Magnetic circular dichroism spectrum of 9,9'-Spirobifluorene

The m.c.d. spectrum of the 9,9?-spirobifluorene molecule is presented and discussed and the lowest excited state is shown to be doubly degenerate. CI wavefunctions are computed by the Pariser-Parr-Pople method with the inclusion of the ?-? interactions at the spiro carbon atom, and the values of the m.c.d. A and B constants are evaluated for the various molecular excited states. To evaluate the m.c.d. B constants we propose a procedure based on the direct solution, by a CI expansion method, of the perturbative equations, which provide the first-order corrections to the ground- and excited-state wavefunctions in the presence of a static magnetic field. It is shown that the ?-? interactions, by breaking the pairing symmetry of alternant hydrocarbons, provide a large contribution to the intensity and to the m.c.d. A constant of the lowest electronic transition. An assignment of the u.v. spectrum of spirobifluorene is proposed, consistent with the spectral data available from absorption, linear dichroism and magnetic circular dichroism measurements.