This paper reports on materials whose second order nonlinear optical (NLO) properties can be switched by light. We synthesized and studied the properties of a series of substituted N-salicylidene anilines which are noncentrosymmetric and photochromic crystals. All compounds exhibit second harmonic generation (SHG). One of the compounds exists in two different cristalline forms, one being photochromic and the other thermochromic. Photoswitching of SHG was performed : irradiation by UV light lead to a decrease of SHG up to 60% of the SHG signal at 1064 nm and 10% at 1907 nm.