Self-assembly by hydrogen bonding and pi-pi interactions in the crystal ofa porphyrin - Attempts to mimic bacteriochlorophyll c

Porphyrins bearing undecyl side chains, which confer solubility, and 4-(met hoxycarbonyl)phenyl substituents in the meso positions have been prepared. The 4-(methoxycarbonyl) substituent could be transformed into groups tailor ed for self-assembly. The crystal structure of the dihydroxymethyleneporphy rin, bis[4-(hydroxymethyl)phenyl]porphyrin 7, shows strong hydrogen bonds b etween the hydroxy group and pyrrole nitrogen atoms in neighboring molecule s, resulting in a two-dimensional network of hydrogen-bonded porphyrins. pi -pi Interactions are also encountered, bringing porphyrins into laddered st acks with edge-to-edge contacts of 3.5 Angstrom. The isomeric diesters Zn-3 and Zn-4 show different tendencies in pi-pi stacking. Models of bacterioch lorophyll c Zn-8 and Zn-11, possessing a formyl group, a hydroxymethyl grou p, and a central zinc atom, have been synthesized. In contrast to bacterioc hlorophyll c, which self-assembles both in the natural antenna system of gr een photosynthetic bacteria and in nonpolar solvents, the self-assembly Zn- 8 and Zn-11 could not be observed in solution. The results provide informat ion for the design of better mimics of natural light-harvesting arrays form ed by self-assembling porphyrins.