Effects of hydrogen bond on 2-aminopyridine and its derivatives complexes in methanol solvent

Turning enol* emission of SBOH via restricting twisted intramolecular charge transfer behavior by pressure

Stimuli-responsive luminogens with aggregation-induced emission and excited state intramolecular proton transfer (ESIPT) properties have applications in storage devices, anti-counterfeiting, imaging, and sensors. Nevertheless, group rotation appears in twisted intramolecular charge transfer (TICT) state, resulting in decreased fluorescence intensity. Inhibiting TICT remains a challenge based on their intrinsic molecular configuration. Herein, we present a simple facile pressure-induced method to restrict the TICT behavior. Steady-state spectroscopy measurement shows that fluorescence enhancement and color shifts can be achieved under high pressure. Combined with in situ high-pressure ultrafast spectroscopy and theoretical calculations, the TICT behavior was restricted in two aspects. The ESIPT process was damaged, hence more particles stored in the E* state, and transferred to the TICT state hardly. Also, the rotation of (E)-dimethyl5-((4-(diethylamino)-2-hydroxybenzylidene)amino)isophthalate (SBOH) was restricted, significantly increasing the fluorescence intensity. This approach provides a new strategy for the development of stimulus-responsive materials.

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.

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.

The mechanisms of a bifunctional fluorescent probe for detecting fluoride and sulfite based on excited-state intramolecular proton transfer and intramolecular charge transfer

The mechanisms of 2-(Benzo[d]thiazol-2-yl)phenol-based bifunctional probe (HBT-FS) for detecting fluoride (F^-) and sulfite (SO₃ ^2-) based on excited-state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) have been theoretically studied. Laplacian bond order of HBT-FS indicates that the F^- ion cleaves the Si-O bond and then forms Compound 2 possessing a six-membered ring with a hydrogen bond. Potential energy curves and dynamic simulations confirm that ESIPT in Compound 2 occurs along with this hydrogen bond and forms a keto structure with an emission at 623 nm, which agrees with the observed experimental value (634 nm) after adding F^-. Therefore, the fluorescence red-shift (from 498  to 634 nm) of HBT-FS observed in experiment after adding F^- is caused by ESIPT. The SO₃ ^2- ion is added to the C₅ site of HBT-FS, which is confirmed by orbital-weighted dual descriptor, and then forms Compound 3 with fluorescence located at 404 nm. The experimentally measured fluorescence at 371 nm after adding SO₃ ^2- is assigned to Compound 3. Charge transfer analyses indicate that the ICT extent of Compound 3 is relatively weak compared with that of HBT-FS because of the destruction of the conjugated structure by the addition reaction of SO₃ ^2-, which induces the blue-shift of the fluorescence of HBT-FS from 498 to 371 nm. The different fluorescence responses make HBT-FS a fluorescent probe to discriminatorily detect F^- and SO₃ ^2-.

Solvent polarity dependent excited state hydrogen bond effects and intramolecular double proton transfer mechanism for 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d']bisimidazole system

In this work, we probe into the photo-induced excited state hydrogen bonding interactions and excited state proton transfer (ESPT) behaviors for a representative benzo[1,2-d:4,5-d']bisimidazole derivative (i.e., 2-hydroxyphenyl-substituted benzo[1,2-d:4,5-d']bisimidazole (HPBB)) compound. In view of aprotic solvents with different polarities, cyclohexane (CYH), dichloromethane (DCM) and acetonitrile (MeCN) solvents are considered. Analyzing hydrogen-bond geometrical parameters, infrared (IR) vibrational spectra, Mayer bond order and predicting hydrogen bonding energy (E_(HB)), we verify dual hydrogen bonds of HPBB are strengthened in S₁ state. Particularly, in nonpolar solvent, the enhanced excited state hydrogen bonds become more obvious. The intriguing charge redistribution and frontier molecular orbitals (MOs) reveal hydrogen bonding acceptance ability of acceptor moieties becomes stronger, which plays a crucial role in capturing hydroxyl proton via photoexcitation. To check and explore ESIPT mechanism, we present the solvent polarity dependent asynchronous excited state intramolecular double proton transfer (ESIDPT) mechanism. That is, nonpolar solvent promotes excited state intramolecular single proton transfer (ESISPT) process for HPBB, while polar solvent contributes to ESIDPT behavior with the primary single proton-transfer product in S₁ state. This work not only makes a rational attribution to experimental phenomena, but also clarifies detailed excited state behaviors for HPBB and presents regulating ESIPT mechanism via solvent polarity.

Revised the excited-state intramolecular proton transfer direction of the BTHMB molecule: A theoretical study

The spectroscopic properties of 3-(benzo[d]-thiazol-2-yl)-2-hydroxy-5-methoxy benzaldehyde molecule were investigated [J. Phys. Chem. A 2019, 123, 10246-10253]. The result shows that the excited-state intramolecular proton transfer (ESIPT) was driven toward the N center over the O center. In this research, the density functional theory and time-dependent density functional theory method were used to calculate molecule structures. Through our calculations, the ESIPT process toward N atom is proved to be feasible. Moreover, the emission peak we obtained of ESIPT process from the OH proton to aldehyde O atom is located at 564 nm, which is attributed to 500 nm in previous research. From the potential energy curves, the 0.35 kcal/mol energy barrier indicates the ESIPT process could occur when excited to S1 state from the OH proton to aldehyde O atom. In addition, the frontier molecular orbitals analysis and IR spectrum were also calculated. Finally, we revise the direction of BTHMB molecule, the two directions of ESIPT are both feasible when excited to S₁ state.

Revised excited-state intramolecular proton transfer of the 3-Aminophthalimide molecule: A TDDFT study

The spectroscopic properties of 3-Aminophthalimide (3AP) molecule were investigated [Chem. Phys. 2002, 283, 249, New J. Chem. 2018, 42, 1181]. The result was that the 3AP molecule was exhibiting excited-state intramolecular proton transfer (ESIPT). In the research, we revised previous result using time-dependent density functional theory (TDDFT) method. The fluorescence spectrum shows that the only fluorescence peak is from initial enol form, which is different from the traditional case of ESIPT. The red shift of characteristic peaks in infrared vibration spectra is not induced by ESIPT process. The change in the vibration mode of the amino group causes the red shift of characteristic peak in the infrared spectrum. Energy curves indicate that the barrier (19.71 kcal/mol) is anomalously high in the first excited state. In addition, there are not stable points to lead the ESIPT to form a keto isomer. Together, these results demonstrate that there is not an ESIPT process happening of 3AP molecule.

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

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

Theoretical insights into excited-state hydrogen bonding effects and intramolecular proton transfer (ESIPT) mechanism for BTS system

In this work, N,N'-bis(salicylidene)-(2-(3',4'-diaminophenyl)benzothiazole) (named as "BTS") system was studied about its excited-state intramolecular proton transfer (ESIPT) process. The analyses about reduced density gradient (RDG) reveal the formation of two intramolecular hydrogen bonds in BTS system. Bond lengths and angles, infrared (IR) vibrations as well as frontier molecular orbitals (MOs) using TDDFT method indicate that the strength of hydrogen bond should be enhanced in the S1 state. Particularly, hydrogen bond O1-H2···N3 undergoes larger variations compared with O4-H5···N6, which infers that hydrogen bond O1-H2···N3 may play a decisive role in the ESIPT process of BTS system. Given the two hydrogen bonds of BTS molecule, two types of potential energy curves have been constructed, which confirms that only single proton transfer process occurs due to lower energy barrier along with O1-H2···N3 rather than O4-H5···N6. This work not only presents a reasonable explanation for previous experiment, but also clarifies the specific ESIPT mechanism for BTS system.

A new interpretation of the ESIPT mechanism of 2-(benzimidazol-2-yl)-3-hydroxychromone derivatives

The present study demonstrates the excited-state intramolecular proton transfer (ESIPT) mechanism of 2-(benzimidazol-2-yl)-3-hydroxychromone (DH3B2) is based on density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods. We find that DH3B2-C is the main conformation to occur ESIPT. Moreover, we get the different results of DH3B2 for the ESIPT mechanisms in comparison with the previous reports. We have optimized three isomers (DH3B2-A, DH3B2-B and DH3B2-C), and calculated absorption and fluorescence spectra, which agree well with the experimental data. Furthermore, we prove the hydrogen bond is enhanced in the S₁ state by comparing infrared vibrational spectra, the relevant bond length and bond angle. In our calculations, the results of the three levels of calculations (CAM-B3LYP/TZVP, B3LYP/TZVP and PBEPBE/TZVP) indicate that DH3B2-C is the most stable conformation, by compared the single point energy of three isomers. By constructed the potential energy surfaces (PESs), we find the converted relationship among the three isomers; DH3B2-C is the main conformation in which DH3B2 exists. Furthermore, combination with reduced density gradient (RDG) function, the hydrogen bond of DH3B2-C is stronger than that of DH3B2-A and DH3B2-B, which proves that DH3B2-C form is the most favorable form for ESIPT among the three isomers. Meanwhile, we have further investigated the ESIPT mechanisms of DH3B2, via constructing the potential energy curves (PECs). These results have shown that DH3B2-C is easier to ESIPT occur than DH3B2-A and DH3B2-B. Therefore, the proton receptors of the ESIPT are mainly the benzimidazole nitrogen atoms.

Excited state intramolecular proton transfer (ESIPT) luminescence mechanism for 4-N,N-diethylamino-3-hydroxyflavone in propylene carbonate, acetonitrile and the mixed solvents

In this work, density functional theory (DFT) and time density functional theory (TDDFT) methods were employed to investigate the nature of the double fluorescence emission of DEAHF in these three solvents. We analyzed the geometric structures, vibrational frequencies, frontier molecular orbitals (MOs), molecular electrostatic potential surface (MEPS), calculated absorption and fluorescence spectra and the potential-energy curves for DEAHF. All the results show that the intramolecular hydrogen bond of DEAHF is strengthened from S₀ to S₁ and the electron density redistribution occurs between the proton acceptor and donor, which can facilitate ESIPT. Moreover, the geometric structures, absorption and emission spectra, MEPS and potential-energy curve of DEAHF are identical. It reveals theoretically that ACN and PC can maintain the polarity of the solvent with 1:1 mixing, which is consistent with the experimental results.

Solvent viscosity induces twisted intramolecular charge transfer state lifetime tunable of Thioflavin-T

Excited-state deactivation dynamics of Thioflavin-T (ThT) in gradual viscosity solvents were investigated. Femtosecond transient absorption spectra and dynamic decay curves both present significant distinction of ThT in different volume ratios binary mixtures solvents. Dynamics fitting lifetime of twisted intramolecular charge transfer (TICT) state is strongly dependent on solvents viscosity. Compared to rotation corresponding time of ThT in low viscosity solvent (0.6 cp) experimentally coincident well with Stokes-Einstein-Debye (SED) equation, the relation between rotation corresponding time and relatively high viscosity (5.9 cp to 1091.2 cp) is more consistent with fractional SED equation. Combined with optimized geometric structures of ThT by density functional theory and time-dependent density functional theory, further understand TICT state lifetime increases with increasing solvents viscosity. Our work provides a comprehensive understanding of fluorescence molecular rotor (FMR) deactivation process in different viscosity solvents and is helpful to design new FMR.

An amino-functionalized ordered mesoporous polymer as a fiber coating for solid phase microextraction of phenols prior to GC-MS analysis

An amino-functionalized ordered mesoporous polymer (OMP-NH₂) was synthesized and applied as a fiber coating for solid phase microextraction of polar phenols from environmental samples. The fiber coating was prepared by loading the OMP-NH₂ powder onto a stainless steel wire through silicone gel. Combined with GC-MS, the fibers were employed to quantify trace of phenols in water through headspace-SPME. The characterization showed the OMP-NH₂ to have a large specific surface area (420 m² g^-1) and good thermal stability (>400 °C). Due to its mesoporous structure and favorable interactions via hydrogen bonding and π stacking interactions with phenols, the sorbent represents a promising candidate for the separation and enrichment of polar phenols. Extraction conditions, such as temperature, extraction time, salt concentration, pH value and desorption time, were fully optimized. Under the optimized conditions, the coating exhibits an enrichment effect that is ~2-10 times better than that of a commercial polyacrylate coating. Figures of merit include (a) low limits of detection (0.05-0.16 ng L^-1), (b) wide linear ranges (0.2-10,000 ng L^-1), and (c) high enrichment factors (510-2272). The relative standard deviations for one fiber and fiber-to-fiber were in the range of 4.0-6.1% and 4.6-7.4%, respectively. The method was applied to the determination of phenols in water samples and gave satisfactory recoveries between 85.4 and 112.2%. Graphical abstract An amino-functionalized ordered mesoporous polymer (OMP-NH₂) was synthesized by a solventless method and applied as headspace solid phase microextraction (HS-SPME) fiber coating for the extraction of polar phenols from the environmental samples.

Reevaluating the effects of reorganization energy on electron transfer rate for quantum dot-molecular acceptor complexes in different solvents

The electron transfer (ET) rate in quantum dot (QD)-molecular acceptor systems is dependent upon system reorganization energy (RE, λ), which comprises contributions from solvent (λ₀) and reactants (λ_i). However, to date, the effect of λ_i on ET rate has been largely ignored. Herein, the ET from CdSe/ZnS QDs to 1-chloroanthraquinone (1-CAQ) in different solvents was investigated using ultrafast transient absorption spectroscopy as a means to evaluate the effect of λ_i on ET rate. The results revealed that ET rate is strongly solvent dependent. Amazingly, the ET rate in carbon disulfide is 300-times higher than that in n-dodecane. Theoretical calculations indicated that the λ_i contribution from 1-CAQ alone accounts for a large proportion of system RE and varies greatly in different solvents. Furthermore, the ET rate increases first and, then, decreases with the λ value in different solvents. This trend was interpreted consistently in terms of Marcus theory by adding λ_i to λ for different solvents. Thus, our present work demonstrates that the RE of the acceptor molecule has a non-negligible effect on ET rate, providing new insight into the mechanisms of ET process and for the development of QD-based devices.

Theoretical study on excited-state intramolecular proton transfer process of cyanide group substituted 2-(2-hydroxyphenyl)benzothiazole

In this work, the influence of cyanide group substituted on the excited-state intramolecular proton transfer of 2-(2-hydroxyphenyl)benzothiazole (HBT) has been theoretical studied. It is found that the intramolecular hydrogen bonds of HBT and its cyanide substituted derivatives are significantly strengthened in the first excited (S₁) state, while this strengthening tends to decrease with the substitution of cyano group. The natural population analysis (NPA) is performed to interpret this phenomenon and the calculated results show that the introduction of cyano groups induces intramolecular charge transfer, leading to the decrease of electrostatic interaction of intramolecular hydrogen bond in the S₁ state compared to isolated HBT. Therefore, proton transfer of HBT and its derivatives become difficult with the introduction of cyano groups, which is consistent with the calculated potential barriers (1.95, 2.71 and 2.81 kcal/mol respectively) of the three molecules in the S₁ state. This work interprets the hindrance of cyanide group on the proton transfer of HBT and its derivatives in the S₁ state, which will bring new insight into the study of analogous molecular system.

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 theoretical study of the ESIPT mechanism of 3-hydroxyflavone derivatives: solvation effect and the importance of TICT for its dual fluorescence properties

As the dielectric constant decreases, the ESIPT reaction occurs more easily and TICT is good at emitting double fluorescence.

TDDFT study on excited state intramolecular proton transfer mechanism in 2-amino-3-(2'-benzazolyl)-quinolines

The intramolecular proton transfer reaction of the 2-amino-3-(2'-benzoxazolyl)-quinoline (ABO) and 2-amino-3-(2'-benzothiazolyl)-quinoline (ABT) molecules in both S₀ and S₁ states at B3LYP/6-311++G(d,p) level in ethanol solvent have been studied to reveal the deactivation mechanism of the tautomers of the two molecules from the S₁ state to the S₀ state. The results show that the tautomers of ABO and ABT molecules may return to the S₀ state by emitting fluorescence. In addition, the bond lengths, angles and infrared spectra are analyzed to confirm the hydrogen bonds strengthened upon photoexcitation, which can facilitate the proton transfer process. The frontier molecular orbitals (MOs) and natural bond orbital (NBO) are also calculated to indicate the intramolecular charge transfer which can be used to explore the tendency of ESIPT reaction. The potential energy surfaces of the ABO and ABT molecules in the S₀ and S₁ states have been constructed. According to the energy potential barrier of 9.12kcal/mol for ABO molecule and 5.96kcal/mol for ABT molecule, it can be indicated that the proton transfer may occur in the S₁ state.

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.

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.

Less stable tautomers form stronger hydrogen bonds: the case of water complexes

Hydrogen bonding in cyclic complexes of water with tautomeric pairs of molecules M⁰ and M¹ is calculated to be stronger by more than 25% for the less stable tautomer M¹ in all cases where the energy gap between the two tautomers is large (ΔE(M⁰ − M¹) > 10 kcal mol⁻¹).

A competitive excited state dynamical process for the 2,2′-((1E,1′E)-((3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)-bis(azanylylidene))bis(methanylylidene))-diphenol system

By applying DFT and TDDFT methods, we theoretically investigate the excited state dynamical process for the 2,2′-((1E,1′E)-((3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)-bis(azanylylidene))bis(methanylylidene))-diphenol system.

Determination of patulin in apple juice using magnetic solid-phase extraction coupled with high-performance liquid chromatography

An efficient magnetic sorbent consisting of benzofuran-2-carboxylic acid-loaded magnetic nanocomposite was successfully synthesised for pre-concentration of patulin from apple juice. The prepared magnetic nanocomposite was characterised by scanning electron microscopy, transmission electron microscopy and Fourier-transform infrared spectroscopy. Determination of enriched patulin was performed by high-performance liquid chromatography. The best adsorption conditions were 40 mg of sorbent, 50 ml of apple juice sample, pH 5, ambient temperature and 25 min; the elution conditions were 500 μl methanol, pH 5, ambient temperature, and 4 min. Under optimised conditions, pre-concentration factor was 100, linearity range was 1-400 μg l^-1 of patulin, limit of detection was 0.15 μg l^-1 and limit of quantification was 0.5 μg l^-1. When samples were determined 20 times, the recovery was 93.9-102.6% and the relative standard deviation was below 5.3%. In terms of proposed procedure, the developed method was successfully applied for patulin detection in apple juice samples.

Theoretical investigation on ESIPT mechanism of a new fluorescent sensor in different solvents

In the present work, a new phenylbenzimidazole derivatized fluorescent sensor (L) (J. Lumin. 147 (2014) 179), has been investigated on the excited state proton transfer (ESPT) based on the time-dependent density functional theory (TDDFT) method. The calculated absorption and fluorescence spectra based on the TDDFT method are in agreement with the experimental results. Two kinds of structures of L chromophore are found in the first excited (S1) state, which may be due to the proton transfer reactive. Hydrogen bond strengthening has been testified in the S1 state based on comparing staple bond lengths and bond angles involved in hydrogen bonding between the S0 state and the S1 state. In addition, the calculated infrared spectra at the N-H stretching vibrational region and calculated hydrogen bond energy also declare the phenomenon of hydrogen bond strengthening. The frontier molecular orbitals (MOs) and Mulliken's charge distribution analysis method as well as natural bond orbital (NBO) demonstrate the charge distribution, which provides the tendency of ESIPT reaction. The potential energy surfaces of the S0 and S1 states are constructed to explain the mechanism of the proton transfer in the excited state in detail. In addition, the ESIPT process of sensor L is dependent on different solvents.

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

The excited state intramolecular proton transfer (ESIPT) mechanisms of 2-(2-hydroxyphenyl)benzoxazole (HBO), bis-2,5-(2-benzoxazolyl)-hydroquinone (BBHQ) and 2,5-bis(5'-tert-butyl-benzoxazol-2'-yl)hydroquinone (DHBO) have been investigated using time-dependent density functional theory (TDDFT). The calculated vertical excitation energies based on the TDDFT method reproduced the experimental absorption and emission spectra well. Three kinds of stable structures were found on the S1 state potential energy surface (PES). A new ESIPT mechanism that differs from the one proposed previously (Mordzinski et al., Chem. Phys. Lett., 1983, 101, 291. and Lim et al., J. Am. Chem. Soc., 2006, 128, 14542.) is proposed. The new mechanism includes the possibility of simultaneous double proton transfer, or successive single transfers, in addition to the accepted single proton transfer mechanism. Hydrogen bond strengthening in the excited state was based on primary bond lengths, angles, IR vibrational spectra and hydrogen bond energy. Intramolecular charge transfer based on the frontier molecular orbitals (MOs) also supports the proposed mechanism of the ESIPT reaction. To further elucidate the proposed mechanism, reduced dimensionality PESs of the S0 and S1 states were constructed by keeping the O-H distance fixed at a series of values. The potential barrier heights among the local minima on the S1 surface imply competitive single and double proton transfer branches in the mechanism. Based on the new ESIPT mechanism, the observed fluorescence quenching can be satisfactorily explained.