H-bonded Structures Research Articles

Propanol Clustering in Argon Matrix: 2D FTIR Correlation Spectroscopy

The dynamic FTIR spectra of propanol in the argon matrix have been measured using a thermal perturbation by heating the sample from T = 11 K to 30 K stepping by 1 K. The 2D Fourier-transform infrared (FTIR) absorption correlation analysis has been carried out, the main attention being concentrated to the region of propanol O–H stretching vibrations at 3000–3700 cm–1. The peaks of monomers, two conformers of open dimer, and the cyclic structures from a dimer to a pentamer have been resolved and studied in more details. Analyzing the dependences of the integral band intensities of various aggregates on the temperature, it has been deduced that monomers and dimers act in the initial clustering stage as the main "building units" whose diffusion sustains the formation of the higher H-bond structures in the matrix. The full width at half height (FWHH) for each band has been processed as afunction of the aggregation number (n). It is found that the FHWW is a perfectly linear function of n for all cyclic aggregates n ≥ 2. The resonance broadening has been proposed to be the most reliable mechanism for the formation of diffuse O–H stretching bands in the matrix isolated clusters.

Identification of Valence Electronic States Reflecting the Hydrogen Bonding in Liquid Ethanol.

The temperature-dependent X-ray emission spectra of liquid ethanol were calculated theoretically using a semi-classical approximation to the Kramers-Heisenberg formula, which includes the dynamical effects induced by a core-hole. Soft X-ray emission spectroscopic measurements were performed to discern the changes in the hydrogen bonding (h-bonding) structure of liquid ethanol using a temperature-controlled liquid cell at 241 and 313 K. The relative intensities of the peaks at approximately 526.5 and 527.1 eV varied with temperature, and the corresponding behavior was reproduced theoretically, although the variation with temperature in the calculated spectra were more enhanced than that in the experiment. The two peaks can be attributed to the 3a″ + 10a' mixed state and pure 3a″ state, respectively, depending on the behavior of the local h-bonding structure. The splitting of the 3a″ component occurred because of the h-bonding behavior of liquid ethanol. Furthermore, the size of the ethanol cluster decreased with an increase in temperature, mainly due to the breaking of the one-donor/one-acceptor type h-bonding. Our studies suggest that the electronic state of liquid ethanol reflects several types of h-bonding structures, and the ratios of these h-bonding types vary with temperature.

Do lipid nanodiscs assemble cell membranes? A study of interfacial H-bond structure and dynamics

In comparison to other membrane mimetics, lipid nanodiscs provide substantial advantages for experimental studies on membrane proteins. Nanodiscs are widely used in the structural and spectroscopic characterization of proteins. However, protein structure is heavily influenced by their local environment. The molecular environments of lipid nanodiscs have unfortunately remained underexplored. In our studies, we compare the hydrogen bond structure and dynamics in various lipid nanodiscs using ultrafast two-dimensional infrared (2D IR) spectroscopy and temperature-dependent studies to understand the influence of lipid composition and size on the local H-bond environment.

How does coal interact with organic groups in an aqueous solution?

Interactions between coal matrix and organics are vital for adsorption [1], surfactant adsorption on lignite [2-5], and solid–liquid separation of coal slurry [6-10]. Herein, for studying the interaction between hydrophobic surface (coal surface) and small molecules (organics), we measured interactions between organics [nitro and phenyl, pyridine, phenyl, alkyl chain (hexyl) or cycloalkyl (hexyl)] and the coal matrix (liptinite or inertinite) using atomic force microscopy. The strength of interaction followed the order: nitro and phenyl > pyridine > phenyl > alkyl chain (hexyl) > cycloalkyl (hexyl), and liptinite > inertinite. The liptinite has a greater hydrophobicity than inertinite, the greater the attraction force. The lignite surface and coal slurry were modified in an aqueous solution using five surfactants (dodecyl trimethyl ammonium chloride, DTAC; hexadecyl trimethyl ammonium chloride, CTAC; octadecyl trimethyl ammonium chloride, STAC; hexadecyl dimethyl benzyl ammonium chloride, HDBAC; and cetyl pyridine chloride, CPC). Subsequently, the moisture adsorption on treated lignite coal surface and the sedimentation of the treated tails was compared to characterize hydrophobic modifications which showed the order of CTAC > HDBAC > DTAC > CPC > STAC. Thus, the interaction between coal surface and organics needs to consider two rules: 1) The desolvation or the hydrophobic property of the coal surface; 2) The VDW force of the organics. The nature of the difference between rules No. 1 and 2 is that with the size of hydrophobic molecules increased to a surface, the H-bond structure between water molecules in the around would change a lot, which formats great hydrophobic property or desolvation. SynopsisInteraction between organic reagents and coal reveals the mechanism of restraining moisture reabsorption of lignite and solid–liquid separation of coal slurry by organics.

Homeopathic Potencies vary from each other with respect to relative proportions of free and bound water molecules

ABSTRACT
 Homeopathic potencies 12CH and above cross the Avogadro number, and as such do not contain any original drug molecules in their aqueous ethanol medium. It is thought H-bonded water structures preserved by ethanol carry the information of initial drug molecules. Potentized drugs show some differences with respect to their infrared (IR) absorption spectra. In a water-ethanol solution, free water molecules vary according the concentration of ethanol. In the present study the concentration of ethanol has been kept constant at 0.03 molar fraction in 6 different homeopathic potencies. To see whether different homeopathic potencies having fixed ethanol content show variation in free water molecules. Two potencies like 8CH and 32CH of three homeopathic drugs Natrum mur, Cantharis and Nux vomica were used in the study, and their ethanol concentration was kept fixed at 0.03 molar fraction. The control was considered to be aquous ethanol at the same concentration. Spectrum of pure water was also taken. Fourier transform infrared (FTIR) absorption spectra were obtained in the wave number region of 4000 – 2800 cm-1. The half-width at half-maximum was measured for each spectrum. The intensity of each spectrum was normalized at 3410 cm-1 close to the peak. The difference spectrum (absorbance of drug solution – absorbance of pure water) for each drug and the control was obtained. FTIR spectra showed variation in absorbance intensity on both the high and low frequency side of the O-H stretching band in different drugs as well as the control. The C-H stretching band of 2977 cm-1 also showed variation in intensity in different drugs. In the difference spectra the absorbance intensity at the dip at 3630 cm-1 varied in different drugs and the control. The decrease in intensity at 3630 cm-1 and subsequent rise in intensity at lower frequency region represent the level of free water molecules and strong alcoholic O-H band around 3250 cm-1, respectively. The drug and the control solutions show distinct variation in their FTIR spectra. The drugs have different levels of bound and free water molecules although their ethanol concentration is same.
 Keywords-Homeopathic potencies, FTIR spectra, free water molecules, intensity and difference spectrum.

Solid-State NMR and Impedance Spectroscopy Study of Spin Dynamics in Proton-Conducting Polymers: An Application of Anisotropic Relaxing Model.

The 1H–13C cross-polarization (CP) kinetics in poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) was studied under moderate (10 kHz) magic-angle spinning (MAS). To elucidate the role of adsorbed water in spin diffusion and proton conductivity, PMETAC was degassed under vacuum. The CP MAS results were processed by applying the anisotropic Naito and McDowell spin dynamics model, which includes the complete scheme of the rotating frame spin–lattice relaxation pathways. Some earlier studied proton-conducting and nonconducting polymers were added to the analysis in order to prove the capability of the used approach and to get more general conclusions. The spin-diffusion rate constant, which describes the damping of the coherences, was found to be strongly depending on the dipolar I–S coupling constant (DIS). The spin diffusion, associated with the incoherent thermal equilibration with the bath, was found to be most probably independent of DIS. It was deduced that the drying scarcely influences the spin-diffusion rates; however, it significantly (1 order of magnitude) reduces the rotating frame spin–lattice relaxation times. The drying causes the polymer hardening that reflects the changes of the local order parameters. The impedance spectroscopy was applied to study proton conductivity. The activation energies for dielectric relaxation and proton conductivity were determined, and the vehicle-type conductivity mechanism was accepted. The spin-diffusion processes occur on the microsecond scale and are one order faster than the dielectric relaxation. The possibility to determine the proton location in the H-bonded structures in powders using CP MAS technique is discussed.

Variation in free and bound water molecules in different homeopathic potencies as revealed by their Fourier Transform Infrared Spectroscopy (FTIR)

ABSTRACT
 
 BACKGROUND
 Homeopathic potencies 12CH and above cross the Avogadro number, and as such do not contain any original drug molecules in their aquous ethanol medium. It is thought H-bonded water structures preserved by ethanol carry the information of initial drug molecules. Potentized drugs show some differences with respect to their infrared (IR) absorption spectra. In a water-ethanol solution, free water molecules vary according the concentration of ethanol. In the present study the concentration of ethanol has been kept constant at 0.03 molar fraction in 6 different homeopathic potencies.
 
 OBJECTIVE
 To see whether different homeopathic potencies having fixed ethanol content show variation in free water molecules.
 
 METHODS
 Two potencies like 8CH and 32CH of three homeopathic drugs Natrum mur, Cantharis and Nux vomica were used in the study, and their ethanol concentration was kept fixed at 0.03 molar fraction. The control was considered to be aquous ethanol at the same concentration. Spectrum of pure water was also taken. Fourier transform infrared (FTIR) absorption spectra were obtained in the wave number region of 4000 – 2800 cm-1. The half-width at half-maximum was measured for each spectrum. The intensity of each spectrum was normalized at 3410 cm-1 close to the peak. The difference spectrum (absorbance of drug solution – absorbance of pure water) for each drug and the control was obtained.
 
 RESULTS
 FTIR spectra showed variation in absorbance intensity on both the high and low frequency side of the O-H stretching band in different drugs as well as the control. The C-H stretching band of 2977 cm-1 also showed variation in intensity in different drugs. In the difference spectra the absorbance intensity at the dip at 3630 cm-1 varied in different drugs and the control. The decrease in intensity at 3630 cm-1 and subsequent rise in intensity at lower frequency region represent the level of free water molecules and strong alcoholic O-H band around 3250 cm-1, respectively.
 
 CONCLUSION
 The drug and the control solutions show distinct variation in their FTIR spectra. The drugs have different levels of bound and free water molecules although their ethanol concentration is same.
 
 KEY WORDS
 Homeopathic potencies, FTIR spectra, free water molecules, intensity and difference spectrum.

Molecular Structure and Vibrational Spectra of Water Molecules Sorbed in Poly(2-methoxyethylacrylate) Revealed by Molecular Dynamics Simulation

Molecular dynamics (MD) simulations of water sorption in poly(2-methoxyethylacrylate) (PMEA) are carried out to elucidate the hydrogen bonding (H-bonding) structures of the water molecules and the side chains of PMEA. A PMEA model incorporating lone-pair virtual sites on the carbonyl and methoxy oxygens of the side chain of PMEA, which are the key interaction sites in a biocompatible polymer, is newly developed. The PMEA model well reproduces the experimentally observed features in the infrared spectra of the hydrated polymer, as well as the radial distribution function of the water molecules in contact with the polymer, as calculated by ab initio MD simulations. The MD simulation results reveal that water molecules tend to form H-bonds with the carbonyl oxygen and the methoxy oxygen of the side chain of PMEA simultaneously, which enhance the "head-to-tail" stacking structure of the side chains at a low concentration range of water. Further penetration of water into the PMEA structure gradually increases the water-water H-bonding state and promotes the formation of water clusters even below the equilibrium water content.

Molecular Mechanism of Cell Membrane Protection by Sugars: A Study of Interfacial H-Bond Networks.

Sugars function as bioprotectants by stabilizing biomolecules during dehydration, thermal stress, and freeze-thaw cycles. A buildup of sugars occurs in many organisms upon their exposure to extreme conditions. Understanding sugar's bioprotective effects on membranes is achieved by characterizing the H-bond networks at the lipid-water interface. Here, we report the headgroup H-bond populations, structures, and dynamics of 1,2-dimyristoyl-sn-glycero-3-phosphocholine vesicles in concentrated glucose solutions using ultrafast two-dimensional infrared spectroscopy in conjunction with molecular dynamics simulations. H-Bond populations and dynamics at the ester carbonyl positions are largely unaffected even at very high, 600 mg/mL, sugar concentrations. In addition, dynamics exhibit a slight nonmonotonic dependence on sugar concentration. Simulations, which are in near-quantitative agreement with measured dynamics, show that the H-bond structure remains largely intact by the existence of sugar. This study shows that the bioprotection of sugar is realized through stable lipid-saccharide-water H-bond networks at the membrane interface that mimic the H-bond networks in pure water.

Accurate prediction of hydration free energies and solvation structures using molecular density functional theory with a simple bridge functional.

This paper assesses the ability of molecular density functional theory to predict efficiently and accurately the hydration free energies of molecular solutes and the surrounding microscopic water structure. A wide range of solutes were investigated, including hydrophobes, water as a solute, and the FreeSolv database containing 642 drug-like molecules having a variety of shapes and sizes. The usual second-order approximation of the theory is corrected by a third-order, angular-independent bridge functional. The overall functional is parameter-free in the sense that the only inputs are bulk water properties, independent of the solutes considered. These inputs are the direct correlation function, compressibility, liquid-gas surface tension, and excess chemical potential of the solvent. Compared to molecular simulations with the same force field and the same fixed solute geometries, the present theory is shown to describe accurately the solvation free energy and structure of both hydrophobic and hydrophilic solutes. Overall, the method yields a precision of order 0.5kBT for the hydration free energies of the FreeSolv database, with a computer speedup of 3 orders of magnitude. The theory remains to be improved for a better description of the H-bonding structure and the hydration free energy of charged solutes.

Proton Conduction Mechanism for Anhydrous Imidazolium Hydrogen Succinate Based on Local Structures and Molecular Dynamics.

Anhydrous organic crystalline materials incorporating imidazolium hydrogen succinate (Im-Suc), which exhibit high proton conduction even at temperatures above 100 °C, are attractive for elucidating proton conduction mechanisms toward the development of solid electrolytes for fuel cells. Herein, quantum chemical calculations were used to investigate the proton conduction mechanism in terms of hydrogen-bonding (H-bonding) changes and restricted molecular rotation in Im-Suc. The local H-bond structures for proton conduction were characterized by vibrational frequency analysis and compared with corresponding experimental data. The calculated potential energy surface involving proton transfer (PT) and imidazole (Im) rotational motion showed that PT between Im and succinic acid was a rate-limiting step for proton transport in Im-Suc and that proton conduction proceeded via the successive coupling of PT and Im rotational motion based on a Grotthuss-type mechanism. These findings provide molecular-level insights into proton conduction mechanisms for Im-based (or -incorporated) H-bonding organic proton conductors.

Search for H-Bonded Motifs in Liquid Ethylene Glycol Using a Machine Learning Strategy.

Trajectories of atomic positions derived from ab initio molecular dynamics (AIMD) simulations of H-bonded liquids contain a wealth of information on dominant structural motifs and recurrent patterns of association. Extracting this information from a detailed search of the trajectories over multiple time frames is, however, a daunting exercise. Here, we use a machine learning strategy based on the neural inspired approach of the self-organizing maps (SOM), a type of artificial neural network that uses unsupervised competitive learning, to analyze the AIMD trajectories of liquid ethylene glycol (EG). The objective was to find whether there are H-bonded fragments, of two or more H-bonded EG molecules, that are recurrent in the liquid and to identify them. The SOM represents a set of high-dimensional data mapped onto a two-dimensional, grid of neurons or nodes, while preserving the topological properties of the input space. We show here that clustering of the fragments by SOM in terms of the molecular conformation of the individual EG molecules of the fragment and their H-bond connectivity pattern facilitates the search for H-bonded motifs. Using this approach, we are able to identify a H-bonded cyclic dimer and a bifurcated H-bonded structure as recurring motifs that appear in the longer H-bonded fragments present in liquid EG.

Matrix Isolation-Vibrational Circular Dichroism Spectroscopic Study of Conformations and Non-Covalent Interactions of Tetrahydro-2-Furoic Acid.

The conformational landscape and aggregation behaviour of tetrahydro-2-furoic acid (THFA) were investigated by using matrix isolation-vibrational circular dichroism (MI-VCD). The well-resolved experimental MI-IR and MI-VCD features in an argon matrix at 10 K allow one to identify two dominant monomeric conformations as trans-THFA where the hydroxyl and carbonyl groups of COOH are at opposite sides, as well as one cis-conformer. At 24 K and 30 K deposition temperatures, the experimental IR and VCD spectral features reveal further growth of the binary THFA aggregates. Systematic conformational searches identified three vastly different binary binding topologies, resulting in a few hundred stable (THFA)2 conformers. Interestingly, the main binary structures observed correspond to an unusual type of structure which is made of two trans-THFA subunits, in contrast to the usual double H-bonded ring binary structures, identified in a previous solution study. The present work showcases the power of MI-VCD spectroscopy in revealing unusual structures formed in a cold rare gas matrix.

Scrutiny of the supramolecular structure of bio-sourced fructose/glycerol/water ternary mixtures: Towards green low transition temperature mixtures

This work deals with fully green ternary mixtures composed of three low cost, bio-sourced and safe molecules, namely Fructose (F), Glycerol (G), and Water (W), able to interact to each other by H-bonds. Through an eco-friendly approach, we prepared a series of FGW mixtures differing in their composition in order to identify the combinations exhibiting a long-lasting macroscopic homogeneity. To well understand how water interferes in such as mixtures, we focused on thermal transitions, changes in viscosity and activation energies, and on internal organization at the molecular scale through NOESY and PFG-NMR experiments. It resulted that, as a function of the water content, the FGW mixtures can be defined either i) as a cooperatively H-bonded supramolecular structure in which water is actively involved in the interactions, thus leading to stable solvents designed as low transition temperature mixtures (LTTM), or ii) as aqueous solutions due to the diluting effect of water which disrupts the H-bonded organization and promotes the overall mobility. This comprehensive study is a prerequisite for a further industrial implementation of such eco-designed solvents in various applicative attractive domains, as in cosmetic field.

Protein nanofibril design via manipulation of hydrogen bonds

The process of amyloid nanofibril formation has broad implications including the generation of the strongest natural materials, namely silk fibers, and their major contribution to the progression of many degenerative diseases. The key question that remains unanswered is whether the amyloidogenic nature, which includes the characteristic H-bonded β-sheet structure and physical characteristics of protein assemblies, can be modified via controlled intervention of the molecular interactions. Here we show that tailored changes in molecular interactions, specifically in the H-bonded network, do not affect the nature of amyloidogenic fibrillation, and even have minimal effect on the initial nucleation events of self-assembly. However, they do trigger changes in networks at a higher hierarchical level, namely enhanced 2D packaging which is rationalized by the 3D hierarchy of β-sheet assembly, leading to variations in fibril morphology, structural composition and, remarkably, nanomechanical properties. These results pave the way to a better understanding of the role of molecular interactions in sculpting the structural and physical properties of protein supramolecular constructs.

Coronene-uracil complexes embedded in argon matrices: FTIR spectroscopy and quantum-mechanical calculations

We employ low-temperature matrix-isolation FTIR spectroscopy and quantum chemical calculations to study the interaction between nucleobase uracil and coronene which models the graphene surface. To observe the dimer FTIR spectrum, we use a quartz microbalance that allows us to produce matrix samples with precisely determined concentrations of coronene and uracil (with the concentration ratio of 2.5:1:1000 for coronene:uracil:argon). The interaction between coronene and uracil results in spectral shifts of uracil spectral bands. These shifts do not exceed 10 cm−1. The maximum shifts are observed for the C=O stretching and NH out-of-plane vibrations of uracil. The structures and interaction energies of stacked and H-bonded coronene-uracil complexes are calculated at the DFT/B3LYP(GD3BJ)/aug-cc-pVDZ and MP2/aug-cc-pVDZ levels of theory. In total, 19 stable stacked and two H-bonded coronene-uracil dimer structures are found in the calculations. The interaction energy obtained for the most stable stacked dimer is −12.1 and −14.3 kcal/mol at the DFT and MP2 levels, respectively. The interaction energies of the H-bonded dimers do not exceed − 3 kcal/mol. The IR spectra of the studied monomeric molecules and of all the dimers are calculated at the DFT/B3LYP(GD3BJ)/aug-cc-pVDZ level of theory. The spectral shifts of the most stable stacked coronene-uracil dimer obtained in the calculations are in good agreement with the experimental results.

Effect of Surface Orientation on Methanol Adsorption and Thermally Induced Structural Transformations on Copper Surfaces

Molecular adsorption of methanol on Cu(111), Cu(100), and Cu(110) surfaces in the 90–200 K temperature range is studied by a combination of infrared (IR) spectroscopy, thermal desorption analysis, and low-energy electron diffraction. Our results reveal the occurrence of a structural transformation in the H-bonded methanol assembly following heating from 90 to 120 K and demonstrate the effect of Cu surface orientation on the produced H-bonded structures, as well as thermal stability and ordering. At 90 K, the IR spectra indicate that similar H-bonded structures, presumably linear chains, are formed on all three surfaces. However, on Cu(111) the chains are assembled in ordered domains, whereas on Cu(100) and Cu(110) the chains are disordered. Heating to 120–130 K causes prominent changes in the IR spectrum of methanol on all surfaces, but there are significant differences between Cu(111) and the two other surfaces. We believe that such differences originate from different H-bonded methanol structures obtained on each surface after thermal annealing. On Cu(111) we suggest that cyclic structures (probably hexamers) are prevalent, whereas on Cu(100) and Cu(110) both cyclic and chain structures may coexist. The H-bonded structures produced at 120 K exhibit no long-range order on all three surfaces and show stronger adsorption (higher desorption temperate) on Cu(111). The higher stability and ordering (only at low temperatures) of adsorbed methanol on Cu(111) are attributed to the matching between the geometry of H-bonded methanol clusters and the surface structure.

Microhydration of ionized building blocks of DNA/RNA: infrared spectra of pyrimidine^{+}-(hbox {H}_{2}hbox {O})_{text {1-3}} clusters

Hydration of biomolecules is an important physiological process that governs their structure, stability, and function. Herein, we probe the microhydration structure of cationic pyrimidine (Pym), a common building block of DNA/RNA bases, by infrared photodissociation spectroscopy (IRPD) of mass-selected microhydrated clusters, hbox {Pym}^{+}-hbox {W}_{n} (W=hbox {H}_{2}hbox {O}), in the size range n=1–3. The IRPD spectra recorded in the OH and CH stretch range are sensitive to the evolution of the hydration network. Analysis with density functional theory calculations at the dispersion-corrected B3LYP-D3/aug-cc-pVTZ level provides a consistent picture of the most stable structures and their energetic and vibrational properties. The global minima of hbox {Pym}^{+}-hbox {W}_{n} predicted by the calculations are characterized by H-bonded structures, in which the H-bonded hbox {W}_{n} solvent cluster is attached to the most acidic C4–H proton of hbox {Pym}^{+} via a single CH...O ionic H-bond. These isomers are identified as predominant carrier of the IRPD spectra, although less stable local minima provide minor contributions. In general, the formation of the H-bonded solvent network (exterior ion solvation) is energetically preferred to less stable structures with interior ion solvation because of cooperative nonadditive three-body polarization effects. Progressive hydration activates the C4–H bond, along with increasing charge transfer from hbox {Pym}^{+} to hbox {W}_{n}, although no proton transfer is observed in the size range nleqslant 3. The solvation with protic, dipolar, and hydrophilic W ligands is qualitative different from solvation with aprotic, quadrupolar, and hydrophobic hbox {N}_{2} ligands, which strongly prefer interior ion solvation by uppi stacking interactions. Comparison of hbox {Pym}^{+}-W with Pym-W and hbox {H}^{+}Pym-W reveals the drastic effect of ionization and protonation on the Pym...W interaction.Graphic

Structured water: effects on animals.

This review focuses on the effects of structured water (SW) on animals when it is consumed on a daily basis. SW is liquid water that is given altered H-bonding structure by treatment with various forms of energy including magnetic fields and light. While most of the research has been conducted on ‘magnetized’ water, which has structure of short duration, recent research has examined effects of a SW with stability of at least 3.5 mo. A variety of laboratory and farm animals have been studied over the past 20 yr. Consistent (3 or more studies) responses among animals consuming SW for 1 mo or more include increased rate of growth, reduced markers of oxidative stress, improved glycemic and insulinemic responses in diabetics, improved blood lipid profile, improved semen and spermatozoa quality, and increased tissue conductivity as measured using bioelectrical impedance analysis. While it is known that fluids in and around cells and molecules are structured, it remains unknown if this endogenous water structuring is influenced by drinking SWs. The mechanisms by which SW affects biological systems are unknown and require investigation. Effects of SW, when taken up by biological systems, are likely associated with altered water structuring around biological surfaces, such as proteins and membranes.

The effect of hydrogen bonds on the ultrafast relaxation dynamics of a BODIPY dimer.

The influence of hydrogen bonds (H-bonds) in the structure, dynamics, and functionality of biological and artificial complex systems is the subject of intense investigation. In this broad context, particular attention has recently been focused on the ultrafast H-bond dependent dynamical properties in the electronic excited state because of their potentially dramatic consequences on the mechanism, dynamics, and efficiency of photochemical reactions and photophysical processes of crucial importance for life and technology. Excited-state H-bond dynamics generally occur on ultrafast time scales of hundreds of femtoseconds or less, making the characterization of associated mechanisms particularly challenging with conventional time-resolved techniques. Here, 2D electronic spectroscopy is exploited to shed light on this still largely unexplored dynamic mechanism. An H-bonded molecular dimer prepared by self-assembly of two boron-dipyrromethene dyes has been specifically designed and synthesized for this aim. The obtained results confirm that upon formation of H-bonds and the dimer, a new ultrafast relaxation channel is activated in the ultrafast dynamics, mediated by the vibrational motions of the hydrogen donor and acceptor groups. This relaxation channel also involves, beyond intra-molecular relaxations, an inter-molecular transfer process. This is particularly significant considering the long distance between the centers of mass of the two molecules. These findings suggest that the design of H-bonded structures is a particularly powerful tool to drive the ultrafast dynamics in complex materials.