Host-guest cocrystal

Lauher and Fowler et al. have proposed an elegant strategy for the control of topochemical polymerization based on the host-guest cocrystal concept. They used the ureylene and oxalamide functionality to form layered supramolecu-lar structures for the topochemically controlled polymerization of diacetylenes and 1,3-butadienes in the solid state [62,63]. 

Another approach is cocrystallization of a substituted urea (host) and a diacetylene (guest) [48]. Substituted ureas are used to prepare layered diacetylene crystals. Two examples are shown in Scheme 21. Such a host-guest/cocrystal approach to supramolecular synthesis should be general. In this strategy, the host is used to control the structure and the guest provides the function (optical, electrical, chemical, or physical). 

When the intermolecular forces are relatively strong and directional, crystalline molecular compounds (crystalline molecular complexes) are formed. They have fixed stoichiometries and ordered structures. These two-component molecular crystals are also called cocrystals or maybe adduct crystals. Hydrogen-bonded cocrystals, donor-acceptor crystals (charge transfer crystals), and inclusion crystals (host-guest crystals) are examples of crystalline molecular complexes. Crystalline organic salt is a special case of hydrogen-bonded cocrystal or donor-acceptor crystal, i.e., proton (or electron) transfer from the acid (or donor) to base (or acceptor) occurred. 

Rizzo P, Daniel C, Girolamo Del Mauro A, Guerra G. (2007) New Host Polymeric Framework and Related Polar Guest Cocrystals. Chem. Mater. 19 3864-3866... 

Already, more than 25 years ago, Toda et al. reported the mechanochemical preparation of crystalline host-guest inclusion compounds and of charge-transfer (CT) systems. CT systems have also been studied by Kuroda et al., who were able to obtain multicomponent systems based on racemic bis-)8-naphtol, benzoquinone, and anthracene the cocrystal formation could be easily followed by the change in color of the solid mixtures. Importantly, this material was not accessible from conventional solution crystallization procedures and required structure determination from X-ray powder diffraction. ... 

Rizzo, P, Daniel, C., De Girolamo Del Mauro, A., Guerra G. New host polymeric framework and related polar guest cocrystals. Chem. Mater., 19(16), 3864-3866 (2007). 

Figure 10.6 Schematic presentation of the orientation of the co-crystal chain axes (c) and of guest molecules, as obtained by p-nitro-aniline absorption in e form (a) and 6 form (b) films, presenting an ideal an Ci uniplanar orientation. (Reproduced with permission from Rizzo, E, Daniel, C., De Girolamo Del Mauro, A., Guerra, G. New host polymeric framework and related polar guest cocrystals. Chem. Mater., 19,3864—3866 (2007) [54]).

TGA of the toluene co-crystal 2 showed a mass loss of 14.8% at 183 °C. The host crystals with the empty channels were soaked in toluene for 36 hours and then subjected to TGA. These crystals showed a mass loss of 9.2% at 166 °C, a behaviour similar to that of the benzene co-crystal, 1. TGA of the /7-xylene cocrystal, 5, showed a mass loss (12.2%) due to the removal of the aromatic guest around 167 °C. After reintroduction of /7-xylene in the channels, the mass loss occurred around 139 °C, but the magnitude of mass loss was smaller... 

Itagaki, H., Sago, T., Uematsu, M., Yoshioka, G, Correa, A., Venditto, V, Guerra, G. Guest orientation in uniplanar-axial polymer host films and in cocrystal unit-cell, determined by angular distributions of polarized guest fluorescence. Macromolecules, 41, 9156-9164 (2008). 

---