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 omega similar or equal to 95 degrees) are reported, and the results are compared with them 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 sigma sigma* and the other of sigma pi* type in the planar cis and anti limits, and they undergo a strongly avoided crossing as co changes from 0 degrees to 180 degrees, keeping their energies nearly constant but interchanging their oscillator strengths.