Proof of Chirality of J-Aggregates Spontaneously and Enantioselectively Generated from Achiral Dyes

The recently published claim 1,2 that the achiral 5,5',6,6'- tetrachlorobenzimidacarbocyanine chromophore spontaneously and enantioselectively generates chiral J-aggregates when it is substituted in the 1,1'-position with n -alkyl groups longer than hexyl and in the 3,3'-position with ω-acido-ethyl or ω-acido- n -propyl groups has been proved by embedding the J-aggregates of such a dye ( 1a ) in a polymeric PVA film and measuring the CD signal at defined observation angles. Dye 1b, which is assumed to give achiral J-aggregates, has been investigated for comparison as well. Possible contibutions of linear dichroism and birefringence of ordered molecules to the measured CD signal have been excluded by careful checks using both a plain PVA film and a film containing monomers of a dye ( 2 ) of known chirality.