Femtosecond transient absorption study of merocyanine J-aggregates

A delocalization length and dimensionality of Frenkel excitons in 6-methyl merocyanine J-aggregates in Langumuir–Blodgett films have been investigated by means of both an absorption experiment and a pump-probe method. We unambiguously determine a delocalization size from a transition energy of monomers, and exciton energies of the lowest and second-lowest states in J-aggregates. It is found that the J-aggregate structure is two-dimensional, and the exciton state is extended over 26×40 molecules.