Crystal Structures of Organic Compounds

Abstract

afforded by their interesting and peculiar helix-like structure, resulting from the condensation of aromatic (and/or heteroaromatic) rings, all of them in ortho position.For example, the formula and crystal structures of tetrathia-[7]-helicene 1 are shown in Figures 1 and2, respectively.The compound 1 has been synthesized in three step by starting material of benzo [1,2-b:4,3-b']dithiophene and is shown in Scheme 1 " (Maiorana et al., 2003)" and this compound showed second-order non-linear optical (NLO) properties and has been investigated (Clays et al., 2003).In particular, carbohelicenes only include benzene rings, and also in heterohelicenes one or more aromatic rings are heterocyclic (pyridine, thiophene, pyrrole and etc.) " (Miyasaka et al., 2005;Rajca et al., 2004)".With increasing number of condensed rings (typically, n > 4), the steric interference of the terminal rings forces the molecule to be a helicoidal form.For n > 4 the energetic barrier is such that the two enantiomers can be separated and stored " (Martin, 1974;Newman, et al., 1955Newman, et al., , 1967;;Newman & Lednicer, 1956;Newman & Chen, 1972)".Of course, the conjugation of π system decreases with decreasing of planarity; however, in longer helicenes π-stack interactions can also take place between overlapping rings " (Caronna et al., 2001;Liberko et al., 1993)".All helicenes (generally, n > 4) are chiral molecules and exhibit huge specific optical rotations " (Nuckolls et al., 1996(Nuckolls et al., , 1998)" since the chromophore itself, in this case the entire aromatic molecule, is inherently dissymmetric (right-hand or left-hand helix), having a twofold symmetry axis, C2, perpendicular to its cylindrical helix (in carbohelicenes), or inherently asymmetric (in heterohelicenes) "(Wynberg, 1971)".