Hybrid Langmuir and Langmuir–Blodgett films of a viologen derivative and TCNQ in a mixed valence state: preparation route and characterization Santiago Mart  ın, Pilar Cea, Carlos Lafuente, F elix M. Royo, Mar  ıa C. L opez * Departamento de Qu  ımica Org anica–Qu  ımica F  ısica, Facultad de Ciencias, Plaza de San Francisco, Ciudad Universitaria, 50009 Zaragoza, Spain Received 8 March 2004; accepted for publication 15 June 2004 Available online 2 July 2004 Abstract Hybrid Langmuir and Langmuir–Blodgett (LB) films containing two moieties of great chemical and electrochemical interest, namely a viologen derivative and tetracyanoquinodimethane (TCNQ) in a mixed valence state, were fabricated. To do so, positively ionized monolayers of 1,1 0 -dioctadecyl 4,4 0 -bipyridilium were prepared onto aqueous solutions of tetracyanoquinodimethane in a mixed valence state. Surface pressure vs. area (p–A), surface potential vs. area (DV –A), and Brewster angle microscope (BAM) images were recorded and interpreted in terms of molecular interactions as well as the incorporation of the hydrophobic anions into the monolayer. After a comprehensive study, a 10 6 M TCNQ aqueous solution was chosen as the best one to build hybrid LB films. Thus, the floating films were transferred onto solid substrates that were characterized using several techniques including ultraviolet–visible (UV–vis), infrared (IR), scanning electron microscopy (SEM), and atomic force microscopy (AFM) proving the incorporation of the TCNQ onto the film. These films show a good optical conductivity as well as a high degree of order and layers with a constant architecture. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Aromatics; Surface structure, morphology, roughness, and topography; Scanning electron microscopy (SEM); Atomic force microscopy 1. Introduction 1,1 0 -Disubstituted-4,4 0 -bipyridilium salts, also called ‘‘viologens’’ (abbreviated as bipm followed by the redox state), have focused the attention of many research groups due to both their theoretical interest and practical applications. Among their more interesting characteristics we could highlight their three oxidation states (bipm 2þ fi bipm þÅ fi bipm 0 ), the high reversibility of their redox reac- tions (specially the first one), and the fact that they can be cycled many times without significant side reactions. They have been used in a wide variety of investigations including studies of electron transfer rate at the electrode substrate film interface [1], the * Corresponding author. Tel.: +34-976-76-11-96; fax: +34- 976-76-12-02. E-mail address: mcarmen@unizar.es (M.C. L opez). 0039-6028/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2004.06.168 Surface Science 563 (2004) 27–40 www.elsevier.com/locate/susc