Self-assembly by hydrogen bonding and π-π interactions in the crystal of a porphyrin - Attempts to mimic bacteriochlorophyll c

Porphyrins bearing undecyl side chains, which confer solubility, and 4-(methoxycarbonyl)phenyl substituents in the meso positions have been prepared. The 4-(methoxycarbonyl) substituent could be transformed into groups tailored for self-assembly. The crystal structure of the dihydroxymethyleneporphyrin, bis[4-(hydroxymethyl)phenyl]porphyrin 7, shows strong hydrogen bonds between the hydroxy group and pyrrole nitrogen atoms in neighboring molecules, resulting in a two-dimensional network of hydrogen-bonded porphyrins. π-π Interactions are also encountered, bringing porphyrins into laddered stacks with edge-to-edge contacts of 3.5 Å. The isomeric diesters Zn-3 and Zn-4 show different tendencies in π-π stacking. Models of bacteriochlorophyll c Zn-8 and Zn-11, possessing a formyl group, a hydroxymethyl group, and a central zinc atom, have been synthesized. In contrast to bacteriochlorophyll c, which self-assembles both in the natural antenna system of green photosynthetic bacteria and in nonpolar solvents, the self-assembly Zn-8 and Zn-11 could not be observed in solution. The results provide information for the design of better mimics of natural light-harvesting arrays formed by self-assembling porphyrins. © Wiley-VCH Verlag GmbH, 2000.