Alternating pyridine ± pyrimidine oligomers 1, 2, and 3, composed of nineteen, twenty one, and twenty seven heterocycles, respectively, have been synthesized in stepwise fashion and characterized. Examination of their 1 H NMR spectra revealed that these achiral nonbiological oligomers organize spontaneously into multiturn helical structures 1 A ± 3 A in solution, as indicated by marked upfield shifts of the aromatic protons coupled with distinct NOE effects. In view of their potential electron-acceptor properties compounds 1 ± 3 also represent coiled wires of nanometric lengths, up to about 90 for 3 in its extended form. The helical structure has been confirmed for 1 in the solid state by X-ray crystallography; a chiral channel arrangement involving only one helical enantiomer was found despite of the lack of chiral center in the strand. The oligomers exhibit a broad structureless fluorescence with a large Stokes shift, attributable to intramolecular pyridine excimer emission resulting from the helical ordering. Variable-temperature 1 H NMR experiments revealed that the oligomers exist as equilibrating mixtures of right-handed and left-handed helices. The barrier for helical handedness reversal was found to be independent on the length of the strand; two-(6), three- (1), and four-turn (3) helices showed comparable free energies of activation. Based on these observations, a stepwise folding mechanism involving two perpendicularly twisted pyridine ± pyrimidine units in the transition states may be proposed for the helicity inversion processes. The present results together with earlier work indicate that the pyridine ± pyrimidine sequence may be considered as a helicity codon, enforcing the formation of helical structures on the basis of intramolecular structural information.

The crystallization of 2,4,6-triethynyl-1,3,5-triazine (2) leads to a pi-stacked layered structure of a C(sp)-H...N hydrogen-bonded unique hexagonal network structure, which may be regarded as a supramolecular analogue of a hitherto unknown graphyne network: in-plane intermolecular interactions are short and linear H...N contacts (2.31 and 2.34 A) and the interlayer separation is 3.23 A.

The programmed self-assembly of a multimetallic grid-type supramolecular architecture and its hierarchical organization on graphite are described. The doubly functionalized 4,6-bis(2,2Ј-bipyridyl-6-yl)-2-phenylpyrimidine derivative 1, equipped with CH 2 OC 16 H 33 moieties at the terminal pyridine rings, was designed as a new bis(tridentate) ligand and synthesized using Stille-type coupling reactions. Treatment of ligand 1 with Fe(BF 4 ) 2 ·6H 2 O in CHCl 3 /CH 3 CN led to the spontaneous formation of a [2 × 2]-grid-type Fe II complex 2 in quantitative yield. The self-assembled tetranuclear complex 2 was adsorbed onto highly oriented pyrolytic graphite (HOPG) and studied by scanning tunneling microscopy (STM), which showed two morphologies of highly ordered two-dimensional arrays of the metallo-supramolecular archi-