A versatile, divergent route for the synthesis of ABAC tetraazaporphyrins: molecularly engineered, push–pull phthalocyanine-type dyes

Abstract

A method to prepare key synthetic intermediates of ABAB- and ABAC-phthalocyanines and other tetraazaporphyrin analogues of interest for molecular electronics is described. These intermediates consist of ABAB-tetraazaporphyrins wherein the two B-subunits are endowed with iodine substituents at the meta-positions, whereas the substitution of the A-subunits is not conditioned to sterically demanding ortho-substituents, but rather can include a variety of functional groups at meta- or ortho-positions. The synthetic procedure includes the assembly of an ABAB tetraazaporphyrin by crossover cyclotetramerization of 5(6)-nitro-1,1,3-trichloroisoindolenine with a diiminoisoindoline or diiminopyrroline derivative, followed by a three-step conversion of the two peripheral nitro groups into iodine substituents. The effectiveness of the method is based on two principles. On one hand, the inability of the trichloroisoindolenine derivative to self-condense, together with the mild reaction conditions, favors the preferential formation of an ABAB derivative, without conditioning the substitution at the pyrroline or isoindoline A-precursor. On the other hand, the opposite diiodo-tetraazaporphyrin can be easily functionalized through a variety of metal-catalyzed cross-coupling reactions, thus being a valuable synthetic intermediate for ABAB and ABAC tetraazaporphyrins. A molecularly engineered phthalocyanine-like dye, which features the prototypical structure of a donor–π-bridge–acceptor, was prepared using this method.