Quantum yields photochromism

Stokes-shifted tautomer emission and high quantum yield in solid state (10%), DHBO was successfully used as an energy transfer donor in the photochromic switching system [88]. 

QUANTUM YIELD PHOTOCHROMISM PHOTO-CROSS-LINKING PHOTOAFFINITY LABELING PHOTODIMERIZATION PHOTOISOMERIZATION PHOTOLYSIS FLASH PHOTOLYSIS... 

Photochromism may be defined as the reversible light-induced interconversion of a system between two forms having distinguishably different absorption spectra.181 The generalized process is outlined in equation (55) where represents the quantum yield for photochemical reaction and... 

The contribution of fluorescence to the deactivation of the excited singlet state of the phytochromobilin chromophores in Pr and Pfr (see Section II.D) is negligible in quantitative terms. The total fluorescence quantum yield of the photochromic P and P3 components amounts only to from Tables 1 and 4). This means that deactivation proceeds predominantly through nonradiative channels, i.e., via internal conversion back to the electronic ground state of Pr and via primary photoreaction(s). Nevertheless, the fluorescence efficiency suffices to serve as a sensitive tool to monitor certain aspects of the competing primary reaction(s) of P (see Sections III.A and III.C). 

Photochromicity as well as lifetimes, relative amplitudes and quantum yields of P and Pr2, which account for 99% of the total amplitude of the fluorescence decay, were the same in HzO and DzO at 275 K (Table 4) and... 

A0 is expressed by A0 = Sqf coiMCFIo (at low concentration), where k depends on the experimental conditions, Sqf is the molar absorptivity of the open form at Amax, co is the photocoloration quantum yield, and [CF]0 is the initial concentration of the closed form. All the photochromic parameters are very sensitive to the substitution on the naphthopyran ring. There is an important bathochromic shift of the absorption wavelength of the open forms (up to 88 nm) when a bithienyl entity is fixed at position 3. Moreover, the closed-form absorption spectra show minor but significant variations with a maximum shift of 20 nm. 

Upon irradiation with UV light, the new absorption band of the closed-ring appeared at 650 nm. The conversion to the closed-ring form was determined to be 92%. The photochromism in the film on a quartz glass reached to 60% conversion at a photostationary state. The quantum yield in toluene was determined to be 0.40. The quantum yield of the polymer in the film was determined to be 0.26. 

My reservations were based on the fatigue of the HABIs, the weak colors that were formed, and the likelihood that these colors would fade, rather than reverse themselves. I also suspected that the fact that no one had really succeeded in commercializing any photochromic materials was because the fatigue reaction was too considerable an obstacle. I said to some of my friends that the academic scientists who reported in this field were content with a few dozen reversals, which might be achieved by low quantum yields of formation, and so there was an ample reservoir of dimers, provided one did not push too hard. 

V. Pimienta, G. Levy, D. Lavabre, A. Samat, R. Guglielmetti, and J. C. Micheau, Computer analysis of the thermoreversible photochromism of spiropyran compounds evaluation of absorption spectrum and quantum yield, Mol. Cryst. Liq. Cryst., 246, 283-286 (1994). 

Substituent Effects on the Quantum Yield of Photochromic Reactions of... 

Heller and Langan38 reported that the quantum yield for photocoloration (c) of fulgide 35 to the 7,7a-DHBF (36) in toluene was 0.20, and the E->c value appeared to be wavelength independent over the range 313-366 nm. Temperature (10-40°C) had little effect on the quantum yield for photocoloration. Furthermore, the cycles of photochromism of 35-36-35 did not affect the (E - c) value. These results showed that fulgide 35 is well suited for chemical actinometry in the near-UV and visible spectral region. 

A highly efficient photochromic process is essential for organic photochromic compounds used as optical recording materials.39,40 That means that the quantum yields for the photocoloring and bleaching reactions should be high, but the quantum yield for the side reactions should be as low as possible. In order to solve the problems mentioned earlier, extensive studies have been carried out, and some promising results have been obtained so far. 

Structure modifications of fulgide molecules play an important role in increasing the values of quantum yields of the photochromic reactions. These modifications... 

In a polar polymer, i.e., cellulose acetate (CA) or nitrocellulose (NC) 35E, 35Z, and 36 had a relatively longer absorption maximum wavelength than in less polar matrices. In NC the of 36 shifts to 528 nm, which is also longer than in organic solvents. The role of polymer films in the quantum yields of photoreactions is not clear. In a comparison of the photochemical properties of 35 in polymer films and in solvents, it was found that the E c in polymer matrices was substantially smaller than that in the corresponding solvent with similar polarity. However, the decoloration quantum yield Oc e in a polymer film was larger than that in solvents. In conclusion, the polymer matrix properties, such as polarity, viscosity, and glass transition temperature (Tg) are quite important for photochromic reactions and applications. The coloration, E — Z and Z —> E isomerizations were suppressed in polymer matrices. 

Substituent effects on the quantum yield of photoreactions of indolyl fulgide 58 have also been reported by Uchida et al.,15 as shown in Table 4.14. The molecular structure and photochromic reactions are shown in Scheme 18. 

Oxazolyl (64), 4-pyrazolyl (65), 4-thiazolyl (66), and 4-isoxazolyl (67)-substituted fulgides have been prepared and their photochromic reactions investigated by several groups. The absorption spectra data and quantum yields of their photoreactions are listed in Table 4.19. 

Source Based on V, Deblauwe and G. Smets, Quantum yields of the photochromic reactions of heterocyclic fulgides and fulgimides, Makromol. Chem, 189, 2503-2512 (1988). 

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