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.

本研究揭示：带有两个处于非中心对称位置甲酯基团的π共轭环状化合物十八脱氢三苯并[14]轮烯（octadehydrotribenzo[14]annulene，[14]DBA）可形成五种多晶型晶体：晶型I（片状，P2₁/c）、晶型II（块状，P2₁/c）、晶型II′（块状，P1̅）、晶型III（针状，P2₁/c）以及晶型IV（针状，C2/c），该数量为DBA家族多晶型晶体的最高纪录。可通过调控结晶条件选择性制备得到晶型I、III与IV。本次多晶型现象源于甲酯基团的构象灵活性与多样化的CH/O相互作用模式。我们对各多晶型的热稳定性、荧光特性与光电导率开展了表征研究，上述性质均受多晶型超分子结构的影响。此外，我们证实多晶型会表现出物理或化学缺陷，且缺陷程度取决于DBA平面间的π重叠强度；荧光与电子性质受缺陷影响显著。尤为值得关注的是，晶型I在530 nm处呈现出显著的荧光发射带，该现象大概率源于晶体内部存在缺陷。