Switchable electrical properties

A three-level switching device has been demonstrated in which photochromic properties are used to control electrical properties, and vice versa. Such a system has been realized in the form of thiophene bisphenol [90, 91]. Conversion of the open (8a) to the closed (8b) form of the thiophene was achieved by absorption of 312 nm light, and revered by absorption of 600 nm light. The bisphenol oxidation occurs at +0.735 V (vs. SCE), forming the closed-ring bisquinone, compound 8c. This species has large absorptions at 400 and 534 nm. The optical properties of the quinone phenol couple have previously been used in a bianthrone-based system [87]. The bisquinone (8c) cannot be converted to the open thiophene, and locks the system in the closed form. The thiophene has also been incorporated as a component in two-level molecular switches [99, 128] and switchable molecular wires [30]. 

Among the various interesting and useful properties of the new class of polymers, their switchable electrical conductivity has proven the most attractive to the community of chemists and physicists, and so it is understandable that these polymers are called conducting polymers. Much effort has been spent on measurements of their electrical conductivity and on determinations of the factors that affect its value [44,113,119,124,151,213,314,327,328,344,345,393 17]. The use of the conventional ex situ dc four-point method [44,393,398,399] or the ac impedance technique in a metal [polymer metal sandwich arrangement [119,124,410] for measurements of the conductivity of dry polymer samples is straightforward. However, the conductivities of dry polymers are affected by humidity and any gas present. Indeed, this is the property that is utilized in gas sensors. Conductivity can also be measured in situ, i.e., under controlled electrochemical and chemical conditions [151,394,395,397,400,402,406,408,417]. 

A unique electrically conductive material, distinguished by a long term stability and fast switchable electrophysical properties, is poly(3,4-ethylenedioxythiophene) with specific electrical conductivities up to 30 S cm" and more. It can be used for antistatic coatings (see section 10.3), as electrode material for solid state capacitors (see section 10.5), for electrochromic devices (see section 10.8) and as IR absorber [11]. 

The ferroelectric materials show a switchable macroscopic electric polarization which effectively couples external electric fields with the elastic and structural properties of these compounds. These properties have been used in many technological applications, like actuators and transducers which transform electrical signals into mechanical work [72], or non-volatile random access memories [73]. From a more fundamental point of view, the study of the phase transitions and symmetry breakings in these materials are also very interesting, and their properties are extremely sensitive to changes in temperature, strain, composition, and defects concentration [74]. 

The signal-triggered functions of these molecular assemblies have to be first characterized in bulk solution. Then, extensive efforts have been directed to integrate these photoswitchable chemical assemblies with transducers in order to tailor switchable molecular devices. The redox properties of photoisomerizable mono-layers assembled on an electrode surface are employed for controlling interfadal electron transfer [16]. Specifically, electrical transduction of photonic information recorded by photosensitive monolayers on electrode supports can be used in developing monolayer optoelectronic systems [16-19]. Electrodes with receptor sites exhibiting controlled binding of photoisomerizable redox-active substrates from the solution [20] also allow the construction of molecular optoelectronic devices. 

Anisotropic particles, such as patchy, multicompartment, and Janus particles, have attracted significant attention in recent years because of their novel morphologies and diverse potential applications. The noncentrosymmetric features of these particles make them a unique class of nano-or microcolloidal materials. Patchy particles usually have different compositional patches in the corona, whereas multicompartment particles have a multiphasic anisotropic architecture in the core domain. In contrast, Janus particles, named after the double-faced Roman god, have a strictly biphasic geometry of distinct compositions and properties in the core and/or corona. This property of Janus particles allows for distinct shape, composition, chemistry, polarity, functionality, electrical, and properties, making them suitable for applications in switchable display devices, interface... 

Tremendous activity [33] has characterized the field of research on polymer-dispersed liquid crystals (PDLCs) which are potentially useful for a variety of electrooptical applications including light shutters and switchable windows, displays, and other devices. These materials consist of micrometer-size nematic droplets dispersed in a polymer matrix and their optical response is based on the electrically controlled light-scattering properties of the droplets. 

PEDOT is a widely used / -type semiconductca . Typical applications include hole injection layers in OLEDs and LCD, antistatic coatings, electrically switchable windows, and polymer solar cells. In most of these applications, PEDOT-poly(styrenesulfonic acid) (PEDOT-PSS) copolymer is used because of improved solubility, film forming properties and stability [118]. 

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