We have performed dynamic light scattering studies on compounds from two classes of thermotropics (bent core and tetrapod compounds), which reportedly exhibit uniaxial to biaxial nematic phase transitions [1,2]. The experiments utilize scattering geometries where the uniaxial fluctuation modes are expected to be “dark”. At high temperatures in the nematic range, we observe a weakly scattering, relatively fast nonhydrodynamic mode that can be attributed to biaxial order parameter fluctuations (which are still orders of magnitude slower than fluctuations of uniaxial order detected in the isotropic phase). Interestingly, no critical dependence of the relaxation rate of this mode is seen on lowering temperature toward the biaxial phase. Instead, the scattering intensity of the mode continuously decreases relative to a significant increase in the strength of hydrodynamic modes that apparently represent a combination of optic axes' fluctuations. The behavior is similar for the bent core and tetrapod samples, but is different from the critical slowing down of order parameter fluctuations at the uniaxial to biaxial transition observed much earlier in lyotropics [3]. This would seem to suggest a possible evolution from weak to stronger biaxial structure over the nematic range in the thermotropic samples.

[1] Veena Prasad, Shin-Woong Kang, K. A. Suresh, Leela Joshi, Qingbing, and Satyendra Kumar; J. Am. Chem. Soc. 2005, 127,17224. [2] K. Merkel, A. Kocot, J.K. Vij, R. Korlacki, G.H. Mehl, and T. Meyer, Phys. Rev. Lett. 2004, 93, 237801. [3] M. B. Lacerda Santos, Y. Galerne, and G. Durand, Phys. Rev. Lett.1984, 53, 787. Supported,in part, by the National Science Foundation grant DMR-03-12792. Any opinions, findings, and conclusions or recommendations expressed here are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.