Polymorphism is the ability of a molecule to adopt more than one crystalline form and the control of polymorphism is of importance to the fine chemical industry. Complimentary computational crystal structure prediction and experimental crystallisation techniques have been used to investigate the polymorphism of four organic molecules, none of which were previously known to be polymorphic. For each molecule computational crystal structure prediction produced possible crystal structures, which could correspond to new polymorphs. Manual crystallisation techniques were employed in three instances, and an automated crystallisation platform was used in the fourth, to discover new polymorphs. The crystal structures of all new polymorphs and solvates were fully determined, where possible, by single crystal X-ray diffraction for comparison to the predicted structures. The 5-fluorouracil crystallisation screen produced one new polymorph which corresponded to a low energy predicted structure. For 5-fluorocytosine, where no anhydrous forms had previously been determined, two new polymorphs were discovered, one of which was predicted by the computational results. The study on 3-azabicyclo 3.3.1 nonane-2,4-dione aimed to find a new hydrogen bond dimer-based polymorph inspired by the results of earlier prediction studies. The crystallisation screen produced one new polymorph which was structurally related to the previously reported chain-based structure, along with a high temperature plastic phase. Four polymorphs of 4-hydroxycoumarin were discovered, of which two were fully characterised by single crystal X-ray diffraction and two were identified by powder X-ray diffraction. Many of the newly discovered solvates of these molecules had their hydrogen bonding rationalised in terms of the hydrogen bonded motifs found in the predicted structures of the parent molecule. The viability of computationally predicting monohydrate structures was investigated, using 5-azauracil monohydrate as a test system. This proved a success, with the known crystal structure found by the computational method to be energetically competitive with the other hypothetical structures.

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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.

It is well known that the specific crystallographic form of inorganic crystalline materials can determine their physico-chemical properties. Furthermore, it is known that many biological processes use bio-organic macromolecules to control the growth and form of bio-inorganics with the result that many biominerals exhibit specific crystallochemical properties linked to their function. For example, the deposition of hydroxyapatite (HAp), a mineral in both bone and enamel, leads to the genesis of biocomposites whose identity is largely defined by the size and shape of the constituent inorganic crystals. This study has shown that the design of polymers (e.g. chemical identity, structure, etc) can have a significant effect upon crystal formation.

In this PhD project, the solvent effects on polymorphism were studied. This was done using an enantiotropic compound. By working at the transition temperature of the compound, it allowed for the separation of supersaturation effects from that of the solvent. p-Aminobenzoic acid was chosen as the model compound. p-Aminobenzoic acid (PABA) is an enantiotropic compound with a transitior temperature of 24°C. It has two polymorphs, a needle shaped α-form which is stable above the transition temperature and a prismatic shaped β-form stable below the transition temperature.