Excited-state proton transfer from pyranine to acetate in gamma-cyclodextrin and hydroxypropyl gamma-cyclodextrin

Excited-state proton transfer (ESPT) from pyranine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) to acetate has been studied by picosecond and femtosecond emission spectroscopy in gamma-cyclodextrin (gamma-CD) and 2-hydroxypropyl-gamma-cyclodextrin (HP-gamma-CD) cavities. In both the CDs, ESPT from HPTS to acetate is found to be very much slower (90 and 200 ps) than that in bulk water (0.15 and 6 ps). From molecular modeling, it is shown that in the cyclodextrin cavity the acetate is separated from the OH group of HPTS by water bridges. As a result, proton transfer in the cavity requires rearrangement of the hydrogen-bond network involving the cyclodextrin. This is responsible for the marked slowdown of ESPT. ESPT of HPTS in substituted gamma-CD is found to be slower than that in the unsubstituted one. This is attributed to the hydroxypropyl groups, which prevent close approach of acetate to HPTS.     

Excited-state intramolecular proton transfer in 2,4,5-triarylimidazole molecules

Excited-state intramolecular proton transfer (ESIPT) in the 2,4,5-triarylimidazole molecules was studied by spectral-luminescent technique. For 4,5-diphenyl-(2-hydroxyphenyl)imidazoles, the ESIPT occurs in both liquid and glassy matrices at 77 K. For 4,5-diphenyl-(2-hydroxynaphthyl)imidazole, the ESIPT requires rotation of molecular fragments and is not observed at 77 K.     

Excited state (formal) intramolecular proton transfer (ESIPT) in p-hydroxyphenyl ketones mediated by water

Evidence is presented that show p-hydroxyphenyl ketones 6–8 undergo excited state intramolecular proton transfer (ESIPT, via the singlet excited state), mediated by water, which formally transfers the phenol proton to the carbonyl oxygen of the ketone. ESIPT was not observed in neat CH₃CN. The ESIPT process in aqueous media generates the corresponding p-quinone methides 9–11 (and the corresponding conjugate bases (phenolate ions) 12–14), as detected by laser flash photolysis (LFP). It competes effectively with intersystem crossing to the excited triplet state. The respective p-methoxyphenyl ketones 15 and 16 failed to undergo the reaction consistent with the expected lack of proton transfer in these systems. Results for the biphenyl ketone 8 indicate that formal ESIPT can also take place over an extended range, suggesting that the process is likely general for all p-hydroxyaromatic ketones which opens up the possibility for designing photoswitchable processes based on this general phenomenon.     

Excited-state intramolecular proton transfer driven by conical intersection in hydroxychromones

Nonradiative decay pathways associated with vibronically coupled S₁(ππ*)–S₂(nπ*) potential energy surfaces of 3- and 5-hydroxychromones are investigated by employing the linear vibronic coupling approach. The presence of a conical intersection close to the Franck–Condon point is identified based on the critical examination of computed energetics and structural parameters of stationary points. We show that very minimal displacements of relevant atoms of intramolecular proton transfer geometry are adequate to drive the molecule toward the conical intersection nuclear configuration. The evolving wavepacket on S₁(ππ*) bifurcates at the conical intersection: a part of the wavepacket moves to S₂(nπ*) within a few femtoseconds while the other decays to S₁ minimum. Our findings indicate the possibility of forming the proton transfer tautomer product via S₂(nπ*), competing with the traditional pathway occurring on S₁(ππ*).     

Intra- and intermolecular proton transfer reactions in 2-phenyl substituted benzazoles

The present review describes the salient features of inter- and intramolecular proton transfer reactions of 2-(2′-aminophenyl)-, 2-(3′-aminophenyl)-, 2-(4′-aminophenyl)-, 2-(2′-hydroxyphenyl)-, 2-(3′-hydroxyphenyl)- and 2-(4′-hydroxyphenyl)-benzimidazoles, benzoxazoles and benzothiazoles. Fluorescence quantum yield of the phototautomer produced by the intramolecular hydrogen bonding decreases on going from benzimidazole to benzoxazole to benzothiazole. This indicates that the rate of internal conversion increases in the order of compounds as mentioned above. The biprotonic phototautomerism or the presence of intermolecular proton transfer has led to the formation of (i) nonfluorescent zwitterions in case of hydroxyphenyl derivatives and the ground state precursor of this species in neutral molecules, (ii) nonfluorescent monoanions from fluorescent monoanions and (iii) nonfluorescent monocations from monocations in case of aminophenyl derivatives. In the case of 2-(4′-aminophenyl)-substituted compounds, the first protonation has always led to the formation of two types of monocations; one by protonating the amino group and the other by protonating the tertiary nitrogen atom. The former is more stable in aqueous media and the latter in non-polar media.     

Theoretical studies of conjugation and substituent effect on intramolecular proton transfer in the ground and excited states

The ground- and excited-state intramolecular proton transfer (GSIPT and ESIPT) for 8-hydroxy-4H-naphthalen-1-one (HNA), 5-hydroxynaphthoquinone (HNQ), 1-hydroxy-anthraquione (HAQ), 7-hydroxy-1-indenone (7HIN), 5,8-dihydroxynaphthoquinone (DHNQ) and 4,9-dihydroxyperylene-3,10-quinone (DHP) are studied at B3LYP/6-31G(d,p) and TD B3LYP/6-31G(d,p) level. The calculated results show that the PES of GSIPT for HNA, HNQ and HAQ exhibit a single minimum in the enol zone, while for 7-HIN, DHNQ and DHP exhibit a double minimum and a high barrier between the two minima. The barrierless ESIPT for HNA is predicted, however, the PES of ESIPT for HNQ, HAQ, 7HIN, DHNQ and DHP exhibit a high barrier in the S₁ tautomerism.     

Excited-state intramolecular proton transfer in 2-(2′,6′-dihydroxyphenyl)benzoxazole: effect of dual hydrogen bonding on the optical properties

2-(2′,6′-Dihydroxyphenyl)benzoxazole (DHBO) has been synthesized by using palladium-catalyzed oxidative cyclization. The compound utilizes both O-H···N and O-H···O bonds to ensure a coplanar structure between the benzoxazole and phenol fragments. Optical comparison with the parent compound 2-(2′-hydroxyphenyl)benzoxazole (HBO) reveals that the dual hydrogen bonding in DHBO plays an essential role in raising the desirable keto emission for ESIPT and tuning the polarity sensitivity toward the molecular environment. DHBO also exhibits a higher quantum yield (?_fl = 0.108 in methanol) than HBO (?_fl = 0.0025) in the same solvent.     

Theoretical study of the mechanism of proton transfer in tautomeric systems: Alloxan

Semiempirical SCF-MO studies of tautomerism in alloxan preclude the possibility of direct proton transfer in the gas phase due to the strain in the four-centred transition state, in which the proton being transferred is forced to come close to the positively charged carbon atom at the opposite corner of the four-membered ring. However, in aqueous solution, the activation barrier reduces appreciably, not only due to reduction in strain, but also due to charge separation in the transition state, which is stabilized due to ionic resonance. The N-H bond is almost broken, while the O-H bond is only partially formed in the transition state. The other stabilizing effect in aqueous solution is due to bulk solvent dielectric effects, which stabilize the transition state to a greater extent due to its higher dipole moment. Although the transition states for proton transfer to the neighbouring oxygen atoms on either side have comparable energies, as the mechanisms of proton transfer leading to the formation of the 2-hydroxy and 4-hydroxy tautomers are similar, bulk solvent effects are larger in the latter due to the higher dipole moment of the transition state. The reason is the almost complete separation of the two entities, i.e. the alloxan anion and the hydronium ion in the latter case, indicating that in this case a dissociative mechanism of the kind encountered in acid-base equilibria is operating.     

Solvent-dependent photoacidity state of pyranine monitored by transient mid-infrared spectroscopy.

We investigate with femtosecond mid-infrared spectroscopy the vibrational-mode characteristics of the electronic states involved in the excited-state dynamics of pyranine (HPTS) that ultimately lead to efficient proton (deuteron) transfer in H2O (D2O). We also study the methoxy derivative of pyranine (MPTS), which is similar in electronic structure but does not have the photoacidity property. We compare the observed vibrational band patterns of MPTS and HPTS after electronic excitation in the solvents: deuterated dimethylsulfoxide, deuterated methanol and H2O/D2O, from which we conclude that for MPTS and HPTS photoacids the first excited singlet state appears to have charge-transfer (CT) properties in water within our time resolution (150 fs), whereas in aprotic dimethylsulfoxide the photoacid appears to be in a non-polar electronic excited state, and in methanol (less polar and less acidic than water) the behaviour is intermediate between these two extremes. For the fingerprint vibrations we do not observe dynamics on a time scale of a few picoseconds, and with our results obtained on the O-H stretching vibration we argue that the dynamic behaviour observed in previous UV/Vis pump-probe studies is likely to be related to solvation dynamics.     

枝状含联苯胺基片段的有机生色团的激发态分子间质子转移性质研究

通过Suzuki反应、胺醛缩合两步反应合成了开链的枝状的含联苯胺基片段的有机染料分子.利用核磁氢谱和碳谱、红外光谱、高分辨质谱和元素分析等手段表征了产物的分子结构,结合X-ray单晶结构分析、核磁氢谱分析、紫外-可见吸收光谱的系统研究发现,目标分子C1和C3存在分子间氢键作用.荧光发射光谱证实了只有目标分子C3能在非质子性溶剂中发生激发态分子间质子转移（Excited-state Intermolecular Proton Transfer,ESPT）反应,且随着溶液浓度的增大,C3发生ESPT反应的能力增强.在固态下同样可以发生ESPT反应.     

A ratiometric fluorescent probe for fluoride ion employing the excited-state intramolecular proton transfer

A ratiometric fluorescent probe 1 for fluoride ion was developed based on modulation of the excited-state intramolecular proton transfer (ESIPT) process of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) through the hydroxyl group protection/deprotection reaction. The probe 1 was readily prepared by the reaction of HPBI with tert-butyldimethylsilyl chloride (TBS-Cl) and shows only fluorescence emission maximum at 360 nm. Upon treatment with fluoride in aqueous DMF solution, the TBS protective group of probe 1 was removed readily and ESIPT of the probe was switched on, which resulted in a decrease of the emission band at 360 nm and an increase of a new fluorescence peak around 454 nm. The fluorescent intensity ratio at 454 and 360 nm (I₄₅₄/I₃₆₀) increases linearly with fluoride ion concentration in the range 0.3-8.0 μmol L⁻¹ and the detection limit is 0.19 μmol L⁻¹. The proposed probe shows excellent selectivity toward fluoride ion over other common anions. The method has been successfully applied to the fluoride determination in toothpaste and tap water samples.     

Monitoring intracellular pH fluctuation with an excited-state intramolecular proton transfer-based ratiometric fluorescent sensor

Intracellular pH is a key parameter related to various biological and pathological processes. In this study, a ratiometric pH fluorescent sensor ABTT was developed harnessing the amino-type excited-state intramolecular proton transfer (ESIPT) process. Relying on whether the ESIPT proceeds normally or not, ABTT exhibited the yellow fluorescence in acidic media, or cyan fluorescence in basic condition. According to the variation, ABTT behaved as a promising sensor which possessed fast and reversible response to pH change without interference from the biological substances, and exported a steady ratiometric signal (I₄₇₈/I₅₄₆). Moreover, due to the ESIPT effect, large Stokes shift and high quantum yield were also exhibited in ABTT. Furthermore, ABTT was applied for monitoring the pH changes in living cells and visualizing the pH fluctuations under oxidative stress successfully. These results elucidated great potential of ABTT in understanding pH-dependent physiological and pathological processes.     

Energy barriers of proton transfer reactions between amino acid side chain analogs and water from ab initio calculations

Proton transfer reactions were studied in all titratable pairs of amino acid side chains where, under physiologically reasonable conditions, one amino acid may function as a donor and the other one as an acceptor. Energy barriers for shifting the proton from donor to acceptor atom were calculated by electronic structure methods at the MP2/6-31++G(d,p) level, and the well-known double-well potentials were characterized. The energy difference between both minima can be expressed by a parabola using as argument the donor-acceptor distance R(DA). In this work, the fit parameters of the quadratic expression are determined for each donor-acceptor pair. Moreover, it was found previously that the energy barriers of the reactions can be expressed by an analytical expression depending on the distance between donor and acceptor and the energy difference between donor and acceptor bound states. The validity of this approach is supported by the extensive new data set. This new parameterization of proton transfer barriers between titratable amino acid side chains allows us to very efficiently estimate proton transfer probabilities in molecular modelling studies or during classical molecular dynamics simulation of biomolecular systems.     

Exploration on regulating factors for proton transfer along hydrogen-bonded water chains.

Proton transfer along a single-file hydrogen-bonded water chain is elucidated with a special emphasis on the investigation of chain length, side water, and solvent effects, as well as the temperature and pressure dependences. The number of water molecules in the chain varies from one to nine. The proton can be transported to the acceptor fragment through the single-file hydrogen-bonded water wire which contains at most five water molecules. If the number of water molecule is more than five, the proton is trapped by the chain in the hydroxyl-centered H(7)O(3) (+) state. The farthest water molecule involved in the formation of H(7)O(3) (+) is the fifth one away from the donor fragment. These phenomena reappear in the molecular dynamics simulations. The energy of the system is reduced along with the proton conduction. The proton transfer mechanism can be altered by excess proton. The augmentation of the solvent dielectric constant weakens the stability of the system, but favors the proton transfer. NMR spin-spin coupling constants can be used as a criterion in judging whether the proton is transferred or not. The enhancement of temperature increases the thermal motion of the molecule, augments the internal energy of the system, and favors the proton transfer. The lengthening of the water wire increases the entropy of the system, concomitantly, the temperature dependence of the Gibbs free energy increases. The most favorable condition for the proton transfer along the H-bonded water wire is the four-water contained chain with side water attached near to the acceptor fragment in polar solvent under higher temperature.     

Direct observation of the nuclear motion during ultrafast intramolecular proton transfer

The skeletal motions contributing to the reaction path of the ultrafast excited state intramolecular proton transfer (ESIPT) are determined directly from time resolved measurements. We investigate the ESIPT in the compounds 2-(2′-hydroxyphenyl)benzothiazole, 2-(2′-hydroxyphenyl)benzoxazole and ortho-hydroxybenzaldehyde by UV–visible pump-probe spectroscopy with 30 fs resolution. The proton transfer is observed in real time and a characteristic ‘ringing’ of the molecule in a small number of vibrational modes is found after the reaction. The results show that a bending motion of the molecular skeleton reduces the proton donor–acceptor distance and an electronic configuration change occurs at a sufficient contraction leading to the bonds of the product conformer. The process evolves as a ballistic wavepacket propagation on an adiabatic potential energy surface. The proton is shifted by the skeletal motions from the donor to the acceptor site and tunneling has not to be considered.     

Explicit proton transfer in classical molecular dynamics simulations

We present Hydrogen Dynamics (HYDYN), a method that allows explicit proton transfer in classical force field molecular dynamics simulations at thermodynamic equilibrium. HYDYN reproduces the characteristic properties of the excess proton in water, from the special pair dance, to the continuous fluctuation between the limiting Eigen and Zundel complexes, and the water reorientation beyond the first solvation layer. Advantages of HYDYN with respect to existing methods are computational efficiency, microscopic reversibility, and easy parameterization for any force field. © 2014 Wiley Periodicals, Inc.     

细菌紫红质中氢键相互作用和质子转移机理的分子模拟

用CHARMM程序以细菌紫红质1R84晶体为模型, 模拟了在等温定容条件下细菌紫红质在1 ps过程中的变化, 分析了与质子传递相关的ASP85, ASP212和水分子与视黄醛间氢键的结构变化情况. 考虑到氨基酸残基和席夫碱质子的不同距离, 考察了EC和PC两种结构的变化情况, 探讨了紫红质中质子传递的可能途径. 模拟结果表明1R84中可能的质子连续传递的机理是质子由席夫碱向水传递, 再由水向ASP85传递. 发现Asp212在模拟过程中保持EC结构, 这样可能更有利于顺序质子传递.