Molecular Crystals and Liquid Crystals

A series of new heterocyclic stable free radicals based on the thioaminyl fragment 1 was designed in order to introduce a x-delocalized spin to the rigid cores of mesogenic molecules. The design allows for preparation of calamitic and discotic liquid crystals. Computational analysis and first experimental results of the radicals (ESR) are presented. The thioaminyl group' (1) has been featured as a structural element of chemically and thermally stable organic free radicals (Figure 1). Almost all species derived from 1 are ndelocalized radicals with the odd electron in the anti-bonding orbital, and they form acyclic compounds2" such as 2-5, and also cyclic derivatives6-*0 such as 6-10 and 11-14. Some of these radicals melt above 150 "C without decomposition? some exist as liquids9 and some have been explored as possible unidimensional conductors, 1-14 and molecular magnetic materials.12 For instance, radical 3 and its meta isomer show weak antiferromagnetic interaction^'^ but in a similar compound with chlorine atoms as substituents weak ferromagnetic interactions have been observed. Another study involving derivatives of 10 re~ea led '~* '~ that they behave as diamagnetic substances in the solid phase with some paramagnetic impurities ( ~ 1 % ) . In the liquid phase, however, the magnetic moment ( ~ ~ 4 . 8 ~ 10-6 cgs) of radical 9 is essentially temperature independent. The antiferromagnetic spin-spin exchange in the solid phase has been attributed to the dimerization at the sulfur center and formation of a weak S-S bond (e.g., A&imer =-3733 W/mol for 79). 0.. .@ +S.. .. '-3* a ..