Kinetics of Photobleaching of β-Carotene in Chloroform and Formation of Transient Carotenoid Species Absorbing in the Near Infrared

Abstract

Upon laser flash photolysis of β–carotene in chloroform instantaneous bleaching of β–carotene and concomitant formation of near infrared absorbing species are observed. One species, absorbing with maximum at 920 nm, is formed during the laser pulse (10 ns) and is practically gone in one millisecond, the decay showing a bi-exponential behaviour. The second species, absorbing with maximum at 1000 nm, is formed from the species absorbing at 920 nm by first order kinetics with a rate constant of 4.9·10⁴ s⁻¹ at 20°C. This second species decays by second order kinetics and is gone within a few milliseconds. An additional slow bleaching of β–carotene and formation of the species absorbing at 920 nm is observed. This slow bleaching/formation of transient absorption is probably due to processes involving free radicals generated during the instantaneous bleaching. The species absorbing at 920 nm is suggested to be either (i) a free radical adduct formed from β–carotene and chloroform or (ii) β–carotene after abstraction of a hydrogen atom. The species absorbing at 1000 nm is most likely the radical cation. Formation and decay of the near infrared absorbing species and bleaching of β–carotene are independent of whether oxygen is present or absent in the solutions.

Key Words:

• β-carotene
• β-carotene radicals
• laser flash photolysis
• transient absorption