In complexes where the energy difference between the high and low spin state of the metal is of the order of k(_b)T, temperature can be used to induce a spin crossover transition (SC). In some cases, at very low temperatures, irradiation can induce excitation to a meta-stable high spin state. At low temperatures, this Light-Induced Excited Spin-State Trapped (LIESST) state is generally long lived, enabling structural examination. The results presented herein refer to a wide range of iron(II) spin crossover materials which have been structurally studied in both thermal and light induced states. These fall into three categories; mononuclear, dinuclear and polymeric. The mononuclear complexes studied include FeL[H(_2)B(Pz)(_2)](_2) L = 2,2'-bipyridine (1) and 1,10-phenanthroline (2), of which 2 was found to undergo a change in symmetry in addition to the change in spin state, leading to a novel light induced polymorphism that has not been previously seen. Two dinuclear compounds with step transitions have been examined. While {[N(CN)(_2)](FeBpl)(_2)}(PF(_6))(_3) undergoes a gradual transition, the transitions in [Fe(Btz)(NCS)(_2)](_2)Bpmd are abrupt and the latter also undergoes LIESST but with a rapid relaxation that has not been observed previously with any other technique. While the origin of this relaxation is uncertain, the structure of this excited state has been studied under constant irradiation. The largest structural study of three dimensional SC materials has been carried out, including bimetallic polymers with [Au(CN)(_2)], [Ag(CN)(_2)]-, [Ag(_2)(CN)(_3)]- and [Pd(CN)(_4)](^2-) bridging ligands. These anionic bridges have been shown to enhance cooperativity between iron centres leading to abrupt transitions and hysteresis. These materials have been shown to undergo LIESST, crystalline state allosterism, and thermo-chromism. Such multi-property materials have a high potential for technological applications
