Journal of Colloid and Interface Science 243, 156–164 (2001) doi:10.1006/jcis.2001.7886, available online at http://www.idealibrary.com on Langmuir–Blodgett Films Containing TCNQ Incorporated from the Aqueous Subphase Pilar Cea, Hector Artigas, Jose S. Urieta, Maria C. Lopez, and Felix M. Royo 1 Departamento de Qu´ ımica Org´ anica–Qu´ ımica F´ ısica, Facultad de Ciencias, Plaza de San Francisco, Ciudad Universitaria, 50009 Zaragoza, Spain Received February 20, 2001; accepted July 26, 2001 Films containing TCNQ (tetracyanoquinodimethane) anions were prepared by incorporating it from a LiTCNQ aqueous sub- phase. The TCNQ moiety was transferred from the subphase as the counterion of a positively charged monolayer of DPOP + (diphenyl- bis(octadecylamino)phosphonium). The isotherm shape, the area per molecule, the collapse pressure, and the monolayer stability were dependent on the LiTCNQconcentration in the subphase. The obtained Langmuir–Blodgett films were classified into three types depending on the TCNQ oxidation state. The oxidation state de- pends on the transference pressure as well as on the monolayer age. The degree of both orderand architecture of the films was studied by means of SEM, IR, UV–vis spectroscopy, and X-ray diffraction. Conductivity studies were also performed. C  2001 Academic Press Key Words: Langmuir–Blodgett films; deposition; tetracyano- quinodimethane. INTRODUCTION Following with our line of work in the field of electroac- tive molecules (1) arranged in thin films (2–4) as well as in the technique of incorporation of ions from the subphase at- tracted by a charged monolayer (5–7), we present here the results of detailed research into the properties of Langmuir and Langmuir–Blodgett (LB) films containing tetracyanoquin- odimethane (TCNQ). Since the discovery of the highly con- ducting charge-transfer (CT) complex TTF(tetratiafulvalene)- TCNQ in the early 1970s, the efforts of many scientists have been focused on these kinds of compounds, opening a new field of interdisciplinary research with far-reaching consequences for material science. Inside this field of research, TCNQ as well as its derivatives have been extensively used as electron acceptor molecules in a large number of organic conducting systems (8), MIM (metal/insulator/metal) devices (9), COS (conducting or- ganic salt) electrodes and biosensors (10, 11), and potentiomet- ric sensors (12, 13), and even incorporated into electrochromic devices (14). To our knowledge all routes to incorporate TCNQ in LB films have implied the spreading of a TCNQ derivative in an original 1 To whom correspondence should be addressed. E-mail: femer@posta. unizar.es. neutral state (15), a TCNQ salt (16–19), or even a compound containing TCNQ in a direct mixed oxidation state (20–22), namely the homodoping route. The compulsory requirement in all cases was the synthesis of a TCNQ derivative containing long alkyl chains to prevent the compound from being dissolved in the subphase. We introduce here a different way of preparing films contain- ing TCNQ, in which no TCNQ derivative is spread onto water. This method was originally applied to achieve TCNQ-ordered films and it is based on the well-known route of incorporating an ion from the subphase when an ionized monolayer is obtained at the gas–liquid interphase. This route has been used to prepare both organic/organic (23, 24) and inorganic/organic layers (5–7, 25). Our aim in this work is to see if it is possible to incorporate the TCNQ moiety from an aqueous subphase and we will try to analyze the advantages and disadvantages of this method versus the traditional ways mentioned above. Obviously one advan- tage is the synthesis process. With this method we only need a water-soluble TCNQ salt easily obtainable (e.g., LiTCNQ) and an organic salt capable of forming a stable ionized monolayer at the air–water interface (there are many salts commercially available). This fact makes it possible to experimentally assay many combinations, a useful ability given the importance of both theoretical and practical uses of thin films (26) containing TCNQ. Another advantage is that with this method we can de- crease the number of alkyl chains (these chains often hinder the use of films for practical applications). A good example might be the preparation of MIM devices formed by hetero-LB films containing TCNQ. These devices require the presence of several molecules containing each one of the needed functional groups (for example sensitizer/electron acceptor) and traditionally they are prepared using alternating troughs (9). The incorporation of one of the moieties from the subphase would simplify the experimental work and decrease the number of alkyl chains. The two molecules employed in this paper, DPOPBr and LiTCNQ, were chosen for the following reasons. DPOPBr is a molecule capable of forming very stable Langmuir films (27) and is easily transferable onto a solid substrate yielding to LB films to a highly ordered degree. Besides, the cation DPOP + has been used as a counterion in TCNQ salts (28–31) with good conductivity values (although not arranged on LB films, but in a 3D salt). The lithium salt of TCNQ ·− was used because it 156 0021-9797/01 $35.00 Copyright C  2001 by Academic Press All rights of reproduction in any form reserved.