On the polarization of the green emission of polyfluorenes

An experimental and theoretical study of the anisotropic optical properties of polyfluorenes (PFs) bearing ketonic defects is presented. Polarized emission experiments performed on photooxidized aligned PF layers indicate that the transition dipole of the green CT pi-pi* transition of the keto-defect is oriented parallel to the chain direction. It is further observed that the polarization ratio of the green emission is slightly smaller than that of the blue emission component originating from undisturbed chains. Quantum mechanical calculations have been performed to support these observations. It is shown that the transition dipole moment of the CT pi-pi* transition of the defect is slightly misaligned with respect to the pi-pi* transition of the undisturbed PF chain, and that the angle between both depends on the chain conformation. For the most probably 5/2 helical conformation, this angle is, however, smaller than 5degrees. Further, polarized PL spectroscopy with polarized excitation has been performed to determine the extent of energy migration prior to emission from the keto-defect. For excitation at 380 nm, the polarization ratio of the green emission is essentially independent of the excitation polarization, indicating almost complete depolarization of the excitation before it is captured at a defect site. In contrast to this, energy migration after direct excitation of the keto-defect is inefficient or even absent. (C) 2003 American Institute of Physics.