Polymorphism of Dehydrobenzo[14]annulene Possessing Two Methyl Ester Groups in Noncentrosymmetric Positions

We revealed that the π-conjugated cyclic compound octadehydrotribenzo[14]annulene ([14]DBA), with two methyl ester groups in noncentrosymmetric positions, formed five polymorphic crystals: forms I (plate, P21/c), II (block, P21/c), II′ (block P1̅), III (needle, P21/c), and IV (needle, C2/c), which is the largest number for polymorphic crystals of the DBA family. Forms I, III, and IV can be obtained selectively upon the crystallization conditions applied. The present polymorphism was provided by conformational flexibility and varied CH/O interaction methods of the methyl ester groups. The thermal stability, fluorescence properties, and photoconductivity of the polymorphs were investigated. Such properties are affected by the polymorphic supramolecular structure. Furthermore, we demonstrate that the polymorphs exhibit physical or chemical defects depending on the magnitude of the π-overlap of the DBA planes and that the fluorescence and electronic properties are strongly affected by the defects. Particularly, form I shows a significant fluorescence band at 530 nm, probably due to defects in the crystals.