The review focuses on various hydrogen producing and storing methods that can be employed for creating a hydrogen economy. The latest advancements that have been made on different hydrogen storing materials and hydrogen storing technologies which have proven useful both on gravimetric and volumetric basis, have been highlighted. The encouraging and hopeful aspect of their developments is that the most of the materials are approaching the hydrogen storing capacity requirement that have been laid down by DOE. The classification of different systems has been done on basis of their storage mechanism, keeping in mind their advantages and disadvantages while they tend to store hydrogen both in the atomic and molecular form.

Hydrogen storage: Materials, methods and perspectives

Incorporation of main group elements into the π-conjugated frameworks is a sophisticated strategy to alter the fundamental nature of the parent conjugated π-systems, giving rise to attractive electronic and photophysical properties that are otherwise inaccessible with classic carbon- or metal-based materials. Out of all π-conjugated heterocycles, those that are structurally constrained by tethered aryl substituents surrounding the main group center deserve a great deal of attention because not only do they commonly possess the maximum efficiency of π-conjugation and intermolecular interaction, but they also enjoy remarkable thermal and morphological stabilities that are especially crucial for solid-state performances. In certain cases, elucidation of the behavior of such compounds may additionally provide sufficient perspective toward graphene materials doped with main group elements, which are widely considered as potential next-generation optoelectronic materials. In this review, we will specifically focus on historical developments of structurally constrained polycyclic π-electron systems particularly of those with boron, nitrogen, silicon, or phosphorus atoms annulated directly into the center of π-conjugated systems.

Structurally Constrained Boron-, Nitrogen-, Silicon-, and Phosphorus-Centered Polycyclic π-Conjugated Systems.

In this review, we discuss the rich chemistry of helicenes and helicenoids containing main-group elements. Enantioenriched helicenic derivatives containing main-group elements B, Si, N, and P, either incorporated within the helical backbone or grafted to it, will be thoroughly presented. We will describe their synthesis, resolution, and asymmetric synthesis, their structural features, electronic and chiroptical properties, and emission, together with other photochemical properties and applications.

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Enantioenriched Helicenes and Helicenoids Containing Main-Group Elements (B, Si, N, P).

Ionic liquids (ILs) and deep eutectic solvents (DESs) have been suggested as eco-friendly alternatives to organic solvents. A trace amount of water is often unavoidable as impurity, and water is also added on purpose to reduce their problematically high viscosity and lower their high price. Understanding the distinct effects of water on the properties of ILs/DESs is highly important. In this review, we collect published experimental and theoretical results for IL/DES–H2O systems at varied water concentrations and analyze them. Results from mechanistic studies, thermodynamic modelling and advanced experiments are collected and critically discussed. Six commonly studied IL/DES–H2O systems were selected to map experimental observations onto microscopic results obtained in mechanistic studies. A great variety of distinct contours of the excess properties can be observed over the entire compositional range, indicating that the properties of IL/DES–H2O systems are highly unpredictable. Mechanistic studies clearly demonstrate that the added H2O rapidly changes the heterogeneous 3D structures of pure ILs/DESs, leading to very different properties and behaviour. There are similarities between aqueous electrolytes and IL/DES solutions but the bulky and asymmetric organic cations in ILs/DESs do not conform to the standard salt dissolution and hydration concepts. Thermodynamic modelling previously assumes ILs/DESs to be either a neutral ion-pair or completely dissociated ions, neglecting specific ion hydration effects. A new conceptual framework is suggested for thermodynamic modelling of IL/DES–H2O binary systems to enable new technologies for their practical applications.

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The peculiar effect of water on ionic liquids and deep eutectic solvents

From first-principles calculations, we predict that a single ethylene molecule can form a stable complex with two transition metals (TM) such as Ti. The resulting TM-ethylene complex then absorbs up to ten hydrogen molecules, reaching to gravimetric storage capacity of $\ensuremath{\sim}14\text{ }\text{ }\mathrm{wt}\text{ }%$. Dimerization, polymerizations, and incorporation of the TM-ethylene complexes in nanoporous carbon materials are also discussed. Our results are quite remarkable and open a new approach to high-capacity hydrogen-storage materials discovery.

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Transition Metal-Ethylene Complexes as High-Capacity Hydrogen Storage Media

A comprehensive theoretical study was carried out on a series of aryldimesityl borane (DMB) derivatives using Density Functional theory. Optimized geometries and electronic parameters like electron affinity, reorganization energy, frontiers molecular contours, polarizability and hyperpolarizability have been calculated by employing B3PW91/6-311++G (d, p) level of theory. Our results show that the Hammett function and geometrical parameters correlates well with the reorganization energies and hyperpolarizability for the series of DMB derivatives studied in this work. The orbital energy study reveals that the electron releasing substituents increase the LUMO energies and electron withdrawing substituents decrease the LUMO energies, reflecting the electron transport character of aryldimesityl borane derivatives. From frontier molecular orbitals diagram it is evident that mesityl rings act as the donor, while the phenylene and Boron atom appear as acceptors in these systems. The calculated hyperpolarizability of secondary amine derivative of DMB is 40 times higher than DMB (1). The electronic excitation contributions to the hyperpolarizability studied by using TDDFT calculation shows that hyperpolarizability correlates well with dipole moment in ground and excited state and excitation energy in terms of the two-level model. Thus the results of these calculations can be helpful in designing the DMB derivatives for efficient electron transport and nonlinear optical material by appropriate substitution with electron releasing or withdrawing substituents on phenyl ring of DMB system.

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Electron Transport and Nonlinear Optical Properties of Substituted Aryldimesityl Boranes: A DFT Study

DFT investigation on nonlinear optical (NLO) properties of novel borazine derivatives

This work involved the design of a new series of triarylaminehelicenes (TAH) with significant hole transport capacity and enhanced nonlinear optical response. The geometries, electronic properties and nonlinear response of TAH derivatives were studied using density functional theory at the B3PW91/6-311++G (2d, 2p) level. Charge transfer and nonlinear optical response were analyzed and correlated with modifications in geometry and energy levels. Calculations indicated that trivial changes in the torsional angle occur in TAH derivatives with electron-donating substituents as compared to those with electron-withdrawing substituents, resulting in lower reorganization energies for TAH derivatives 2–6. TAH derivatives with an –N(CH3)2 group have the greatest highest occupied molecular orbital (HOMO) level, and thus the least ionization potential, indicating significant hole transfer efficiency as compared to unsubstituted TAH. A decrease in the HOMO–LUMO gap occurs upon substitution with electron-releasing groups, whereas there is an increase in the case of –NO2, –COOH, and –CN TAH derivatives. Topological analysis of the HOMOs of the neutral molecules revealed that these orbitals are concentrated mainly in the helicene backbone, with an important contribution from fused phenyl rings, nitrogen atoms and carbonyl groups. However, the lowest unoccupied molecular orbitals (LUMO) are invariably constituted by fused phenyl rings without any contribution from the central nitrogen atom. Studying the effect of substitution on the nonlinear optical response of TAH derivatives, the calculated polarizability and hyperpolarizability at B3PW91/6-311++G (2d,2p) level of theory exhibited a prominent improvement as compared to unsubstituted TAH. Both electron-donating groups and electron-withdrawing groups result in a red shift in the electronic absorption bands of the substitution derivatives, in particular those with –N(CH3)2 and –NH2 groups.

Optoelectronic and nonlinear optical properties of triarylamine helicenes: a DFT study

Lanthanide (=Ce, Pr, Nd and Tb) ions substitution at calcium sites of hydroxyl apatite nanoparticles as fluorescent bio probes: Experimental and density functional theory study

Precise determination of thermodynamic properties of binary mixtures containing ionic liquids (ILs), plays an important role in process design and potential engineering applications of these mixtures. Thermodynamic studies can provide an insight into the nature of intermolecular interactions occurring between the solute and solvent in solutions. In this regard, thermodynamic properties of 1,8-diazabicyclo[5.4.0]undec-7-en-8-ium trifluoroacetate [DBUTFA] in two organic solvents viz. N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) have been evaluated at different temperatures (293.15 to 328.15) K. Apparent molar property data were fitted to a Redlich–Mayer type of equation to compute infinite dilution values and empirical parameters. The experimental studies suggest that the nature of the interactions between DBUTFA and organic solvents are influenced by temperature. DBUTFA interacts more strongly with DMF at low temperature ( 308.15 K). Density functional level of theory (DFT) was employed to investigate intra-ionic and inter-ionic interactions between the ions of PIL and organic solvents. The computational results are in good agreement with experimental results.

Nasarul Islam

Papers

Electron Transport and Nonlinear Optical Properties of Substituted Aryldimesityl Boranes: A DFT Study

DFT investigation on nonlinear optical (NLO) properties of novel borazine derivatives

Optoelectronic and nonlinear optical properties of triarylamine helicenes: a DFT study

Lanthanide (=Ce, Pr, Nd and Tb) ions substitution at calcium sites of hydroxyl apatite nanoparticles as fluorescent bio probes: Experimental and density functional theory study

Elucidation of molecular interactions between a DBU based protic ionic liquid and organic solvents: thermophysical and computational studies