EVIDENCE FOR THE CYCLODEXTRIN MEDIATED AGGREGATION OF CYANINE DYES INTO OLIGOMERS

Based on the analysis of UV/Vis-and CD spectra of several cyanine dyes in beta- and gamma-cyclodextrin a case is made for the aggregation of the dyes into oligomers. The criteria for this new type of aggregates are: (i) the appearance of a concentration and temperature dependent shoulder on the high energy side of the dimer band in the UV/Vis-spect ra; (ii) two oppositely signed high intensity CD absorptions, one cent ered at the shoulder, the other at about the energy of the monomer abs orption. Calculations based on coupled-oscillator theory support the c ontention that chiral dye aggregates are formed in the presence of cyc lodextrins, most probably trimers and tetramers. From UV/Vis- and CD-d ata a helically twisted sandwich structure is suggested for these aggr egates. In contrast to the reported inclusion complexes these aggregat es must form - at least partly - outside the chiral host.