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We have carried out a crystallographic study of the spin-Peierls distorted phase (T, =17. 4 K) ofNeutrondiffraction data~ere collected at 6 K from a deuterated sample. At the transition the unit cell axis along the dimerized TCNQ stack is doubled, hence giving rise to tetramers. Refinement of rigid molecules converged to 8=0.162 for 340 super-reflections. The structural change consists of a transverse shift 6v =0.17L + 0.10M of every second dimer in the stack. Calculations of transfer integrals confirmed the significance of the distortion.

A Commentary on the paper ''Homologous series of liquid-crystalline metal free and copper octa-n-alkoxyphthalocyanines'', by . Our colleagues wanted to study the deformation modes in liquid-crystalline phases of disclike mesogens and approached us for the compounds. We had never been involved with mesogenic compounds and decided, after a literature search, to focus on the synthesis of mesogenic triphenylenes and truxenes. We discovered, by chance, a new truxene derivative that displayed an interesting physical property, namely, a re-entrant isotropic phase [1]. Such behaviour had been observed before, but only in mixtures of liquid-crystalline compounds and not in a pure mesogen. Experimental studies revealed that for the nematic discotic (N D ) phases of our truxenes the bend mode was the easiest deformation mode as had been predicted theoretically. Our physicist colleagues were quite happy with the results obtained and for us these studies opened a completely new field. We decided to embark on some general studies of liquid-crystalline compounds, but also to focus specifically on electron transport in stacks of discotic mesogenic molecules, which had not been studied before. We carried out some initial experiments on the triphenylene derivatives [2] we had synthesized, and then looked for other disc-shaped compounds, in particular discs that could bind metal centres. We hoped that these would be better conductors than the triphenylenes, which turned out to be rather poor conducting materials. This is how we stumbled upon the liquid-crystalline phthalocyanines. As is described in our 1989 Liquid Crystals paper and in a paper by the group of Simon in Paris [3], these compounds form well-behaved liquid-crystalline phases, which, according to our complex impedance spectroscopy measurements, were semi-conducting (s510 23 -10 28 S/m depending on whether or not they were doped) with a conductivity that was somewhat lower in the mesogenic phase than in the crystalline phase. In subsequent studies we also synthesized and investigated different types of polymerized liquid-crystalline phthalocyanines and studied their physical properties including electrical properties [4]. Having established that liquid-crystalline phthalocyanines and their polymerized counterparts were electron conducting we looked to set up a collaboration with experts on electrical measurements. This brought us in contact with Drs. John Warman and Mathijs de Haas of Delft University of Technology. John and Mathijs had developed a very useful Time Resolved Microwave Conductivity Technique, which was subsequently used to study, in great detail, the electrical properties of our liquid-crystalline phthalocyanines. The properties studied included the hopping time of charges in the stacks and the recombination of charges (free electrons and electrons residing on separate stacks) as a function of the distance between the stacks. The latter could be controlled by varying the length of the hydrocarbon chains attached to the phthalocyanine discs. A paper summar...

Résumé -Les intégralesde transfert sont de grande importance pour la compréhension des propriétés physiques des sels de TCNQ, particulièrement parce que leurs structures cristallographiques varient considérablement. Cette note donne une méthode sûre pour leur calcul.Abstract -Transfer integrals are of prime importance in understanding the physical properties of TCNQ-salts, particularly since the crystal structures vary widely. This paper gives a reliable method of their calculation. IntroductionFor a fairly long time it has been known that the properties of TCNQ-salts vary widely. Depending on whether one considers 1:1 or 1:2 salts, insulators and metals have been found, 4kF and 2kF phase transitions occur and many different crystal structures have been determined even of chemically very similar compounds. As a case in point we may take the derivatives of R1R2-morpholinium-(TCNQ)2, where R1 and R2 may be varied (note elsewhere in these proceedings the papers of De Boer et al. and Oostra et al. for a review of crystal structures and physical properties.

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