Anthraquinone-biimidazole based ruthenium(II) complexes as selective multichannel anion sensors and multi-readout molecular logic gates and memory devices: Combined experimental and DFT/TD-DFT study

摘要

In this work we will report anion sensing properties as well as multiple Boolean logic operations of recently reported Ru(II) complexes based on a biimidazole-anthraquinone ligand (Anq-BiimzH_2). Covalent linking of electron accepting 9,10-anthraquinone moiety to the biimidazole unit followed by its coordination to Ru(II) center, leads to enhance the acidity of the imidazole NH protons in coordinated Anq-BiimzH_2. Two NH protons which adopt a syn conformation in the complexes can be utilized for sensing of selective anions either via hydrogen bonding interaction or by complete proton transfer. The anion sensing studies of the complexes were thoroughly investigated in both organic and pure aqueous media through multiple channels viz. absorption, steady state and time-resolved emission, and ~1H NMR spectroscopic techniques as well as by cyclic and square wave voltammetry. The complexes behave as highly sensitive sensors for F~−, AcO~−, CN~−, and to some extent for H_2PO_4 ~− in acetonitrile without much selectivity. In contrast to acetonitrile, the complexes posses high sensitivity and selectivity towards CN~− even in presence of excess of other studied anions in water with detection limit lying in the range between 5.24 × 10~(−7) and 6.67 × 10~(−7) M, making them useful for practical application. By utilizing the optical output signals upon interaction with a particular set of anions (F~−, CN~−, AcO~− and OH~−) and H~+, the functions of two-inputs INHIBIT and IMPLICATION logic gates as well as three inputs NOR logic gate have been nicely demonstrated. More importantly, the complicated function of memory device was also mimicked by the complexes. Computational investigation with the help of density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were performed to understand and interpret the experimentally observed optical properties of the complexes. Moreover, the experimental trend of the red-shifts of absorption and emission bands upon successive deprotonation of the complexes in presence of selective anions was nicely reproduced by calculations.