2D assemblies

For discotic molecules with long alkyl side chains, 2D assembly also often leads to mirror-symmetry-broken structures. In addition to steric hindrance and superlattice formation, the mechanism involves conformational mobility of the side chain [34], This has been shown for the Cu-phathlocyanine derivative CuPcOC8 on hopg using STM (Fig. 13). The C4v-symmetric molecule forms dense layers with interdigitated alkyl chains. This forces the molecules into a rotated structure [35,36]. 

Figure 4. TEM images of (A) a 2D assembly of the 11 nm Co nanoparticles, and (B) a 3D assembly of the 8 nm cobalt nanoparticles on an amorphous carbon surface.

So-called superlattices of 5mn alkylthiolate protected silver particles having truncated octahedral shapes as well as thiol-stabilized 5 6 mn gold particles can be obtained from solution. Fractional crystallization is a very usual method to separate chemical compounds from other compounds and impurities. Mixtures of thiolate stabilized gold nanoparticles between 1.5 and 3.5 nm could successfully be fractioned into real monodisperse species containing 140, 225, 314, and 459 atoms. 2D assemblies have also become available of these fcc-structured nanoparticles. The decisive criterion to successhilly fraction and crystallize metal nanoparticles is to protect them perfectly by strongly bound ligand molecules in order to avoid coalescence. 

Fig. 13 X-ray crystal structures of two 2D assemblies with components that radiate from a central core (a) tris(imidazolium) triflate salt and (b) phloroglucinol and 2,4-dimethyl-pyridine solid.

Whereas most 2D finite crystalline assemblies exhibit a cyclic structure, 2D assemblies with components that radiate, or branch, from a central core have also been reported (Fig. 13).30,31 In particular, Kraft and Frohlich have reported a tris(imidazolium) triflate salt with anions that assemble along the exterior of a 1,3,5-trisubstituted benzene (Fig. 13a).30 Three imidazoline groups directed the assembly... 

Whereas most functional 2D assemblies have been constructed using hydrogen bonds, Atwood et al. have recently described a functional 2D assembly sustained via van der Waals forces.52... 

D assemblies from propeller-like Fe11 complexes with chelating 2,2 -bipyridine ligands (appended with urea-like substituents) and inter-connected through NH---0 hydrogen bonds yielded 2D sheets of alternating chirality.56 Despite the presence of solvent-filled 1 nm cavities, the material maintained crystallinity to 564 K. 

Figure 9 2D assembly from a combination of linear Cu1 coordination polymers and amide1 "amide... 

Very available and promising square panels are tetrapyridyl porphyrins 15 and 16. In fact, 4-pyridyl-substituted porphyrin 15 has been frequently employed for infinite assemblies. In contrast, there seems to be difficulty in constructing discrete 3D assemblies from tetrapyridyl porphyrins, though there are some examples of 2D assemblies such as porphyrin squares 17 [27] and 18 [28] from dipyridyl porphyrins. 

The preparation of desired 2D assembly of nanoparticles is the second step of free and complicated patterning in the nanometer order, of course, the first step is the ID assembles. For the purpose, the unique 2D shape and designed molecules... 

---