The near-infrared (NIR) spectral region has long been somewhat of terra incognita for photochemistry, at least in the laboratory, although nature has employed bacteriochlorophylls for NIR light harvesting and energy transduction in anoxygenic photosynthesis for several billion years. The ability to capture and utilize the energy derived from NIR light is important for solar energy conversion and also holds prospects for applications in photomedicine, particularly in clinical diagnostics and deep-tissue imaging. We have been developing the synthetic chemistry of bacteriochlorins, which are tailorable analogues of bacteriochlorophylls, Nature’s NIR absorbers. The general molecular design relies on use of a gem-dimethyl group in each reduced ring to secure the bacteriochlorin chromophore. The general synthetic strategy relies on (1) self-condensation of a dihydrodipyrrin-acetal, (2) bromination of the resulting bacteriochlorin macrocycle, (3) Pd-mediated installation and tailoring of substituents including annulated rings, and where desired, (4) construction of arrays comprised of multiple chromophores. As examples, bacteriochlorin dyads of interest here are joined by a phenylethynyl linker and afford closely spaced long-wavelength absorption bands, which together enable effective capture of NIR light and ensuing Förster resonance energy transfer (FRET) between the bacteriochlorins with transfer times ~1 ps and quantum efficiency &gt;99.9%. The energy levels of the respective bacteriochlorins can be tuned with few-nanometer exactitude by placement of appropriate substituents (R<sup>1</sup>–R<sup>3</sup>) at appropriate sites, thereby controlling FRET properties. Bacteriochlorins afford an attractive combination of NIR spectral features, robust photochemistry, and synthetic malleability. The focus of this presentation concerns advances in synthetic methodology for bacteriochlorin preparation and elaboration.

Chromophores and architectures for ultrafast FRET in the NIR region – synthesis

ACS Spring 2022

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