Photoirradiation Irradiation

Fig. 6. UV-Vis absorption spectral change of trans-25 (0.126 mM) in acetonitrile under a nitrogen atmosphere upon photoirradiation with three bright lines (Amaj[ = 365, 436, and 546, nm) of a super-high-pressure Hg lamp. The spectra are depicted at 10 min intervals of photoirradiation. The irradiation with each bright line was continued for 30 min in ascending order of wavelength. (Reprinted with permission from Ref. 153.)...

Various enol silyl ethers and quinones lead to the vividly colored [D, A] complexes described above and the electron-transfer activation within such a donor/acceptor pair can be achieved either via photoexcitation of charge-transfer absorption band (as described in the nitration of ESE with TNM) or via selective photoirradiation of either the separate donor or acceptor.41 (The difference arising in the ion-pair dynamics from varied modes of photoactivation of donor/acceptor pairs will be discussed in detail in a later section.) Thus, actinic irradiation with /.exc > 380 nm of a solution of chloranil and the prototypical cyclohexanone ESE leads to a mixture of cyclohexenone and/or an adduct depending on the reaction conditions summarized in Scheme 5. 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

Quast et al. investigated the deprotonation followed by irradiation of a series of annulated tetrazolium salts <1998EJ0317>. Attempts at deprotonation of trimethylenetetrazolium hexafluorophosphates 52 and isolation of 5-alkylidcnc l,5-dihydro-l //-tctrazolcs 53 proved to be unsuccessful (Equation 3). Deprotonation of tetrazolium salt 52 (R = H) in TII l -z/H occurred at low temperature (—50 °C) however, slow decomposition of the product occurred, which precluded its isolation but permitted its characterization by NMR spectroscopy. Photoirradiation of the product at — 60 °C (A >320nm) led to a complex mixture of unidentified products. Deprotonation of 52 (R = Me) also occurred at —45°C in THF-4 but led to unidentified products upon irradiation. 

FIGURE 28. 283-MHz 19F NMR spectra of isomers of 8-F-rhodopsin in CFIAPS before (lower) and after photoirradiation (upper) (a) 11-cis (pulse delay, D5 = 5.0 s, number of acquisitions, NA = 5200, line broadening, LB = 80 Hz) (b) 9-cis (D5 = 50 ms, NA = 160000, LB = 80 Hz). Disappearance of the excess 9-cis aldehyde was due to repeated formation and bleaching of pigment during the irradiation process. Reprinted with permission from Reference 48. Copyright (1996) American Chemical Society... 

Irradiation of toluene (80 ppm) by UV light (A, = 200-300 nm) on titanium dioxide in the presence of oxygen (20%) and moisture resulted in the formation of benzaldehyde and carbon dioxide. Carbon dioxide concentrations increased linearly with the increase in relative humidity. However, the concentration of benzaldehyde decreased with an increase in relative humidity. An identical experiment, but without moisture, resulted in the formation of benzaldehyde, carbon dioxide, hydrogen cyanide, and nitrotoluenes. In an atmosphere containing moisture and nitrogen dioxide (80 ppm), cresols, benzaldehyde, carbon dioxide, and nitrotoluenes were the photoirradiation products (Ibusuki and Takeuchi, 1986). 

The photocatalytic system is shown in Scheme 5, where BNAH is oxidized by the ZnP + moiety in the radical ion pair ZaP -Ceo (ki) produced upon photoirradiation of ZnP-Ceo, whereas HV " is reduced to HV by the Ceo" moiety of ZnP +-C6o ki). These individual electron-transfer processes compete, however, with the BET in the radical ion pair (/cbet)- This pathway was experimentally confirmed by photolysis of the ZnP-Ceo/BNAH/HV and ZnP-H2P-C6o/BNAH/HV + systems with visible light (433 nm) in deoxyge-nated PhCN [70], For instance. Fig. 4 depicts the steady-state photolysis in deoxy-genated PhCN, in which the HV absorption band (X ax = 402 and 615 nm) increases progressively with irradiation time. By contrast, no reaction occurs in the dark or in the absence of the photocatalyst (i.e., ZnP-Ceo or ZnP-H2P-C6o) under photoirradiation [70]. Once HV+ is generated in the photochemical reaction, it was found to be stable in deoxygenated PhCN. The stoichiometry of the reaction is established as given by Eq. (3), where BNAH acts as a two-electron donor to reduce two equivalents of HV [70] ... 

Fig. 8. Effect of photoirradiation on the polymerization of VCZ initiated by NaAuClj 2H20 at 30° C,in the presence of air. Solvent nitrobenzene. [VCZ] = 0.25 M, [AunlJ = 1.0 x 10- M. (1) (- -) time-conversion, polymerization in the dark (2) (-o-) time-conversion, a hard glass, cylindrical polymerization vessel was irradiated by a 300 W mercury lamp from a distance of 28 cm (3) (-O-) polymerization time-molecular weight, polymerization in dark...

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