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Organic Porphyrin Nanoparticles with Induced Optical Activity: Ion-Based Synthesis from Achiral Chromophore and Chiral Counterions

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journal contribution
posted on 08.02.2011, 00:00 by Hiroshi Yao, Hisamu Sasahara, Keisaku Kimura
Organic porphyrin nanoparticles with induced optical activity are successfully synthesized via ion-association between the achiral meso-tetrakis(1-methylpyridinium-4-yl) porphine (TMPyP) cation and the chiral hexacoordinated phosphate (D3-symmetric TRISPHAT or C2-symmetric BINPHAT with Δ or Λ chirality, respectively) anion in the presence of poly(vinylpyrrolidone). The particle size ranges from 30 to 50 nm in diameter, which is almost independent of the counteranion used. The Soret band of the porphyrin nanoparticles exhibits a large bathochromic shift in comparison with that of the porphyrin monomer in solution. Simple quantum chemical calculations suggest that the meso-substituent flattening of the TMPyP molecule as well as ruffling/saddling deformation can be an origin of this bathochromic shift. The induced circular dichroism (CD) of the porphyrin nanoparticles exhibits split Cotton effects at the Soret band region, although the split modes are different between the nanoparticles prepared from TMPyP/TRISPHAT and TMPyP/BINPHAT systems. Possible mechanisms that could account for the chiroptical effects observed in the porphyrin nanoparticles are also discussed. We believe this synthetic concept and methodology are applicable to a wide variety of dyes and thus will be attractive to produce optically active organic nanoparticles with simplicity and versatility.