Optical birefringence and dichroism in physical steroid organogels

Abstract

Optical birefringence and dichroism have been studied in physical steroid organogels. We have used two complementary derivatives (SNH and SNO*) in cyclohexane: the former has no chromophoric group in the 220–800 nm wavelength range studied, in contrast with the latter. The dichroism is mainly due to linear contributions. The amplitude of the signal is correlated with the growth of fibrillar structures and specially with the heterogeneities of the gel network which was assumed to be chiral fibrillar junction zones. The characteristic SNO* dichroic absorptions (for C ≈ 2.0 × 10⁻² M) are λ_max = 250nm, |Δϵ| ≈ 6.0 (mainly a linear contribution) and λ_max = 450 nm, Δϵ = +0.45 (mainly a circular contribution). These preliminary observations provide strong evidence for a helical stacking of the steroid molecules within the fibres of the gel samples. The optical dichroic absorption can be a valuable technique to monitor the orientational order of the microdomains crossed by the light beam.