The subject of the present invention is a compound of formula (I) below, the addition salts thereof, the dimer thereof and also the organic or inorganic acid salts thereof, the optical isomers thereof, the geometric isomers thereof, and the solvates thereof such as hydrates: (I) These compounds are sensitive to pH and of use for the treatment of keratin fibers, such as human keratin fibers and in particular the hair.
