A single-crystal Raman and infrared study of the nonlinear optical crystal MBANP

Single‐crystal Raman and polycrystalline thin‐film infrared measurements have been obtained for the polar organic nonlinear optical material 2‐(α‐methylbenzylamino)‐5‐nitropyridine (MBANP). For comparison, thin‐film polycrystalline infrared measurements were also made on 2‐(α‐methylbenzylamino)‐3,5‐dinitropyridine (MBADNP). The long wavelength electronic absorption was measured in several solvents and as a thin solid film. The Raman spectra are dominated by three intense bands attributed to vibrations of the ring, the NO2 substituent, and the NH bond. The most intense scattering and absorption arose from the αbb component of the polarisability tensor. This implies that the most significant contribution to the transition polarisability arises from the electronic transition near 383 nm, polarised along the b‐axis of the crystal. The strongest bands in the infrared spectra are also associated with the same three bands, consistent with the predictions of the effective conjugation coordinate (ECC) theory, implying efficient electron–phonon coupling (or electronic delocalisation) in the conjugated system. DFT calculations of vibrational wavenumbers and eigenvectors were used to assign relevant vibrational features and to derive useful information about the molecular structure. This single‐crystal material is also a strong candidate for an efficient laser Raman converter with a large wavenumber shift of 3404 cm−1 and a high damage threshold. Copyright © 2010 John Wiley & Sons, Ltd. The Raman spectra of the optically nonlinear crystal MBANP were dominated by three intense bands attributed to vibrations of the pyridine ring, NO2 substituent, and NH bond. The largest contribution to the transition polarisability arose from the charge‐transfer transition near 383 nm polarised along the b‐axis of the crystal The strongest bands in the infrared spectrum were also associated with these transitions, implying efficient electron–phonon coupling.