We report the photosubstitution of one cyano group in dicyanobenzene-based photocatalysts and thermally activated delayed fluorescence (TADF) emitters. The reaction is a general degradation pathway for some widely used organic photocatalysts such as 4CzIPN and suggests that the active photocatalyst in many reactions is likely different from 4CzIPN. On the contrary, photosubstitution is a facile route to diverse highly reducing photocatalysts and blue TADF emitters.