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 omega-acido-ethyl or omega-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.