Neutral Conjugated Polymers: Chromism

Neutral Conjugated Polymers: Chromism Since the late 1970s conjugated polymers (polyacetylene, polyaniline, polypyrrole, polyphenylenevinylene, polythiophene, polyfluorene, etc.) have attracted a lot of attention from both academic and industrial laboratories mainly because of their interesting and unusual optical, electrochemical, and electrical properties (Skotheim et al. 1998). For instance, these polymeric materials are now well known for their high electrical conductivity arising on doping (partial oxidation or reduction). The delocalized electronic structure of these polymers is partly responsible for the good stability and mobility of the charge carriers created on doping and electrical conductivities in the range 1–100 S cm−" can be reached in most cases. The conjugated structure is also responsible for a strong absorption (and sometimes emission) in the UV– visible range. Not surprisingly, all these polymeric materials have been used as antistatic coatings, electrodes, transistors, light-emitting diodes, conducting photoresists, solar cells, etc. Moreover, processability 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 enhance the processability of these aromatic polymers but can also modify their electrical, electrochemical, and\or optical properties. As an example, electrochemical redox processes result in important optical changes (electrochromism) from dark red to blue with poly(3-alkylthiophene)s but from dark blue to pale blue-gray with poly(3,4ethylenedioxythiophene). Furthermore, the presence of substituents can even lead to physical phenomena that are not found in the parent, unsubstituted

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Figure 1 Repeat unit of (a) poly(3-alkylthiophene)s, (b) poly(3alkoxy-4-methylthiophene)s, (c) poly(3(alkylthio)thiophene)s, (d) poly(3,3h-di(alkylthio)-2,2hbithiophene)s, (e) polydiacetylenes, and (f) polysilanes.

polymers. For instance, the UV–visible absorption characteristics (both in the solid state and in solution) of some neutral π-conjugated polythiophenes and polydiacetylenes and σ-conjugated polysilanes (see Fig. 1) can be strongly modified by varying the temperature, the pressure, the solvent quality, etc. (Sandman 1994, Ingana$ s 1994, Leclerc and Faı$ d 1997, Skotheim et al. 1998, Okada et al. 1998). It is the purpose of this article to give a detailed description of these intriguing chromic (thermochromism, piezochromism, solvatochromism, ionochromism, photochromism, and affinitychromism) effects and to present applications of these smart polymers in different fields.

1. Thermochromism and Solvatochromism As an example of thermochromism reported for some neutral conjugated polymers, Fig. 2 shows the temperature-dependent UV–visible spectrum of a regioregular poly(3-alkoxy-4-methylthiophene) (poly(3-(ω-(12-crown-4)-methoxyhexyloxy)-4-methylthiophene)), in acetonitrile. At low temperatures, this polymer is highly conjugated with an absorption maximum near 545 nm, the two other absorption peaks being related to a vibronic fine structure. On heating, an important color change (from red-violet to yellow) takes place, which is related to the shift of the maximum of absorption from 545 nm to 425 nm. The color change is reversible (but with the presence of some hysteresis) and, on cooling, the polymer solution recovers its initial absorption spectrum. The thermochromic transition has been studied as a function of time, and no variation in the absorption spectrum of the polymer has been observed for a fixed temperature. Clearly, these optical effects cannot be explained by a degradation of the polymer. In dilute solutions, no dependence on the polymer concentration has been observed, which suggests that aggregation is not necessarily responsible for the thermochromic transition although aggregation and precipitation of the polymer chains can be observed at very low temperatures and\or after a long period of time. The presence of an isosbestic point indicates a cooperative effect and, therefore, the coexistence of two different chromophores. However, it is impossible to determine whether these two chromophores are on different parts of the same polymer chain or on different polymer chains. Very similar optical features can be obtained in the solid state but, usually, for a given polymer, at higher temperatures than those observed in a good solvent. The higher temperature ranges can be partly explained by a more viscous medium. Interestingly, it is possible to decrease the temperature range of these solid-state thermochromic effects by increasing the length and\or the flexibility of the side chains. The kinetics of this conformational transition is complicated by the pres1