Multicomponent Molecular Systems Research Articles

Stiff, and Sticky in the Right Places: Binding Interactions in Isolated Mechanically Interlocked Molecules Probed by Mid-Infrared Spectroscopy

We report the results of high-resolution spectroscopic studies on isolated, jet-cooled [2]rotaxanes. Employing IR absorption spectroscopy, we show how these noncovalently bound, multicomponent molecular systems that so far have been out of reach from high-resolution techniques, can now be characterized at an unprecedented level. IR absorption spectra of prototypical hydrogen-bond assembled rotaxanes as well as their associated threads and macrocycles are shown to provide a direct view on the effects of interlocking the macrocycle and thread, and to offer a straightforward approach for the study of their structural and dynamical properties.

Multi-component supramolecular assembly structures studied by scanning tunnelling microscopy

In this review, recent progress on the multi-component self-assembled structure and assembling mechanism are discussed. The effects of various interaction types on the assembling structures are summarised to gain better understanding and controlling on the functional molecular assemblies. We are focused on the hydrogen-bonded supramolecular structures, template effects of assemblies and the host-guest architectures in multi-component molecular systems, which are very essential to design and build two-dimensional molecular structures.

“Mechanics” of Molecular Assembly: Real-Time and In-Situ Analysis of Nano-to-Mesoscopic Scale Hierarchical Structures and Nonequilibrium Phenomena

Abstract We have presented studies of the mechanics of molecular assembly for multicomponent molecular systems in the context of nonlinear and nonequilibrium phenomena. The studies cover the following three themes: (i) elucidation of space-time self-assembly of hierarchical structures, in the length scale ranging from nm to μm, of binary molecular mixtures via phase transition by means of real-time and in-situ analyses with various scattering methods and microscopy methods; (ii) elucidation of self-assembly of molecular mixtures under shear flow in the context of open nonequilibrium phenomena by means of newly developed rheo-optical methods, leading us to discover new dissipative structures as well as shear-induced phase transitions both from single-phase to two-phase and from two-phase to single-phase; (iii) elucidation of the influences of chemical bonds between different component polymers in block copolymers on microphase transition and ordered microdomain structures with nano-scale periodicity and various space-group symmetries and exploration of nano-fabrication methods of microdomain structures as templates in the context of nano-science and nano-technology of block copolymers. In each of the three themes, we highlight and emphasize the fact that polymer systems provide very good model systems for basic studies of nonlinear and nonequilibrium phenomena because of the fact that large structures evolve very slowly in polymer systems. The studies may give perspectives for creating advanced soft materials responsive to environmental stimuli and for understanding activities, adaptation, and evolution of living organisms.

Direct calculation of bubble points by Monte Carlo simulation

The calculation of bubble points, i.e, the conditions in which a liquid starts to form a vapour phase, is a problem of industrial interest in petroleum and chemical engineering. Phase equilibrium at specified global composition, temperature and volume—or pressure—can be computed by the Gibbs Ensemble Monte Carlo method (GEMC). However it is not directly applicable to bubble points, which require one to solve the problem where the liquid composition and temperature are specified, and while pressure and vapour composition are sought. In order to provide an efficient algorithm, the present article proposes a pseudo ensemble in which liquid mole numbers, temperature and global volume are imposed, while mole numbers in the vapour phase and phase volumes are allowed to fluctuate. The partition function, probability density and thermodynamic potential are given. This pseudo ensemble satisfies the conditions of mechanical and thermal equilibrium, while chemical equilibrium is satisfied by imposing the chemical potential of the liquid phase to the vapour. A Monte Carlo method is proposed to generate a Markov chain simulating the ensemble, with a specific move for molecule insertion or deletion in the vapour phase. Various solutions are considered to evaluate the chemical potential of the liquid phase using test insertion methods. Finally, we show that an efficient procedure for bubble point calculation is to use the proposed algorithm as an initialization of a GEMC simulation run. The method has been tested on the argon—krypton binary system for which a Lennard-Jones intermolecular potential enables one to reproduce experimental vapour—liquid equilibrium data. It is shown that the method provides a satisfactory prediction of phase diagrams and that it is applicable to near-critical conditions. Its application to determine bubble points of multicomponent molecular systems thus appears promising.

Rotaxanes Incorporating Two Different Coordinating Units in Their Thread: Synthesis and Electrochemically and Photochemically Induced Molecular Motions

Three different multicomponent molecular systems have been synthesized by means of the three-dimensional template effect of copper(I). These systems incorporate both a coordinating ring (2,9-diphenyl-1,10-phenanthroline-containing 30-membered ring) and a molecular string which consists of two different coordination sites (2,9-disubstituted-1,10-phenanthroline and 5,5‘ ‘-disubstituted-2,2‘:6‘,2‘ ‘-terpyridine unit). Each end of the string could be functionnalyzed by a small group or by a bulky stopper (tris(p-tert-butylphenyl)(4-hydroxyphenyl)methane), leading to an unstoppered compound, to a semi-rotaxane, or to a real rotaxane. As in the case of a disymmetrical copper [2]-catenane, large reversible molecular motions have been induced both electrochemically and photochemically. The driving force of the rearrangement processes is the high stability of two markedly different coordination environments for the copper(I) and copper(II) ions. In the copper(I) state, two phenanthroline units (one of the ring, one of the string) interact with the metal ion in a tetrahedral geometry (CuI(4)), whereas in the copper(II) state, one phenanthroline belonging to the ring and the terpyridine of the string afford a five-coordinate geometry (CuII(5)). The rates of the molecular motion processes (from CuII(4) to CuII(5) and from CuI(5) to CuI(4)) are respectively faster and slower (minutes time scale) as compared to those for the catenane species. This result could be interpreted on the basis of structural differences between the rotaxane and catenane systems.

Construction of One‐Dimensional Multicomponent Molecular Arrays: Control of Electronic and Molecular Motions

Transition metals are powerful three-dimensional templates, which can gather various functionalized ligands and orient them in a precise fashion so that complex multicomponent molecular systems can be obtained without constructing covalently-assembled edifices. The compounds thus prepared exhibit precise chemical or physical functions, which are governed by the design of the system. The construction of one-dimensional molecules around ruthenium(II) or osmium(II), using rigid ligands attached to the desired electroactive species, leads to systems that are able to undergo charge separation after photonic excitation. In other related compounds, a ruthenium(II)-based chromophore is, for example, connected to an osmium(II) complex by means of rod-like bridging ligand, thereby ensuring strict control over the Ru···Os distance. By tuning the length and the electronic properties of the bridge, one can control the efficiency of the electronic energy transfer between the two chromophores. In particular, the use of a bis-cyclometallating ligand is very conducive to energy transfer and allows the observation of this process up to a Ru…Os distance of ≈20 A. By combining the building blocks of these inorganic systems with appropriate porphyrins, long-range (centre-to-centre distance between the donor and the acceptor porphyrins ≈30 A) and relatively long-lived photoinduced charge separation has been demonstrated. Finally, with copper(I) as the template, compounds of the rotaxane family are obtained, which consist of a coordinating ring threaded by a string-like component. If this acyclic fragment is end-functionalized by two bulky stoppering groups and if it incorporates two different coordination sites (a bi- and a terdentate chelate site), novel dynamic properties are observed. The movement of a given fragment of the molecule is triggered by changing the metal oxidation state. This one-dimensional motion of the ring along the string on which it is threaded is controlled by redox manipulation, resulting in a primitive molecular machine.

Fullerenes linked to photosynthetic pigments

The synthesis and photochemical characterization of two porphyrin-fullerene dyads, two zinc porphyrin-fullerene dyads, and a carotenobuckminsterfullerene are reviewed. In these molecules, the fullerene first excited singlet state may be formed by direct excitation or by singlet-singlet energy transfer from the attached pigment. In polar solvents, the dominant singlet-state decay pathway is photoinduced electron transfer to yield the pigment radical cation and fullerene radical anion. This charge-separated state has a long lifetime relative to the time constant for charge separation. In toluene, in cases where photoinduced electron transfer is slow for thermodynamic reasons, the fullerene singlet state decays by intersystem crossing, and the resulting triplet energy is partitioned between the components of the dyad according to their triplet energies. The results suggest that fullerenes can be valuable components of photochemically active multicomponent molecular systems.

Transition metals as assembling and templating species: From catenanes and knots to strings threaded through molecular rings

Abstract

Transition metal directed threading of molecular strings into coordinating rings

Multicomponent molecular systems consisting of rings, coordinating open-chain fragments and transition metals can be constructed at will; they consist of one or two rings threaded by the acyclic subunit, the various organic components being assembled via coordination to copper(I).