Protonation-Induced Sign Inversion of the Cotton Effects of Pyridinophanes. A Combined Experimental and Theoretical Study

The
circular dichroisms (CDs) of planar chiral [2.2]- and [3.3]-pyridinophanes
were investigated experimentally and theoretically. Strong multisignate
Cotton effects, typical for cyclophane derivatives, were observed.
The CD spectra of [2.2]- and [3.3]-paracyclophanes closely resembled
in pattern each other, despite the much greater conformational variations
in the latter. Upon protonation, both of the cyclophanes suffered
dramatic CD spectral changes with accompanying complete sign inversion,
which was attributed to the reversal of diploe moment of pyridinium
versus pyridine moiety. This chiroptical property switching driven
by protonation/deprotonation was temperature-dependent and hence applicable
to thermal sensing. The protonated forms of pyridinophanes served
as ideal model systems for studying the cation−π interactions
and their effects on chiroptical properties. Thus, the molar CD (Δε)
of the charge-transfer band of protonated [2.2]­pyridinophane was 10-fold
larger than that of protonated [3.3]­pyridinophane, which exceeds the
increased interplane electronic interactions assessed from the electronic
coupling element values.

本研究针对平面手性[2.2]-与[3.3]-吡啶环蕃（pyridinophanes）的圆二色性（circular dichroisms, CDs）开展了实验与理论层面的系统探究。研究观测到环蕃衍生物典型的强多信号科顿效应（Cotton effects）。尽管[3.3]-对环蕃（paracyclophanes）的构象变化程度远高于[2.2]-对环蕃，但二者的圆二色谱图谱特征高度相似。经质子化处理后，两种环蕃的圆二色谱均发生显著变化，并伴随完全的信号符号反转，该现象归因于吡啶鎓与吡啶基团的偶极矩方向发生反转。这种由质子化/去质子化驱动的手性光学性质切换具有温度依赖性，因此可应用于热传感领域。吡啶环蕃的质子化形式是研究阳离子-π相互作用（cation−π interactions）及其对手性光学性质影响的理想模型体系。具体而言，质子化[2.2]-吡啶环蕃的电荷转移带摩尔圆二色性（Δε）是质子化[3.3]-吡啶环蕃的10倍，这一差值超过了基于电子耦合参数（electronic coupling element）评估得到的平面间电子相互作用增强幅度。