ABSTRACT
A detailed time resolved investigation of the photoluminescence of a thin tetracene film deposited on highly oriented pyrolytic graphite is presented. In agreement with Lim et al. [Phys. Rev. Lett. 92, 107402 (2004)], we find strong evidence for superradiance: an increase of the relative intensity of the pure electronic transition with respect to the vibronic sideband and a concomitant decrease of the radiative lifetime from 10 to 1.83 ns upon cooling from 300 to 4 K. For lower temperatures, a redshift (approximately 200 cm(-1)) of the free exciton is observed. Previously, this shift was attributed to a structural phase transition. Our time resolved spectra reveal that the spectral shift is related to a dynamical relaxation process which occurs within the first 50 ps.
ABSTRACT
The oriented polyfluorene 1 is a promising candidate for a blue polarized emitter in organic optoelectronic devices. By examination of the anisotropic luminescence of thin films of 1, polarization-dependent spectral narrowing was found at high excitation densities, which is of particular interest in reducing the threshold for optically pumped polymer lasers. Femtosecond pumpprobe experiments show optical gain at this spectral position.