Biopolymer-assisted synthesis of assembled g-C3N4 open frameworks with electron delocalization channels for prompted H2 production

Three-dimensional (3D) photocatalysts can overcome the serious two-dimensional (2D) nanosheet aggregation reduced by unavoidable Van del Waals forces. However, its water splitting efficiency needs further improvement via boosting the exciton...

Electron Delocalization on the Silicon Nanoparticles Surface

The specific properties of tubular and fullerenlike silicon nanoparticles depend on theirs electronic structure, which is directly related to the surface geometry. Using density functional approach, a novel dual nature of the surface structure of silicon nanotubes which depends on the type of nanotube have been revevaled. The rippled form of the surface has shown to be a favorable one for (n, n) type structure and the most stable form for (n, 0) Si NT is the nanotube with a smooth-walled graphene-like surface. The phenomenon is explained by the relative position of the non-hybridized p orbitals on the surface.

Three-Dimensional Fully pi-Conjugated Macrocycles: When Truly 3D-Aromatic and when 2D-Aromatic-in-3D?

Recently, several fully pi-conjugated macrocycles with strongly puckered or cage-type structures have been synthesized and found to exhibit aromatic character according to both experiments and computations. Herein, we examine their electronic structures and put them in relation to truly 3D-aromatic molecules (e.g., closo-boranes and certain charged fullerenes) as well as 2D-aromatic polycyclic aromatic hydrocarbons. We use qualitative theory combined with quantum chemical calculations, and find that the macrocycles explored thus far should be described as 2D-aromatic with three-dimensional structures (abbr. 2D-aromatic-in-3D) instead of truly 3D-aromatic. Besides fulfilling the 6n + 2 pi-electron rule, 3D-aromatic molecules with highly symmetric structures (e.g., Td and Oh) have a number of molecular orbital (MO) levels that are (at least) triply degenerate. At lower symmetries, the triple (or higher) orbital degeneracies should be kept in approximate sense. This last criterion is not fulfilled by macrocyclic cage molecules that are 2D-aromatic-in-3D. Their aromaticity results from a fulfillment of Hückel’s 4n + 2 rule for each individual macrocyclic path, yet, their pi-electron counts are coincidentally 6n + 2 numbers for macrocycles with three tethers of equal lengths. We instead link the 3D-macrocyclic molecules explored earlier to naphthalene, motivating their description as 2D-aromatics albeit with 3D structures. It is notable that macrocyclic cages which are 2D-aromatic-in-3D can be aromatic also when the tethers are of different lengths, i.e., when their pi-electron counts differ from 6n + 2. Finally, we identify tetrahedral and cubic pi-conjugated molecules that fulfill the 6n + 2 rule and which exhibit significant electron delocalization. Yet, their properties are similar to those of analogous compounds with electron counts that differ from 6n + 2. Thus, despite that these tetrahedral and cubic molecules show substantial pi-electron delocalization they should not be classified as true 3D-aromatics.

Clustering and Halogen Effects Enabled Red/Near-Infrared Room Temperature Phosphorescence from Aliphatic Cyclic Imides

<a>Pure organics with room temperature phosphorescence (RTP) are urgently demanded in advanced optoelectronic and bioelectronic applications. However, currently reported phosphors are mostly aromatics and restricted to blue to orange colors. It remains an enormous challenge to achieve red and near-infrared (NIR) RTP, particularly for those from nonaromatics. Here we demonstrate a series of cyclic imides derived from succinimide, which can emit red (665, 690 nm) and even NIR (745 nm) RTP with outstanding efficiencies of up to 9.2%, despite their rather limited molecular conjugations. Such unique emission should be ascribed to the presence of the imide unit and heavy atoms, effective molecular clustering, and the electron delocalization of halogens, which not only greatly facilitate intersystem crossing, but also afford significantly extended through-space conjugation and rigidified conformations.</a> These results pave the way to the rational construction of red and NIR nonconventional luminophores through synergistic clustering and halogen effects.

Are Metallacyclopentadienes Always Non-Aromatic?

Even though metallacyclopentadienes (MCPs) are among the most common metallacycles, their electron delocalization (aromaticity) has received far less attention than other metallacycles, such as metallabenzenes. We systematically studied the aromaticity of MCPs with energetic (isomerization stabilization energy), density (delocalization index) and magnetic (current density) aromaticity indices. The indices agree that metallacyclopentadienes are, in general, weakly aromatic at most. The 18e− complexes showed the expected weak aromaticity, and only the d8 molecules are somewhat anti-aromatic. However, the theoretical account of the aromaticity of the 16e− MCPs is more convoluted. We find that the aromatic criteria for a 16e−d4 ruthenacyclopentadiene disagree. The lack of agreement shows that significant electron delocalization is not always related to great stability or to strong diatropic currents.

Chemical Bonding 2: Modern Theories of Chemical Bonding

Chemical Bonding II: Modern Theories of Chemical Bonding explains four bonding theories related to molecular geometry and bonding. Lewis structures and the Valence-Shell Electron-Pair Repulsion (VSEPR) model are used to describe and predict the electron group geometry, molecular geometry and shapes of molecules. The VSEPR model is then used to predict molecular polarity as a function of shape. This leads to Valence Bond Theory, which uses the principles of orbital overlap and hybridization of atomic orbitals to describe chemical bonding. Finally the Molecular Orbital Theory (MOT) based on electron delocalization is discussed in terms of bonding and anti-bonding molecular orbitals.