Models for the Photosynthetic Reaction Center—Synthesis and Structure of Porphyrin Dimers with cis‐ and trans‐Ethene and Skewed Hydroxymethylene Bridges

TiCl3-induced coupling of meso-forrnylporphyrins provides cofacial porphyrin dimers such as 1 (M  Cu, Ni) with bridging cis-CHCH units. These compounds show strong interactions between the porphyrin rings; in the case of M  Ni, the dimer can be easily transformed into the corresponding trans isomer. Similar reactions with β-formyl-porphyrins lead to a third type of dimer, a skewed hydroxymethylene-bridged bisporphyrin. These dimeric compounds are models for the photosynthetic reaction center and should facilitate studies on the influence of the dimer geometry on the photophysical properties.