摘要:

IntroductionSymmetry lowering in organic solid solutions was first demonstrated by X-ray diffraction.1Here we show a systematic approach to polarity formation in H1 − xGx(H = 4,4′-dinitrostilbeneDNS, G = 4-chloro-4′-nitrostilbeneCNS, both crystallising inP21/c) extending abovex> 0 up to 1. Orientational disorder and weak hydrogen bond interactions are resposible for the observed second harmonic effects. Phase sensitive second harmonic microscopy revealed a bi-polar state for crystals as predicted by the Markov model of polarity formation.2Recently, we have presented a general principle3for obtaining molecular crystals showing pyroelectric and other polar properties usually exhibited by particular acentric crystal structures. Essentially, all structures of molecular crystals composed of dipolar (but not necessarily chiral) compounds may grow sector-wise into a polar real-structure exhibiting a characteristic non-geometrical twinning.4Symmetry lowering to acentric space groups by H1 − xGxformation was first confirmed by X-ray diffraction.1As demonstrated by many examples, stochastic polarity formation can be due to functional group interactions during the growth of crystalline materials.5Dipolar molecules entering attachment sites at slow growing crystal surfaces can show orientational selectivity with respect to an “up” or “down” orientation of dipoles.Formally, dipolar molecules may be composed of some acceptor (A)- and donor (D)-type fragments attached to a π-frame. Whatever the A and D fragments are, when entering the crystal structure, interactions –A⋯A– and –D⋯D– may show corresponding probabilitiesPAA,PDDdriving the attachment in the surface layer. As a result ofPAA≠PDDfor attachments at certain surface sites of topologically centric structures, polarity can evolve during growth. Growth induced polarity formation has been reported for a number of channel type inclusion compounds6and other materials.5In this respect, solid solutions represent particularly interesting systems which allow a tuning of polar properties. The basic mechanism for polarity formation in H1 − xGx(where H is a symmetrical and G a dipolar molecule, both crystallising in acentricspace group) can be retrieved fromFig. 1. Two cases are discussed starting crystal growth from a seed obtained from either symmetrical molecules A–π–A (Fig. 1a, molecule —) or dipolar molecules D–π–A (Fig. 1b, molecule →). It is assumed that both native structures are centric. As a matter of specific interactions at the crystal–nutrient interface (seeFig. 3), polar sectors can form. When G molecules A–π–D enter the H (H =e.g.A–π–A) lattice (Fig. 1a) byb-sectors inP21/cthere is a probabilityPAD(–A⋯D– interaction) to align preferably along one orientation, ifPAD≫PAA(PAA= probability of having a packing –A⋯A–) forx≪ 1. Because of dilution effected by H, G molecules do not exceed the influence of lateral interactions, which are responsible for the centric packing of the A–π–D molecules (in most cases). As a matter of fact, effects of polarity should increase forx> 0, as far as lateral interactions between A–π–D guests have a minor influence. Close tox≈ 1, polarity will reach the level of intrinsic orientational disorder in the structure of G. A similar analysis can be made forFig. 1b, where a centric seed obtained from A–π–D molecules transforms into a bi-polar object because of chain inversion introduced by A–π–A (similar for D–π–D) G molecules.Fig. 1a and 1btherefore describe the mechanisms of polarity formation at both ends of the composition of H1 − xGx.Formation of sectors showing polar symmetry in solid solutions starting either from a centric crystal structure of symmetrical molecules or from a centric crystal structure of a dipolar molecule. a) Solid solution of D–π–A (G) e.g.in A–π–A (H), centric seed. b) Solid solution ofe.g.A–π–A (G) in D–π–A (H), centric seed of D–π–A.

ISSN:1466-8033    

DOI:10.1039/b211829g

出版商:RSC    

年代:2002

数据来源: RSC