The effect of the extent of $\pi$ electron conjugation on the primary photophysics in semiconducting polymers is reported. A rapid depolarization of photoluminescence and transient absorption, which indicates a reorientation of the transition dipole moment by $\sim 30°$ on a sub-100 fs time scale, is observed in the fully conjugated polymer poly[2-(2’-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV). In contrast, partially conjugated polymers exhibit a much slower depolarization. The results reveal rapid changes of exciton delocalization in the fully conjugated MEH-PPV driven by structural relaxation.

• Received 28 February 2005

DOI:https://doi.org/10.1103/PhysRevB.72.115214