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

Abstract

The photophysics and photochemistry of R-terthiophene (RT) in one nonionic, three cationic, and one anionic micellar systems have been investigated. If the micelles are large enough, RT 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 RT triplet is dependent on the number of RT'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 RT 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 RT molecule can penetrate the hydrophilic shell between the hydrophobic core of the micelle and the aqueous bulk. The yield of formation of the RT 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 R-hexathiophene occurred in all micellar systems, proving that the radical can leave its host micelle and migrate via the bulk. The R-hexathiophene yield was highest in the anionic micellar system.

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

“…The steady-state emission and excitation spectra were recorded in the front-face mode on a SPEX Fluorolog 1680 combined with a SPEX Spectroscopy Laboratory Coordinator DM1B, and the absorption spectra recorded on a Perkin Elmer l-6 spectrophotometer. The spectra were almost identical to those reported earlier, [6,7] and are not shown here. The transient absorption spectrometer has been described elsewhere.…”

“…The total concentration of aT was kept constant at 40 mm in all experiments. In an earlier contribution, [7] it was shown that aT obeys a Poissonian distribution in SDS micelles at higher micelle concentrations. At lower SDS concentrations, on the other hand, the distribution is influenced by the size limitations of the SDS micelle, which causes dissolution problems for higher occupancies.…”

“…The measurements performed at 16 mm total surfactant concentration were, for that reason, performed to explore the influence of the mixed micelle composition on the capacity of aT dissolution. At the three higher surfactant concentrations, where the aT distribution over the micelles is clearly Poissonian, [7] more than 90 % of the aT molecules will be present as single guests in their host micelles, see Table 2. This distribution yields a situation in which the observed photophysics and photochemistry will emanate from locally excited aT molecules only.…”

“…[3e, 5] We have reported studies of the photophysics of aT in micellar environments. [6,7] The photophysics and photochemistry of aT in one nonionic, three cationic, and one anionic micellar system were investigated. These experiments showed that if the micellar volume is large enough, aT follows a Poissonian distribution amongst the micelles.…”

“…While studies of triplet-state phenomena are often carried out under oxygen-free conditions, the presence of oxygen in our systems is an advantage as it eliminates the possibility of spectral interference from delayed fluorescence due to triplet-triplet annihilation. [7] Further, the quantum yield for formation of the terthienyl radical cation via oxygen quenching of the triplet is known to be very small (< 1%), and the radical cation is unreactive to oxygen. [3a,b] In the event that some radical cations may be generated though oxygen quenching of the triplet, their behavior relative to changes in micellar surface charge will be the same as that of radical cations generated biphotonical- 16 [10] [a] < a > is the micellar aggregation number and cmc is the critical micellar concentration.…”