Water-catalyzed excited-state double proton transfer in 3-cyano-7-azaindole: the resolution of the proton-transfer mechanism for 7-azaindoles in pure water

The effect of the number of conjugated C=C bonds on the ESIPT and ICT reactions of SNCN derivatives

The electronic structure of the molecule is significantly influenced by the number of conjugated C=C bonds. In this work, the influence of the conjugated C=C bonds of the SNCN derivatives on the excited state intramolecular proton transfer (ESIPT) and intramolecular charge transfer (ICT) properties are studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The calculation level is proved to be reasonable by calculating electronic spectra. The hydrogen bond parameters, infrared vibrational frequency (IR), reduction density gradient (RDG) isosurface, topological analysis and potential energy curves of SNCN derivatives in ground state (S0) and the first excited state (S1) are analyzed. According to theoretical research results, ESIPT reaction has a higher likelihood of occurring in the S1 state. Moreover, the ESIPT reaction becomes more challenging to occur with the number of conjugated C=C bonds rising. Finally, the analyses of the frontier molecular orbitals (FMOs), dipole moment and charge transfer transition confirm that the ICT effect is aided by the increased number of conjugated C=C bonds. This work indicates that the number of conjugated C=C bonds can regulate the ESIPT and ICT processes, which provides guidance for the study of fluorescent groups with similar characteristics.

Long-range hydrogen-bond relay catalyses the excited-state proton transfer reaction

Solvent (e.g., water)-catalyzed proton transfer (SCPT) via the relay of hydrogen (H)-bonds plays a key role in proton migration. In this study, a new class of 1H-pyrrolo[3,2-g]quinolines (PyrQs) and their derivatives were synthesized, with sufficient separation of the pyrrolic proton donating and pyridinic proton accepting sites to probe excited-state SCPT. There was dual fluorescence for all PyrQs in methanol, i.e., normal (PyrQ) and tautomer 8H-pyrrolo[3,2-g]quinoline (8H-PyrQ) emissions. The fluorescence dynamics unveiled a precursor (PyrQ) and successor (8H-PyrQ) relationship and the correlation of an increasing overall excited-state SCPT rate (kSCPT) upon increasing the N(8)-site basicity. kSCPT can be expressed by the coupling reaction kSCPT = Keq × kPT, where kPT denotes the intrinsic proton tunneling rate in the relay and Keq denotes the pre-equilibrium between randomly and cyclically H-bonded solvated PyrQs. Molecular dynamics (MD) simulation defined the cyclic PyrQs and analyzed the H-bond and molecular arrangement over time, which showed the cyclic PyrQs incorporating ≧3 methanol molecules. These cyclic H-bonded PyrQs are endowed with a relay-like proton transfer rate, kPT. MD simulation estimated an upper-limited Keq value of 0.02-0.03 for all studied PyrQs. When there was little change in Keq, the distinct kSCPT values for PyrQs were at different kPT values, which increased as the N(8) basicity increased, which was induced by the C(3)-substituent. kSCPT was subject to a deuterium isotope effect, where the kSCPT of 1.35 × 1010 s-1 for PyrQ-D in CH3OD was 1.68 times slower than that (2.27 × 1010 s-1) of PyrQ in CH3OH. MD simulation provided a similar Keq for PyrQ and PyrQ-D, leading to different proton tunneling rates (kPT) between PyrQ and PyrQ-D.

A Solvent-Mediated Excited-State Intermolecular Proton Transfer Fluorescent Probe for Fe3+ Sensing and Cell Imaging

Constructing excited-state intermolecular proton transfer (ESIPT-e) fluorophores represents significant challenges due to the harsh requirement of bearing a proton donor-acceptor (D-A) system and their matching proton donating-accepting ability in the same molecule. Herein, we synthesized a new-type ESIPT-e fluorophor (2-APC) using the "four-component one-pot" reaction. By the installing of a cyano-group on pyridine scaffold, the proton donating ability of -NH2 was greatly enhanced, enabling 2-APC to undergo ESIPT-e process. Surprisingly, 2-APC exhibited dual-emissions in protic solvents ethanol and normal fluorescence in aprotic solvents, which is vastly different from that of conventional ESIPT-a dyes. The ESIPT emission can be obviously suppressed by Fe3+ due to the coordination reaction of Fe3+ with the A-D system in 2-APC. From this basis, a highly sensitive and selective method was established using 2-APC as a fluorescent probe, which offers the sensitive detection of Fe3+ ranging from 0 to 13 μM with the detection limit of 7.5 nM. The recovery study of spiked Fe3+ measured by the probe showed satisfactory results (97.2103.4%) with the reasonable RSD ranging from 3.1 to 3.8%. Moreover, 2-APC can also exhibit aggregation-induced effect in poor solvent or solid-state, eliciting strong red fluorescence. 2-APC was also applied to cell-imaging, exhibiting good cell-permeability, biocompatibility and color rendering. This multi-mode emission of 2-APC is significant departure from that of conventional extended p-conjugated systems and ESIPT dyes based on a flat and rigid molecular design. The "one-pot synthesis" strategy for the construction of ESIPT molecules pioneered a new route to achieve tricolor-emissive fluorophores.

Double Proton Transfer during a Novel Tertiary α-Ketol Rearrangement in Ketol-Acid Reductoisomerase: A Water-Mediated, Metal-Catalyzed, Base-Induced Mechanism

(KARI) catalyzes the conversion of (S)-2-acetolactate or (S)-2-aceto-2-hydroxybutyrate to 2,3-dihydroxy-3-alkylbutyrate, the second step in the biosynthesis of branched chain amino acids (BCAAs). Because the BCAA biosynthetic pathway is present in bacteria, plants, and fungi, but absent in animals, it is an excellent target for the development of new-generation antibiotics and herbicides. Nevertheless, the mechanism of the KARI-catalyzed reaction has not yet been fully solved. In this study, we used iterative molecular dynamics (MD) flexible fitting-Rosetta techniques to optimize the three-dimensional solution structure of archaea KARI from Sulfolobus solfataricus (Sso-KARI) determined from cryo-electron microscopy. On the basis of the structure of the Sso-KARI/2Mg²⁺/NADH/(S)-2-acetolactate complex, we deciphered the catalytic mechanism of the KARI-mediated reaction through hybrid quantum mechanics/molecular mechanics MD simulations in conjunction with umbrella sampling. With an activation energy of only 6.06 kcal/mol, a water-mediated, metal-catalyzed, base-induced (WMMCBI) mechanism was preferred for deprotonation of the tertiary OH group of (S)-2-acetolactate in Sso-KARI. The WMMCBI mechanism for double proton transfer occurred within a proton wire route with two steps involving the formation of hydroxide: (i) Glu233 served as a general base to deprotonate the Mg²⁺-bound water, forming a hydroxide-coordinated Mg²⁺ ion; (ii) this hydroxide ion acted as a strong base that rapidly deprotonated the ternary OH group of the substrate. In contrast, the direct deprotonation of the substrate by Glu233 was kinetically unfavorable. This mechanism suggests a novel approach for designing catalysts for deprotonation and provides clues for the development of new-generation antibiotics and herbicides.

3-Hydroxyflavone derivatives: promising scaffolds for fluorescent imaging in cells

As a typical class of excited-state intramolecular proton transfer (ESIPT) molecules, 3-hydroxyflavone derivatives (3HF, also known as flavonols) have received much attention recently. Thereinto, the role of hydrophobic microenvironment is significant importance in promoting the process and effects of ESIPT, which can be regulated by the solvents, the existence of metal ions and proteins rich with α-helix structures or the advanced DNA structures. Considering that plenty of biological macromolecules offer cellular hydrophobic microenvironment, enhancing the ESIPT effects and resulting in dual emission, 3HF could be a promising scaffold for the development of fluorescent imaging in cells. Furthermore, as the widespread occurance of compounds with biological activity in plants, 3HF derivatives are much more secure to be cellular diagnosis and treatment integrated fluorescent probes. In this review, multiple regulatory strategies for the fluorescence emission of 3HF derivatives have been collectively and comprehensively analyzed, including the solvent effects, metal chelation, interaction with proteins or DNAs, which would be beneficial for ESIPT-promoting or ESIPT-blocking processes and then enhance or control the fluorescence emission of 3HF effectively. We expect that this review would provide a new perspective to develop novel 3HF-based fluorescent sensors for imaging in cells and plants.

Could Chemical Reaction at the Molecular Level Show Distinction between Two Liquid-Water States? Study of the Excited-State Water-Catalyzed Proton Transfer Reaction Provides a Clue

The two liquid-water states, which lead to some anomalies when temperature crosses over 50 ± 10 °C at the atmospheric pressure, have been continuously catching popular attention. In this study, using the excited-state proton transfer (ESPT) catalyzed by water molecules as a prototypical reaction, we demonstrate that the kinetics of ESPT indeed is influenced by the two liquid-water states. In the water-catalyzed ESPT of 3-cyano-7-azaindole (3CAI), a repetitive and comprehensive temperature-dependent study of ESPT in H₂O from 0 to 90 °C shows anomalous behavior. The plot of the logarithm of ESPT rate constant as a function of inverse of absolute temperature deviates from a straight line. The convex-Arrhenius behavior manifests the activation free energy for water-assisted ESPT being dependent on temperature and hence the liquid water structure. To simplify the discussion, the plot is well fitted by using two straight lines that are crossed over in the vicinity of 40 °C. The free energy difference between water-solvated 3CAI and the 1:1 H₂O:3CAI complex is deduced to be 2.29 ± 0.04 and 1.96 ± 0.04 kcal·mol^-1 in the regions of 0-40 and 40-90 °C water, respectively, which also results in different frequency factors, i.e., the proton transfer/tunneling rates of (5.83 ± 0.36) × 10¹⁰ and (3.48 ± 0.27) × 10¹⁰ s^-1, respectively. In a qualitative manner, the results are then rationalized by the different types of H-bonding configuration as proposed for two liquid-water phases, rendering experimental evidence to support the different water phases in ambient temperatures at 1 bar.

Hydrogen-Bond Free Energy of Local Biological Water

Here, we propose an experimental methodology based on femtosecond-resolved fluorescence spectroscopy to measure the hydrogen (H)-bond free energy of water at protein surfaces under isothermal conditions. A demonstration was conducted by installing a non-canonical isostere of tryptophan (7-azatryptophan) at the surface of a coiled-coil protein to exploit the photoinduced proton transfer of its chromophoric moiety, 7-azaindole. The H-bond free energy of this biological water was evaluated by comparing the rates of proton transfer, sensitive to the hydration environment, at the protein surface and in bulk water, and it was found to be higher than that of bulk water by 0.4 kcal mol^-1 . The free-energy difference is dominated by the entropic cost in the H-bond network among water molecules at the hydrophilic and charged protein surface. Our study opens a door to accessing the energetics and dynamics of local biological water to give insight into its roles in protein structure and function.

Is the origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals single or double proton transfer?

The origin of green fluorescence in unsymmetrical four-ring bent-core liquid crystals (BCLCs) is not understood clearly.

Room-Temperature Dual Fluorescence of a Locked Green Fluorescent Protein Chromophore Analogue

A structurally locked green fluorescent protein (GFP) chromophore with a phenyl group at C(2) of the imidazolone has been synthesized. Rotation around the exocyclic double bond is hindered, resulting in room-temperature fluorescence. The quantum yield in water is 500 times greater than that of unlocked analogues. Unlike the methyl-substituted analogue, the phenyl analogue exhibits a dual emission (cyan and red) that can be used for ultrasensitive ratiometric measurements and fluorescence microscopy. To explain this dual emission, DFT calculations were carried out along with fluorescence upconversion experiments. The Z-isomer was found to be emissive, while the origin of the dual emission was dependent on the phenyl group in the Z-isomer, which stabilizes the Franck-Condon state, resulting in a cyan fluorescence, while the zwitterionic tautomer fluoresces red. These results bring important new insights into the photophysics of the GFP chromophore and provide a new scaffold capable of dual emission with utility in biotechnology.

Halogen substituent effect on the water-assisted excited-state tautomerization of 2, 7-diazaindole-H2O complex in aqueous solution: A theoretical study

We studied the ESPT process in the 2-DAI-H₂O complex theoretically for the first time, and compared the kinetics of 2-DAI-H₂O with those features of 7-DAI-H₂O. The substituted effect on the dynamics of excited-state double proton transfer in 2-DAI-H₂O and 7-DAI-H₂O clusters in water were also investigated at the TD-M06-2X/6-311+G(d, p) level. In this work, 2,7-DAI-H₂O is also expressed as 2-DAI-H₂O and 7-DAI-H₂O, in which correspond to different ESPT reactions and generate two tautomers (N₂H form and N₇H form). In both the 2-DAI-H₂O and 7-DAI-H₂O complexes, ESPT processes happened in a concertedly but asynchronously protolysis pathway. The ESPT process preferred to occur in the 7-DAI-H₂O complex due to its lower barrier height. For the 3-X-2-DAI-H₂O and 3-X-7-DAI-H₂O (X = H, F, Cl, Br) complexes, the replacement of halogen atom did not influence the ESPT mechanism. However, the replacement of halogen atom changed the structural parameters evidently, reduced the barrier height up to 4-5 kcal/mol, and enlarged the asynchronicity of ESPT apparently. ∆(R₁+R₂) values in the 3X-2-DAI-H₂O and 3X-7-DAI-H₂O complexes have linear correlation to the ZPE-corrected ESPT barrier height linearly.

Effect of different alkyl groups on excited-state tautomerization of 7AI-azaindole-H2O: A theoretical study

The effect of substituted alkyl groups at different substituted position on the first excited-state proton transfer of nR7AI-H₂O (n = 2-6; R = CH₃, C₂H₅, CF₃) complexes were theoretically investigated at the TD-M06-2X/6-31 + G(d, p) level. Here n value denoted the substituted position C_n of R group. The replacement of alkyl R group had no effect on the features of HOMO and LUMO, but influenced the S₀ → S₁ adiabatic transition energies of the nR7AI-H₂O complex. Through computation, we found that the double proton transfer took place in a concerted but asynchronous protolysis pattern regardless of substituted group R and substituted position in the nR7AI-H₂O complex. The vibrational-mode specific nature of ESPT was verified. The alkyl group R changed the geometrical parameters of TS, and resulted in enlarging/narrowing the asynchronousity of ESPT. The ESPT barrier height was also affected by the substituted group and position.

Theoretical research on excited-state intramolecular proton coupled charge transfer modulated by molecular structure

At the TD-B3LYP/TZVP/IEFPCM theory level, we have theoretically studied the excited-state intramolecular proton coupled charge transfer (ESIPCCT) process for both 4′-N,N-diethylamino-3-hydroxyflavone (3HFN) and 2-{[2-(2-hydroxyphenyl)benzo[d]oxazol-6-yl]methylene}malononitrile (diCN-HBO) molecules. Our calculated hydrogen bond lengths and angles sufficiently confirm that the intramolecular hydrogen bonds O₁–H₁⋯O₂ and O₁–H₁⋯N₁ formed at the S₀ states of 3HFN and diCN-HBO should be significantly strengthened in the S₁ state, which is further supported by the results obtained based on the analyses of infrared spectra shifts, molecular orbitals and charge density differences maps. The significant strengthening of intramolecular hydrogen bonds O₁–H₁⋯O₂ and O₁–H₁⋯N₁ upon photoexcitation should facilitate the ESIPCCT process of the two title molecules. The scanned potential energy curves and confirmed excited-state transition states for both 3HFN and diCN-HBO show that the proton can be easily transferred from O₁ to O₂ (N₁ for diCN-HBO) through the strengthened intramolecular hydrogen bonds upon photoexcitation to the S₁ state.

Charge transfer from O₁ to O₂ of 3HFN results in proton H₁ transfer from O₁ to O₂ in S₁ state, while small energy barrier facilitates proton H₁ transfer from O₁ to N₁ in S₁ state of diCN-HBO, which results in charge transfer from O₁ to di-cyano.

Combined effect of hydrogen bonding interactions and freezing of rotameric equilibrium on the enhancement of photostability

Rigid structure and hydrogen-bonding interactions provide a higher photostability of organic chromophores.

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).

Optically Triggered Stepwise Double-Proton Transfer in an Intramolecular Proton Relay: A Case Study of 1,8-Dihydroxy-2-naphthaldehyde

1,8-Dihydroxy-2-naphthaldehyde (DHNA), having doubly intramolecular hydrogen bonds, was strategically designed and synthesized in an aim to probe a long-standing fundamental issue regarding synchronous versus asynchronous double-proton transfer in the excited state. In cyclohexane, DHNA shows the lowest lying S0 →S1 (π-π*) absorption at ∼400 nm. Upon excitation, two large Stokes shifted emission bands maximized at 520 and 650 nm are resolved, which are ascribed to the tautomer emission resulting from the first and second proton-transfer products, denoted by TA* and TB*, respectively. The first proton transfer (DHNA* → TA*) is ultrafast (< system response of 150 fs), whereas the second proton transfer is reversible, for which the rates of forward (TA* → TB*) and backward (TA* ← TB*) proton transfer were determined to be (1.7 ps)(-1) and (3.6 ps)(-1), respectively. The fast equilibrium leads to identical population lifetimes of ∼54 ps for both TA* and TB* tautomers. Similar excited-state double-proton transfer takes place for DHNA in a single crystal, resulting in TA* (560 nm) and TB* (650 nm) dual-tautomer emission. A comprehensive 2D plot of reaction potential energy surface further proves that the sequential two-step proton motion is along the minimum energetic pathway firmly supporting the experimental results. Using DHNA as a paradigm, we thus demonstrate unambiguously a stepwise, proton-relay type of intramolecular double-proton transfer reaction in the excited state, which should gain fundamental understanding of the multiple proton transfer reactions.

New insights into the solvent-assisted excited-state double proton transfer of 2-(1H-pyrazol-5-yl)pyridine with alcoholic partners: a TDDFT investigation

Excited-state double proton transfer (ESDPT) in the hydrogen-bonded 2-(1H-pyrazol-5-yl)pyridine with propyl alcoholic partner (PPP) was theoretically investigated by time-dependent density functional theory (TDDFT) method. Great changes have taken place for the calculated geometric structures, the electron density features and vibrational spectrum of PPP system in S0 and S1 state. Our results have demonstrated that ESDPT reaction happens within the system upon photoexcitation. We also found that the ESDPT process is facilitated by the electronically excited state intermolecular hydrogen bond strengthening. Particularly, after the photoexcitation from HOMO(π) to the LUMO(π(∗)), the rearrangement of electronic density distribution of frontier molecular orbitals (MOs) on pyridine and the pyrazol moieties exhibits a very important positive factor for the ESDPT. Furthermore, by the investigation of the stretching vibrations of NH and OH groups, the infrared (IR) spectroscopic results provide us not only a theoretical evidence of ESDPT, but also a considerable clue to characterize the nature of intermolecular reaction. In addition, efforts have also been devoted towards calculating the absorption peak, which shows good consistency with the experimental result of the studied system.

A new insight into the chemistry of iridium(iii) complexes bearing phenyl phenylphosphonite cyclometalate and chelating pyridyl triazolate: the excited-state proton transfer tautomerism via an inter-ligand PO–H⋯N hydrogen bond

Ir(pdpit)(pppo)(bptz) complex (3) reveals a PO–H–N inter- ligand H-bond from which proton transfer takes place.

Water-catalyzed excited-state proton-transfer reactions in 7-azaindole and its analogues

The mechanism of the water-catalyzed excited-state proton-transfer (ESPT) reaction for 7-azaindole (7AI) has long been investigated, but there are some controversial viewpoints. Recently, owing to the superiority of sensing biowaters in proteins by a 7AI analogue, 2,7-diazatryptophan, it is timely to reinvestigate water-catalyzed ESPT in 7AI and its analogues in an attempt to unify the mechanism. Herein, a series of 7AI analogues and their methylated derivatives were synthesized to carry out a systematic study on pKa, pKa*, and the associated fluorescence spectroscopy and dynamics. The results conclude that all 7AI derivatives undergo water-catalyzed ESPT in neutral water. However, for those derivatives with -H (7AI) and a electron-donating substituent at C(3), they follow water-catalyzed ESPT to form an excited N(7)-H proton-transfer tautomer, T*. T* is rapidly protonated to generate an excited cationic (TC*) species. TC* then undergoes a fast deactivation to the N(1)-H normal species in the ground state. Conversely, protonation in T* is prohibited for those derivatives with an electron-withdrawing group at the C(2) or C(3) or with the C(2) atom replaced by an electron-withdrawing nitrogen atom (N(2) in, e.g., 2,7-diazatryptophan), giving a prominent green T* emission. Additional support is given by the synthesis of the corresponding N(7)-CH3 tautomer species, for which pKa* of the cationic form, that is, the N(7)-CH3N(1)-H(+) species, is measured to be much greater than 7.0 for those with electron-donating C(3) substituents, whereas it is lower than 7.0 upon anchoring electron-withdrawing groups. For 7AI, the previously missing T* emission is clearly resolved with a peak wavelength at 530 nm in the pH interval of 13.0-14.3 (H- 14.2).

Probing water micro-solvation in proteins by water catalysed proton-transfer tautomerism

Scientists have made tremendous efforts to gain understanding of the water molecules in proteins via indirect measurements such as molecular dynamic simulation and/or probing the polarity of the local environment. Here we present a tryptophan analogue that exhibits remarkable water catalysed proton-transfer properties. The resulting multiple emissions provide unique fingerprints that can be exploited for direct sensing of a site-specific water environment in a protein without disrupting its native structure. Replacing tryptophan with the newly developed tryptophan analogue we sense different water environments surrounding the five tryptophans in human thromboxane A₂ synthase. This development may lead to future research to probe how water molecules affect the folding, structures and activities of proteins.

Excited-state proton coupled charge transfer modulated by molecular structure and media polarization

Charge and proton transfer reactions in the excited states of organic dyes can be coupled in many different ways. Despite the complementarity of charges, they can occur on different time scales and in different directions of the molecular framework. In certain cases, excited-state equilibrium can be established between the charge-transfer and proton-transfer species. The interplay of these reactions can be modulated and even reversed by variations in dye molecular structures and changes of the surrounding media. With knowledge of the mechanisms of these processes, desired rates and directions can be achieved, and thus the multiple emission spectral features can be harnessed. These features have found versatile applications in a number of cutting-edge technological areas, particularly in fluorescence sensing and imaging.

Recent experimental advances on excited-state intramolecular proton coupled electron transfer reaction

Proton-coupled electron transfer reactions form the basis of many important chemical processes including much of the energy conversion that occurs within living cells. However, much of the physical chemistry that underlies these reaction mechanisms remains poorly understood. In this Account, we report on recent progress in the understanding of excited-state intramolecular proton-coupled electron transfer (PCET) reactions. The strategic design and synthesis of various types of PCET molecules, along with steady-state and femtosecond time-resolved spectroscopy, have uncovered the mechanisms of several excited-state PCET reactions in solution. These experimental advancements correlate well with current theoretical models, in which the proton has quantum motion with a high probability of tunneling. In addition, the rate of proton transfer is commonly incorporated within the rate of rearrangement of solvent molecules. As a result, the reaction activation free energy is essentially governed by the solvent reorganization because the charge redistribution is considered based on a solvent polarity-induced barrier instead of the height of the proton migration barrier. In accord with this theoretical basis, we can rationalize the observation that the proton transfer for many excited-state PCET systems occurs during the solvent relaxation time scale of 1-10 ps: the highly exergonic reaction takes place before the system reaches its equilibrium polarization. Also, we have used various derivatives of proton transfer molecules, especially those of 3-hydroxyflavone to clearly demonstrate how researchers can tune the dynamics of excited-state PCET through changes in the magnitude or direction of the dipole vector within the reaction. Subsequently, using 2-(2'-hydroxyphenyl)benzoxazole as the parent model, we then report on methods for the development of an ideal system for probing PCET reaction. Because future biomedical applications of such systems will likely occur in aqueous environments, we discuss various 7-azaindole analogues, for which proton transfer requires the assistance of protic solvent molecules. These results provide a unique contrast to the ubiquitous studies on the dynamic solvent effects of PCET molecules that undergo intrinsic intramolecular proton motion.

Direct ab initio dynamics calculations for rates and the kinetic isotope effects of multiproton transfer in ClONO2 + HCl --> HNO3 + Cl2 reactions with water clusters: breakdown of the rule of the geometric mean

We performed high-level quantum mechanical calculations and direct ab initio reaction dynamics calculations for multiple proton transfers in ClONO(2)+HCl-->HNO(3)+Cl(2) with water clusters containing one to two water molecules, which can be used as a model of the reactions occurring on ice surface in stratospheric clouds. The energy barriers of these reactions depend on the number of water molecules involved. Two and three protons in these reactions with one and two water molecules, respectively, were transferred concertedly and asynchronously. The potential energy barrier at the MP2/6-311++(3df,3pd)//MP2/6-31G(d,p) level was 4.8 kcal/mol for the triple proton transfer involving two water molecules with a rate constant of 1.6x10(3) s(-1) at 197 K. The potential energy curve near the saddle points was very flat and the tunneling effect on the proton transfer was negligible. The primary HH/DH kinetic isotope effect for the double proton transfer involving one water molecule was lower than unity due to the enhanced force constant at the transition state. The rule of the geometric mean for the concerted proton transfer does not hold in these reactions because the zero-point energy changes of each proton in flight at the transition state are not the same in the highly asynchronous processes.

Synthesis of 7-azaserotonin: its photophysical properties associated with excited state proton transfer reaction

We report the synthesis of 3-(2-aminoethyl)-5-ol-1H-pyrrolo[2,3-b]pyridine (7-azaserotonin), which may potentially serve as an agonist or antagonist of serotonin receptors. In alcohols, the solvent (e.g., ethanol) catalyzed proton-transfer reaction takes place for 7-azaserotonin in the excited state, resulting in dual emission. Conversely, excited-state deprotonation takes place in neutral aqueous solution. The unique excitation behavior makes 7-azaserotonin versatile as a potential bioprobe.

7-Azamelatonin: Efficient Synthetic Routes, Excited-State Double Proton Transfer Properties and Biomedical Implications

On the basis of a seven-step synthetic route, the total synthesis of 7-azamelatonin, an analogue of melatonin, has been achieved with an overall yield of approximately 9.2%. In aqueous solution, 7-azamelatonin exhibits a unique excited-state double proton transfer (ESDPT) property, resulting in dual emission bands (405 and 560 nm). The ESDPT property makes 7-azamelatonin superb as a potential molecular probe for future bioapplication and for pharmacological research.

Probing the Behavior of Confined Water by Proton-Transfer Reactions

The picosecond dynamics of a bifunctional and H-bonding molecule, 7-hydroxyquinoline (7HQ), has been studied in a reverse micelle with increasing water content. The fluorescence kinetics has a complex behavior as the water content is changed. All reactions are irreversible, and a two-step mechanism is invoked to explain the observations. H2O/D2O exchange and excitation energy effects show that the second step has a higher barrier and that the corresponding reaction occurs through tunneling. The results clearly indicate two regimes of water nanopool behavior switching at W0 approximately 5 (W0 = [water]/[surfactant]). Water collective dynamics explains these observations. The lower fluidity of confined water within the reverse micelle with respect to normal bulk water alters the related H-bond network dynamics and therefore is responsible for the slower proton-transfer processes.

Excited-State Double Proton Transfer of 7-Azaindole in Water Nanopools

The excited-state proton-transfer dynamics of 7-azaindole occurring in the water nanopools of reverse micelles has been investigated by measuring time-resolved fluorescence spectra and kinetics, as well as static absorption and emission spectra, with varying water content and isotope. 7-Azaindole molecules are found to exist in the bound-water regions of reverse micelles. The rate constant and the kinetic isotope effect of proton transfer are smaller than those in bulk water although both increase with the size of the water nanopool. The retardation of proton transfer in the bound regions is attributed to the increased free energy of prerequisite solvation to form a cyclically H-bonded 1:1 7-azaindole/water complex.

Proton Transfer of Excited 7-Azaindole in Reverse-Micellar Methanol Nanopools: Even Faster than in Bulk Methanol

The methanol-catalyzed double-proton transfer of photoexcited 7-azaindole in the free cores of solvation-restricted reverse micelles takes place on the time scale of 90 ps, even shorter than in bulk methanol. This anomalous rate increase with a large kinetic isotope effect of 5 experimentally proves the widely discussed two-step model for the overall reaction of solvent-mediated proton transfer. On the other hand, the molecules in the bound layers and in the headgroup layers relax in 900 and 6000 ps, respectively, without going through proton transfer. The tautomerization and the relaxation of excited 7-azaindole can be exploited to probe the nanopools of methanol reverse micelles.

Excited-State Tautomerization Dynamics of 7-Hydroxyquinoline in β-Cyclodextrin

The excited-state tautomerization dynamics of 7-hydroxyquinoline encapsulated in beta-cyclodextrin is compared with that in pure water by measuring isotope-dependent fluorescence kinetics as well as absorption and emission spectra. The normal species tautomerizes stepwise via forming anionic intermediate species in both systems. However, the enol-deprotonation time (40 ps in water) becomes as large as 170 ps whereas the imine-protonation time of the anionic intermediate (160 ps in water) becomes as short as 85 ps in beta-cyclodextrin. The slow formation and the fast decay of the anionic species are attributed to the unstability of the charged species in hydrophobic cages. Encapsulation can be utilized to enhance fluorescence enormously and to accelerate selective reactions by retarding other processes.