Development of ultrafast broadband UV-Vis circular dichroism spectroscopy for investigating the photoinduced dynamics of chiral molecular systems

瞬时圆二色谱(TrCD)是一种很有前途的跟踪光激发手性体系瞬时演化的方法。这是通过监测左、右圆偏振光之间光诱导吸收变化的差异来实现的。在这个项目的第一个资助期，我们建立了覆盖260-700 nm波长范围的超快TrCD光谱装置，时间分辨率为170-200 fS。它被成功地应用于获得关于手性聚合物薄膜的光诱导动力学和相干光谱的新信息。在这份更新建议中，我们希望通过采用两项新的技术开发来显著提高现有UV-Vis TrCD装置的灵敏度：(1)基于Pockels池的先进设计和(2)采用光弹性调制器的替代设计。这两种方法都将把设置的时间分辨率提高到约90ps，并将目前的信噪比提高至少10倍。这些新的TrCD光谱装置将被用来阐明三类手性体系的动力学：(1)表现出大的本征手性或巨大手性诱导的光激发胆甾型荧咔唑共聚物排列的电子和结构动力学，(2)螺烯类分子和薄膜在激发电子态下的瞬时CD响应，以及(3)手性钙钛矿半导体在光激发下对导带的动力学。使用超快宽带UV-VIS-NIR瞬时吸收的测量将补充使用TrCD光谱的研究。这些实验将通过复杂的全局动力学分析技术和基于密度泛函理论的计算进行分析。我们相信，这些先进的超快瞬变CD光谱装置将适用于探索各种手性体系的结构和电子动力学。

Transient circular dichroism (TrCD) spectroscopy is a promising method for tracking the transient evolution of photoexcited chiral systems. This is made possible by monitoring the difference in the photoinduced changes of absorption between left and right circularly polarized light. In the first funding period of this project, we established a setup for ultrafast TrCD spectroscopy covering the wavelength range 260-700 nm with a time resolution of 170-200 fs. It was successfully applied to obtain new information regarding the photoinduced dynamics and the coherent spectroscopy of chiral polymer thin films. In this renewal proposal, we would like to improve the sensitivity of the existing UV-Vis TrCD setup considerably by employing two new technical developments: (1) an advanced Pockels-cell-based design and (2) an alternative design involving a photoelastic modulator. Both approaches will improve the time resolution of the setup to ca. 90 fs and the present signal-to-noise ratio by at least a factor of ten. These new setups for TrCD spectroscopy will then be applied to elucidate the dynamics of three classes of chiral systems: (1) the electronic and structural dynamics of photoexcited cholesteric fluorene- and carbazole-based copolymer arrangements showing either large intrinsic chirality or giant chiral induction, (2) the transient CD response of helicene-type molecules and thin films in the excited electronic state and (3) the dynamics of chiral perovskite-inspired semiconductors upon photoexcitation to the conduction band. Measurements employing ultrafast broadband UV-Vis-NIR transient absorption will complement the studies using TrCD spectroscopy. The experiments will be analyzed by sophisticated global kinetic analysis techniques and calculations based on density functional theory. We are convinced that these advanced setups for ultrafast transient CD spectroscopy will be applicable to probing the structural and electronic dynamics of various chiral systems.