Photoinduced protonation of azo dyes in halogenated solvents and polymers

Abstract

We show that protonation and/or photocyclisation of azo dye molecules, which are often used as chromophores for applications in non-linear optics, can be induced by optical irradiation either in solution or in polymer films. Irradiation causes dissociation of halogenated solvents or polymers, generating an acid which then protonates the dye molecule. In guest–host polymer systems consisting of azo dyes in halogenated polymers this type of protonation is shown also to result from the application of an electric field during the poling required for secondorder applications. In both halogenated and non-halogenated systems a relatively slow bleaching process has been observed and attributed to photoinduced 6π-cyclisation of the azo dye. We investigate the conditions under which protonation or cyclisation of azobenzenes occurs and identify the form of the protonated molecule.