Photodissociation of Haloaromatics: Detection, Kinetics, and Mechanism of Arylmethyl Radical Formation

E. F. Hilinski, S. V. Milton, P. M. Rentzepis, D. Huppert, D. F. Kelley

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The photodissociation of 1- and 2-(halomethyl)naphthalenes in room-temperature hexane solution was studied by means of picosecond emission and absorption spectroscopy. Excitation of the (halomethyl)naphthalenes at 266 or 299 nm into the S2(π,π*) state caused the molecules to undergo carbon-halogen bond homolysis. Emission from the 1- and 2-naphthylmethyl radicals was observed when a 355-nm, 25-ps pulse was directed into the sample at times ranging from 0 ps to 40 ns after the arrival of the first photolysis pulse. A broad absorption in the spectral region between 325 and 395 nm was observed for the 1-naphthylmethyl radical with a maximum near 365 nm and for the 2-naphthylmethyl radical with a maximum near 380 nm which appeared within the laser pulse width and persisted for times longer than 500 ps after excitation. A mechanism incorporating the experimental data is proposed for the photodissociation of (halomethyl)naphthalenes which is quite generally applicable to haloaromatic compounds. Excitation of the S2(π,π*) state results in energy dissipation through two channels: (1) internal conversion to S1(π,π*) followed by fluorescence to S0 and (2) intersystem crossing to an upper triplet state which is itself, or crosses to, a dissociative (σ,σ*) triplet state that leads to homolysis of the carbon-halogen bond. The branching ratio of channel 2 to channel 1 was found to be ∼6:1 for 2-(chloromethyl)naphthalene in hexane at room temperature.

Original languageEnglish
Pages (from-to)1951-1957
Number of pages7
JournalJournal of the American Chemical Society
Issue number7
StatePublished - 1984


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