Photoexcitations in Conjugated Copolymers

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Abstract
We report in this short paper a preliminary account on the optical characterization and photoexcitation of a class of novel conjugated polymers consisting of regularly alternating thiophene and benzene sequences, which might represent models for one-dimensional superlattices. Optical absorption and emission spectra suggest that the barriers produced by the phenylene moieties do not interrupt the conjugation but can produce localization effects. Resonant Raman spectra indicate that a selective response of the thiophene and phenylene moieties occurs by varying the excitation energy from visible to ultraviolet. Moreover these measurements confirm the presence of a confinement on the thiophene blocks, while the biphenyl moiety does not appear to feel the energy barrier and behaves as a poly(paraphenylene) oligomer of 5-6 ring length. CW photomodulation spectra show that the formation of long-lived bipolarons takes place only if the thienylenic moiety is long enough to accomodate the lattice distortion.