Products μβ of the first order hyperpolarisabilities β of both isomers of dithienyl ethene derivatives by their ground state dipole moments μ have been determined experimentally by the EFISHG technique, then calculated by a semi-empirical method. Both calculations and measurements put into evidence a large increase in μβ in going from the open isomer (OF) to the largely π conjugated closed isomer (CF). This photoinduced change in second order NLO coefficient is used to modulate the Second Harmonic Generation (SHG) signal generated by a PMMA film doped with one diarylethene under alternate irradiation with UV (325 nm) and visible (514 nm) light.