Thermochromic devices

Reliable micro-scale measurement and control of the temperature are required in developing thermal micro-devices. Available measurement techniques can be largely classified into contact and non-contact groups. While the resistance thermometer, thermocouples, thermodiodes, and thermotransistors measure temperature at specific points in contact with them, infrared thermography, thermochromic liquid crystals (TLC), and temperature-sensitive fluorescent dyes cover the whole temperature field (Yoo 2006). 

Research on Fe(II) 1,2,4-triazole polynuclear SCO complexes has undergone renewed activity over the last ten years since their potential for being incorporated in memory devices and displays was outlined by Kahn in collaboration with an industrial partner [7]. Towards this end, spin transition (ST) materials showing wide hysteresis effects around room temperature along with thermochromic behaviour are currently being sought [8]. 

In these polymeric species, the M,AT2-1,2,4-triazole linkage is rigid, and allows an efficient transmission of cooperative effects. Consequently, abrupt ST with broad thermal hysteresis loops have been observed [26, 32-34]. The absorption spectra of these compounds show a broad band at 520 nm corresponding to the Aig Trg d-d transition in the LS state whereas no band is found in the visible region in the HS state, the 5T2g-5Eg transition being located around 850 nm [7a]. The ST is thus accompanied by a thermochromic effect, purple (LS) and white (HS). These characteristics make these compounds potential candidates for practical applications, e.g. thermal display devices [7, 8, 17]. Such behaviour has been observed, for example, in the compound [Fe(4-amino-l,2,4-triazole)3](NC>3)2 [32] whose SCO is associated with a hysteresis loop of width 35 K, centred above room temperature [8]. 

Selective reflection of light Thermochromic cholesteric devices, gel displays... 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

In one such approach we seek to exploit the potential signal transduction properties of the thermochromic Dawson [40] polyoxometalates and combine this with a fluorescent POM-hybrid to produce a device that could respond optically as a function of the local environment, see Figure 2.12. In this example, the clusters would be positioned on a gold surface using SAMs (Self-assembled monolayer) with a cationic tail and local positional control could be aimed using self-assembly, or even by means of an atomic force microscope tip. 

Proton tautomerism in the solid state has drawn attention from the aspect of application as well. Further investigations in related areas, such as photochromism, thermochromism, photochemical hole-burning and hydrogen-bonded dielectrics, may open a horizon of protonic molecular devices. 

Materials of this type may find application in thermochromic or liquid crystal devices. 

Sage I (1991) Thermochromic liquid crystals in devices. In Bahadur B (ed) Liquid crystals applications and uses. World Scientific, Singapore... 

I. Sage, Thermochromic liquid crystal devices in liquid crystals-applications and uses, Vol. 3, ed. B. Bahadur (World Scientific, New Jersey, 1990). 

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