Exciton delocalization length of merocyanine J-aggregates in Langmuir–Blodgett films studied from linear and nonlinear absorption measurements

A delocalization length and dimensionality of Frenkel excitons in 6-methyl merocyanine J-aggregates in Langmuir–Blodgett films have been investigated by means of linear and nonlinear absorption spectroscopy. The absorption spectrum of dye molecules in an LB film shows the energy shift due to the formation of J-aggregates with respect to the transition energy of monomers. From the differential absorption spectra measured by the femtosecond pump and probe experiment, we have obtained exciton energies of the lowest and second lowest states in J-aggregates. Using both spectroscopic data and a dipole model of molecular arrangement, we have determined the exciton delocalization size of two-dimensional J-aggregates (26(±1)×40(±9) molecules).