Photochemistry of quinoxaline derivatives and mechanism of the triplet state quenching by electron-poor alkenes

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 × 109 M−1 s−1) to CrN (1.70 × 105 M−1 s−1) with DMQ and from FN (1.77 × 109 M−1 s−1) to CrN (1.20 × 105 M−1 s−1) 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 ΔGct for different alkenes are linear and yield slopes −2.75 eV−1 for DMQ, −2.69 eV−1 for TMQ and −2.99 eV−1 for DMN in the endergonic region (ΔGct > 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 σ constant.