PHOTOLUMINESCENCE IN PYRIDINE-BASED POLYMERS - ROLE OF AGGREGATES

We present a study of the morphology dependence of the photoluminescen ce (PL) properties of the pyridine-based polymers poly(p-pyridyl vinyl ene), poly(p-pyridine), and poly(p-pyridyl vinylene p-phenylene vinyle ne) (PPyVPV). The photoluminescence of solution samples is characteriz ed by high quantum efficiency (>70% in PPyVPV), weak coupling to vibra tional modes (Huang-Rhys parameter similar to 0.5) and a single-expone ntial decay (radiative lifetime similar to 1 ns). On the other hand, f ilm samples display strongly redshifted, featureless emission with low quantum yield (<20%) and highly nonexponential decay dynamics. Throug h consideration of absorption and excitation spectra, the ''site-selec tivity'' of the FL, and the concentration dependence of the PL spectru m, we demonstrate that the redshifted film spectra are a result of the formation of low-energy aggregate sites due to strong interchain inte ractions. Time-resolved measurements suggest a longer radiative lifeti me for the aggregate vs solution, leading to the lower efficiency. Agg regate formation is found to be morphology dependent, and is minimal i n ''powder'' samples which are precipitated after polymerization.