In fact, their photoinduced transformations can beexploited to switch the color of liquid solutions, rigid polymermatrices or even crystals with optical inputs. Furthermore,these photoinduced processes are generally accompanied bysignificant structural modifications at the molecular level,which can translate into pronounced changes in the refractiveindex of the host matrix. Indeed, a diversity of photonic mate-rials and devices have already been designed around theunique absorptive and dispersive effects associated withphotochromic compounds.