Time-resolved ESR study on the photochemistry of naphthoquinones included in cyclodextrins

The photochemical reactions of 1,4-naphthoquinones included in alpha-, beta - and gamma -cyclodextrins (CDs) are studied using a time-resolved ESR meth od. The CIDEP (chemically induced dynamic electron polarization) spectra ob served in the case of 1,4-naphthoquinone (NQ) in alpha- and gamma -CDs show the main formation of the NQ anion radical, contrary to the case of beta - CD that shows the naphthosemiquinone neutral radical. The carbon-centered r adicals of the CDs are also identified in all the cases. The spin polarizat ion patterns of these species prove that the reaction takes place via the e xcited triplet state of NQ, and the NQ neutral and anion radicals are eject ed from the CD cavity. The initial reaction of these systems is the hydroge n abstraction from the CD framework and some NQ neutral radicals lose the p roton to form the anion radical. In the case of 2-methyl-1,4-naphthoquinone in CDs, similar results are obtained. As for beta -CD both NQ and MNQ syst ems show the appearance of a broad emissive spectrum immediately after phot olysis. This is tentatively assigned to a strongly coupled radical pair for med inside of the beta -CD cavity. Accordingly, the initial photochemical r eaction takes place inside of the CD cavities and the NQ and MNQ radicals a re ejected to the aqueous phase.