Preparation and characterization of Langmuir and Langmuir–Blodgett films from a pyridine-terminated stilbene

Abstract

Langmuir and Langmuir–Blodgett films of a pyridine-terminated stilbene derivative, namely 4-hexyloxy-4′-[N-(4-pyridilmethylene)amine] stilbene (abbreviated as HPAS), have been fabricated and characterized at various surface pressures, with surface pressure and Brewster Angle Microscopy being used to map the different phases of the stilbene monolayer at the air–water interface. UV–vis reflection spectroscopy showed a strong blue shift of 48 and 93 nm of the reflection spectrum of the Langmuir film with respect to the spectrum of a solution of HPAS, which indicates that two dimensional H-aggregates are formed at the air–water interface. These structures represent a minimum free energy conformation for the system, as they are observed even before the compression process starts. The sequential transfer of monolayers of HPAS onto solid substrates results in a Y-type deposition. Single layer LB films transferred at several surface pressures onto mica substrates have been analyzed by means of atomic force microscopy, from which it can be concluded that 20 mN/m is an optimum surface pressure of transfer, giving well-ordered homogeneous films without three dimensional defects and a low surface roughness. The optical properties of the LB films have been determined, with significant blue-shifted absorption spectra suggesting that the two dimensional H-aggregates at the air–water interface are transferred undisturbed onto the solid substrates. In addition, absorption and emission spectra are indicative of a strongly coupled excitonic interaction between the HPAS H-aggregates. X-ray reflectivity (XRR) experiments yielded a film thickness consistent with one monolayer in which the molecules are tilted 73° with respect to the solid substrate.