Chain Length and Substituent Effects on the Formation of Excimer-Like States in Nanoaggregates of CN-PPV Model Oligomers

The effects of aggregate formation on the photophysical properties of alkoxy and cyano-substituted polyphenylene phenylene vinylene oligomers (CN-PPVs) were studied in bulk solution to better understand the consequences of aggregation for the emission properties of the polymer. Nanoaggregates of oligomers from 5 to 13 repeat units in length were formed using a solvent reprecipitation method. The propensity for these aggregates to exhibit excimer-like emission in solution was found to be a strong function of oligomer chain length and the solvents used in the reprecipitation process. Short-chain oligomers produced nanoaggregates with absorption and fluorescence spectra and emission lifetimes essentially identical to those of the monomer. The aggregates of long-chain oligomers have broad and red-shifted emission spectra and relatively long emission lifetimes, both of which are characteristic of excimer states. However their absorption spectra are also perturbed suggesting that the oligomer chains in these aggregates interact strongly in their electronic ground states as well. For intermediate chain lengths, dual monomer-like (green) and excimer-like (red) emission is observed. Single aggregate dispersed emission spectra from aggregates deposited onto glass coverslips demonstrate that, in the absence of solvent, the predominant emitters are monomer-like rather than excimer-like. Moreover, the monomer-like emitters are found to be far more photostable than the analogous non-CN substituted aggregates, whereas the photostability of the excimer-like emitters is exceptionally poor under the illumination conditions used for microscopy. Comparisons between the properties of these nanoaggregates and the corresponding CN-substituted polymer are drawn

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

Aggregation Effects on the Emission Spectra and Dynamics of Model Oligomers of MEH-PPV

The effects of aggregate formation on the photophysical properties of oligomers of MEH-PPV were studied in bulk solution to better understand the effects of aggregation on the emission properties of the polymer. Nanoaggregates of oligomers from 3 to 17 repeat units in length were formed using a solvent reprecipitation method. The spectra are not readily modeled using the classical dipole−dipole coupling picture of interchain interactions. A strong dependence of the photophysics on the oligomer chain length is also observed. Short-chain oligomers produce nanoaggregates with absorption and emission spectra essentially identical to those of the monomer. Long-chain oligomers form aggregates having more strongly perturbed absorption and fluorescence spectra and decreased emission yields. In these aggregates, the size of the 0−0 band relative to that of the vibronic replicates is a sensitive function of aggregate size and solvent precipitation conditions. Their fluorescence lifetimes are also strongly wavelength dependent. These trends are explained in terms of a core−shell model that postulates the existence of “single-chain-like” and “aggregate-like” emitters within a single aggregate

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 (<i>J. Phys. Chem. C</i> <b>2009</b>, <i>113</i>, 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 (<i>J. Phys. Chem. C</i> <b>2011</b>, <i>115</i>, 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