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'-tetrachloroben zimidacarbocyanine chromophore spontaneously and enantioselectively generat es chiral J-aggregates when it is substituted in the 1,1'-position with n-a lkyl groups longer than hexyl and in the 3,3'-position with omega-acido-eth yl or omega-acido-n-propyl groups has been proved by embedding the J-aggreg ates 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 contibut ions of linear dichroism and birefringence of ordered molecules to the meas ured 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.