Color reversible reaction

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

The reverse reaction, the photochemical ring opening of sphopyranes (22b), takes place by absorption ia the short-wave uv region of the spectmm and the merocyanine isomer (22a) is obtained. The electron transition of (22a) is ia the visible spectral region, whereas (22b) is colorless. As a result, the dye solution can change from colorless to a colored solution (87,88). These photochromic reactions can be used for technical appHcations (89). 

The species shown in red, HB and HA, act as Bronsted-Lowry acids in the forward and reverse reactions, respectively A and B (blue) act as Bronsted-Lowry bases. (We will use this color coding consistently throughout the chapter in writing Bronsted-Lowry acid-base equations.)... 

From a process point of view, the direct neutralization is clearly preferred moreover, the product quality (color) and free oil content deteriorates with aging (Table lb). The fact that the free oil and the inorganic sulfate level increase simultaneously upon aging is due to the fact that the formation of p-sultones from olefins is a reversible reaction [28], in competition with thermal rearrangement to alkenesulfonic acid and y- and 8-sultone. The effects of the reverse reaction of p-sultones are less with AOS because the rearrangement rates of AO-derived sultones are higher [29,35]. 

The color-formation reaction is not irreversible but reversible. Thus, the colored form can easily reproduce the colorless form by treating with base. 

Figure 6.3. Reversible color-formation reaction between fluoran 9 and Bisphenol A.

When a sample of N2O4, a colorless and poisonous gas, is placed in a closed container, a reddish-brown color develops due to the presence of nitrogen dioxide, NO2. As soon as some NO2 is formed, the reverse reaction can occur simultaneously with the forward reaction. 

Bisulfite ions, HS03", condense with anthocyanins. This reversible reaction decreases the color by forming a colorless compound (12) (16). This effect is less evident in strongly acid media because the bisulfite ions are not as numerous since they are being converted to the undissociated acid. This property explains the decolorization of red wines following sulfite treatment but, since it is reversible, the color gradually reappears as the free S02 (bisulfite ions) disappears. The major role of tannins in the color of old wines explains their insensitivity to color change with SOo. 

The concentration of the colored form at steady state concentration is largely dependent on the intensity of the incident radiation, quantum yield, kinetics of the reverse reaction, and temperature and solvent sensitivity of both the forward and reverse reactions. Normally the kinetics for the reverse reaction will be first or second order, although some systems are considerably more complex. Most reverse reactions are thermally sensitive and a few are accelerated by irradiation. 

The above properties (a and b) are interpreted by Cohen and Schmidt (21) on the basis of a detailed crystallographic study of photochromic and thermochromic anils. They conclude that photochromic crystals involve structures in which the central portion of adjacent molecules are essentially isolated from one another, so that, to a first approximation the energetics are that of an isolated molecule. On the other hand, when the alignment of the molecular dipoles is such as to give strong intermolecular interactions then the transition to the quinoid form requires much less energy and can occur thermally. For crystals in which thermochromism occurs, the photochemical isomerization is still possible but the reverse reaction is so rapid that no buildup of color is observed. In fact, fluorescence measurements on the thermochromic 5 -chlorosalicylidene-aniline (Fig. 5) indicate that photochemical isomerization precedes the luminescence process via the photochromic route ... 

None of the steroid molecules is colored so direct electronic spectrophotometric analyses must be done in the near UV range. The major chromophores are, of course, the aromatic ring, ketone, conjugated ketones, and ethynyl groups. With some exceptions prior derivatization reactions are generally required for the determinations of sterols. These reactions can be either reversible or irreversible. Ketal formation, for instance, is an example of the latter type color induction reactions are examples of the second kind. 

This chapter relates to the photochromism of viologens and thionine dyes proceeding by electron transfer. The colored species spontaneously disappear in a liquid phase, because the rate of back electron transfer (dotted line in Figure 9.1) is rapid and is accelerated by the existence of oxygen in the system. If the compound is dispersed in a solid-state or layered system, the reversible reaction can be controlled. Since the color development via electron transfer is generally very rapid and the reverse electron transfer can also be carried out electrochemically,4 the combination of both photoreduction and electrooxidation will receive much attention for application in recording devices. 

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