Molecules with well-defined shapes and multiple sites that engage in strong directional interactions are now widely used to build new ordered materials by design. 1 Such molecules, which have been called tectons, 2 associate spontaneously to form networks with architectures that can often be predicted in detail. Tectons are a prolific source of engineered three-dimensional (3D) and twodimensional (2D) crystals with predefined structures and properties. 3Trimesic acid (1) and its salts are prototypic tectons. 4 3D and 2D crystallization of the acid is typically directed by association of the three -COOH groups as cyclic hydrogen-bonded pairs, which generates the planar hexagonal network represented by structure I (Figure 1). 4,5 Expanded versions of the network can be built from molecules derived from triacid 1 by inserting spacers between the -COOH groups and the 1,3,5-trisubstituted phenyl core. 6 However, the utility of tecton 1 and its extended analogues is limited by (1) the inability of normal pairwise association of the -COOH groups to program the construction of networks other than polymorph I 7 and (2) the small number of hydrogen bonds (6) in which each molecule can participate.Related tetraacids 2-4, as well as other derivatives with multiple isophthalic acid groups grafted to suitable cores, can provide planar networks that are both richer in variety and more robust. In particular, we have found that 2D crystallization of tectons 2-4 can be controlled to produce two polymorphs, parallel network II or Kagomé network III. 8 Moreover, the special connectivity of these networks allows their growth to be interrupted by a smooth transition to the alternative polymorph, without necessarily introducing defects in which tectons are missing, improperly oriented, or unable to form the optimal number of hydrogen bonds with neighbors. We show that this can frustrate crystallization, particularly when tectons 2-4 are mixed.Tetraacid 2 was prepared by the reported method, 9 and elongated analogues 3 and 4 were synthesized by standard procedures. 10 Specifically, Sonogashira coupling of diethyl 5-iodo-1,3-benzenedicarboxylate with diethyl 5-ethynyl-1,3-benzenedicarboxylate, 11 followed by hydrolysis, provided tecton 3 in 71% overall yield. An analogous route converted diethyl 5-iodo-1,3-benzenedicarboxylate and 1,4-diethynylbenzene into extended tecton 4 in 68% overall yield.In a typical 2D crystallization, a droplet of a saturated solution of tecton 2 in heptanoic acid was placed on freshly cleaved highly oriented pyrolytic graphite (HOPG) at 25°C, and the resulting physisorbed assembly was imaged by scanning tunneling microscopy (STM) in the constant-height mode (Figure 2). Figure 2a reveals the formation of an open parallel network in which the tectons associate by hydrogen bonding according to motif II ( Figure 1), with unit cell parameters a ) b ) ∼1.36 nm and γ ) ∼90°. This interpretation is reinforced by the observation of homologous sheets in the 3D crystal structure of tecton 2. 9 In the 3D structure, tecto...

The molecular orientation and conformation of methyl pyruvate on Ni(111) was studied in the temperature range 105-220 K. The full monolayer formed at 105 K was found to be almost exclusively in the bidentate cis-conformation, with the molecular plane oriented perpendicular to the surface. In contrast, the low coverage layer at 105 K was found to be composed of a mixture of trans-and cis-methyl pyruvate. However, direct exposure at 200-220 K yielded exclusively cis-bidentate adsorption at all coverages. The observation of the preferred cis-bidentate species is at odds with the adsorption geometry and conformation usually assumed in rationalizations of the enantioselective hydrogenation of methyl pyruvate on chiral-compound modified platinum metal particle catalysts.

We calculated the effect of varying the length of a metal-semiconductor carbon nanotube junction on its electrical properties. Joining a metallic (5,5) tube to a semiconducting (10,0) tube leads to the creation of new states near the Fermi energy and produces a larger conductance gap (about 2 eV) than the band gap of the semiconducting segment (about 1 eV). The new states reflect the charge transfer from the (5,5) to the (10,0) segment. The larger conductance gap is due to the mismatch in the conducting states of the (5,5) and (10,0) segments. Although the number of states in the vicinity of E F increases significantly with increasing nanotube length, the electrical behavior of the junction does not acquire the characteristics of the semiconducting segment. The calculations suggest that the (5,5)/(10,0) nanotube junction could behave as an intrinsic diode for lengths as small as 4 nm.

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