Synthetic Metallobacteriochlorins with Site-specific Isotopic Substitution for Probing Photosynthetic Function.

摘要

Bacteriochlorophylls -- natural pigments for absorption of near-infrared (NIR) light -- underlie light-absorption and energy transduction in photosynthetic bacteria. Understanding photosynthetic function including the capture and utilization of light is essential for applications ranging from artificial photosynthesis to photomedicine (photodynamic therapy, imaging, and diagnostics). Previous synthetic limitations have largely precluded access to bacteriochlorin model compounds suitable for probing photosynthetic function. A recently developed de novo synthesis of stable bacteriochlorins employs the self-condensation of a dihydrodipyrrin--acetal, which is prepared from simple precursors such as a pyrrole-2-carboxaldehye and an alpha,beta-unsaturated ketone--acetal. The success of the de novo synthesis to synthetic bacteriochlorins has opened the door to the study of fundamental properties and pursuit of a range of photochemical applications. However, accessibility to metallobacteriochlorins has remained limited.;Access to metallobacteriochlorins is essential for investigation of a wide variety of fundamental photochemical processes, yet relatively few synthetic metallobacteriochlorins have been prepared. Members of a set of synthetic bacteriochlorins bearing 0--4 carbonyl groups (1, 2, or 4 carboethoxy substituents, or an annulated imide moiety) were examined under two conditions: (i) standard conditions for zincation of porphyrins [Zn(OAc)2˙2H 2O in DMF at 60--80 °C], and (ii) treatment in THF with a strong base (e.g., NaH or LDA) followed by a metal reagent MXn. Zincation of bacteriochlorins that bear 2--4 carbonyl groups proceeded under the former method whereas those with 0--2 carbonyl groups proceeded with NaH or LDA/THF followed by Zn(OTf)2. The scope of metalation (via NaH or LDA in THF) is as follows: (a) for bacteriochlorins that bear two electron-releasing aryl groups, M = Cu, Zn, Pd, and InCl (but not Mg, Al, Ni, Sn, or Au); (b) for bacteriochlorins that bear two carboethoxy groups, M = Ni, Cu, Zn, Pd, Cd, InCl, and Sn (but not Mg, Al or Au); and (c) a bacteriochlorin with four carboethoxy groups was metalated with Mg (other metals were not examined). Altogether, 15 metallobacteriochlorins were isolated and characterized. The availability of diverse metallobacteriochlorins should prove useful in a variety of fundamental photochemical studies and applications.;The concise de novo synthesis enables site-specific incorporation of isotopes about the bacteriochlorin macrocycle; for a singly substituted dihydrodipyrrin--acetal, the corresponding bacteriochlorin contains a pair of isotopic substitutions located at positions symmetrically disposed by 180° rotation about the normal to the plane of the macrocycle. With successful methods for preparing metallobacteriochlorins in hand, a series of synthetic metallobacteriochlorins have been prepared that bear minimal substituents (to minimize spectral complexity) and incorporate 13C/ 15N isotopes in the inner resonance frame of bacteriochlorin pi-system. In total, 24 isotopically substituted bacteriochlorins were synthesized. The Cu chelates of the isotopologues have been examined by IR and resonance Raman spectroscopies with the aim of assigning the vibrations of the macrocycle. The pi-cation radicals of the Zn chelates of the isotopologues have been examine via electron paramagnetic resonance spectroscopy with the aim of elucidating the spin density in the macrocycle. Together, the vibrational and spin-density characteristics should aid in probing photosynthetic function of bacteriochlorophylls in natural systems.