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44317-AC4
The Effects of Aggregation on the Electronic Properties of Oligomers Designed for Organic LED's: A Stark and Microscopy Study
Organic materials have great potential for electronic and photo-physical applications such as flat-screen displays, organic transistors, and photocells. The realization of this potential relies on a more thorough understanding of the underlying photophysics, including the effects of aggregation. The photophysics of such amorphous materials are complex, since disorder leads to substantial heterogeneity in both the chromophores themselves and the coupling between these chromophores, all of which can have large impacts on the spectral properties and fluorescence yields. This ACS-PRF supported project explores the underlying mechanisms of such effects by studying structures that span the range from isolated chromophores to bulk, while generating a variety of data that can help clarify the underlying photophysical mechanisms. The specific systems under study are aggregates comprised of oligomers of the well-known electroluminescent polymer MEH-PPV, formed as suspensions by solvent poisoning. Structures spanning from isolated molecule to bulk are obtained by controlling: the length and substitution pattern of the oligomers or polymer, the degree of aggregation, and the environment. Our findings with regard to the somewhat surprising effects of aggregation on the absorption and emission properties of these systems will be described below. During the funding period, the technique of dispersed fluorescence microscopy was implemented in our lab and applied to characterizing individual aggregates so that the heterogeneity in the optical properties of these systems is probed as well. The funded project also includes the use of electrofluorescence (EF) spectroscopy to measure field-induced quenching in the oligomer and polymer systems. This phenomenon has been thought to be a measure of exciton dissociation, at least in polymeric systems. Our studies comparing the degree of fluorescence quenching in isolated oligomers to that of the polymer showed the unexpected result that applied electric fields quench the emission of both and that the degree of quenching observed is readily explained by a model in which the applied field alters the rate of internal conversion. These findings are published and will be summarized briefly below. Our studies represent a systematic study of the effects of oligomer length, aggregate size and solvent precipitation conditions on the absorption and emission properties of PPV-like aggregates. We find that in the case of the 9 ring oligomer aggregate, there is little or no change in the absorption or emission spectrum on aggregation whereas, in the 13 ring oligomer, both spectra are obviously perturbed. The perturbation is indicative of an increase in the Franck-Condon factor for the C=C stretching mode. Moreover, the degree to which the emission spectrum is perturbed depends on aggregate size with larger aggregates showing a smaller perturbation. Very similar intensity patterns are seen in spectra of individual oligomer aggregates indicating that the emission spectra are not strongly sensitive to the expected heterogeneity in the aggregate populations. Our current model for understanding the changes observed in these spectra is that the longer chain oligomers form aggregates with more highly localized excitons or more accessible trap states than do the smaller chain oligomers. More detailed analyses of the spectra are proceeding in collaboration with Professor David Yaron, an electronic structure theorist in my Department and a manuscript on the experimental findings is being written. In addition, our group has been studying field-induced perturbations of the absorption and emission spectra of MEH-PPV and its oligomers over the last few years. One unexpected finding was that the applied field quenches the emission of isolated oligomer molecules as well as the polymer when these systems are present in micromolar concentrations in organic glasses. The degree of fluorescence quenching exhibits a nearly linear dependence on inverse chain length. The observation of emission quenching in the oligomers and in single polymer chains was unexpected because the probability is small either of finding free charges or of dissociating the exciton in the high dielectric environment of an organic solvent glass. These are the two mechanisms that have been invoked to explain field-induced emission quenching in the polymer species. In order to understand the origin of this phenomenon a model was developed that relates the magnitude of the quenching to the expected change in the non-radiative rate of the molecule due to the Stark shift of the excited state in the presence of a field. This model, which achieves quantitative agreement with the experimental findings, presents a new paradigm for the origin of field-induced quenching in organic semi-conductor materials at the low concentration limit.
