Influence of aggregation on the optical spectra of a polymethine dye in single crystals Original Research Article

Reflectivity spectra of several crystal faces of bis(dimethylamino)-heptamethinium perchlorate, BDH+ClO4−, and bis(dimethylamino)-heptamethinium tetraphenylborate, BDH+BPh4−, have been analyzed with a polariton model to derive transition energies and strength of the molecular π → π∗ transitions. Different stacking leads to considerable shift of the transitions with respect to spectra in solution in qualitative agreement with models based on dipolar interaction. The contribution of these transitions to the dielectric susceptibility is very different in both materials. It is shown that this is solely due to the packing of the molecules while their large oscillator strength f=5.5 remains the same. Finestructure related to molecular vibrations is absent in the spectra of the densily packed BDH+ClO4− which is interpreted by fast energy loss of polariton resonances.