Photodimerization of cinnamoyl

Fig. 24. Photodimerization of cinnamoyl group bearing polymer pendant crown-ethers...

Photocross I inking of poly(vinyl-p-azidocinnamate) (XIII) is reported to involve competitive photodimerization of cinnamoyl groups to cyclobutenes along with the dominant nitrene reactions of insertion and radical abstraction. The reaction can be triplet-sensitized (19). 

Figure 8.2. Photoalignment of silica mesochannels by photoreactive polymer layers based on the photoisomerization of Az (/eft) and photodimerization of cinnamoyl unit right).

Fig. 8. Photodimerization in o-ethoxy cinnamic acid Two centrosymmetric molecules of the a-form before (full lines) and after (dashed lines) the reaction. Hydrogen atoms other than the hydroxyl are omitted for the sake of clarity. The cinnamoyl double bonds are spaced at —4.3 A and following the cycloaddition, a new pair of bonds is formed which are slightly longer (—1.57 A) than typical C-C bonds. The inset shows the deformation density (at 0.075 ek 3) in the plane of the cyclobutane ring in the a-dimer.

The photodimerization reaetion concept was thus adapted hy Mink and co-workers in the design of a poly(vinyl cinnamate) resist system. Cross-links are formed hy photoaddition between an excited ( ) cinnamoyl group of one chain with a ground state cinnamoyl group belonging to another chain (Scheme 6.3). ... 

Photocrosslinking may be carried out by exposure to UV radiation. Crosslinks are formed by 2 + 2 photodimerization between an excited cinnamoyl group with another in the ground state. It is expected that a sufficient number of these pairs will belong to the same or different molecules to form a crosslinked network. The intermolecular crosslinking reaction between the photosensitive chromophores is represented in Figure 3. 

Cinnamoyl groups were introduced into the amino- and carboxyl-bearing blocks of the copolymers, because we initially intended to take advantage of the photodimerization reaction of the cinnamoyl units (Guo et al, 1996 Liu et al, 1999 Stewart and Liu, 2000) and thus stitch together the associated polymer chains. However, the amidization reaction was later found to be more efficient than the cinnamoyl dimerization reaction. Therefore, the cinnamoyl units performed no apparent function in this study. 

Coumarin and its derivatives, which are found in many plants, also show photodimerization by irradiation of UV light with longer wavelength (X = 300 350 nm) than a cinnamoyl group. Trenor et al wrote a detailed... 

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