Formation kinetics and spectra of aromatic radicals in solution

Spectra and kinetics of aromatic radicals have been obtained by means of a two-color laser-induced fluorescence experiment. The rates and dissociation mechanism of 1-(chloromethyl)naphthalene and 1-(bromomethyl)-naphthalene following 266-nm photoexcitation have been established experimentally by the direct observation of the rate of formation of the 1-naphthylmethyl radical product. The radicals have been identified from their emission spectra induced by excitation with a 355-nm picosecond pulse. It is shown that the excited state energy dissipation process of the parent species involves a dual path: (1) direct dissociation from the S₂(π,π*) state of the naphthalene into the T(σ*) state of the C-X bond with subsequent fragmentation to a 1-naphthylmethyl radical and a halogen, and (2) internal conversion and vibrational relaxation to the S₁ with subsequent fluorescence and intersystem crossing to the bound triplet manifold.