Optosensing properties of fac-Re(CO)3(dpknph)Cl (dpknph=di-2-pyridyl ketone p-nitrophenyl hydrazone)

Optical and thermodynamic measurements on fac-Re(CO)3(dpknph)Cl in polar non aqueous solvents revealed the existence of two interlocked conformational forms for fac-Re(CO)3(dpknph)Cl. The equilibrium distribution of the low (α-) and high (β-) energy conformations is solvent dependent, controlled by the dipole moment of the solvent molecules and their orientation around the total dipole of fac-Re(CO)3(dpknph)Cl. The interplay between the α- and β-conformations of fac-Re(CO)3(dpknph)Cl, allowed calculations of their extinction coefficients, by forcing the equilibrium to shift to one conformation, using chemical stimuli. In DMSO and DMF extinction coefficients of 87 000±2000 and 35 000±2000 M−1 cm−1 were calculated for the β- and α-conformations of fac-Re(CO)3(dpknph)Cl at λmax., respectively. Thermo-optical measurements on fac-Re(CO)3(dpknph)Cl, allowed calculations of the activation parameters for the interconversion between the α- and β-conformations of fac-Re(CO)3(dpknph)Cl. In DMSO and DMF changes in enthalpy (ΔHø) of −11.2±1.3 and 10.9±0.5 kJmol−1, entropy (ΔSø) of −12.7±4.3 and 29.4±1.7 JK−1 mol−1, and free energy (ΔGø) of −7.5±0.2 and+2.2±0.2 kJmol−1 and hence equilibrium constants of 20.9±1.7 and 0.4±0.1 were calculated for fac-Re(CO)3(dpknph) at 295 K. The high values for the extinction coefficients and low values for the activation parameters for the interconversion between the α- and β-conformations of fac-Re(CO)3(dpknph)Cl, in polar non aqueous solvents allowed the use of these systems as molecular sensors to probe their structural relaxation and interactions with their surroundings. These systems (fac-Re(CO)3(dpknph)Cl and surrounding solvent molecules) optically sense chemical and physical stimuli and their sensing power depends on the intensity and nature of these stimuli, i.e. the systems exhibit a high degree of sensitivity and selectivity.