NIR-FT Raman, FT-IR and surface-enhanced Raman scattering spectra of organic nonlinear optic material:p-hydroxy acetophenone

Abstract

The NIR-FT Raman, FT-IR spectral and SERS analysis of potential NLO material, p-hydroxy acetophenone is carried out by density functional computations. The SHG efficiency is tested using a Q-switched Nd:YAG laser of wavelength λ at 1064 nm, which is approximately 1.2 times that of urea. Optimized geometry reveals that the OH and COCH3 groups substituted in the para position of a phenyl ring are coplanar, which predicts maximum conjugation of the molecule with donor and acceptor groups. Vibrational analysis is used to investigate the electronic effects of hyperconjugation and induction of the methyl group with the π orbital of an aromatic ring system and a resulting increase of stretching mode wavenumbers and decrease of infrared intensities. The simultaneous IR and Raman activation of the phenyl ring modes also provides evidence for the charge transfer interaction between the donors and the acceptor group through the π-system. The π-electron cloud movement from donor to acceptor can make the molecule highly polarized, and the intramolecular charge transfer interaction must be responsible for the NLO properties of PHA. The splitting of the carbonyl mode is attributed to the intramolecular association on the basis of CO···H type hydrogen bonding in the molecule. The doublet of the CO mode band with the traces of crystal splitting originates from two differently associated CO groups. The conjugation and influence of the intermolecular hydrogen bonding-type network in the crystal results in lowered CO stretching wavenumber. The large enhancement of in-plane ring stretching and ring stretching modes in the surface-enhanced Raman scattering spectrum indicates that the molecule is adsorbed on the silver surface in a stand-on orientation of PHA on a silver surface. Copyright © 2005 John Wiley & Sons, Ltd.