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 C 01 ≡ 〈 cos ⁡ θ 〉 and C 03 ≡ 〈 5 cos ⁡ 3 θ − 3 cos ⁡ 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.