Stubbornly Committed to PBG

So, I finally had my Photobase Generator, but lots of questions.

First, why did it seem to release acid upon irradiation? Upon reviewing the paper in which this molecule was synthesized, I discovered that they had published no actual pH data, only that the product had decomposed upon irradiation into two products, one of which was cyclohexylamine, the basic compound. Because cyclohexylamine was released, it had been classified as a photobase generating solution, which made sense. However, 6-nitrocoumarin was another product, and I couldn’t find any record of it’s acidic properties. My assumption was that perhaps the nitrocoumarin was actually more acidic than the cylclohexylamine was basic. Perhaps that would account for the drop of pH. However, quick tests proved that the nitrocoumarin was relatively basic, so that couldn’t be the solution.

After one irradiation, I noticed that the pH kept changing, never settling even after sitting for more than an hour without any external stimulus. Additionally, I had seen that in every trial, every peak on the absorbance spectrum increased, even though the literature clearly stated, that all but one peak should decrease in amplitude. On my next trial, I irradiated, and then checked for changes to the UV-Vis absorbance spectrum over time after irradiation. It showed that even more than an hour after irradiation, the absorbance peaks continued to increase in amplitude, when they should have remained static.  A possible reason for that would be the phenomenon of photobleaching, wherein a compound is over-irradiated and releases free radicals, causing a number of unwanted sidereactions. It was clear that something was going wrong.

Decreasing the intensity of light proved ineffective at mitigating the photobleaching. If the light was too low, no change would occur. If the light was too high, the same phenomena stated above would happen. It proved impossible to find a point where this did not happen. Without the tools to further analyze these phenomena, I decided to change the current focus of my project.