Miki Kohei, Mao Niimura, Ryosuke Tsutsumi, Naoya Kumagai
Systematic comparison of hydroxy-, amino-, amide-, sulfonamide-, and phosphinamide-substituted C4N4 fluorophores revealed that fluorescence is modulated not simply by hydrogen-bond donor strength, but by positional, geometrical, and conformational accessibility of intramolecular hydrogen bonding to the pyrimidine core. This insight was translated into an acetylation-responsive turn-on probe, providing a design basis for responsive C4N4 fluorophores.