Electron Delocalization in Saturated Molecules

美国国家科学基金会化学分部的化学结构、动力学和机理B项目支持科罗拉多大学博尔德分校化学和生物化学系Josef Michl教授的研究。Michl教授和他的学生正在研究硅原子链在电子激发态下的键合性质。这些研究对于理解非晶硅太阳电池暴露于紫外线辐射下的疲劳以及理解由硅链组成的材料的导电性具有实际意义。该项目为对激光、光与物质相互作用以及国际科学感兴趣的本科生、研究生和博士后研究人员提供了培训机会。重点是吸引学生进入STEM领域，包括那些目前代表性不足的群体。目前对饱和分子（如低聚硅烷和聚硅烷（饱和烃的硅类似物））中电子离域的理解远远落后于共轭pi电子系统。光吸收引起的过烷基低聚硅烷链的几何畸变特别有趣。荧光显示，它们在长链中可以忽略不计，而在含有少于7个硅原子的链中则非常明显。计算表明，这种扭曲可以被描述为一个或两个硅原子从正常的四面体几何形状转变为三角双锥体几何形状，其中硅使用了五个“价”轨道而不是通常的四个。硅价壳的这种膨胀对化学键理论具有根本性的重要性。Michl教授的研究小组正试图利用短寿命激发态的振动光谱学，通过实验来验证这一理论预测。瞬态振动光谱学的相同方法对其他光物理问题也很有用，特别是在研究已知能进行有效单线态裂变的材料时，如高级多烯和异苯并呋喃的衍生物。这项工作包括持续的国际合作，为本科生提供研究经验，并为高中生提供课堂演示。

The Chemical Structure, Dynamics, and Mechanism B Program of the NSF Chemistry Division supports the research of Professor Josef Michl in the Department of Chemistry and Biochemistry at the University of Colorado, Boulder. Professor Michl and his students are examining the nature of bonding in electronically excited states of chains of silicon atoms. These studies have practical implications for understanding fatigue in amorphous silicon solar cells exposed to ultraviolet radiation as well as understanding conductivity in materials comprised of silicon chains. The project is providing opportunities for the training of undergraduate and graduate students and postdoctoral researchers interested in lasers, in the interaction of light with matter, and in international science. Emphasis is being placed on attracting students to STEM fields, including those from groups that are presently underrepresented. The current understanding of electron delocalization in saturated molecules such as oligosilanes and polysilanes (silicon analogs of saturated hydrocarbons) lags far behind that in conjugated pi-electron systems. Geometrical distortions induced in peralkylated oligosilane chains by light absorption are particularly intriguing. Fluorescence reveals that they are negligible in long chains and very pronounced in chains containing fewer than about seven silicon atoms. Calculations suggest that the distortions can be described as a transformation of one or two of the silicon atoms from the normal tetrahedral to trigonal bipyramidal geometry, in which silicon used five "valence" orbitals instead of the usual four. Such expansion of the silicon valence shell would have fundamental importance for bonding theory. Professor Michl's group is attempting to verify the theoretical prediction experimentally, using vibrational spectroscopy of the short-lived excited species. The same methods of transient vibrational spectroscopy are useful for other photophysical problems, in particular in the investigation of materials known to undergo efficient singlet fission, such as the higher polyacenes and derivatives of isobenzofuran. The work involves continuing international collaborations, providing undergraduate students a research experience, and giving classroom demonstrations to high school students.