Green self-assembling porphyrins and chlorins as mimics of the natural bacteriochlorophylls c, d, and e

Novel porphyrins and chlorins that self-assemble in nonpolar solvents in a manner similar to that of the bacteriochlorophylls c, d, and e have been synthesized by a common protective group approach. The supramolecular assemblies have broad and red-shifted absorption spectra in comparison to those of the monomeric building blocks. The presence of a carbonyl group in conjugation with the tetrapyrrolic macrocycle produces green colors both in the free bases and in their zinc complexes, which, after self-assembly, are thus perfect artificial mimics of the chlorosomal antennas encountered in green photosynthetic bacteria. Enantiopure building blocks produce large helical aggregates with M or P helicity determined by the chirality of the 1-hydroxyethyl substituent. It is demonstrated that the groups essential for self-organization to occur - namely the hydroxy group, the zinc metal atom and the carbonyl group - do not have to be collinear, as has been presumed until now. Surprisingly, the chlorin derivative does not show hyperchromicity relative to similarly substituted porphyrins. This fact allows us to conclude that the more readily available porphyrins may be used for efficient artificial antennas of potential use in solar devices; otherwise it is necessary to increase the number of synthetic steps in order to incorporate into chlorins the annulated cyclopentanone ring, a structural feature carefully optimized by evolution in all (bacterio)chlorophylls.