Chromonic mesophases

Current Opinion in Colloid and Interface Science 8 (2004) 480–490

Chromonic mesophases John Lydon University of Leeds, The School of Biochemistry and Molecular Biology, Leeds LS2 9JT, UK

Abstract Over the last 10 years, there has been a growing acceptance of the concept of chromonic phases and a wider recognition that they form a well-defined family of lyotropic liquid crystalline phases, with a package of properties distinct in almost every aspect, from those of conventional amphiphiles. New chromonogenic compounds have appeared and new technological uses for chromonic systems are being actively explored. Recent promising investigations include the synthesis of a chromonic dye, C.I. Direct Blue 67, which has an N phase of high order parameter and which can be dried down to give well-oriented films of solid. When dried down on a ‘command surface’ of photoaligned substrate this can produce a highly patterned film. The use of chromonic phases in the construction of compensating plates for improving the viewing characteristics of twisted nematic displays has been explored. Although this technology may not be suitable for commercially exploitation in its present form, the success of the devices is significant. It is suggested that current studies of the way in which the temperature range of thermotropic discotic mesophases is enhanced in 1:1 CPI mixtures may well lead to improved formulations for chromonic dyes. It is predicted that the marriage of chromonic phase technology with current biochemical analytical techniques will give rise to a new generation of medical diagnostic tests. 䊚 2004 Elsevier Ltd. All rights reserved.

1. Introduction About two decades ago it was becoming clear that there is an extensive and well-defined family of lyotropic mesogens with properties distinct from those of conventional amphiphiles. This family consists of various drugs, dyes, nucleic acids, antibiotics, carcinogens and anticancer agents. In almost every respect the properties are different to those of ordinary lyotropic mesogens of the soapydeterergentyphosphilipid type. The molecules have aromatic rather than aliphatic structures. They are rigid rather than flexible and planar disc-like or planklike, rather than rod-like. The hydrophilic solublising groups are disposed around the periphery of the molecules rather than at one end. The molecules aggregate in solution, not into micelles, but into columns. They have distinctive optical textures (involving characteristic types of paramorphosis). Thermodynamic measurements indicate that the driving force for the aggregation is enthalpic rather than entropic. Their phase diagrams tend to be of the peritectic rather than eutectic type. They do not show cmc’s or Krafft points. These are the chromonic mesophases E-mail address: [email protected] (J. Lydon).

This review is intended to be read as an update of two previous papers. The first w1●● x, in The Handbook of Liquid Crystals, gives details of the prehistory of chromonic phases, the patterns of molecular aggregation in N an M phases and NyM phase diagrams and includes a description and analysis of characteristic optical textures. The second w2●●x, in this journal 5 years ago, described the newer brickwall and chimney types of aggregation encountered in some chromonic dyeywater systems and discussed the extension of the Israelachvili approach from lyotropic amphiphile systems to chromonics (Figs. 1 and 2). The recent developments discussed in this review fall into three categories. The first concerns the use of an optical alignment technique to produce detailed patterns of alignment in dried down films of dyes. The second concerns the experimental use of chromonic phases in compensating plates to improve the performance of the standard twisted nematic display devices. The third concerns CPI mixtures—a theme of current interest in thermotropic discotic systems, which I predict will become of importance in the future technological exploitation of chromonic systems.

1359-0294/04/$ - see front matter 䊚 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cocis.2004.01.006

J. Lydon / Current Opinion in Colloid and Interface Science 8 (2004) 480–490

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Fig. 1. The Structures of Chromonic N and M Phases. Chromonic mesogens can be regarded as being insoluble in one dimension. They tend to aggregate face-to-face, producing a variety of stacked structures. The classic chromonic phases are the nematic, N phase and the hexagonal, M phase. In both of these, the molecules are stacked in columns. In the N phase, these lie in a nematic array (i.e. the columns are more or less parallel, but there is no positional order and there is no orientational order of the columns about their long axes). In the M phase, the columns lie on a lattice with statistical hexagonal symmetry and have long-range order. Other chromonic structures have been identified by Tiddy et al. and Harrison et al. where the molecules are aggregated into brickwork patterns or cylindrical chimneys w2,18,19x.

2. Outline of chromonic phase structure and properties The name chromonic was derived from the bischromone structure of the widely marketed anti-asthmatic known in the UK as INTAL and in the US as Chromolyn w3–10x—by no means the first mesogen of this type to be reported—but one of the most extensively studied. It was considered (by its creator at least) to be a particularly good name because of the fortuitous combination of connotations of the word, with both colour (with reference to dyestuffs) and with chromosomes (with reference to nucleic acids). Chromonic mesophases are the lyotropic counterparts of the discotic mesophases-and there are some parallels in their history. In both cases, the definition of the concept came far later than one would have expected. The so-called ‘carbonaceous phases’ had been known and characterised by the coking industry for decades and there were predictions of ‘negative nematic’ liquid crystalline phases years before Chandrasekhar’s classic work. Similarly, references to the aggregation of dye molecules, stacking like piles of pennies or packs of cards were scattered in the dye chemistry literature for almost a century. Terms such as H- and J-aggregates were widely used in the industry—but there was scarcely ever a mention of liquid crystalline properties. In many aspects, chromonic systems are closer to thermotropic systems than to conventional amphiphiles. In both cases, the driving force causing liquid crystalline phase formation is the face-to-face aggregation of molecules forming columns—and the geometrical aspects of the packing of these columns are more or less the same in the two cases—the difference being that in one case the columns lie in a sea of alkyl chains and in the

other, they lie in a sea of water. I had expected that by now, chromonic counterparts of all of the thermotropic discotic phases would have been found. Bearing in mind the extensive literature of the dye industry concerning tilted stacks of dye molecules (J-aggregates), I find it surprising that there are, as yet, no well-authenticated tilted chromonic systems. In general, there is a strong tendency for chromonic molecules to aggregate into columns, even in very dilute solution—just as conventional lyotropic mesogens form micelles before a mesophase is formed w11,12x. Although there may be a threshold concentration before significant aggregation begins to occur, there is no specific optimum column length and, therefore no analogue of the critical micelle concentration (cmc) of

Fig. 2. A snapshot plan view of the chromonic M phase. The array shown in this sketch has orthorhombic symmetry – but the rotational disorder of the columns between the three possible orientations (as indicated for the central column) leads to the phase having overall hexagonal symmetry. The hexagonal lattice spacing is approximately half the length of the molecule plus half the width of the molecule plus the thickness of the interlying water w1,4,10,29x. For the dye ˚ molecule shown in Fig. 3, this is approximately 24 A.

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conventional amphiphiles. The term ‘isodesmic’ (first used in the study of the aggregation of nucleic acids in solution) has been applied to the steady build up of chromonic aggregates where the addition or removal of one molecule to a stack is always associated with the same increment of free energy w13,14x. This is in direct contrast with the situation for conventional amphiphile association, where the micelle represents a free energy minimum—and there is a cost to the system in having either larger or smaller units. A further fundamental distinction between conventional amphiphile and chromonic systems concerns what happens at the lower temperature limit of mesophase formation. In conventional amphilphiles, there are two micro-phase regions; the aqueous and the hydrophobic, aliphatic parts. As the temperature is lowered, the alkyl chain motion in the hydrophobic region usually freezes out, forming a gel phase. The system becomes too brittle to be able to pack into the micelles required for mesophase formation. This gives a lower temperature limit characterised by its Krafft temperature. In chromonic systems the opposite happens. Because of the absence of alkyl chains in chromonic systems (or at least the absence of significant lengths of alkyl chains), chromonic systems do not show a Krafft point and the lower temperature is limit is marked by the appearance of ice—usually a few degrees below 0 8C. Note that there is evidence that one can access monotropic chromonic phases at sub-zero temperatures by adding an antifreeze to the system w7,9x. Misciblity is a feature of liquid crystalline systems – and in general, an understanding of the rules which govern miscibility is crucial to our understanding of the factors which determine the dynamics of each type of mesophase. The chromonic analogue of miscibility is intercalation where guest molecules can be accepted randomly into chromonic stacks. Although there have been no extended systematic studies of chromonic miscibility it appears that this is as widespread as miscibility in other mesophase systems w15x. There is another related pattern of behaviour of chromonic mixtures (which is discussed in more detail below). This occurs where there is a strong preference for an ABAB alternating arrangement of the two components in every stack w47–53x. This occurs to such a pronounced extent that the alternating column must be regarded as the structural unit of the phase. Since at least some of these CPI ‘compounds’ give mesophases with enhanced stability (and since the aromatic cores of the compounds in both families of mesophase are similar), it is an obvious suggestion that the search for similar effects in chromonic systems would be worthwhile. Chromonic systems can be doped with small soluble chiral compounds to give a chiral N phase (in an

Fig. 3. The molecular structure of C.I. Direct Blue 67. This new dye forms a chromonic N phase of high order parameter and which can be dried down to give an aligned solid film. Matsunaga et al. w29x have shown that a highly patterned film can be produced by drying down the N phase on a photaligned command substrate as sketched in Fig. 5.

analogous way to the chiral doping of thermotropic nematics). Since the twist produced is proportional to the concentration of the dopant, this property has been proposed as a practical assay for chiral compounds w7,9x. A new exploitation of the chirally-doped N phase is in compensating devices for TN cells as described below w38–40x. 3. New chromonic materials The range of compounds which form chromonic phases now includes xanthoses (used as antiasthmatic drugs, azo dyes, cyanine dyes, nucleic acids (guanosine derivatives) and perylenes w16–26x. Over the last few years, reports of new chromonic mesophases have grown from a trickle to a stream. These include a report of a new metallo-mesogen and excitingly, a non-aqueous chromonic system (where the solvent is dimethyl formamide) w27,28x (Fig. 3). Significant amongst new chromonic materials is the azo dye, C.I Direct 67 w29●x. This shows typical NyM phase behaviour promises to become a useful new chromonic compound. The chromonic phases formed by aqueous solutions of this dye were investigated by means of temperature-controlled X-ray diffraction, polarised light microscopy and UV–visible spectroscopy. The dye molecules form columnar aggregates, even at low concentrations. The structural study gave results very similar to those of earlier work on the INTALywater system w5,6,10x. As one would expect, the stacking repeat was ˚ (and was found to be independent of concentration 3.4 A and temperature). The diameter of the column was found to be comparable with the length of the molecule—suggesting that the column is a unimolecular stack. One interesting feature of this investigation was the observation that the addition of a small amount of anionic surfactant (0.01% by wt.) was found to enhance the stability of the nematic phase (at the expense of the


Fig. 4. The production of an aligned dye coated polymer film using a ‘command plate’ produced by the Weigert effect. A spin-coated film of the azo compound is photaligned with a beam of plane polarised light to produce a ‘command plate’.

M phase). Clearly at this concentration there is no suggestion of the added amphiphile having a direct structural effect on the mesophase such as coating the columns. The effect must be more subtle. Perhaps in some way it stabilises column ends or simply alters the chemical potential of the counter ions.

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Over the last 10 years, a photoalignment approach has been developed which appears to have the potential for achieving this w30–37x. This technique utilises the Weigert effect. This is the sensitivity of some photohemical reactions to the orientation of the plane of polarised light striking the molecule. Reactions for which the Weigert effect has been observed include photobleaching, photodimerization and photoisomerism. In the photoalignment of photoisomerisable molecules the final state of the sample has the molecular director aligned normal to the electric vector of the incident light (Figs. 4 and 5). Azobenzene has been a favourite compound for producing aligned films using the Weigert effect. Unfortunately, it is only weakly absorbing in the visible wavelengths range and it is, therefore by no means ideal. There is a way round this problem, however. It has been found that a film of phoaligned azobenzene molecules is able to epitaxially align liquid crystalline phases. The photoaligned substrate can, therefore be used as a ‘command surface’, which is in turn able to direct the alignment if the mesophase. Photoinduced alignment of this kind was first demonstrated with thermotropic mesophases (using films of azodoped polymer, polymers with azobenzene side groups or polymers with cinnamic acid side groups). However, it also works for lyotropic phases and can be used to align the chromonic N phase. In their recent paper, Matsunga et al. w30●x describe the production of a patterned film of dye using this approach. They used a striped template command surface prepared from the photoinduced alignment of a polyamide with azo side-groups. The chromonic N phase of the dye, C.I. Direct Blue 67 is aligned by this surface and then dried down to give a patterned film. They

4. The production of patterned dye films In the production of optical elements such as polarisers, retarders and optical compensators, it is necessary for us to be able to control the alignment of birefringent material embedded in (or deposited on) a film. A variety of manufacturing processes have been tried, including classical mechanical methods such as stretching the film, shearing, rubbing the surface and newer approaches such as electric field poling. The disadvantage of all of these approaches is that they are only able to produce surface films with same alignment over the entire area of film being treated. What would be much more desirable is a technique which could align small areas of film in different orientations, ultimately giving us the ability to align individual pixels in any required orientation.

Fig. 5. The epitaxial alignment of a chromonic N phase by a photoaligned command plate. (redrawn from Ref. w30x).

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Fig. 6. Conventional (uncompensated) twisted nematic cells in the on and off states. There are two forms of the twisted nematic cell: normally white (NW) and normally black (NB) cells – the difference lies only in the orientation of the upper polarising layer plate.

scale inversion’ can be severe. This arises where the contrast between ‘on’ and ‘off’ areas falls to zero and is then reversed, as the viewing angle is changed. The defects of the standard TN cell can, in some measure, be corrected by the addition of a compensator. This is an optical system, which matches the optical characteristics of the sample and reduces the contrast loss as the viewing angle is increased. An ideal compensating device would ‘correct’ the optics of both the ‘on’ and ‘off’ states, but this is an over-ambitious target at the present time. Current compensators are passive devices, which are designed to correct only the more critical of the two states of the device. In an uncompensated TN cell there is both leakage of light through the dark state and a decrease of intensity of the light state. However, the effects of these deficiencies are not symmetrical and the leakage of light through the dark state is the more serious. Because of this, compensators are specifically designed to improve only the dark state optics. The structures of the two variants of the standard (i.e. non-compensated) TN cell are shown in Fig. 6. In a cell in the ‘off’ state, the twisting molecular alignment rotates the plane of polarisation of the light through 908. In the ‘on’ state, the molecules are realigned to give a homeotropic state where the molecular axes lie perpendicular to the cell surface. This arrangement does not change the polarisation state of the light. There are two geometries that are used in these devices—with the two polarisers lying either parallel or perpendicular. The parallel arrangement gives a ‘normally black’ (NB) device in the absence of a field and the perpendicular arrangement gives a ‘normally white’ (NW) device. Compensators have, therefore been devised to improve the optics of the ‘off’ state of the NB device and the ‘on’ state of the NW device.

were able to produce a solid dye film consisting of alternating 300-mm wide rows of orthogonally aligned dye material with impressively sharp-edged resolution. 5. The use of planar and twisted lyotropic chromonic liquid crystal cells as optical compensators for twisted nematic cells The twisted nematic (TN) device continues to dominate the flat panel liquid crystal display market. However, it is not without its shortcomings. The characteristics of an ideal device are high contrast, a wide viewing angle, good colour rendition and a completely achromatic dark state. The standard (uncompensated) TN device fails to live up to every one of these ideals. In particular, the phenomenon known as ‘grey-

Fig. 7. A more accurate view of the mesophase alignment of the NW cell in the ‘on’ mode. To a first approximation, the applied field aligns the mesophase in a homeotropic pattern as sketched in Fig. 6. However, molecules near to the surfaces tend to retain a parallel alignment and the phase adopts a complex splay pattern shown.


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7. Compensating a normally white cell For a NW cell, the situation is more complex. In the dark ‘on’ state the director field is not perfectly homeotropic and the arrangement sketched in Fig. 6 is only an approximation to reality. In practice, the situation is more like that sketched in Fig. 7 where the molecules near to the upper and lower substrate surfaces retain a parallel alignment and, as the director field curves towards the homeotropic region in the centre, there is appreciable splay distortion. A plate compensating specifically for this splay has been developed by Fuji. In addition to the splay distortion, the optics require correction for birefringence and Lavetrovitch et al. w40●x have explored the use of chromonic cells to compensate for the birefringence and twist. The layout of their complete device is shown in Fig. 9.

Fig. 8. The design of a compensated NB cell (Lavrentrovich et al. w40x). Compensators have been added to improve the optics of the dark ‘off’ state. The twisted compensating cell is filled with a chirally doped N* phase, the compensating A-plate contains a parallel-aligned chromonic mesophase. (redrawn from Ref. w40x).

6. Compensating a normally black TN cell The root cause of the optical problems of a TN cell is the large positive birefringence of the cell. One can improve the optical performance of a NB display by compensating this with a passive retarder. The compensating plate should have negative birefringence and a twisted structure that mirrors that of the cell in the ‘off’ mode. An early attempt at constructing a compensator with these properties consisted of several superposed polymer films with negative birefringence. These were stacked in a twisted arranged to match the twist director twist of the cell in the ‘off’ state. More sophisticated compensators have been constructed by Laventrovitch et al. using liquid crystalline phases w38–49●x. To counteract the positive birefringence of the nematic phase, phases with negative optical anisotropy are required. This suggests the use of thermotropic discotic and lyotropic chromonic phases—and both have been tried. The design of an experimental compensated NB TN cell devised by Laventovitch et al. w40●x is shown in Fig. 8. Note that this employs a combination of a planar N-phase chromonic cell and a twisted N* chromonic cell to match the uncompensated optics.

Fig. 9. The design for a compensated NW cell (Lavrentrovich et al. w40x). Compensators have been added to improve the optics of the dark, ‘on’ state. The Fuji plates compensate for the splay shown in Fig. 7. The compensating A-plates contain a parallel-aligned chromonic mesophase. (redrawn from Ref. w40x).

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I have given prominence to the use of chromonic compensators in this review. This was not done, because I believe that devices of the kind described using the chromonic solutions are immediately feasible for largescale commercial production. The narrow temperature range of the chromonic phase used would alone make this impracticable. The importance lies in the way it stresses the versatility of this family of mesophases. 8. Complementary polytopic interaction (C.P.I.) in discotic systems The so-called p – p interactions which hold chromonic molecules face to face have been discussed by Hunter et al. w41–43x and Bates and Luckhurst w44x. They argue that these interactions are in fact a combination of van der Waals forces and electrostatic interactions and that no specific properties of p systems need be invoked. In recent studies of thermotropic columnar mesophases, in the search for enhanced electronic conductors, a phenomenon has been encountered which suggests that in certain cases, two different molecules can form 1:1 ‘compounds’ with enhanced mesophase-forming properties w45–53x. In these cases, the structural unit is thought to be the (AB)n column. The reason why these alternating columns are ‘better’ mesogenic units than those of either of the two separate species is not immediately evident. Some form of ‘bonding’ must be occurring between the two types of molecules, which is not describable in classical chemical terms. The interaction appears to be more subtle than a simple covalent, hydrogen bonding or electrostatic effect. There is no evidence that the electronic structure of either molecule is significantly disturbed and there is no detectable charge-transfer interaction. The explanation for the complementary nature of the two types of molecule appears to lie in the sum total of a large number of atom-byatom van der Waals type interactions. The term complementary polytopic interaction (C.P.I.) has been coined. Computer modelling of the interactions between the two types of molecule in such systems has been carried out using the XED program and the results appears to confirm this view. The two component phase diagram of two compounds which interact in this fashion, recently reported by Boden, Bushby and Lozman w53●x is shown in Fig. 10. Note that one of these compounds is mesogenic on its own, and the other, although having a similar overall structure (with a polyaryl core and a halo of alky chains) is not. The thin vertical region at the centre of the diagram represents the 1:1 C.P.I. compound. The extreme narrowness of this single phase area implies that there is a very positive interaction between the two types of molecule and that we are dealing with a system qualitatively different to one based simply on the mutual

solubility of the two component. Note in particular, that the discotic columnar temperature range is enhanced in both directions, with the mesophase extending to higher and lower temperatures than those of the pure mesogenic component. The result of computer modelling the docking of an ABA unit of the column using the XED program is shown in Fig. 11. The reason for highlighting a property of thermotropic mixtures in this review is that I can see no reason why such effects should not arise in chromonic systems— especially since in some cases, the core polyaryl parts of the molecules are the same. In the past, the suggestion that it ought be possible to convert intransigent, insoluble dyes into conveniently soluble mesophases by the addition of some magic colourless agent was greeted with dismissive scepticism by the (British) dye industry. Perhaps it now looks one increment more plausible. 9. The future The development of chromonic mesohase studies has not proceeded in the way I had expected. Bearing in mind the large industry concerned with the production and use of dyestuffs for printing fabric and paper and the apparently widespread occurrence of chromonic phases amongst dyes, I had expected that over the last decade there would have emerged a large literature concerned with enhancing the solubility properties (i.e. the mesophase-forming properties) of dyes, the investigation of mesophase enhancing mixtures, the fine tuning of colour by the addition of non-dye chromonic additives, the specalist use of the alignment of mesophases on prepared surfaces and with electric and magnetic fields for printed material—perhaps for high grade security printing such as banknotes, credit cards and identification. As far as I am aware, little of this has occurred. However, the production of aligned dye films has materialised (albeit requiring a command surface). In the previous review, I referred to two grails. The first was of these is the production of cheap electrically conducting organic material, the second, a viable lightharvesting device. With the development of techniques to control the alignment of chromonic phases to produce multilayer stacks of aligned phases w54x, and to produce highly-aligned films by drying down chromonic phases (enabling circuits to be produced by ink jet printer), both appear to have been brought a step closer. The use of chromonic phases as compensating plates for TN devices came as a surprise—but since an optically negative nematic phase was required, there were not a lot of alternative systems to choose from. Polymer films and discotic and chromonic mesphases are the obvious three—and all have been investigated. The major distinction between chromonic phases and thermotropic phases is of course the fact that they are


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Fig. 10. The production of ABAB stacks by complementary polytopic interactions (CPI). In some mixtures of thermotropic discotic and ‘near discotic’ compounds, it appears that the 1:1 mixture gives a columnar phase with a wider temperature range (and enhanced conductivity and photoconductivity) than that of either of the two separate compounds. It is presumed that the two molecules complement each other in some way and that the enhanced stability of the mesophase is due to the enhance stability of the ABAB columns. This example of the CPI effect is for a binary system of a mesogen, hexaalyltryphenylene(HAT6) – based derivative and the non-mesogenic compound, hexakis(4-nonylphenyl)dipyrazino w2,3-f:2.3.hxquoinoxalene( PDQ9). The nematic phase of the CPI 1:1 mixture occurs as a narrow vertical band at the centre of the phase diagram. The shaded area at the bottom of this band indicates where the chromonic N phase of the CPI compound is in a glassy state. (redrawn from Ref. w53x).

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proposed). However, there are still books being produced which purport to give a broad overview of ‘soft matter’, which omit all mention of chromonics. A single large-scale commercial application of chromonics will of course change this picture overnight. The widespread technological use of chromonic systems has not yet materialised, but the continuing discovery of unique properties and versatility of these systems promises much. I would hazard the guess that wherever nanotechnology takes us, the liquid crystalline state will never be far away—and chromonic systems will have something vital to offer. Acknowledgments I am indebted to Richard Bushby, Owen Lozman and to Gordon Tiddy for their continuing encouragement. References and recommended reading ● of special interest ●● of outstanding interest

Fig. 11. Orthogonal views of the optimum stacked columnar structure of a sequence of HAT1–PDQ1–HAT1 molecules as predicted by the XED program. The material in this figure is taken from Ref. w53x. The XED program is described in Refs. w41–43x and Vinter JC, Saunders MR: Ciba F Symp 1991, 158:249–265, Vinter JC: J Comp-Aid Mol Des 1994, 8:653–668, Vinter JC: J Comp-Aid Mol Des 1996, 10:417–426.

‘water based’. This should makes possible a marriage between established display technology and established biochemical techniques. I foresee the diagnostic biomedical tools of the next century operating via a combination of liquid crystal display technology and biochemical recognition. 10. Conclusion The recognition of chromonic mesophases as an important and distinct class of lyotropic mesophases is still patchy. Papers are now appearing where the term is used without the authors feeling the need to define it (and to refer to the literature where the word was first

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