Dilip K. Maity

Microhydration of NO3-: A Theoretical Study on Structure, Stability and IR Spectra

A systematic study on the structure and stability of nitrate anion hydrated clusters, NO3(-) x n H2O (n = 1-8) are carried out by applying first principle electronic structure methods. Several possible initial structures are considered for each size cluster to locate equilibrium geometry by applying a correlated hybrid density functional with 6-311++G(d,p) basis function. Three different types of arrangements, namely, symmetrical double hydrogen bonding, single hydrogen bonding and inter-water hydrogen bonding are obtained in these hydrated clusters. A structure having inter-water hydrogen bonding is more stable compared to other arrangements. Surface structures are predicted to be more stable over interior structures. Up to five solvent H2O molecules can stay around solute NO3(-) anion in structures having an inter-water hydrogen-bonded cyclic network. A linear correlation is obtained for weighted average solvent stabilization energy with the size (n) of the hydrated cluster. Distinctly different shifts of IR bands are observed in these hydrated clusters for different kinds of bonding environments of O-H and N=O stretching modes compared to isolated H2O and NO3(-) anion. Weighted average IR spectra are calculated on the basis of statistical population of individual configurations of each size cluster at 150 K.

Cd(II) based metal–organic framework behaving as a Schottky barrier diode

A metal-organic framework (MOF) of cadmium(ii) is reported here which is the first example of an experimentally achieved MOF based electronic device, and in the present case it is a Schottky diode.

Change of boron substitution improves the lasing performance of Bodipy dyes: a mechanistic rationalisation.

Bodipy laser dyes are highly efficient but degrade rapidly in solution by reacting with in situ generated singlet oxygen ((1)O(2)). To increase the lasing lifetimes of these dyes, we have designed and synthesised two different congeners of the widely studied Pyrromethene 567 (PM567) by substitution at the boron centre and/or at both the boron centre and the meso position. The two new dyes showed high lasing efficiencies with increased photostability. The results of theoretical and pulse radiolysis studies revealed that the substitution at the boron centre reduced the (1)O(2) generation capacity of these dyes as well as their rate of reaction with (1)O(2), thereby enhancing their lifetimes even under lasing conditions.

Effect of five membered versus six membered meso-substituents on structure and electronic properties of Mg(II) porphyrins: a combined experimental and theoretical study.

Meso-tetrasubstituted Mg(II) porphyrins containing six membered phenyl groups (MgTPP) and five membered thienyl (MgTThP) and furyl groups (MgTFP) were synthesized and structurally characterized, and the effects of meso-substituents on electronic properties were studied using NMR, absorption, fluorescence spectroscopy, and electrochemical studies. Density functional theory (DFT) calculations were carried out to correlate with experimental observations. The three Mg(II) porphyrins MgTPP, MgTThP, and MgTFP were crystallized as hexa-coordinate systems with Mg(II) ion in the center of the porphyrin plane and having two tetrahydrofuran molecules as axial ligands. The X-ray studies clearly showed that the meso-furyl groups adopt a conformation in which they are more in-plane with the porphyrin plane whereas the thienyl and phenyl groups prefer an orthogonal arrangement with respect to the porphyrin plane. This arrangement of meso-substituents with the porphyrin plane helps in the enhancement of porphyrin π-delocalization in MgTFP compared to MgTThP and MgTPP. The differences in their structures are clearly reflected in their spectral and electrochemical properties. The absorption and fluorescence bands experienced bathochromic shifts on moving from six membered phenyls to five membered thienyl and furyl group, and the maximum effects were observed for meso-tetrafuryl Mg(II) porphyrin. The electrochemical studies indicated that the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases as we move from six membered phenyl groups to five membered thienyl and furyl groups, which explains the bathochromic shifts observed in absorption and fluorescence bands. Results on structure and electronic properties based on DFT studies are in agreement with experimental observations.

Supramolecular Interaction of Coumarin 1 Dye with Cucurbit[7]uril as Host: Combined Experimental and Theoretical Study

Molecules of the coumarin family have fluorescence characteristics that are highly sensitive to their environment, and thus, they have been used as fluorescent sensors in chemical and biological systems. However, the very poor fluorescence yield of most coumarin dyes in aqueous media limits their applications. We have adopted a supramolecular strategy to improve the fluorescence intensity of coumarin dye through its interaction with the relatively new host cucurbit[7]uril (CB[7]). The virtually nonfluorescent coumarin 1 (Φ(f) = 0.04) was converted into a highly fluorescent (Φ(f) = 0.52) entity in water upon addition of the nonfluorescent host CB[7]. Various spectroscopy techniques, namely, UV-vis absorption and steady-state and time-resolved fluorescence spectroscopies, established the formation of a strong 1:1 dye-CB[7] inclusion complex with a high binding constant of (1.2 ± 0.1) × 10(5) M(-1) for the dye. The stable inclusion complex of the neutral molecule was supported by density-functional-theory- (DFT-) based quantum chemical calculations. Energy decomposition analysis of various interaction factors in the host-guest complex revealed that key components providing stability to the complex were electrostatic, polarization, and charge-transfer energies. These new results on the formation of a strong inclusion complex of the versatile fluorophore coumarin 1 with the nontoxic host CB[7] could lead to the design of efficient molecular-scale biological probes, sensors, and photostable aqueous UV dye lasers.

Five diverse bivalent metal coordination polymers based on benzene dicarboxylate and bent dipyridyl ligands: syntheses, structures, and photoluminescent properties

Five new mixed ligand coordination polymers, {[Co0.5(H2O)(1,4-bdc)(3-bpdb)Co0.5(H2O)2]·(H2O)3}n (1), [Co(3-bpdh)(1,4-bdc)(H2O)2]n (2), [Zn(3-bpdb)(1,4-bdc)]n (3), {[Zn(3-bpdh)(1,4-bdc)]·(3-bpdh)0.5}n (4) and [Cd(3-bpdb)(1,4-bdc)(H2O)]n (5) [where 3-bpdb = 1,4-bis-(3-pyridyl)-2,3-diaza-1,3-butadiene; 3-bpdh = 2,5-bis-(3-pyridyl)-3,4-diaza-2,4-hexadiene and 1,4-bdc = benzene-1,4-dicarboxylate] were synthesized at room temperature. The structures of 1–5 were determined by single crystal X-ray diffraction analysis and were further characterized by elemental analysis, infrared spectroscopy (IR) and powder X-ray diffraction (PXRD). Compound 1 exhibits a one-dimensional (1D) chain structure with monodentate pendant 1,4-bdc ligands which further extend to a three-dimensional (3D) supramolecular structure by H-bonding and π⋯π interactions. Compound 2 displays a 4-connected two-dimensional (2D) framework with the point symbol {44·62}. Moreover, the 2D structure of 2 is also ultimately packed into 3D supramolecular frameworks through H-bonding and π–π stacking interactions. Compound 3 shows a 5-fold interpenetrated diamondoid net with 66 topology. Compound 4 is a 2D 3-connected net having a point symbol {63} and is extended to a supramolecular 3D structure through C–H⋯π and π⋯π interactions with lattice 3-bpdh ligands. Compound 5 features a 3-fold interpenetrated diamondoid net with 66 topology. The thermal stabilities and luminescent properties of 3–5 were also studied in detail. The complexes exhibit ligands based photoluminescence properties at room temperature.

A comparative ab initio study of Br2•− and Br2 water clusters

The work presents ab initio results on structure and electronic properties of Br2*-.nH2O(n=1-10) and Br2.nH2O(n=1-8) hydrated clusters to study the effects of an excess electron on the microhydration of the halide dimer. A nonlocal density functional, namely, Beckes half-and-half hybrid exchange-correlation functional is found to perform well on the present systems with a split valence 6-31++G(d,p) basis function. Geometry optimizations for all the clusters are carried out with several initial guess structures and without imposing any symmetry restriction. Br2*-.nH2O clusters prefer to have symmetrical double hydrogen-bonding structures. Results on Br2.nH2O(n>or=2) cluster show that the O atom of one H2O is oriented towards one Br atom and the H atom of another H2O is directed to other Br atom making Br2 to exist as Br+-Br- entity in the cluster. The binding and solvation energies are calculated for the Br2*-.nH2O and Br2.nH2O clusters. Calculations of the vibrational frequencies show that the formation of Br2*- and Br2 water clusters induces significant shifts from the normal modes of isolated water. Excited-state calculations are carried out on Br2*-.nH2O clusters following configuration interaction with single electron excitation procedure and UV-VIS absorption profiles are simulated. There is an excellent agreement between the present theoretical UV-VIS spectra of Br2*-.10H2O cluster and the reported transient optical spectra for Br2*- in aqueous solution.

Microhydration of X2 Gas (X = Cl, Br, and I): A Theoretical Study on X2·nH2O Clusters (n = 1−8)

Structure and properties of hydrated clusters of halogen gas, X2.nH2O (X = Cl, Br, and I; n = 1-8) are presented following first principle based electronic structure theory, namely, BHHLYP density functional and second-order Moller-Plesset perturbation (MP2) methods. Several geometrical arrangements are considered as initial guess structures to look for the minimum energy equilibrium structures by applying the 6-311++G(d,p) set of the basis function. Results on X2-water clusters (X = Br and I) suggest that X2 exists as a charge separated ion pair, X+delta-X-delta in the hydrated clusters, X2.nH2O (n > or = 2). Though the optimized structures of Cl2.nH2O clusters look like X2.nH2O (X = Br and I) clusters, Cl2 does not exist as a charge separated ion pair in the presence of solvent water molecules. The calculated interaction energy between X2 and solvent water cluster increases from Cl2.nH2O to I2.nH2O clusters, suggesting solubility of gas-phase I2 in water to be a maximum among these three systems. Static and dynamic polarizabilities of hydrated X2 clusters, X2.nH2O, are calculated and observed to vary linearly with the size (n) of these water clusters with correlation coefficient >0.999. This suggests that the polarizability of the larger size hydrated clusters can be reliably predicted. Static and dynamic polarizabilities of these hydrated clusters grow exponentially with the frequency of an external applied field for a particular size (n) of hydrated cluster.

Spectral Signatures of Intramolecular Charge Transfer Process in β-Enaminones: A Combined Experimental and Theoretical Analysis

In this paper, we present spectroscopic signatures of intramolecular charge transfer (ICT) and effects of solvent on the ICT process in 3-(phenylamino)-2-cyclohexen-1-one (PACO), a member of the well-known molecular family, the beta-enaminones. The dual fluorescence in the steady state emission spectra of the molecule in polar solvents indicates the occurrence of ICT, which is further supported by time-resolved studies, using time correlated single photon counting technique with picosecond resolution. To understand the nature of the charge transfer, pH dependent studies of the probe in water were performed, where a quenching of fluorescence was observed even in the presence of very low concentrations of acids. Solvent induced fluorescence quenching was observed in ethanol and methanol. The ICT process was also investigated by quantum chemical calculations. To understand the role of solvents in the ICT process, we have theoretically studied the macroscopic and microscopic solvation of the probe in water. The absorption spectra of the molecule in the gas phase as well as in water were simulated using time dependent density functional theory with cc-pVTZ basis set and self-consistent reaction field theory that models macroscopic solvation. The possibility of microscopic solvation in water was probed theoretically and the formation of 1:3 molecular clusters by PACO with water molecules has been confirmed. Our findings could have a bearing on pH sensing applications of the probe.

C-H···Y hydrogen bonds in the complexes of p-cresol and p-cyanophenol with fluoroform and chloroform.

In this work, we present spectroscopic investigations of hydrogen bonded complexes of CHF3 and CHCl3 with p-cresol and p-cyanophenol. The systems were chosen as the potential candidates bound by C-H···Y type hydrogen bonds that are known to exhibit unconventional blue shifts in the C-H stretching frequency. The two phenol derivatives chosen offer multiple hydrogen bonding acceptor sites. They also differ from each other in regard to the electron-donating/withdrawing ability of the para substituents which could dictate the global minimum structure in each case. The complexes were formed using the supersonic jet expansion method and were investigated using a variant of the IR-UV double resonance technique, namely fluorescence depletion IR (FDIR) spectroscopy. It was found that in the case of p-cresol the complexes were C-H···π bound in which the C-H stretch was blue-shifted. In the case of p-cyanophenol the complexes were C-H···N bound. In its fluoroform complex the C-H frequency was blue-shifted by 27 cm(-1), whereas the chloroform complex gave an example of zero-shifted hydrogen bond. The ab initio computational studies indicated that for the CHCl3 complexes it is necessary to optimize the structures on the BSSE-corrected PES using the counterpoise method to correctly predict the magnitudes of the C-H frequency shift.