SYMMETRY LOWERING IN CRYSTALLINE SOLID-SOLUTIONS - A STUDY OF CINNAMAMIDE-THIENYLACRYLAMIDE BY X-RAY AND NEUTRON-DIFFRACTION AND SOLID-STATE PHOTOCHEMISTRY

Principles are outlined for symmetry lowering of a mixed crystal compo sed of host and tailor-made additive molecules, based on selective occ lusion of the latter through a subset of surface sites of the growing crystal, the symmetry of the surface generally being lower than that o f the bulk. A survey is given of the various methods and approaches us ed to detect the reduction in symmetry. These include changes in cryst al morphology, detection of enantiomeric segregation of chiral additiv es in 'centrosymmetric' crystals, generation of second-harmonic optica l signals, optical birefringence, asymmetric photoreactions in the cry stalline state and X-ray and neutron diffraction. The last two methods are applied to mixed crystals of cinnamamide (host) and thienylacryla mide (additive). The diffraction analysis demonstrated that the mixed crystals are composed of six sectors of reduced symmetry, from monocli nic centrosymmetric P2(1)/c to triclinic P1 in four sectors and possib ly Pc in the remaining two. The X-ray diffraction data were not suffic iently accurate to permit assignment of the absolute structures of the P1 sectors with the use of anomalous X-ray scattering. Thus, by this method one could not ascertain the absolute orientation of the guest m olecules on the surface sites through which they were selectively occl uded. This ambiguity was resolved by assignment of the absolute config uration of the chiral heterophotodimers, between host and guest, in en antiomeric excess in the P1 sectors, after irradiation with UV light. These results led to the definite conclusion that the selective occlus ion of thienylacrylamide arises from a replacement of attractive C-H . .. pi (electron) interactions between host molecules by repulsive sulf ur (lone-pair electron) ... pi (electron) interactions between guest a nd host at the crystal surfaces.