Photoluminescence in pyridine-based polymers: Role of aggregates

We present a study of the morphology dependence of the photoluminescence (PL) properties of the pyridine-based polymers poly(p-pyridyl vinylene), poly(p-pyridine), and poly(p-pyridyl vinylene p-phenylene vinylene) (PPyVPV). The photoluminescence of solution samples is characterized by high quantum efficiency (>70% in PPyVPV), weak coupling to vibrational modes (Huang-Rhys parameter similar to 0.5) and a single-exponential decay (radiative lifetime similar to 1 ns). On the other hand, film samples display strongly redshifted, featureless emission with low quantum yield (<20%) and highly nonexponential decay dynamics. Through consideration of absorption and excitation spectra, the ''site-selectivity'' of the FL, and the concentration dependence of the PL spectrum, we demonstrate that the redshifted film spectra are a result of the formation of low-energy aggregate sites due to strong interchain interactions. Time-resolved measurements suggest a longer radiative lifetime for the aggregate vs solution, leading to the lower efficiency. Aggregate formation is found to be morphology dependent, and is minimal in ''powder'' samples which are precipitated after polymerization.