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Competitive Decay Pathways of the Radical Ions Formed by Photoinduced Electron Transfer between Quinones and 4,4′‐Dimethoxydiphenylmethane in Acetonitrile
Author(s) -
Del Giacco Tiziana,
Baciocchi Enrico,
Lanzalunga Osvaldo,
Elisei Fausto
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20010716)7:14<3005::aid-chem3005>3.0.co;2-9
Subject(s) - chemistry , photochemistry , chloranil , flash photolysis , quantum yield , radical , ion , electron transfer , yield (engineering) , acetonitrile , benzoquinone , reaction rate constant , organic chemistry , kinetics , physics , materials science , quantum mechanics , metallurgy , fluorescence
The reactivity of the cation radical of (4‐MeOC 6 H 4 ) 2 CH 2 photosensitized by 1,4‐benzoquinone (BQ), 2,5‐dichloro‐1,4‐benzoquinone (Cl 2 BQ), and tetrachloro‐1,4‐benzoquinone (chloranil, CA) was investigated in acetonitrile. The main photoreaction products obtained by steady‐state irradiation were identified to be: (4‐MeOC 6 H 4 ) 2 CHOC 6 H 4 OH, sensitized by BQ; (4‐MeOC 6 H 4 ) 2 CHCl, sensitized by Cl 2 BQ; (4‐MeOC 6 H 4 ) 2 CHOH, sensitized by CA. The mechanism of their formation was investigated by nanosecond laser flash photolysis that allowed transient species (radical ions, neutral radicals, and ions) to be detected and characterized in terms of absorption spectra, formation quantum yields, and decay rate constants. For all systems, the interaction between the triplet quinone (Q) and (4‐MeOC 6 H 4 ) 2 CH 2 produced the corresponding radical ions (quantum yield ϕ ≥0.72) which mainly decay by back electron transfer processes. Less efficient reaction routes for the radical ions Q .− and (4‐MeOC 6 H 4 ) 2 CH 2 .+ were also: i) the proton‐transfer process with the formation of the radical (4‐MeOC 6 H 4 ) 2 CH . by use of Cl 2 BQ; ii) the hydrogen‐transfer process with the formation of the cation (4‐MeOC 6 H 4 ) 2 CH + in the case of CA. Instead, BQ sensitized a much higher yield of BQH . and (4‐MeOC 6 H 4 ) 2 CH . , mainly by the direct interaction of triplet BQ with (4‐MeOC 6 H 4 ) 2 CH 2 . It was also shown that the presence of salts decreases significantly the rate of the back electron transfer process and enhances the quantum yields of formation of the neutral radicals and ions when Cl 2 BQ and CA are used, respectively. The behavior of BQ .− , Cl 2 BQ .− , and CA .− appears to be mainly determined by the Mulliken charges on the oxygen atom obtained from quantum mechanical calculations with the model B3LYP/6‐311G(d,p). Spin densities seem to be much less important.