The vibronic structure of fluorescence excitation spectra of single molecules of dibenzoterrylene (DBT) in anthracene (Ac) and in 2,3-dimethylanthracene (2,3-DMA) crystals were studied at 5 K. The vibronic patterns observed for different single molecules in the same crystal site were explained as fingerprints of the molecular geometry, which according to the ONIOM calculations are strongly dependent on the local insertion cavity. The 'dipolar' disorder encountered in the 2,3-DMA crystals led to a broad frequency distribution of the purely electronic lines of single molecules. The saturation curves for the (0, 0) and vibronic transitions were studied. (C) 2012 Elsevier B.V. All rights reserved.