A Comparative Study into Two Dual Fluorescent Mechanisms via Positional Isomers of N-hydroxyarene-1,8-naphthalimides

Three isomers of hydroxy substituted N-aryl-1, 8-naphthalimides based on N-aryl naphthalic anhydride fluorophore have been synthesized. The decrease in fluorescence intensity from ortho to para substitution of hydroxy group on N-aryl reveals that para substituted isomer undergoes ESEC (Excited State with Extended Conjugation) mechanism which is proved by low quantum yield and appearance of dual emission. The ortho isomer, however, has high quantum yield and no tautomer emission, indicating ESIPT (Excited State Intramolecular Proton Transfer) mechanism is not operating. Similarly, all these isomers show strong fluorescence quenching in presence of strong H-bonding solvents like DMSO and pyridine, but there was neither the shift of emission bands nor the appearance of new bands for proton transfer to these solvents. Thus, it also indicates the absence of excited state proton transfer mechanism. Both the ortho isomer, and to a greater degree the meta isomer, showed larger quenching constants (Kapp) with pyridine than DMSO. This trend opposes the hydrogen-bond affinity for these solvents with phenol and points to a 2-point recognition interaction. In addition, a naphthalimide derivative using 2-aminoimidazole was prepared and examined for optimal positioning of a six-membered ring hydrogen bond pattern. No dual fluorescence was observed for this compound either.