Effect of Molecular Distortion on the Optical Properties of Carotenoid-Based Nanoparticles

Tuning the conformations of flexible molecules has recently shown promise as a strategy for creating new materials. However, the creation of soft-molecule materials remains challenging because these materials are typically characterized without fully considering the contribution of molecular distortion. Carotenoids are flexible, polyene-based molecules that are prone to molecular distortion, which has not yet been considered a factor that controls their optical properties; rather, it has only been explained via simulations as causing differences in their molecular arrangements. Here, we performed an experimental structural characterization of the solid-state properties of carotenoids (β-carotene, lycopene, astaxanthin, and lutein) to illustrate the influence of molecular distortion on their optical properties. Interestingly, the carotenoid nanoparticles prepared via reprecipitation exhibited blue-shifted absorption spectra compared to those of their corresponding bulk crystals. We also found, via transmission electron microscopy and electron diffraction analyses, that all the nanoparticles composed of two domains with different crystallinities containing distorted or less-distorted molecules, as determined by their effective π-conjugation lengths obtained via Raman spectroscopy. Furthermore, the correlation between the absorption spectra and effective π-conjugation lengths of the carotenoids suggests that molecular distortion strongly affects their solid-state properties. This unveils that actively controlling molecular distortion in flexible molecules can potentially provide new materials.