Photoisomerism of Azobenzenes

Abstract

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Thermal cis-to-trans isomerisation of triazene dyes in doped polymer films

“…Nevertheless, one may consider, similarly to the case of azobenzenes [6], the following two (limiting) mechanisms for thermal cis-to-trans isomerisation of DPT: a rotation mechanism (as illustrated in Scheme 2 for 1-phenyltriazene derivatives) or an inversion mechanism (via a transition state involving a linear sp hybridized nitrogen). Owing to the contribution of a 1,3-dipolar resonance form to the structure of the triazeno moiety (eN 1 ] N 2 eN 3 < 4 eN À eN]N þ <), which decreases the N 1 ]N 2 bond after lamp irradiation.…”

“…In this context, previous studies by the present authors have focussed on the mechanism of thermal relaxation of photochemically generated cis isomeric forms of 1,3-diphenyltriazenes and of 1-phenyltriazenes derived from cyclic amines (i.e., trisubstituted triazenes) in homogenous solution. Thermally driven cis-to-trans isomerisation of 1,3-diphenyltriazenes dissolved in aqueous media involves an acid/base-promoted 1,3-prototropic rearrangement (Scheme 1) [3], analogous to acid/base-catalyzed keto-enolisation [4]; thermal cis-to-trans isomerisation of 1-phenyltriazenes derived from cyclic amines, in contrast, proceeds in organic solvents via an internal rotation mechanism (Scheme 2) involving heterolytic rupture of the N]N p-bond and twisting around the CeN]NeN dihedral angle [5], in a manner similar to the isomerisation of donor/ acceptor-substituted ('pushepull') azobenzenes [6].…”