How Water Transfers Proton to Pyrrolo[1,2- a ]quinoxalines in the Presence of TNT: A Mechanistic Study and a Possible Method for TNT Detection

Light-driven proton transfer offers several opportunities to study analytes, given the availability of multiple excited-state structures. In this work, we present the water-mediated protonation of pyrroloquinoxaline (PQN) derivatives in the presence of trinitrotoluene (TNT). PQN photobase molecules demonstrated a preferential response toward TNT over other nitroaromatics. On photoexcitation, a charge transfer toward the pyrazine moiety of PQN increases the electron density on the ring nitrogen atom. This photobasicity triggers the attachment of a water proton to the nitrogen atom, forming PQNH + * with the simultaneous formation of sigma complexes, such as [TNT–OH]–C1 or [TNT–OH]–C3. The excited state (S 1 ) of PQNH + is more planar and exhibits better charge delocalization compared to that of PQN, which explains the higher fluorescence intensity of the former. On the other hand, PQNs are found to interact with 2,4,6-trinitrophenol (TNP) through a ground-state complex formation, characterized by a strong N–H bond and a weakly stretched O–H bond. Here, the observed quenching is due to the transfer of electronic charge from PQNH + to the TNP – moiety. The work reported opens the possibility of expanding the scope of PQN systems as potential turn-on TNT probes.