内消旋二苯甲酰酒石酸主导苝酰亚胺聚集体超分子手性的反Z-型相关

Negative nonlinear CD-ee dependence in dibenzoyltartaric acid induced aggregates of perylenebisimide

CD-ee关系曲线常用以描述聚集体的超分子手性特征,其中线型和S(Z)-型相关已有诸多报道,但反S(Z)-型相关却鲜见报道.我们提出通过引入手性诱导剂的内消旋体,构建呈反Z-型相关的超分子聚集体.在手性二苯甲酰酒石酸对映体存在下,修饰以胍基的苝酰亚胺(PBI)分子因为胍基-羧酸根离子间的盐桥氢键作用,聚集形成超分子聚集体, CD信号与二苯甲酰酒石酸对映异构体的对映体过量值(ee)间呈Z-型相关,表现为手性放大特征;于对映体中引入内消旋二苯甲酰酒石酸,摩尔分数达到0.4或超过后观察到罕见的反Z-型相关."将军与士兵"法则实验亦呈现凹型相关曲线.吸收和CD光谱表明,对映异构体与内消旋体分别诱导的聚集体的结构和稳定性相近,后混合体系中聚集体之间无分子交换作用;但荧光光谱实验指示内消旋体更利于PBI染料分子的聚集,与对映体共存时主导聚集体的形成,导致超分子手性的反Z-型相关.研究结果为构筑反S(Z)-型相关的超分子聚集体提供了路径,有助于全面理解超分子手性.

Supramolecular chirality of aggregates can be described in terms of the CD signals of the aggregates as a function of the enantiomeric excess of the chiral building block or chiral inducer. While many examples have been reported that show a positive nonlinear CD-ee relationship, indicating chiral amplification, the negative nonlinear dependence has rarely been observed. We recently proposed to build such supramolecular aggregates by using the mesomer of the enantiomeric inducers. Indeed we found that in the presence of enantiomeric dibenzoyltartaric acids(DboTar), achiral perylenebisimide(PBI) dye equipped with two terminal guanidinium groups in alkaline aqueous buffer solution is induced to form chiral supramolecular aggregates, via the known guanidinium-carboxylate salt bridge,that show a positive nonlinear CD-ee relationship, and is turned into negatively nonlinear when 40% or more molar fraction of meso-DboTar was introduced. Absorption and CD spectral titrations suggest that the aggregates led by enantiomers and by mesomer are similar in terms of the supramolecular structure and stability, and when the two kinds of aggregates are mixed no molecular exchange occurs between them. Fluorescence spectral traces of the aggregates show that the mesomer is more powerful in inducing the aggregation of the PBI dyes, and thus dominates the aggregation. Our findings that the mesomer of the enantiomeric inducer could lead to aggregates to exhibit a negative nonlinear CD-ee relationship provide a promising approach to such supramolecular aggregates, which could be of significance for better insight into the mechanism leading to the negative nonlinear dependence and eventually a comprehensive understanding of the supramolecular chirality.