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Smart Materials Based on Functionalized Conjugated Polymers.

This article reviews how some conjugated polymers can detect and transduce chemical or physical information into an optical or electrical signal.

There is hardly an aspect of our lives that is not touched by synthetic polymers. In the field of optics and electronics, these materials were traditionally used in applications such as packaging, electrical insulators, and photoresists. However, electroactive and photoactive conjugated polymers (polyacetylene, polyaniline, polyphenylene, polypyrrole, polyphenylenevinylene, polythiophene, polyfluorene, etc.) have recently received a lot of attention from both academic and industrial laboratories mainly because of their unique optical, electrochemical, and/or electrical properties. [l]

The delocalized electronic structure of these polymers is partly responsible for the good stability and mobility of the charge carriers created upon doping (i.e. partial oxidation or reduction); electrical conductivities in the range of 1-1000 S/cm being reached in most cases. This conjugated structure is also responsible for a strong absorption (and often emission) in the UV-visible range. Not surprisingly, these polymeric materials have been used as antistatic coatings, electrodes, transistors, light-emitting diodes, conducting photoresists, photoconducting cells, etc. [1]

Moreover, solubility and/or fusibility has been obtained through the incorporation of relatively long and flexible side chains. The introduction of various substituents along the conjugated backbone can not only improve the processability of these aromatic polymers but can also modify their physical properties; it can even lead to physical phenomena that are not found in the parent unsubstituted polymers. For instance, the optical and/or electrochemical properties of some functionalized conjugated polymers (polydiacetylenes, polythiophenes, etc.) can be strongly modified by varying the temperature, the pressure, the solvent, the electrolyte, etc. In other words, these smart polymers can detect, transduce, and, sometimes, amplify a chemical or physical information into an optical or electrical signal. [2,3]

The development of such integrated polymeric systems is an important research topic in our laboratory (mainly on functionalized polythiophenes) and should lead to the development of efficient display devices, sensors, diagnostic tools, etc.

Thermochromism

The first example of transduction of a physical information (temperature) into an optical signal from polythiophenes was related to the so-called thermochromic effect. This intriguing effect (observed both in solution and in the solid state) has been extensively studied and has led to the development of various thermochromic polythiophene derivatives. To a first approximation, these interesting optical effects have been attributed to a cooperative transition between a planar (highly conjugated, red-violet) form and a nonplanar (less conjugated, yellow) conformational structure of the backbone (Figure 1). These assumptions are based on the fact that there is a close relationship between the backbone conformation and the electronic structure of conjugated macromolecules, any twisting of the main chain from a planar conformation leading to a decrease of the effective conjugation length associated with a blue shift of the absorption in the UV- visible range.

Although the chromic effects are obviously the consequence of the main chain conformation (and/or interchain interactions), it has been also found that these thermochromic properties are strongly dependent upon the nature and the position of the side chains along the backbone. More precisely, it has been suggested that these optical effects are driven by a delicate balance between repulsive steric interactions and attractive interchain (or intrachain, due to a chain folding) interactions; the latter being necessary to get a planar conformation in the case of thermochromic polythiophenes. [4] On the basis of this model, this cooperative conformational transition could be explained by the formation of assemblies of well-defined conjugated macromolecules that are disrupted upon side-chain disordering.

Ionochromism and Photochromism

These results obtained on thermochromic polythiophenes support the assumption that side-chain disordering can be the driving force for the twisting of the conjugated backbone and, therefore, using appropriately functionalized conjugated polymers, it can be expected that a large number of external stimuli can also lead to chromic effects. First, experiments have been carried out with conjugated polymers bearing ether or crown- ether substituents which are well known for their ability to make complexes with alkali metal cations. [3] Interesting ionochromic effects have been indeed obtained with some new functionalized polythiophenes, in solution (Figure 2).

Following a similar scheme, it can be expected that side-chain disordering (or ordering) could be induced photochemically. For instance, by introducing photo-isomerizable (trans-cis) azobeuzene substituents in poly(3-alkoxy-4-methylthiophene)s, it has been possible to photo-induce a conformational transition of the conjugated backbone. In other words, the trans-cis isomerization of the photoactive side chains not only modifies their own UV-visible absorption features but induces also a modification of the optical properties associated with the conjugated backbone, leading to a so-called dual photochromic effect.[3]

Affinity Chromism

From these results, it can be, therefore, anticipated that any complexation (e.g. host-guest interactions, lock and key interactions, etc.) between some binding sites present in the side chains and some chemical or biochemical targets could lead to significant colour changes. Along these lines, particularly promising biochromic sensors have been developed from functionalized polydiacetylenes and polythiophenes. [3] For instance, water-soluble biochromic poly(3-alkoxy-4-methylthiophene)s (based on biotin-avidin interactions) have been developed and have led to the optical detection of as low as [10.sup.-11] mole of the protein avidin, in water. [5] Moreover, it has been shown that by electrostatically binding (a non-covalent modification of the side chains from a polythiophene precursor) adequate responsive substituents, thermochromic, ionochromic, photochromic, and affinitychromic materials can be easily developed. These new possibilities may even lead to a combinatorial approach for the development of arrays of different polymeric sensors.

Luminescence and Electrochemistry

In some cases, both absorption and emission characteristics of conjugated polymers are modified by external stimuli. A first type of applications is based on the fact that the planar (aggregated) and non-planar conformational structures may have a different emission spectrum or a different fluorescence quantum yield. For instance, with ionochromic poly(3(oligo(oxyethylene)-4-methylthiophene), the intensity of fluorescence at 542 nm is a function of the concentration of alkali metal cations. [3] Fortunately, a modification of the conformation of a conjugated polymer does not only alter its absorption and/or emission properties but can also modify its electrical or electrochemical properties. This is particularly true with electroactive polythiophenes. For instance, a modified electrode was easily prepared from an aminosilane-treated ITO electrode and a partially pre-neutralized acidic polythiophene derivative (Figure 3) and the interaction between protein avidin and the biotinylated poly( 3-alkoxy-4- methylthi ophene) was detected (in the femtomole range) by electrochemical means. Once again, the detection mechanism is assumed to be related to a conformational transition of the conjugated backbone.

Conclusions

On the basis of all these examples, it has been clearly shown that the optical (UV-visible absorption and/or emission) and electrochemical properties of some functionalized conjugated polymers can be dramatically altered by several external stimuli (heat, solvent, light, chemicals, etc.) The development of these smart polymers is just beginning and examples reported above must not be seen as the limitations for this novel class of polymeric materials but rather as some possible applications. This great potential is also now backed by a better understanding of the molecular mechanisms that drive the observed optical and electrochemical effects. For all these reasons, it is believed that conjugated polymeric assemblies should allow the versatile design of novel integrated chemical systems (incorporating a trigger, a transducer, and an amplifier) and should, therefore, lead, in the near future, to new applications in the areas of sensors, diagnostics, and drug screening.

Mario Leclerc, MCIC is a professor with the D[acute{e]]partement de Chimie, Centre de Recherche en Sciences et Ing[acute{e}]nierie des Macromol[acute{e}]cules at Universit[acute{e}] Layal, Qu[acute{e}]bec, QC.

References

(1.) Skotheim, T., J.R. Reynolds, and R.L. Elsenbaumer., Eds., Handbook of Conducting Polymers, 2nd ed., Marcel Dekker, New York, 1998.

(2.) Leclerc, M., and K. Fa[ddot{i}]d, 'Conformation-Induced Chromism in Conjugated Polymers' in Handbook of Conducting Polymers, 2nd ed., T. Skotheim, JR. Reynolds, R.L. Elsenbaumer, Eds., Marcel Dekker, New York, 1998, p. 695.

(3.) Leclere, M., 'Optical and Electrochemical Transducers Based on Functionalized Conjugated Polymers', Adv. Mater. 11:1491, 1999.

(4.) Di C[acute{e}]sare, N., M. Bellet[hat{e}]te, C. Durocher, and M. Leclerc, 'Towards a Theoretical Design of Thermochromic Polythiophenes', Chem. Phys. Lett. 275:533, 1997.

(5.) Fa[ddot{i}]d, K., and M. Leclere, 'Responsive Supramolecular Polythiophene Assemblies', J. Am. Chem. Soc. 120:5274, 1998.
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Author:Leclerc, Mario
Publication:Canadian Chemical News
Date:Jun 1, 2000
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