The author replies to
Rademacher
I thank Paul Rademacher for drawing my attention to recent publications that concern
the structure of the blue substance that is produced by exposure of
2-(2,4-dinitrobenzyl)pyridine to the light. Apparently, while exposure of the "CH" substance to light produces both the "OH" and "NH" substances, it is the "NH" material that is responsible
for the blue color.
  
We could speculate that the deep blue color is a result of the effective resonance
delocalization that is present in the NH form:
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