THE EVOLUTION OF CHARGE-INDUCED GAP STATES IN DEGENERATE AND NONDEGENERATE CONJUGATED MOLECULES AND POLYMERS AS STUDIED BY PHOTOELECTRON-SPECTROSCOPY

We report the results of ultraviolet photoelectron spectroscopy (UPS) studies of the interaction between sodium and conjugated systems for a series of diphenylpolyenes and different oligomers of poly(p-phenylen evinylene) (PPV). The diphenylpolyenes include molecules containing tw o (i.e., stilbene) to 14 carbon atoms in the polyene part; stilbene it self can also be considered as a phenyl-capped monomer of PPV. Further more, a PPV oligomer with three phenylene units, as well as PPV itself , has been studied. The experimental results are interpreted with the help of quantum-chemical calculations using the Hartree-Fock semi-empi rical Austin Model 1 (AM1) and valence-effective Hamiltonian (VEH) met hods. An important result is that all the systems react strongly with sodium; at high doping levels two new doping-induced states are detect ed above the valence band edge of the pristine material. In the case o f saturation-doped diphenylpolyenes (i.e., two sodiums per molecule), the new states can be discussed in terms of soliton-antisoliton pairs confined within the polyene part of the molecules; in contrast, the se lf-localized states induced in PPV and its oligomers have to be referr ed to as bipolarons.