Quantum yield of photoreaction

Scheme 9 shows in fulgide 39, replacement of the isopropylidene group (IPD) by a dicyclopropylidene group (DCP) caused a bathochromic shift in the absorption band of the 7,7a-DHBF, owing to the partial double-bond character of the DCP group. It is obvious that the steric hindrance of R1 and R2 also affected the max of the colored form. There was not a definite role for the substituent effect on the quantum yield of photoreactions. Table 4.8 shows the absorption spectroscopic data and quantum yields of photoreactions of fulgides with different R R2 groups in toluene. 

Table 4.9. Absorption Spectra Data and Quantum Yields of Photoreactions of Fulgides 41 (E,Z) and its Colored Form (42) (C) with Bulky Substituents in Toluene...

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. 

Table 4.14. Effects of Substituents on Quantum Yield of Photoreactions and Spectroscopic Properties oflndolyl Fulgide 58 (Z,E) and its Colored...

The steric hindrance of R1, R2, and R3 dramatically affects the quantum yields of photoreactions, e->c andOc -> i , but for group R4, the influence on the quantum yield of the photoreactions of fulgide has been attributed to the electronic... 

Table 4.30. Quantum Yields of Photoreactions of Indolyl Fulgides 90...

Y. Yokoyama, T. Serizawa, S. Suzuki, Y. Yokoyama, and Y. Kurita, Fulgenolides thermally irreversible photochromic lactones with large quantum yields of photoreactions, Chem. Lett., 1995, 17-18. 

Gauglitz G. Azobenzene as a convenient actinometer for the determination of quantum yields of photoreactions. J Photochem 1976 541-547. 

Besides these physical methods, chemical methods can also be used to monitor the photon exposure of a sample. Chemical actinometry is a very precise method. A publication of lUPAC describes numerous actinometric systems useful for several different wavelength ranges (10). Chemical actinometry is important for the determination of the reaction kinetics and quantum yield of photoreactions in experimental photochemistry as well as for the control and quantification of the overall exposure in photostability testing systems. A more detailed presentation of chemical actinometry is given in Chapter 8 of this book. 

Gauglitz, G. (1975). Azobenzene as a Convenient Actinometer for the Determination of Quantum Yields of Photoreactions. Photochem. 5, 41- 6. 

Effect of solvent polarity on the quantum yields of photoreaction and intersystem crossing and fluorescence peak wavelength of A -phenylphthalimide. Reproduced with permission from Macromolecules 1992 25 6651. 1992 American Chemical Society [173]... 

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