Ionochromism ionochromes

Levesque I, Leclerc M (1996) Ionochromic and thermochromic phenomena in a regioregular polythiophene derivative bearing oligo(oxyethylene) side chains. Chem Mater 8 2843-2849... 

Marsella MJ, Swager TM (1993) Designing conducting polymer-based sensors - selective ionochromic response in crown-ether containing polythiophenes. J Am Chem Soc 115 12214-12215... 

The chemistry and applications of the colour change grouping, containing all the well-known isms of chromic phenomena, namely photochromism, thermo-chromism, ionochromism, electrochromism and solvatochromism, as well as the lesser-known ones such as tribochromism and vapochromism, are covered in Chapter 1. These chromic phenomena impinge on our everyday life, e.g. in photo-chromic spectacle lens, thermochromic temperature indicators, fax paper, smart windows and mirrors and in visual displays. 

Typically the chromophores used are pH sensitive, i.e. acidochromic or ionochromic (see section 1.4), and the medium is one whose pH varies with temperature. A variety of pH sensitive chromophores are known that are readily synthesised and can be modified to provide a wide range of shades. Application of these pH sensitive dyes in pressure and thermally sensitive papers for digital printing, e.g. fax papers, where the whole object is for the process to be irreversible, will be covered under ionochromism. The main area of application for reversible indirect thermochromic materials is as thermochromic organic pigments, as will be described below (section 1.3.4.1). 

Polysilanes with several oxygen atoms in the side chains have been studied by Cleij and coworkers.83 84 These polysilanes may become water soluble, as is the case for [MeSi(CH2)3(OCH2CH2)3OCFl3] . In THF solution at 20 °C this polymer absorbs at 305 nm, but in water the UV band is shifted to 281 nm. The water may cause tighter coiling of the polymer chains, perhaps due to micelle formation. Between 46 and 70 °C, the water solution undergoes phase separation. The UV band shifts to 320 nm, consistent with partial straightening of the polymer chain after phase transition. These polymers also display ionochromism. 

Figure 5.17 UV spectrum versus temperature for a film of IC2H5OCILCI I2OfCH2Ts 25>i ] (a) neat (b) with the addition of 75 mol % of LiS03CF3, showing a strong positive ionochromic effect.

In this chapter, the first section will deal with the complex question of conformations of polysilanes. This will be followed by sections on the chromotropism of polysilanes, beginning with their UV thermochromism. Despite intensive study this phenomenon is still not well understood, and is the subject of important recent findings. Solvatochromism, ionochromism and other types of chromotropic behavior of polysilanes will then be considered. A final brief section will deal with the electrical conductivity of polysilanes, a topic which has not previously been reviewed. 

An opposite type of behavior, positive ionochromism, is shown by the polymer [C2H50CH2CH20(CH2)5]2Si n52- In this case, as shown in Figure 9, addition of lithium triflate to a thin film of the polymer promotes a bathochromic shift, increasing the midpoint temperature of the thermochromic transition from —45 to +50 °C Probably in this example the lithium ions, complexed to the ethylene oxide units, lock the polymer... 

The generality of these positive and negative ionochromic effects has not been explored further studies are needed. 

FIGURE 10. UV spectra for [C2H50CH2CH20(CH2)5]2Si n in dibutyl ether at 23 °C, showing ionochromic effect in solution. Numbers indicate the molar ratio of LiSC>3CF3 Si. Reprinted with permission from Reference 54. Copyright 2000 American Chemical Society... 

Poly(methylsilylene-/>-phenylene)s carrying Si-linked benzocrown ether and ethoxy pendant groups 86 (parameters x and y are associated with the benzo crown vs. ethoxy ratio present in the polymer) showed ionochromic behavior, and responded selectively to alkali and alkaline earth metal ions in the emission spectra, depending on the crown ether ring size. These polymers also display solvatochromic properties by changing the solvent polarity in the absence of metal ions <2006OM2225>. 

Another interesting behavior observed in polysilane films is ionochromism where the thermochromic frequency shift is either inhibited (negative inonochromism) or accelerated (positive ionochromism) upon addition of lithium salts. ... 

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