The geometry of hindered stilbenes in their ground and excited states

The torsional angles about the 1-α single bond and about the α-α double bond in hindered stilbene derivatives were calculated for the ground and for the excited states. The calculated ground state geometries form a basis for a π-electron-SCF-LCAO-CI-MO calculation of the singlet excitation energies which agree accurately with the experimental values. The calculated potential curves for torsion about the 1-α bonds provide a qualitative explanation for the significant red shift and increase in intensity of the first singlet absorption band observed in these compounds at low temperatures.