- Email: [email protected]
Engineering supramolecular artificial edifices designed for a specific function
Biosensors & Bioelectronics 9 (1994) 617-624
Enclineerincr suwamolecular artificial sdifices >esfgned for a specific function Andre Barraud Commissariat
a 1’Energie Atomique,
Service de Chimie Moleculaire, France
C.E. Saclay, 91191, Gif sur Yvette,
Abstract: The Langmuir-Blodgett technique and its variants (alternate layers, self-organising mixtures, the semi-amphiphilic technique, the peculiar solid state chemistry in L.B. films) are collective methods which allow physical chemists, with a very small amount of synthetic chemistry, to build up molecular assemblies exhibiting not only the properties of each of their components, but also extra properties which arise from the architecture: cooperativity, anomalous chemical properties, molecular recognition, etc. These new tailored molecular edifices are the basic “bricks” of tomorrow’s molecular electronics and fine chemistry. These strategies are exemplified here by two active supramolecular edifices which have been successfully designed and built up: an artificial dioxygen trap based on the same principle as hemoglobin, and one molecule thick conductors. Promising applied results have already been obtained in the field of gas sensing with these new conductors, owing to molecular architectural amplification. Keywords: supramolecular architecture, modular chemistry, molecular electronics, conducting Langmuir-Blodgett films, gas sensors, solid state chemistry
INTRODUCTION
Recently, many questions about the working mechanisms of biological molecular machineries have been answered. Huge efforts have also been
made to determine the corresponding molecular architectures. This has inspired and encouraged physical chemists to undertake the design of artificial supramolecular edifices, tailored to carry out simple functions. These functional constructions are made up of several molecules held together by physical forces: electrostatic attraction forces; comnlexation forces; hydrogen bonding; 0956-5X3/94. $07.OC@ 1994 Elsevier Science Ltd.
interactions between permanent or induced electrical dipoles, and so on. Such supramolecular engineering requires little synthetic chemistry, but because physical forces are relatively weak, great care must be taken to ensure that the correct architecture is attained.
CONSTRUCTION
TECHNIQUES
As yet, molecules cannot be ordered at an individual level; only collective methods of molecular organisation are at the physical chemist’s disposal. One of the most simple and efficient 617
A. Barraud
techniques with which to order molecules collectively is the Langmuir-Blodgett method (Langmuir, 1920; Blodgett, 1935; Gaines, 1966) and its physical and chemical variants. This method uses amphiphilic molecules; a one molecule thick film is made at the water surface and then transferred onto a solid substrate; the process is repeated so that subsequent layers are deposited on top of other previously deposited layers. If the amphiphilic molecule has been well designed, the resulting lamellar film (called a Langmuir-Blodgett or LB film) is well organized and all the molecules have the same orientation and position relative to each other and to the substrate. Several variants (Barraud, 1987) allow the construction of LB films with more than one type of molecule, for example, the technique of alternate LB layers, the use of self-organizing mixtures, and the semi-amphiphilic technique. Alternate LB films are obtained using a doublecompartmented Langmuir trough in which two independent films are formed (Barraud et al., 1985). After transfer the polar heads of two different molecules face each other in adjacent layers. The technique of self-organizing mixtures takes advantage of couple formation to assemble different molecules in the same monolayer, contrary to the technique of alternate LB layers which groups two molecules from different layers. A further degree of freedom is provided by the semi-amphiphilic technique (Barraud et al., 1985). Here also, the stability of a semi-amphiphilic compound* is provided by physical forces, usually by electrostatic attraction between two molecular ions. The whole semi-amphiphilic compound behaves as a normal amphiphilic molecule and, if properly designed, can give rise to high quality LB films. The above techniques are complemented by a specific solid-state chemistry, which can be performed in situ in the LB films to give the supramolecular assembly its final properties. With this type of chemistry, reactions are lattice controlled and, in this context, semi-amphiphilic compounds give the designer an extra degree of freedom: the reaction control is provided by the unperturbed hydrophobic lattice of the amphi-
Biosensors & Bioelectronics
philic partner (which does not participate in the reaction), while the polar partner, which undergoes the reaction, can freely (because it has no hydrophobic tail) reorganize to the postreactional minimum energy configuration aimed at by the designer, without endangering the lamellar structure (Barraud et al., 1985). Another peculiar feature makes this chemistry very attractive: because LB films are very thin, small molecules can easily diffuse from outside, which is not the case for classical bulk solid-state chemistry. Hence, chemistry in LB films combines the high degree of control found in solid-state chemistry with the high versatility and richness of solution chemistry.
THE GOAL OF SUPRAMOLECULAR ARCHITECTURE The goal of supramolecular architecture is to design the molecular edifice in such a way that an extra property is promoted: this property is not brought by any of the constituents in particular, but arises from a molecular assembling which changes the environment of the molecules and gives them an unexpected chemical or physical behaviour. This is how the biological molecular machineries work. The challenge is not to copy nature, but to design and build up simple, totally artificial structures that exhibit non-trivial properties even in rough environmental conditions, which are representative of industrial requirements. Examples of these properties are: enhancement of the chemical reactivity of normally inert molecules; l electrical conduction and magnetism or, in general terms, cooperative phenomena; 0 molecular recognition.
l
These properties, which in the past were specific to biology, are now being understood and transferred to very simple artificial edifices. Two examples will be used to illustrate the concepts and strategies followed: the d&oxygen trap and the LB conducting film.
THE DIOXYGEN * A semi-amphiphilic compound is made of several partner molecules, some being amphiphilic, some nonamphiphilic (water-soluble molecules, for instance). 618
TRAP
This molecular machinery applies the same working principle as haemoglobin, but is totally
Artificial edifices designed for a specific function
Biosensors & Bioelectronics
artificial, with no protein or water present, so that it functions in hostile conditions. The active centre of haemoglobin is an Fe” porphyrin. Fe” is six-coordinated: its four equatorial bonds keep it fixed at the centre of the porphyrin macrocycle, while its two axial bonds, which prefer to bind shnuItaneously, are normally less reactive and unable to bind dioxygen. Haemoglobin is activated when the two Fen axial bonds are forced to bind unsymmetrically: when the surrounding protein offers a histidine to one of the bonds, the other one is activated and complexes dioxygen. For practical reasons, a very simple molecular edifice was adopted, the heart of which was the amphiphilic porphyrin shown in Fig. 1. This porphyrin macrocycle is naturally designed to anchor flat on water or on a hydrophilic substrate owing to its four hydrophilic carboxyl groups. This well-defined structure was maintained in the absence of protein. Histidine was replaced by an amphiphilic imidazole, which also possesses a complexable nitrogen; Fe” was replaced by Co”, also six-coordinated but less oxidizable. One symmetrical and three unsymmetrical supramol-
ecular assemblies (Fig. 1) were built up using the above components (Lecomte, 1985):
(4
Normal superimposed porphyrin LB layers: This structure was not intended to break the cobalt axial bond symmetry and, as expected, did not complex dioxygen. Alternate LB layers of amphiphilic imidaz(b) ole and porphyrin: In this structure imidazole could only contact cobalt on one side of the macrocycle. As expected, this dissymmetrical molecular assembly was very sensitive to dioxygen which was complexed at pressure as low as O-1 mbar. (cl A self-organizing equimolar mixture of amphiphilic porphyrin and imidazole: Upon film compression, imidazole, which likes to be complexed by cobalt, “climbs” on top of the macrocycle. This breaks the symmetry of the cobalt axial bonds. The single layer assembly is complemented by an alternate layer of an inert molecule, stearic acid. As expected, this system complexes dioxygen at pressures as low as a fraction of a mbar.
>
CO-OH
Fig. 1. The dioxygen trap. Only molecular edifices which are dissymmetrical (B and C) complex di-oxygen. the Co” amphiphilic tetraphenyl-porphyrin seen in profile in the diagrams on the right.
Left:
619
A. Barraud
Biosensors & Bioelectronics
(d) Same dissymmetrical assembly as C, but imidazole made bulky by adding a methyl side group: Imidazole still “climbs” onto the macrocycle but, because of the limited room between the four aliphatic chains of the porphyrin, imidazole cannot come close enough to the cobalt ion to activate the axial bond significantly. As expected, this bulky system was hindered and did not complex dioxygen, even at atmospheric pressure. These four molecular edifices provide clear evidence that assemblies involving several molecules held together by only physical forces operate according to textbook predictions.
THE LB CONDUCTING
FILM
Two conditions must be fulfilled for an organic molecule, which is by nature insulating, to become conducting:
(1) The
molecule must be in a mixed valence state4.e. the number of electrons it bears must not be an integer (for instance, when its highest occupied orbital bears O-5 electrons) (Simon & Andre, 1985) . The molecules must be stacked and tightly (2) coupled to one another in a regular array. This gives rise to energy bands in which electrons are quasi-free to move, unless the band is either completely full or empty of electrons. Tetracyanoquinodimethane (TCNQ) is a planar, fully conjugated molecule which stacks easily, especially in the mixed valence state. Although it is not the best molecule for conduction applications, it has been chosen because of the large amount of information concerning TCNQ that has been gathered by physicochemists. TCNQ is a fragile molecule: it is best used in its non-modified form; consequently, the semiamphiphilic method was the better method with which to make TCNQ monolayers, even though the molecule is not amphiphilic. When associated with an amphiphilic pyridinium cation and TCNQO, TCNQ- gives rise to a semi-amphiphilic compound (Fig. 2, left) which is a conducting powder. Unfortunately, this mixed valence compound is destroyed at the surface of water, so
that the resulting film was insulating (TCNQO dissolves in water). Following the failure of the above direct strategy, we resorted to an indirect one, which circumvented the exposure of the mixed valence compound to water. The semi-amphiphilic compound was again a salt of N-docosyl-pyridinium+ (NDP’) and TCNQ-, but no TCNQO was added (Fig. 2, right). The compound was not in a mixed valence state so that, as expected, both the powder and the LB film were insulating (Barraud, 1985). The LB film was then submitted to iodine vapour for a few seconds. Iodine picks out the negative charge from TCNQ, and tends to form TCNQO while transforming into I<. If the reaction was left to its natural thermodynamics, it would yield only TCNQO, and no mixed valence state would be obtained. But due to the peculiarities of the chemistry in LB films, the reaction is controlled by the lattice of the hydrophobic chains of NDP, and ceases when all the lattice sites available to iodine are filled with I;. This happens when one TCNQ out of two has been deprived of its electron (Fig. 3). Thus, when iodine exposure is carried out properly, it automatically yields mixed valence TCNQ, which matches the first requirement for conduction, described above. A spectacular reorganization of the TCNQs takes place spontaneously upon this iodine “doping”: the disk-like TCNQ molecules, which normally lie flat on the substrate, rotate by a quarter of a revolution and become vertical (Barraud et al., 1985), forming long stacks with a horizontal axis in which molecules are closely packed and coupled. This structure fulfils the for conduction. This second requirement reorganization was purposely favoured in the design of the strategy, and the semi-amphiphilic method is quite helpful in this respect because TCNQ bears no hindering aliphatic chain. The reorganization is indeed crucial for the success of the strategy. Quantum mechanics tells us that the electronic energy of a mixed valence stack is lower than that of the same molecules in isolation: if the molecules can re-organize, they will automatically re-arrange into the desired stacks, which form the structure with the lowest electronic energy. This is a direct consequence of band formation: forming an energy band and taking away half of its electrons results in an energy gain that is roughly equal to half the band-width, i.e. around O-25 eV in the case of TCNQ. This gain in electronic energy is enough
Artificial edifices designed for a specific function
Biosensors & Bioelectronics
Fig. 2. Semi-amphiphilic compounds: Left, a mixed valence docosylpyridinium (DCP+) tetracyanoquinodimethane (TCNQ-, TCNQO) semi-amphiphilic complex; Right, a DCP+ TCNQ- semi-amphiphilic salt. These semi-amphiphilic compounds, which behave like an amphiphilic molecule, allow the fabrication of monolayers of TCNQ, even though TCNQ bears no aliphatic chain.
u
TCNQ
CJ
P,r,dlni
0
1;
m
TCNQ-
urn
Fig. 3. The oxidation of TCNQ- by iodine is controlled by the lattice of the amphiphilic pyridinium. It stops when all the lattice sites available to iodine are filled with I>. This happens when one TCNQ out of two has lost its electron. This reaction is accompanied by a rotation of the TCNQ’s, which form conducting horizontal mixed valence stacks.
to overcome lattice friction in the polar plane of semi-amphiphilic LB films, but is not high enough to destroy their lamellar structure. The resulting
films are stable for years. They exhibit infrared, high frequency and d.c. semi-conduction, with a d.c. resistivity of around 10 Ohm.cm (Richard et al., 1986). Their structure has been completely determined owing to the remarkable molecular order present in the films: the aliphatic chains are tilted and interdigitated and iodine is located at the end of the aliphatic chains, in the cavity left by the interdigitation (Belbeoch et al., 1985). These films and several variants have found an application which was one of the first outcomes of molecular electronics: gas sensors. In the presence of phosphine (PH,), they undergo a drastic resistivity increase. The sensitivity of the films to phosphine is better than 1 part per million (ppm) (Henrion et al., 1989) which lies in the correct range for worker protection. They also selectively distinguish phosphine from ammonia (NH,), a very closely related molecule, with a good selectivity factor (4000). The tremendous sensitivity of the films results from their specific molecular architecture. In each stack, band conduction, which is a cooperative phenomenon, requires all the molecules to be 621
A. Barraud
exactly identical. When a phosphine molecule comes into contact with a TCNQ molecule, it modifies it so that the system is no longer periodic (this modified TCNQ behaves as a defect). Because the TCNQ stacks are highly one dimensional in LB films,* charge carriers cannot circumvent the defect and conduction is killed over the whole stack. Whatever the precise nature and location of the defect,? this mechanism, which allies low dimensionality and molecular cooperativity, probably explains why the electrical sensitivity of these sensors is a hundred- to a thousand-fold higher than their intrinsic chemical sensitivity (seen from spectral modification). This “architectural amplification” has no equivalent in classical electronics, because it takes place in the transducing material itself. These thin conducting films are typical of the close relationship between the design of the supramolecular architecture and the final properties of a material. Every step in the elaboration strategy must be carefully designed to help tailor the material and bring as little disorder as possible. Indeed, disorder is one of the basic enemies. Although the iodine doped conducting LB films exhibit marvellous X-ray diffraction patterns (Belbeoch et al., 1985), they show some anomalous electrical properties, which appeared to be related to defects arising upon iodine treatment (Barraud & Vandevyver, 1991). For instance, a doped mono- or bi-layer is not d.c. conducting, while its infrared spectrum is characteristic of a conducting material; only above five to six bilayers is the film conducting (Fig. 4, left). This anomaly can be explained by insulating grain boundaries in which molecules damaged by side reactions accrete: a few percent of this “dead” material is enough to make the whole sample insulating. In order to obtain a material with less defects, so as to render a single bilayer conducting, a new, doping free, hence direct strategy (Bourgoin et al., 1992) was developed. In spite of the complex synthetic task involved, an aliphatic chain was grafted to TCNQO to reduce its solubility and a direct strategy, similar to the * TCNQ stacks in LB films do not exhibit the slight secondary 2D and 3D characters they show in bulk, because of a different arrangement of the stacks. t For energy minimisation reasons, these defects are thought to migrate to stack ends or to grain boundaries, which does not change their general behaviour. 622
Biosemors & Bioelectronics
one used initially, was set up: a semi-amphiphilic salt of amphiphilic octadecyl-sulphonium (ODS+) and TCNQ- was mixed with amphiphilic (C,,) TCNQO. The mixture (powder) exhibits a semiconducting infrared absorption spectrum, indicating that (i) a mixed valence state and (ii) stacks of mixed valence TCNQ are obtained. After spreading on water and transfer onto an insulating substrate, this mixture gives rise to infrared and d.c. conducting single bilayers, (100 to 1000 Ohm.cm, same value as for multiple bilayers, Fig. 4, right) and even to d.c.conducting monolayers. This new strategy, which has been given the name “homodoping”, is in fact quite general, and applies even to compounds impossible to dope with iodine (Bourgoin, 1991). The homodoped films are not defect-free, but the defect density is low enough to allow charge carriers to percolate in a single polar plane. This provides a way to form molecule thick electrodes; these are required to study the effect of high electric fields on molecules-a basic field in molecular electronics. A very recent improvement in conducting TCNQ LB films concerns their orientational order. Orienting mixed valence TCNQ stacks along a preferential direction in the plane of the film is impossible in the iodine doping strategy, and inefficient in the homodoping strategy. However this can be achieved by co-spreading a mesogenic molecule with amphiphilic TCNQ at the water surface. The pyrilium dye shown in Fig. 5 is a discotic molecule that also gives rise to LB films in which it is organized in stacks in the plane of the film. Upon transfer from water to a solid support, these stacks orient parallel to the transfer direction and induce their orientation to the TCNQ stacks. In addition, pyrilium modifies the redox properties of water locally (the reducing power of the HzO/OZ couple is increased at the surface due to the strong local pH increase), so that the initially neutral amphiphilic TCNQ can be controllably reduced by water. For a mixing ratio of two TCNQs for one pyrilium molecule, stacks of mixed valence TCNQ are obtained, which are fully oriented by pyrilium upon transfer (Perez, 1993) as shown by the in-plane dichroic absorption spectra (Fig. 6). The example of the LB conducting films shows that strategies must, for the greatest efficiency, take account of all the energies involved in the
Artificial edifices designed for a specific function
Biosensors & Bioelectronics
loor
m
, 10
.
,
,
20
30
.
, 40
.
, 50
.
,Ol I 60
0
5
10
15
20
25
30
number of bilayers
number of bilayers
Fig. 4. Conductivity (conductance per layer) vs. the number of layers in the case of: left: iodine doped conducting films; right: homodoped conducting films. The arrows point to the case of a single bilayer.
Fig. 5. @-ilium,
a discotic molecule which also gives rise to LB jilms.
construction and transformations of the system. In particular, advantage can be taken of systems that predictably evolve towards the required structure spontaneously; in such a case the strategy must favour this evolution, while forming boundaries against undesired evolution routes.
REFERENCES
CONCLUSION Of the two examples described, one is biomimetic and was chosen to show how an activation mechanism, which is successfully used by nature in a highly sophisticated and precise biological environment, can operate in a much simpler, cruder,
purely
conducting properties in bio-systems do not involve these principles. For this reason, the strategies were designed ab initio from models of organic conduction that are validated through Combining the collective these experiments. properties obtained (intermolecular cooperation, molecular recognition) with the structural peculiarities of these organic conductors resulted in remarkable transducing performances that have already been successfully applied to gas sensors. These new materials are of great help to demonstrate the basic concepts of molecular electronics. It already appears that the very large choice of organic molecules available, and the quasi-infinite variety of combinations of molecules is a major trump for the design of active materials dedicated to specific functions. Mother Nature has widely exploited supramolecular architecture already, but there are many combinations other than those found in biological systems. Molecular electronics is expected to be one of the first fields to benefit from this new field of physical chemistry.
chemical system. The second one in biology; the magnetic or
has no equivalent
Barraud, A., Lesieur, P., Richard, J., RuaudelTeixier, A. & Vandevyver, M. (1985a). Structure and properties of an N-docosylpyridinium-tetracyanoquinodimethane salt in Langmuir-Blodgett multilayers, Thin Solid Films, 133, 125-131. Barraud, A. Lesieur, P., Ruaudel-Teixier, A. & Vandevyver, M. (1985b). Conducting LangmuirBlodgett films, Thin Solid Films, 134, 195-W. 623
A. Barraud
Biosensors & Bioelectronics
Fig. 6. Infrared dichroic absorption spectra taken on an LB film of a 2:l TCNQlPyrilium mixture with the electric field of the light respectively perpendicular (top) and parallel (bottom) to the dipping direction. Only the bottom spectrum exhibits the features characteristic of conduction, showing that the TCNQ stacks are: in a mixed valence state; oriented parallel to the dipping direction.
Barraud, A., Leloup, J., Gouzerh, A. & Palacin, S. (198%). An automatic trough to make alternate layers, Thin Solid Films, 133, 117-123. Barraud, A. & Vandevyver, M. (1987). In: Nonlinear Optical Properties of Organic Molecules and Crystals, Vol. 1, Academic Press, New York, pp.
357-383. Barraud, A. (1987). La technique de LangmuirBlodgett: un outil pour l’ingenierie supramoleculaire en film mince, J. Chimie Phys., ?34,1105-1111. Barraud, A. & Vandevyver, M. (1991). In: Condensed Systems of Low Dimensionality, 771-777, Plenum Press, New York. BelbCoch, B., Roulliay, M. & Toumarie, M. (1985). Evidence of chain interdigitation in LangmuirBlodgett films. Thin Solid Films, 134, 89-99. Blodgett, K. (1935). Films built by deposing successive monolayers on a solid substrate. .I. Am. Chem. Sot., 57, 1007.
Bourgoin, J.P. (1991). Films de Langmuir-Blodgett conducteurs: dopage a l’iode ou autodopage?. Ph.D. Thesis, Univ. Paris XI. Bourgoin, J.P., Ruaudel-Teixier A., Vandevyver M., Roulliay M., Barraud A., Lequan M. & Lequan R. M. (1992). The homodoping strategy towards Langmuir-Blodgett conducting films: mixture of
624
amphiphilic octadecyl-tetracyanoquinodimethane with various semi-amphiphilic tetracyanoquinodimethane salts. Thin Solid Films, 210/211,250-252. Gaines Jr. G. (1966). In: Znsoluble Monolayers at Liquid-Gas Interface. Interscience, John Wiley, New York. Henrion, L., Derost G., Barraud A. & RuaudelTeixier A. (1989). Detection of phosphine with a Langmuir-Blodgett film of N-octadecyl pyridinium TCNQ. Sensors & Actuators, 17, 493-498. Langmuir, I. (1920). Trans. Faraday Sot., 15, 62. Lecomte, C., Baudin C., Berleur F., Ruaudel-Teixier A., Barraud A. & Momenteau M. (1985). An example of molecular building: alternate Langmuir-Blodgett films of cobalto-meso-porphyrins designed to bind dioxygen. Thin Solid Films, 133, 103-112.
Perez, J. (1993). Comportement de sels colonnaires a l’interface air-eau. Films de Langmuir-Blodgett anisotropes. Ph.D. Thesis, Univ. Paris XI. Richard, J., Vandevyver M., Lesieur P., Barraud A. & Holczer K. (1986). Electronic transport properties in conducting Langmuir-Blodgett films, J. Phys. D: Appl. Phys., 19, 2421-2430. Simon, J. & Andre, J.J. (1985). Molecular SemiConductors, Springer-Verlag.
Enclineerincr suwamolecular artificial sdifices >esfgned for a specific function Andre Barraud Commissariat
a 1’Energie Atomique,
Service de Chimie Moleculaire, France
C.E. Saclay, 91191, Gif sur Yvette,
Abstract: The Langmuir-Blodgett technique and its variants (alternate layers, self-organising mixtures, the semi-amphiphilic technique, the peculiar solid state chemistry in L.B. films) are collective methods which allow physical chemists, with a very small amount of synthetic chemistry, to build up molecular assemblies exhibiting not only the properties of each of their components, but also extra properties which arise from the architecture: cooperativity, anomalous chemical properties, molecular recognition, etc. These new tailored molecular edifices are the basic “bricks” of tomorrow’s molecular electronics and fine chemistry. These strategies are exemplified here by two active supramolecular edifices which have been successfully designed and built up: an artificial dioxygen trap based on the same principle as hemoglobin, and one molecule thick conductors. Promising applied results have already been obtained in the field of gas sensing with these new conductors, owing to molecular architectural amplification. Keywords: supramolecular architecture, modular chemistry, molecular electronics, conducting Langmuir-Blodgett films, gas sensors, solid state chemistry
INTRODUCTION
Recently, many questions about the working mechanisms of biological molecular machineries have been answered. Huge efforts have also been
made to determine the corresponding molecular architectures. This has inspired and encouraged physical chemists to undertake the design of artificial supramolecular edifices, tailored to carry out simple functions. These functional constructions are made up of several molecules held together by physical forces: electrostatic attraction forces; comnlexation forces; hydrogen bonding; 0956-5X3/94. $07.OC@ 1994 Elsevier Science Ltd.
interactions between permanent or induced electrical dipoles, and so on. Such supramolecular engineering requires little synthetic chemistry, but because physical forces are relatively weak, great care must be taken to ensure that the correct architecture is attained.
CONSTRUCTION
TECHNIQUES
As yet, molecules cannot be ordered at an individual level; only collective methods of molecular organisation are at the physical chemist’s disposal. One of the most simple and efficient 617
A. Barraud
techniques with which to order molecules collectively is the Langmuir-Blodgett method (Langmuir, 1920; Blodgett, 1935; Gaines, 1966) and its physical and chemical variants. This method uses amphiphilic molecules; a one molecule thick film is made at the water surface and then transferred onto a solid substrate; the process is repeated so that subsequent layers are deposited on top of other previously deposited layers. If the amphiphilic molecule has been well designed, the resulting lamellar film (called a Langmuir-Blodgett or LB film) is well organized and all the molecules have the same orientation and position relative to each other and to the substrate. Several variants (Barraud, 1987) allow the construction of LB films with more than one type of molecule, for example, the technique of alternate LB layers, the use of self-organizing mixtures, and the semi-amphiphilic technique. Alternate LB films are obtained using a doublecompartmented Langmuir trough in which two independent films are formed (Barraud et al., 1985). After transfer the polar heads of two different molecules face each other in adjacent layers. The technique of self-organizing mixtures takes advantage of couple formation to assemble different molecules in the same monolayer, contrary to the technique of alternate LB layers which groups two molecules from different layers. A further degree of freedom is provided by the semi-amphiphilic technique (Barraud et al., 1985). Here also, the stability of a semi-amphiphilic compound* is provided by physical forces, usually by electrostatic attraction between two molecular ions. The whole semi-amphiphilic compound behaves as a normal amphiphilic molecule and, if properly designed, can give rise to high quality LB films. The above techniques are complemented by a specific solid-state chemistry, which can be performed in situ in the LB films to give the supramolecular assembly its final properties. With this type of chemistry, reactions are lattice controlled and, in this context, semi-amphiphilic compounds give the designer an extra degree of freedom: the reaction control is provided by the unperturbed hydrophobic lattice of the amphi-
Biosensors & Bioelectronics
philic partner (which does not participate in the reaction), while the polar partner, which undergoes the reaction, can freely (because it has no hydrophobic tail) reorganize to the postreactional minimum energy configuration aimed at by the designer, without endangering the lamellar structure (Barraud et al., 1985). Another peculiar feature makes this chemistry very attractive: because LB films are very thin, small molecules can easily diffuse from outside, which is not the case for classical bulk solid-state chemistry. Hence, chemistry in LB films combines the high degree of control found in solid-state chemistry with the high versatility and richness of solution chemistry.
THE GOAL OF SUPRAMOLECULAR ARCHITECTURE The goal of supramolecular architecture is to design the molecular edifice in such a way that an extra property is promoted: this property is not brought by any of the constituents in particular, but arises from a molecular assembling which changes the environment of the molecules and gives them an unexpected chemical or physical behaviour. This is how the biological molecular machineries work. The challenge is not to copy nature, but to design and build up simple, totally artificial structures that exhibit non-trivial properties even in rough environmental conditions, which are representative of industrial requirements. Examples of these properties are: enhancement of the chemical reactivity of normally inert molecules; l electrical conduction and magnetism or, in general terms, cooperative phenomena; 0 molecular recognition.
l
These properties, which in the past were specific to biology, are now being understood and transferred to very simple artificial edifices. Two examples will be used to illustrate the concepts and strategies followed: the d&oxygen trap and the LB conducting film.
THE DIOXYGEN * A semi-amphiphilic compound is made of several partner molecules, some being amphiphilic, some nonamphiphilic (water-soluble molecules, for instance). 618
TRAP
This molecular machinery applies the same working principle as haemoglobin, but is totally
Artificial edifices designed for a specific function
Biosensors & Bioelectronics
artificial, with no protein or water present, so that it functions in hostile conditions. The active centre of haemoglobin is an Fe” porphyrin. Fe” is six-coordinated: its four equatorial bonds keep it fixed at the centre of the porphyrin macrocycle, while its two axial bonds, which prefer to bind shnuItaneously, are normally less reactive and unable to bind dioxygen. Haemoglobin is activated when the two Fen axial bonds are forced to bind unsymmetrically: when the surrounding protein offers a histidine to one of the bonds, the other one is activated and complexes dioxygen. For practical reasons, a very simple molecular edifice was adopted, the heart of which was the amphiphilic porphyrin shown in Fig. 1. This porphyrin macrocycle is naturally designed to anchor flat on water or on a hydrophilic substrate owing to its four hydrophilic carboxyl groups. This well-defined structure was maintained in the absence of protein. Histidine was replaced by an amphiphilic imidazole, which also possesses a complexable nitrogen; Fe” was replaced by Co”, also six-coordinated but less oxidizable. One symmetrical and three unsymmetrical supramol-
ecular assemblies (Fig. 1) were built up using the above components (Lecomte, 1985):
(4
Normal superimposed porphyrin LB layers: This structure was not intended to break the cobalt axial bond symmetry and, as expected, did not complex dioxygen. Alternate LB layers of amphiphilic imidaz(b) ole and porphyrin: In this structure imidazole could only contact cobalt on one side of the macrocycle. As expected, this dissymmetrical molecular assembly was very sensitive to dioxygen which was complexed at pressure as low as O-1 mbar. (cl A self-organizing equimolar mixture of amphiphilic porphyrin and imidazole: Upon film compression, imidazole, which likes to be complexed by cobalt, “climbs” on top of the macrocycle. This breaks the symmetry of the cobalt axial bonds. The single layer assembly is complemented by an alternate layer of an inert molecule, stearic acid. As expected, this system complexes dioxygen at pressures as low as a fraction of a mbar.
>
CO-OH
Fig. 1. The dioxygen trap. Only molecular edifices which are dissymmetrical (B and C) complex di-oxygen. the Co” amphiphilic tetraphenyl-porphyrin seen in profile in the diagrams on the right.
Left:
619
A. Barraud
Biosensors & Bioelectronics
(d) Same dissymmetrical assembly as C, but imidazole made bulky by adding a methyl side group: Imidazole still “climbs” onto the macrocycle but, because of the limited room between the four aliphatic chains of the porphyrin, imidazole cannot come close enough to the cobalt ion to activate the axial bond significantly. As expected, this bulky system was hindered and did not complex dioxygen, even at atmospheric pressure. These four molecular edifices provide clear evidence that assemblies involving several molecules held together by only physical forces operate according to textbook predictions.
THE LB CONDUCTING
FILM
Two conditions must be fulfilled for an organic molecule, which is by nature insulating, to become conducting:
(1) The
molecule must be in a mixed valence state4.e. the number of electrons it bears must not be an integer (for instance, when its highest occupied orbital bears O-5 electrons) (Simon & Andre, 1985) . The molecules must be stacked and tightly (2) coupled to one another in a regular array. This gives rise to energy bands in which electrons are quasi-free to move, unless the band is either completely full or empty of electrons. Tetracyanoquinodimethane (TCNQ) is a planar, fully conjugated molecule which stacks easily, especially in the mixed valence state. Although it is not the best molecule for conduction applications, it has been chosen because of the large amount of information concerning TCNQ that has been gathered by physicochemists. TCNQ is a fragile molecule: it is best used in its non-modified form; consequently, the semiamphiphilic method was the better method with which to make TCNQ monolayers, even though the molecule is not amphiphilic. When associated with an amphiphilic pyridinium cation and TCNQO, TCNQ- gives rise to a semi-amphiphilic compound (Fig. 2, left) which is a conducting powder. Unfortunately, this mixed valence compound is destroyed at the surface of water, so
that the resulting film was insulating (TCNQO dissolves in water). Following the failure of the above direct strategy, we resorted to an indirect one, which circumvented the exposure of the mixed valence compound to water. The semi-amphiphilic compound was again a salt of N-docosyl-pyridinium+ (NDP’) and TCNQ-, but no TCNQO was added (Fig. 2, right). The compound was not in a mixed valence state so that, as expected, both the powder and the LB film were insulating (Barraud, 1985). The LB film was then submitted to iodine vapour for a few seconds. Iodine picks out the negative charge from TCNQ, and tends to form TCNQO while transforming into I<. If the reaction was left to its natural thermodynamics, it would yield only TCNQO, and no mixed valence state would be obtained. But due to the peculiarities of the chemistry in LB films, the reaction is controlled by the lattice of the hydrophobic chains of NDP, and ceases when all the lattice sites available to iodine are filled with I;. This happens when one TCNQ out of two has been deprived of its electron (Fig. 3). Thus, when iodine exposure is carried out properly, it automatically yields mixed valence TCNQ, which matches the first requirement for conduction, described above. A spectacular reorganization of the TCNQs takes place spontaneously upon this iodine “doping”: the disk-like TCNQ molecules, which normally lie flat on the substrate, rotate by a quarter of a revolution and become vertical (Barraud et al., 1985), forming long stacks with a horizontal axis in which molecules are closely packed and coupled. This structure fulfils the for conduction. This second requirement reorganization was purposely favoured in the design of the strategy, and the semi-amphiphilic method is quite helpful in this respect because TCNQ bears no hindering aliphatic chain. The reorganization is indeed crucial for the success of the strategy. Quantum mechanics tells us that the electronic energy of a mixed valence stack is lower than that of the same molecules in isolation: if the molecules can re-organize, they will automatically re-arrange into the desired stacks, which form the structure with the lowest electronic energy. This is a direct consequence of band formation: forming an energy band and taking away half of its electrons results in an energy gain that is roughly equal to half the band-width, i.e. around O-25 eV in the case of TCNQ. This gain in electronic energy is enough
Artificial edifices designed for a specific function
Biosensors & Bioelectronics
Fig. 2. Semi-amphiphilic compounds: Left, a mixed valence docosylpyridinium (DCP+) tetracyanoquinodimethane (TCNQ-, TCNQO) semi-amphiphilic complex; Right, a DCP+ TCNQ- semi-amphiphilic salt. These semi-amphiphilic compounds, which behave like an amphiphilic molecule, allow the fabrication of monolayers of TCNQ, even though TCNQ bears no aliphatic chain.
u
TCNQ
CJ
P,r,dlni
0
1;
m
TCNQ-
urn
Fig. 3. The oxidation of TCNQ- by iodine is controlled by the lattice of the amphiphilic pyridinium. It stops when all the lattice sites available to iodine are filled with I>. This happens when one TCNQ out of two has lost its electron. This reaction is accompanied by a rotation of the TCNQ’s, which form conducting horizontal mixed valence stacks.
to overcome lattice friction in the polar plane of semi-amphiphilic LB films, but is not high enough to destroy their lamellar structure. The resulting
films are stable for years. They exhibit infrared, high frequency and d.c. semi-conduction, with a d.c. resistivity of around 10 Ohm.cm (Richard et al., 1986). Their structure has been completely determined owing to the remarkable molecular order present in the films: the aliphatic chains are tilted and interdigitated and iodine is located at the end of the aliphatic chains, in the cavity left by the interdigitation (Belbeoch et al., 1985). These films and several variants have found an application which was one of the first outcomes of molecular electronics: gas sensors. In the presence of phosphine (PH,), they undergo a drastic resistivity increase. The sensitivity of the films to phosphine is better than 1 part per million (ppm) (Henrion et al., 1989) which lies in the correct range for worker protection. They also selectively distinguish phosphine from ammonia (NH,), a very closely related molecule, with a good selectivity factor (4000). The tremendous sensitivity of the films results from their specific molecular architecture. In each stack, band conduction, which is a cooperative phenomenon, requires all the molecules to be 621
A. Barraud
exactly identical. When a phosphine molecule comes into contact with a TCNQ molecule, it modifies it so that the system is no longer periodic (this modified TCNQ behaves as a defect). Because the TCNQ stacks are highly one dimensional in LB films,* charge carriers cannot circumvent the defect and conduction is killed over the whole stack. Whatever the precise nature and location of the defect,? this mechanism, which allies low dimensionality and molecular cooperativity, probably explains why the electrical sensitivity of these sensors is a hundred- to a thousand-fold higher than their intrinsic chemical sensitivity (seen from spectral modification). This “architectural amplification” has no equivalent in classical electronics, because it takes place in the transducing material itself. These thin conducting films are typical of the close relationship between the design of the supramolecular architecture and the final properties of a material. Every step in the elaboration strategy must be carefully designed to help tailor the material and bring as little disorder as possible. Indeed, disorder is one of the basic enemies. Although the iodine doped conducting LB films exhibit marvellous X-ray diffraction patterns (Belbeoch et al., 1985), they show some anomalous electrical properties, which appeared to be related to defects arising upon iodine treatment (Barraud & Vandevyver, 1991). For instance, a doped mono- or bi-layer is not d.c. conducting, while its infrared spectrum is characteristic of a conducting material; only above five to six bilayers is the film conducting (Fig. 4, left). This anomaly can be explained by insulating grain boundaries in which molecules damaged by side reactions accrete: a few percent of this “dead” material is enough to make the whole sample insulating. In order to obtain a material with less defects, so as to render a single bilayer conducting, a new, doping free, hence direct strategy (Bourgoin et al., 1992) was developed. In spite of the complex synthetic task involved, an aliphatic chain was grafted to TCNQO to reduce its solubility and a direct strategy, similar to the * TCNQ stacks in LB films do not exhibit the slight secondary 2D and 3D characters they show in bulk, because of a different arrangement of the stacks. t For energy minimisation reasons, these defects are thought to migrate to stack ends or to grain boundaries, which does not change their general behaviour. 622
Biosemors & Bioelectronics
one used initially, was set up: a semi-amphiphilic salt of amphiphilic octadecyl-sulphonium (ODS+) and TCNQ- was mixed with amphiphilic (C,,) TCNQO. The mixture (powder) exhibits a semiconducting infrared absorption spectrum, indicating that (i) a mixed valence state and (ii) stacks of mixed valence TCNQ are obtained. After spreading on water and transfer onto an insulating substrate, this mixture gives rise to infrared and d.c. conducting single bilayers, (100 to 1000 Ohm.cm, same value as for multiple bilayers, Fig. 4, right) and even to d.c.conducting monolayers. This new strategy, which has been given the name “homodoping”, is in fact quite general, and applies even to compounds impossible to dope with iodine (Bourgoin, 1991). The homodoped films are not defect-free, but the defect density is low enough to allow charge carriers to percolate in a single polar plane. This provides a way to form molecule thick electrodes; these are required to study the effect of high electric fields on molecules-a basic field in molecular electronics. A very recent improvement in conducting TCNQ LB films concerns their orientational order. Orienting mixed valence TCNQ stacks along a preferential direction in the plane of the film is impossible in the iodine doping strategy, and inefficient in the homodoping strategy. However this can be achieved by co-spreading a mesogenic molecule with amphiphilic TCNQ at the water surface. The pyrilium dye shown in Fig. 5 is a discotic molecule that also gives rise to LB films in which it is organized in stacks in the plane of the film. Upon transfer from water to a solid support, these stacks orient parallel to the transfer direction and induce their orientation to the TCNQ stacks. In addition, pyrilium modifies the redox properties of water locally (the reducing power of the HzO/OZ couple is increased at the surface due to the strong local pH increase), so that the initially neutral amphiphilic TCNQ can be controllably reduced by water. For a mixing ratio of two TCNQs for one pyrilium molecule, stacks of mixed valence TCNQ are obtained, which are fully oriented by pyrilium upon transfer (Perez, 1993) as shown by the in-plane dichroic absorption spectra (Fig. 6). The example of the LB conducting films shows that strategies must, for the greatest efficiency, take account of all the energies involved in the
Artificial edifices designed for a specific function
Biosensors & Bioelectronics
loor
m
, 10
.
,
,
20
30
.
, 40
.
, 50
.
,Ol I 60
0
5
10
15
20
25
30
number of bilayers
number of bilayers
Fig. 4. Conductivity (conductance per layer) vs. the number of layers in the case of: left: iodine doped conducting films; right: homodoped conducting films. The arrows point to the case of a single bilayer.
Fig. 5. @-ilium,
a discotic molecule which also gives rise to LB jilms.
construction and transformations of the system. In particular, advantage can be taken of systems that predictably evolve towards the required structure spontaneously; in such a case the strategy must favour this evolution, while forming boundaries against undesired evolution routes.
REFERENCES
CONCLUSION Of the two examples described, one is biomimetic and was chosen to show how an activation mechanism, which is successfully used by nature in a highly sophisticated and precise biological environment, can operate in a much simpler, cruder,
purely
conducting properties in bio-systems do not involve these principles. For this reason, the strategies were designed ab initio from models of organic conduction that are validated through Combining the collective these experiments. properties obtained (intermolecular cooperation, molecular recognition) with the structural peculiarities of these organic conductors resulted in remarkable transducing performances that have already been successfully applied to gas sensors. These new materials are of great help to demonstrate the basic concepts of molecular electronics. It already appears that the very large choice of organic molecules available, and the quasi-infinite variety of combinations of molecules is a major trump for the design of active materials dedicated to specific functions. Mother Nature has widely exploited supramolecular architecture already, but there are many combinations other than those found in biological systems. Molecular electronics is expected to be one of the first fields to benefit from this new field of physical chemistry.
chemical system. The second one in biology; the magnetic or
has no equivalent
Barraud, A., Lesieur, P., Richard, J., RuaudelTeixier, A. & Vandevyver, M. (1985a). Structure and properties of an N-docosylpyridinium-tetracyanoquinodimethane salt in Langmuir-Blodgett multilayers, Thin Solid Films, 133, 125-131. Barraud, A. Lesieur, P., Ruaudel-Teixier, A. & Vandevyver, M. (1985b). Conducting LangmuirBlodgett films, Thin Solid Films, 134, 195-W. 623
A. Barraud
Biosensors & Bioelectronics
Fig. 6. Infrared dichroic absorption spectra taken on an LB film of a 2:l TCNQlPyrilium mixture with the electric field of the light respectively perpendicular (top) and parallel (bottom) to the dipping direction. Only the bottom spectrum exhibits the features characteristic of conduction, showing that the TCNQ stacks are: in a mixed valence state; oriented parallel to the dipping direction.
Barraud, A., Leloup, J., Gouzerh, A. & Palacin, S. (198%). An automatic trough to make alternate layers, Thin Solid Films, 133, 117-123. Barraud, A. & Vandevyver, M. (1987). In: Nonlinear Optical Properties of Organic Molecules and Crystals, Vol. 1, Academic Press, New York, pp.
357-383. Barraud, A. (1987). La technique de LangmuirBlodgett: un outil pour l’ingenierie supramoleculaire en film mince, J. Chimie Phys., ?34,1105-1111. Barraud, A. & Vandevyver, M. (1991). In: Condensed Systems of Low Dimensionality, 771-777, Plenum Press, New York. BelbCoch, B., Roulliay, M. & Toumarie, M. (1985). Evidence of chain interdigitation in LangmuirBlodgett films. Thin Solid Films, 134, 89-99. Blodgett, K. (1935). Films built by deposing successive monolayers on a solid substrate. .I. Am. Chem. Sot., 57, 1007.
Bourgoin, J.P. (1991). Films de Langmuir-Blodgett conducteurs: dopage a l’iode ou autodopage?. Ph.D. Thesis, Univ. Paris XI. Bourgoin, J.P., Ruaudel-Teixier A., Vandevyver M., Roulliay M., Barraud A., Lequan M. & Lequan R. M. (1992). The homodoping strategy towards Langmuir-Blodgett conducting films: mixture of
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amphiphilic octadecyl-tetracyanoquinodimethane with various semi-amphiphilic tetracyanoquinodimethane salts. Thin Solid Films, 210/211,250-252. Gaines Jr. G. (1966). In: Znsoluble Monolayers at Liquid-Gas Interface. Interscience, John Wiley, New York. Henrion, L., Derost G., Barraud A. & RuaudelTeixier A. (1989). Detection of phosphine with a Langmuir-Blodgett film of N-octadecyl pyridinium TCNQ. Sensors & Actuators, 17, 493-498. Langmuir, I. (1920). Trans. Faraday Sot., 15, 62. Lecomte, C., Baudin C., Berleur F., Ruaudel-Teixier A., Barraud A. & Momenteau M. (1985). An example of molecular building: alternate Langmuir-Blodgett films of cobalto-meso-porphyrins designed to bind dioxygen. Thin Solid Films, 133, 103-112.
Perez, J. (1993). Comportement de sels colonnaires a l’interface air-eau. Films de Langmuir-Blodgett anisotropes. Ph.D. Thesis, Univ. Paris XI. Richard, J., Vandevyver M., Lesieur P., Barraud A. & Holczer K. (1986). Electronic transport properties in conducting Langmuir-Blodgett films, J. Phys. D: Appl. Phys., 19, 2421-2430. Simon, J. & Andre, J.J. (1985). Molecular SemiConductors, Springer-Verlag.