Photoreactivity and emission properties of liquid-crystalline twin dimers containing cinnamic acid moieties at both ends of ethylene glycol spacers. I. Synthesis, thermotropic properties and photoreactivity

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Abstract
Twin dimers containing 4-methoxycinnamic acid moieties at both ends of various lengths of ethylene glycol spacers (nEGMC) were found to exhibit liquid-crystalline phase behaviour, while those with cinnamic acid moieties (nEGC) at both ends did not show the liquid-crystalline phase behaviour. The liquid-crystalline phase behaviour of nEGMC depended on the length of the ethylene glycol spacers, n. Photoirradiation of nEGMC at >300nm resulted in the formation of photodimers through cyclobutane ring formation at the C=C part of the 4-methoxycinnamic acid moieties. The photoreaction was explored by following a decrease in vC=C (FTIR), formation of the photodimers (GPC) and structural analysis of the photoproducts (NMR). It was found that the morphology of the systems strongly affected the photoreactivity and the head-to-head photodimer was predominantly formed. The photoreactivity of nEGMC is discussed in relation to the morphology of the systems.