The one- and two-photon optical absorption spectra as well as the frequency dependent third-harmonic generation response of a side-chain polymer containing 4-dialkylamino-4’-nitro-stilbene (DANS) as its side group have been measured over a wide range of frequency. A three-state model, based on an intermediate neglect of differential overlap/multireference double-configuration interaction description of the excited states, provides a coherent picture of the dynamic response of DANS, but fails in reproducing the red shift observed when going from the linear to the nonlinear absorption spectra; including vibronic coupling within the Franck-Condon approximation improves the fit.