PHOTOINDUCED ABSORPTION IN POLYACETYLENE POLYNORBORNENE BLOCK COPOLYMERS

We report photoinduced absorption (PIA) measurements on three different types of diblock copolymers. They consist of polyacetylene (PA) sequences of different degrees of polymerization (namely 16, 26 and 48 double bonds) synthesized via a prepolymer route. The other constituent is polynorbornene (PN) with a constant chain length of 500 repeat units. We focused on the photoinduced electronic transition at about 4500 cm-1 (low energy peak). The influence of sample composition and excitation wavelength as well as the lifetime of the observed photoinduced states are discussed. In conjugated systems the length of a region along a chain, where maximum overlap of the carbon p(z)-orbitals is allowed, is called the effective conjugation length (l(c, eff)). To determine l(c, eff) we use Raman and absorption data in the visible range. In contrast to the chain length, which is determined by synthesis, l(c, eff) seems to be almost identical for all the samples. The contribution of one PA-CH unit to the PIA is found to increase significantly with increasing PA chain length.