jeanflop/post_ocr_correction2 huggingface dataset:
Phys., 36 (2), 1965, p. 518-522. Laboratoire d'Électricité, Université "Al. I. Cuza", Bd. Copou 11, 6600, Iasi, Roumanie. Chimie physique/ Physical Chemistry (Chimie des surfaces/Surface Chemistry) Réalisation de films de Langmuir-Blodgett conducteurs : généralisation de la technique d'autodopage à partir de composés amphiphiles du tétracyanoquinodiméthane (TCNQ) Odile FICHET, Béatrice AGRICOLE, Jacques AMIELL, Vassilis GIONIS et Pierre DELHAES Résumé En vue d'obtenir des conducteurs organiques de structure bidimensionnelle, la technique d'autodopage est appliquée sur des films de Langmuir-Blodgett (L-B) d'une série de sels ioniques composés d'un même cation mais d'anions différents (I-, TCNQ' et ~TCNQF"). Les films L-B sont obtenus à partir d'une monocouche de Langmuir contenant un des sels et la molécule neutre C18H37 TCNQ. Leurs propriétés spectroscopiques et électroniques sont indépendantes de l'anion utilisé et le même matériau final est obtenu. Une réaction chimique, qui n'a lieu qu'à l'interface gaz-eau, conduit à des composés conducteurs formés de dimères à l'état de valence mixte de type (C18TCNQ)2 Ce type de matériau ne peut être obtenu par aucune autre technique connue aujourd'hui. Conducting Langmuir-Blodgett films: a generalized approach of the homodoping technique applied on amphiphilic tetracyanoquinodimethane (TCNQ) salts Abstract In order to obtain organic conductors with a two-dimensional structure, the homodoping technique is applied on L-B films of ionic salts with the same cation but with different anions (I -, TCNQ and TCNQF4). LB films are obtained from a Langmuir monolayer containing one of the salts and the neutral molecule C18 TCNQ. Their spectroscopic and electronic properties are independent of the anion used and the same final material is obtained. A chemical reaction taking place only at the gas-water interface gives conductor compounds formed of dimers in the mixed valence state ~(C18 TCNQ); for example. This kind of material cannot be obtained with any other known technique. Abridged English Version Conducting Langmuir-Blodgett (L-B) films with in-plane electronic transport properties is an attractive possibility to develop components for molecular electronic devices. The requirement of mixed valence state is a well established principle concerning the molecular conducting and superconducting systems. Different strategies have been elaborated to create new molecular associations, with a mixed
technique is applied on L-B films of ionic salts with the same cation but with different anions (I -, TCNQ and TCNQF4). LB films are obtained from a Langmuir monolayer containing one of the salts and the neutral molecule C18 TCNQ. Their spectroscopic and electronic properties are independent of the anion used and the same final material is obtained. A chemical reaction taking place only at the gas-water interface gives conductor compounds formed of dimers in the mixed valence state ~(C18 TCNQ); for example. This kind of material cannot be obtained with any other known technique. Abridged English Version Conducting Langmuir-Blodgett (L-B) films with in-plane electronic transport properties is an attractive possibility to develop components for molecular electronic devices. The requirement of mixed valence state is a well established principle concerning the molecular conducting and superconducting systems. Different strategies have been elaborated to create new molecular associations, with a mixed valence state in a layered system, which are based on the synthesis of amphiphilic Π-donor and acceptor derivatives. Among them, it appears that the homodoping process is a new approach to obtain a conducting L-B film with mixed valence clusters brought to a controlled ratio between fully ionic semi-amphiphilic (1:1 stoichiometry) salts and its amphiphilic neutral analogue [3]. Following these leads, we have carried out a series of experiments using a new series of an ionic semi-amphiphilic TCNQ (tetracyanoquinodimethane) salt that we have recently synthesized [6] looking specifically for the anion or radical anion influence. The experimental system, as presented on scheme 1, is the 4,5-bis (octadecylthio )-3-methylthio-1,2-dithiolium cation associated with iodide anion, TCNQ radical anion or its fluorinated analogue (TCNQF4) to form a salt. Mixtures of all three salt solutions with the neutral octadecyl TCNQ derivative solution (salt/C18TCNQ1 : n, n = 2 for this study) were prepared and L-B films of these materials were obtained following the standard procedure, i.e. solutions prepared in chloroform, spreading out on the Langmuir trough, ATEMETA 105 (Z-type films with a transfer ratio close to unity for 50 deposits). Note présentée par Adolphe PACAULT. 0764-4450/93/03160335 $ 2.00 0 Académie des Sciences A specific
stoichiometry) salts and its amphiphilic neutral analogue [3]. Following these leads, we have carried out a series of experiments using a new series of an ionic semi-amphiphilic TCNQ (tetracyanoquinodimethane) salt that we have recently synthesized [6] looking specifically for the anion or radical anion influence. The experimental system, as presented on scheme 1, is the 4,5-bis (octadecylthio )-3-methylthio-1,2-dithiolium cation associated with iodide anion, TCNQ radical anion or its fluorinated analogue (TCNQF4) to form a salt. Mixtures of all three salt solutions with the neutral octadecyl TCNQ derivative solution (salt/C18TCNQ1 : n, n = 2 for this study) were prepared and L-B films of these materials were obtained following the standard procedure, i.e. solutions prepared in chloroform, spreading out on the Langmuir trough, ATEMETA 105 (Z-type films with a transfer ratio close to unity for 50 deposits). Note présentée par Adolphe PACAULT. 0764-4450/93/03160335 $ 2.00 0 Académie des Sciences A specific molecular association appears, at the air-water interface only, giving rise to a mixed valence cluster which is, in principle, a dimer (C18 TCNQ) [7]. This result implies specific redox reactions which cannot occur in a homogeneous medium and therefore, need to be explained (TCNQF4 is a much stronger electron acceptor than TCNQ [12]). Finally, this generalization of the homodoping process opens new ways to create organized and conducting thin films. I. INTRODUCTION. — For the obtention of molecular crystals, conductors or even superconductors, two essential criteria must be satisfied: the presence of a mixed valence state and a delocalization associated with an overlap of π orbitals in at least one direction of space [1]. An alternative possible to the crystallogenesis of this type of materials is the employment of the technique of Langmuir-Blodgett (L-B) which imposes a molecular organization particular in the form of a monolayer at the gas-water interface. For this, it is necessary that the synthesized molecules present a double character, both electroactive (donor or acceptor of electrons) and amphiphilic (polar of intrinsic nature or ionic linked to the formation of a salt). Their repetitive transfer onto a solid substrate leads to an organized system in multilayers [2].
than TCNQ [12]). Finally, this generalization of the homodoping process opens new ways to create organized and conducting thin films. I. INTRODUCTION. — For the obtention of molecular crystals, conductors or even superconductors, two essential criteria must be satisfied: the presence of a mixed valence state and a delocalization associated with an overlap of π orbitals in at least one direction of space [1]. An alternative possible to the crystallogenesis of this type of materials is the employment of the technique of Langmuir-Blodgett (L-B) which imposes a molecular organization particular in the form of a monolayer at the gas-water interface. For this, it is necessary that the synthesized molecules present a double character, both electroactive (donor or acceptor of electrons) and amphiphilic (polar of intrinsic nature or ionic linked to the formation of a salt). Their repetitive transfer onto a solid substrate leads to an organized system in multilayers [2]. Within the framework of realizing conducting monolayers and multilayers, several strategies have been developed in recent years, the most promising being that of autodoping [3]. This approach permits, indeed, to create a mixed valence state thanks to the mixture, prealable to the formation of the molecular film, of a semi-amphiphilic salt of tetracyanoquinodimethane (TCNQ: R+) with a derivative amphiphile of the same molecule at the neutral state (octadecyltetracyanoquinodimethane, C18H37TCNQ) that we will call C18 TCNQ. This strategy has been revealed of a rather general character, independent of the cation used (R+), and leading to an appreciable electrical conductivity [4]. It has been shown, indeed, that the planar conductivity of such a monolayer could reach, at room temperature, 10-5 S. cm-1 [5] and grow by more than two orders of magnitude after the deposition of several layers. These interesting results have led to ask, on the one hand, what were the conditions for the formation of a mixed valence state, in general dimers, trimers or tetramers of TCNQ [1], and on the other hand, what was the role played by the diamagnetic or paramagnetic anion present in the starting salt? II. REALIZATION OF L-B FILMS. — We have applied the technique
salt of tetracyanoquinodimethane (TCNQ: R+) with a derivative amphiphile of the same molecule at the neutral state (octadecyltetracyanoquinodimethane, C18H37TCNQ) that we will call C18 TCNQ. This strategy has been revealed of a rather general character, independent of the cation used (R+), and leading to an appreciable electrical conductivity [4]. It has been shown, indeed, that the planar conductivity of such a monolayer could reach, at room temperature, 10-5 S. cm-1 [5] and grow by more than two orders of magnitude after the deposition of several layers. These interesting results have led to ask, on the one hand, what were the conditions for the formation of a mixed valence state, in general dimers, trimers or tetramers of TCNQ [1], and on the other hand, what was the role played by the diamagnetic or paramagnetic anion present in the starting salt? II. REALIZATION OF L-B FILMS. — We have applied the technique of autodoping to a series of ionic semi-amphiphilic salts based on the cation 4,5-bis(octadecylthio)-3-methylthio-1,2-dithiolium associated with an iodide, a TCNQ or its fluorinated homolog (TCNQF4) (see scheme) [6]. The salt of dithiolium and the molecule C18 TCNQ are dissolved separately in chloroform (c= 10-4 mole/l). Their mixture, in proportions variable characterized by their composition (1 : n) (see scheme), constitutes the spreading solution. We have already shown with the salt of TCNQ that the result obtained was independent of the chemical composition within the interval 1 < n < 3 [7]. For this study, we have chosen a constant value median n = 2 for the three salts presented. We propose to examine the L-B films obtained from an anion diamagnetic (I-) and from a radical anion (TCNQF4) possessing a stronger electron acceptor character than the TCNQ [1]. After spreading and evaporation of chloroform, the monolayers formed at the surface of water under an atmosphere of nitrogen, are homogeneous and thermodynamically stable; they present identical compression isotherms irrespective of the anion used [8]. The transfer pressure is fixed at 25 mN. m-1 with a transfer speed equal to 6 mm. min-1 onto hydrophilic substrates of calcium fluoride or glass. The
characterized by their composition (1 : n) (see scheme), constitutes the spreading solution. We have already shown with the salt of TCNQ that the result obtained was independent of the chemical composition within the interval 1 < n < 3 [7]. For this study, we have chosen a constant value median n = 2 for the three salts presented. We propose to examine the L-B films obtained from an anion diamagnetic (I-) and from a radical anion (TCNQF4) possessing a stronger electron acceptor character than the TCNQ [1]. After spreading and evaporation of chloroform, the monolayers formed at the surface of water under an atmosphere of nitrogen, are homogeneous and thermodynamically stable; they present identical compression isotherms irrespective of the anion used [8]. The transfer pressure is fixed at 25 mN. m-1 with a transfer speed equal to 6 mm. min-1 onto hydrophilic substrates of calcium fluoride or glass. The multilayers are essentially of type Z [6] with a good transfer ratio (t>0.8) at emersion. The different samples obtained after 50 cycles of transfer are green and of good optical quality. We have examined their vibrational, electronic and magnetic properties. III. SPECTRA OF ABSORPTION ELECTRONIC AND VIBRATIONNAL. — The spectra of absorption are recorded in the UV-visible and infrared domains. They are, for the three salts, identical, showing the presence of molecular associations partially ionic. In particular, in the infrared (fig. 1) these spectra clearly show the presence of a broad band centered around 3,000 cm-1 corresponding to a charge transfer band associated with a mixed valence state. At lower energies, a series of vibrational modes of the TCNQ (agv2, agv3, agvA, ag vs) normally inactive in infrared spectroscopy appear. They are the manifestation of an electron-vibration coupling Fig. 1. Absorption spectra of L-B films (1,2-dithiolium-R) (C18TCNQ)2 with R = I, TCNQ, TCNQF4 (note the change of the abscissa scale at 2,000 cm-1). Fig. 1. Absorption spectra of (1,2-dithiolium-R) (C18TCNQ)2 L-B films with R = I, TCNQ, TCNQF4 (note the change of the abscissa scale at 2,000 cm-1). molecular (e-mv) already observed in the crystals conductors and the films semi-amphiphilic
of absorption are recorded in the UV-visible and infrared domains. They are, for the three salts, identical, showing the presence of molecular associations partially ionic. In particular, in the infrared (fig. 1) these spectra clearly show the presence of a broad band centered around 3,000 cm-1 corresponding to a charge transfer band associated with a mixed valence state. At lower energies, a series of vibrational modes of the TCNQ (agv2, agv3, agvA, ag vs) normally inactive in infrared spectroscopy appear. They are the manifestation of an electron-vibration coupling Fig. 1. Absorption spectra of L-B films (1,2-dithiolium-R) (C18TCNQ)2 with R = I, TCNQ, TCNQF4 (note the change of the abscissa scale at 2,000 cm-1). Fig. 1. Absorption spectra of (1,2-dithiolium-R) (C18TCNQ)2 L-B films with R = I, TCNQ, TCNQF4 (note the change of the abscissa scale at 2,000 cm-1). molecular (e-mv) already observed in the crystals conductors and the films semi-amphiphilic of TCNQ doped with iodide [9]. The interpretation theory [7] from a model of linear coupling e-mv demonstrates the existence of dimers carrying a single charge. Moreover, the spectra of optical absorption are independent of the starting anion: thus it appears as probable that entities (C18 TCNQ)2 are present. This result is confirmed by the measurement of the electrical conductivity in continuous current which, for the three samples, gives a value at room temperature to a little constant, of the order of 5. 10-3 S. cm-1, value usual in the L-B films fabricated by the technique of autodoping [4]. IV. ELECTRONIC PARAMAGNETIC RESONANCE. — The three L-B films conductors already defined present, at room temperature, a line of resonance fine (△H= 1-2 Gs) situated at a value of the factor g equal to 2.0029 close to that of the free electron. Afin de mieux caractériser l'origine de ce signal paramagnétique obtenu avec un spectromètre classique fonctionnant en bande X, nous avons examiné son comportement en fonction de la température et de l'orientation du substrat dans la cavité de résonance pour les deux préparations issues des sels de TCNQ et de TCNQF4 (fig. 2). Les variations thermiques de la susceptibilité du
result is confirmed by the measurement of the electrical conductivity in continuous current which, for the three samples, gives a value at room temperature to a little constant, of the order of 5. 10-3 S. cm-1, value usual in the L-B films fabricated by the technique of autodoping [4]. IV. ELECTRONIC PARAMAGNETIC RESONANCE. — The three L-B films conductors already defined present, at room temperature, a line of resonance fine (△H= 1-2 Gs) situated at a value of the factor g equal to 2.0029 close to that of the free electron. Afin de mieux caractériser l'origine de ce signal paramagnétique obtenu avec un spectromètre classique fonctionnant en bande X, nous avons examiné son comportement en fonction de la température et de l'orientation du substrat dans la cavité de résonance pour les deux préparations issues des sels de TCNQ et de TCNQF4 (fig. 2). Les variations thermiques de la susceptibilité du spin, effectuées entre 300 et 4 K peuvent se décomposer comme la somme de deux termes, l'un constant, l'autre de type Curie (la constante de Curie calculée correspond à 5,7 de la valeur théorique pour un spin S = 1/2 par mole). En outre, les largeurs de raie présentent une variation spécifique en fonction de l'angle θ entre la normale au substrat et le champ magnétique de résonance H0 lorsque l'axe de rotation est situé dans le plan du substrat [10]. Pour les deux échantillons étudiés, un comportement identique est détecté, complètement différent de celui observé pour les films L-B des sels avant autodopage [8]. Un comportement particulier de la largeur de raie est observé à l'angle magique (θm = 55°) révélant la présence d'interactions dipolaires magnétiques. En effet, nous avons pu rendre Fig. 2. Variations de la largeur de raie RPE △H à T = 295 K en fonction de l'orientation θ du substrat par rapport au champ magnétique H0. (a) (1,2-dithiolium-TCNQ) (C18TCNQ)2; (b) (1,2-dithiolium-TCNQF4) (C18 TCNQ)2. Les lignes en traits pointillés représentent le meilleur accord entre les points expérimentaux et l'équation donnée sur cette figure. Fig. 2. Variations of the ESR linewidth △H at T = 295 K
variation spécifique en fonction de l'angle θ entre la normale au substrat et le champ magnétique de résonance H0 lorsque l'axe de rotation est situé dans le plan du substrat [10]. Pour les deux échantillons étudiés, un comportement identique est détecté, complètement différent de celui observé pour les films L-B des sels avant autodopage [8]. Un comportement particulier de la largeur de raie est observé à l'angle magique (θm = 55°) révélant la présence d'interactions dipolaires magnétiques. En effet, nous avons pu rendre Fig. 2. Variations de la largeur de raie RPE △H à T = 295 K en fonction de l'orientation θ du substrat par rapport au champ magnétique H0. (a) (1,2-dithiolium-TCNQ) (C18TCNQ)2; (b) (1,2-dithiolium-TCNQF4) (C18 TCNQ)2. Les lignes en traits pointillés représentent le meilleur accord entre les points expérimentaux et l'équation donnée sur cette figure. Fig. 2. Variations of the ESR linewidth △H at T = 295 K versus the orientation θ of the substrate in relation to the magnetic field H0. (a) (1,2-dithiolium-TCNQ) (C18TCNQ)2; (b) (1,2-dithiolium-TCNQF4) (C18TCNQ)2. The dashed lines represents the best fit between the experimental points and the equation given in the figure. compte de ces figures de rotation en utilisant l'expression indiquée sur les figures 2 a et b qui est caractéristique de systèmes magnétiques de type Heisenberg bidimensionnel [11]. Nous obtenons donc, quel que soit le radical anion du sel de départ, un système magnétique identique présentant une géométrie planaire associée à des interactions magnétiques inter-dimères. V. DISCUSSION ET CONCLUSION. — Pour compléter cette étude, nous avons également mené à bien une série de contre-expériences sur les solutions d'épandage. D'une part, nous avons vérifié par spectroscopie d'absorption dans le visible qu'aucune bande nouvelle pouvant être attribuée à un transfert de charge intermoléculaire n'apparaissait. D'autre part, nous avons détecté la présence d'un signal RPE inchangé dans le sel de TCNQF4 avec une raie intrinsèque plus large que dans les films autodopés (ΔH = 5,5 Gs) dont seule l'intensité diminue lorsque la concentration en C18 TCNQ augmente, ou bien son absence pour les mélanges d'iodure de 1,2-dithiolium avec C18 TCNQ qui demeurent diamagnétiques en solution.
figures 2 a et b qui est caractéristique de systèmes magnétiques de type Heisenberg bidimensionnel [11]. Nous obtenons donc, quel que soit le radical anion du sel de départ, un système magnétique identique présentant une géométrie planaire associée à des interactions magnétiques inter-dimères. V. DISCUSSION ET CONCLUSION. — Pour compléter cette étude, nous avons également mené à bien une série de contre-expériences sur les solutions d'épandage. D'une part, nous avons vérifié par spectroscopie d'absorption dans le visible qu'aucune bande nouvelle pouvant être attribuée à un transfert de charge intermoléculaire n'apparaissait. D'autre part, nous avons détecté la présence d'un signal RPE inchangé dans le sel de TCNQF4 avec une raie intrinsèque plus large que dans les films autodopés (ΔH = 5,5 Gs) dont seule l'intensité diminue lorsque la concentration en C18 TCNQ augmente, ou bien son absence pour les mélanges d'iodure de 1,2-dithiolium avec C18 TCNQ qui demeurent diamagnétiques en solution. Ainsi, avons-nous montré que les réactions chimiques conduisant à ces films L-B ne se passaient pas en solution homogène mais sûrement à l'interface azote-eau. Si nous examinons les valeurs des potentiels redox correspondant à la première vague de réduction dans une phase organique et homogène [12] aucun transfert d'électron ne peut avoir lieu entre le TCNF4 et le C18 TCNQ à l'état neutre. Il y a donc un processus redox spécifique à la surface de l'eau qui conduit à la formation de dimères à état de valence mixte de type (C18 TCNQ)2 indépendants de l'anion utilisé qui ne se trouverait plus dans les couches mais apparemment serait en partie solubilisé dans la phase aqueuse : dans ce cas, les protons pourraient jouer un rôle essentiel. Une autre possibilité à considérer est la préformation de dimère (C18TCNQ)2 qui provoquerait un abaissement du potentiel redox par rapport à la valeur initiale observée sur le monomère; néanmoins, à notre connaissance, il n'y a pas d'évidence spectroscopique concernant la formation de tels dimères. Ce résultat surprenant ouvre la voie à de nouvelles organisations supramoléculaires qui, a priori, ne peuvent pas être réalisées par des méthodes conventionnelles de cristallogenèse. Enfin, la compréhension de ces mécanismes