Nature of the mesophase of a conducting charge transfer complex : neither discotic nor calamitic

2014 We describe the structure of the mesophase of a conducting charge transfer complex formed by a donor substituted by four long paraffinic chains and TCNQ as acceptor. In the mesophase the donor cores remain in stacks with a short core-to-core distance (3.6 A). The presence of paraffinic chains favours a layer structure, the stacks being localized in the lamellae and TCNQ likely filling the space between stacks. Therefore the organization in this mesophase is intermediate between those of smectic and columnar discotic liquid crystals. J. Physique LETTRES 44 (1983) L-823 L-828 ler OCTOBRE 1983, 1 Classification Physics Abstracts 61.30E A new class of liquid crystals formed by disc-like molecules, (the discotic), was discovered [1] some years ago. Disc-like molecules in which a flat aromatic core is surrounded by paraffinic chains can display a mesomorphic state with a columnar structure. The aromatic moieties are stacked more or less regularly in the central part of the column. The paraffinic chains are in a fluid state and fill the external part of each column in such a way that the columns are parallel and form a two-dimensional crystal without any positional correlation between columns along a direction parallel to their axis. Such a columnar organization presents some similarities with the segregated stacking of ID organic conductors. Therefore it was tempting to synthetize organic conductors in which one (or two) conducting chains have a discogen character. The introduction of long chain substituents induces mesogenic properties in a few precursors of organic conductors : di-substituted tetrathiofulvalene [2, 3] or metal complexes dithiene [4] show smectic or nematic liquid crystalline phases without any columnar organization. A hexagonal columnar mesophase has been identified in a (*) Laboratoire associe au CNRS. L-824 JOURNAL DE PHYSIQUE LETTRES metallophtalocyanine substituted by eight aliphatic chains, each rigid part being surrounded by a large paraffinic zone [5]. The synthesis of tetra substituted cores may also be an interesting approach for this problem and we prepared disk-like n donors obtained by introducing long chain substituents R into the phenyl rings of the heterocyclic system [6, 7]. For example, in (1) with R = CnH2n+1’ X = 0 the high temperature phase seems to be a 2D crystal in which successive layers are uncorrelated [8]. The existence of columns of (1) in each lamella is not excluded; in such a case we will have stacks of molecules which are not fully surrounded by a paraffinic region. Compounds (1) give stable and isolable complexes with TCNQ. The electrical behaviour of the obtained complexes depends on the nature of the hetero atom X (oxygen or sulphur, [9, 10]). Namely we obtained a conducting TCNQ complex ((J II = 0.7 (Q cm) -1 ) at room temperature in the following case : The phase transformations of this complex were first studied by DTA experiments. The first heating curve (Fig. 1) for the complex is characteristic of a mesogen compound showing a unique mesophase : the enthalpy of the first transformation at ~ 123 ~C (melting) is larger than the enthalpy corresponding to the clearing point at 154oC. On cooling, the peaks appear very diffuse; moreover the DTA curves obtained with a further heating show that the sample has not recovered its previous crystalline state. EPR measurements [10] show a discontinuity of the magnetic susceptibility at the melting point. Texture observation of the complex in a transmission light polarizing microscope is not conclusive since the compound is opaque even in a thin layer. In order to characterize the mesophase, we have performed X-ray diffraction studies with technics usually employed for the observation of mesophases. 1. X-ray diffraction experiments. The sample is put in a Lindeman glass capillary of 1.5 mm diameter. An oven maintains the temperature of the sample constant within 1 ~C. The X-ray CuKa monochromatic beam is reflected by a double bent pyrolitic graphite and a cylindrical film at 60 mm from the sample collects the diffracted radiations. The sample and film holders are surrounded by a shield which can be evacuated and which is put between the two conic poles of an electromagnet The magnetic L-825 A LIQUID CRYSTALLINE CONDUCTING COMPLEX Fig. 1. Variation of the DTA signal versus temperature in degrees C during the first thermal cycle. The heating and cooling curves (2~/min. rate) are pointed out by arrows. We must notice that the temperature of the first DTA peak obtained by heating can fluctuate between 120 and 127 °C depending on the sample. field is perpendicular to the X-ray beam and to the capillary tube. Its maximum value at the sample level is of 1.8 T. Oriented diffraction patterns are obtained when the sample is cooled from the isotropic liquid under a magnetic field of 1.7 T (Fig. 2). Along the field direction one can see equally spaced Bragg spots (3 orders of reflection). In a direction perpendicular to the magnetic field one can find three diffuse maxima. When we suppress the magnetic field and make a n/2 turn of the sample around the capillary axis the Bragg spot disappears and the diffuse spots become three circular rings. This picture has similarity both with the diffraction patterns of smectic and discotic mesophases : the two outer diffuse lines are similar to the two outer diffuse lines of the pattern of some Fig. 2. X-ray diffraction pattern of an aligned sample of the complex in the mesophase temperature