Photo-oxidation of diglycine in confined media Relaxation of longitudinal magnetization in spin correlated radical pairs

Electrostatic control of spin exchange between mobile spin-correlated radical pairs created in micellar solutions

A series of photoinduced H-atom abstraction reactions between anthraquinone-2,6,-disulfonate, disodium salt (AQDS) and differently charged micellar substrates is presented. After a 248 nm excimer laser flash, the first excited triplet state of AQDS is rapidly formed and then quenched by abstraction of a hydrogen atom from the alkyl chain of the micelle surfactant, leading to a spin-correlated radical pair (SCRP). The SCRP is detected 500 ns after the laser flash using time-resolved (direct detection) electron paramagnetic resonance (TREPR) spectroscopy at X-band (9.5 GHz). By changing the charge on the surfactant headgroup from negative (sodium dodecyl sulfate, SDS) to positive (dodecyltrimethylammonium chloride, DTAC), TREPR spectra with different degrees of antiphase structure (APS) in their line shape were observed. The first derivative-like APS line shape is the signature of an SCRP experiencing an electron spin exchange interaction between the radical centers, which was clearly observable in DTAC micelles and absent in SDS micellar solutions. Solutions with surfactant concentrations well below the critical micelle concentration (cmc) or solutions where micellar formation had been disrupted (1:1 v/v CH(3)CN/H(2)O) also showed no APS line shapes in their TREPR spectra. These results support the conclusion that electrostatic forces between the sensitizer (AQDS) charge and the substrate (surfactant) headgroup charge are responsible for the observed effects. The results represent a new example of electrostatic control of a spin exchange interaction in mobile radical pairs.

Photoredox chemistry of AOT: electron transfer and hydrogen abstraction in microemulsions involving the surfactant

Time-resolved magnetic resonance experiments (TREPR and CIDNP) are used to investigate previously unobserved redox chemistry of the surfactant dioctyl sulfosuccinate ester (AOT) using the photoexcited triplet state of anthraquinone 2,6-disulfonate (3AQDS*). Several different free radicals resulting from two independent oxidation pathways (electron transfer and hydrogen abstraction) are observed. These include the radical ions of AQDS and sulfite from electron-transfer processes, carbon-centered radicals from H-atom abstraction reactions, and an additional carbon-centered radical formed by electron transfer from the AOT sulfonate head group followed by the loss of SO3. The radicals exhibit intense chemically induced dynamic electron spin polarization (CIDEP) in their TREPR spectra. The intensity ratios of the observed TREPR signals for each radical depend on the water pool size and temperature, which in turn affect the predominant CIDEP mechanism. All signal carriers are accounted for by simulation, and CIDNP results provide strong supporting evidence for the assignments.