Isomérisation photochimique de l'azobenzène en solution. I

Abstract

The photochemical isomerization of azobenzene can be sensitized by aromatic compounds (chrysene, phenanthrene, naphthalene) or by carbonyls (benzil, biacetyl). Comparison of the photostationary states and the initial quantum yields of isomerization leads to the deduction of a mechanism for this sensitization. The triplet levels Tβt and Tβc of the trans and cis forms are populated by diffusion-controlled, triplet–triplet energy transfer. These levels are deactivated nonradiatively without giving rise to isomerization. The Tβc level of the cis form undergoes a quantitative internal conversion to another triplet level Tαc, while the Tβt level of the trans form undergoes a nonradiative transition to the ground state. The internal conversion Tβt → Tαt in the trans isomer occurs with low probability (0.04 to 0.06). The levels Tαt and Tαc are the intermediates in the isomerization; they are deactivated to the two isomeric forms of azobenzene with equal probability. [Journal translation]