Conformational effects in poly(p-phenylene vinylene)s revealed by low-temperature site-selective fluorescence

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Abstract
Low-temperature site-selective fluorescence (SSF) spectroscopy is employed to study morphological effects on the conformation of poly(p-phenylene vinylene) (PPV) and its phenyl-substituted, soluble derivative poly(phenylphenylenevinylene) (PPPV). Samples of PPV prepared as spin-coated thin films and stretch-aligned free-standing films, and samples of PPPV prepared as cast films and as blends with poly(methylmethacrylate) and polycarbonate have been studied. The results that the authors present are considered with the notion that each polymer sample consists of an array of ordered chain segments whose average length reflects the perfection of the local structure. The statistical distribution of the segment lengths is responsible for inhomogeneous broadening of the optical spectra (absorption and emission). The dominant electronic excitation created by photoexcitation across the pi - pi * energy gap is a singlet exciton that can execute a random walk among the chain segments. SSF spectroscopy allows the authors to distinguish the contributions to the apparent fluorescence Stokes shift that arise from energy relaxation through excitation migration (spectral diffusion) and from structural relaxation of the polymer chain (self-localization). The structural contribution to the Stokes shift approaches zero in well aligned PPV and reaches values of up to 500 cm-1 in highly disordered PPPV films. The SSF method also provides a means of assessing the extent of phase separation that occurs in PPPV blends.