Controlling the helicity of tubular J-aggregates by chiral alcohols

The achiral 3,3'-bis(3-carboxypropyl)-5,5',6,6'-tetrachloro- 1,1'-dioctylbenzimidacarbocyanine dye (C8O3) self-aggregates in aqueous solution to form optically active helical J-aggregates. A three-phasic circular dichroism (CD) spectrum with positive Cotton effect at 568 run, negative Cotton effect at 582 nm, and positive Cotton effect at 605 nm (+,-,+) is usually found and ascribed to molecular excitons. The predominance of the (+,-,+) signature means that the generation of chirality is enantioselective. The CD signal increases by a factor of about 100 when the chiral (R)-2-octanol is added to an aggregated solution, and the CD signal is further amplified by a factor of about 5 when the alcohol is present in the solvent prior to aggregation. The signature of the CD signal reverses if the enantiomeric (S)-2-octanol is used instead, and for a molar alcohol/ dye ratio R of about 20, the spectra become mirror images in good approximation; i.e., the chirality of the alcohol and the CD spectra of aggregates are correlated. The net effect of chiral alcohols is an increased excess of aggregates with a particular handedness due to induced circular dichroism. Samples containing (S)-2-octanol showed that their reversed CD spectra ((-,+,-) signature) are not fully stable when stored for longer times at ambient temperature. The spectral changes indicate a gradually increasing contribution from aggregates with the opposite sense of handedness ((+,-,+) signature). Electron micrographs reveal left helix handedness for the majority of aggregates in the presence of (R)-2-octanol ((+,-,+) signature), while right helix handedness predominates for the aggregates in the presence of (S)-2-octanol ((-,+,-) signature).