α-Terthiophene in Micellar Solutions: Influence of Micellar Size and Charge on the Terthiophene Photophysics and Photochemical Reactivity

Abstract

The photophysics and photochemistry of α-terthiophene (αT) in one nonionic, three cationic, and one anionic micellar systems have been investigated. If the micelles are large enough, αT follows a Poissonian distribution among the micelles. The extinction coefficients and the fluorescence quantum yields are independent of the surfactant forming the host micelle. The survival probability of the αT triplet is dependent on the number of αT's per micelle; the lower this number is, the longer the triplet survives. The triplet decays multiexponentially at times close to the excitation event but monoexponentially at longer times. The triplet remains in its host micelle due to a substantially slower intermicellar migration as compared to that of the αT radical cation. Delayed fluorescence emission, due to triplet−triplet annihilation, was detected in all systems. The rate constant of the delayed fluorescence could be correlated to the micellar volume of the ionic surfactants. For the nonionic surfactant, it was assumed that the αT molecule can penetrate the hydrophilic shell between the hydrophobic core of the micelle and the aqueous bulk. The yield of formation of the αT radical cation is the highest in the anionic system and the lowest in the neutral one. For the cationic surfactant micelles, the yield is intermediary and approximately equal. The formation of the radical is found to be biphotonic. The formation of the radical coupling product α-hexathiophene occurred in all micellar systems, proving that the radical can leave its host micelle and migrate via the bulk. The α-hexathiophene yield was highest in the anionic micellar system.