Nonlinear optical properties of push-pull stilbenes based on an alkylamino donor and a strong carbocation acceptor moieties are analyzed in the present study. The highly electron-acceptor character of the carbocation group makes this kind of molecule promising material for second-order nonlinear optical applications. Photoinduced intramolecular charge-transfer is studied by second harmonic generation in solution using time-resolved nondegenerate six-wave mixing. The fully characterized nonlinear dye Disperse Red 1 is used as a reference. The second-order nonlinear properties of the carbocation molecules are compared to a more standard amino-nitro push-pull stilbene. Kerr ellipsometry measurements aiming at a better understanding of the nonlinear excited state spectroscopy of the molecules are also reported. They reveal a very short excited state lifetime of the molecules. Optimization of this lifetime should lead to an improvement of the six-wave mixing response of such compounds.