UV, MCD, and LD Spectra of a Conformationally Constrained ortho-Tetrasilane: Support for the Avoided Crossing Model of Conformational Effects on Excited States

UV absorption, magnetic circular (MCD), and linear (LD) dichroism of an n -tetrasilane configurationally constrained to the ortho conformation (calculated SiSiSiSi dihedral angle ω ≃ 95 °) are reported, and the results are compared with the theoretical expectation of four low-energy transitions. Two strong UV absorption maxima with similar intensity and positive MCD B terms are assigned to states of B symmetry. A third calculated transition to an 2 1 A state was clearly detected in the LD spectrum, but a calculated weak transition to a 3 1 A state at a higher energy was not observed. The results support a previous rationalization of the conformational properties of the n -tetrasilane chromophore, according to which one of two low-energy transitions into B-symmetry states is of σσ* and the other of σπ* type in the planar cis and anti limits, and they undergo a strongly avoided crossing as ω changes from 0° to 180°, keeping their energies nearly constant but interchanging their oscillator strengths.