Electrochemistry of organometallic lyotropic chromonic liquid crystals

Electrochemistry Communications 19 (2012) 50–54

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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

a r t i c l e

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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

a b s t r a c t 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 intermolecular stacking energy on the order of 10 kBT) at high monomer concentrations in water, to afford a nematic (N) phase (aggregates exhibit 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 tetrasulfonatedphthalocyanine motif [6–10]. We observe that the general alignment of the phase relative to that of the electrode surface can be determined ⁎ Corresponding author. E-mail address: [email protected] (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

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 system (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 Kα radiation, graphite monochromator, λ=1.54 Å, 130–300 mm detector-sample distance, with exposure time of 30 min). The samples were heated (between 297 and 355 K) in the presence of a magnetic field using a home-built capillary furnace.

J.E. Halls et al. / Electrochemistry Communications 19 (2012) 50–54

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Fig. 1. (a) Schematic illustration of the N and M phases of chromonic liquid crystals based on tetrasulfonated phthalocyanines. Note that the suggestion that all aggregate stacks have D4h symmetry has not been verified experimentally; some aggregates may have defects. Also illustrated is the general structural motif of individual mesogens (M2+ = Ni2+ or Cu2+). (b) Optical polarising microscope image (crossed-polarisers, magnification ×100) of the N-phase (left) of 0.26 M nickel(II) phthalocyanine tetrasulfonic acid tetrasodium salt in water at pH 8 (made up with sodium hydroxide), and the M-phase (right) of 0.88 M copper(II) phthalocyanine tetrasulfonic acid tetrasodium salt in water under ambient temperature. (c) X-ray scattering patterns obtained from the samples in (b): N-phase (left) and M-phase (right). The primary beam is shown at 2θ= 0.4°. Traces illustrate the change from low to high temperatures.

Electrochemical experiments were undertaken using a μAutolab Type III potentiostat, employing a silver/silver chloride reference electrode (BAS), a nickel spiral counter electrode, and a glassy carbon or platinum working electrode (of diameter 3.0 mm, 11.0 μm or 10.0 μm, BAS). Samples were degassed with argon only for reductive electrochemistry, and working electrode was cleaned and polished before every experiment, so that a clean surface was exposed to different locations of the sample for every change in experimental variable. Chronoamperometric transients were averaged over at least two runs prior to data analysis. 3. Results and discussion We characterise two chromonic phases of copper(II)- or nickel(II)phthalocyanine tetrasulfonic acid tetrasodium salt.

3.1. Structural characterisation Preliminary experiments in preparing aqueous-based lyotropic chromonic liquid crystals of the above revealed the presence of the N-phase for the nickel(II) complex only at 20 wt.% (0.26 M) provided the aqueous component contained sodium hydroxide at pH 8 (Fig. 1b illustrates the observed Schlieren textures under crossed-polarisers); less concentrated solutions formed an isotropic phase — the systems appeared dark when viewed through crossed-polarisers, whilst more concentrated solutions exhibited birefringence, but appeared to be crystal suspensions on closer inspection. The presence of oxygen within the system was not found to be critical for N phase formation. Birefringent samples for the copper(II) system only occurred for samples >27 wt.% (0.38 M), with the M phase occurring at 46 wt.% (0.88 M, Fig. 1b), a system visibly more viscous than the N-phase

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Ni(II) one. For both phases, it was observed that evaporation of solvent was rapid and difficult to control, when the system existed in thin volumes. The one-dimensional resistivity of both N and M phases was observed to be comparable with 0.1 M aqueous KCl (Table 1), indicating partial ionisation of the stacks. UV–visible spectrophotometry of the dark blue nematic phases indicated systems of very high optical absorbance, as expected for dye aggregates [11]; the use of dilute solutions in the range −6 ≤ lg(c0/ M) ≤ −4.3 revealed extinction coefficients of 46,970 M− 1 cm− 1 (λmax = 623 nm) for the Ni(II) system, with 25,236 M− 1 cm− 1 (λmax = 613 nm) for the Cu(II) system. X-ray scattering measurements (Table 1) using Cu Kα radiation (1.54 Å) of the nickel system revealed several features (Fig. 1c), as anticipated from literature measurements with similar systems [6–10], suggesting the presence of the α-polymorph within the chromonic system [7]. Following the Bragg equation, we suggest that the wideangle features represents the intramolecular macrocyle separation within aggregates (δ) is 3.4 Å, a value close to the packing distance observed within the solid state [6,8], with the small-angle data implying an aggregation correlation length of 43.5 Å (Ni) or 30.7 Å (Cu), suggesting that, on average, there are 13 (Ni) or 9 (Cu) molecules-per-aggregate, and with the stacks being of single molecule widths of 22.1–21.7 Å, in agreement with that estimated for the diameter of a single phthalocyanine molecule using ChemDraw 12.0. The temperature-independence of these data (Fig. 1c) indicates that the system remains within the chromonic liquid crystal phase over the range 298–325 K (Ni) or 297–355 K (Cu). For the M-phase, up to six orders of the hexagonal pattern are seen (Table 1), although the higher orders are very weak, and diffuse.

exhibits a single oxidation wave, as also observed in dilute aqueous solution [12,14], also thought to originate from ligand oxidation in a complex, multi-electron process. In both systems, the voltammetry is consistent with quasi-reversible electrode kinetics, and of a diffusioncontrolled nature (the peak currents are directly proportional to the square-root of the scan rate, see insets in Fig. 2a). These data are entirely consistent with aggregate diffusion or charge transport between aggregate stacks, given that copper(II) phthalocyanine itself is considered to be a prototypical organic semiconductor [14], so that intra-aggregate charge transport should be very fast. Reduction of the M phase at a Pt microelectrode afforded waves, which are not at steady-state (Fig. 2a), but are consistent with a two-electron process with following slow, pffiffiffiffiffiffiffiffiffiffiffi irreversible chemical reaction [12], with Dr Dz ∼10−13 m2 s−1 [15]. In order to assess the occurrence of diffusion anisotropy within these ordered, ionic aggregates, microdisc potential-step chronoamperometry was undertaken, potentiostating the electrode after the two-electron waves. Following previous work on the electrochemical characterisation of anisotropic diffusion [15], the resulting current– time (i–t) transients were fitted to the adimensional expression, qffiffi 0:7824 pffi ipffiffiffiffiffiffiffiffi = τ, ψ¼ where n = 2, ¼ 0:7854 þ 12 πτ þ 0:214e− 4nFr 0 c0

Dr Dz

F=96484.6 C mol− 1, r0 is the microelectrode radius, c0 is the effective homogeneous concentration of the redox system, with diffusion coefficients in the directions normal (Dz) and tangential (Dr) to the electrode surface, and for a dimensionless time variable τ ¼ 4 Drr2t . The fitting pro0

cedure employed involved iterative optimisation of Dr and Dz (in the range −17≤lg(Di/m2 s− 1)≤−8, with ca. 10% uncertainty) through comparison of experimental data with that given above over the whole normalised temporal domain (of dummy variable s), subject to jψexpt −ψtheory j 1 a difference-minimised parameter, P ¼ ∑s ∑ . The results, ψexpt s

3.2. Electrochemical characterisation Fig. 2a illustrates voltammograms corresponding to the oxidation of the two systems. For the Ni(II) system, two one-electron waves are observed, as in dilute solutions [12,13], and which are attributed to phthalocyanine ligand-based oxidations. In contrast, the Cu(II) system

in reduced space, are illustrated in Fig. 2b, where it is evident that both systems fit reasonably well over the whole temporal domain with, for the Ni(II) N-phase Dr = 3.8 ± 0.4 × 10 − 8 cm 2 s − 1 and Dz = 2.2 ± 1.5 × 10 − 9 cm 2 s − 1, whereas the Cu(II) M-phase appears to be more ordered, with Dr = 1.2 ± 0.9 × 10 − 8 cm 2 s − 1 and Dz = 4.4 ± 1.7 × 10 − 11 cm 2 s − 1. In noting that both systems afford

Table 1 Physical characteristics of the N and M lyotropic chromonic liquid crystals prepared. Phase

ρa/Ω cm

Rb/Ω

Cdc/mF cm− 2

X-ray scattering data 2θ/°

dd/Å

Assignment qe/Å− 1

f

q/q0

N Nickel(II) Phthalocyanine Tetrasulfonic acid Tetrasodium salt 0.26 M in H2O pH 8 M Copper(II) Phthalocyanine Tetrasulfonic acid Tetrasodium salt 0.88 M in H2O

12.70

5773

12.3

2.03 3.99 26.5

43.5 22.1 3.36

Aggregate length Aggregate width Aggregate spacing

9.17

4585

2.1

2.88 4.07 6.81 10.5 13.0 15.6 21.2 24.4 26.3

30.7 21.7 13.0 8.45 6.78 5.68 4.18 3.64 3.39

Aggregate length Aggregate width q0pffiffiffi q0 3 2qp 0 ffiffiffi q0 7 3q0pffiffiffi 2q0 3 Aggregate spacing

Isotropic solution 0.1 M aqueous KCl

11.29

a b c d e f

0.485 0.744 0.926 1.11 1.50 1.73

Resistivity measured at 293 ± 1 K. Resistance determined using R ¼ 4rρ0 with r0 = 5.5 μm (N-phase) or 5.0 μm (M-phase). Specific double-layer capacitance inferred from cyclic voltammetry. Fundamental crystal spacing determined using d=A ¼ 21:54 sinθ. Scattering vector estimated through q ¼ 2π . d q0 is the fundamental repeat distance in the hexagonal system (viz. the centre-to-centre separation between [cylindrical] aggregates).

1.00 1.53 1.91 2.28 3.10 3.56

J.E. Halls et al. / Electrochemistry Communications 19 (2012) 50–54

larger diffusion coefficients in the direction radial to the electrode surface compared with that in the perpendicular direction, with anisotropic ratios of DDrz e20 or 300 for the Ni(II) or Cu(II) system, respectively, we suggest that the presence of the electrical field at the electrode orients the stacks so that the peripherial functionalities are located close to the electrode, viz. the stacks adopt an homogenous alignment in the electrochemical system, so that in both cases Dr represents transport along the stacks (on the same order of magnitude in both phases), limited by thermal basculation of stacks in this direction, and Dz indicates that lateral transport between columnar aggregates is important, with the smaller value for the M-phase indicating worse lateral aggregate electronic couplings, as anticipated. In noting that aggregate distances are at least on the order of q0, a Dahms–Ruff view suggests bimolecular electron hopping kinetics between aggregates in the direction perpendicular to the stack ordering

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of 106 M− 1 s− 1 (Ni) or 6×103 M− 1 s− 1 (Cu), allowing for the inference of Einstein mobilities in this direction of 2×10− 7 cm2 s− 1 V− 1 (Ni) or 3×10− 9 cm2 s− 1 V− 1 (Cu). The lower values of D here compared with dilute aqueous solution are consistent with the greater viscosities of these highly concentrated redox liquid crystals. We suggest the reason that the diffusion model employed above (which assumes slow follow-on homogeneous kinetics) overestimates the current at short times is either because of adsorption, or due to an electro-induced orientation effect— the experimental data only match-up with the model after τ ~ 0.2 (Ni) or 0.5 (Cu), and are not attributable to Ohmic losses (see Table 1) nor large time constants: we estimate these as being ~70 μs (Ni) or 10 μs (Cu). Such effects may stem from the degree of gegen-ion condensation onto the stacks, with the

Fig. 2. (a) Cyclic voltammograms (v = 0.1 V s− 1, five consecutive cycles) corresponding to the oxidation of (i) the Ni(II) sample, and (ii, iii) the Cu(II) sample in Fig. 1, at (i, ii) a glassy carbon electrode (3.0 mm diameter) or (iii) 10.0 μm diameter platinum microdisc electrode. The arrows indicate the direction of the initial sweep. The insets in (i) and (ii) correspond to the variation of the peak oxidative potential and the peak oxidative current of the first cycle with experimental timescale, with circles corresponding to the first wave, and squares for the second (see text); open symbols refer to oxidative processes, filled symbols refer to reductive signals.(b) Reduced space chronoamperometric transients corresponding to (i) the two-electron oxidation of the Ni(II) sample in (a)(i), with the 11.0 μm diameter glassy carbon microelectrode held at 1.06 V vs. Ag/AgCl/Cl−, and (ii) the two-electron reduction of the Cu(II) sample in (a) (iii), with the 10.0 μm diameter Pt microelectrode held at − 0.7 V vs. Ag/AgCl/Cl-,. In both cases the solid line represents the theoretical fit with the open circles corresponding to the experimental data (see text).

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Cu(II) system being less ionised than the Ni(II) aggregates (as suggested by the lower double-layer capacitance despite the higher mesogen concentration). Such effects are known, and have been modelled recently for the sunset yellow FCF chromonic liquid crystal [16]. Furthermore, the degree of ionisation may be envisaged to aid charge transport within the columns in providing localised energy minima through counter-ion availability [17]. 4. Conclusions Self-assembling chromonic liquid crystals within the nematic phase allow routes for diffusion (physical transport or through carrier hopping) to occur in at least two dimensions. Mobilities are likely affected by the size of the gaps between aggegates, mesogens stacks and stack defects. It is not clear whether the nature of the metal centre is significant compared with the order of the structured liquid nanosystem; the archetypal molecular semiconductor, copper(II) phthalocyanine is known [14] to have a complex electronic structure caused by overlap of the metal 3 d levels with the ligand 2p orbitals, leading to both localised and delocalised states at the Fermi level, which can be distorted by the presence of dioxygen. The exact role played by peripheral group dissociation has not been unravelled; the extent of this may be controlled by the applied electric field, encouraging phase alignment and migrative structuring of the phase, in a manner similar to that proposed for room temperature ionic liquids [18], or through local changes in proton concentration. Acknowledgements This work has been financed through EPSRC (grant number EP/ G020833/1). RWB thanks University of Hull 80th Anniversary Fund for a PhD studentship; TR expresses gratitude to The Leverhulme

Trust for a Visiting Fellowship; YZ acknowledges partial funding from Hull University. MGT and GHM thank EU for funding in the context of the FP7 project “BIND”.

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