Photochemistry in the Solid State

SCHEME 6. Photochemical reaction of polymer 6 in the absence of an external trapping reagent. 

These data suggest that oxygen is necessary for the solid-state photochemical reaction to occur. It was proposed that oxygen traps the metal radicals produced in the photolysis of the metal-metal bonds, thereby preventing radical recombination (eq. 25). If oxygen diffusion is rate limiting then 

FIGURE 2. The absorbance at 508 nm vs. time for four thin films of the polyurethane synthesized by equation 3. Only the sample exposed to both light and air degraded. The absorbance at 508 nm is characteristic of the metal-metal bond. The differences in absorbance at time = 0 are due to different thicknesses of the various samples. 

An especially interesting degradation is that of polymer 2.33 (This polymer was synthesized by the route in equation 8. The isocyanate prepolymer is Hypol 2000, and the diol prepolymer is PEG-1000.) The solid polymer was readily degraded by ambient laboratory light over the course of 5 h, and as degradation occurred the solid polymer was converted back into the liquid prepolymers  

FACTORS CONTROLLING THE RATE OF POLYMER PHOTOCHEMICAL DEGRADATION IN THE SOLID STATE 

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This review concentrates on chiral templates that transfer the chiral information in a ground state complex in solution, and it considers results published before the beginning of 2003. Reviews focusing on chiral auxiliaries, chiral sensitizers, CPL as well as on enantioselective photochemistry in the solid state, in confined media, or in molecular aggregates can be found elsewhere in this book. Moreover, it should be mentioned that this account deals with synthetic templates. Natural templates such as cyclodextrins, bovine serum albumin, and DNA will be accounted for separately. 

Styrylcoumarin has been shown to crystallize out of chloroform and hexane in two different polymorphs [30], The crystal structures of the two forms differed in the nature of the molecular packing, and these differences were sufficient to permit different pathways of photochemistry in the solid state. A summary of the crystallographic data obtained on the two forms is located in Table 10. 

Cheung, E., Netherton, M. R., Scheffer, J. R., and Trotter, J., Linearly conjugated benzocyclohexa-dienone photochemistry in the solid state ionic chiral auxiliary mediated asymmetric induction. 

Scheffer JR, Xia W (2005) Asymmetric Induction in Organic Photochemistry via the Solid-State Ionic Chiral Auxiliary Approach. 254 233-262 Schenning APHJ, see Ajayaghosh A (2005) 258 83-118 Schepers U, see Hahn F (2007) 278 135-208 Schindler J, see Faigl F (2007) 269 133-157... 

Table 2 Asymmetric induction in the solid-state photochemistry of compounds 29a-d ...

The X-ray analyses reveal that, as in the case of salt 37b, enantio- and dia-stereoselectivity in the solid-state photochemistry of salt 39 are conformationally... 

Sakamoto et al. also reported the solution photochemistry for la-d, which gave the corresponding thietane-fused -lactams 2a-d as shown in Table 2 [30,31]. For tigloyl derivatives Ic and Id, P-lactams were obtained as a mixture of two stereoisomers (entries 6 and 8). The anti isomers were preferably formed, in ratios of 0.8 and 0.7, respectively. In the solid-state photoreaction of... 

Asymmetric Induction in Organic Photochemistry via the Solid-State Ionic Chiral Auxiliary Approach... 

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