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Excited state (excitation) interactions in polyene aggregates. Resonance Raman and absorption spectroscopic evidence

1977; Wiley; Volume: 6; Issue: 6 Linguagem: Inglês

10.1002/jrs.1250060605

1097-4555

V. R. Salares, N. Martin Young, Paul Carey, H. J. Bernstein,

Nonlinear Optical Materials Studies

Abstract The carotenoids astaxanthin, lutein and zeaxanthin form high molecular weight aggregates in aqueous solvent systems. The absorption spectra of the aggregates, compared to those of the monomer molecules, show significant bandwidth narrowing and blue shifts of 600–800 Å. The resonance Raman spectra of these aggregated polyenes are anomalous in that ν CC occurs near 1518 cm −1 whereas a well‐established ν CC vs 1/λ max correlation predicts that ν CC should be near 1560 cm −1 . The Raman and absorption data together demonstrate that strong excited state interactions are taking place and these results illustrate the utility of the resonance Raman technique for recognizing excited state perturbations. Raman excitation profiles (the variation of Raman intensity with wavelength) on aggregated astaxanthin and low temperature (−100 °C) absorption spectra on aggregates of astaxanthin and lutein establish that a pronounced change in vibronic structure occurs upon aggregation. In contrast to astaxanthin or lutein monomers whose electronic absorption spectra are made up of 0–0, 0–1, 0–2,… transitions, only the 0–0 transition is seen in the aggregates. The results are explained in terms of the molecular exciton model.