Thermal fading

Copper (I) (cuprous) ion serves as a catalyst for both the photochemical darkening and thermal fading reactions (1). Therefore, a smaU amount of cuprous ion is normally added to the glass batch. 

Thin films of photochromic glass containing silver haUde have been produced by simultaneous vacuum deposition of siUcon monoxide, lead siUcate, aluminum chloride, copper (I) chloride, and silver haUdes (9). Again, heat treatment (120°C for several hours) after vacuum deposition results in the formation of photochromic silver haUde crystaUites. Photochemical darkening and thermal fade rates are much slower than those of the standard dispersed systems. 

Photochromic silver—copper haUde films were produced by vacuum evaporation and deposition of a mixture of the components onto a sUicate glass substrate (13). The molar ratio of the components was approximately 9 1 (Ag Cu) and film thicknesses were in the range of 0.45—2.05 p.m. Coloration rate upon uv exposure was high but thermal fade rates were very slow when compared with standard silver haUde glass photochromic systems. 

Photochromic compounds that can be thermally faded have also been used in engineering studies to visuali2e flows in dynamic fluid systems (44,45). 

Phenols of stmcture (4) are also claimed to show markedly improved dye stabiUty both in the presence of ferrous ion and, with a second carbonamido group in the 5-position, to simple thermal fade (62). Numerous substituent variations are described in the Hterature to adjust dye hue. A perfluoroacylamido in the 2-position shifts the hues bathochromicaHy while maintaining thermal stabHity of the dyes (63). Phenols of stmcture (5) are said to show outstanding light stabHity, which makes them especiaHy suitable for display materials like color paper (64). 

The substituent effect in the phenyl group at the 3-position is also observed in the benzothiazoline series.2,17 The thermal fading rate increases with the bulky group at the 3-position, but the colored forms of 3-methoxy and 3-phenoxy derivatives are largely stabilized by an intramolecular interaction with 5-hydrogen atom.2... 

The photocolored form is assumed to form via the heterolytic cleavage of the C—O bond. Studies on the thermal fading kinetics have shown 2,50... 

In the case of CHR2, no steric crowding of protons occurs for the TT isomer, which can therefore adopt a planar geometry. In this case, the TT isomer is extremely stable and does not thermally fade [30,77],... 

The use of photochromies in novelty printing is a signiheant commercial outlet. Because of their ability to show resistance to thermal fade rates fulgides have been the class most commonly used for this application. Typical uses are on children s toys and for logos on T-shirts. 

Steady-state photoinduced electrochromism was achieved in organic solutions [34,35], microcrystals [36,37], LB films [7,8,38,39], and polymer films [40- 48] which was due only to the photoinduced electron transfer reaction via the excited state of specific IPCT complexes [49,50], The photochemical coloring and the thermal fading due to the reverse electron transfer were highly reversible in a deaerated atmosphere in all systems [7,8,34 18], The lifetime of colored (blue)... 

Trimethylindolenine can also be N-alkylated by a-haloketones such as bromoacetone (58% yield). For a series of l -acetonylBIPS, the thermal fade rate constant was several times larger than for the corresponding l -methylBIPS. The absorption spectra exhibited bathochromic shifts in ethanol, but negligible shifts in... 

The nitration of 5-chloromethylsalicylaldehyde in the absence of water gave the 3-nitro derivative in 43% yield, which was very easily converted to the hydroxymethyl and acetoxymethyl compounds. These gave the corresponding photochromic 8-nitro-6-substituted BIPS, which had Xmax 590 and 600 nm, respectively. Compared with 8-nitroBIPS itself, the thermal fade rates of the compounds having the additional 6-substituents were slightly faster in toluene and slightly slower in dioxane.109... 

The absorption maxima in toluene and ethanol, the thermal fade rate constants in toluene and dioxane, and the kinetic parameters AH and AS for a series of F-phenyl- and T-carbethoxymethyl-6-nitro BIPS having also methoxy, bromo, or nitro substituents in the 8-position have been recorded. The effects of the substituents on the Xmax and the rate constants are discussed. The preparation of 1-carbethoxy-methylFischer s base is given in detail.112... 

The thermal fading rates were measured in toluene at 10°C for a series of 6-nitroBIPS having additional substituents (nitro, methoxy, chloro, bromo, carbo-methoxy, and carboethoxy) in the 5 - and 8-positions. The rate constants were fitted to an equation of the form log (k/k0) =piOi + p2a2 with a correlation coefficient / =().85. This relatively low correlation coefficient, which is not improved by attempts to include the specific steric effects of the 8 substituent or by including the cross-term p12ai02, implies the complexity of the substituent effects on the fading rate.113... 

In a search for visible-light photoinitiators for stereolithography, the 6,8-diiodo-, 6-nitro-8-iodo-, and 6-nitro-7-methoxy-8-iodo- and T-benzyl-6-nitro-7-methoxy-8-iodoBIPS [respective Xmax (nm) and first-order half-lives for thermal fading (s) 606, 35 572, 330 568, 70 and 578, 15] were converted to their colored forms by heat or UV irradiation in solution in trimethylolpropane triacrylate containing added coinitiators. The benzyl compound with added A -phcnylglyciiie was the best initiator when irradiated with a 632-nm laser beam.114,115... 

The thermal fade rates of 6-nitroBIPS and its 8-bromo, 8-iodo, and 8-methoxy derivatives were measured in 13 solvents of varying polarity. Plotting the log of the rate constants vs. the ET values of the solvents gave linear plots, not double-branched ones as found later by this research group. The ring closure involves a zwitterionic transition state. Semiempirical molecular orbital (MO) calculations predict that the most stable merocyanine configuration is the cis-trans-trans-cis form.121122... 

A series of seven substituted BIPS were treated in toluene and ethyl acetate with phenol and with 4-nitrophenol as proton donors. The changes in absorption spectra indicated the presence of H-bonded complexes with the colored form. The equilibrium constants and thermodynamic parameters for their formation were determined. The complex formation slowed the thermal fade rate significantly.126... 

Thus, the condensation of 2-phenyl-l,3,6-trimethyl-2-azaazulenium perchlorate with 5-nitrosalicylaldehyde gave the cationic styryl dye (A,max 510 nm) in 56% yield.139 This dye is the salt of the open form of the corresponding spiropyran, and upon treatment with base, a reaction which was not originally reported, was found to give the expected dye (126), which absorbs at 733 and 536 nm. This material did not thermally fade or photobleach to any significant extent with visible light in either polar or nonpolar solvents.7... 

The usual kinetic equations for the calculation of quantum yields and thermal fade rate constants have been extended by taking into account the information contained in the experimentally recorded absorbance vs. time curves recorded under continuous irradiation and by adding additional kinetic terms representing photodegradation or other mechanistic complications. The extraction of the rate constants and quantum yields from the experimental curve requires numerical integration and iterative calculations.176... 

Practical applications of photochromism at first (ca. 1955-70) concentrated on the spiropyrans, and especially on BIPS compounds, because of their ready availability, photosensitivity, convenient thermal fade rates, and good color contrast when perceived by the human eye. However, applications in which the dye was required to cycle very many times (e g., an optical binary switch for a photochemical computer memory) or be irradiated continuously (e g., sunglasses, vehicle windshields) were impractical because of the rapid fatigue of these dyes.196... 

The thermal fading of spirooxazines has been shown to be dependent on the matrix. Tateoka et al. 42 reported that the fading rate for the colored form could be increased by adding a plasticizer, for example, dibutyl phthalate to poly(vinyl... 

The kinetic scheme for analyzing the thermal fade rate (see Scheme 12) is given by Eq. 6.1. Neglecting a photochemical back reaction 9 —> 7 (see earlier discussion), the system can be characterized using the rate constants of the thermal steps k l,k2 and k 2. All spectroscopic data (see later discussion) indicate that the equilibrium Bi, B2 is very fast (k first-order analysis, the thermal fade rate should depend only on k-. The values obtained experimentally clearly follow first-order kinetics, and the parameters obtained by standard evaluation were given in Ref. 3. 

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