Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
9605513 | Journal of Photochemistry and Photobiology A: Chemistry | 2005 | 8 Pages |
Abstract
The properties of the lowest excited triplet states of quinoxaline derivatives 2,3-dimethylquinoxaline (DMQ) and 2,3,6,7-tetramethylquinoxaline (TMQ) in acetonitrile have been investigated by using time-resolved laser flash photolysis at 266 nm. The transient absorption spectra and the self-quenching rate constants (ksq) of the excited sensitizers have been obtained. The excited triplet states of DMQ and TMQ are efficiently quenched by some electron-poor alkenes, such as fumaronitrile (FN), dimethyl fumarate (DMF), diethyl fumarate (DEF), α-acetoxyacrylonitrile (AAN), acrylonitrile (AN) and crotononitrile (CrN). Obtained kq values decrease significantly from FN (2.50 Ã 109 Mâ1 sâ1) to CrN (1.70 Ã 105 Mâ1 sâ1) with DMQ and from FN (1.77 Ã 109 Mâ1 sâ1) to CrN (1.20 Ã 105 Mâ1 sâ1) with TMQ, which are in accordance with the reduction potential (Ered) and electron cloud density of these electron-poor alkenes. Plots of log kq versus free energy change ÎGct for different alkenes are linear and yield slopes â2.75 eVâ1 for DMQ, â2.69 eVâ1 for TMQ and â2.99 eVâ1 for DMN in the endergonic region (ÎGct > 0). These shallower slopes indicate processes involving only partial charge transfer. The failure to detect the ion radicals also supports that triplet exciplex is formed during the processes. In addition, the electronic effects of ortho-substituents are estimated according to the good linear relation between log kq and Hammett Ï constant.
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Authors
Yang Pan, Zhenyu Sheng, Xiaodong Ye, Zhuo Ao, Gaosheng Chu, Jinghua Dai, Shuqin Yu,