Electrochemistry of organometallic lyotropic chromonic liquid crystals Jonathan E. Halls 1 , Richard W. Bourne, Kevin J. Wright, Lee I. Partington, M. Gabriela Tamba, Yan Zhou, Thippeswamy Ramakrishnappa, Georg H. Mehl, Stephen M. Kelly, Jay D. Wadhawan ⁎ Department of Chemistry, The University of Hull, Cottingham Road, Kingston-upon-Hull HU6 7RX, United Kingdom abstract article info Article history: Received 15 December 2011 Received in revised form 12 February 2012 Accepted 22 February 2012 Available online 3 March 2012 Keywords: Liquid metal–organic frameworks Anisotropic diffusion Chronoamperometry Two aqueous lyotropic chromonic liquid crystals made from nickel(II) or copper(II) phthalocyananine tetrasulfonic acid tetrasodium salt are prepared with characterisation through optical polarising microscopy, X-ray diffraction and conductivity. These consist of ordered molecular aggregates of ~ 10 molecules, held together by π–π stacking. Electrochemistry within these dynamic, optically anisotropic columnar systems reveals that diffusion (physical transport or charge carrier hopping) can occur within two-dimensions. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The development of lightweight, self-assembling, self-“healing” and flexible molecular wires, over which long-range electron transport may occur, is currently of interest [1] since these empower, inter alia, fast and efficient communications [2], “hi-tech” redox-based security systems [3], and, ambitiously, towards redox-controlled logic for molecular computers [4] — systems that can be moulded into the geometries/ volumes needed for the pragmatic and ergonomic technologies currently revolutionising modern lifestyles. Chromonic lyotropic liquid crystals [5] based on transition metal phthalocyanines [6–10] represent an interesting class of metal–organic liquid nanomaterials which may provide a framework for long-range electron transport for technological exploitation. These systems (Fig. 1a), which neither exhibit a Kraft point nor cmc, autoassemble through approximately isodesmic π-stacked H-aggregates (with inter- molecular stacking energy on the order of 10 k B T) at high monomer concentrations in water, to afford a nematic (N) phase (aggregates ex- hibit orientational order), or, at higher concentrations, an hexagonal (M) phase (aggregates possess orientational and positional order). Aggregates within these systems are generally considered to be single molecular columnar stacks, so that charge transport within aggregates has been perceived as occurring merely within one dimension [1,2]. In this communication, we report our studies into the electrochemistry of two chromonic liquid crystals based on a tetrasulfonated- phthalocyanine motif [6–10]. We observe that the general alignment of the phase relative to that of the electrode surface can be determined through the anisotropy of diffusive mass transport — we stress the importance of lateral diffusion in these systems, in contrast to current approaches to charge carrier mobility measurements in columnar systems. 2. Experimental All chemical reagents were purchased from Sigma-Aldrich in the purest commercially-available grade and used as received. Water, with a resistivity of not less than 18 M Ω cm, was taken from an Elgastat sys- tem (Vivendi). Nitrogen and argon were obtained from BOC Gases, UK. Concentrated solutions and chromonic liquid crystals were prepared by mixing the required mass of tetrasulfonated-phthalocyanine with a water/aqueous solution in the appropriate wt.% ratio in screw-capped vials, followed by heating with stirring to approximately 345 K for between 30 and 60 min, thereby achieving sample homogenisation. The samples were then allowed to cool to ambient temperature (296±2 K) prior to further experimentation at this temperature. Solution resistivity was measured using a CDM210 conductivity meter equipped with a four-pole CDC511T conductivity cell (Radiometer) inserted vertically into the sample. All samples were examined using an Olympus BX-51 optical polarising microscope, equipped with a digital camera for image capture. Ultraviolet–visible spectrophotometry was undertaken using a Perkin–Elmer Lambda-25-Scan-UV–VIS instrument, using a quartz cell of 1.0 cm path length. X-ray scattering measurements were undertaken through filling capillary tubes with the viscous sample, placed into a MAR345 diffractometer with a 2D image plate detector (Cu K α radiation, graphite monochromator, λ = 1.54 Å, 130–300 mm detector-sample distance, with exposure time of 30 min). The samples were heated (be- tween 297 and 355 K) in the presence of a magnetic field using a home-built capillary furnace. Electrochemistry Communications 19 (2012) 50–54 ⁎ Corresponding author. E-mail address: j.wadhawan@hull.ac.uk (J.D. Wadhawan). 1 Present address: Department of Chemistry, The University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom. 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.02.032 Contents lists available at SciVerse ScienceDirect Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom