The present theoretical and experimental studies of the molecular second order nonlinear optical response of donor-acceptor substituted conjugated compounds which consist of heteroaromatic moieties in place of the traditional benzenoid moieties. Electric-field-induced second harmonic generation measurement at a fundamental wavelength of 1.907 mu m demonstrated a twofold increase of the off-resonant molecular hyperpolarizability of a heteroaromatic compound compared to its benzenoid counterpart. Results of theoretical calculations based on ab initio scheme are in good agreement with the experimental results.