Triplet exciton state and related phenomena in the β-phase of poly(9,9-dioctyl)fluorene

Abstract

Using both time-resolved emission and cw photoinduced absorption spectroscopy as a function of temperature, the aggregation phenomena ($\ensuremath{\beta}$-phase formation) observed in poly(9,9-dioctyl)fluorene is studied. All spectra of the $\ensuremath{\beta}$ phase, including absorption, prompt and delayed fluorescence, phosphorescence, and photoinduced triplet absorption feature very narrow linewidths, which are unique within the class of conjugated polymers. From the comparison of the latter data with amorphous polyfluorene, poly(9,9-diethylhexyl)fluorene, as well as with the fully planar ladder-type poly(paraphenylene), we conclude that the origin of the $\ensuremath{\beta}$ phase cannot simply be an extended intrachain conjugation, but interchain interactions are involved. Furthermore, the $\ensuremath{\beta}$ phase acts as an energetic trap for both singlet and triplet excitons initially created on amorphous chain segments. The delayed fluorescence kinetics of the $\ensuremath{\beta}$ phase were measured at different temperatures. From the analysis of these decays within the framework of dispersive triplet migration in a Gaussian density of states distribution, further evidence is provided that the delayed fluorescence originates from triplet-triplet annihilation. At room temperature, it is clear that triplet excitons migrate over large distances, exceeding that of singlet excitons. Also, the segregation time between dispersive triplet migration and classical thermally activated hopping, is in the case of $\ensuremath{\beta}$-phase containing samples, dependent on the separation of the $\ensuremath{\beta}$-phase domains.