Hierarchical structure of merocyanine J-aggregates prepared by Langmuir–Blodgett and spin-coating methods

We have investigated structures of 6-methyl merocyanine (6Me-DS) J-aggregates self-assembled in Langmuir–Blodgett (LB) films and spin-coated polyvinyl alcohol (PVA) films prepared from a mixed solution of 6Me-DS and arachidic acid (AA). The coherent size of J-aggregate depends on the fabrication method. The coherent sizes in the LB and spin-coated film for the same concentration ratio of 6Me-DS:AA = 1:2 are 26 (±1) × 40 (±9) molecules and 31 (±1) × 22 (±3) molecules, respectively. This result indicates that the well-defined molecular arrangement in the LB film provides a larger coherent size. Microscopy observation shows that J-aggregates in spin-coated film form domains with sizes on a submicron scale. Domain shape is a grain-type if the molar concentration ratio of 6Me-DS to AA is less than 1:8 for 6Me-DS:PVA = 1:6. Increasing AA concentration to 6Me-DS:AA = 1:12, the domain shape drastically changes to a fiber type, indicating that the concentration of AA is a key parameter determining the shape of J-aggregate domain.