Supramolecular Chromotropism of the Crystalline Phases of 4,5,6,7-Tetrafluorobenzo-2,1,3-telluradiazole

The remarkable effect that secondary bonding interactions can have on the macroscopic properties of a material is illustrated by two polymorphs of the title compound. The phase which is most stable under ambient pressure and temperature consists of puckered supramolecular ribbon polymers assembled by Te--N secondary bonding interactions and displays a characteristic red-orange color. A second yellow phase consists of ribbons with alternating short and long intermolecular Te--N secondary bonding distances and is metastable; at 127 °C the material undergoes an exothermic irreversible transition to the red polymorph. A third phase consists of pyridine-solvated supramolecular dimers; it is also yellow and transforms into the red phase after the crystals effloresce. Computational DFT studies indicate that the observed changes in optical properties are related to intermolecular mixing of π orbitals enabled by the supramolecular interactions and the symmetry of the supramolecular synthon.

次级键相互作用对材料宏观性能的显著影响，可通过标题化合物的两种多晶型得到直观例证。常温常压下最稳定的晶相为由Te—N次级键相互作用组装形成的褶皱状超分子带状聚合物，呈现特征性的红橙色外观。第二种黄色晶相由分子间Te—N次级键键长交替变化的带状结构构成，属于亚稳态；在127℃时，该材料会发生放热不可逆转变，转化为红色多晶型。第三种晶相为吡啶溶剂化的超分子二聚体，同样呈黄色，当晶体发生风化（失去结晶溶剂）后，会转化为红色晶相。密度泛函理论（Density Functional Theory, DFT）计算研究表明，实验中观测到的光学性能变化，与超分子相互作用以及超分子合成子（supramolecular synthon）的对称性所促成的π轨道分子间杂化密切相关。