Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Understanding self-assembly and the associated noncovalent interactions that connect complementary interacting molecular surfaces in biological aggregates is a central concern in structural biochemistry. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions of 1 to 10(2) nanometers (with molecular weights of 10(4) to 10(10) daltons). Structures in the upper part of this range of sizes are presently inaccessible through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication.

Molecular self-assembly and nanochemistry: A chemical strategy for the synthesis of nanostructures

Zwar steht der Entwicklung von nanometergrosen Strukturen prinzipiell nichts im Wege, doch setzt sich immer mehr die Auffassung durch, das sich Strukturminiaturisierungen unter die gegenwartig durch lithographische Techniken erreichbare 1-μ-Grenze als nicht mehr praktikabel erweisen werden. Es wurde daher deutlich, das nur durch ein grundlegendes Verstandnis der Selbstorganisation von funktionellen makroskopischen biologischen Strukturen mit Abmessungen im Nanometerbereich und sogar darunter (Verkleinerungsansatz) und durch die Erweiterung unseres Wissens uber die chemische Synthese von mikroskopischen Strukturen (Vergroserungsansatz) die Brucke zwischen Anspruch und Wirklichkeit bei Nanosystemen geschlagen werden kann. Die Konstruktion von Nanostrukturen und -systemen aus kleinen Molekulbausteinen ist das „engineering-up” zum Aufbau von molekularen Funktionseinheiten. Bedeutende Fortschritte konnen auf dem Gebiet der Nanowissenschaften erzielt werden, wenn die Konzepte, die in der Biologie gefunden wurden, auf die Chemie ubertragen werden. Im Zentrum dieser Aufgabe steht die Entwicklung von einfachen chemischen Systemen, die sich selbst durch gegenseitige Erkennung zu groseren Molekulaggregaten organisieren konnen. Die genaue Programmierung derartiger Erkennungsprozesse und somit auch der korrekte Aufbau der Uberstrukturen setzen ein fundamentales Verstandnis und die Nutzung inter- sowie intramolekularer nichtkovalenter bindender Wechselwirkungen voraus. Die supramolekulare Chemie – eine Chemie, die in jeder Hinsicht uber die Chemie der Molekule hinausgeht – hat begonnen, den grosen Graben zwischen molekularen und makromolekularen Strukturen zu schliesen. Durch Nutzung von so unterschiedlichen Wechselwirkungen wie aromatischen π-Stapel- und Metall-Ligand-Koordinationswechselwirkungen als Informationsquellen der Aufbauprozesse haben Chemiker in den letzten zehn Jahren biologische Konzepte wie die Selbstorganisation zur Konstruktion von Nanostrukturen und Uberstrukturen mit einer Vielzahl von Formen und Funktionen herangezogen. Wir wollen hier einen Eindruck davon vermitteln, wie die Selbstorganisation in naturlichen Systemen funktioniert und wie diese Prinzipien nutzbringend auf nichtnaturliche Systeme angewendet werden konnen.

Selbstorganisation in natürlichen und in nichtnatürlichen Systemen

As a result of cooperative noncovalent bonding interactions (namely, π–π stacking, [CH…O] hydrogen bonding, and [CH…π] interactions) supramolecular complexes and mechanically interlocked molecular compounds—in particular pseudorotaxanes (precatenanes) and catenanes—self-assemble spontaneously from appropriate complementary components under thermodynamic and kinetic control, respectively. The stereoelectronic information imprinted in the components is crucial in controlling the extent of the formation of the complexes and compounds in the first place; moreover, it has a very significant influence on the relative orientations and motions of the components. In other words, the noncovalent bonding interactions—that is, the driving forces responsible for the self-assembly processes—live on inside the final superstructures and structures, governing both their thermodynamic and kinetic behavior in solution. In an unsymmetrical [2]catenane, for example, changing the constitutions of the aromatic rings or altering the nature of substituents attached to them can drive an equilibrium associated with translational isomerism in the direction of one of two or more possible isomers both in solution and in the solid state. Generally speaking, the slower the components in mechanically interlocked compounds like catenanes and rotaxanes move with respect to each other, the easier it is for them to self-assemble.

Controlling Self-Assembly

Big and little Meccano

A novel class of preorganized U-shape calix[4]arene clefts, dicationic salts 3a,c,e2Cl, 3b2PF(6)(), and 3f2Br, consisting of one cone calix[4]arene and two bipyridine residues being linked by an aliphatic chain, have been designed and synthesized as precursors for self-assembly of calix[4]arene [2]catenanes by utilizing pi-stacking interactions between the hydroquinone and bipyridinium units Conformationally flexible 62PF(6)() and cone 102Cl, whose conformation is fixed by two propyloxy groups on the lower rim, were also prepared in order to explore the effects of conformation and hydrogen bonding of the calix[4]arene moiety on self-assembly For all reactions, bis-p-phenylene-34-crown ether-10 (11) was employed as the donor component Alternate cone [2]catenane 134Cl is obtained in 8% yield from reaction of ethylene-incorporating 3a2Cl and 1,4-bis(bromomethyl)benzene (12a) Three cone and one conformationally flexible [2]catenanes were obtained in moderate to good yields from reactions of propylene-incorporating 3bPF(6)() and 3c2Cl with 12a, 1,3-bis(bromomethyl)benzene (12b) or 4,4'-(bromomethyl)biphenyl (12c) Both cone and partial cone [2]catenanes were generated in moderate yields from butylene-incorporating 3c2Cl with four tert-butyl groups on the calix[4]arene moiety and with 12a In contrast, only cone [2]catenane was obtained from similar tert-butyl-free cleft 3d2Cl Cone and conformationally flexible [2]catenanes were obtained in moderate yields, respectively, from the reactions of 3d2Cl and 3e2Cl with 12c No catenanes were isolated from reaction of phenylene-incorporating 3f2Br or 62Cl, whereas reaction of 102Cl afforded cone [2]catenane in low yield It was demonstrated that hydrogen bonding, which may be destroyed after catenation, within the calix[4]arene moiety is crucial for efficient self-assembly of the [2]catenanes The dynamic (1)H NMR and absorption spectra and luminescent properties of the [2]catenanes were investigated, which reveal that incorporation of calix[4]arene into the tetracationic cyclophane reduces pi-stacking interactions between the donor and acceptor units and catenation has substantial influence on conformational distributions of the calix[4]arene moiety The results demonstrate the versatility of calix[4]arene derivatives as building blocks in the construction of supramolecular structures

Isobel Iriepa

Papers

An optically-active [2]catenane made to order