Molecular structure and optosensing behavior of di-2-pyridyl ketone benzoylhydrazone in non-aqueous solvents

Crystals of di-2-pyridyl ketone benzoylhydrazone (dpkbh) obtained from a dimethylsulfoxide (DMSO) solution of dpkbh are in the monoclinic space group, P_l/c. Structural analysis reveals anti-parallel, non-planar, one-dimensional tapes of dpkbh with a network of intra- and intermolecular hydrogen bonds. Optical measurements on dpkbh in polar non-aqueous solvents revealed the presence of interlocked low-(α-) and high-(β-) energy intraligand charge-transfer electronic states may be due to dpk π-π* followed by dpk to benzoyl charge-transfer. The equilibrium distribution of the low-(α-) and high-(β-) energy electronic states is solvent dependent, controlled by the hydrogen bonding capacity of surrounding solvent molecules. The interplay between the α- and β-conformations of dpkbh allowed calculations of their extinction coefficients by forcing the equilibrium to shift to one conformation using chemical stimuli. In N,N-dimethylformamide (DMF), extinction coefficients of 45,000 ± 2000 and 15,000 ± 2000 M^-1 cm^-1 were calculated for the α- and β-conformations of dpkbh at λ_max, respectively, when excess acid was added to a DMF solution of dpkbh. The weak bond energies of the hydrogen bonds, the reversibility and the sensitivity of the high- and low-energy electronic states to their surroundings in polar non-aqueous solvents allow for the use of this system as molecular sensor for a variety of stimuli that include metal ions, acids and bases. Metal ions in concentrations < 10 -7 M can be detected and determined using this system (dpkbh and surrounding solvent molecules).