Influence of alkoxy substituents on the exciton binding energy of conjugated polymers

Scanning tunneling microscope (STM) spectroscopy and optical absorption mea surements 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 signifi cantly higher than the value of E-b reported previously for an alkoxy-PPV d erivative. Furthermore, an analysis of photoluminescence (PL) and STM excit ed cathodoluminescence (CL) spectra, performed on PPV and on alkoxy-PPV der ivatives, 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 interactio n as well as the molecular structural relaxation energy of the exciton. (C) 2000 Elsevier Science S.A. All rights reserved.