The thermal behavior, including the cold crystallization, of alkylderivatized Sudan III (1-[4-(phenylazo)phenylazo]-2-naphthol), SD3-OCn, was investigated. Its structural flexibility due to the two azo groups caused the formation of plural crystal structures, which further resulted in complex cold crystallization. A metastable nonflat structure and a thermodynamically stable flat structure resulted in the two sets of crystallization and melting behavior during the heating process. Further, the change in the alkyl chain length caused variations in the crystallization rate and crystallinity, thus systematically changing the cold crystallization behavior. It was proven that the high mobility of the phenyl group triggered supercooling and cold crystallization and that the alkyl chain correspondingly controlled the thermal behavior.
Molecules exhibiting cold crystallization, an exothermic phenomenon in heating following supercooling, can be used as a heat storing material. On the other hand, examples in non-polymers are still few. A chiral Schiff-base nickel(II) complex, which had a characteristic methyl group and long alkyl chains, exhibited complicated thermal behavior including cold crystallization. The methyl group was a steric barrier to dimerization and molecular stacking, leading to the supercooled state. In addition, the thermal behavior of enantiomer was different from that of racemate.
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