Because of its fundamental importance in many branches of science, hydrogen bonding is a subject of intense contemporary research interest. The physical and chemical properties of hydrogen bonds in the ground state have been widely studied both experimentally and theoretically by chemists, physicists, and biologists. However, hydrogen bonding in the electronic excited state, which plays an important role in many photophysical processes and photochemical reactions, has scarcely been investigated.Upon electronic excitation of hydrogen-bonded systems by light, the hydrogen donor and acceptor molecules must reorganize in the electronic excited state because of the significant charge distribution difference between the different electronic states. The electronic excited-state hydrogen-bonding dynamics, which are predominantly determined by the vibrational motions of the hydrogen donor and acceptor groups, generally occur on ultrafast time scales of hundreds of femtoseconds. As a result, state-of-the-art femtos...

Hydrogen bonding in the electronic excited state.

Supramolecular chemistry has expanded dramatically in recent years both in terms of potential applications and in its relevance to analogous biological systems. The formation and function of supramolecular complexes occur through a multiplicity of often difficult to differentiate noncovalent forces. The aim of this Review is to describe the crucial interaction mechanisms in context, and thus classify the entire subject. In most cases, organic host-guest complexes have been selected as examples, but biologically relevant problems are also considered. An understanding and quantification of intermolecular interactions is of importance both for the rational planning of new supramolecular systems, including intelligent materials, as well as for developing new biologically active agents.

Binding Mechanisms in Supramolecular Complexes

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Hydrogen Bonding in the Electronic Excited State

Various present and future specialized applications of magnets require monodisperse, small magnetic particles, and the discovery of molecules that can function as nanoscale magnets was an important development in this regard. These molecules act as single-domain magnetic particles that, below their blocking temperature, exhibit magnetization hysteresis, a classical property of macroscopic magnets. Such 'single-molecule magnets' (SMMs) straddle the interface between classical and quantum mechanical behaviour because they also display quantum tunnelling of magnetization and quantum phase interference. Quantum tunnelling of magnetization can be advantageous for some potential applications of SMMs, for example, in providing the quantum superposition of states required for quantum computing. However, it is a disadvantage in other applications, such as information storage, where it would lead to information loss. Thus it is important to both understand and control the quantum properties of SMMs. Here we report a supramolecular SMM dimer in which antiferromagnetic coupling between the two components results in quantum behaviour different from that of the individual SMMs. Our experimental observations and theoretical analysis suggest a means of tuning the quantum tunnelling of magnetization in SMMs. This system may also prove useful for studying quantum tunnelling of relevance to mesoscopic antiferromagnets.

Exchange-biased quantum tunnelling in a supramolecular dimer of single-molecule magnets

Upon cooling, water freezes to ice. This familiar phase transition occurs widely in nature, yet unlike the freezing of simple liquids, it has never been successfully simulated on a computer. The difficulty lies with the fact that hydrogen bonding between individual water molecules yields a disordered three-dimensional hydrogen-bond network whose rugged and complex global potential energy surface permits a large number of possible network configurations. As a result, it is very challenging to reproduce the freezing of 'real' water into a solid with a unique crystalline structure. For systems with a limited number of possible disordered hydrogen-bond network structures, such as confined water, it is relatively easy to locate a pathway from a liquid state to a crystalline structure. For pure and spatially unconfined water, however, molecular dynamics simulations of freezing are severely hampered by the large number of possible network configurations that exist. Here we present a molecular dynamics trajectory that captures the molecular processes involved in the freezing of pure water. We find that ice nucleation occurs once a sufficient number of relatively long-lived hydrogen bonds develop spontaneously at the same location to form a fairly compact initial nucleus. The initial nucleus then slowly changes shape and size until it reaches a stage that allows rapid expansion, resulting in crystallization of the entire system.

Molecular dynamics simulation of the ice nucleation and growth process leading to water freezing

Ab initio calculations at the MP2/6-311++G** level on model systems (N-methylpyridinium complexes of dimethyl ether and dimethyl phosphate anion) provide quantitative measures of the large stabilization energies that arise from [C-H...O] contacts in charged systems. These attractive interactions control (i) the self-assembly of bipyridinium-based catenanes and rotaxanes in solution, (ii) the self-organization of left-handed Z-DNA with alternating [dC-dG] sequences in the solid state, and (iii) the binding of pyridinium derivatives with single- and double-stranded DNA. Slightly attractive interactions occur between the donor ether and phosphate moieties and a neutral pyridine molecule in the gas phase. Electrostatic potential and solvation calculations demonstrate that [C-H...O] interactions which involve a cationic [C-H] donor are dominated by electrostatic terms.

The magnitude of [C-H...O] hydrogen bonding in molecular and supramolecular assemblies.

One of the major challenges in supramolecular chemistry [1] is the characterization of the (super)structural properties of the (supra)molecular species resulting from self-assembly [2,3] processes Although the stereoelectronic properties of the supramolecular synthons are responsible for determining the geometries of the final self-assembled (super)strucures, in most instances the experimental conditions (eg, solvent, temperature, pressure, and stoichiometry) play a dominant role As a result, the prediction of the geometries of supramolecular entities is difficult and, more often than not, impossible even by means of computational methods [4] Experimental techniques that are able to establish the (super)structural properties of (supra)molecular species unequivocably are much in demand X-Ray crystallography has played a major role in determining the superstructures of supermolecules and supramolecular arrays, as well as the structures of large and complex molecular assemblies, in the solid state [5] The characterization of self-assembled monolayers at the air/water interface — and on solid supports — has been achieved employing sophisticated microscopic techniques eg, Brewster angle microscopy [6], scanning and transmission electron microscopies (SEM and TEM) [7], atomic force microscopy (AFM) [8] and scanning tunneling microscopy (STM) [9]} The combination of absorption and emission spectroscopies [10], along with electrochemical techniques [11], has permitted the characterization of self-assembled products in solution Chromatographic methods eg, high performance liquid chromatography (HPLC) [12] and gel permeation chromatography (GPC) [13]}, often coupled with mass spectrometric techniques eg, fast atom bombardment and liquid secondary ion mass spectrometries (FABMS and LSIMS) [14], electrospray mass spectrometry (ESMS) [15], and matrix-assisted laser desorption ionization time-of-flight mass spectometry (MALDI-TOF-MS) [16]}, are becoming more and more powerful analytical methods for the characterization of (supra)molecular species

Probing Self-Assembly by NMR

The goal of this proposal is to cast biomolecular motors as the model for designing wholly artificial motormolecules to serve as the building blocks for synthetic muscles. At the level of the single molecule, the artificial machines will be constructed from chemically powered motor-molecules with mechanically interlocked components. On the molecular ensemble level, the motormolecules will be coherently self-organized on to surfaces in order to harness the cooperative operation of each motormolecule in unison, consuming chemical energy to perform mechanical work up to the micro scale. These structures could be stacked in series to form artificial muscle tissue displaying large distance contraction and extension and to serve as a standard module for analogous applications on the meso scale. In the first phase of this work, the synthesis of the motor-molecules for attachment on surfaces is proposed, together with demonstrable nanoscale actuation as the target deliverable.

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J. F. Stoddart

Papers

The magnitude of [C-H...O] hydrogen bonding in molecular and supramolecular assemblies.

Probing Self-Assembly by NMR

Biomimetic Molecules as Building Blocks for Synthetic Muscles - A Proposal