Highly organized aromatic molecular systems using Langmuir-Blodgett films: Structure, optical properties and probable epitaxy of anthracene-derivative multilayers

Abstract

The preparation of Langmuir-Blodgett multilayers of lightly substituted derivatives of a model aromatic (anthracene) has recently been reported; the anthracene was 9,10-substituted with a polar group (typically C 2H 4COOH) and an alkyl chain (from C 4H 9 to C 12H 25). We discuss here the optical absorption, fluorescence and X-ray diffraction properties of these films. The absorption and X-ray data are analysed to yield the detailed (and highly organized) molecular arrangement in the multilayers; the fluorescence appears to be a rather unusual excimer emission, which is explicable in terms of the unusual structure of the films. The deduced molecular arrangement confirms an ealier model of the water-borne monolayer structure and possesses several unique features; these include (i) probable close-packed interpenetration of the hydrocarbon chains of successive monolayers (which in turn would imply that an epitaxial orientation effect occurs during deposition, to bring the molecular L axes of successive layers parallel to one another), and (ii) two substantial tilts of the anthracene nuclei and of the chains away from the “upright ring” configuration which is generally observed with substituted-aromatic ( e.g. dye) multilayers. Excellent agreement is found between the observed structure and that predicted from space-filling models and from the operating point on the surface pressure-area isotherm; this, together with the existence of well-defined X-ray lattice spacings, demonstrates the predictability and perfection with which thin highly organized systems of largely aromatic molecules can now be constructed, using Langmuir-Blodgett techniques.