Characterization of aggregates of nonamphiphilic anthracene assembled in ultrathin supramolecular Langmuir-Blodgett films

This paper reports the behavior of pure and mixed films of nonamphiphilic anthracene and stearic acid (SA) at the air-water interface studied by conventional surface pressure (II) versus area per molecule (A) isotherm measurements. In addition, the spectroscopic characteristics of mixed films of anthracene and SA transferred onto solid substrates as Langmuir-Blodgett (LB) films are also reported. II-A isotherms show that pure anthracene is incapable of forming stable films at the air-water interface and collapses readily at low surface pressures. Mixed films of anthracene and SA, however, readily formed stable films at the air-water interface which are easily transferred onto solid substrates. The average area per molecule of the mixed films at the air-water interface is observed to decrease with increasing mole fraction of anthracene, which indicates the formation of aggregates. Detailed studies revealed that the anthracene aggregates formed are squeezed out of the air-water interface to remain partly sandwiched between the SA chains and partly on the surface of the film. Atomic force microscopic studies confirmed the existence of three-dimensional anthracene crystallites on the surface of the mixed LB films. This aggregation process is attributed to the strong cohesive forces existing between anthracene molecules and weak adhesive forces between the anthracene and SA molecules resulting in the formation of microscopic crystallites which provide the nuclei for their growth into large crystals. Spectroscopic studies confirm a high degree of molecular ordering and orientation in these aggregates, and polarization studies suggest that very Likely the long axis of the anthracene molecule makes a small angle with the dipping direction of the substrate. Anisotropy measurements confirm the existence of ordered aggregates. Energy transfer from the energetically higher aggregates to the lowest one results in emission from this energetically lowest aggregate that is manifested as a large shift in the emission spectrum and a small value of the anisotropic ratio.