Recognition of acids involved in Krebs cycle by 9-anthrylmethyl-di(6-acetylamino-2-picolyl)amine: a case of selective fluorescence enhancement for maleic acid

Abstract

Anthracene coupled di(6-acetylamino-2-picolyl)amine, 1 is designed and synthesized for selective sensing of maleic acid among the series of carboxylic acids involved in the Krebs cycle. The interaction of sensor 1 with acids is examined using ¹H NMR, UV-vis and fluorescence spectroscopic methods. In UV-vis, 1 shows six isosbestic points on titration with maleic acid and this result is similar to the result obtained with trifluoroacetic acid (TFA). Sensor 1 shows significant fluorescence enhancement upon treatment with maleic acid with red shift (Δλ = 10 nm) in the λ_max whereas other acids show very marginal change. Significant amide shifts in 1 : 1 ¹H NMR and appreciable binding constant values of sensor 1 with monotetrabutylamonium salts of guest acids and also with tetrabutylammonium salts of guest acids by fluorescence suggest the binding affinity towards the anions. In the solid state, 1 shows two polymorphs in the same space group and in co-crystal A (1 with fumaric acid), aliphatic tertiary nitrogen of 1 is protonated and this proton forms two intramolecular hydrogen bonds with nitrogen of pyridine rings. Thus pyridine rings of 1 remain inert towards intermolecular hydrogen bonding to fumaric acid and only amide hydrogens of the sensor 1 are available for intermolecular hydrogen bonding and form a polymeric supramolecular network.