Abstract
Highly resolved fluorescence, phosphorescence and phosphorescence excitation spectra of tetrahelicene (THL) and hexahelicene (HHL) in polycrystalline matrices were obtained at liquid helium temperature. The observed concentration effects as well as the influence of the crystallinity of the medium on the fluorescence-to-phosphorescence intensity ratios are explained assuming the presence of fluorescent but not phosphorescent preaggregates and aggregates of the helicenes in crystalline matrices. The vibronic structures of the site-selected THL phosphorescence excitation spectra point to site-dependent distortions of the THL molecules. Calculations of electronic transitions of the HHL molecule were performed with the modified Warshel-Karplus method. The activity of out-of-plane skeletal modes in S 1↔S 0 and T 1→S 0 HHL spectra correlates with the calculated HHL helix pitch changes in S 1 and T 1 states, respectively.
Published Version
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