Photoprocesses of chloro-substituted p-benzoquinones.
The photochemistry of chloro-(ClBQ), dichloro-(2,5- and 2,6-Cl 2BQ), and trichloro-1,4-benzoquinone (Cl 3BQ) was studied in aqueous solution and/or in mixtures with acetonitrile. Final products are the corresponding hydroquinones (QH 2s) and 2-hydroxy-1,4-benzoquinones (QOHs). Three transients were detected by UV-vis absorption spectroscopy. The triplet state appears within the 20 ns 248 nm pulse and is converted within 0.1-1 micros into a photohydrate (HI aq). HI aq which is spectroscopically and kinetically separated from the triplet state decays within 5 ms, whereas the anion of the hydroxyquinone (QO (-)) grows in at ca. 500 nm in the 0.1-1 s time range. The proton formation and decay kinetics within 0.1-10 micros were observed by transient conductivity in the course of the reaction of the triplet state with water en route to HI aq at pH 4-9. Formation of QO (-) results in a permanent conductance. The efficient photoconversion of Cl n BQs at low concentrations (<0.2 mM) into QH 2s and HOQs is due to a redox reaction of Q with rearranged HI aq. The quantum yield of photoconversion at lambda irr = 254 nm is 0.8-1.2 for ClBQ or Cl 2BQs in aqueous acetonitrile and smaller (0.4) for Cl 3BQ. The yield of semiquinone radical ( (*)QH/Q (*-)) of Cl n BQs ( n = 1-4) in acetonitrile-water (1:1) is low (<20%) at low substrate concentration but is significantly increased upon addition of an H-atom donor, for example, 2-propanol. Other mechanisms involving (*)QH/Q (*-) radicals, such as quenching of the triplet state at enhanced Cl n BQ concentrations and H-atom abstraction from an organic solvent in mixtures with water, have also to be considered.
Görner
Helmut
H
Max-Planck-Institut für Bioanorganische Chemie, D-45413 Mülheim an der Ruhr, Germany. goerner@mpi-muelheim-mpg.de
von Sonntag
Clemens
C
D000097:Acetonitriles / Q000737:chemistry; D016227:Benzoquinones / Q000138:chemical synthesis / Q000737:chemistry* / Q000528:radiation effects*; D002703:Chloranil / Q000031:analogs & derivatives / Q000737:chemistry / Q000528:radiation effects; D004553:Electric Conductivity; D005609:Free Radicals / Q000737:chemistry; D006843:Hydrocarbons, Chlorinated / Q000737:chemistry* / Q000528:radiation effects*; D006873:Hydroquinones / Q000138:chemical synthesis / Q000737:chemistry / Q000528:radiation effects; D015394:Molecular Structure; D010777:Photochemistry; D010782:Photolysis; D011789:Quantum Theory; D012996:Solutions / Q000737:chemistry; D012997:Solvents / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D014466:Ultraviolet Rays; D014867:Water / Q000737:chemistry
D000097:Acetonitriles; D016227:Benzoquinones; D005609:Free Radicals; D006843:Hydrocarbons, Chlorinated; D006873:Hydroquinones; D012996:Solutions; D012997:Solvents; D002703:Chloranil; D014867:Water; C029582:trichloro-4-benzoquinone; C054093:2-hydroxy-1,4-benzoquinone; C032159:acetonitrile