Time-Resolved ESR Study on Complex Radical Pairs Formed in the Photolysis of Methylene Blue Included in Water Soluble Sulfonated Calixarenes

The photochemical reactions of methylene blue (MB) included in water soluble sulfonated calix[n]arenes (n=4, 6, 8) are studied using a time-resolved ESR method. The CIDEP (chemically induced dynamic electron polarization) spectra show the formation of the complex radical pair composed of the MB monocation radical and calixarene (phenoxyl-type) radical. The lifetime and broadened spectral shape are dependent on the size of the calixarene and are due to the longitudinal and transverse relaxation mainly induced by the tumbling motion of the radical pair with the spin dipole-dipole interaction. The pair dissociates in a few hundreds of nanoseconds in cases of n = 6 and 8. * To whom correspondence should be addressed. Fax: +81-54-238-4753; E-mail