TY - JOUR
T1 - Photodissociation of Haloaromatics
T2 - Detection, Kinetics, and Mechanism of Arylmethyl Radical Formation
AU - Hilinski, E. F.
AU - Milton, S. V.
AU - Rentzepis, P. M.
AU - Huppert, D.
AU - Kelley, D. F.
PY - 1984
Y1 - 1984
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0021754838&partnerID=8YFLogxK
U2 - 10.1021/ja00319a009
DO - 10.1021/ja00319a009
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AN - SCOPUS:0021754838
SN - 0002-7863
VL - 106
SP - 1951
EP - 1957
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 7
ER -