Vapochromism

In vapochromic systems dyes change colour in response to a vapour or an aroma, and therefore an alternative name is aromachromic. Systems based on this principle can be considered as belonging to the family of synthetic nose devices. 

In a recent example metalloporphyrins are used as the sensing dyes for a wide variety of odorants, including alcohols, amines, arenes, ethers, halocarbons, ketones, phosphines, thioethers and thiols. An array of four different metalloporphyrins are placed on a substrate and exposed to the vapours for 30 s. The various vapour molecules coordinate onto the central metal atoms of the porphyrins, causing them to change colour and producing a unique four-colour array. The resulting colour array is compared with a library of known chemicals or mixtures and identification is achieved. 

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Extended linear chain inorganic compounds have special chemical and physical properties [60,61], This has led to new developments in fields such as supramolecular chemistry, acid-base chemistry, luminescent materials, and various optoelectronic applications. Among recent examples are the developments of a vapochromic light emitting diode from linear chain Pt(II)/Pd(II) complexes [62], a luminescent switch consisting of an Au(I) dithiocarbamate complex that possesses a luminescent linear... 

With the complex where L = pyridine an optical nanosensor was developed [135-137], the method used to fix the vapochromic material to the optical fiber was the electrostatic self assembling method (ESA) and the light source used was an 850 nm LED. The sensor was tested for two different alcohols (ethanol and methanol) and it was possible to distinguish between different concentrations. It was also possible to discriminate between the two different alcohols. 

Applications of the pentafluorophenylgold complexes have been mentioned when appropriate throughout the chapter. Catalytic reactions, vapochromic behavior. 

Bariain, C., Mati as, I.R., Romeo, L, Garrido, J. and Laguna, M. (2000) Detection of volatile organic compound vapors by using a vapochromic material on a tapered optical fiber. Applied Physics Letters, 77(15), 2274-2276. 

Bariain, C Matias, I.R., Fdez-Valdivielso, C Elosiia, C Luquin, A., Garrido, J. and Laguna, M. (2005) Optical fiber sensors based on vapochromic gold complexes for environmental applications. Sensors and Actuators B, 108, 535—541. 

Square-planar ds platinum complexes could perhaps be used as the chemically sensitive layer for a chemical sensor system. These complexes are robust and form colored solid-state materials that respond spectroscopically to a wide range of volatile organic compounds (VOCs) this process has been named vapochromism. The vapochromic shifts occur in the solid-state UV-vis,... 

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. 

Gasochromism is a colour change brought about by a gas (see also vapochromism). 

The building block [Au(CN)2] has been revealed as a versatile unit in the formation of heteronuclear arrays which, in some cases, display unusual and interesting luminescent properties, such as vapochromic behavior. In many of these complexes the cyanide group uses the carbon and nitrogen ends as Lewis bases able to coordinate two different metal cations (i.e., Au-CN-M) [82-86]. These ideas may be illustrated by the reaction of K[Au(CN)2] with MX2 salts (M = Cu, Ni, Co, Mn ... 

Connick and co-workers have found that the N,N-dimclhylated bzimpy Pt(II) complex, 42, displays interesting vapochromic effects [101]. When the counterion is Cl , thin films respond to vapors of MeOH, CHCI3, EtOH and MeCN by changing color within a few seconds from yellow to red. The PF6 salt, on the other hand, responds only to MeCN, changing from yellow to violet. Emission spectra are also affected by the vapors, albeit more weakly. On the basis of the spectroscopic evidence, it appears that the vapochromic response is associated with a decrease in the Pt - Pt separation upon uptake of the vapor, favoring the MMLCT [da -> n ] transitions. 

Leznoff and co-workers reported the structural characterization and the polymorphic nature of a gold(I)-copper(II) coordination polymer, [Cu Au(CN)2 2(dmso)2] 00 [125]. In a green polymorphic form it possessed a 1-D chain structure with five-coordinate copper(II) centers, while a 2-D corrugated sheet structure with six-coordinate copper(II) centers was characterized in the blue polymorph, both of which are linked together to form a 3-D structure via aurophilic interactions. These two polymorphs exhibited virtually identical vapochromic behavior towards water, MeCN, dioxane, dmf, pyridine and ammonia, with the formation of [Cu Au(CN)2 2(solvent)x]oc,. 

SCHEME 1. Vapochromic bahavior of complexes [AuAg(C6F5)2L]2 . 

As far as we are aware, the number of luminescent extended systems with gold-platinum interactions is reduced to only one report. This is a puzzling situation since the situation of platinum in the periodic table suggests considerable relativistic effects for this atom and extended chains of squared-planar Pt(II) cation-anion complexes built by metal-metal interactions are not strange. In fact, salts such as [Pt(CNR)4][Pt(CN)4] ,67 which are luminescent and display vapochromic behavior, and the modified form of the Magnus salt [Pt(NH3)4][Pt(CN)4] ,68 which shows semiconducting properties, are examples of this type of supramolecular systems. Therefore, the stable combination of gold and platinum in cation/anion-acid/base systems should be anticipated. 

Moreover, the complex displays a completely reversible vapochromic behavior with reversible changes of color when the solid is exposed to a variety of organic vapors, which is even deeper under UV light. These results illustrate the potential applications of this complex for the detection of VOCs. 

The dyes 29 and 31, 32, 33 and 34 (Scheme 9) are not only vapochromic but also thermochromic and mechanochromic. Modification of the packing state by a thermal process, applied mechanical stress, and exposure to vapour is transduced to alteration of the luminescent properties of the dye according to the mechanisms discussed above. 

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