H-Aggregate of a Long-Chain Crystal Violet Dye in Langmuir-Blodgett Films.

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The molecular orientation and aggregate formation in Crystal Violet (CV) dye monomolecular layers have been investigated by means of polarized reflection and absorption spectral measurements at an incident angle of 60°. A relatively sharp absorption band at 2.04 eV and a broad band around 2.23 eV assignable to the J-dimer and the H-dimer, respectively, were observed in the optical measurement geometry with polarized light either parallel(p) or perpendicular(s) to the incident plane. A sharp band at 2.61 eV, assigned to the H-aggregate (tetramer), was detected by p-polarized spectral measurement, but not s-polarized measurement. The J- and H-dimers have horizontal transition moments and the H-aggregate has a vertical transition moment. The H-aggregate peak shifted from 2.53 eV (10 mN m<SUP>−1</SUP>) to 2.61 eV (40 mN m<SUP>−1</SUP>) with increasing surface pressure. This is due to an increase in the slip angle between the transition dipole moments and their center-to-center lines because the chromophoric planes become to be gradually oriented normal to the water surface. The behavior of these aggregates is interpreted in terms of the extended dipole model.
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