Hydrogen-abstraction reactivity of excited-state para-substituted benzyl radicals in solution at room temperature

Photochemistry of radicals and biradicals

“…A similar cleavage has also been suggested for 1,1,3,3tetraarylpropane-l,3-diyl biradicals generated by addition of diaryl carbenes to diaryl olefins.11 In this case the products of laser-jet irradiation indicate that excitation of the biradical leads to a cleavage reaction that regenerates carbene plus olefin (reaction 19), in contrast to the usual thermal ring closure reaction for the ground-state biradical. Recent laser-jet results for another 1,3-biradical derived from a cyclopropyl-substituted anthrone (reaction 20) suggest that cleavage of excited 1,3-biradicals may be a relatively general phenomenon.10 In this case only cleavage of bond b (reaction 20) to generate diphenylcarbene plus 10-methylene anthrone is observed.…”

“…In the case of the 4-cyanobenzyl radical the 45-nm shift in the absorption spectrum has been attributed to a change in the nature of the lowest excited doublet from IA2 to 2B2, as discussed above. 19 Monitoring the decays of either the transient absorption spectrum or the fluorescence gives results which are identical within experimental error for all radicals which have been examined using both techniques.…”

“…These results indicate that the Di and D2 states are sufficiently separated that nonradiative relaxation via the D2 state does not contribute significantly to the 120-ns excited-state lifetime for this radical at room temperature.21 Anomalous behavior has been observed for the 4-cyanobenzyl radical, both in terms of its absorption spectrum and chemical reactivity, as discussed below; these effects have been attributed to the fact that the 2B2 state is the lowest excited state in this radical. 19 Support for this hypothesis comes from fluorescence polarization studies at 77 K which confirm that the lowest excited state is 1A2 for benzyl and 4-fluorobenzyl but 2B2 for 4-cyanobenzyl radicals. 44 Little information is currently available on the effects of multiple substitution on benzyl radical fluorescence lifetimes.…”

“…In one example, a large substituent effect has been measured for the fluorescence quenching of excited benzyl radicals with 1,4cyclohexadiene in hexane. 19 The rate constants varied from 3.2 X 109 for 4-cyano to 9.4 X 105 M'1 s'1 for 4-methoxy, with intermediate values for 4-chloro and 4-fluoro. For the 4-cyano radical transient absorption measurements showed the formation of 1,4-cyclohexadienyl radical, demonstrating that at least some of the fluorescence quenching leads to hydrogen atom abstraction (reaction 27) for this particular radical.…”

Characterization of the excited-state reactivity of a persistent aryl-substituted allyl free radical

“…The origin of this effect is not clearly understood, but cyano substitution on the benzyl radical also produces hypsochromic shifts. 13 This effect has been explained as arising from an inversion of the energies of the two lowest lying excited states. Overall, the 5-30-nm absorption shifts for lb-d (relative to la) are comparable to shifts observed upon substitution of diphenylm-ethyl14•16 and diphenylketyl radicals.6•8 Semiempirical molecular orbital calculations (AMI) produce a minimized energy for la when the C2-phenyl is rotated 59.1°from the plane of the central allyl core (dihedral angle of Ci-Cr-Cie-Cn).…”