Electron Delocalization in Saturated Molecules

NSF化学部的化学结构、动力学和机制B计划支持科罗拉多大学博尔德分校化学和生物化学系的Josef Michl教授的研究。米克尔教授和他的学生正在研究硅原子链中电子激发态的成键性质。这些研究对于了解非晶硅太阳能电池在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。该项目为培养对激光、光与物质相互作用和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机会。重点是吸引学生进入STEM领域，包括来自目前任职人数不足的群体的学生。目前对低聚硅烷和聚硅烷(饱和烃的硅类似物)等饱和分子中电子离域的认识远远落后于共轭pi电子体系。光吸收在全烷基低聚硅烷链中引起的几何扭曲特别耐人寻味。荧光显示，它们在长链中可以忽略不计，而在硅原子少于7个的链中则非常明显。计算表明，这种扭曲可以描述为一个或两个硅原子从正常的四面体几何结构转变为三角双锥体几何结构，其中硅使用了五个“价”轨道，而不是通常的四个。硅价壳层的这种展开将对成键理论具有重要意义。Michl教授的团队正试图通过实验验证这一理论预测，他们使用的是这种短寿命激发物种的振动光谱。相同的瞬变振动光谱方法对其他光物理问题也很有用，特别是在研究已知经历有效单线态裂变的材料时，例如高级聚苯和异苯并呋喃的衍生物。这项工作包括继续进行国际合作，为本科生提供研究体验，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.