Visualizing Core–Shell Structure in Substituted PPV Oligomer Aggregates Using Fluorescence Lifetime Imaging Microscopy (FLIM)

The use of fluorescence lifetime imaging microscopy (FLIM) is introduced as a means of directly imaging core–shell structured organic aggregates through the gradient observed in their emission wavelength and lifetime as a function of distance from their center to their exterior. The aggregates studied consist of alkoxy-substituted oligomeric PPVs (OPPVs) 7 and 13 rings in length that are formed via reprecipitation in a mixture of methyl tetrahydrofuran (MeTHF) and methanol (MeOH). Prior bulk fluorescence spectroscopy and wavelength-dependent lifetime measurements on these aggregates ( J. Phys. Chem. C 2009, 113, 18851–18862) showed that their properties are consistent with the presence of two types of emitters, one that behaves identically to the monomer with the other having the longer emission wavelengths and shorter lifetimes characteristic of aggregated chains. These two emitters were postulated to be the components of “core-shell”-like structures in which the core consists of aggregated chains and the shell consists of monomer-like chains that are in direct contact with the surrounding solvent. The FLIM images of individual aggregates presented here are consistent with this model which had been developed on the basis of measurements on bulk samples. The uniformity of the emission spectra of these aggregates is also demonstrated using single-aggregate dispersed emission spectroscopy.