Homochiral Helices of Oligonaphthalenes Inducing Opposite-Handed Cholesteric Phases

• Published in: Chemistry : a European journal Vol. 12; no. 4; pp. 1121 - 1126
• Main Authors: Pieraccini, Silvia, Ferrarini, Alberta, Fuji, Kaoru, Gottarelli, Giovanni, Lena, Stefano, Tsubaki, Kazunori, Spada, Gian Piero
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 23-01-2006; WILEY‐VCH Verlag
• Subjects: chirality; Circular Dichroism; helical structures; helical twisting power; liquid crystals; Models, Molecular; Molecular Conformation; Naphthalenes - chemistry; oligonaphthalene; Stereoisomerism
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200500683

The helical structure of the chiral nematic phases (cholesterics) obtained by doping nematic solvents with chiral non‐racemic compounds is a macroscopic proof of the solute chirality. Oligonaphthalene (tetra‐, hexa‐, octa‐) derivatives linked at the 1,4‐positions have been used as chiral dopants: When the chirality axes are configurationally homogeneous (that is, all‐S), the molecular structures correspond to right‐handed helices. Yet, we have found series of derivatives with the surprising property that the handedness of the induced cholesteric phase alternates from positive to negative and to positive again, on passing from tetra‐ to hexa‐ and to octanaphthalene. A comparison with oligonapthalene derivatives, which do not exhibit this twisting ability, points to the importance of the substitution pattern. Both the possibility of inducing oppositely‐handed cholesteric phases by homochiral helices of different length, and the role played of substituents, are confirmed by calculations performed with the surface chirality model.
Alternating handedness was observed for the cholesteric phase of a series of homochiral (all‐S)‐oligo‐1,1′‐naphthalenes. This surprising property was obtained by doping nematic phases and resulted in the alternation from right to left and to right again (see scheme), on passing from tetra‐ to hexa‐ to octanaphthalene.