A new approach based on fluorescence quenching of a chromophore, pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt, POH), was established for monitoring the synthesis of the pyranine-substituted phthalonitrile derivative POPht. 4-Nitrophthalonitrile binds to the POH through nitro elimination, by means of a nucleophilic aromatic substitution reaction in a basic medium, and forms phthalonitrile (Pht) bearing deprotonated POH units, Pht + POH -> POPht. This binding process results in a considerable blue shift (from 515 to 430 nm) in the fluorescence emission spectra of POH. Besides, the fluorescence intensities of unreacted pyranines and POPht molecules in the reacting mixture decrease as they are quenched by nitrite appearing as one of the byproducts. POH, therefore, acts as both an aromatic substituent for Pht and a fluorescence probe for monitoring the substitution reaction. Thus, the change in the fluorescence spectra introduces a novel method for in situ monitoring of the synthesis of the phthalonitrile derivative POPht. This new material may have a potential to synthesize the phthalocyanine having an enhanced fluorescence property and solubility in an aqua medium that would result in a desired candidate molecule in many applications of phthalocyanines.