Long-lived quinone-naphthalene triplet excited complexes: direct measure of rates of proton transfer within intimate radical ion pairs

The quenching of the triplet state of chloranil (tetrachloro-p-benzoquinone) by derivatives of naphthalene in benzene results in the formation of relatively long-lived (/tau/ = 100-350 ns) radical ion pair intermediates. For the electron donor naphthalenes having aliphatic side chain substitution, intrapair proton transfer can be directly monitored by laser flash photolysis. Measurement of radical ion pair (triplet excited complex) decay times and the quantum yields of resultant semiquinone (OH*) radicals provide rate constants for decay via intersystem crossing (k/sub isc/) and intra-ion-pair proton transfer (k/sub H/). The data reveal relative kinetic acidities of quinone-complexed radical cations and include activation parameters and isotope effects associated with the proton-transfer step.