Langmuir–Blodgett films of indandione‐l,3 pyridinium betaine. II: Molecular structure and properties of monolayers; simulation and optical absorption data

The molecular structure and optical properties of a monolayer at the air/water interface of novel amphiphilic derivatives of indandione‐1,3 pyridinium betaine (IPB) with different lengths of the aliphatic tail, namely C_1lIPB and C₁₇IPB, have been studied using optical absorption techniques and computer simulation approaches. The compression π‐A isotherm of the C₁₇IPB monolayer and computer simulation of its molecular structure show that there may exist two energetically stable molecular configurations, one with antiparallel orientation of the dipole moments of the C₁₇IP ‘heads’ in the low‐pressure region at π = 5–32 mN m⁻¹ and the second (after a distinct phase transition at π = 33 mN m⁻¹) with parallel orientation of the dipoles, with different tilt angles and areas per molecule. For C₁₁IPB only the first structural phase is observable. The compression‐induced changes in spectral characteristics of the two structural phases go in diametrically opposite directions. In the low‐pressure phase compression induces a red shift and an increase in intensity of the S₁ absorption band, while in the high pressure phase a blue shift and a decrease in the intensity of this band are observed. These spectral changes correlate reproducibly with the compression π‐A isotherms. Measurements of absorption dichroism confirm the change in the tilt angle at the phase transition pressure. The compression‐induced spectral changes have been substantiated by the results of quantum chemical calculations.