Picosecond spectroscopy and hyperlinear photoluminescence in poly(para-phenylene)-type ladder polymers

In our investigations of the photoluminescence in poly-(para-phenylene)-type ladder-polymer films a highly nonlinear dependence of the emission spectra is observable at elevated excitation densities. A spectral redistribution results in the hyperlinear increase of emission efficiency in the blue region, as the vibronic 0-1 transition (lambda=492 nm) becomes the most predominant radiative relaxation. The detailed study of temporal decay and spectral evolution of luminescence provided by streak camera measurements performed in the temperature range T=20-300 K reveals the spectrum being composed by the contributions of two different emitting species, namely, S-1-singlet states and aggregates. The material is found to be photochemically stable and luminescence does not seem to be limited by saturation effects even under high-excitation conditions, which is essential for laser application.