Photopolymerization action spectrum

Fig. 20. Comparison between UV-absorption spectra of a paraffinsubstitiled diacetylene (m = 8, n = 8) incorporated in a multilayer assembly and the action spectrum of photopolymerization (from Ref.

In course of subsequent work Bubeck, Tieke, and Wegner discovered that the action spectrum for photopolymerization of undoped diacetylene multilayers extends into the visible provided some polymer formed in course of previous UV-irradiation is present. Since obviously excitation of the polymer can sensitize the reaction this effect has been termed self-sensitization. Checking the absorption spectrum of the polymer produced via self-sensitization assured that the final product is identical with the product obtained under UV excitation of the monomer. Later work by Braunschweig and Bassler demonstrated, that the effect is not confined to multilayer systems but is also present in partially polymerized single crystalline TS-6, albeit with lower efficiency. Interestingly, the action spectrum of self-sensitization follows the action spectrum for excitation of an electron from the valence band of the polymer backbone to the conduction band rather than the excitonic absorption spectrum of the polymer which is the dominant spectral feature in the visible (see Fig. 21). The quantum yield is independent of the electric field, whereas in a onedimensional system the yield of free carriers, determined by thermal dissociation of optically produced, weakly bound geminate electron-hole pairs, is an linear function of an applied electric field 29.30,32,129) Apparently, the sensitizing action does not... 

Fig. 21. a. Action spectra for photopolymerization of neat (triangles) and partially polymerized (circles) multilayer systems. AOD is the change in optical density at a given irradiation dose b Action spectrum of intrinsic photoconductivity of a fully polymerized multilayer assembly from Ref. 

Fig. 23. Reflexion spectra of the 2 1 complex of 2.4 heptadecadiynoic acid and phenazine before (a), and after sample irradiation at 410 nm for 5 min (b), 10 min (c) and 20 min (d). Spectrum e was obtained after sample annealing at 90 °C. f is the action spectrum for photopolymerization at X > 300 nm in arbitrary units (from Ref.

Because of their high transparency and exactly defined thickness multilayers have been proven to be excellently suited for quantitative studies of the polymerization process. The action spectrum and quantum yield of the photopolymerization were determined. Self-sensitization as well as senatization of the photopolymerization by surfactant dye molecules were studied in great detail. 

Fig. 20. Diffuse reflectance spectrum of monomeric complex 86 and its action spectrum of UV/VIS-photopolymerization.

We made attempts to use photopolymerized ChLC films to demonstrate the presence of twist defects experimentally [109]. However, even though the principle is straightforward, the realization is not easy. Ryotaro Ozaki, a doctoral student at that time (now at Ehime University) spent half a year to fix the films with a good enough precision to see the localized defects in the transmission spectrum. Furthermore, he could also confirm a laser action from this defect mode, which is interesting because it is a different mechanism to produce laser action [109-111]. 

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