ARTICLE pubs.acs.org/JPCC
Self-Assemblies of Novel Magnesium Porphyrins Mimicking Natural Chlorosomal Bacteriochlorophylls Je˛drzej Szmytkowski,†,‡,*,#,r,O Jonas Conradt,O,†,‡ Hendrik Kuhn,† Chilla Malla Reddy,§,|| Mihaela Carmen Balaban,‡,§ Teodor Silviu Balaban,*,‡,§,^ and Heinz Kalt*,†,‡ †
Institute of Applied Physics, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany § Institute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Campus Nord, Postfach 3640, D-76021 Karlsruhe, Germany Indian Institute of Science Education and Research, Kolkata, West Bengal, 741252 India ^ Universite Paul Cezanne Aix-Marseille III, ISM2 - UMR 6263 Chirosciences, France
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ABSTRACT: Self-assembling porphyrins are promising materials to mimic natural bacteriochlorophylls c, d, or e encountered in the chlorosomes of photosynthetic bacteria. We have studied four novel magnesium porphyrins mimicking this chlorosomal antenna system. In contrast to previous articles reporting synthetic Zn-porphyrins, our studies focus on porphyrins with Mg as the central atom, which mimic more closely the natural bacteriochlorophylls. The analysis of time-resolved photoluminescence dynamics based on the decay-associated emission spectra shows a short-lived component associated with the supramolecular architecture. It confirms the ordered nature of the investigated porphyrins and suggests that these self-assemblies can be used as artificial antennae in light-harvesting devices like biomimetic solar cells.
’ INTRODUCTION Chlorosomes of green sulfur photosynthetic bacteria formed by bacteriochlorophyll (BChl) c, d, or e pigments are the most efficient natural light-harvesting systems that are able to capture photons even in the lowest illumination conditions.1 Their architecture is very different from other photosynthetic organisms like purple bacteria, cyanobacteria, algae, or plants, which are equipped with antennae organized by pigmentprotein interactions. The self-assemblies of BChls allow forming efficient antenna systems in the absence of a protein scaffold. Still one finds efficient collection of light due to an increased photon absorption cross section and an energy transfer to the reaction center without significant loss, both resulting from the selfassembly. The exact molecular structure of BChl aggregates in chlorosomes is still not well understood.2 Several models, sometimes controversial, have been proposed to explain their architecture.314 The process of self-assembly can be investigated using artificial mimics of natural BChls, which was presented in our previous studies.12,15,16 Other groups have major contributions in mimicking BChls, either with semisynthetic mimics, mostly derived from BChl a,17,18 or from fully synthetic porphyrins.19,20 Synthetic metalloporphyrins are chemically more stable than natural bacteriochlorophyllous pigments and can be synthesized with different recognition groups to achieve self-assembly. They easily self-aggregate in nonpolar organic solvents, like n-heptane and form oligomers in the same manner as the natural BChls.15 Besides clarification of the r 2011 American Chemical Society
self-assembly algorithm, synthetic metalloporphyrins are also interesting for the progress of organic photovoltaics based on a functional blueprint. The mimics of BChls are promising candidates to be used in biomimetic solar cells as light-harvesting antennae.16,2124 In the present article, we report on four novel magnesium porphyrins. These metalloporphyrins, with a central magnesium atom, are more closely related to the natural bacteriochlorophylls than the synthetic mimics consisting of Zn-porphyrins. A timeresolved photoluminescence technique was used to study the new materials. This method allows identifying the self-assemblies by a net differentiation of the fluorescence decays due to the monomers and to the ordered self-assembled species. Similarly to natural photosynthetic BChls, the ordered nature of the synthetic self-assemblies reduces the concentration quenching observed in not well-ordered structures.10 The finding of wellordered self-assemblies is an essential prerequisite for the use of synthetic self-assembling materials in biomimetic solar cells.
’ EXPERIMENTAL SECTION Materials and Synthesis. The natural BChls c, d, or e are chlorins with Mg as the central atom. Their molecular structure is presented in part a of Figure 1. To mimic the BChls pigments, Received: December 23, 2010 Revised: March 23, 2011 Published: April 07, 2011 8832
dx.doi.org/10.1021/jp112244m | J. Phys. Chem. C 2011, 115, 8832–8839
The Journal of Physical Chemistry C
ARTICLE
Figure 1. (a) Typical chlorosomal bacteriochlorophylls c, d, and e. The long chain alcohol esterifying the 17-propionic acid residue is depicted as farnesol although other lipophilic alcohols, such as stearol, cetol, geranyl-geraniol, or phytol may be encountered in different photosynthetic bacteria. The fifth ligand of the Mg atom can be a water molecule (R0 = H), methyl alcohol (R0 = CH3), or another BChl ligated through the 31 secondary hydroxyl group giving rise to extended aggregated species. Additionally, this fifth ligand can be either R, on the opposite side of the 17-propionic acid residue, or β, as drawn. (b, c) Molecular structures of the BChl mimics discussed in this work. The central Mg atom is depicted in the four porphyrins as four coordinated but it can be either five- or six-coordinated by hydroxy groups and/or carbonyl groups.
four magnesium porphyrins (called in this article 3,13-ketol; 3,17-ketol; 3,13-diacetyl; 3,17-diacetyl) were synthesized. The molecular structures of ketol and diacetyl compounds are presented in parts b and c of Figure 1, respectively. The syntheses for 3,13- and 3,17-Mg ketols described herein are very similar and consisted in a two step reaction sequence: (A) the monoreduction with sodium borohydride of the corresponding diacetyl compounds, which were prepared as previously described,25,26 followed by (B) Mg insertion using the excellent methodology developed by Lindsey and Woodford.27 Briefly, 79 mg of 3,13-diacetyl-10,20-di-tert-butylphenyl porphyrin free base was dissolved under nitrogen in 35 mL dry
dichloromethane and 7 mL methanol. After degassing with nitrogen for 2 min, sodium borohydride (22 mg) was added at once. The reaction was monitored by TLC and stopped after 30 min when the lower migrating spot of the racemic and meso-diols starts to be visible (Rf =
