σ Bonds:: Electronic structure, photophysics, and photochemistry of oligosilanes

The making and breaking of sigma bonds is an integral part of almost all photochemical reactions. Yet, the electronic states of sigma electrons are not nearly as well understood as the states of pi-electron systems. Efforts in our laboratory to enhance the current state of their understanding are described, using the specific example of oligosilanes. We address the intrinsically cyclic nature of sigma delocalization and its dependence on chain length and conformation, both in terms of theory and spectroscopic experiments, from the simplest disilane chromophore to the spectral properties of the individual conformers of permethylated heptasilane. We also describe a new low-energy luminescence from certain conformers of permethylated oligosilanes.