Effects of solvent properties on the spectroscopy and dynamics of alkoxy-substituted PPV oligomer aggregates

Conjugated systems are frequently studied in their nanoaggregate form to probe the effects of solvent and of film formation on their spectral and dynamical properties. This article focuses on the emission spectra and dynamics of nanoaggregates of alkoxy-substituted PPV oligomers with the goal of interpreting the vibronic emission envelopes observed in these systems (J. Phys. Chem. C2009, 113, 18851-18862). The aggregates are formed by adding a nonsolvent such as methanol (MeOH) or water to a solution of the oligomers in a good solvent such as methyl tetrahydrofuran (MeTHF) or tetrahydrofuran (THF). The emission spectra of aggregates formed using either of these combinations exhibit a vibronic pattern in which the ratio of the intensity of highest-energy band to that of the lower energy peaks depends strongly on the ratio of good to poor solvent. In aggregates formed from MeTHF:MeOH, this was shown to be due to the presence of both aggregate-like and monomer-like emitters forming a "core" and surrounding "shell"-like structure, respectively, within a single aggregate (J. Phys. Chem. C2011, 115, 15607-15616). In support of this model, the monomer-like emission is shown here to be significantly decreased by changing the solvent pair to the more polar THF:water. This suggests that nanoaggregates formed in THF:water contain a much smaller proportion of monomer-like chains than those formed in MeTHF/MeOH, as would be expected from using a more highly polar nonsolvent. Results from bulk steady-state and time-resolved emission measurements as well as fluorescence lifetime imaging microscopy (FLIM) of the aggregates are shown to be consistent with this interpretation.