Structures, vibrational frequencies, and normal modes of substituted azo dyes: Infrared, Raman, and density functional calculations

The vibrational structures of 4-nitro-azobenzene (NAB), 4-(dimethylamino)-azobenzene (DAB), and 4-nitro,4'-(dimethylamino)-azobenzene (NDAB) are of interest due to their importance in optoelectronic applications as well as the unique isomerization mechanism involving the inversion process (at the nitrogen site). In this paper, we present the Fourier transform infrared (FTIR) and Raman spectral studies of NAB, DAB, and NDAB and also report their equilibrium structures, harmonic frequencies, and normal mode assignments, employing the hybrid Hartree-Fock/density functional (HF/DF) method with the 6-31G basis set. The results of the optimized molecular structure obtained on the basis of B3LYP with 6-31G basis set suggest a greater conjugation and pi-electron delocalization for the substituted azo dyes in comparison to the parent azo molecule, viz. ti-ans-azobenzene (TAB). It is found that the B3LYP/6-31G method is very accurate in predicting harmonic vibrational frequencies and the normal modes for the substituted azo dyes and their isotopic analogues. On the basis of the B3LYP/6-31G force field, the infrared intensities for NAB, DAB, and NDAB, and their isotopomers, are calculated and then compared with those observed experimentally. Finally, the main differences in the vibrational spectra of the substituted azobenzene derivatives are discussed from an analysis of the normal modes.