Oligothiophene Nanoparticles: Photophysical and Electrogenerated Chemiluminescence Studies

Thiophene oligomer nanoparticles (NPs) were studied by fluorescence spectroscopy and electrogenerated chemiluminescence (ECL). Distinct spectroscopic differences between aggregates or NPs of thiophene hexamers having differing substitution patterns of solubilizing alkyl groups were observed. The α,ω-unsubstituted thiophene hexamer, Hexamer-2, exhibited fluorescence properties that were similar in solution and as colloidal NPs; there was only a small red shift compared with what was observed for the discrete system dissolved in tetrahydrofuran (THF). In contrast, the oligomer substituted in the α,ω-positions with branched alkyl substituents (Hexamer1) displayed a gradual bathochromic shift of the fluorescence maximum in proportion to the amount of a poor solvent (water) added to the THF solution. Moreover, the fluorescence characteristics for the oligomer(s) dissolved in a mixture of THF and water were similar to those seen by annihilation ECL in a mixture of benzene/acetonitrile. On this basis, we conclude that annihilation ECL may be a useful technique for monitoring the formation of organic nanoparticles. SECTION: Spectroscopy, Photochemistry, and Excited States Aggregates of various sizes have been studied for decades, and understanding the factors in their formation and structure is of significance in self-organization and supramolecular chemistry. On the basis of this prior work, it is now appreciated that the use of mixtures of “good” and “poor” solvents provides a useful technique for obtaining aggregates having different shapes as well as differing photophysical and structural properties. Oligothiophenes are good model compounds for such studies because they are π-conjugated and able to form highly ordered assemblies. They have also been actively studied for many applications, ranging from transistors and light-emitting diodes to photovoltaic devices. Ellinger et al. studied the aggregation behavior of α-mono and α,ω-disubstituted oligothiophenes with linear, branched, and mixed alkyl substituents From UV/vis absorbance and fluorescence measurements, as well as AFM imaging, they showed that branched α,ω-substituted thiophene oligomers form aggregates in 1,1,2,2-tetrachloroethane with and without addition of a poor solvent, methanol. Here, we present photophysical and ECL studies of aggregates of two oligothiophene hexamers with different substitution patterns. The structures of the compounds studied in this work, Hexamer-1 and Hexamer-2, are presented in Scheme 1. Detailed synthetic, electrochemical, and photophysical procedures for these compounds are described elsewhere. Both oligomers contain the same number of thiophenes (six) as well as multiple long alkyl chains to enhance their solubility in polar organic solvents. The two compounds are distinguished by their alkyl substitution patterns, and we hypothesized that these structural differences would affect their self-assembly behavior. Hexamer-1 is characterized by long, branching alkyl chains in the α,ω-positions, while Hexamer-2 is unsubstituted at the α,ω-positions and contains alkyl substituents along the oligomer backbone. To test whether Hexamer-1 and Hexamer-2 were capable of forming NPs in solution, a poor solvent for the oligomers, water, was gradually added to independent THF solutions of the oligomers. The addition process was accompanied by a change in the color of the solutions from green to orange. Images of the UV-irradiated solutions of Hexamer-1 and Hexamer-2 in 1:9 v/v THF/water as well as in pure THF are shown in Figure 1. The observed color change upon the addition of water was consistent with aggregate formation. Moreover, the behavior was dependent on solute concentration, as well as the volume fraction of water (Figure S1 in the Supporting Information). To probe these correlations further, fluorescence spectra of Hexamer-1 and Hexamer-2 with different compositions of THF/water were acquired at several different concentrations. In general, there was a large variation in the fluorescence spectra of Hexamer-1 depending on both the concentration and solvent composition (Figure 2a−c and Supporting Information Figure S2a). This was not true for Hexamer-2, which did not show a pronounced change in its fluorescence behavior even in the presence of large quantities of water (Figure 2d). In contrast, the fluorescence spectra of Hexamer-1 Received: June 26, 2012 Accepted: July 11, 2012 Published: July 11, 2012 Letter