Saied Md Pratik

Computational design of concomitant type-I and type-II porphyrin sensitized solar cells

Structures and electronic properties of porphyrins with various Donor (D)-Acceptor (A) functionalization adsorbed on TiO2 (anatase) nanoparticles are studied using DFT calculations. Adsorption of porphyrin leads to a substantial loss of planarity (puckering) for the porphyrin rings due to the stabilization of the system by the interaction of the lone-pair of electrons of the N-atoms of porphyrin with the Ti-atoms of anatase. For free porphyrin, the mode of binding to anatase is from the top-site, while for the binding of D-A functionalized porphyrin, side-wise interactions are stabilized via anchoring groups on the A (-SO3H, -PO3H2 and -CO2H). Adsorption of porphyrin on TiO2 changes the relative ordering of HOMO and LUMO levels compared to that of the free molecule and bare TiO2 nanoparticles which critically effects their performance for dye sensitized solar cells (DSSCs). The relative energy differences between the LUMO of the free molecule and LUMO of molecule···TiO2 complex (ε2) and LUMO of molecule···TiO2 complex and CB (conduction band) of bare TiO2 (ε3) are proposed as two key parameters for determining the suitability of the material for functioning as a DSSC material. Coupling of the porphyrin ring with nanoparticles leads to the appearance of additional optically-active states due to dye → TiO2 charge-transfer (CT) transitions. This leads to a possibility for these dyes to act as type-II DSSC materials as well. We suggest that there exists no such general rule that only different sets of molecules are suitable for type-I and type-II DSSCs. Concentration dependent UV-Vis absorption spectra measurements can be a simple experimental test to detect a mechanistic switch-over between type-I and type-II DSSC processes in dyes.

Topochemical Transformations of CaX2 (X=C, Si, Ge) to Form Free-Standing Two-Dimensional Materials.

Topochemical transformations of layered materials CaX2 (X=Si, Ge) are the method of choice for the high-yield synthesis of pristine, defect-free two-dimensional systems silicane and germanane, which have advanced electronic properties. Based on solid-state dispersion-corrected calculations, mechanisms for such transformations are elucidated that provide an in-depth understanding of phase transition in these layered materials. While formation of such layered materials is highly favorable for silicane and germanane, a barrier of 1.2 eV in the case of graphane precludes its synthesis from CaC2 topochemically. The energy penalty required for distorting linear acetylene into a trans-bent geometry accounts for this barrier. In contrast it is highly favorable in the heavier analogues, resulting in barrierless topochemical generation of silicane and germanane. Photochemical generation of the trans-bent structure of acetylene in its first excited state (S1 ) can directly generate graphane through a barrierless condensation. Unlike the buckled structure of silicene, the phase-h of CaSi2 with perfectly planar silicene layers exhibits the Dirac cones at the high symmetry points K and H. Interestingly, topochemical acidification of the cubic phase of calcium carbide is predicted to generate the previously elusive platonic hydrocarbon, tetrahedrane.

Janus all-cis-1,2,3,4,5,6-Hexafluorocyclohexane: A Molecular Motif for Aggregation-Induced Enhanced Polarization.

Recently synthesized all-cis-1,2,3,4,5,6-hexafluorocyclohexane is the least stable among all possible configurational isomers of 1,2,3,4,5,6-hexafluorocyclohexane. This molecule has a remarkably large dipole moment (6.2 D) as well as high facial polarization. Solid-state, dispersion-corrected DFT (DFT-D3) calculations are performed on the crystalline phase of all-cis-1,2,3,4,5,6- hexafluorocyclohexane, which reveal that dispersion interactions play a crucial role in its stabilization. A number of thermodynamically favorable orientations of dimers, trimers and tetramers are demonstrated for this molecule. Parallel-stacked aggregates, from dimers to higher-order aggregates, which are absent in the crystal, are found to be thermodynamically most favorable due to the presence of strong short-range C-H⋅⋅⋅F-C intermolecular hydrogen-bonding networks. Because of their cooperative nature, binding energies, dipole moments, and polarizations per molecule increase from monomer to tetramer, whereas the HOMO-LUMO gaps follow the opposite trend. Based on the DFT-D3 calculations, it is proposed that this parallel-stacked arrangement can be further extended to prepare stable a 1D crystal such that a large dipole moment and macroscopic polarizations can arise, which might be useful in designing electronic and nonlinear optical devices. Because the molecule has conformational flexibility, the potential energy surface is investigated for ring flipping and the effects of fluorine substitution are studied by comparing the barrier with respect to cyclohexane and all-cis-1,2,3-trifluorocyclohexane.

Nonequimolar Mixture of Organic Acids and Bases: An Exception to the Rule of Thumb for Salt or Cocrystal

Formation of salt and/or cocrystal from organic acid-base mixtures has significant consequences in the pharmaceutical industry and its related intellectual property rights (IPR). On the basis of calculations using periodic dispersion corrected DFT (DFT-D2) on formic acid-pyridine adduct, we have demonstrated that an equimolar stoichiometric ratio (1:1) exists as a neutral cocrystal. On the other hand, the nonequimolar stoichiometry (4:1) readily forms an ionic salt. While the former result is in agreement with the ΔpKa rule between the base and the acid, the latter is not. Calculations reveal that, within the equimolar manifold (n:n; n = 1-4), the mixture exists as a hydrogen bonded complex in a cocrystal-like environment. However, the nonequimolar mixture in a ratio of 5:1 and above readily forms salt-like structures. Because of the cooperative nature of hydrogen bonding, the strength of the O-H···N hydrogen bond increases and eventually transforms into O(-)···H-N(+) (complete proton transfer) as the ratio of formic acid increases and forms salt as experimentally observed. Clearly, an enhanced polarization of formic acid on aggregation increases its acidity and, hence, facilitates its transfer to pyridine. Motion of the proton from formic acid to pyridine is shown to follow a relay mechanism wherein the proton that is far away from pyridine is ionized and is subsequently transferred to pyridine via hopping across the neutral formic acid molecules (Grotthuss type pathway). The dynamic nature of protons in the condensed phase is also evident for cocrystals as the barrier of intramolecular proton migration in formic acid (leading to tautomerism), ΔH(⧧)tautomer = 17.1 kcal/mol in the presence of pyridine is half of that in free formic acid (cf. ΔH(⧧)tautomer = 34.2 kcal/mol). We show that an acid-base reaction can be altered in the solid state to selectively form a cocrystal or salt depending on the strength and nature of aggregation.

Color polymorphism: Understanding the diverse solid-state packing and color in dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate

Dimethyl-3,6-dichloro-2,5-dihydroxyterephthalate (MCHT) is known to exist in three differently packed crystals having three different colors, namely yellow (Y), light yellow (LY), and white (W). Apart from the difference in their color, the molecules in the crystals also differ in their intramolecular O-H⋅⋅⋅O and O-H⋅⋅⋅Cl hydrogen bonds. Time-dependent DFT calculations reveal the role of the various types of hydrogen bonds in controlling the color of the polymorphs. Mechanistic pathways that lead to such transformations in the crystal are elucidated by solid-state dispersion-corrected DFT studies. Relative stabilities of the various polymorphs rationalize the experimentally observed transformations between them. Calculations reveal that the minimum-energy pathway for the conversion of the Y form to a W form is through stepwise disrotatory motion of the two -OH groups through a hybrid intermediate having one intramolecular OH⋅⋅⋅O and one O-H⋅⋅⋅Cl bond. The LY form is shown to exist on the higher-energy pathway involving a concerted Y→W transformation.

Exploring Ultrashort Hydrogen–Hydrogen Nonbonded Contacts in Constrained Molecular Cavities

Confined molecular chambers such as macrocycle bridged E1-H···H-E2 (E1(E2) = Si(Si), 1) exhibit rare ultrashort H···H nonbonded contacts (d(H···H) = 1.56 Å). In this article, on the basis of density functional theory and ab initio molecular dynamics simulations, we propose new molecular motifs where d(H···H) can be reduced to 1.44 Å (E1(E2) = Si(Ge), 3). Further tuning the structure of the macrocycle by replacing the bulky phenyl groups by ethylenic spacers and substitution of the H-atoms by -CN groups makes the cavity more compact and furnishes even shorter d(H···H) = 1.38 Å (E1(E2) = Ge(Ge), 8). These unusually close H···H nonbonded contacts originate from the strong attractive noncovalent interactions between them, which are evident from various computational indicators, namely, NCI, Wiberg bond index, relaxed force constant, quantum theory of atoms in molecules, and natural orbitals for chemical valence combined with the extended transition state method analyses. Substantial stabilization of the in,in-configuration (exhibiting short H···H contacts) compared with the out,out-configuration (by ∼5.7 kcal/mol) and statistically insignificant fluctuations in ⟨d(H···H)⟩ and ⟨θav⟩(θ(E1(E2)-H···H = 152°) at room temperature confirm that the ultrashort H···H distances in these molecules are thermodynamically stable and would be persistent under ambient experimental conditions.

Synthesis, structure, photocatalytic and magnetic properties of an oxo-bridged copper dimer

Dimeric copper complex, [GuH]4[Cu2II(Cit)2]·2H2O (where GuH = monoprotonated guanidine and Cit = citrate anion) was synthesized solvothermally and characterized by single crystal X-ray diffraction. The Cu atoms are bridged by alkoxide oxygen atoms of the citrate ligand, forming the dimer. The Cu atom adopts a distorted square pyramidal geometry. Two CuO5 units are connected edge-wise to form the dimer, which is capped by the citrate ligand. This molecular dimer is strongly H-bonded with guanidine cation and a water molecule to form a supramolecular structure. The optical band gap energy data exhibit its semiconductor behavior. We have explored this material as a photocatalyst for the degradation of organic dye. The magnetic measurements show that compound 1 behaves like a paramagnet. Detailed theoretical investigations reveal that inter-molecular H-bonding is responsible for the stability of this structure.

Coexistence of Normal and Auxetic Behavior in a Thermally and Chemically Stable sp3 Nanothread: Poly[5]asterane

A one-dimensional nanostructure with sp3 -hybridized carbon atoms, namely, poly[5]asterane (PA), is predicted by means of electronic structure calculations and reactive molecular dynamics simulations. Thermochemical analysis based on homodesmotic reactions showed that the formation of poly[5]asterane is more favorable than that of polytriangulane and comparable to that of polytwistane. A plane-wave DFT approach gave a computed Youngs modulus of about 0.84 TPa, which is quite promising and comparable to those of other sp3 -hybridized nanothreads. Simulations of the desorption of hydrogen atoms from PA showed a high activation energy (Ea ≈52 kcal mol-1 ), which again indicates substantial chemical stability. Interestingly, PA was shown to exhibit auxetic behavior (negative Poissons ratio). Thus, PA is advocated as a new mechanically and chemically stable nanothread with exotic auxetic behavior.