors on proton transfer were observed. The only structural difference among the three substituted hydroxyfluorenes is the heteroatom in the azole ring (oxygen, sulfur and selenium, respectively). We have scanned the potential energy curve of the ESIPT process, and compared the potential barrier, it is found that the heavier chalcogen atoms are more favorable for proton transfer. At the same time, the potential application of changing heteroatoms in the azole ring by walking down the chalcogenic group in crystal luminescence color regulation is also discussed.

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.

Elaborating the excited-state double proton transfer mechanism and multiple fluorescent characteristics of 3,5-bis(2-hydroxypheny)-1H-1,2,4-triazole

Recently, Krishnamoorthy and coworkers reported a new type of proton transfer, which was labeled as 'proton transfer triggered proton transfer', in 3,5-bis(2-hydroxypheny)-1H-1,2,4-triazole (bis-HPTA). In this work, the excited-state double proton transfer (ESDPT) mechanism and multiple fluorescent characteristics of bis-HPTA were investigated. Upon photo-excitation, the intramolecular hydrogen bonding strength changed and the electron density of bis-HPTA redistributed. These changes will affect the proton transfer process. In S₀ state, the proton transfer processes of bis-HPTA were prohibited on the stepwise and concerted pathways. After vertical excitation to the S₁ state, the ESIPT-II process was more likely to occur than the ESIPT-I process, which was contrary to the conclusion that the ESIPT-II process is blocked and the ESIPT-II process takes place after the ESIPT-I process proposed by Krishnamoorthy and coworkers. When the K2 tautomer was formed through the ESIPT-II process, the second proton transfer process on the stepwise pathway was prohibited. On another stepwise pathway, after the ESIPT-I process (form the K1 tautomer), the second proton transfer process should overcome a higher potential barrier than the ESIPT-I process to form ESDPT tautomer. On the concerted pathway, the bis-HPTA can synchronous transfer double protons to form the ESDPT tautomer. The ESDPT tautomer was unstable and immediately converted to the K2 tautomer via a barrierless reverse proton transfer process. Thus, the fluorescent maximum at 465 nm from the ESDPT tautomer reported by Krishnamoorthy and coworkers was ascribed to the K2 tautomer. Most of the fluorophores show dual fluorescent properties, while the bis-HPTA undergoing ESDPT process exhibited three well-separated fluorescent peaks, corresponding to its normal form (438 nm), K1 tautomer (462 nm) and K2 tautomer (450 nm), respectively.

Deciphering the dual emission in the photoluminescence of Au14Cd(SR)12: A theoretical study using TDDFT and TDDFT + TB

Determining excited state processes for small nanoclusters, specifically gold, aids in our ability to fine-tune luminescent materials and optical devices. Using TDDFT and TDDFT + TB, we present a detailed theoretical explanation for the dual emission peaks displayed in Au₁₄Cd(S-Adm)₁₂ (Adm = adamantane). As dual emission is relatively rare, we decipher whether the mechanism originates from two different excited states or from two different minima on the same excited state surface. This unique mechanism, which proposes that the dual emission results from two minima on the first excited state, stems from geometrical changes in the bi-tetrahedron core during the emission process.