Time Resolved ESR Study on the Photochemical Reactions of Pyrene and Nitroxide Radical System in Micelle; Formation of Spin-Correlated Pyrene Cation–Nitroxide Pairs

Time-resolved ESR (TR-ESR) spectra were measured in the laser excitation of aromatic compounds such as xanthone, bromopyrene, fluoranthene (Flt) and pyrene (Py) in sodium dodecyl sulfate (SDS) aqueous micellar system in the presence of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). Net emission (Em) signals of triplet hyperfine (hf) lines of TEMPO were observed in xanthone– and bromopyrene–TEMPO systems. The chemically induced dynamic electron polarization (CIDEP) was explained by radical-triplet pair mechanism. Antiphase signals of TEMPO with Em in low-field side and enhanced absorption in high-field side (EA) of each triplet hf line were superimposed on net Em signals in Flt– and Py–TEMPO systems. The square laser power dependence of the EA signal suggests that the two-photon ionization of Flt and Py is related to the EA antiphase signal. Rate constants of the reactions occurring in SDS solution were determined by analysis of transient absorption decays in the Py–TEMPO system. The reaction rates of Py+–TEMPO and 3Py*–TEMPO in SDS solution were determined as 1.2 × 106 s−1 and 3.0 × 105 s−1, respectively, at the TEMPO concentration of 2 mM (M = mol dm−3). According to these measurements, it was concluded that the EA CIDEP is due to spin-correlated radical pair (SCRP) of TEMPO and Py+ which was formed in the electron transfer reaction from TEMPO to Py+. Simulation of TR-ESR spectra was carried out using SCRP theory. From kinetic analysis of intermediates together with the simulation of TR-ESR spectra, it was concluded that both SCRP of Py+–TEMPO and RTPM of 3Py*–TEMPO contribute to the CIDEP of TEMPO.