Charge-conjugation symmetry breaking in poly (phenylenevinylene) derivatives

When the phenylene monomer of poly (phenylenevinylene) (PPV) is substituted to form one of the well-known PPV derivatives, the charge-conjugation symmetry (CCS) of the unsubstituted polymer is broken, giving rise to a new optical absorption band. A model Pariser-Parr-Pople-like Hamiltonian neglecting the side groups would fail to allow for the breaking of this symmetry and would miss the general property that the CCS-even excitations of the unsubstituted polymer acquire a transition dipole moment whenever CCS is broken.