Photophysical properties of a series of poly(ladder-type phenylene)s

The photophysical properties, i.e., the fluorescence and phosphorescence of a series of blue light-emitting poly(ladder-type phenylene)s have been investigated employing continuous-wave (cw) and time-resolved photoluminescence (PL) spectroscopy in solid state and dilute solution. The chemically well-defined polymers vary from two to five bridged phenyl-rings per monomer unit bearing aryl- or alkyl-substitution at the bridge-head carbon atoms. It has been found that the fluorescence energy of the polymers and of the corresponding monomers deviates from a simple 1/N dependence, if the number N of bridged-phenylene rings is increased beyond a certain limit. Time-resolved fluorescence spectroscopy on thin films showed that apart from the blue fluorescence of the polymers an additional lower energy emission feature exists, which cannot be assigned to keto-defects and which seems to be an inherent solid state property of this class of materials. Delayed time-resolved photoluminescence spectroscopy allowed the detection of phosphorescence energies and lifetimes for all investigated polymers. Photoinduced absorption spectroscopy on thin films showed that the triplet-triplet absorption red-shifts with increasing monomer length but reaches a constant value for polymers with N≥4. Amplification of light via amplified spontaneous emission (ASE) from thin film slab waveguide structures could be demonstrated for all ladder-type polymers but the onset threshold value for ASE varies significantly with the polymer structure.