Porphyrins perform diverse functions in Nature (Figure 1). For example, the characteristic red color of blood and green color of plants are due to porphyrinic macrocycles (heme and chlorophyll respectively). The rich diversity of porphyrin function arises from the variety of mechanisms available for the fine tuning of macrocycle properties. The identity of the central metal atom and axial ligands are important. The protein matrix surrounding the porphyrin ring also influences macrocycle properties. And of particular interest to the organic chemist, the core structure of the porphyrin can be substituted, reduced, heteroatom modified, isomerized, expanded, and/or contracted relative the prototypical porphyrin structure. Such structural alterations give rise to a large family of molecules that display diverse properties (Figure 2). Some of the general structures shown in Figure 2 are found in Nature. Others have been created in the laboratory in an effort to produce porphyrinoids of fundamental interest, and materials useful for a wide range of commercial applications including molecular scale electronic devices, solar energy, photodynamic cancer therapy, ion selective sensors, and catalysis.
In the Geier laboratory, we have a number of ongoing projects involving many of the macrocycles shown in Figure 2. Two current studies are summarized here. (1) We are developing syntheses of porphyrinic macrocycles that possess a direct linkage between two adjacent pyrrole rings (e.g. corrole and sapphyrin in Figure 2). This direct linkage affects the dimensions of the central core producing molecules with properties distinct from porphyrin itself. For example, corrole is known to stabilize metal ions in unusually high oxidation states. (2) We are developing syntheses of macrocycles that possess an sp3 hybridized carbon atom (e.g. phlorin in Figure 2). The introduction of one or more sp3 hybridized carbon atoms disrupts macrocycle aromaticity, and disturbs macrocycle planarity—producing species with unique properties. Unfortunately, this modification can also result in poor macrocycle stability. We are investigating syntheses that are intended to provide macrocycles with improved stability so that their properties can be better studied.
Recent accomplishments in the Geier laboratory include studies of mild acid catalysis in the condensation of a dipyrromethane-dicarbinol with pyrrole leading to meso-substituted corroles (Scheme 1, J. Org. Chem. 2004, 69, 4159-4169), and an investigation of the condensation of a dipyrromethane-dicarbinol with 2,2´-bipyrrole leading to a novel meso-substituted octaphyrin (Scheme 2, J. Org. Chem. 2004, 69, 6404-6412).
Throughout our studies, we utilize a broad range of experimental techniques including preparative organic synthesis, parallel analytical scale reactions, computer molecular modeling, analytical (GC) and preparative chromatography, and a variety of spectroscopic tools (NMR, UV-vis, IR, fluorimetry, EI-MS, CI-MS, and LD-MS).
Research in the Geier laboratory is supported by Colgate University and grants from the American Chemical Society—Petroleum Research Fund and Research Corporation. Please make an appointment with Professor Geier to discuss current research opportunities in the Geier laboratory.
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