Synthetic approach for elucidation of relationship between photophysical properties and conformation of oligosilanes

Recent theoretical and experimental studies on conformational dependence of the oligosilane chromophore are described. CIS/3-21 G* calculations suggested that a tetrasilane has four transitions into valence excited states. Two of them are of B symmetry, one at lower and one at higher energy. It is not so much the excitation energies as the oscillator strengths of these transitions that are functions of the SiSiSiSi dihedral angle omega (avoided crossing model). This model has been confirmed by experimental efforts such as spectral separation of n-Si4Me10 conformers in low temperature matrices and measurements on conformationally constrained tetrasilane compounds, e.g., tetrasilanes incorporated into five-to-eight-membered carbosilane ring systems and ones composed of configurationally constrained bicyclic disilane unit(s). In addition to tetrasilanes, three hexasilane conformers were investigated as well. The examination revealed that incorporation of a syn-segment into an anti chain raises the excitation energy of the first transition.