Field Responsive Polymers - American Chemical Society

Chapter 8

Field-Responsive Conjugated Polymers Karim Faïd and Mario Leclerc

Downloaded by CORNELL UNIV on October 28, 2016 | http://pubs.acs.org Publication Date: August 19, 1999 | doi: 10.1021/bk-1999-0726.ch008

Département de Chimie, Université de Montréal, C.P. 6128, Succ. Centre Ville, Montréal, Québec H3C 3J7, Canada

Neutral, highly regioregular polythiophene derivatives undergo striking optical changes upon exposure to various external stimuli. These optical changes are believed to be related to a conformational transition of the polymer backbone,froma planar to non-planar form, triggered by adequately functionalized side-chains. In addition to the well-known chromic transitions induced by heating (thermochromism) or solvent quality changes (solvatochromism), novel phenomena have been generated including the detection of alkali metal cations (ionochromism), UV-induced dual photochromism and molecular recognition of chemical or biological moieties (affinitychromism).

In addition to the use of colorimetric detection, due to the change in the absorption characteristics of the polymer backbone, electrochemical techniques can be also advantageously employed. The recognition or binding events, between the functionalized side chains and the external stimuli, could be detected and measured by taking advantage of the large difference in the electronic structure between a planar and a nonplanar form of the polymer backbone. This results in a very significative shift of the oxidation potentials, allowing the design of highly selective and efficent electrochemical sensors.

The search for smart materials is an exploding researchfielddue to the high demand for materials capable of carrying out increasingly complex tasks. One of the main requirements is the obtention of materials that can perform various functions while shrinking cost requirements. Field-responsive materials are one of these fast developing areas and can be defined as materials in which a given property might be changed in a

© 1999 American Chemical Society Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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114 measurable way through its interaction with some external stimuli. The detectable characteristic can be any optical, electrical or magnetic properties while the external stimuli can be any form of energy or matter. Amongfield-responsivematerials, functionalized regioregular conjugated polythiophene derivatives have beenfoundto be a very promising family, with impressive conformational changes upon exposure to specific stimuli. These conformational changes, which are believed to be related to a planar to non-planar transition of the conjugated backbone, result in very pronounced chromic effects,fromdeep violet to bright yellow. The utilization of suchfield-inducedchromic effects will be the subject of this chapter and we will review some examples in which the external stimuli can be varied from heat, light, ions or biological moieties while the side-chain moieties are tuned accordingly. A brief presentation of conjugated polymers will be also provided as well as a presentation of different chromic polymers andfinallysome possible applications will be discussed. Conjugated Polymers During the last twenty years, conjugated polymers (Figure 1), such as polyacetylenes, polyanilines, polypyrroles, polythiophenes, etc., have attracted tremendous attention, mainly because of their interesting optical, electrochemical and electrical properties. These properties may lead to a variety of applications such as information storage, electroluminescent devices, optical signal processing, solar energy conversion materials, electrochemical cells, EMI shieldings, antistatic coatings, bioelectronic devices, etc.[1-4]. For instance, these materials are well known for their high electrical conductivity arising upon doping (oxidation, reduction, protonation). The delocalized electronic structure of these polymers is partly responsible for the stabilization of the charge carriers created upon doping and electrical conductivities in the range of 1-1000 S/cm can be reached in most cases. Moreover, processability and a high level of conjugation have been obtained through the incorporation of alkyl side chains on polythiophenes[5-70]. However, the asymmetric nature of the starting monomers usually leads to the

Polyacctylcne

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Polythiophene

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H

H

H

H

Polypyrrole

-O-O-OO-O-O-OFigure 1: Structure of Different conjugated polymers

Khan and Harrison; Field Responsive Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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occurrence of head-to-tail, head-to-head or tail-to-tail couplings upon polymerization, which can yield up to four different triads along the backbone[77-74] (Figure 2).

Figure 2:

Head-Tail/Head-Head

Tail-Tail/Head-Head

Head-Tail/Head-Tail

Tail-Tail/Head-Tail

Different regiochemical structure in poly(3-alkylthiophene)s

Highly conjugated and fully substituted poly(3-alkoxy-4-methylthiophene)[75-77] have been designed in such a way that the presence of a second substituent was made possible by the introduction of the small oxygen atom in the vicinity of the thiophene backbone[77]. Moreover, the asymmetric reactivity of the oxidized monomers[7#-79] (Figure 3) allowed the preparation of poly(3-alkoxy-4-methylthiophene)s in good yields with a high degree of regioregularity compared to poly(3-alkylthiophene)s which are poorly regioregular when polymerized by oxidative means.

Polymerization

i

>95% Head to Tail

Figure 3:

Effect of the side-chain on the spin density distribution of oxidized thiophene monomers and on the regioregularity of the resulting polymers



116 The introduction of various substituents on the backbone can not only enhance the processability of some of these polymers but also modulate their electrical, electrochemical and optical properties. Electrochemical redox processes result usually in strong changes in the visible absorption spectra (electochromism),fromdark red to light blue in the case of poly(3-alkylthiophene)s[20]fromdark blue to transparent light blue in alkoxy-substituted poly(thienylenevinylene)s[27] and poly(3,4ethylenedioxythiophene)s[22]. On the other hand, the UV-visible absorption characteristics of neutral conjugated polymers can be varied by tuning their conformational structure. It has been shown that the backbone conformation has strong effects on the electronic structure of conjugated molecules and, therefore on their absorption characteristics[23-24]. For example, striking reversible chromic effects[25-29] have been reported in polythiophene derivatives upon heating both in solid state and solution (thermochrornism) or when the solvent quality is altered (solvatochromism). The dependence of the electronic structure of conjugated polymers upon their conformation have been fully described[2,24] and can explain the interesting optical effects that have been attributed to a reversible transition between a coplanar (highly conjugated) form and a nonplanar (less conjugated) conformational structure of the backbone[30]. Field-induced Chromism in Polythiophene Derivatives Many experimental results have suggested that the conformational modification of the polythiophene backbone can be induced through order-disorder transitions of the sidechains [30]. It was then postulated that various external stimuli could perturb the sidechain organization and consequently induce some chromic effects. These side-chain transitions can be induced by heating (thermochrornism), varying the solvent quality (solvatochromism), ion complexation (ionochromism), photo-induced isomerization (photochromism) and affinity binding (affinity or biochromism) givingriseto a novel class offieldresponsive materials. Thermochrornism. Neutral poly(3-alkylthiophene)s and poly(3-alkoxy-4methyhhiophene)s exhibit strong chromic effect upon heating both in the solid state and in solution[25-29]. Two types of thermochromic behavior can be observed and are correlated to the substitution pattern of the polymers. As an example of thefirsttype (the two-phase behavior), the temperature dependence of the absorbance of a thinfilmof poly[3-oligo(oxyethylene)-4-methylthiophene] [31] is shown in Figure 4. At room temperature this polymer is highly conjugated with an absorption maximum around 550 nm. Upon heating, a new absorption band, centered around 426 nm is increasing while the band at 550 nm is decreasing. This strong blue shift of the maximum of absorption upon heating could be related to a conformational transition from a highly conjugated form (coplanar or nearly planar, deep violet in color) at low temperatures to a less conjugated form (nonplanar and yellow in color) at higher temperatures [30]. At afixedtemperature, there is no evolution as a function of the elapsed time and, since these optical effects are also reversible, they cannot be therefore attributed to a degradation of the polymer.


117


T

- i

300

1

1

1

1

i

i

400

1

1

i

i

500

1

1

i

i

600

r

L_ 700

Wavelength (nm) Figure 4:

Temperature-dependent UV-visible absorption spectra of highly regioregular poly(3-(oligo(oxyethylene)-4-methylthiophene) in the solid state

A clear isosbestic point is also observed indicating the coexistence of two phases in the material while it is impossible, however, to determine whether these phases do exist on different parts of the same polymer chain or on different ones. A similar effect has been observed in solution, both in good and poor solvents [57]. In a good solvent (tetrahydrofuran), at room temperature, the maximum absorption of rx)ly[3-ofigo(oxyethylene)-4-methylthiophene] takes place at 426 nm, indicating that the polymer chains are already in a twisted conformation. Upon cooling, the color of the solution shiftsfromyellow at room temperature to violet at -100°C, the transition being fiiUy reversible [57]. Poly[3-oligo(oxyethylene)-4-methylthiophene] can be dissolved in poor solvents, such as methanol, but the solution is then violet at room temperature with the maximum of absorption at 550 nm, indicating that the polymer chains are mostly planar. Upon heating, the 426 nm band is increasing while the band at 550 nm is disappearing (Figure 5) in a very similar manner to the situation observed in tetrahydrofuran and in the solid state, although at different temperature ranges [37]. Differential scanning calorimetric measurements have revealed well-defined thermal transitions which are well correlated with the observed optical transitions[29]. Temperature-dependent X-ray diffraction analyses have revealed a rather amorphous structure for this polymer that is not strongly affected by the heating process[2P].



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Wavelength (nm) Figure 5:

Temperature-dependent UV-visible absorption spectra of highly regioregular poly(3-(oligo(oxyethylene)-4-methylthiophene) in poor solvent (methanol)

This two-phase thermally induced behavior have been also observed in other amorphous poly(3-alkoxy-4-methyltniophene)s and in semicrystalline poly(3-alkylthiophene)s which exhibit a strong chromic transition upon heating, with the occurrence of a clear isosbestic point, indicating the coexistence of two distinct conformational structures [29]. Fairly good correlation have been established between the melting of these polymers and the chromic transitions by DSC, FTIR and X-ray analyses[30], indicating that a similar twisting of the main conjugated chain can be observed in both the amorphous and crystalline phases, although the transition speed and temperature of the crystalline regions are expected to deviatefromthat of the amorphous zones, explaining the absence of clear isosbestic point in some semicrystalline polythiophene derivatives[30]. This two-phase chromic behavior has been observed in relatively well-defined regioregular polymers, with head-to-tail couplings rangingfrom80 to 98%. The head-totail structure refers to couplings between the 2-position of a thiophene monomer and the 5-position of a second one, leading to structures shown in Figure 2. Depending on the nature of the monomers used, different radiochemical structures can be obtained that can lead to fairly different properties. By adequately designing the polymerization procedures[32-33] of 3-alkyhhiophenes, highly regioregular polymers have been obtained which display a two-phase chromic behavior[29]. Highly regioregular poly(3-alkoxy-4methylthiophene)s have also been obtained by the simple and straightforward oxidative polymerization of the corresponding monomers. The asymmetric reactivity of these


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monomers[/9] leads to a high symmetry along the main polymeric chains, giving rise to a simple way of obtaining highly regioregular functionalized polymers (Figure 3). In contrast to highly regioregular polythiophene derivatives which display a two-phase behavior upon heating, only weak and monotonic shifts of the absorption maximum are observed upon heating of non-regioregular polythiophene derivatives[30]. For instance, poly(3-hexylthiophene) containing only 50% head-to-tail couplings, poly(3-dodecyl-2,2bithiophene), poly(3-butoxy-3'-decyl-bithiophene) and poly(3\4-

Field Responsive Polymers - American Chemical Society

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