State Intramolecular Proton Transfer Mechanism Research Articles

Reversible ratiometric fluorescence probe for the detection of HClO/H2S based on excited state intramolecular proton transfer mechanism

The redox balance between HClO and H2S is essential for the physiological and pathological properties of organisms. Therefore, monitoring the redox process between HClO and H2S is of great significance. Shen et al. synthesised a reversible ratiometric fluorescence probe BT-Se based on excited state intramolecular proton transfer (ESIPT) reaction for the detection of HClO and H2S in previous experiments. When BT-Se detects HClO, it will be oxidised to a new substance which is named BT-SeO, and BT-SeO can be reduced to BT-Se by detecting H2S. BT-Se and BT-SeO emit different colours of fluorescence to realise detection. In this paper, the attribution of the fluorescence peak and the luminescence mechanism are studied. The bond parameters, infrared (IR) vibrational frequency, interaction region indicator (IRI) and the potential energy curve are analysed to demonstrate that the excited state is easier to conduct the ESIPT process. The combination of frontier molecular orbitals (FMOs) and hole–electron analysis reveals the charge excitation characteristics. This work suggests that the yellow and blue fluorescence observed in the experiment is emitted by the keto structures of BT-Se and BT-SeO, respectively. Our research will provide an important theoretical basis for the detection of photophysical phenomena in reversible ratiometric fluorescent probes.

Solvent conditions effect on the excited state intramolecular proton transfer mechanism and photophysical property of 1′-hydroxy-2′-acetonaphthone: A DFT/TD-DFT analysis

Recently, a fluorescence probe 1′-hydroxy-2′-acetonaphthone (HAN) detecting of metal ions is synthesized and characterized experimentally (Malini J, J. Photochem. Photobiol. A Chem., 2021, 418: 113431.). However, comprehensive research on its excited state intramolecular proton transfer (ESIPT) mechanism and photophysical property is required. In the present work, we select four solvents with different dielectric constant to investigate the effect on ESIPT process and photophysical property of HAN molecule. By means of the density functional theory (DFT) and time-density functional theory (TDDFT) methods, all theoretical calculations are accomplished. Through insights into the hydrogen bond geometrical parameters, infrared (IR) vibrational spectra, molecular electrostatic potential (MEP) surfaces, intramolecular hydrogen bond (IHB) strengthening mechanism can be verified, which providing powerful contribution to the occurrence of ESIPT reaction. Based on the analysis of frontier molecular orbitals (FMOs) and Mulliken’s charges and hole–electron, the intramolecular charge transfer (ICT) character is explained. Potential energy curves (PECs) reveal the proton transfer reaction take place more favorably in the S1 state under all solvent conditions. In addition, the energy barriers of QS molecule have ascended higher and higher from toluene (TOL) to sulfoxide (DMSO). The ESIPT behavior is preferred to proceed in the solvent with high dielectric constant instead of solvent with low dielectric constant.

Theoretical study on excited state intramolecular proton transfer mechanism of thiazole complex in different kinds of solvents

AbstractThe excited state intramolecular proton transfer (ESIPT) processes of 3‐(benzo[d]‐ thiazol‐2‐yl)‐2‐hydroxy‐5‐methoxybenzaldehyde (BTHMB) in four kinds of solvents were studied using density functional theory (DFT) and time‐dependent DFT (TDDFT) method. As the solvent dielectric constant decreases, the strength of hydrogen bonds (H‐bonds) in the excited state decreases. After photoexciation, the electron density rearrangement could be affected by the solvents, while the degree of charge transfer and the hydrogen bond interaction in the excited state became lower as the solvent dielectric constant decreased. The potential energy curves displayed the complete process of ESIPT, indicating that the ESIPT process of BTHMB was affected by the solvent polarity. When the solvent dielectric constant decreased, the tendency of proton transfer followed the trend: acetonitrile (ACN) > dichloromethane (DCM) > chloroform (CHCl3) > 1,4‐dioxane (DOX). Therefore, the ESIPT process was affected by the solvent polarity and became more difficult to occur as the solvent dielectric constant decreased.

Theoretical insights into the excited state processes of a novel fluorescent probe for thiophenol with large Stokes shift

Based on a new designed hydroxylflavone-quinoline fluorophore, a novel fluorescent probe FQDNP for detecting thiophenol with large Stokes shift has been developed recently. Despite the excited state intramolecular proton transfer mechanism has been mentioned in the report, the detailed excited state processes for the compound FQOH produced by FQDNP reacting with PhSH are still ambiguous. Typically, in which direction will the proton of hydroxyl group transfer in the excited state? Oxygen or nitrogen atom, which one is more likely to interact with the proton in hydroxyl group by an intramolecular hydrogen bonding? In this work, we have explored the excited state processes of the probe detecting for thiophenol with the density functional theory (DFT) and the time-dependent density functional theory (TDDFT) methods. The calculated photo-induced changes in the optimized geometry parameters, intramolecular hydrogen bonding energies, stretching vibration frequencies, as well as the energy barriers for FQOH imply the proton of hydroxyl group is more likely to transfer to nitrogen atom rather than to oxygen atom in the S1 state. The calculations on the probe FQDNP reveals the fluorescence is quenched by the twisting and photoinduced electron transfer process. Our calculations provide a detailed description for the excited state processes of the probe sensing to thiophenol, which is expected to be applied in designing more effective fluorescent probes.

Excited state intramolecular proton transfer mechanism of a benzothiazole derivative fluorescent probe: Spontaneous ESIPT process

We have investigated the fluorescent mechanism of 4-(benzo[d]thiazol-2-yl)-2-(1,3-dithian-2-yl) phenol (HBT-Hg) and the excited state intramolecular proton transfer processes (ESIPT) for hydroxyphenyl benzothiazole-based fluorescent probe (HBH) by DFT/TDDFT method. After optimizing the geometries of HBH-Enol and HBH-Keto in the ground (S0) and first excited (S1) states at B3LYP/TZVP/IEFPCM computation level, we demonstrated that the intramolecular hydrogen bond was strengthened in S1 state and would facilitate the ESIPT process of HBH, which were further verified by the electron spectroscopy and the frontier molecular orbital results. In addition, the spectral results also show that the isomerism form (HBH-Keto) plays the role in the fluorescence emission. The potential energy curve analyses according to variational O-H coordinate also support proton transfer process because of the ultra-low potential energy barriers for HBH in S1 state. Thus, the intramolecular proton transfer is more likely to occur in the excited state and it turns out to be a spontaneous process.

7-OH quinoline Schiff bases: are they the long awaited tautomeric bistable switches?

Two novel, 7-hydroxyquinoline based, Schiff bases have been synthesized and their spectral properties have been investigated by combined use of optical and NMR spectroscopy and theoretical DFT calculations. The results indicate that 8-((phenylimino)methyl)quinolin-7-ol exists as a mixture of enol and two keto tautomers in solution, while 8-(((pentarfluorophenyl)imino)methyl)quinolin-7-ol is presented as a single enol form. Upon irradiation, in both compounds, through excited state intramolecular proton transfer mechanism, a rotation around the Cquin-CH bond occurs, indicating that they are suitable to be used as bistable switches. The back relaxation is faster comparing to the theoretical expectations, due to the additional flexibility around the azomethine bond. As a result of forming complexes with the acidic counter ion, no intramolecular rotation happens upon protonation with trifluoroacetic acid.

Unveiling the effect of solvent polarity on the excited state intramolecular proton transfer mechanism of new 3-hydroxy-4-pyridylisoquinoline compound.

The new 3-hydroxy-4-pyridylisoquinoline compound is attractive and promising lead structure in drug discovery. The pronounced sensitivity of its emission property toward solvent polarity effect was presented in experiment (J. Org. Chem, 2019, 84, 3011). Nevertheless, the experiment was lack of solvent polarity effect on the excited state intramolecular proton transfer (ESIPT) mechanism in detail. In this study, the ESIPT process of this molecule in different polarity solvents were comprehensively expounded by density functional theory (DFT) and time-dependent DFT (TDDFT) methods. In order to ensure the accuracy of the experiment and roundly explore in theoretical level, two ESIPT pathways (1 and 2) based on the N1 and N2 forms of studied molecule were proposed, among which the ESIPT pathway 1 was derived from experiment. The calculated electronic spectrum of both N1 and N2 forms were rather comparable with the experiment. The calculated intramolecular hydrogen bond (IHB) parameters and infrared (IR) vibration spectra determined the enhancement of IHBs at the S1 state under different solvents for both N1 and N2 forms. The frontier molecular orbitals (FMOs) analysis proved that the intramolecular charge transfer (ICT) taken place during photoexcitation. The potential energy curves (PECs) at the S0 and S1 states were constructed to illustrate the solvent polarity effect on ESIPT mechanism. According to potential energy barriers (PEBs) on the PECs at S1 state, it is concluded that the ESIPT pathway 1 was forbidden with exceedingly high PEBs (24.585-25.322 kcal/mol), while the ESIPT pathway 2 was feasible with enough low PEBs (0.100-0.510 kcal/mol), which suggested the inconsequence of the experiment. Based on the PEBs of ESIPT pathway 2 in different solvent, the effect of solvent polarity on ESIPT mechanism was depicted. The results are as follows: the S1 state IHB intensity was enhanced with increasing solvent polarity; the extent of ICT was decreased with the increment of solvent polarity; the S1 state PEB was decreased as the solvent polarity increased. Indeed in short, the ESIPT reaction became more and more likely as the solvent polarity enhanced. We believe that this investigation will be useful to the utilization and development of property for such photochemical substances.

A 3-hydroxythalidomide-based fluorescent probe for detection of fluoride in water sample and bioimaging in living cells

Thalidomide and its derivatives were drawing attentions as an old drug since the cereblon (CRBN) as a thalidomide-binding protein was identified. Herein, we reported its novel application in ion detection for the first time. And a turn-on fluorescent fluoride probe, (4-((tert-butyldimethylsilyl)oxy)-2-(2,6-dioxopiperidin-3- yl)isoindoline-1,3-dione was prepared using a protection-deprotection design strategy taking 3-hyroxythalidomide as parent compound. High selectivity and sensitivity for fluoride ion were observed with significant green fluorescent responses and a remarkable Stokes shift (about 178 nm). The fluorescent change was based on the exited state intramolecular proton transfer mechanism, which had been established by in-situ Fourier transform infrared spectroscopy and 1H NMR techniques. In addition, the probe was successfully applied to detect the fluoride ion in lake water. More importantly, the probe was almost nontoxic to the cultured cells and was applied in bioimaging in living cells.

Unravelling the solvent polarity effect on the excited state intramolecular proton transfer mechanism of the 1- and 2-salicylideneanthrylamine. A TD-DFT case study.

Time dependent density functional theory has been used to investigate the photochemical and photophysical processes involved in the excited states relaxation of 1- and 2-salicylideneanthrylamine in different solvent environments. This investigation reveals that the pathways involved in the relaxation of the first excited state depend on the solvent polarity. The emission spectrum in acetonitrile and methanol is dominated by the cis-keto tautomers, while in cyclohexane, the spectrum is dominated by the fluorescence emission of the locally excited trans-enol form. Our results showed that, for each compound, two nearly isoenergetic trans-enol conformers can coexist in equilibrium, which upon photoexcitation, can relax by two competitive processes: rotation about the azomethine N[double bond, length as m-dash]C bond leading to the twisted-enol conformer, and the excited state intramolecular proton transfer leading to the fluorescent cis-keto tautomer, which can undergo a cis-trans isomerization producing the trans-keto photochromic product. The TD-DFT relaxed potential energy profiles for the ESIPT show that the effect of changing the solvent from polar to nonpolar solvents results on an increment of the energy barrier, and therefore, the ESIPT become kinetically less favoured. In constrast, this change favours the relaxation of the excited trans-enol form towards the twisted conformers, in both the enol and keto regions.

Theoretical insights into the excited state intramolecular proton transfer mechanism for a novel 4‐methoxy‐3‐hydroxyflavone system

A novel 3‐hydroxyflavone derivative, 4‐methoxy‐3‐hydroxyflavone (4M3HF), is theoretically investigated regarding its excited state intramolecular proton transfer (ESIPT) mechanism based on density functional theory (DFT) and time‐dependent DFT (TDDFT) methods. By comparing the solvent polarity, we selected acetonitrile and toluene in this work. First, based on atoms in molecules (AIM) analyses, we verified the formation of intramolecular hydrogen bond of the 4M3HF structure. On investigating the geometrical parameters (i.e., bond lengths and bond angles), we found that the hydrogen bond strength should be enhanced in the S1 state. By calculating the infrared vibrational spectra, we confirmed the strengthening of the intramolecular hydrogen bond in the S1 state. We further studied the excitation process using the frontier molecular orbitals method, based on which we concluded that the charge redistribution in solvents facilitates the ESIPT tendency. For exploring the ESIPT behavior, we constructed the potential energy curves along with ESIPT paths in both solvents (acetonitrile and toluene). We found that low potential energy barriers facilitate the ESIPT process in the 4M3HF system, which was also verified by searching the transition state (TS) structure. Comparing these two solvents, we concluded that solvent effects play little role in the ESIPT reaction in the 4M3HF system. And we could successfully explain the previous experimental result.

A detailed DFT/TDDFT study on excited‐state intramolecular hydrogen bonding dynamics and proton‐transfer mechanism of 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol

AbstractIn this present work, using density functional theory and time‐dependent density functional theory methods, we theoretically study the excited‐state hydrogen bonding dynamics and the excited state intramolecular proton transfer mechanism of a new 2‐phenanthro[9,10‐d]oxazol‐2‐yl‐phenol (2PYP) system. Via exploring the reduced density gradient versus sign(λ2(r))ρ(r), we affirm that the intramolecular hydrogen bond O1‐H2⋯N3 is formed in the ground state. Based on photoexcitation, comparing bond lengths, bond angles, and infrared vibrational spectra involved in hydrogen bond, we confirm that the hydrogen bond O1‐H2⋯N3 of 2PYP should be strengthened in the S1 state. Analyses about frontier molecular orbitals prove that charge redistribution of 2PYP facilitates excited state intramolecular proton transfer process. Via constructing potential energy curves and searching transition state structure, we clarify the excited state intramolecular proton transfer mechanism of 2PYP in detail, which may make contributions for the applications of such kinds of system in future.

Solvent effects on excited state intramolecular proton transfer mechanism in 4-(N,N-dimethylamino)-3-hydroxyflavone

Abstract The excited state intramolecular proton transfer (ESIPT) dynamics and photophysical properties of 4-(N,N-dimethylamino)-3-hydroxyflavone (DMAF) in acetonitrile (ACN) and ethanol (EtOH) were investigated by density-functional theory (DFT) and time-dependent density-functional theory (TDDFT) methods at B3LYP/6-31G (d,p) level. The calculation results show that hydrogen bond in the first excited state (S1) is strengthened, which can facilitate the process of proton transfer. The absorption and emission spectra obtained through the vertical excitation energies agree well with the experiment, which confirm the rationality of the choice of theoretical methods. In addition, the frontier molecular orbitals (MOs) analysis indicates the intramolecular charge transfer, which can reveal the tendency of ESIPT reaction. In order to explore the mechanism of the ESIPT process, the potential energy curves of the ground state (S0) and the S1 state have been constructed. Based on the energy potential barrier of 3.48 kcal/mol for DMAF molecule and 5.48 kcal/mol for 4-(N,N-dimethylamino)-3-hydroxyflavone/ethanol (DMAF-EtOH) complex in the S1 state, we can find that the proton transfer occurs more easily in acetonitrile solvent.

Excited state intramolecular proton transfer mechanism of o-hydroxynaphthyl phenanthroimidazole**Project supported by the Shandong Provincial Higher Educational Science and Technology Program, China (Grant No. J17KA186), the Taishan Scholar Project of Shandong Province, China, the Natural Science Foundation of Liaoning Province, China (Grant No. 20170540408), and the Science and Technology Plan Project of Shenyang City, China (Grant No. 17-231-1-06).

By utilizing the density functional theory (DFT) and the time-dependent density functional theory (TDDFT), the excited state intramolecular proton transfer (ESIPT) mechanism of o-hydroxynaphthyl phenanthroimidazole (HNPI) is studied in detail. Upon photo is excited, the intramolecular hydrogen bond is obviously enhanced in the S1 state, which thus promotes the ESIPT process. Hydrogen bond is shown to be strengthened via comparing the molecular structures and the infrared vibration spectra of the S0 and S1 states. Through analyzing the frontier molecular orbitals, we can conclude that the excitation is a type of the intramolecular charge transfer excitation, which also indicates the trend of proton transfer in S1 state. The vertical excitation based on TDDFT calculation can effectively repeat the absorption and fluorescence spectra of the experiment. However, the fluorescence spectrum of normal structure, which is similar to the spectrum of isomer structure is not detected in the experiment. It can be concluded that the fluorescence measured in the experiment is attributed to both structures. In addition, by analyzing the potential energy curves (PECs) calculated by the B3LYP functional method, it can be derived that since the molecule to cross the potential barrier in the S1 state is smaller than in the S0 state and the reverse proton transfer process in the S1 state is more difficult than in the S0 state, the ESIPT occurs in the S1 state.

Excited state behavior of benzoxazole derivatives in a confined environment afforded by a water soluble octaacid capsule

Abstract This feature article presents a brief description of the excited state intramolecular proton transfer mechanism (ESIPT), describes the solvent dependence of the ESIPT phenomenon and present the potential applications that can be envisaged for these fluorophores. Photophysics of three dye molecules exhibiting ESIPT were examined free in solution and within a water soluble supramolecular host, octaacid (OA) by steady-state and time resolved fluorescence emission techniques. Inclusion of the above dyes within OA was confirmed by 1H NMR spectra. The free and octaacid complexed dyes had electronic absorption in the UV region. Within the OA container the fluorophores showed fluorescence emission similar to that in benzene, dichloromethane and acetonitrile. Moreover, the observed large Stokes’ shift in the emission of the dyes when confined within OA capsule was unlike that in aqueous medium. The results suggest that a confined medium is a powerful tool to tailor the fluorescence emission of ESIPT compounds in aqueous media.

A novel excited-state intramolecular proton transfer (ESIPT) dye with unique near-IR keto emission and its application in detection of hydrogen sulfide.

A new ESIPT dye of benzothiazole with a conjugative electron acceptor was discovered and synthesized. Due to its unique NIR keto emission and a large Stokes shift, it was used to develop a fluorescent probe for sensitive and selective detection of hydrogen sulfide.

An ESIPT-based ratiometric fluorescent probe for the imaging of nitroxyl in living cells

A novel fluorescent probe based on the excited state intramolecular proton transfer mechanism for the detection of nitroxyl was developed for the first time.

Novel fluorescent copolymers of styrene with benzazole chromophores

Novel green fluorescent copolymers of styrene and 2-[(5′- N-acryloyl)-2′-hydroxyphenyl]benzoxazole or 2-[(5′- N-acryloyl)-2′-hydroxyphenyl]benzothiazole were synthesized by radical polymerization. The copolymers were characterized by size exclusion chromatography, thermal analyses (DSC, TGA), UV–Vis and fluorescence spectrophotometry. The glass transition temperatures of the fluorescent copolymers were similar to that of the parent polystyrene and the average molecular weight was lower due to the chain transfer effect of the benzazole dyes. The maximum fluorescence emission wavelength of the copolymers in the solid state or in chloroform solution accessed by fluorescence spectrophotometry was also similar. The Stokes shift of the styrene–benzoxazole or –benzothiazole copolymers was similar to those presented by fluorochromes that exhibit an excited state intramolecular proton transfer mechanism.

The first silica aerogels fluorescent by excited state intramolecular proton transfer mechanism (ESIPT)

Five silyl-functionalized benzazole dyes, fluorescent by excited state intramolecular proton transfer (ESIPT) mechanism, were synthesized by reaction of amino benzazole derivatives with 3-(triethoxysilyl)propyl isocyanate. Fluorescent silica gels were prepared and monolithic aerogels (d ≈ 0.18 g cm−3) were obtained via supercritical CO2 drying of the fluorescent gel. The photophysical behaviour of the dyes and fluorescent silica aerogels was investigated by UV–vis and steady-state fluorescence spectroscopy.

New phototautomerizing systems: non-symmetric derivatives of [2,2′-bipyridyl]-3,3′-diol

Six new phototautomerizing compounds, structural analogues of [2,2′-bipyridyl]-3,3′-diol (BP(OH) 2) and [2,2′-bipyridyl]-3-ol, were synthesized. The electron-donating or electron-withdrawing substituent introduced next to the reactive proton transfer site (N atom) can modify the status of the internal hydrogen bonds, controlling the excited state intramolecular proton transfer mechanism in a predictable way. Ab initio RHF/6-31 G ** and semiempirical (PM3) calculations for the S 0 ground state were performed to determine the structural features that `prepare' the molecular system for its fate in the S 1 excited state. The calculated molecular geometries and Mulliken net charges were useful for interpreting photophysical data. The dramatic changes of phototautomeric fluorescence quantum yields are dependent on (1) molecular symmetry lowering of the substituted BP(OH) 2 system and (2) electron density on the N atom(s).

Ring closing and photooxidation in nitrogen analogues of 3-hydroxyflavone

An epoxide intermediate in the Algar–Flynn–Oyamada (AFO) synthesis of flavones is reaffirmed through the use of quinolone analogues to 3-hydroxyflavone (3HF). Stepwise synthesis of analogues 3-hydroxy-2-phenyl-1,4-dihydro-4-quinolone 11 and 3-hydroxy-1-methyl-2-phenyl-1,4-dihydro-4-quinolone 12, via chalcone formation, epoxidation, ring closing and final oxidation, has been accomplished. The intermediary of an epoxide is further supported by blocking cyclization with methoxy substitution at the 2′-position (1A). Absorption/emission spectroscopy of 11 and 12 shows large red shifts, as seen in 3HF, indicative of an excited state intramolecular proton transfer mechanism. Nitrogen analogues demonstrate photooxidative stability similar to that of 3HF.