Synthesis, Resolution, Configurational Stability, and Properties of Cationic Functionalized [5]Helicenes

A straightforward approach to the synthesis of two different series of cationic [5]­helicenes has been achieved including, in dioxa series, the possibility to introduce aromatic functional groups at the periphery of the helical structure. While photophysical study highlights that the introduction of aryl substituents at position 23 of the helical moieties has a negligible impact on the optical properties, styryl substituents allow a welcoming extension of the conjugation pathways. Finally, a red shift of the optical properties was evidenced upon introduction of nitrogen atoms in the helicene scaffold, leading to particularly good fluorescence efficiencies in the red domain for a helicenic dye. Detailed information on racemization kinetics was collected for the most stable species upon direct high-performance liquid chromatography (HPLC) resolution or, when configurational lability was too high, through VT-HPLC analysis on the chiral stationary phase (ΔG‡ values ranging from 85.0 to 137.1 kJ·mol–1 and above).本研究成功开发了一种简便合成策略，可制备两类不同的阳离子[5]螺烯（cationic [5]-helicenes）；其中二氧杂系列可在螺旋结构的外围引入芳香官能团。光物理研究表明，在螺旋片段的23位引入芳基取代基对其光学性能的影响可忽略不计，而苯乙烯基取代基则可有效拓展共轭路径。此外，在螺烯骨架（helicene scaffold）中引入氮原子可使光学性能发生红移，最终使该螺烯类染料在红光区域展现出优异的荧光量子效率。针对最稳定的物种，本研究通过直接高效液相色谱（high-performance liquid chromatography, HPLC）拆分获取了其消旋动力学的详细信息；当构型易变性过高时，则通过手性固定相上的变温高效液相色谱（VT-HPLC）分析完成表征，测得其吉布斯自由能活化能垒（ΔG‡）介于85.0至137.1 kJ·mol–1之间，部分样本甚至更高。