Circular dichroism and theoretical calculations of pinacyanol dimer inclusion in γ-cyclodextrin

Aggregation of pinacyanol chloride is strongly enhanced through inclusion into gamma-cyclodextrin, which was studied using both UV/Visible, derivative and Circular Dichroism (CD) spectroscopy. The intensities at 546 and at 507 nm were increased with the growing ratio of gamma-cyclodextrin to dye. Both aggregates were shown to be optically active. CD spectra with two and three oppositely signed excitonic bands were shown. The structures of the aggregates have been analysed and discussed in terms of qualitative H-type aggregation and quantitative based on OSCI program. Applying eight different parameter sets aggregates of pinacyanol chloride ranging in size from 2 to 20 dye molecules were considered. The calculated energy splitting was adjusted, via the transition dipole length, to the experimentally observed band shift. Rotational strengths associated with the CD absorptions were calculated and used to obtain an estimate of the inherent twist of the dimer aggregate inside the gamma-cyclodextrin cavity.