220529-50-0Relevant articles and documents
Mimics of the self-assembling chlorosomal bacteriochlorophylls: Regio- and stereoselective synthesis and stereoanalysis of acyl(1-hydroxyalkyl)porphyrins
Balaban, Teodor Silviu,Bhise, Anil Dnyanoba,Bringmann, Gerhard,Buerck, Jochen,Chappaz-Gillot, Cyril,Eichhoefer, Andreas,Fenske, Dieter,Goetz, Daniel C. G.,Knauer, Michael,Mizoguchi, Tadashi,Moessinger, Dennis,Roesner, Harald,Roussel, Christian,Schraut, Michaela,Tamiaki, Hitoshi,Vanthuyne, Nicolas
body text, p. 14480 - 14492 (2010/03/02)
Diacylation of copper 10,20-bis(3,5-di-tert-butylphenylporphyrin) using Friedel-Crafts conditions at short reaction times, high concentrations of catalyst, and 0-4°C affords only the 3,17-diacyl-substituted porphyrins, out of the 12 possible regioisomers. At longer reaction times and higher temperatures, the 3,13-diacyl compounds are also formed, and the two isomers can be conveniently separated by normal chromatographic techniques. Monoreduction of these diketones affords in good yields the corresponding acyl(1-hydroxyalkyl) porphyrins, which after zinc metalation are mimics of the natural chlorosomal bacteriochlorophyll (BChl) d. Racemate resolution by HPLC on a variety of chiral columns was achieved and further optimized, thus permitting easy access to enantiopure porphyrins. Enantioselective reductions proved to be less effective in this respect, giving moderate yields and only 79% ee in the best case. The absolute configuration of the 31-stereocenter was assigned by independent chemical and spectroscopic methods. Self-assembly of a variety of these zinc BChl d mimics proves that a collinear arrangement of the hydroxyalkyl substituent with the zinc atom and the carbonyl substituent is not a stringent requirement, since both the 3,13 and the 3,17 regioisomers self-assemble readily as the racemates. Interestingly, the separated enantiomers self-assemble less readily, as judged by absorption, fluorescence, and transmission electron microscopy studies. Circular dichroism spectra of the self-assemblies show intense Cotton effects, which are mirror-images for the two 3 1-enantiomers, proving that the supramolecular chirality is dependent on the configuration at the 31-stereocenter. Upon disruption of these self-assemblies with methanol, which competes with zinc ligation, only very weak monomeric Cotton effects are present. The favored heterochiral self-assembly process may also be encountered for the natural BChls. This touches upon the long-standing problem of why both 31-epimers are encountered in BChls in ratios that vary with the illumination and culturing conditions.
Resonance Raman characterization of excitonically coupled meso,meso-linked porphyrin arrays
Bhuiyan, Anwar A.,Seth, Jyoti,Yoshida, Naoya,Osuka, Atsuhiro,Bocian, David F.
, p. 10757 - 10764 (2007/10/03)
Resonance Raman (RR) spectra are reported for dimeric, trimeric, and tetrameric porphyrin arrays in which the porphyrins are directly linked at the meso position. The RR spectra of two monomeric building blocks of the arrays are also examined. The close proximity and orthogonal orientation of adjacent porphyrins in the arrays result in exceptionally strong excitonic interactions along the axis defined by the meso,meso-linkage-(s) and negligible interactions along the orthogonal axis. The coupling scheme breaks the degeneracy of the B excited state and leads to two B(0,0) absorption features. One feature, designated Bx(0,0), is the supermolecule absorption that is strongly red-shifted, due to exciton coupling. The other feature, designated By(0,0), is the superposition of absorptions of the orthogonal, individual porphyrins. This latter absorption occurs at approximately the same energy as the B(0,0) band of a monomeric porphyrin, due to the absence of excitonic interactions. The exciton coupling in the Q state is much weaker than that in the B state, due to the smaller oscillator strength of the former transition. The B-state excitation RR spectra of the meso,meso-linked arrays exhibit a complex and unusual scattering pattern. The most striking features are (1) the appearance of only polarized and anomalously polarized modes in the RR spectrum, (2) the intensity enhancement of anomalously polarized vibrations with B-state excitation, and (3) the large differential enhancement of symmetric versus nontotally symmetric vibrations with excitation. across the B-state absorptions. All of these scattering characteristics are due to the effects of symmetry lowering. The asymmetric meso substitution pattern inherent to the meso,meso-linked arrays contributes to symmetry lowering in both the ground and excited electronic states. The strong uniaxial excitonic interactions make an additional contribution to symmetry lowering in the excited state(s). This latter characteristic promotes novel Franck-Condon and vibronic scattering mechanisms in the B state(s) of the arrays. Collectively, the studies of the meso,meso-linked arrays provide insight into the type of RR scattering that might be anticipated for other types of systems that exhibit strong excitonic interactions among the constituents.
