2,7,12,17-Tetra(p-butylphenyl)-3,6,13,16-tetraazaporphycene: The first example of a straightforward synthetic approach to a new class of photosensitizing macrocycles

Selected on the basis of computational studies and synthetic feasibility, the title compound 9c has been obtained by cross-coupling of 4,4′-bis(p-butylphenyl)-2,2′-biimidazole-5,5′- dicarbaldehyde (28c) followed by oxidative aromatization. The introduction of a Suzuki coupling protocol opens the way to 2,7,12,17-tetraryl-substituted 3,6,13,16-tetraazaporphycenes avoiding the development of a de novo synthesis whenever a new peripheral substituent is desired. As predicted by computational studies, oxidation of the non-aromatic precursor 33c to yield the azaporphycene macrocycle 9c is more favourable than in the case of porphycene itself. The absorption spectrum of 9c shows a substantial bathochromic shift relative to porphycene 1a, revealing a synergism between aza substitution in the macrocycle and phenyl substitution at its periphery.