Structure-fluorescence properties of some naphthoylene-benzimidazole-based Langmuir-Blodgett films

Naphthoylene benzimidazoles (NBIs) and related polymers possess promising spectral properties for photonic applications. In the present study we focus on the possibilities of a controlled variation of molecular aggregation of NBI chromophores of various chemical compositions and symmetries in Langmuir-Blodgett (LB) films. Very different pi-A isotherms are found for the NBI-stearic acid (SA) complexes with different NBI isomers (cis-, trans- or a 1/2-NBI chromophore units). The interface behavior is explained by consideration of differences in molecular conformations at the water surface under lateral pressure, This addresses also a different type of chromophore aggregation within the LB films. Spectroscopic, atomic force microscopy (AFM), and X-ray scattering methods are used thoroughly to study mono- and multilayers of NBI:SA complexes. We propose a flat-on arrangement for trans-NBI molecules and an edge-on orientation for cis- and 1/2NBI chromophores at the water surface and then in the LB films, A strong tendency for aggregation in the solution leads to a formation of different types of dimers and n-mers (depending on the isomer used) in the LB films. The type of aggregation of NBI chromophores in molecular films affects their spectroscopic properties. J-aggregation formation is revealed from the fluorescence spectra.