Design, synthesis, and photodynamics of light-harvesting arrays comprised of a porphyrin and one, two, or eight boron-dipyrrin accessory pigments

Light-harvesting arrays containing one, two, or eight boron-dipyrrin (BDPY) pigments and one porphyrin (free base or Zn chelate) have been synthesized using a modular building block approach. The reaction of pyrrole and 4-(BDPY)benzaldehyde or 3,5-bis(BDPY)benzaldehyde, prepared by Pd- mediated ethynylation with the corresponding iodo-benzaldehydes, affords the desired BDPY-porphyrin array in yields of 10-58%. The arrays are soluble in organic solvents and have been characterized by static and time-resolved absorption and fluorescence spectroscopy. The blue-green BDPY absorption complements spectral coverage of the porphyrin chromophores and rivals the intensity of the porphyrin Soret band when eight BDPY accessory pigments are present. Efficient energy transfer from the BDPY pigment(s) to the porphyrin (free base or Zn-chelate) is observed in arrays containing one or two (>90%) or eight (>85%) accessory pigments per porphyrin. Biphasic excited-state decay behavior is exhibited by the BDPY pigments in isolated form and in the arrays. The time constants are ~15 and ~500 ps in the reference compounds (both reflecting deactivation to the ground state) and ~2 and ~20 ps in the arrays (both primarily reflecting energy transfer to the porphyrin). The longer-lived kinetic component comprises ~70% of the decay in each case. Ab initio calculations suggest that the two kinetic components are associated with two energetically accessible excited-state conformers involving the boron-dipyrrin unit and the 5-aryl ring (which is integral to the linker in the arrays). The calculations and experimental results indicate that the two excited-state conformers differ from one another in structure (the planarity of the boron-dipyrrin unit and its orientation with respect to the 5-aryl ring), electronic composition (especially the electron density on the 5-aryl group of the boron-dipyrrin unit), radiative and nonradiative coupling to the ground state, and the rate of energy transfer to the porphyrin constituent in the arrays. The high energy-transfer efficiencies together with favorable light-absorption and chemical properties exhibited by the boron-dipyrrin pigments make them amenable for use in porphyrin-based arrays for molecular photonics applications.