Structural basis for the spectroscopy and photophysics of solution-aggregated conjugated polymers

We present fluorescence spectra, quantum yields and structural data from H-1 NMR studies on solutions of a di-alkoxy substituted pentamer of p-phenylenevinylene (5PV) and the corresponding polymer poly-(p-phenylenevinylene) (PPV). We vary the solvent quality continuously using miscible good and poor solvents to study the effects of polymer aggregation as is relevant to properties of films and of the concentrated solutions from which films are formed. We observe a large drop in quantum yield with reduced solvent quality that we attribute to formation of interchain excitations with negligible luminescence when the polymer aggregates. Concomitant with the drop in fluorescence, we observe a red shift in the spectra. The NMR data suggest the red shift is due to an increase in the steric hindrance of the backbone motion that leads to an increase in effective conjugation length. In each material, our data can be described as a linear combination of only two species with distinct spectroscopy and quantum yield, an isolated and aggregated form of the polymer. Studies of the properties of mixtures of the pentamer and polymer demonstrate conclusively that these species mix and that the spectroscopic changes are due to aggregation and not conformational changes of single chains. They also provide insight into recent results from electroluminescent devices based on analogous conjugated polymer blends. (C) 2003 Elsevier B.V. All rights reserved.