Laser flash photolysis study of the triplet reactivity of beta-lapachones.
The photochemical reactivity of beta-lapachone (1), nor-beta-lapachone (2) and beta-lapachone 3-sulfonic acid (3) has been examined by laser flash photolysis. Excitation (lambda = 266 nm) of degassed solutions of , in acetonitrile or dichloromethane, resulted in the formation of detectable transients with absorption maxima at 300, 380 and 650 nm. These transients, with lifetimes of 5.0 micros, were quenched by beta-carotene at a diffusion-controlled rate constant and assigned to the triplet excited states of 1-3. Addition of hydrogen donors, such as 2-propanol, 1,4-cyclohexadiene, 4-methoxyphenol or indole led to the formation of new transients, which were assigned to the corresponding ketyl radicals obtained from the hydrogen abstraction reaction by the triplets 1-3 . In the presence of triethylamine it was observed the formation of the long-lived anion radical derived from , which shows absorption maxima at 300 and 380 nm. The low values observed for the hydrogen abstraction rate constants for the beta-lapachones 1-3 using 2-propanol and 1,4-cyclohexadiene as quenchers led us to conclude that their triplet excited states show pi pi* character.
Netto-Ferreira
José Carlos
JC
Departamento de Química-Universidade Federal Rural do Rio de Janeiro, Antiga Rio São Paulo, km 47, Seropédica, 23970-000, RJ, Brazil. jcnetto@ufrrj.br
Bernardes
Bauer
B
Ferreira
Aurelio Baird Buarque
AB
Miranda
Miguel Angel
MA
D007834:Lasers; D008969:Molecular Sequence Data; D009285:Naphthoquinones / Q000737:chemistry*; D010782:Photolysis / Q000528:radiation effects*; D013053:Spectrophotometry
D009285:Naphthoquinones; C014638:beta-lapachone