The near-infrared (NIR) spectral region has been comparatively under-utilized for diverse materials and medical applications owing to the lack of chromophores that afford stability, solubility, synthetic malleability, and tunable photophysical features. Bacteriochlorins are attractive candidates in this regard; however, preparation via modification of naturally occurring bacteriochlorophylls or reduction of porphyrins or chlorins has proved cumbersome. To overcome such limitations, a dibromobacteriochlorin (BC-Br 3 Br 13 ) was prepared de novo by the acid-catalyzed condensation of an 8-bromodihydrodipyrrin-acetal. BC-Br 3 Br 13 bears (1) a geminal dimethyl group in each reduced ring to block adventitious dehydrogenation, and (2) bromo groups at the 3-and 13-positions for further chemical modifications. BC-Br 3 Br 13 was subjected to four types of Pd-mediated coupling reaction (Suzuki, Stille, Sonogashira, dehalogenation) to give bacteriochlorins bearing substituents at the 3-and 13-positions (phenyl, vinyl, acetyl, phenylethynyl), and a benchmark bacteriochlorin lacking such substituents. The 3,13divinylbacteriochlorin was transformed to the 3,13-diformylbacteriochlorin. Depending on the substituents at the 3-and 13-positions, the position of the long-wavelength absorption maximum (Q y (0,0) band) lies between 713 and 771 nm, the fluorescence emission maximum lies between 717 and 777 nm, and the fluorescence quantum yield ranges from 0.15 to 0.070. The ability to introduce a wide variety of functional groups via Pd-mediated coupling reactions and the tunable absorption and emission spectral properties suggest that synthetic bacteriochlorins are viable candidates for a wide variety of photochemical applications.

We report two crystal structures of a synthetic porphyrin molecule which was programmed for self-assembly. The same groups which ensure that bacteriochlorophylls c, d, and e can self-assemble into the chlorosomal nanorods, the photosynthetic antenna system of some green bacteria, have been engineered into desired positions of the tetrapyrrolic macrocycle. In the case of the 5,15-meso-substituted anchoring groups, depending upon the concentration, by using the same crystallization solvents, either a tetragonal or a layered structure of porphyrin stacks were encountered. Surprisingly, pi-pi interactions combined with extensive dispersive interactions, which also encompass cyclohexane, one of the crystallization solvents, win over putative hydrogen bonding. We are aware that our compounds differ considerably from the natural bacteriochlorophylls, but based upon our findings, we now question the hydrogen-bonding network, previously proposed to organize stacks of bacteriochlorophylls. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS) on various isomeric compounds support our challenge of current models for the chlorosomal antenna as these show structures, astonishingly similar to those of chlorosomes.

ZuschriftenMit dem Ziel, die Selbstorganisation der natürlichen Bacteriochlorophylle c, d und e nachzuahmen, wurden verschiedene neue Porphyrine synthetisiert. Diese künstlichen Antennensysteme sollten Sonnenlicht sogar bei geringen Strahlungsintensitäten, angedeutet durch die Sonnenfinsternis im Bildhintergrund, absorbieren können. Darüber berichten T. S. Balaban et al. auf den folgenden Seiten.

The synthesis of porphyrin precursors requires the successive introduction of substituents at the pyrrole alpha- and alpha'-positions (2- and 5-, respectively). An alpha-pyrrole substituent that serves as a temporary masking agent and is not deactivating would greatly facilitate such syntheses, particularly for beta-(3,4)-unsubstituted pyrroles, but has heretofore not been available. A series of alpha-RS groups (R = Me, Et, n-decyl, Ph) have been investigated in this regard, including the determination of the kinetics of substitution at the pyrrolic 3-, 4-, and 5-positions and the application to dipyrromethane formation. The RS group was readily introduced into the pyrrole alpha-position by the reaction of 2-thiocyanatopyrrole (prepared from pyrrole, ammonium thiocyanate, and iodine) and the corresponding Grignard reagent RMgBr. Each 2-alkylthio group activated the pyrrole ring toward deuteration at the 3- or 5- (vs 4-) position. The dipyrromethane synthesis was carried out using a 2:1 ratio of 2-(RS)pyrrole/benzaldehyde with a catalytic amount of InCl3 at room temperature in the absence of any solvent. The alpha-RS group was removed by hydrodesulfurization using Raney nickel or nickel complexes. This stoichiometric synthesis using the alpha-RS-protected pyrrole is in contrast to the traditional synthesis that employs an aldehyde and 25-100 mol equiv of pyrrole. Six meso-substituted dipyrromethanes were prepared by the reaction of 2-(n-decylthio)pyrrole/aldehyde/InCl3 (2.2:1:0.2 ratio) followed by hydrodesulfurization. Other reactions of the 1,9-bis(RS)dipyrromethane include oxidation to give (i) the 1,9-bis(RS)dipyrrin or (ii) the 1,9-bis(RSO2)dipyrromethane, which underwent subsequent complexation with dibutyltin dichloride. In summary, under mild reaction conditions, the 2-alkylthio group is readily introduced to the pyrrole nucleus, directs electrophilic substitution to the 5-position, and is readily removed as required for elaboration of porphyrinic precursors.

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,