Regulating Charge-Transfer in Conjugated Microporous Polymers for Photocatalytic Hydrogen Evolution

Bandgap engineering in donor–acceptor conjugated microporous polymers (CMPs) is a potential way to increase the solar-energy harvesting towards photochemical water splitting. Here, the design and synthesis of a series of donor–acceptor CMPs [tetraphenylethylene (TPE) and 9-fluorenone (F) as the donor and the acceptor, respectively], F_0.1CMP , F_0.5CMP , and F_2.0CMP , are reported. These CMPs exhibited tunable bandgaps and photocatalytic hydrogen evolution from water. The donor–acceptor CMPs exhibited also intramolecular charge-transfer (ICT) absorption in the visible region (λ_max=480 nm) and their bandgap was finely tuned from 2.8 to 2.1 eV by increasing the 9-fluorenone content. Interestingly, they also showed emissions in the 540–580 nm range assisted by the energy transfer from the other TPE segments (not involved in charge-transfer interactions), as evidenced from fluorescence lifetime decay analysis. By increasing the 9-fluorenone content the emission color of the polymer was also tuned from green to red. Photocatalytic activities of the donor–acceptor CMPs ( F_0.1CMP , F_0.5CMP , and F_2.0CMP ) are greatly enhanced compared to the 9-fluorenone free polymer ( F_0.0CMP ), which is essentially due to improved visible-light absorption and low bandgap of donor–acceptor CMPs. Among all the polymers F_0.5CMP with an optimum bandgap (2.3 eV) showed the highest H₂ evolution under visible-light irradiation. Moreover, all polymers showed excellent dispersibility in organic solvents and easy coated on the solid substrates.     

Bio-based tri-functional epoxy resin (TEIA) blend cured with anhydride (MHHPA) based cross-linker: Thermal,mechanical and morphological characterization

A novel renewable resource based tri-functional epoxy resin from itaconic acid (TEIA) was blended with petroleum based epoxy resin (DGEBA) and fabricated at different ratios. Then, it was by thermally cured with methylhexahydrophthalic anhydride (MHHPA) in presence of 2-methylimidazole (2-MI) catalyst. The tensile, modulus, strength of virgin epoxy resin (41.97 MPa, 2222 MPa) increased to 47.59 MPa, 2515 MPa, respectively, with the addition of 30% of TEIA. The fracture toughness parameter, critical stress intensity factor (K_IC) revealed enhancement of toughness in the TEIA bio-based blends system. The thermomechanical properties of TEIA (tri-functional epoxy resin from itaconic acid) modified petroleum-epoxy networks were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). The fracture morphology was also studied by the scanning electron microscopy and atomic force microscopy respectively.     

Nonlinear optical properties in calix[n]arenes: orientation effects of monomers

We investigate the nonlinear optical (NLO) polarizations for various geometric arrangements of the dipolar chromophores in a calix[n]arene geometry. The interactions were studied by using (HF)3 as model compounds both in parallel and frustrated orientations. The interdipolar angle between the monomer molecules is varied so that many different arrangements of the dipoles can be realized which is associated with the opening up of the calix[n]arene rings. Quantum-chemical calculations at both ab initio and semiempirical levels, show that the all-parallel arrangement of the dipoles exhibit the maximum magnitude for the nonlinear optical coefficients at small interdipolar angles for all interdipolar distances. In the frustrated orientations however, the NLO response is maximum at large interdipolar angles corresponding to the flattened calix[n]arene rings. The role of hydrogen bonding in stabilizing these assemblies in various conformations has been investigated. Crystallographic database analysis for calix[3]arenes shows that optical properties in real molecular systems can be modeled based on our simple interaction theory.     

Multivalency and the mode of action of bacterial sialidases

Although complex modular proteins are encountered frequently in a variety of biological systems, their occurrence in biocatalysis has not been widely appreciated. Here, we describe that bacterial sialidases, which have both a catalytic and carbohydrate-binding domain, can hydrolyze polyvalent substrates with much greater catalytic efficiency than their monovalent counterparts. The enhancement of catalytic efficiency was due to a much smaller Michaelis constant and rationalized by a model in which the catalytic and lectin domains interact simultaneously with the polyvalent substrate, leading to an enhancement of affinity. Inhibition studies have shown that galactosides released by the action of the sialidase can act as the ligand for the lectin domain. This knowledge has been exploited in the design of a potent polyvalent inhibitor of the sialidase of Vibrio cholerae, which displayed exquisite selectivities for sialidases that have a lectin domain.     

Role of the spin magnitude of the magnetic ion in determining the frustration and low-temperature properties of kagome lattices

In view of the variety of low-temperature magnetic properties reported recently for kagome lattices with transition-metal ions in different oxidation states, we have investigated the low-energy spectrum and low-temperature thermodynamic properties of antiferromagnetic kagome lattices with varying magnitudes of site spins, employing quantum many-body Heisenberg models. The ground state and the low-lying excitation spectrum are found to depend strongly on the nature of the spin magnitude of the magnetic ions. The system remains highly frustrated if spins are half-odd-integer in magnitude, while the frustration is very weak or almost absent for integer spins or mixed-spin systems. In fact, for a mixed-spin kagome system with a certain magnitude, the whole system behaves as a classical magnet with a ferrimagnetic ground state without any frustration. These theoretical findings are consistent with a few experimental observations recently reported in the literature and would be of value in designing new kagome systems with unusual and interesting low-temperature magnetic properties.     

Odd-even oscillations in first hyperpolarizability of dipolar chromophores: role of conformations of spacers

Quantum chemical calculations on the linear and nonlinear electric polarizabilities of dipolar molecules separated by the alkyl spacers have been performed on O(2)N-Ph-N=N-Ph-(CH(2))(n)-Ph-N=N-Ph-NO(2), n = 1-12. These molecules exhibit a very strong odd-even behavior in the first hyperpolarizabilities (beta), with large (small) beta for n = odd (n = even). Such odd-even oscillations have been reported experimentally on similar systems, but the origin of such phenomena remains unclear. We propose it to be due to the role of the conformational orientation of the intervening alkyl spacers that leads to eclipsed orientation (parallel) of the dipoles for n = odd chains while staggered orientation (antiparallel) for n = even chains. The energy difference between the two extreme angular forms is approximately 6-8 kcal/mol, clearly more than the thermal fluctuations at room temperature. These conformational orientations will be preserved, leading to different packing arrangements at the macroscopic scale. We believe that it is this interaction at the molecular scale that controls such a macromolecular property.     

Binary and ternary complexes of copper(II) with some dihydroxycoumarins

The complexation behaviour of copper(II) with a series of selected dihydroxycoumarins has been studied. Copper(II) binary complexes with esculetin, daphnetin and their 4-methyl and 4-phenyl derivatives and ternary complexes having a secondary ligand, ammonia or pyridine have bean synthesized. The general composition of binary complexes has bean found to be [Cu (H₂O)₂ (HL)₂] and that of ternary complexes as [Cu (L) (X)₄] (where X = NH₃ or C₅H₅N). The magnetic studies indicate all the complexes to be monomeric in nature. IR studies show that one of the two phenolic hydroxyl groups present in the ligand is lost during complexation. It is concluded that of the four ligands attached to the copper atom dihydroxycoumarin is bound most firmly.     

New examples of metalloaromatic Al-clusters: (Al4M4)Fe(CO)3 (M = Li, Na and K) and (Al4M4)2Ni: rationalization for possible synthesis

Ab initio calculations reveal that all-metal antiaromatic molecules like Al4M4 (M = Li, Na and K) can be stabilized in half sandwich (Al4M4)Fe(CO)3 and full sandwich (Al4M4)2Ni complexes. The formation of the full sandwich complex [(Al4M4)2Ni] from its organometallic precursor depends on the stability of the organic-inorganic hybrid (C4H4)Ni(Al4Li4).