Green Self‐Assembling Porphyrins and Chlorins as Mimics of the Natural Bacteriochlorophylls c, d, and e

Abstract

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-or- IntroductionThe ubiquitous green color of light-harvesting apparata is encountered in (bacterio)chlorophyll-based photosynthetic organisms. In conjunction with other pigments, such as carotenoids, the light absorption characteristics, which include broad wavelength ranges and high extinction coefficients, have been optimized during evolution in order to ensure conversion of light into biochemical energy by adaptation to different habitats. Thus, bacteria that live under the water surface at depths of over 50 m have evolved antenna systems different to those of bacteria or algae living at the surface, and these differ in turn from terrestrial plants in their light-harvesting systems. While the last, more highly evolved, of these species have developed protein complexes to bind chromophores, in the early green photosynthetic bacteria, due to the pressures of synthetic and genetic economy, self-assembly of bacteriochlorophylls c, d, and e (Figure 1) is used. [1,2] This much simpler architectural construct, which is fully functional, is worth mimicking [a] Forschungszentrum Karlsruhe,

Self-aggregation of Synthetic Protobacteriochlorophyll-d Derivatives¶

“…As expected, Zn-1S selfaggregated to form mirror-imaged (anticlockwise-twisted and helical) supramolecules of Zn-1R self-aggregates. (Very recently, Balaban et al have reported similar observation of CD spectra in self-aggregates of chiral porphyrins [33].) Generation of the CD activity by self-aggregation of the 3 1 -chiral compounds is supported by the results that racemic mixture Zn-1 gave no CD peaks in the self-aggregates and by the reports that both Zn-4R/S self-aggregates gave giant CD peaks (13,27).…”

Interaction Between Terminal Zinc–Porphyrins in a U‐Shaped Trimeric Porphyrin Array Bearing Chiral Amino Acid Tails

“…Indeed, the interaction between zinc-porphyrins are well known and there have been increasing examples of the interacting zinc-porphyrins. [25] In connection with this, we have recently reported that a pair of zincporphyrins bound on a cyclic polypeptide showed more distinct interactions compared with a pair of free-base porphyrins on the same template.…”

Red‐Emitting Polymer Microspheres Containing Zinc Porphyrins

“…[30] ZnPor units were sufficiently close and in favorable orientations to form porphyrin assemblies, [16][17][18][35][36][37] similar to those formed in micelles. [16][17][18] Consequently, ZnPor units in polymers displayed a broad Soret band, a new absorption band at 622 nm (labeled as A3 in Figure 2a) and a new emission band at 627 nm (labeled as E3 in Figure 2b) compared with ZnAOTPP monomer.…”