Thermal bleaching

Thin films of photochromic silver complex oxides were prepared by anodic oxidation of silver metal films (15). Complex oxides, such as Ag2V04, Ag SiO, and Ag2P04, darkened by exposure to visible light, but required heating to 150—250°C for thermal bleaching. 

The brown color of the Ni(n-Bu2 Dtc) J cation is photochemically and thermally bleached reversibly in CH3CN solution. Thiuram disulfide and the Ni(II) complex were identified as products of this bleaching process. The kinetics of the reappearance of color were reported to depend on the square of the concentration of the bleached nickel and were inversely proportional to the concentrations of thiuram disulfide and Br (231). 

The various photochromic parameters which were taken into account in the case of 243 were absorption wavelengths of the closed and open forms, rate constants of thermal bleaching (kA) and coloration ability or colorability (measured as the absorbance A0 immediately after the irradiation flash) (00JPO523). 

Quantum mechanical calculations on the 2//-pyran ring opening and closing agree with the different mechanisms for photocoloration and photodecoloration experimentally observed in spiropyrans. The photobleaching proceeds from the 7 state, and mechanistically is very different from thermal bleaching, which proceeds from the S0 state and has a much lower activation of energy barrier. 

Spirooxazines (NISO) have UV activation properties and thermal bleaching properties that are convenient for eyewear applications. A plastic photochromic lens must have several features, such as (1) ultraviolet energy protection, (2) comfortably lightweight, and (3) tint from a fashionably light tint to a functionally darker tinted sun lens. 45... 

The thermal bleaching rate of the photoinduced form of 5-methyl-1,4-naphtha-quinone depended on the solvent polarity (Table 7.13)." It was assumed" that an... 

The substituents in the anthraquinone ring affected the rate constant of thermal bleaching of derivatives of 1-methylphenoxyanthraquinone (Table 7.1).17,18,26 The substituents that increased the Tt-electron density on the hydrogen atom of the methide group decreased the lifetime of the photoinduced form (Table 7.1).18,26 The introduction of methoxy and piperidine substituents stabilized the photoinduced form (Table 7.1). 

The rate constant of thermal bleaching of derivatives of 1-methylanthraquinone depended on the nature of the solvent. Table 7.14 shows that it decreased by several orders or magnitude as the proton-donor ability increased. 

Quantum yields of photoproduction of ana-quinones of acetoxy-substituted anthraquinones with amino substituents in the anthraquinone ring as well as with an acetyl group proved to be the lowest (Table 7.2) As in the case of photochromic alkylanthraquinones, the reverse photoreaction, from ana-quinone to para-quinone for derivatives of acetoxyanthraquinone, proved to be impossible.21 The transition from ana-quinone to para-quinone occurred during freezing out of a sample owing to thermal bleaching. 

Table 7.15. Kinetic Characteristics for Processes of Photocoloration and Thermal Bleaching of 1-Aryloxyanthraquinones (R1 = OC6H4R, R3 = H) in Polymeric Matrix at 290 K (Type II)27...

A peculiar feature of the photochromic behavior of perimidinespirocyclohex-adienones and their structural analogs is an extremely slow thermal bleaching of the colored quinoneimine photoisomer lb caused by its conversion to the ring-closed form la. The effective lifetimes of the colored isomers formed upon UV irradiation of solutions of perimidinespirocyclohexadienones in nonpolar solvents at room... 

Alkyl side chain of p-alkylbenzene sulfonate as counteranion of Bzl-V2+. bAl= values at /.max indicated in parentheses, A50= after 50 cycles of irradiation and subsequent thermal bleaching (70°C for lOmin). 

The thermal decay rates of the colored forms, which correspond to the amount of time required for a UV-activated photochromic to lose its color, typically range from 0.02 to 1.51 mol"1 s"1. However, this decay rate is highly influenced by temperature and the nature of the polymeric host. For instance, at 0°C a SO possesses a thermal fade rate of 2.67 x 10 2, and at 50°C the same molecule demonstrates a thermal decay rate of 3.3221. In fact, the thermal bleach reaction of some SOs ceases at -60°C. 

Chapter 7 (Structural Studies by X-ray Diffraction). The geometric parameters of photochromic compounds have been determined by X-ray diffraction on single crystals and yield interesting correlations with photochromic behavior (colorability, thermal bleaching rates, or absorption spectrum of the colored species). More intensive studies have been carried out on members of the spiropyran series (indolinospiropyrans), examining both closed spiranic forms and permanent merocyanines. The latter served as models of open photomer-ocyanines, the transient species produced by UV irradiation of spiropyrans. 

Table 1 also includes the values of the durability parameter, Vl/2, and the colorability, defined as the apparent molar extinction coefficient EapP = DB//[A], along with the thermal bleaching rate constants k and after the first and nth flash, respectively. It may be noted that k generally increases with the number of flashes. 

A CH3 group on C3 greatly decreases stability whereas QH5 substitution yields stabilization (Table 5, 47 vs. 48). It was noted that, in general, the rate constant k of the thermal bleaching reaction was not affected by the ct Hammett constants of substituents on the indoline moiety whereas for substituents on the chromene moiety EDGs produced an increase of k. 

These compounds were extracted by refluxing the aged photochromic PU films. A kinetic analysis of the thermal bleaching of merocyanine in PU showed that, at variance with that was observed in toluene solution, no acceleration of the ring closure rate took place. The decoloration was described by a biexponential function ... 

Finally, the thermal bleaching rate constant of the merocyanine back to the colorless form can be measured. In the kinetic mode, once the initial transmittance I0 data has been acquired, the flash is fired and the kinetic decay is sampled with preset rate. For biphasic or multiphasic kinetics, the decay can be adequately described by a sum of exponentially decaying functions /) ) ... 

Chirico G, Cannone F. Baldini G. Diaspro A (2003) Two-photon thermal bleaching of single fluorescent molecules. Biophys J 84 588-598... 

The presence of impurities or foreign ions in KC1 has a marked effect on the coagulation of F-eentres. Watson and Scott83 have shown that the introduction of alkaline earth ions into additively coloured KC1 enhances the stability of F-centres. Markham and Hersh84 have found that the optical and thermal bleaching properties of potassium halides are radically affected by doping with thallium ions. 

Trapped electrons are not produced in terf-butyl alcohol by y-irradi-ation in the solid phase (7, 9). They are produced and trapped in this matrix by a special technique of depositing alkali-metal atoms on tert-butyl alcohol at 77°K. as demonstrated by Bennett et ah (5). The ultimate fate of these trapped electrons was not established since no additional radical species were observed on photo- or thermal bleaching (6). In our case tert-butyl radicals are produced on photobleaching the trapped electrons. Possible reasons for this difference are discussed later. 

In contrast to the results obtained by the optical bleaching of y-irradiated samples, thermal bleaching of two samples containing 2.1 and 6 mole % did not produce any CH3 radicals during the warmup. In each case the electron singlet decayed out completely at about 95 °K., and the radical spectrum disappeared at 97 °K. 

The glow curve of a sample which no longer responds to infrared light (owing to photobleaching or thermal bleaching) consists of the second peak only. 

Table I. Evolution of the Photoresponse of a 5 X 10 "4M Solution of Biphenyl in MCH Glass after Thermal Bleaching of the First Peak and Illumination in the 15,500 cm.-1 Band...

The ESR spectrum is easily microwave-power saturated and is obscured somewhat by resonances attributed to an F center (see below). The spectrum is clarified if observations are made at 120°K, since this temperature serves both to thermally bleach the F-center resonance and to make the N° resonance less susceptible to microwave power saturation, due to a shorter spin-lattice relaxation time. The nitrogen atom has an electronic spin of 3/2 due to its three un-... 

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