Topochemical assembly

Dihydrogen bonds can be preorganizing interactions for the topochemical assembly of covalent materials due to solid-state H2 elimination. 

The central problem in organic solid-state photochemistry is the preorganization of molecules satisfying the topochemical postulates. Schmidt coined the term crystal engineering for this problem of supramolecular assembly. Indeed, the importance of crystal engineering is fundamental to areas as diverse as nonlinear optics, high- /) superconductors, and the generation of polymorph forms in pharmaceuticals. 

Figure 15.19 Light induced topochemical polymerisation of a diacetlyene self-assembled layer on HOPG.26...

Topochemical Polymerization Using Self-Assembled Scaffolds... 

These examples serve to highlight that supramolecular self-assembly and topo-chemical diacetylene polymerizations are a perfect match. Topochemical diacetylene polymerizations are an advantageous means of covalent capture for the reasons outlined above. The required order may, on the other hand, be provided by supramolecular self-assembly, which extends the scope beyond singlecrystalline monomers. This aspect becomes particularly important in the case of functional monomers in order to address specific applications. However, in contrast to previous investigations, the targeted preparation of hierarchically structured poly (diace tylene)s with a defined, finite number of strands required the presence of equally well-defined, uniform supramolecular polymers [106] with the propensity to form predictable superstructures, instead of micellar or vesicular ID aggregates. 

The molecular assemblies described above have inspired us, in recent years, to develop finite assemblies in the solid state that exhibit chemical reactivity. Specifically, we,69 and others,70 have been utilizing principles of molecular recognition and self-assembly to develop a method to direct the formation of covalent bonds in organic solids. The method builds on the work of Schmidt on the reactivity of cinnamic acids in the organic solid state.45 Specifically, Schmidt has described topochemical postulates that dictate geometry criteria for a [2 + 2] photodimerization to occur in a solid. The postulates state that two carbon-carbon double (C=C) bonds should be aligned in parallel and separated by a distance <4.2 A to react. 

Last but not least, rigid cydophanes can be used to assemble functional groups in defined orientation in space [15]. Such orientation determines in many cases the stereochemistry of their reactions, so that topochemical reaction control [16], previously restricted to the solid state, was expanded into solution. The use of functionalized cydophanes as stereochemical reaction control elements will be summarized in chapter 5 of this article. 

Custelcean and Jackson have demonstrated that anionic boron hydrides can also serve as hydrogen bond acceptors to assemble novel lattices held together by hydridic-protonic bonds [107, 108]. Cations with hydrogen bond donors include A -[2-(6-aminopyridyl)]acetamidine and triethanolamine(sodium). The former is an NH donor while the latter is an OH donor. These scientists provided evidence for the topochemical conversion of the hydridic-protonic bonds into B-N and B-0 covalent bonds, respectively, when the materials are heated and dihydrogen is driven from the H-H bonds. 

Chart 10.7 Typical diacetylene derivatives capable of undergoing topochemical photopolymerization. Left classical examples [30]. Right Self-assembling bolaamphiphilic diacetylenes [67]. 

In a notable success, a J-shaped dicarboxylic acid was shown by Feldman to form a hydrogen-bonded molecular assembly held together by four O-H-. -O hydrogen bonds, wherein two olefins conform to the topochemical principle and are photoactive(Fig. 2b). UV irradiation produced the expected photoproduct in quantitative yield. The covalently bound naphthalene unit served to preorganize the two double bonds within the discrete complex for reaction. 

The ability of resorcinol to function as a linear template to organize reactants in the solid state for [2 + 2] photoreaction also led to a molecular solid-state synthesis by design.Specifically, cocrystallization of lA-bis[2-(4-pyridyl)ethenyl]benzene (1.4-bpeb) with 5-methoxy-resorcinol (5-OMe-res) yielded a four-component assembly. 2(5-OMe-res) 2(1,4-bpeb), wherein four olefins, as two reaction centers, conformed to the topochemical principle. UV irradiation of the solid produced, regio- and stereoselectively. a targeted [2.2]paracyclophane (yield 60%) (Fig. 4c). 

Combining mechanochemical self-assembly and topochemical [2 + 2] cycloadditions... 

In general, photoinert auxiliary molecules have been used as templating agents to direct double bonds in parallel orientations of single olefins and with distances smaller than 4.2 A. These geometrical parameters are strictly required as previous steps in order to achieve the photoreaction via topochemical control and then the template must be liberated from the expected product. We envisage that an alternative way to increase the level of sophistication in the design of solid state reactions is the possibility to extend the self-assembly of two or more potentially reactive unsaturated molecules controlled by directional supramolecular synthons. 

The combination of topochemical photoreactions with hydrothermal methods opens a window to develop efficient routes for the preparation and/or improvement of the yield of new and conventional cyclobutane stereoisomers that are difficult or impossible to access either in solution or by known solid state strategies, including the possibility of inducing chirality from achiral cyclobutane derivatives. The combination of such alternatives can be very helpful in order to overcome the limitations imposed by the topochemical postulate for obtaining regioselective photoproducts with such stereochemical requirements from crystalline assemblies. 

Polymerization of self-assembled LMOG molecules within the fibers of a molecular gel locks the scaffold-like structures and can make new stable materials [99,100]. Photopolymerization in the gels of diacetylenic cholesteryl derivatives, such as 63X-n [88,89], has been successfully employed to access organic nanowires with potential electrical conductivity, which could be used in molecular electronics. This strategy was based on the known propensity of diacetylenes to undergo topochemical soUd-state 1,4-addition reactions leading to conjugated polymers [101]. 

Moreover, the diyne surfactants represent excellent probes for the study of the physical state of order of surfactant assemblies, due to the anisotropic properties of the polymer, and the topochemical control of the polymerization process, which easily allows to distinguish between ordered and disordered regions. 

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