Functional Molecular System of Bis(pyrazolyl)pyridine Derivatives: Photophysics, Spectroscopy, Computation, and Ion Sensing

A new series of conjugated donor−π–acceptor type of 2,6-bis­(pyrazolyl)­pyridine derivatives (compounds IK-(3–9)) have been synthesized via Horner–Wadsworth–Emmons (HWE) reaction, starting from a common phosphonate precursor and diverse donor aromatic aldehydes and characterized by routine spectral analysis including elemental analysis. Compound IK-2, one of the starting precursors, and molecule IK-3, the first member of the donor−π–acceptor series, are additionally characterized by single-crystal X-ray structure determination. Compounds IK-2 and IK-3 are crystallized in P1̅ (triclinic) and P2₁/c (monoclinic) space groups, respectively. The absorption maxima in the electronic spectra of the title compounds shift mainly due to intramolecular charge transfer (ICT) between different donor (dibutyl and cyclic pyrrolidine) groups and the acceptor moiety [2,6-bis­(pyrazolyl) pyridine]. Solution-state emission spectral studies of all these compounds show large solvent sensitive behavior with significant amounts of Stokes shifts. The large solvent dependence of the emission indicates that the excited state is stabilized in more polar solvents due to the ICT. All chromophores exhibit solid-state fluorescence behavior except compound IK-7. The role of the position and nature of the donor functionalities in the conjugated backbone of overall donor moiety of compounds IK-(3–9), on the electronic absorption properties of the title chromophores has been demonstrated, which has further been corroborated by density functional theory (DFT) and time-dependent DFT (TDDFT) computational studies. The emission spectral results of compounds IK-3, IK-5, and IK-7 have also been supported by the DFT and TDDFT calculations. A fluorescence lifetime study on this series also shows that the excited states are stabilized in more polar solvents. Finally, one of the chromophores (chromophore IK-4) in the title series has been shown to act as a selective molecular sensor (turn-off switch) for the Cu­(II) ion.