Saumik Sen

Charge transfer aided selective sensing and capture of picric acid by triphenylbenzenes

A fluorescent chemo-sensor, 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene was synthesized by substituting the N–H protons of 1,3,5-tris(4′-aminophenyl)benzene with methyl groups. The chemo-sensor shows highly selective and remarkable fluorescence quenching in the presence of picric acid with a detection limit of 1.5 ppm. The origin of the selectivity was investigated using absorption, fluorescence emission and 1H NMR spectroscopic techniques. The solid state structure of 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene and its picric acid complex reveals multiple hydrogen bonds (N–H⋯O and C–H⋯O), π–π interactions and electrostatic interactions between 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene and picric acid. The proton transfer process from picric acid to 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene results in the formation of picrate anions and the triply protonated 1,3,5-tris(4′-(N,N-dimethylamino)phenyl)benzene species containing dimethylammonium (–NHMe2+) groups.

Octanuclear Zinc Phosphates with Hitherto Unknown Cluster Architectures: Ancillary Ligand and Solvent Assisted Structural Transformations Thereof

Structural variations in zinc phosphate cluster chemistry have been achieved through a careful selection of phosphate ligand, ancillary ligand, and solvent medium. The use of 4-haloaryl phosphates (X-dippH2) as phosphate source in conjunction with 2-hydroxypyridine (hpy) ancillary ligand in acetonitrile solvent resulted in the isolation of the first examples of octameric zinc phosphates [Zn8(X-dipp)8(hpy)4(CH3CN)2(H2O)2]·4H2O (X = Cl 2, Br 3) and not the expected tetranuclear D4R cubane clusters. Use of 2,3-dihydroxypyridine (dhpy) as ancillary ligand, under otherwise similar reaction conditions with the same set of phosphate ligands and solvent, resulted in isolation of another type of octanuclear zinc phosphate clusters {[(Zn8(X-dipp)4(X-dippH)4(dhpyH)4(dhpyH2)2(H2O)2]·2solvent} (X = Cl, solvent = MeCN 4; Br, solvent = H2O 5), as the only isolated products. X-ray crystal diffraction studies reveal that 2 and 3 are octanuclear clusters that are essentially formed by edge fusion of two D4R zinc phosphates. Although 4 and 5 are also octanuclear clusters, they exhibit a completely different cluster architecture and have been presumably formed by the ability of 2,3-dihydroxypyridine to bridge zinc centers in addition to the X-dipp ligands. Dissolution of both types of octanuclear clusters in DMSO followed by crystallization yields D4R cubanes [Zn(X-dipp)(DMSO)]4 (X = Cl 6, Br 7), in which the ancillary ligands such as hpy, H2O, and CH3CN originally present on the zinc centers of 2-5 have been replaced by DMSO. DFT calculations carried out to understand the preference of Zn8 versus Zn4 clusters in different solvent media reveal that use of CH3CN as solvent favors the formation of fused cubanes of the type 2 and 3, whereas use of DMSO as the solvent medium promotes the formation of D4R structures of the type 6 and 7. The calculations also reveal that the vacant exocluster coordination sites on the zinc centers at the bridgehead positions prefer coordination by water to hpy or CH3CN. Interestingly, the initially inaccessible D4R cubanes [Zn(X-dipp)(hpy)]4·2MeCN (X = Cl 8, Br 9) could be isolated as the sole products from the corresponding DMSO-decorated cubanes 6 and 7 by combining them with hpy in CH3CN.

Elusive Double-Eight-Ring Zeolitic Secondary Building Unit

The double-eight-ring (D8R), an elusive secondary building unit of zeolites, has been stabilized for the first time, both in solution and solid-state. The present study further establishes that any of the three double-ring building blocks of zeolites, viz. D4R, D6R and D8R ([ArPO3Zn(L)]n (n = 4, 6 or 8)), can be preferentially isolated (over the other two) through a careful choice of metal source, aryl phosphate and ancillary ligand, apart from maintaining a meticulous control on the reaction conditions.

Spectroscopic and Ab Initio Investigation of C-H⋅⋅⋅N Hydrogen-Bonded Complexes of Fluorophenylacetylenes: Frequency Shifts and Correlations.

The C-H⋅⋅⋅N hydrogen-bonded complexes of several fluorophenyacetylenes with ammonia and methylamine were characterized by a redshift in the acetylenic C-H stretching vibration of the phenylacetylene moiety. These redshifts were linearly correlated with the stabilization energies calculated at the CCSD(T)/CBS//MP2-aug-cc-pVDZ level. Analysis of various components of the interaction energy indicated that the observed redshifts were weakly correlated with the electrostatic component. The weaker linear correlation between the frequency shifts and the electrostatic component between two data sets can perhaps be attributed to the marginal differences in the Stark tuning rate and zero-field shifts. The induction and exchange-repulsion components were linearly correlated. However, the dispersion component depends on the nature of the hydrogen-bond acceptor and shows a quantum jump when the hydrogen-bond acceptor is changed from ammonia to methylamine. The observed linear correlation between the redshifts in the C-H stretching frequencies and the total stabilization energies is due to mutual cancellation of deviations from linearity between various components.

π-Stacked Dimers of Fluorophenylacetylenes: Role of Dipole Moment

The homodimers of singly fluorine-substituted phenylacetylenes were investigated using electronic and vibrational spectroscopic methods in combination with density functional theory calculations. The IR spectra in the acetylenic C-H stretching region show a marginal red shift for the dimers relative to the monomers. Further, the marginal red shifts indicate that the acetylenic group in all the dimers is minimally perturbed relative to the corresponding monomer. The observed spectra were assigned to a set of π-stacked structures within an energy range of 1.5 kJ mol-1, which differ in the relative orientation of the two monomers on the basis of M06-2X/aug-cc-pVTZ level calculation. The observed red shift in the acetylenic C-H stretching vibration of the dimers suggests that the antiparallel structures contribute predominantly based on a simple coupled dipole model. Energy decomposition analysis using symmetry-adapted perturbation theory indicates that dispersion plays a pivotal role in π-π stacking with appreciable contribution of electrostatics. The stabilization energies of fluorophenylacetylene dimers follow the same ordering as their dipole moments, which suggests that dipole moment enhances the ability to form π-stacked structures.