13C NMR Investigations of Hairy-Rod-Like π-Conjugated Mesogens

Abstract

Two hairy-rod π-conjugated mesogens comprising dihexylfluorene and didecyloxyterphenyl are synthesized, and their mesophase properties are examined by hot-stage optical polarizing microscope and differential scanning calorimetry techniques. Both mesogens exhibit enantiotropic nematic mesophase with broad mesophase range. A detailed 13C NMR study is carried out in solution as well as in nematic mesophase to understand changes in the orientation of the aliphatic chains. Accordingly, the unusual NMR chemical shift value of one of the methylene carbons of the hexyl chain of fluorene moiety in solution is ascribed to ring current effect changes in nematic mesophase owing to variation in the orientation of the hexyl chain. The change in conformation of lateral and alkyl chains in the nematic phase is clearly noticed compared to isotropic solution as the molecular orientation in mesophase is governed by the orientation of the long molecular axis. Furthermore, the 13C-1H dipolar couplings obtained from 2D separated local field experiments in nematic phase aided the assignments of all of the carbons of the molecules besides offering the local order. To comprehend the orientation of the fluorene unit, three order parameters are necessary whereas for the phenyl rings, two order parameters are found to be sufficient. The molecular biaxiality ( S xx - S yy) for fluorene-based mesogen is found to be higher due to the fused nature of the moiety and the presence of dihexyl chains. The orientational ordering of π-conjugated mesogens is crucial as the optoelectronic properties of them critically depend on the orientation of the constituent chromophores. The complete mapping of order parameters of fluorene, the phenyl rings and the alkyl chains showed that despite both molecules exhibiting nematic mesophase, the orientational constraints are governed by the molecular structure as well as the rigidity of the core unit, i.e., fused versus linearly connected rings.