EXCITON VERSUS BAND DESCRIPTION OF THE ABSORPTION, LUMINESCENCE AND ELECTROABSORPTION OF POLY(PHENYLPHENYLENEVINYLENE) AND POLY(DODECYLTHIOPHENE)

Arguments are presented in favour of an exciton description of the optical absorption of poly(phenylenevinylenes) (PPVs) and polythiophenes (PTs). (i) Absorption spectra can be deconvoluted into a series of inhomogeneously broadened vibronic transitions characteristic of chromophores no longer than about 10 repeat units. (ii) Site-selective fluorescence studies are in accord with the concept of spectral diffusion among chromophores differing in excitation energy. (iii) Electro-absorption measurements bear out a quadratic Stark effect acting on an exciton the spatial extent of which is 2-3 repeat units only; no evidence for transitions among delocalized states is found. (iv) Action spectra of intrinsic photoconductivity can be interpreted in terms of disorder-promoted expansion of singlet excitons to form geminate e-h pairs that can subsequently dissociate. (v) By comparison with the absorption spectra of chemically prepared anions of oligo-and poly(phenylenevinylenes) the photoinduced absorption of PPV is assigned to the absorption of monovalent ordered polymer segments of finite length.