The Topochemical Polymerisation of Radiation-Sensitive 10,12-Pentacosadiynoic Acid as Different Multicomponent Solid Forms

In this work, the relationship between the solid-state photoreactivity of 10,12-pentacosadiynoic acid (PCDA) and its X-ray structure has been established. The characterisation of PCDA was used to elucidate the structures of the different forms of a lithium salt of PCDA (Li-PCDA) as the single-crystal X-ray structures could not be obtained. Li-PCDA is currently used in radiochromic films due to its impressive colour change from colourless to blue upon irradiation. In addition to Li-PCDA, a lithium salt of 5,7-hexadecadiynoic acid was synthesised and displays greater photoreactivity than Li-PCDA upon UV irradiation. Additionally, a sodium salt of PCDA was crystallised and reveals the salt is outside of the topochemical postulate for reactivity and is, therefore, unreactive. Furthermore, a bismuth complex of PCDA was also synthesised, however, further work is required to fully understand the photoresponsive material. To investigate how PCDA would behave as a cocrystal with organic coformers, model compounds of short-chain n-alkyl carboxylic acids cocrystallised with dipyridyl derivatives and were synthesised in solution and by grinding to give new materials. When PCDA was combined with the same dipyridyl coformers, the resulting cocrystals do not respond to UV irradiation. Additionally, when PCDA is combined with an aliphatic bifunctional compound, similar in structure to the bipyridyl coformers, a PCDA cocrystal salt is formed that is also unreactive to radiation. Further PCDA cocrystal salts are synthesised when combined with aliphatic monofunctional coformers that produce a range of colourful powders before and after UV exposure. The enhanced reactivity of the monofunctional cocrystal salts is reinforced by the X-ray structures which confirm that the structures adhere to the topochemical postulate, further emphasising the importance of the criteria to predict the photopolymerisation of diacetylene systems. These photosensitive materials contribute to the improvement of radiochromic materials, such as dosimetric films.