Polarised single crystal infra-red spectra and optical indicatrix rotation in an organic charge transfer molecule

Polarised infrared reflectance spectra have been obtained for oriented single crystals of the polar organic nonlinear optical material 2-(α-methylbenzylamino)-5-nitropyridine (MBANP). The strong charge transfer electronic absorption was measured in several solvents and as a thin solid film. DFT calculations of vibrational frequencies and eigenvectors were used to assign relevant vibrational features and to derive useful information about the molecular structure. The intensities of the bands associated with the donor and acceptor groups are increased by electron phonon coupling with the strong charge transfer transition. This results in unusually strong NO2 and N-H stretching vibrations. The spectra are consistent with a large rotation of the optical indicatrix with increasing wavelength due to the bent shape and strong electron-phonon coupling in the molecule.