Time-resolved Electron Paramagnetic Resonance spectra (TREPR) of radical pairs created photochemically in micelles

PAULA CAREGNATO; MALCOLM D.E. FORBES

CEPEMA-USP, Cubatao, Sao Paulo, Brasil.

Congreso; IX Encuentro Latino-Americano de Fotoquimica y Fotobiologia (IX ELAFOT); 2008

CEPEMA- Universidad de Sao Paulo

Quinone compounds are known to play an important role in photobiological systems. In particular, 9,10- anthraquinone and its derivatives have been investigated to reveal the photochemical properties related to their excited-state nature in solution. The molecules with their lowest triplet state of n,ð* electronic character undergo intermolecular H-atom abstraction and electron transfer. Micelle surfaces play an important role in the study of microheterogeneous reaction system. It is because the solute molecules could be in the surface or inside the micelles, depending on molecular size, shape, and polarity. TREPR has been used to investigate the diffusional dynamics of radical pairs created by UV eximer laser radiation (excitation wavelength 248 y 308 nm) of ionic and neutral quinines in micelles. In the TREPR (CW) technique, the microwave excitation of the electron spin levels is continuous during the experiment (X band, 9.5GHz frequency), and the resolution is achieved by pulsing the production of radicals with a laser and gating the detection system. In a TREPR study several mechanism of chemically induced dynamic electron spin polarization (CIDEP) can be observed: the triplet mechanism (TM), the radical pair mechanism (RPM) and the spin-correlated radical pair mechanism (SCRP). In the SCRP spectrum each individual hyperfine line of the free radicals is split into two components of opposite phase: emission (E) or absorption (A). This behavior is characteristic of micellized radical pairs. Laser flash photolyzed aqueous solution of surfactants molecules containing anthraquinone involves a simple hydrogen abstraction by the triplet state of anthraquinone from the micelle-forming molecules. Micellar solubilization is convenient for restricting free radical diffusion and increasing the probability of radical encounters. In the present work we studied the CIDEP mechanism of the radical pairs produced during the photoreduction of anthraquinone and mono and di-sulfonated anthraquinone in a micellar solution. We used anionic and cationic micelles with different side chain size.