Charge-transfer interaction of 4-(2-pyridylazo)resorcinol with nitroaromatics: Insights from experimental and theoretical results

Molecular charge-transfer (CT) complexation of the donor 4-(2-pyridylazo)resorcinol (1P) with selected nitroaromatic acceptors as like, 2,4-DNP, 3,5-DNSA, 3,5-DNBA, and PA have been studied in methanol at room temperature. Among the selected nitroaromatics we have isolated 1P-3,5-DNSA and 1P-PA complexes and characterized them using FT-IR, 1H NMR studies, and elemental analysis. UV-vis studies revealed that upon interaction of 1P with nitroaromatics a charge-transfer band arises which has remarkably different absorption region than the donor and the acceptor. Titrimetric studies of the ground state show the presence of one isosbestic point which also indicates the strong proton-transfer equilibrium between the molecules. Fluorescence quenching studies were also carried out to find out the Stern-Volmer binding constant (KSV) and the quenching efficiency. FT-IR and 1H NMR studies indicate that the complexation is due to the acute H-bonding interactions. Theoretical calculations (DFT) also propose the formation of the H-bonded adducts.