Site-selective spectroscopy of matrix-isolated conjugated polymers

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Abstract
Fluorescence spectra of poly(phenylphenylenevinylene) (PPPV) and poly(diphenylenediphenylvinylene) (PDPV) incorporated in a glassy methyltetra-hydrofuran matrix have been measured at 6 K upon site-selective excitation. Their shift with excitation energy above what is commonly referred to as band gap is incompatible with the band picture. Instead, localised transitions within segments of the chain have to be invoked for emission as well as for absorption as evidenced by deconvoluted fluorescence excitation spectra characteristic of chromophores sitting in a random environment. Solid film spectra can be rationalised in terms of additional inhomogeneous broadening. In PDPV, yet not in PPPV, the excitation is strongly coupled to a chain deformation associated with a twist vibration of the biphenyl unit. In PPPV the observed Stokes shift is attributed to electronic relaxation in the course of energy transfer. The electron-phonon coupling parameters have been evaluated.