Steering Power of Perfluoroalkyl Substituents in Crystal Engineering: Tuning the π–π Distance While Maintaining the Lamellar Packing Motif for Aromatics with Various Sizes of π‑Conjugation

Previously, we reported that introducing perfluoroalkyl substituents onto aromatics promotes the formation of lamellar π–π stacked crystalline materials with short interplanar distances. In this report, we developed a new synthetic route that effectively prepares perfluoroalkylated N-containing aromatics by eliminating a side perfluoroalkylation reaction occurring on nonsubstituted Csp2–H sites of the corresponding bromoaromatics without regioselectivity. This results in a significant improvement of the yield of target perfluoroalkylated aromatics and facilitates the purification and scale-up processes. X-ray single crystal structural analyses show that lamellar π–π stacked structures with tunable interplanar distances are achieved with fused N-containing aromatics with varying sizes of π-conjugation. Both crystal structures and theoretical calculations demonstrated that the interplanar distance can be fine-tuned with the size of π-conjugation, with larger π-conjugation favoring shorter interplanar distances while still maintaining a lamellar π–π stacked packing motif. Compared to our previous results, we find that simply changing the perfluoroalkyl substituent positions and patterns can change molecular topology to exclusively form lamellar π–π stacked packing motifs through prioritization of specific steric effects. Electrochemical results and absorption spectra confirm that the band gap is reduced due to increasing π-conjugation, and the first reduction potential exhibits a significant positive shift due to both increasing π-conjugation and perfluoroalkylation.

此前我们曾报道，在芳烃上引入全氟烷基取代基（perfluoroalkyl substituents），可促进形成具有短面间距的层状π-π堆积晶体材料（lamellar π–π stacked crystalline materials）。本研究开发了一条全新合成路线，通过抑制对应溴代芳烃上未取代Csp2-H位点发生的无区域选择性（regioselectivity）副全氟烷基化反应，高效制备全氟烷基化含氮芳烃。该策略显著提升了目标全氟烷基化芳烃的产率，同时简化了纯化与放大生产流程。X射线单晶结构分析（X-ray single crystal structural analyses）结果表明，通过调控π共轭体系（π-conjugation）尺寸各异的稠环含氮芳烃，可获得面间距可调的层状π-π堆积结构。晶体结构解析与理论计算均证实，π共轭体系尺寸可对分子面间距进行精准调控：更大的π共轭体系有利于缩短面间距，同时仍可维持层状π-π堆积的堆砌模式。相较于此前的研究结果，我们发现仅通过改变全氟烷基取代基的位置与模式，即可通过特定空间位阻效应（steric effects）的优先作用，改变分子拓扑结构，专一性地形成层状π-π堆积堆砌基元。电化学测试结果与吸收光谱证实，随着π共轭体系扩大，分子带隙（band gap）得以降低；同时，由于π共轭体系扩大与全氟烷基化的共同作用，其第一还原电位（first reduction potential）发生显著正移。