Hydrogen bond-mediated self-assembly is a powerful strategy for generating large structures from smaller subunits. The synthesis of molecules containing two isophthalic acid units covalently attached to a rigid aromatic spacer is described. By normal pairing of carboxylic acids into hydrogen-bonded dimers, these molecules self-assemble in organic solvents to form either a series of linear aggregates or a cyclic hexamer. These molecules were linked to the core of a family of polyether dendrimers, which caused the hexamer to be formed preferentially. The stability of the hexamer depended on the generation number of the dendrimer. The largest of these hydrogen-bonded macromolecular assemblies is roughly disk-shaped with a 9-nanometer diameter and a 2-nanometer thickness. Its size and molecular mass (34,000 daltons) are comparable to that of small proteins.

COMMUNICATIONS \pectroscop\ (AAS) after di\mlution of known quantities of the zeolites materials i n HF'H,SO, AAS mcasureinents &'ere performed with an Instrumentation Labo-Inc. apparatus uith B nitrous oxide,acetylene flame Scanning electron miiphs (SEM) were obtained by using a Jeol Superprobe 733 instrument. while diffuse reflecrmcc spectroscopy (DRS) was performed on a Varian Cary 5 UVWIS! NIR spectrophotometer at room teniperitture. The diffuse reflectance spectra were recorded against A h;ilon white rellectmce standard in the range 2200 -~200 nm. The computer processing of the spectra consisted of the following steps: 1 ) subtraction of the baseline. 2 ) conversion to wavenumber. and 3) calculation of the Kubelka-Munk ( K M ) function. For ESR spectroscopy. a Bruker ESP300E spectrometer at X-band (ca. 9.5 GHz) was used. Quantitative ESR results were obtained by comparison with Cu(acetylacetonate)..KCI reference samples (spin density : l o i h -10" CU" g-'). The molecular models were generated by the commercial software package HyperchemTM of Autodesk.

Heterocyclic unit 2 containing complementary donor-donor-acceptor (DDA) and acceptor-acceptor-donor (AAD) hydrogen bonding arrays at an angle of about 60 degrees was designed to self-assemble into a hexamer. To investigate whether this unit could self-assemble dendrimers, the 2,8-diamino-2-N-ethylpyrimido-(4,5-b)(1,8)naphthyridine-3H-4-one subunit was synthesized with a first (2a), second (2b), and third generation (2c) Fréchet-type dendron attached to the 8-amino group. The synthesis of 2a-c was accomplished in 11 steps beginning with 2,6-diaminopyridine and the corresponding dendron bromide. Studies using (1)H NMR, size exclusion chromatography (SEC), and dynamic light scattering (DLS) support the cooperative formation of cyclic hexamers in apolar solvents. The stability of the self-assembled dendrimers is dependent on the size of the attached dendron, and mixing studies with 2a-c indicate their usefulness in constructing dynamic combinatorial libraries.

A crystalline inclusion complex between 1,3,5-benzenetricarboxylic acid (trimesic acid, 1) and pyrene was grown from diethyl ether/ethanol and its structure was determined by X-ray analysis. Trialkyltrimesic acids 4 b ± 4 e were synthesized in seven steps from 1,3,5-trichlorobenzene (5). Trimethyltrimesic acid (4 a) was synthesized in four steps from mesitylene. All five substituted trimesic acids were crystallized to determine their potential as clathrate hosts and to compare their solidstate structures with the parent trimesic acid. The X-ray structures of 4 a ± 4 d were resolved and are reported. A covalent analogue of a trimesic acid dimer (13) was synthesized in seven steps from 2-butylisophthalic acid and its solid-state structure determined.

No abstract