INFLUENCE OF AGGREGATION ON THE OPTICAL-SPECTRA OF A POLYMETHINE DYE IN SINGLE-CRYSTALS

Reflectivity spectra of several crystal faces of bis(dimethylamino)-he ptamethinium perchlorate, BDH+ClO4-, and bis(dimethylamino)-heptamethi nium tetraphenylborate, BDH(+)BPh(4)(-), have been analyzed with a pol ariton model to derive transition energies and strength of the molecul ar pi --> pi transitions. Different stacking leads to considerable sh ift of the transitions with respect to spectra in solution in qualitat ive agreement with models based on dipolar interaction. The contributi on of these transitions to the dielectric susceptibility is very diffe rent in both materials. It is shown that this is solely due to the pac king of the molecules while their large oscillator strength f = 5.5 re mains the same. Finestructure related to molecular vibrations is absen t in the spectra of the densily packed BDH+ClO4-, which is interpreted by fast energy loss of polariton resonances.