Linear dichroism spectroscopy as a tool for studying molecular orientation in model membrane systems

Journal article, 1977

Ordinary linear dichroism (LD) spectroscopy with rectangular optics is not applicable to thin uniaxial samples having the optical axis perpendicular to the plane of the sample. A method has been developed to study such samples at inclined optical incidence and a general relation derived between the LD (corrected for polarized reflection) and the average orientation of the absorbing transition dipole. For molecules of C2”, D2h, or higher symmetry the usual order parameters can be obtained. The method has been tested on anthracene in a polyethylene standard slab of perfectly axial symmetry. The estimated orientation agreed with previous reports on anthracene in stretched polyethylene sheets. A number of chromophoric molecules solubilized in macroscopically aligned lamellar mesophases of the systems sodium octanoate/decanol/water and sodium di- 2-ethylhexylsulfosuccinate (Aerosol OT)/water have also been studied. In the first system rodlike molecules (diphenylethyne, anthracene, retinal) are oriented with their long axes parallel to the hydrocarbon chains of the amphiphile, while an extremely long molecule (&carotene) as well as water-soluble planar cationic dyes are on the average oriented parallel to the lamellar planes. In the Aerosol OT system all molecules studied were oriented parallel to the lamellar planes.