The photoinduced [4+2] cycloadditions of anthracene to maleic anhydride and
various maleimides yield the Diels-Alder adducts in high (chemical) yields
. Analysis of the efficiency of these photoreactions as a function of the d
ienophile concentration leads to limiting quantum efficiencies of Phi=0.014
and Phi=0.16 for maleic anhydride and N-(pentafluorophenyl)maleimide, resp
ectively. Picosecond time-resolved spectroscopy reveals the one-electron tr
ansfer from excited (singlet) anthracene to the dienophile acceptor (result
ing in the formation of anthracene cation radical and dienophile anion radi
cal) as the critical step prior to cycloaddition. Competition between coupl
ing, back-electron transfer and dissociation of the ion-radical pair which
depends on the solvent polarity and the presence of added (inert) salt, lim
its the quantum yields of adduct formation. (C) 1999 Elsevier Science S.A.
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