The emission efficiency, (dye fluorescence)/(excitation intensity), by multi-photon excitation of 4-(dicyanomethylene)-2-methyl-6-[p-(dimethyl-amino) styryl]-4H-pyran in methanol solution was maximized using an evolutionary algorithm. Without any a priori knowledge on the molecular system, the optimization goal was automatically achieved by improving spectral phases through feedback of experimental outputs. The spectral structure of the optimized pulse was found to have a negative chirp. It is proposed that the efficiency change induced by the negative chirp is due to wavepacket localization on locally excited states.