Vibrational polarization spectroscopy of fluorene: alignment in stretched polymers and nematic liquid crystals

New linear dichroism spectra in the mid-IR region of fluorene aligned in st retched polymers and nematic liquid crystals are compared with earlier expe rimental and theoretical investigations. Linear dichroism spectra of molecu les aligned in such anisotropic solvents are simple to obtain, especially f or small and medium sized molecules. Most molecules obtain a satisfactory a lignment by these methods and the spectra are simple to interpret. The information obtained from these spectra about vibrational transition mo ment directions is often crucial for vibrational assignments in molecules l ike fluorene. Even high quality calculations are still unable to provide sa fe assignments for all fundamentals in fluorene as long as they are only co mpared with traditional spectra. Linear dichroism spectra provide a separat ion of the experimental information according to symmetry classes (particul arly striking for molecules of C-2v and similar symmetries); this reduces t he assignment puzzle drastically. In the present case the result is an almo st complete and safe assignment of all symmetry allowed invibrational funda mentals of fluorene. A number of earlier assignments, which had been questi oned or were doubtful, have been confirmed or corrected. (C) 1999 Elsevier Science B.V. All rights reserved.