Self-assembly is a powerful technique for the bottom-up construction of discrete, well-defined nanoscale structures. Large multicomponent systems (with more than 50 components) offer mechanistic insights into biological assembly but present daunting synthetic challenges. Here we report the self-assembly of giant M24L48 coordination spheres from 24 palladium ions (M) and 48 curved bridging ligands (L). The structure of this multicomponent system is highly sensitive to the geometry of the bent ligands. Even a slight change in the ligand bend angle critically switches the final structure observed across the entire ensemble of building blocks between M24L48 and M12L24 coordination spheres. The amplification of this small initial difference into an incommensurable difference in the resultant structures is a key mark of emergent behavior.

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Security inks have become of increasing importance. They are composed of invisible substances that provide printed images that are not able to be photocopied, and are readable only under special environments. Here we report a novel photoluminescent ink for rewritable media that dichroically emits phosphorescence due to a structural bistability of the self-assembled luminophor. Long-lasting images have been developed by using conventional thermal printers, which are readable only on exposure to ultraviolet light, and more importantly, are thermally erasable for rewriting. Although thermally rewritable printing media have already been developed using visible dyes and cholesteric liquid crystals, security inks that allow rewriting of invisible printed images are unprecedented. We realized this unique feature by the control of kinetic and thermodynamic processes that compete with one another in the self-assembly of the luminophor. This strategy can provide an important step towards the next-generation security technology for information handling.

Two different molecules are selectively included in cucurbit[8]uril to form a stable 1:1:1 ternary complex, which has been characterized by X‐ray crystallography (see picture). The inclusion of a hetero‐guest pair (a pyridinium derivative (blue) and 2,6‐dihydroxynaphthalene (magenta)) in the molecular host is driven and stabilized by a charge‐transfer interaction between the electron‐rich and electron‐deficient guests.

The structure of a molecular square assembling from [Pd(en)(NO&] 3 and 4,4'-bipyridine is confirmed by X-ray crystallography, whereas squares assembling from 3 and larger ligands, py-X-py (py = 4-pyridyl, X = CH=CH, C Z , p-C6H4) are shown to exist in equilibrium with molecular triangles.Since we first reported the self-assembly of square supramolecule 1 in 1990,* there have appeared many molecular squares293 in which transition metals, hypervalent iodine, or organic frameworks provide the 90" angle at the four edges of the square. It seems that these reports have made an impression that the combination of a right-angular component and a linear one always gives rise to the self-assembly of molecular squares. Such an understanding is, however, not fully consistent with our earlier results accumulated before 1990: although we observed the quantitative self-assembly of 1, square molecules existed in equilibrium with another structure when bipyridine or ethylenediamine ligands were replaced with larger ligands. Such observations as well as considerable current interest in the square structure4 prompted us to communicate our earlier5 and recent results on the structure of molecular squares, warning that a reliable method is required for studying their structures. Here, the following aspects are described. (i) Both the solid and solution structures of 1 were fully characterized. (ii) A molecular square exists in equilibrium with another discrete structure most probably assigned as a molecular triangle.

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Optically active phosphines play a most important role as the chiral ligands in various metal-catalyzed asymmetric reactions, and numerous chiral phosphines have been designed and synthesized over the past three decades. 1 Among them, some P-chiral phosphines such as (R,R)-1,2-bis[(o-methoxyphenyl)phenylphosphino]ethane (DIPAMP) were landmark discoveries at an early stage in the history of asymmetric hydrogenation reactions. 2,3 Thereafter, however, relatively less attention has been paid to P-chiral phosphine ligands in the field of asymmetric catalysis. 4 This is largely ascribed not only to the synthetic difficulty of highly enantiomerically enriched P-chiral phosphines but also to the fact that this class of phosphines, especially diaryl-and triarylphosphines, are configurationally unstable and gradually racemize at high temperatures. 5 On the other hand, optically active trialkylphosphines are known to hardly racemize even at considerably high temperature. 6 On the basis of this fact, we designed a new class of P-chiral phosphine ligands, 1,2-bis(alkylmethylphosphino)ethanes (alkyl ) tert-butyl, 1,1-diethylpropyl, 1-adamantyl, cyclopentyl, cyclohexyl) (abbreviated as BisP*) (Figure 1). 7 An important feature of these ligands is that a bulky alkyl group and the smallest alkyl S0002-7863(97)03423-9 CCC: $15.00