A possible route to finding new optically switchable molecular solids is to investigate the photoeffects of compounds that exhibit an abrupt phase transition or hysteresis loop. Such compounds must possess at least bistable states that are separated by a potential barrier in free energy. In fact, on the basis of this idea, we have recently succeeded in identifying three kinds of optically switchable molecular compounds. These include an FeIII spin-crossover complex exhibiting light induced excited spin state trapping effects and a CuII photochromic complex and an FeCo Prussian blue exhibiting photoinduced magnetization. It should be noted that abrupt phase transitions or phase transitions with a hysteresis loop are observed when cooperativity due to an intermolecular interaction operates within the crystals. This suggests that the introduction of intermolecular interactions such as π-π interactions, hydrogen bonding, or an ion-dipole interaction might serve to achieve optical switching effects in molecular compounds.
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