Equilibration of Intrachain and Interchain Excitations in Aggregates of a Cyano-Substituted Phenylene Vinylene Polymer

Aggregates formed in dilute solutions of poly[2,2'-dimethoxy-5,5'-di-(2?,7?-dimethyloctyloxy)-dicyano-di-p-phenylene vinylene], OC1C10-CNPPV, in toluene exhibit emission from both an intrachain and an interchain excitation that equilibrate on the timescale of the excited states. The position of the equilibrium can be adjusted with temperature, and a binding energy of 130 meV for the interchain species has been determined. A repulsion energy of 350 meV demonstrates that the interchain excitation is not bound in the ground state, indicative of an excited dimer or complex. At ambient temperatures the equilibrated state exhibits a mean photoluminescence quantum yield of 19% that actually decreases with a reduction in temperature, but a mean photoluminescence lifetime of 4.2 ns that shows a moderate increase. These counterintuitive observations, which imply a strong dependence upon temperature of the mean natural radiative lifetime, arise from the relative weighting factors of the two individual natural radiative lifetimes of the interacting species and the dependence upon the position of equilibrium.