Experimental and theoretical studies of the electronic structure of substituted and unsubstituted poly(para-phenylenevinylene) (PPV)

The electronic structure of poly(p-phenylenevinylene) and that of its ring-substituted derivatives, poly(2,5-diheptyl-1,4-phenylenevinylene), poly(2,5-dimethoxy-1,4-phenylenevinylene), and poly(2-methoxy-5-(2'-ethylhexoxy)-1,4-phenylenevinylene), are studied by Ultraviolet Photoelectron Spectroscopy, UPS, and X-ray Photoelectron Spectroscopy, XPS. It is observed by UPS that the pi-bands closest to the valence band edge are strongly affected by the presence of the substituents. The influence of the side groups on the experimental spectra is studied theoretically using the Valence Effective Hamiltonian, VEH, model. Calculations are carried out on isolated polymer chains, including full treatment of the aliphatic side groups. Particular attention is paid to the effect of chain torsion angles on the pi-band edge. For the diheptyl derivative, the experimental results can be explained on the basis of side-group-induced torsions of the phenylene rings along the backbone, which influence the pi-band widths and contribute to differences in both optical absorption threshold and binding energy of the valence band edge. For the alkoxy derivatives, the side groups cause strong modifications in the shape of the upper two occupied pi-bands, which results in significant changes in the electronic density of states