The molecular organization in Langmuir-Blodgett (LB) monolayers for five organic dyes with a common chromophore moiety was studied. With the use of transmission and reflection-absorption (FT)IR spectroscopy (RAIRS), the orientation of the chromophore units in the LB monolayer was determined. The results obtained with carbonyl and C=C stretching vibrations exclude a flat-on orientation of the chromophore moiety and may be rationalized in terms of a tilted head-on organization of the chromophore on the Ag, ZnS, and Ge surfaces. The tilt angle, Φ, formed between the direction of the dynamic dipole moment and the normal to the surface of the film was determined from the combined infrared data obtained in a transmission geometry and in RAIRS. The aromatic-aromatic interaction plays a significant role in determining the molecular alignment in the Langmuir and LB monolayers.
Ricardo Aroca, Eric Johnson, and Asim Kumar Maiti, "Molecular Orientation and Stacking of Perylenetetracarboxylic Diimide Derivatives in Langmuir-Blodgett Monolayers: Part I," Appl. Spectrosc. 49, 466-471 (1995)