Electron paramagnetic resonance and proton matrix electron nuclear double resonance studies of N,N,N′,N′-tetramethylbenzidine photoionization in sodium dodecyl sulfate micelles: structural effects of added alcohols

Abstract

An electron nuclear double resonance and electron paramagnetic resonance study of photogenerated N,N,N′,N′-tetramethylbenzidine (TMB) cation in frozen suspensions of sodium dodecyl sulfate micelles containing various concentrations of alcohols has been undertaken. The alcohols used in the study were ethanol, propan-2-ol, butan-1-ol and octan-1-ol. Experiments were performed with both the deuteriated and protonated analogues of these molecules. The photoyields were determined from double integration of the first-derivative electron paramagnetic resonance spectra. The location of the TMB cation relative to the micelle interface was determined from the proton matrix electron nuclear double resonance (H-ENDOR) linewidth at 140 K. In general, the TMB radical cation was observed to have a broader H-ENDOR linewidth for samples in which the protonated alcohol was used. This difference increased when longer-chained alcohols were used. The results demonstrate that the shorter chain length alcohols are located at the micellar interface, whereas both butan-1-ol and octan-1-ol intercalate deeper into the hydrocarbon interior of the micelle. The photoyield grew with alcohol addition up to 50 mmol dm^–3, after which the yield decreased. The maximum yield was obtained in samples with short-chain alcohols. These results showed that the yield depended on the degree of disruption of the micelle/water interface. Although hydration of the interface stabilizes the photogenerated cation thus increasing the yield, this effect competes with the disruption of the interfacial region and concomitant decrease of the negative surface charge. The lower negative surface charge helps increase back electron transfer. The results from this ENDOR study are consistent with recent work involving electron spin echo modulation spectroscopy, which showed greater water penetration into the micellar interface upon addition of alcohol.