Tuning of the α‐Terthiophene Radical Cation Coupling Reaction Using Mixed Micelles with Varying Charge Density

The photophysics and photochemistry of α ‐terthiophene ( α T), compartmentalized in mixed nonionic/anionic micelles, have been investigated with focus on the influence of the micellar surface charge density on the formation of the radical coupling product α ‐hexathiophene ( α H). By varying the ratio of nonionic‐to‐anionic surfactants, and assuming ideal mixing, the charge density of the mixed micelles was varied. From Poisson–Boltzmann calculations, performed using the cell model, the electrostatic potential and the counterion activity were estimated as a function of the distance from the micellar surface. Upon excitation, the triplet state of α T is formed, from which the α T radical cation can be formed by absorption of a second photon. The radical cation can form α H if it encounters another α T radical cation. Under the experimental conditions used, this implies that the α H formation only occurs if the compartmentalized radical cation is able to migrate from its host micelle to another micelle, either via the surrounding bulk or by fusion of two micelles followed by mixing of their contents before micellar fission. The formation yield of the radical cation depends on the charge density of the mixed micelle; a lower charge density, that is, an increased amount of nonionic surfactant, lowers the yield. The yield of the coupling product α H, however, does not follow the same trend. A maximum yield of α H is found at intermediate nonionic surfactant molar ratios. This behavior is understood in terms of the Poisson–Boltzmann simulation results and by comparing charge‐density changes as a function of molar fraction with the changes in counterion activity. The α H yield is a result of the balance between an increased possibility of radical cation bulk migration and a lowered electrostatic stabilization of the radical.