330 Thin SolM Films, 210/211 (1992) 330 331 Langmuir-Blodgett films based on substituted 1,2-dithiolium-tetracyanoquinodimethane radical anion salt O. Fichet, V. Gionis*, J. Amiell, B. Agricole and P. Delhaes Centre de Recherche Paul Pascal, Avenue A. Schweitzer, 33600 Pessae (France) D. Ducharme, A. Perrier and R. M. Leblanc Centre de Reeherche en Photobiophysique, UQTR, G9A 5H7 (Canada) Abstract A new semi-amphiphilic radical-anion salt, 1,2-dithiolium-tetracyanoquinodimethane has been synthesized for developing electroactive LB films. From in situ surface potential and ellipsometric measurements, it has been shown that the monolayer thermodynamic state is crucial for the building up of controlled multilayers. I. Introduction The control of the supramolecular organization in Langmuir-Blodgett (LB) films for obtaining a required physical property is currently a general aim [1]. Re- cently, there has been considerable interest in using the LB technique to match donor and (or) acceptor molecules with the goal of producing either conducting or opto-electronic molecular films. From published re- sults on charge transfer salts, it appears that only a few series of semi-amphiphilic TCNQ salts make LB films of a remarkable quality [2]. Studies were made on a new series of 3,4,5-(alkythio)-l,2-dithiolium salts [3] prepared either with iodine or perchlorate as counter- ions and, also with radical anions such as tetra- cyanoquinodimethane (TCNQ) and its fluorinated analog (TCNQF4). In this report, we present the main monolayer properties of TCNQ compound as measured by surface pressure, surface potential and ellipsometry. We also include the LB film characteristics of this compound. The main goal of our study is to relate the monolayer behaviour at the gas-water interface to the supramolecular organization in the LB films formed for the (1-1) TCNQ salt. H57716 i~ ' 46H"S $,,i~ $cHs 5--$ *Permanent address: Theoretical and Physical Chemistry Institute NHRF 48 Vassileos Constantinon Avenue, Athens 115/35, Greece. 2. Experimental details The experimental surface pressure (n-A), surface potential (AV-A), and ellipsometric (6A-A) isotherms versus area are shown in Fig. 1. The isotherms were obtained at 20 °C on a deionized and purified water subphase with a specific resistivity of 18 Mf~ cm and a surface tension superior to 71 mN/m. Methods and instrumentation concerning surface po- tential and ellipsometric measurements have been previ- ously described [4]. The surface pressure isotherm is classical, with a collapse pressure above 50 mN/m for a molecular area nearly 50/~2/molecule, but does not exhibit any clear phase transition. Surface potentials are informative about the surface density of dipoles. In the present case, we detect three different regimes (Fig. la): (1) The onset of a surface potential for a molecular area equal to 122 AZ/molecule which indicates a G-LE phase transition. (2) A regular increase of A V during the compression for reaching a maximum value of 750 + 10 mV when the molecular area is about 60 A,2/molecule. This varia- tion reveals an increase of the polar head density with a possible reorientation associated with the observed slope change. (3) A slow decrease of A V for the smallest molecular areas which is usually characteristic of no further reor- ganization of polar heads, but rather may indicate a collapse. For a non-absorbing thin film only one of the ellipso- metric angles 6 A varies during the film compression [5]. We observe four different parts with, in particular, a plateau for a molecular area between 72 and 61 A,2/ molecule. 0040-6090/92/$5.00 (' 1992 -- Elsevier Sequoia. All rights reserved