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.     

Nature of C60 and C70 fullerene encapsulation in a porphyrin- and metalloporphyrin-based cage: Insights from dispersion-corrected density functional theory calculations

The search for efficient synthetic hosts able to encapsulate fullerenes has attracted attention with regard to the purification and formation of ordered supramolecular architectures. This study of a porphyrin-based cage as an extension of the well-described ExCage⁶⁺ and BlueCage⁶⁺, involving viologen as sidearms, provides an interesting scenario where the oblate C₇₀ fullerene is preferred in comparison to the spherical C₆₀. Our results expose the nature of the fullerene-cage interaction involving ∼50% of dispersion-type interactions evidencing the strong π⋯π surface stacking, with a complementary contribution by the electrostatic and orbital polarization character produced by a charge reorganization with a charge accumulation facing the porphyrin macrocycles and a charge depletion along the equator formed by the viologens sidearms. Interestingly, the central N₄H₂ ring from each porphyrin contributes to the dispersion term via N-H⋯π interactions, which is decreased when the metallate N₄Zn is evaluated. Thus, the formation of stable and selective fullerene encapsulation can be achieved by taking into account two main driving forces, namely, (a) the extension of the π⋯π and X-H⋯π stacking surface and (b) charge reorganization over the fullerene surfaces, which can be used to control fine tuning of the encapsulation thanks to the introduction of more electron-deficient and electron-rich groups within the host cage.     

Humidity effect on electrical properties of layered α-zirconium phosphate

The electrical properties of alkali ions exchanged zirconium hydrogen phosphate Zr(HPO₄)₂·H₂O (α-ZrP) are investigated by impedance spectroscopy technique. Cole-cole plots are used to describe the characteristic changes of electrical properties with relative humidity. The evaluated bulk resistance from the equivalent circuit shows exponential dependence of relative humidity. The surface protons of layered (α-ZrP) contribute to electrical conduction. The largest sensitivity is obtained for K ion exchanged systems.     

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.     

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.     

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).     

Polymer-supported triacetoxyborohydride: a novel reagent of choice for reductive amination

A novel polymer-supported triacetoxyborohydride reagent for reductive amination of aldehydes and ketones is reported. The bound reagent was found to be shelf-stable and provided broad scope and reactivity in reductive amination reactions under mild reaction conditions. Streamlined protocols for its application in reductive amination reactions using both primary and secondary amines are described.