Owing to their low production cost, conjugated organic molecules such as perylene diimides (PDIs) are an attractive choice as semiconductors in optoelectronic devices. The efficiency of PDIs in these devices is determined by many factors, including molecular aggregation in solution and its effects on thin film morphology. This work presents a detailed analysis of aggregate formation in N-alkylated branched PDIs. Visible spectroscopy methods were used to measure the presence of aggregates in binary solvents. A methanol:water system was used to examine the aggregation of one of the molecules assayed, N,N′-Bis(10-nonadecyl)perylene-3,4,9,10-bis(dicarboximide) (B10). Because of this molecule's solubility in methanol and insolubility in water, increasing the water concentration in the solvent promoted the formation of dimers and small-scale aggregates. Dimerization was characterized by a decrease in overall absorbance intensity and an increase in the 0-2 vibronic transition (490 nm) with respect to the 0-1 vibronic transition (527 nm). For some solvent ratios, a new, broader peak at 540 nm appeared, indicating the presence of larger scale aggregates in solution. Temperature controlled experiments were performed on a 7% water binary mixture of 10 µM B10 to evaluate the energetics of aggregate formation. The decrease in intensity of the monomer spectrum was used to determine the temperature dependent equilibrium constants for this mixture. Through these experiments, the Gibbs free energy of aggregate formation was calculated (Δ<i>G</i> = –175 kJ/mol). The solution phase spectra were then compared to spectra of films drop cast from this solution. Changes between the solution phase and solid state spectra provide information about those electronic interactions which are impacted by aggregate size, such as π stacking. We believe that this study provides the scaffolding for a more complete set of experiments to determine the thermodynamic, molecular geometry, and orbital effects on aggregation in conducting organic thin films.

Spectroscopic characterization of the molecular aggregation of N-alkylated perylene diimides

251st National Meeting (2016)

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