Design, Synthesis, and Photodynamics of Light-Harvesting Arrays Comprised of a Porphyrin and One, Two, or Eight Boron-Dipyrrin Accessory Pigments

Abstract

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 timeresolved 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.

Structural Control of the Photodynamics of Boron−Dipyrrin Complexes

“…4,8, 11 Here we have employed excess TEA and BF 3 ·O(Et) 2 to complete the reaction (Scheme 2). Thus, treatment of a free base dipyrrin 8 with TEA and BF 3 ·O(Et) 2 (10 molar equivalents each) in CH 2 Cl 2 at room temperature for 30 min afforded the desired boron-dipyrrin complex 2 in fair yield (19-68%).…”

Naphthyridyl‐Substituted 4,4‐Difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) Luminophores: Photophysics and Application as Molecular Imaging Probes in Live Cells

“…This behavior is reminiscent of that reported by Birge et al on biphasic emission decays from elegantly conceived light-harvesting arrays composed of multiple BODIPY units and a porphyrin and their precursor BODIPY arrays. [60] They attributed this behavior to two energetically accessible excited-state conformers that differed on the basis of the relative orientation of the BODIPY core and an aryl unit at the meso position.…”

Molecular Rotor Measures Viscosity of Live Cells via Fluorescence Lifetime Imaging

“…9 Furthermore, a DFT study of 2a has confirmed that the dark state is accessed by rotating the phenyl group from the metastable twisted conformation (corresponding to an emissive state) into the plane of the dipyrrin framework. 9 In 2b and 2c, where the free rotation of the phenyl group is restricted, nonradiative decay is prevented, and this leads to an increase in the fluorescent quantum yield which becomes very close to unity. 10 Similarly, in the series of 3a-d, the fluorescence quantum yield increases from 3a < 3b ≈ 3c < 3d, following the decreasing ease of rotation of the meso-phenyl group.…”