Photoinitiated charge transfer in tailor-made molecules studied with 100 kilohertz two-dimensional white-Light spectroscopy

在这项由化学系化学结构动力学和机理A(CSDM-A)计划资助的项目中，威斯康星大学麦迪逊分校的马丁·T·赞尼教授正在使用一种名为二维白光(2D WL)的新技术来研究定制分子中的电荷转移。电荷转移过程在化学中起着核心作用，例如传递电子以加速化学反应。对电荷转移的详细理解是困难的，因为人们必须知道分子的结构，它们包含多少能量，以及电子从一个地方转移到另一个地方的速度有多快。Zanni教授正在通过研究一系列具有明确定义和系统变化的结构的定制分子来应对这一挑战。这一系列分子是由西北大学迈克尔·瓦西列夫斯基教授的研究小组合成的。由于这些分子吸收不同颜色的光，赞尼教授使用白光光谱学来研究分子的能级和电荷转移的速率。白光是可见光的所有其他颜色的组合。吸收光谱是原子或分子如何吸收不同颜色(波长)的光的图形描述。通过用彼此相隔几百飞秒(一飞秒等于一千万亿分之一秒)的短光脉冲激发分子，赞尼教授能够生成看起来像地形图的二维光谱，并提供关于分子内部或分子之间电荷和能量如何运动的线索。这项研究提供的见解可能会促进我们对生命系统中电荷转移过程的理解，并可能对与太阳能技术相关的材料的设计产生影响。Zanni教授和他的团队参与了各种以公共科学教育为重点的活动。他们在温格拉小学科学之夜发表演讲，并主持本科生研究体验(REU)高中教师研究活动，作为研究传播计划的一部分。Zanni教授和他的团队还为威斯康星州公共电视台设计并准备了一个电视讲座，内容是将技术从大学研究过渡到商业领域。Zanni研究小组正在使用该小组开发的2D WL光谱技术研究一系列定制的分子。该系列分子由Wasielewski小组制备，旨在以系统的方式改变电子耦合和电荷转移特性。一组化合物是笼形络合物，旨在捕获和传递多个电荷，用于多电子氧化还原反应。另一组是以特定几何形状持有的聚丙稀二亚胺和二苯二亚胺的桥联二聚体。桥联二聚体系统已经系统地控制了与单重态裂变中的电荷转移相关的电子耦合。二维WL光谱学对脉冲序列使用连续谱生成，创建的光谱几乎覆盖了光谱的整个可见光和近红外区域。白光比光学参数放大器(OPA)的输出要弱得多，因此光谱仪与一个100 kHz的激光光源和一个逐个脉冲整形器配对。这种2D WL光谱仪光谱范围宽，信噪比高，非常适合研究Wasielewski教授的化合物。最初的2D WL光谱包含交叉峰，揭示了以前在吸收光谱中没有观察到的电荷转移跃迁，揭示了这些化合物中电荷转移的光物理途径。这台100 kHz光谱仪的灵敏度如此之高，以至于本研究的最终目的是将其作为一种测量空间分辨率2D WL光谱的显微镜工具来实现。这项研究可能会扩大我们对电荷转移的理解，并开发新的光学技术。通过将这些量身定制的分子与新的2D WL光谱相结合而实现的实验可能会克服现有的挑战，并扩展对化学中电荷转移的理解。

In this project funded by the Chemical Structure Dynamics and Mechanism-A (CSDM-A) program of the Chemistry Division, Professor Martin T. Zanni of the University of Wisconsin-Madison is studying charge transfer in tailor-made molecules using a new technique called two-dimensional white-light (2D WL) spectroscopy. Charge transfer processes play a central role in chemistry, such as for delivering electrons to speed up chemical reactions. A detailed understanding of charge transfer is difficult, because one must know the structures of the molecules, how much energy they contain, and how fast the electrons transfer from one place to another. Professor Zanni is addressing this challenge by studying a series of tailor-made molecules with well-defined and systematically varied structures. This series of molecules is being synthesized by the research group of Professor Michael Wasielewski at Northwestern University. Since these molecules absorb light of different colors, Professor Zanni uses white light spectroscopy to study the energy levels of the molecules and the rates of charge transfer. White light is a combination of all other colors of visible light. Absorption spectra are graphic descriptions of how atoms or molecules absorb light of different colors (wavelengths). By exciting the molecules with brief light pulses separated from each other by a few hundred femtoseconds (a femtosecond is one quadrillionth of a second), Professor Zanni is able to generate two-dimensional spectra, which look like topographical maps, and provide clues about how charge and energy are moving within or between molecules. The insights provided by this research may advance our understanding of charge transfer processes in living systems, and may have implications for the design of materials relevant to solar energy technologies. Professor Zanni and his group are involved in various activities focused on the public science education. They give presentations at the Wingra Elementary School Science Night and host an Research Experiences for Undergraduates (REU) high school teacher as part of their research dissemination plan. Professor Zanni and his group have also designed and prepared a televised lecture for Wisconsin Public Television on transitioning technologies from university research into the commercial sphere.A series of custom-made molecules are being studied by the Zanni research group using a 2D WL spectroscopy technique developed by this group. The series of molecules, prepared by the Wasielewski group, is designed to vary the electronic couplings and charge-transfer character in a systematic way. One set of compounds are caged complexes designed to capture and deliver multiple charges for use in multielectron redox reactions. Another set are bridged dimers of terrylene and perylene diimide, held in specific geometries. The bridged dimer systems have systematically controlled electronic couplings relevant to charge transfer in singlet fission. Two dimensional WL spectroscopy uses continuum generation for the pulse sequence, creating spectra that span nearly the entire visible and near-IR region of the spectrum. White-light is much weaker than the output of an optical parametric amplifier (OPA), and so the spectrometer is paired to a 100 kHz laser source and a shot-to-shot pulse shaper. With a broad spectral range and high signal-to-noise, this 2D WL spectrometer is well-suited for studying Professor Wasielewski's compounds. Initial 2D WL spectra contain cross peaks that reveal charge-transfer transitions that were previously unobserved in absorption spectra, uncovering the photophysical pathway for charge transfer in these compounds. The sensitivity of this 100 kHz spectrometer is so high that the final aim of this research is to implement it as a microscopy tool to measure spatially resolved 2D WL spectra. This research may extend our understanding of charge transfer and to develop new optical technologies. The experiments made possible by the combination of these tailor-made molecules and new 2D WL spectroscopy may overcome existing challenges and extend the understanding of charge transfer in chemistry.