An ESR characterization of α,ω-bis(4'-cyanobiphenyl-4-yl)undecane symmetric dimer mesophases

Liquid crystal (LC) dimers have attracted much interest as model compounds for liquid crystalline polymers [1,2], as well as building blocks in the development of new materials, for example, in the search for biaxial nematics. One example is provided by the class of symmetric dimers nBCB consisting of two cyanobiphenyl mesogens connected by a flexible (CH2)n alkyl chain spacer [3]. These dimeric LCs show pronounced odd-even effect in the nematic-isotropic transition temperatures and entropy. Moreover the possibility that a biaxial nematic phase could occur has been suggested [3]. Here we study the highly flexible odd-spacer dimer α,ω-bis(4'-cyanobiphenyl-4-yl)undecane (11BCB) by means of the Electron Spin Resonance (ESR) spin probe technique, already successfully employed in similar [4] and other complex systems [5] for its high sensitivity. We present results of different ESR experiments performed on 11BCB (here a cholestane-type nitroxide spin probe, 3β-doxyl-5α-cholestane (CSL) is used), with the aim of elucidating its behaviour, both from the point of view of the order and the molecular organization, as well as in terms of dynamics, issues about which very little is known as yet. The possibility that this system, which in view of its odd alkyl spacer should contain a high concentration of bent conformers, exhibits a biaxial phase has also been investigated. The experimental results show the clear presence of more than one contribution to the overall spectrum, which is typical of complex LC systems, that is of materials which display a coexistence of the spin probe close to different sites of the molecules, each one characterized by different degrees of alignment and dynamics [5]. Thus, the spectra can be well reproduced resorting to a model which takes into the account two fractions of the LC molecules, one ordered along a common director and the other mainly randomly oriented. Rotation experiments point out to the possible presence of a smectic (Sm) phase, as the mesogens do not reorient upon changing the magnetic field position. Coupling these findings to those of atomistic Molecular Dynamics simulations performed on the same system, we can deduce the CSL preferred location with respect to the LC dimers and their possible conformations, and therefore suggest a model for the dimer organization. Acknowledgments: This work was supported by the EU-STREP project “Biaxial Nematic Devices” (BIND)FP7-216025. References [1] A. C. Griffin and T. R. Britt, J. Am. Chem. Soc. 103, 4957 (1981). [2] C. T. Imrie and P. A. Henderson, Chem. Soc. Rev. 36, 209 (2007); I. Miglioli, L. Muccioli, S. Orlandi, M. Ricci, R. Berardi, C. Zannoni, Theor. Chem. Acc. 118, 203 (2007) [3] P. J. Barnes, A. G. Douglass, S. K. Heeks and G. R. Luckhurst, Liq. Cryst. 13, 603 (1993). [4] C. J. Dunn, P. J. Le Masurier and G. R. Luckhurst, Phys. Chem. Chem. Phys. 1, 3757 (1999) [5] C. Bacchiocchi, I. Miglioli, A. Arcioni, I. Vecchi, K. Rai, A. Fontecchio and C. Zannoni, J. Phys. Chem. B 113, 5391 (2009)