Molecular sensing behavior of di-2-pyridyl ketone p-aminophenylhydrazone hydrate (dpkabh·H2O) in non-aqueous media

1H NMR studies on di-2-pyridyl ketone p-aminobenzoylhydrazone hydrate (dpkabh·H2O) in non-aqueous solvents show high sensitivity to its surrounding. In protophilic solvents (d6-dmso or d7-dmf), the amine protons are equivalent, while in CDCl3 they are not. Variable temperature analysis in CDCL3 show the NH proton to exhibit high temperature dependence due to strong intra-molecular hydrogen bonding of the type N–H⋯N between the amide (NH) and N atom of a pyridine ring. The temperature dependence for the same proton in d6-dmso and d7-dmf is due to hydrogen bonding of the type N–H⋯O between the amide proton and oxygen atom of the solvent. Optical measurements on dpkabh·H2O show one intra-ligand charge transfer (ILCT) transition in CH2Cl2 and in protophilic solvents, two ILCT of the donor–acceptor type due to dpkabh·H2O and its conjugate base appeared. Variable temperature studies on protophilic solution of dpkabh·H2O confirm the sensitivity of dpkabh·H2O to its surroundings and show facile reversible inter-conversion between dpkabh·H2O and its conjugate base. Changes in enthalpy (ΔHϕ) of −5.2 ± 0.4 and −24.2 ± 1.20 kJ mol−1, entropy (ΔSϕ) of +9.6 ± 0.5 and −63.0 ± 2.0 JK−1 mol−1, and free energy (ΔGϕ) of +2.3 ± 0.2 and +5.4 ± 0.2 kJ mol−1 were calculated for dpkabh·H2O at 298 K in dmso and dmf, respectively. When stoichiometric amounts of NaBH4 or MCl2 (M = Zn, Cd or Hg) were added to protophilic solution of dpkanh·H2O conversion from the high to low energy electronic transition was observed and show that substrates in low concentrations can be detected and determined using protophilic solution of dpkabh·H2O.