Ironmixed-valencecomplex,(n-C3H7)4N[FeIIFeIII(dto)3](dto=C2O2S2),showsa spin-entropydrivenphasetransitioncalledchargetransfer phasetransitionaround120Kandaferromagnetictransitionat6.5K.Thesephase transitionsremarkablydependonthehexagonalringsizein two-dimensionalhoneycombnetworkstructureof[FeIIFeIII(dto)3]∞.Inorderto controlthemagneticpropertiesandtheelectronicstateinthe dto-bridgedironmixed-valencesystembymeansofphoto-irradiation,wehave synthesizedaphoto-sensitiveorganic–inorganichybridsystem, (SP)[FeIIFeIII(dto)3](SP=spiropyran),andinvestigatedthephoto-inducedeffecton themagneticpropertiesbythemeasurementsofmagnetic susceptibility,ESR,IR,UV–visand57FeM