Synthesis of porphyrins tailored with eight facially-encumbering groups. An approach to solid-state light-harvesting complexes

Article abstract of DOI:10.1016/S0040-4020(01)89413-8

Synthetic models of the photosynthetic antenna complexes must achieve long-range 3-dimensional order encompassing a large number of porphyrinic pigments with limited direct contact of the pigments. In order to develop solid-state antenna complexes, we have synthesized porphyrins bearing benzyloxy groups projecting over both faces and optionally also around the periphery of the porphyrin. Routes have been established for prefunctionalizing benzaldehydes with various benzyloxy groups. Reaction of 2,6-bis, 3,5-bis, or 2,4,6-tris(benzyloxy)benzaldehydes with pyrrole via the room temperature two-step one-flask porphyrin reaction provides direct access to the facially-encumbered porphyrins. The benzyloxybenzaldehydes react as efficiently as methoxybenzaldehydes, indicating the utility of the -OCH₂- unit for introducing large substituents near the face of the porphyrin. The octakis and dodecakis(benzyloxy)porphyrins exhibit characteristic porphyrin absorption and fluorescence properties in solution. The crystal structure of meso-tetrakis[2,6-bis(2,3,4,5,6-pentalfuorobenzyloxy)phenyl]porphyrin has been determined. The pentafluorobenzyloxy substituents provide a cavity on each side of the porphyrin plane which has an approximate cylindrical shape with a diameter of ~7.5 A and a height of ≥4.5 A. The porphyrin core parameters are those obtained for free base derivatives in which the inner hydrogen atoms are ordered. Crystal data: a=14.759 (1) A, b=25.519 (2) A, c=13.100 (1) A, α=100.04 (1), β=99.83 (1), γ=88.25 (1), V=4767.3 (6) A³, all measurements at 127 K, triclinic, space group P₁, Z=2 R₁(F)=0.097, for 10020 'observed' data, and wR₂(F²)=0.275 for 17761 total unique (all) data.