Plus
The two relevant reactants were dissolved in acetonitrile, and transferred into a Pyrex photo reactor immersion well. The solution was degassed for 10-15 minutes by the bubbling of nitrogen through the solution. Irradiation of the solution under a nitrogen atmosphere for the specified length of time was achieved by the use of a 125 W medium pressure mercury discharge lamp. After the reaction was complete, the solvent was removed in vacuo and the resultant crude photoadduct was purified by flash column chromatography using the solvent system stated.
Why do you suppose they degassed the solution?
Finally, for one experiment..
A solution of benzotriazole (2.68 g, 22.5 mmol) and maleimide (0.43 g, 4.43 mmol) in CH3CN (100 ml) was irradiated according to the general photolysis procedure for 16 h. After this time an insoluble brown precipitate had formed, this was collected by suction filtration and washed with EtOAc (2 × 50 ml) to give 2a as a brown solid (0.37 g, 39%; mp 289-290 °C);
2.68 + .43 - .37 = 2.74
Where do you suppose that 2.74 grams went?
It doesn't matter. The point is to show that light can induce a reaction to produce a more complex molecule. It doesn't matter where the particular reactants in this case comes from, and it doesn't matter if this particular reaction doesn't have quantitative yield. The point is that 1. The argument has been made that the second law of thermodynamics requires that light always break down molecules outside a living system and 2. Actual reality shows this is not true. You're once again shifting the goal posts--first it was "that's impossible," now it's "this particular reaction has crappy yield, especially in the presence of oxygen." Umm . . . different point, and one not half as compelling.
And I don't have to suppose anything, I know. :-D