Structural, electrochemical and optical properties of di-2-pyridyl ketone 2-furoic acid hydrazone (dpkfah)

Article abstract of DOI:10.1016/S0022-2860(03)00048-6

Crystals of di-2-pyridyl ketone 2-furoic acid hydrazone (dpkfah), obtained from a dmso (dimethylsulfoxide) solution of dpkfah, are in the monoclinic space group, P2₁/n. Structural analysis reveals planar hydrazone moiety, non-coplanar pyridine rings and infinite chains of anti-parallel dpkfah dimers interlocked via a web of hydrogen bonds. Electrochemical measurements on dpkfah in non-aqueous solvents show solvent dependence, single and multi-electronic transfers and electrochemical transformation(s) following the first oxidative or reductive electronic transfer. Optical measurements on dpkfah in non-aqueous solvents show strong solvent dependence. In non-polar solvent such as CH₂Cl₂ a single electronic absorption band with extinction coefficient of 18,600 ± 2000 M^-1 cm^-1 appeared at 325 nm and in polar solvents a low-energy absorption band at ～396 nm and a high-energy absorption band at ～320-335 nm appeared that are concentration, temperature and salt dependent. In the presence and absence of NaBF₄ and NaBH₄, extinction coefficients of 21,000 ± 2000 and 22,500 M^-1 cm^-1, and 17,200 ± 2,000 and 23,000 M^-1 cm^-1 were calculated for the low and high energy electronic states of dpkfah in dmf and dmso, respectively at 295 K. Thermo-optical measurements on dpkfah in dmso and dmf confirmed the reversible interconversion between the high and low energy electronic states of dpkfah and allowed calculations of their thermodynamic activation parameters and gave changes in enthalpy (ΔH^?) of +47.5 ± 1.2 and -16.3 ± 0.4 kJ mol^-1, entropy (ΔS^?) of +147.7 ± 3.8 and -64.4 ± 1.64 J mol^-1 K^-1 and free energy (ΔG^?) of +3.49 ± 0.2 and +2.85 ± 0.2 kJ mol^-1 and hence equilibrium constant (K) of +0.25 ± 0.05 and +0.32 ± 0.05 in dmso and dmf, respectively. The reversible BH₄^-/BF₄^- interconversion of the electronic states of dpkfah points to weak non-covalent interactions between these species and dpkfah and possible use of dpkfah as a spectrophotometric sensor for a variety of physical and chemical stimuli. Chemical stimuli in concentrations <1.00 × 10^-6 M can be detected and determined using dpkfah in non-aqueous media.