The perishable nature of 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in aqueous medium is always considered as a side-effect. The rapid color fading of PTAD inspired us to explore the mechanism of its hydrolysis. Herein, photogenerated nitrile imine is discovered to be an ultra-fast trapping agent to “click” with a ring-opening hydrolysate with an umpolung effect, which resulted in an unprecedented