Polarity of organic supramolecular materials: A tunable crystal property

Basic principles are reviewed which provide new access to the synthesis of acentric molecular crystals along with an application to the design of inte rmolecular interactions and the crystallization of polar host-guest materia ls. Because of the process of crystal growth, the primary confinement for t he alignment of dipolar molecules is given by a surface-nutrient interface and not by the bulk state as usually assumed for the prediction of crystal structures by computational methods. Subject to defined restrictions, spont aneous polarity formation in slowly growing crystals can be regarded as a s imple isomerization reaction, down arrow reversible arrow up arrow(down arr ow, up arrow, orientations of the dipole moments in crystals). Following re sults of Monte Carlo simulations, performed for a surface layer (adlayer) o n a nonrelaxing substrate layer, me conclude the following: (i) Although bi nding motifs (synthons) are important to induce 1D or 2D order into chains, ribbons, and planes, lateral interactions between such structural elements enter polarity formation by a much higher weight factor. (ii) Particular l ateral interactions favoring down arrow ... down arrow are not necessary to obtain polarity in some molecular crystals. Channel-type inclusion materia ls represent a solution to (ii), Experimental confirmation is provided by, e.g., a large number of polar inclusion compounds of perhydrotriphenylene ( PHTP). It is shown that in general a combination of van der Waals interacti ons for a 2D confinement (alignment of molecular frames) and one of the mos t nonbonding recognition motifs (down arrow vs up arrow orientation) can op timize polarity formation in host-guest lattices. In essence, me review tha t in some molecular crystals polarity is a tunable property, and that a sup ramolecular synthesis can produce a material and a property by parallel rea ctions. (C) 2000 Academic Press.