Electronic transitions of fluorene, dibenzofuran, carbazole, and dibenzothiophene: From the onset of absorption to the ionization treshold

A comparative study of the electronic transitions of fluorene and its hetero-analogues dibenzofuran, carbazole, and dibenzothiophene was performed in a wide energy range. Gas phase, crystal phase, and linear dichroism electronic transmittance spectra were measured with synchrotron radiation. Prediction of electronic transitions to excited singlet states was performed by using time-dependent density functional theory TD-B3LYP/6-31+G(d,p). Based on the experimental and theoretical results, symmetry assignments of electronic transitions in the vacuum and near-UV region are suggested.  The correspondence between excited states in these molecules, similarities, and differences between their electronic spectra are discussed.