Reorganization Of Hydrogen Bonds Research Articles

Terahertz spectroscopic study of the dehydration process and hydrogen bonding network of DL-glutamic acid and its monohydrate

The dehydration process involves the dissociation and reorganization of hydrogen bonds, and for the characterization of the dehydration process of amino acid-based hydrates, the analysis of their molecular vibrational modes and hydrogen bonds is still challenging nowadays. In this paper, the dynamic process of the dehydration of DL-glutamic acid monohydrate was monitored using terahertz spectroscopy. Quantitative simulations were performed on DL-glutamic acid and its monohydrate, with vibrational mode assignments and hydrogen bonding network analysis based on Density Functional Theory (DFT) for terahertz absorption peaks. The results show that the hydrogen bond stretching vibration is intermixed with atomic vibration. Specifically, the intermolecular hydrogen bond O4-H9···O2 of DL-glutamic acid exhibits a stronger vibration at 2.43 THz. Furthermore, the intramolecular hydrogen bond O5-H11···O4 in DL-glutamic acid monohydrate shows significant changes in bond length and bond angle at 1.85 THz. These findings validate the effectiveness of terahertz spectroscopy in monitoring the transformation process of the hydrate and the analysis of the hydrogen bond.

Interaction of pepper numbing substances with myofibrillar proteins and numbness perception under thermal conditions: A structural mechanism analysis

This study examined the interaction between myofibrillar proteins (MPs) and the numbing substance hydroxy-α-sanshool (α-SOH) in a thermal environment, and provided an explanation of the numbness perception mechanism through muti-spectroscopic and molecular dynamics simulation methodology. Results showed that addition of α-SOH could reduce the particle size and molecular weight of MPs, accompanied by changes in the tertiary and secondary structure, causing the α-helix of MPs transitioned to β-sheet and β-turn due to the reorganization of hydrogen bonds. After a moderate heating (60 or 70 °C), MPs could form the stable complexes with α-SOH that were associated with attachment sites and protein wrapping. The thermal process might convert a portion of α-SOH’ into hydroxy-β-sanshool’ (β-SOH’). When docking with the sensory receptor TRPV1, the RMSD, RMSF and binding free energy all showed that β-SOH’ demonstrated a low affinity, thereby reducing the numbing perception. These findings can provide a theoretical foundation for the advanced processing of numbing meat products.

Hydrogen Bond Reorganization‐Enabled Dielectric Pulsing Effects in Polar Polymers

AbstractPhase transition and relaxation, both essentially molecular motions, are critical mechanisms that determine the mechanical, electrical, optical, and thermal behaviors of polymer materials. For decades, hydrogen bonding has played an essential role in the modulation of molecular motions and material properties. However, in the design of stimulus‐responsive dielectric materials and the modulation of their properties, the role of hydrogen bonding has been rarely investigated, and its effect on dielectric response behavior remains elusive. Here, observations and proof that hydrogen bond reorganization is able to act as the origin of the dielectric pulsing effect in polar semicrystalline polymers by in situ testing is presented. A two‐step hydrogen bond reorganization drives molecular chain relaxation in polar semicrystalline polymers causing a strong dielectric response behavior, opening up the possibility of modulating dielectric response behavior through hydrogen bonding. Moreover, electrode polarization can synergize with interfacial and dipolar polarizations to enhance the dielectric pulsing effect. This study also provides a blow‐spinning method for preparing large‐area, flat, flexible, lightweight, and recyclable thermo‐responsive dielectric films by continuous, efficient, and low‐cost processing.

2D-IR Spectroscopy of Nitrosyl Stretch of Sodium Nitroprusside Reveals the Elusive Two Anomalous Regions in the DMSO–Water Mixture

DMSO-water mixtures provide an intriguing hydrogen-bonding environment which has been a subject of various theoretical and experimental investigations. The structural dynamics of aqueous DMSO solutions has been investigated, using nitrosyl stretch of sodium nitroprusside (SNP, Na2[Fe(CN)5NO]) as a local vibrational probe, with the help of infrared (IR) absorption spectroscopy, vibrational pump-probe spectroscopy, and two-dimensional IR spectroscopy (2D-IR). Fourier transform infrared spectra of the nitrosyl stretch of SNP reveals that both the peak position and spectral broadening are very sensitive to the composition of the DMSO-water mixture and the subsequent structural changes occurring due to the addition of DMSO to water. The vibrational lifetime of the nitrosyl stretch displays two different linear variation regimes as a function of mole fraction of DMSO which has been assigned presumably to two different predominant structures at these compositions. However, the rotational depolarization measurements show that the reorientational times follow a bell-shaped profile, imitating the changes in the composition-dependent physical properties (viscosity) of DMSO-water solvent mixtures. To get a holistic picture of the system, 2D-IR spectroscopy of the NO stretch of SNP has been employed to study time scales of hydrogen-bond reorganization dynamics existing at different compositions. The analysis of frequency-frequency correlation function (FFCF) decay times reveal that the dynamics gets slower in intermediate DMSO concentrations than that of pure DMSO or pure water. A careful analysis reveals two anomalous regions of hydrogen-bond dynamics: XDMSO ∼0.2 and 0.4, which indicates that different hydrogen-bonded structures exist in these regions that can be effectively probed by SNP which has remained mostly elusive to previous vibrational probe-based investigations.

Dynamics of hydrogen bond reorganization in the S1(ππ*) state of 9-Anthracenecarboxaldehyde

Effect of solvent polarity and hydrogen bonding interaction on the excited state dynamics of 9-Anthracenecarboxaldehyde (9-ACD) have been investigated using steady state and time-resolved electronic and vibrational spectroscopic techniques. Photophysical properties and dynamics of the excited states are seen to be more sensitive to intermolecular hydrogen bonding ability of the solvent, rather than its polarity. FTIR studies reveal formation of complexes with the protic solvents via formation of conventional hydrogen bond at the carbonyl site of 9-ACD molecule. In strong hydrogen bond donating solvents, namely, 2, 2, 2-trifuoroethanol (TFE) and 1, 1, 1, 3, 3, 3-hexafluoroisopropanol (HFIP), nearly all the molecules of 9-ACD in solution remain engaged in complex formation leaving insignificant number of molecules remaining free in solution. In aprotic solvents, the S1 state is short-lived (the lifetime is only a few tens of ps) and intersystem crossing (ISC) is the major deactivation pathway (the triplet yield is near unity). However, in strong hydrogen bond donating solvents, the S1 state lifetime is long (a few ns) because the deactivation pathway for the S1 state via the triplet manifold is blocked. Both the ultrafast time-resolved electronic and vibrational spectroscopy studies in TFE and HFIP reveal that hydrogen bond reorganization process following photoexcitation of the hydrogen bonded complex is the only relaxation process undergone by the S1 state in sub-100 ps time domain. However, the time-resolved IR (TRIR) spectroscopy studies further reveal the possibility of formation of aromatic π- hydrogen bonding and reorganization of the hydrogen bond at this site makes the major contribution towards the S1 state relaxation process in strong hydrogen bond donating solvents.

Tuning an Imine Reductase for the Asymmetric Synthesis of Azacycloalkylamines by Concise Structure-Guided Engineering.

Although imine reductases (IREDs) are emerging as attractive reductive aminases (RedAms), their substrate scope is still narrow, and rational engineering is rare. Focusing on hydrogen bond reorganization and cavity expansion, a concise strategy combining rational cavity design, combinatorial active-site saturation test (CAST), and thermostability engineering was designed, that transformed the weakly active IR-G36 into a variant M5 with superior performance for the synthesis of (R)-3-benzylamino-1-Boc-piperidine, with a 4193-fold improvement in catalytic efficiency, a 16.2 °C improvement in Tm , and a significant increase in the e.e. value from 78 % (R) to >99 % (R). M5 exhibits broad substrate scope for the synthesis of diverse azacycloalkylamines, and the reaction was demonstrated on a hectogram-scale under industrially relevant conditions. Our study provides a compelling example of the preparation of versatile and efficient IREDs, with exciting opportunities in medicinal and process chemistry as well as synthetic biology.

Tuning an Imine Reductase for the Asymmetric Synthesis of Azacycloalkylamines by Concise Structure‐Guided Engineering

AbstractAlthough imine reductases (IREDs) are emerging as attractive reductive aminases (RedAms), their substrate scope is still narrow, and rational engineering is rare. Focusing on hydrogen bond reorganization and cavity expansion, a concise strategy combining rational cavity design, combinatorial active‐site saturation test (CAST), and thermostability engineering was designed, that transformed the weakly active IR‐G36 into a variant M5 with superior performance for the synthesis of (R)‐3‐benzylamino‐1‐Boc‐piperidine, with a 4193‐fold improvement in catalytic efficiency, a 16.2 °C improvement in Tm, and a significant increase in the e.e. value from 78 % (R) to >99 % (R). M5 exhibits broad substrate scope for the synthesis of diverse azacycloalkylamines, and the reaction was demonstrated on a hectogram‐scale under industrially relevant conditions. Our study provides a compelling example of the preparation of versatile and efficient IREDs, with exciting opportunities in medicinal and process chemistry as well as synthetic biology.

Effect of cation structure on the formation of hydrogen bond between ionic liquids and solute molecules

In this work, we have studied the thermochemistry of the hydrogen bond formation between ionic liquids and solute molecules. The solution enthalpies of several organic compounds (benzene, acetone, 3-picoline dimethylsulfoxide, formamide, N-methylformamide, acetamide, N-methylacetamide) in the series of ionic liquids with common anion bis(trifluoromethanesulfonyl)imide and different cations (1-butyl-3-methylimidazolium [BMIM][NTf2], 1-butyl-1-methylpyrrolidinium [BMPYR][NTf2], 1-butylpyridinium [BPY][NTf2], 1-butyl-1-methylpiperidinium [BMPIP][NTf2], trimethylpropylammonium [TMPAm][NTf2]) were measured by solution calorimetry at infinite dilution. The enthalpies of hydrogen bond between solutes and ionic liquids were determined by the previously proposed approach. The hydrogen bond enthalpies were correlated with the proton acceptor ability of ionic liquids from the Kamlet-Abboud-Taft equation and the proton donor ability of amides from the Abraham model. The proton donor ability and hydrogen bond reorganization of studied ionic liquids was analyzed. The formation of the hydrogen bond complexes of linear amides with ionic liquids with different compositions was shown. The effect of the ionic liquid structure on the strength of the hydrogen bond of linear amides with ionic liquids has been established.

A Study of the Structural Organization of the Surface Layer and the State of Water in Composite Ultrafiltration Membranes

The structural organization of the surface (active) layer and the state of water in composite ultrafiltration membranes have been investigated by means of the methods of vibrational IR spectroscopy and differential scanning calorimetry (DSC). Analysis of the shape of the absorption band of stretching vibrations of hydroxyl groups at ν = 3366.2 cm–1 with an asymmetry coefficient of ~1 and the activity of vibrations of methyl groups with ν = 2884.02 and 2942.35 cm—1 has demonstrated that the bulk supramolecular structure of a cellulose acetate layer of a dry sample is formed by two types of hydrogen bonds and dipole–dipole interactions of carbonyl groups. The interaction of macromolecules in the equatorial plane is formed by a lattice of hydrogen bonds of the (OH…O) type with participation of the only hydroxyl group of a pyranose ring in cellulose acetates. In the axial direction, the supramolecular structure is organized by hydrogen bonds between methine and carbonyl groups of the (CH…O=C) type and dipole interactions of coplanar ordered dipoles of (C=O) groups. The decrease in the intensity and the coefficient of asymmetry down to 0.81 of the absorption band of hydroxyl groups and the absorption band of methyl groups with frequencies ν = 2884.02–2942.35 cm–1 by 2.56 and 3.3 times in water-saturated samples occurs due to the destruction of the supramolecular structure and the reorganization of hydrogen bonds of active groups of cellulose acetate and water molecules blocking intermolecular interactions of the macromolecules. The manifestation of hydroxyl groups with frequencies of 3302.7 and 33 105 sh cm–1 and ν = 3600.6 and ν = 3598.8 sh cm–1 on the absorption band indicates the interaction of associated water molecules of the bound and capillary forms. The absence of the absorption band with ν = 873.53 cm–1 in the water-saturated sample is explained as an empirical indicator of the conformational transition of macromolecules into a linear form. Deconvolution of the endothermic peak on the DSC curve in the temperature range from 50 to 110°C into five Gaussians has demonstrated the multilevel structural organization of water molecules sorbed in pores and between macromolecules of the amorphous phase.

Managing of hydrogen bonding in aqueous poly(vinyl alcohol) solutions: new perspectives towards preparation of more homogeneous poly(vinyl butyral)

This paper follows up investigations in the field of solution properties of poly(vinyl alcohol). Here we present a combined investigation based on both theoretical and various experimental methods focused on peculiarities in hydrogen bond reorganization in aqueous poly(vinyl alcohol) solutions discussed as a function of temperature. It was demonstrated that the specific polymeric effect takes place in binary poly(vinyl alcohol)/water systems. The effect is characterized by the delay in response of the polymer system on heating until the critical temperature is reached. It was demonstrated in this paper that hydrogen bond reorganization in binary aqueous systems may lead to an increase in the degree of undesirable intermolecular cross-linking reaction. This finding was in contrast to previously made proposition based on hydrogen bond reorganization in ternary poly(vinyl alcohol) solutions. This assumption based on the theoretical studies was confirmed experimentally. Thus, a series of poly(vinyl butyral) samples were synthesized in various solvents using traditional catalysts (HCl) as well as the modern thermosensitive polymeric catalyst. It was demonstrated by using GPC and viscometry experiments that poly(vinyl butyral) samples with a low degree of acetalization (<10%) prepared in water in the absence of co-solvent additives are characterized by large molecular weights and bimodal molecular weight distribution. On the contrary, polymers prepared in mixed (ternary) systems and in the presence of a thermosensitive polymeric catalyst demonstrate relatively narrow molecular weight distribution. This indicates the preparation of more homogeneous products which also facilitates mechanisms of hydrogen bond reorganizations observed in binary and ternary poly(vinyl alcohol) solutions.

Effects of concentration and pressure on the aqueous solvation structure of ammonia and composition dependent ion solvation scenario in water-ammonia mixtures

The hydrogen bonding structure of water and its dynamics in aqueous ammonia solution is investigated by using classical molecular dynamics simulations. We have considered eight different mole-fraction of ammonia, ranging from xNH3 = 0.02 to 0.30, and the pressure variation is carried out on the concentrated ammonia solution (xNH3=0.30), ranging from P = 0.1 MPa to 800 MPa, at 293K. The radial distribution function indicates that the addition of ammonia facilitates the structural organization of water, which is further supported by increasing values of asphericity parameter. But, the tetrahedral order parameter values suggest an angularly disordered arrangement of water molecules. The self-diffusion coefficient values of water pass through shallow minima, whereas it increases for ammonia with ammonia concentration in the solution. The application of pressure causes a sharp decline in translation mobility of ammonia compared to water, but at very high pressure (above 600 MPa), both the species diffuse at the same rate. The rotational relaxation of ammonia is approximately three times faster than water in the solution, which further increases along with concentration as well as pressure. A similar trend is also observed in the case of structural reorganization of water-water and water-ammonia hydrogen bonds. The water-water and water-ammonia hydrogen-bond lifetimes are increasing with ammonia concentration, whereas it passes through shallow maxima with the application of pressure. In this study, we have compared two different Lennard-Jones (LJ) combination rule, which causes negligible differences in the calculated properties. The solvation scenario of different ionic (Na+, Cl−) and neutral (Cl0) solutes are also analyzed along with the coordination number. It is observed that the hydration shell of ion is mainly dominated by water molecules in the solution.

High-yield cellulose hydrolysis by HCl vapor: co-crystallization, deuterium accessibility and high-temperature thermal stability

Hydrochloric acid hydrolysis in its gas form was used to produce cellulose nanoparticles from flax shives with a very high yield (> 90%). The efficiency of the transformation was examined by gravimetry, atomic force microscopy and transmission electron microscopy. A novel purpose-built anisotropic line-broadening X-ray diffraction model was used. This XRD method uses a parametric shape in order to address the issue of strongly broadened and overlapping Bragg rays that are characteristic of nano-sized crystallites. This method demonstrated the remarkable stability of the crystallite shapes during hydrolysis and its sensibility was sufficient to detect a minor co-crystallization along the hydrophilic faces. The presence of amorphous material strictosensu in the form of individual and randomly oriented chains was not necessary to describe the diffractograms accurately. Thermal FTIR with isotopic exchange was also performed using deuterium oxide to characterize the accessibility of the materials between 20 and 260 °C. Further, back-exchange experiments were performed in order to quantify the hysteretic amount of deuterium that was trapped by microstructural reorganization. These experiments showed that hydrolysis cancelled any form of deuterium trapping (water-induced co-crystallization). For the first time, thermal FTIR demonstrated that isotopic labelling of cellulose sources can produce false positives when conducted at room temperature and thermal FTIR can unambiguously distinguish between labelled cellulose groups and free deuterium oxide, which is paramount when measuring the higher accessibility of the nanocelluloses. It was also demonstrated that the high-temperature hydrogen bond reorganization and thermal degradation of the cellulose chains strongly depend on the hydrolysis and on the microstructure of the substrate.

Influence of Annealing in the Close Vicinity of Tg on the Reorganization within Dimers and Its Impact on the Crystallization Kinetics of Gemfibrozil.

In this paper, broadband dielectric spectroscopy (BDS) has been applied to study the molecular dynamics and crystallization kinetics of the antihyperlipidemic active pharmaceutical ingredient (API), gemfibrozil (GEM), as well as its deuterated (dGEM) and methylated (metGEM) derivatives, characterized by different types and strengths of intermolecular interactions. Moreover, calorimetric and infrared measurements have been carried out to characterize the thermal properties of examined samples and to probe a change in the H-bonding pattern in GEM, respectively. We found that the dielectric spectra of all examined compounds, collected below the glass transition temperature (Tg), reveal the presence of two secondary relaxations (β, γ). According to the coupling model (CM) predictions, it was assumed that the slower process (β) is of JG type, whereas the faster one (γ) has an intramolecular origin. Interestingly, the extensive crystallization kinetics measurements performed after applying two paths, i.e., the standard procedure (cooling and subsequently heating up to the appropriate temperature, Tc), as well as annealing at two temperatures in the vicinity of Tg and further heating up to Tc, showed that the annealing increases the crystallization rate in the case of native API, while the thermal history of the sample has no significant impact on the pace of this process in the two derivatives of GEM. Analysis of the dielectric strength (Δε) of the α-process during annealing, together with the results of Fourier transform infrared spectroscopy (FTIR) measurements, suggested that the reorganization within dimeric structures formed between the GEM molecules is responsible for the observed behavior. Importantly, our results differ from those obtained by Tominaka et al. (TominakaS.; KawakamiK.; FukushimaM.; MiyazakiA.Physical Stabilization of Pharmaceutical Glasses Based on Hydrogen Bond Reorganization under Sub-Tg TemperatureMol. Pharm.20171426427310.1021/acs.molpharmaceut.6b00866.28043129), who demonstrated that the sub-Tg annealing of ritonavir (RTV), which is able to form extensive supramolecular hydrogen bonds, protects this active substance against crystallization. Therefore, based on these contradictory reports, one can hypothesize that materials forming H-bonded structures, characterized by varying architecture, may behave differently after annealing in the vicinity of the glass transition temperature.

Entropic barriers in the kinetics of aqueous proton transfer.

Aqueous proton transport is uniquely rapid among aqueous processes, mediated by fluctuating hydrogen bond reorganization in liquid water. In a process known as Grotthuss diffusion, the excess charge diffuses primarily by sequential proton transfers between water molecules rather than standard Brownian motion, which explains the anomalously high electrical conductivity of acidic solutions. Employing ultrafast IR spectroscopy, we use the orientational anisotropy decay of the bending vibrations of the hydrated proton complex to study the picosecond aqueous proton transfer kinetics as a function of temperature, concentration, and counterion. We find that the orientational anisotropy decay exhibits Arrhenius behavior, with an apparent activation energy of 2.4 kcal/mol in 1M and 2M HCl. Interestingly, acidic solutions at high concentration with longer proton transfer time scales display corresponding decreases in activation energy. We interpret this counterintuitive trend by considering the entropic and enthalpic contributions to the activation free energy for proton transfer. Halide counteranions at high concentrations impose entropic barriers to proton transfer in the form of constraints on the solution's collective H-bond fluctuations and obstruction of potential proton transfer pathways. The corresponding proton transfer barrier decreases due to weaker water-halide H-bonds in close proximity to the excess proton, but the entropic effects dominate and result in a net reduction in the proton transfer rate. We estimate the activation free energy for proton transfer as ∼1.0 kcal/mol at 280 K.

Disentangling Time Scales of Vibrational Cooling, Solvation, and Hydrogen Bond Reorganization Dynamics Using Ultrafast Transient Infrared Spectroscopy of Formylperylene.

Unraveling dynamics of solvation and hydrogen bond (H-bond) reorganization between a solute and solvent is crucial to understand the importance of specific and nonspecific interactions in a solution-phase chemical reaction. Ultrafast time-resolved infrared (TRIR) spectroscopy provides direct opportunity to monitor site-specific intermolecular dynamics on a real-time scale by probing vibrational marker bands in the excited state of a solute. Herein, we report the real-time dynamics of vibrational cooling, solvation, and hydrogen bond reorganization of formylperylene (FPe) through TRIR spectroscopy of carbonyl (C═O) stretching mode in nonpolar, polar aprotic, and polar protic solvents. High sensitivity of the C═O stretch frequency (υ̅C═O) to photoinduced intramolecular charge transfer processes induced by specific and nonspecific solvent interactions led us to monitor the dynamics of dipolar solvation, site-specific H-bond formation, and reorganization processes by the TRIR method. In nonpolar cyclohexane, the υ̅C═O stretch band appears at 1610 cm-1 and exhibits negligible spectral shift over several tens of picoseconds. In acetonitrile, the υ̅C═O peak shifts to 1594 cm-1 and exhibits a further temporal red shift of about 5 cm-1 with a characteristic solvation time scale of acetonitrile (τ ∼ 0.5 ps). In methanol, υ̅C═O exhibits two bands corresponding to free and H-bonded FPe in early time scale. The free FPe population converts to the hydrogen-bonded population with a lifetime of about 10 ps. Vibrational cooling (τvc ∼ 12-20 ps) in the excited electronic state of FPe could independently be monitored from the temporal dynamics of the ring vibration mode, which is less sensitive to solvation and hydrogen bonding. The present study provides insight into the specific and nonspecific solvation-controlled charge transfer dynamics in aprotic and protic solvents using FPe as a probe.

Picosecond Proton Transfer Kinetics in Water Revealed with Ultrafast IR Spectroscopy.

Aqueous proton transport involves the ultrafast interconversion of hydrated proton species that are closely linked to the hydrogen bond dynamics of water, which has been a long-standing challenge to experiments. In this study, we use ultrafast IR spectroscopy to investigate the distinct vibrational transition centered at 1750 cm-1 in strong acid solutions, which arises from bending vibrations of the hydrated proton complex. Broadband ultrafast two-dimensional IR spectroscopy and transient absorption are used to measure vibrational relaxation, spectral diffusion, and orientational relaxation dynamics. The hydrated proton bend displays fast vibrational relaxation and spectral diffusion timescales of 200-300 fs; however, the transient absorption anisotropy decays on a remarkably long 2.5 ps timescale, which matches the timescale for hydrogen bond reorganization in liquid water. These observations are indications that the bending vibration of the aqueous proton complex is relatively localized, with an orientation that is insensitive to fast hydrogen bonding fluctuations and dependent on collective structural relaxation of the liquid to reorient. We conclude that the orientational relaxation is a result of proton transfer between configurations that are well described by a Zundel-like proton shared between two flanking water molecules.

Perspective: Structural fluctuation of protein and Anfinsen's thermodynamic hypothesis.

The thermodynamics hypothesis, casually referred to as "Anfinsen's dogma," is described theoretically in terms of a concept of the structural fluctuation of protein or the first moment (average structure) and the second moment (variance and covariance) of the structural distribution. The new theoretical concept views the unfolding and refolding processes of protein as a shift of the structural distribution induced by a thermodynamic perturbation, with the variance-covariance matrix varying. Based on the theoretical concept, a method to characterize the mechanism of folding (or unfolding) is proposed. The transition state, if any, between two stable states is interpreted as a gap in the distribution, which is created due to an extensive reorganization of hydrogen bonds among back-bone atoms of protein and with water molecules in the course of conformational change. Further perspective to applying the theory to the computer-aided drug design, and to the material science, is briefly discussed.

From vine to wine: photophysics of a pyranoflavylium analog of red wine pyranoanthocyanins

Abstract In the ground state, the p-methoxyphenyl-substituted pyranoflavylium cation I, prepared by the reaction of the 5,7-dihydroxy-4-methylflavylium cation with p-methoxybenzaldehyde, is a weak acid (pKa=3.7±0.1). In its lowest excited singlet state, I is a moderate photoacid (pKa*=0.67) in 30% methanol-water acidified with trifluoroacetic acid (TFA). In comparison to anthocyanins and 7-hydroxyflavylium cations, the photoacidity of I is much less pronounced and the rate of proton loss from the excited acid form of I much slower (by a factor of up to 100). In 50% ethanol:0.10 mol dm−3 HClO4, the excited state of the acid form of I undergoes fast (12 ps) initial relaxation (potentially in the direction of an intramolecular charge transfer state), followed by much slower (340 ps) adiabatic deprotonation to form the excited base. The excited base in turn exhibits a moderately fast relaxation (70 ps), consistent with solvent hydrogen-bond reorganization times, followed by slower but efficient decay (1240 ps) back to the ground state. As in uncomplexed anthocyanins and 7-hydroxyflavylium cations, the photophysical behavior of I points to excited state proton transfer as the dominant excited state deactivation pathway of pyranoanthocyanins, consistent with relatively good photostability of natural pyranoanthocyanins.

Ultrafast Dynamics of Hydrogen Bond Breaking and Making in the Excited State of Fluoren-9-one: Time-Resolved Visible Pump-IR Probe Spectroscopic Study.

The fluoren-9-one (FL) molecule, with a single hydrogen bond-accepting site (C═O group), has been used as a probe for investigation of the dynamics of a hydrogen bond in its lowest excited singlet (S1) state using the subpicosecond time-resolved visible pump-IR probe spectroscopic technique. In 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), a strong hydrogen bond-donating solvent, the formation of an FL-alcohol hydrogen-bonded complex in the ground electronic (S0) state is nearly complete, with a negligible concentration of the FL molecule remaining free in solution. In addition to the presence of a band due to the hydrogen-bonded complex in the transient IR spectrum recorded immediately after photoexcitation of FL in HFIP solution, appearance of the absorption band due to a free C═O stretch provides confirmatory evidence of ultrafast photodissociation of hydrogen bonds in some of the complexes formed in the S0 state. The peak-shift dynamics of the C═O stretch bands reveal two major relaxation pathways, namely, vibrational relaxation in the S1 state of the free FL molecules and the solvent reorganization process in the hydrogen-bonded complex. The latter process follows bimodal exponential dynamics involving hydrogen bond-making and hydrogen bond-reorganization processes. The similar lifetimes of the S1 states of the FL molecules, both free and hydrogen-bonded, suggest establishment of a dynamic equilibrium between these two species in the excited state. However, investigations in two other weaker hydrogen bond-donating solvents, namely, trifluoroethanol (TFE) and perdeuterated methanol (CD3OD), reveal different features of peak-shift dynamics because of the prominence of the vibrational relaxation process over the hydrogen bond-reorganization process during the early time.

Three-dimensional protonic conductivity in porous organic cage solids.

Proton conduction is a fundamental process in biology and in devices such as proton exchange membrane fuel cells. To maximize proton conduction, three-dimensional conduction pathways are preferred over one-dimensional pathways, which prevent conduction in two dimensions. Many crystalline porous solids to date show one-dimensional proton conduction. Here we report porous molecular cages with proton conductivities (up to 10−3 S cm−1 at high relative humidity) that compete with extended metal-organic frameworks. The structure of the organic cage imposes a conduction pathway that is necessarily three-dimensional. The cage molecules also promote proton transfer by confining the water molecules while being sufficiently flexible to allow hydrogen bond reorganization. The proton conduction is explained at the molecular level through a combination of proton conductivity measurements, crystallography, molecular simulations and quasi-elastic neutron scattering. These results provide a starting point for high-temperature, anhydrous proton conductors through inclusion of guests other than water in the cage pores.