Chiroptical Properties of Indolenine Squaraines with a Stereogenic Center at Close Proximity

A series of four indolenine squaraines
bearing a chiral center
at the 3-position of the indolenine moiety, with either an n-propyl or a phenyl group alongside a methyl group, were synthesized and obtained in a high
purity of ≥98% for the desired stereoisomer. The indolenine
precursors with a phenyl group attached at the chiral center were
asymmetrically synthesized using a pericyclic-reaction cascade and
obtained in a high ee of 98%, whereas the ones with an n-propyl group were prepared by kinetic resolution through asymmetric
hydrogenation, resulting in an ee of up to 98%. X-ray crystallography
revealed a slightly twisted geometry for the phenyl-substituted cisoid
squaraine derivative, whereas the n-propyl-substituted
derivative possessed the expected planar geometry. Variation of the
substitution also influenced the optical properties, where the introduction
of phenyl groups caused a progressive red-shift and reduction in squared
transition moments, as well as reduced fluorescence quantum yields,
Stokes shifts, and fluorescence lifetimes. All of the investigated
compounds exhibited strong ECD signals, with Δε values
of up to 24 M–1 cm–1 for the HOMO–LUMO
transition. DFT calculations indicated that this was due to both large
electric and magnetic transition moments, although the two vectors
were mutually almost orthogonal.

本研究合成了四种吲哚啉方酸菁（indolenine squaraines）系列化合物，其吲哚啉基团的3号位带有手性中心（chiral center），且在手性中心处同时连接甲基与正丙基或苯基；目标立体异构体的纯度均≥98%。其中，手性中心连有苯基的吲哚啉前体通过周环反应级联实现不对称合成，对映体过量值（enantiomeric excess，ee）可达98%；而手性中心连有正丙基的前体则通过不对称氢化动力学拆分制备，其对映体过量值最高达98%。X射线晶体衍射（X-ray crystallography）结果显示，苯基取代的顺式方酸菁衍生物呈现轻微扭曲的几何构型，而正丙基取代的衍生物则呈现预期的平面几何结构。取代基的变化同样会影响其光学性质：引入苯基会使光谱逐步红移，平方跃迁矩降低，同时荧光量子产率、斯托克斯位移（Stokes shifts）及荧光寿命均有所下降。所有被测化合物均表现出较强的电子圆二色性（Electronic Circular Dichroism，ECD）信号，其最高占据分子轨道-最低未占据分子轨道（HOMO-LUMO）跃迁的摩尔消光系数差（Δε）可达24 M⁻¹ cm⁻¹。密度泛函理论（Density Functional Theory，DFT）计算表明，该现象源于较大的电跃迁矩与磁跃迁矩的共同作用，尽管二者几乎相互正交。