Synthesis of β-substituted porphyrin building blocks and conversion to diphenylethyne-linked porphyrin dimers

A recent hypothesis concerning effects of orbital ordering on electronic communication (excited-state energy transfer, ground-state hole-hopping) in covalently linked porphyrin arrays prompted the development and application of methodology for the synthesis of beta-linked porphyrin dimers. Reaction of a beta-substituted pyrrole with 2-hydroxymethylpyrrole led to the dipyrromethane bearing a single beta-substituent and no meso- nor alpha-substituents. Condensation of the beta-substituted dipyrromethane with an aldehyde and a meso-substituted dipyrromethane gave the desired beta-substituted porphyrin building block, albeit in low yield. Four building blocks were prepared with a p-iodophenyl or p-ethynylphenyl group at one beta-position, no substituent at the flanking meso-position, and mesityl or pentafluorophenyl groups at the three non-flanking meso-positions. The porphyrin building blocks were coupled via Pd-mediated reactions, affording diphenylethyne-linked dimers with the linker attached at beta-positions. This approach provided access to zinc-free base porphyrin dimers and bis-zinc dimers bearing mesityl or pentafluorophenyl groups at the three non-linking meso-positions. The availability of these dimers and monomeric benchmarks enabled a critical test of the orbital ordering hypothesis. This methodology for preparing porphyrin building blocks bearing a lone, non-hindered beta-substituent complements existing methods for preparing meso-substituted porphyrin building blocks. The ability to position the linker at the meso- or beta-positions provides a desirable level of versatility for incorporating porphyrinic molecules with an a(2u) or a(1u) HOMO in various molecular devices. (C) 1999 Elsevier Science Ltd. All rights reserved.