Analysis of the orientational structure of monolayers comprised of banana-shaped achiral molecules at an air-water interface

The orientational structure of monolayers comprised of 1-butyl-2,6-bis-[2-(4-dibutylaminophenyl)-vinyl]- pyridiniumiodide (M53) banana-shaped achiral molecules was studied by Maxwell displacement current (MDC) and optical second harmonic generation (SHG) measurements. In the MDC-SHG measurement, gradual increase of the orientational order parameters C01≡〈cos⁡θ〉 and C03≡〈5cos⁡3θ−3cos⁡θ〉/2 (θ: tilt angle, 〈〉: thermodynamic average) was detected as the molecular area was decreased. In order to clarify this result, a model of monolayers comprised of banana-shaped achiral molecules was constructed taking into account the electrostatic Coulomb interaction between constituent molecules and induced image dipole at an air-water interface, and the entropy with respect to positional and orienational distribution of constituent molecules. A good agreement between the model calculation and the experimental result demonstrated that the geometrical constraint imposed on the orientation of constituent molecules at an air-water interface (interfacial binding effect) plays an important role in the determination of orientational structure of monolayers comprised of banana-shaped achiral molecules.