Analysis of the UV/Vis and CD spectral line shapes of carotenoid assemblies: spectral signatures of chiral H-aggregates.

Using vibronic exciton theory we evaluate the absorption and CD spectra of lutein and lutein diacetate aggregates, which have previously been described as card-packed H-aggregates and head-to-tail J-aggregates, respectively. The dramatically different spectral line shapes for both aggregates are shown to arise from strongly and weakly coupled H-aggregates consisting of helical arrays of chromophores. For lutein, the aggregates consist of tightly packed stacks of individual carotenoid molecules, while lutein diacetate aggregates resemble nematic liquid crystals. For both aggregates the agreement between experiment and theory is excellent. Analytical expressions for the absorption and CD spectra are presented, highlighting the spectral signatures of weakly coupled H (and J)-aggregation arising from distortions in the single-molecule Franck-Condon progression.