Synthesis of 7,8-dehydropurpurin dimers and their conversion into conformationally constrained β-to-β vinylene-bridged porphyrin dimers

7,8-Dehydropurpurin has attracted much attention owing to the dual 18π- and 20π-electron circuits in its macrocyclic conjugation. The two-fold Pd-catalyzed [3+2] annulation of meso-bromoporphyrin with 1,4-diphenylbutadiyne furnished 7,8-dehydropurpurin dimers. The 8^a,8^a-linked dimer displays a red-shifted and enhanced absorption band in the NIR region and a small electrochemical HOMO-LUMO band gap as a consequence of efficient conjugation between the two coplanar 7,8-dehydropurpurin units. Treatment of this dimer with N-bromosuccinimide in chloroform and ethanol gave β-to-β vinylene-bridged porphyrin dimers. Owing to the highly constrained conformations, these dimers exhibit perturbed absorption spectra, small Stokes shifts, and high fluorescence quantum yields. Bridging the networks: 7,8-Dehydropurpurin dimers were synthesized by twofold Pd-catalyzed [3+2] annulation of meso-bromoporphyrin with 1,4-diphenylbutadiyne. Treatment of this dimer with N-bromosuccinimide (NBS) in CHCl₃ and ethanol gave the title porphyrin dimers, which exhibited well-conjugated electronic networks and constrained conformations.