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.
