Electronic Spectra and Luminescence Properties of Helicenes in Crystalline Matrices at 4.2K

Fluorescence, phosphorescence and excitation spectra, of [4]-, [5]-, [6]-,and [7]-helicenes in policrystalline matrices were obtained at 4.2 K. Highly-resolved electronic spectra were observed for [4]-, [5]-,and [6]-helicene whereas for [7]-helicene only broad bands were monitored in emission and excitation spectra. Irradiation of [5]-helicene at 4. 2 K leads to its photocy- clization to dihydrobenzoperylene. The observed concentration effects in [4]-, and [6]-helicene as well as influence of the crystallinity of the medium on the nuorescence-to-phosphorescence intensity ratios are explained assuming the presence of the fluorescent but not phosphorescent preaggregates. The vibronic structures of the site-selected [4]-, and [6]-helicene phosphorescence excitation spectra point to site-dependent distortions of the helicene molecules. Calculations of electronic transitions helicene molecules were performed with the modified Warshel-Karplus method.