Photophysical Properties of a Series of Poly(ladder-type phenylene)s

Authors


  • One of the authors (F.L.) acknowledges a postdoctoral research fellowship of the German Research Foundation (DFG) and a Non-stipendiary Clare Hall College Research Fellowship of the Sciences. Another author (M.R.R.) thanks the DAAD for a graduate scholarship. All authors thank U. Scherf for the kind provision of MeLPPP and E. Hoke for the PIA data of polyfluorene. F.L. and A.P. thank P. E. Keivanidis for useful discussions.

Abstract

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.

Ancillary