Cationic polythiophene-surfactant self-assembly complexes: phase transitions, optical response, and sensing

Aggregation properties of p-phenylene vinylene based conjugated oligoelectrolytes with surfactants

The amphiphilic properties of conjugated oligoelectrolytes (COE) and their sensitivity to the polarity of their microenvironment lead to interesting aggregation behavior, in particular in their interaction with surfactants. Photoluminescence (PL) spectroscopy, liquid-phase atomic force microscopy, small-angle neutron scattering, small-angle X-ray scattering, and grazing-incidence X-ray diffraction were used to examine interactions between cationic p-phenylene vinylene based oligoelectrolytes and surfactants. These techniques indicate the formation of COE/surfactant aggregates in aqueous solution, and changes in the photophysical properties are observed when compared to pure aqueous solutions. We evaluate the effect of the charge of the surfactant polar headgroup, the size of the hydrophobic chain, and the role of counterions. At low COE concentrations (micromolar), it was found that these COEs display larger emission quantum efficiencies upon incorporation into micelles, along with marked blue-shifts of the PL spectra. This effect is most pronounced in the series of anionic surfactants, and the degree of blue shifts as a function of surfactant charge is as follows: cationic < nonionic < anionic surfactants. In anionic surfactants, such as sodium dodecyl sulfate (SDS), the PL spectra show vibronic resolution above the critical micelle concentration of the surfactant, suggesting more rigid structures. Scattering data indicate that in aqueous solutions, trimers appear as essentially 3-dimensional particles, while tetra- and pentamers form larger, cylindrical particles. When the molar ratio of nonionic C12E5 surfactant to 1,4-bis(4-{N,N-bis-[(N,N,N-trimethylammonium)hexyl]amino}-styryl)benzene tetraiodide (DSBNI) is close to one, the size of the formed DSBNI-C12E5 particles corresponds to the full coverage of individual oligomers. When these particles are transferred into thin films, they organize into a cubic in-plane pattern. If anionic SDS is added, the formed DSBNI-SDS particles are larger than expected for full surfactant coverage, and particles may thus contain several oligomers. This tendency is attributed to the merging of DSBNI oligomers due to the charge screening and, thus, reduced water solubility.

Polythiophene-based optical sensors for small molecules

Water-soluble, regioregular polythiophene (PT) derivatives are conformational sensitive to external stimuli, and their spectral responses are determined by conformation changes, π-π interactions and scattering of visible lights originated from PT aggregates. In this paper, we review the synthesis of water-soluble PTs and their photophysical properties, and then highlight our efforts toward the development of PT-based optical sensors for small molecules. Mechanism studies indicate that the interplay between side-chain steric/electrostatic repulsive and interchain attractive interactions is responsible for the assembly and disassembly of water-soluble PTs in aqueous media. Consequently, the planarization/deplanarization of PT backbones provides the possibility for colorimetric and fluorescent dual detection of small bioanalytes and chemical species.