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

Abstract

New linear dichroism spectra in the mid-IR region of fluorene aligned in stretched polymers and nematic liquid crystals are compared with earlier experimental and theoretical investigations. Linear dichroism spectra of molecules aligned in such anisotropic solvents are simple to obtain, especially for small and medium sized molecules. Most molecules obtain a satisfactory alignment by these methods and the spectra are simple to interpret.The information obtained from these spectra about vibrational transition moment directions is often crucial for vibrational assignments in molecules like fluorene. Even high quality calculations are still unable to provide safe assignments for all fundamentals in fluorene as long as they are only compared with traditional spectra. Linear dichroism spectra provide a separation of the experimental information according to symmetry classes (particularly striking for molecules of C2v and similar symmetries); this reduces the assignment puzzle drastically. In the present case the result is an almost complete and safe assignment of all symmetry allowed invibrational fundamentals of fluorene. A number of earlier assignments, which had been questioned or were doubtful, have been confirmed or corrected.