Influence of Alkoxy Substituents on the Exciton Binding Energy of Conjugated Polymers

Scanning tunneling microscope spectroscopy and optical absorption measurements were used to determine the exciton binding energy (E(b)) of poly(p-phenylenevinylene) (PPV). We find E(b) = 0.48 +/- 0.14 eV, which is significantly higher than the value of E(b) reported previously for an alkoxy-PPV derivative. Furthermore, an analysis of photoluminescence and scanning-tunneling-microscope-excited cathodoluminescence spectra, performed on PPV and on alkoxy-PPV derivatives, reveals that the energy separation between the dominant peaks in the vibronic progression is significantly smaller in the alkoxy-substituted compounds. The reduction of E(b) appears to be related to the influence of the alkoxy side-chains, inducing a reduction of both the Coulomb interaction as well as the molecular structural relaxation energy of the exciton.