X-ray crystallographic and optosensing studies of di-2-pyridyl ketone p-nitrophenylhydrazone (dpknph) in dimethylsulfoxide (dmso)

Crystals of di-2-pyridyl ketone p-nitrophenylhydrazone (dpknph), obtained from a dmso (dimethylsulfoxide) solution of dpknph, are in the monoclinic space group, P2₁/n. Structural analysis reveals non-coplanar pyridine rings and infinite one-dimensional chains of dpknph with a network of classic and non-classic hydrogen bonds. Optical measurements on dpknph in dmso in the presence and absence of CdCl₂ and NaBH₄ gave extinction coefficients of 91,000 ± 2000 and 35,000 ± 2000 M^-1 cm^-1 for the high and low energy electronic states of dpknph at 295 K. Thermo-optical measurements on dpknph in dmso established the reversible interconversion between the high and low energy electronic states of dpknph and allowed calculations of their thermodynamic activation parameters and gave changes in enthalpy (ΔH^∅) of - 49.05 ± 1.25 kJ mol^-1, entropy (ΔS^∅) of +0.16 ± 0.04 kJ mol^-1 and free energy (ΔG^∅) of - 96.30 ± 2.45 kJ mol^-1 at 295 K. Manipulation of the equilibrium distribution of the high and low energy electronic states of dpknph allowed for the use of the system as cation-anion and acid-base sensor. Basic anions such as BH₄^- and OH^- favor the low energy electronic state and electron deficient anions such as BF₄^-, metal ions and protons favor the high-energy electronic state. The reversible BH₄^-/BF₄^- and OH^-/H⁺ interconversion points to physical interactions between these species and dpknph and the use of dpknph as a spectrophotometric sensor for a variety of physical and chemical stimuli. Metal ions in concentrations as low as 1.00 x 10^-7 M can be detected and determined using dpknph in dmso.