Small-scale photoreactions

As mentioned above, small-scale photoreactions are quite often carried out in quartz or Pyrex tubes, by external irradiation. However, this is certainly not an optimal solution for maximizing the exploitation of the emitted radiation. Internal irradiation is obviously better from the geometric point of view, but (relatively) large-scale preparations must take into account all of these factors and achieve optimal light and mass transfer. These elements are not taken into account in exploratory studies or small-scale syntheses, just as is the case for thermal reactions, where the optimization is considered at a later stage the essential requirement is that the explorative study is carried out under conditions where occurrence of the reaction is not prevented. Thus, it is important that the source is matched with the reagent absorption, the vessel is of the correct material, and the solvent does not absorb competitively (unless it acts also as the sensitizer). Figure 1.7 and Table 1.1 may help in this choice, in conjunction with the U V spectra of all of the compounds used (it is recommended that the spectra are measured on the actual samples used, in comparison with those taken from the literature, in order to check for absorption by impurities). 

The time scale over which an event occurs determines wlm molecular motitms and interactions influence a particular event In the case of photoisomerization, the reaction proceeds only with the molecules which lie at the sites where the reactant is mobile dining the exdted lifetime. Consequently a very small quantum yield or an apparent rate coefficient is often observed for the photoreaction whidi requires a large critical free volume, e.g. for the photodecoloration of the merocyanine form of spirobenzo-pyran in polycarbonate film... 

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