Crystal structure and UV spectra of a 1,2-disubstituted benzimidazolium chloride.

1-(2-Hy-droxy-benz-yl)-2-(2-hy-droxy-phen-yl)-1H-benzimidazol-3-ium chloride, C20H17N2O2+·Cl-, was prepared by reaction of salicyl-aldehyde with o-phenyl-enedi-amine in the presence of tri-methyl-silyl chloride acting as a source of HCl. As a result of steric hindrance, the cation in the crystal is far from planar: the benzimidazole ring system makes dihedral angles of 55.49 (9) and 81.36 (8)° with the planes of the phenolic groups. The crystal packing is dominated by O-H⋯Cl and N-H⋯Cl hydrogen bonds, which link the cations and anions into four-membered rings and then into chains along [100]. The title compound exhibits two transitions in the UV region, which are revealed in the solid state and solution spectra as an absorption maximum at 280 nm and a shoulder at 320 nm. According to the results of TD-DFT calculation, both transitions have a π-π* nature and the mol-ecular orbitals involved in these transitions are mostly localized on the benzimidazole ring system and on the phenyl ring attached to it at the 2-position.

The Prospect of Salophen in Fluorescence Lifetime Sensing of Al3.

We have assessed the potential of salophen, a tetradentate Schiff base, in fluorescence sensing of Al3+ ions. While performing this investigation, we have noticed conflicting literature reports on the fluorescence spectral maximum and quantum yield of salophen. So, the compound has been purified by repeated crystallization. Fluorescence studies have been performed on samples in which the absorption and excitation spectra are completely superimposable. The purified compound exhibits a feeble fluorescence at 545 nm, associated with an ultrafast fluorescence decay. This is rationalized by excited state proton transfer and torsional motions within the molecule, which provide efficient nonradiative channels of deactivation of its excited state. The fluorescence quantum yield increases upon complexation of salophen with Zn2+ as well as Al3+. The increase is significantly more upon complexation with Al3+. However, fluorescence maxima are similar for the two complexes. This indicates that fluorescence intensity may not be a good parameter for Al3+ sensing by salophen, in the presence of a large excess of Zn2+. This problem can be circumvented if fluorescence lifetime is used as the sensing parameter, as the lifetime of the Al3+ complex is in the nanosecond time regime while that of the Zn2+ complex is in tens of picoseconds. The significant difference in the fluorescence quantum yield and lifetime between the two complexes is explained as follows: the Al3+ complex is monomeric, but the Zn2+ complex is dimeric. Quantum chemical calculations indicate a higher density of states near the locally excited state for the dimeric complex. This may lead to more efficient nonradiative pathways.

Binding and ratiometric dual ion recognition of Zn(2+) and Cu(2+) by 1,3,5-tris-amidoquinoline conjugate of calix[6]arene by spectroscopy and its supramolecular features by microscopy.

Lower rim amide linked 8-amino quinoline and 8-amino naphthalene moiety 1,3,5-triderivatives of calix[6]arene L1 and L2 have been synthesized and characterized. While the L1 acts as a receptor molecule, the L2 acts as a control molecule. The complexation between L1 and Cu(2+) or Zn(2+) was delineated by the absorption and electrospray ionization (ESI) MS spectra. The binding ability of these molecules toward biologically important metal ions was studied by fluorescence and absorption spectroscopy. The derivative L1 detects Zn(2+) by bringing ratiometric change in the fluorescence signals at 390 and 490 nm, but in the case of Cu(2+), it is only the fluorescence quenching of 390 nm band that is observed, while no new band is observed at 390 nm. The stoichiometry of both the complexes is 1:1 and was confirmed in both the cases by measuring the ESI mass spectra. The isotopic peak pattern observed in the ESI MS confirmed the presence of Zn(2+) or Cu(2+) present in the corresponding complex formed with L1. Among these two ions, the Cu(2+) exhibits higher sensitivity. The density-functional theory (DFT) studies revealed the conformational changes in the arms and also revealed the coordination features in the case of the metal complexes. The arm conformational changes upon Zn(2+) binding were supported by nuclear Overhauser effect spectrometry (NOESY) studies. The stronger binding of Cu(2+) over that of Zn(2+) observed from the absorption study was further supported by the complexational energies computed from the computational data. While the L1 exhibited spherical particles, upon complexation with Cu(2+), it exhibits chain like morphological features in scanning electron microscopy (SEM) but only small aggregates in the case of Zn(2+). Thus, even the microscopy data can differentiate the complex formed between L1 and Cu(2+) from that formed with Zn(2+).