Macrocyclic π-systems with C2-symmetry as the sum of π-systems with hückel and möbius topology

The π-system of a macrocycle with twofold rotation symmetry and 2N conjugated π-electron can conveniently be regarded as the sum of two cyclic N π-electron systems having Hückel and Möbius topology, respectively.     

Dynamical screening by Σ-electrons in π-electron transitions: Treatment in the equations-of-motion method

Dynamical screening in π-electron systems is studied by the equations of motion method. By using a partitioning technique on the equations of motion we can obtain simple expressions for the effect of dynamical screening directly on the transition energies and transition moments in π-electron systems. These results are used to study the effect of such screening in the N → V transition in ethylene. This procedure can be used to extend the equations-of-motion method to larger π-electron systems.     

π-Electron currents in fixed π-sextet aromatic benzenoids

In view of different patterns of π-electron density currents in benzenoid aromatic compounds it is of interest to investigate the pattern of ring currents in various classes of compounds. Recently such a study using a graph theoretical approach to calculating CC bond currents was reported for fully benzenoid hydrocarbons, that is, benzenoid hydrocarbons which have either π-sextets rings or “empty” rings in the terminology of Clar. In this contribution we consider π-electron currents in benzenoid hydrocarbons which have π-electron sextets and C=C bonds fully fixed. Our approach assumes that currents arise from contributions of individual conjugated circuits within the set of Kekulé valence structures of these molecules.     

吡咯与HX(X=F,Cl,Br)分子间多种氢键的电子密度拓扑研究

采用密度泛函B3LYP/6-311＋＋G(d,p)方法, 对吡咯与HX (X＝F, Cl, Br)形成的经典氢键和π型氢键, 从其几何参数、电子密度的拓扑性质和电子积分等方面进行了研究. 在对π型氢键的讨论中我们将π电子与σ电子分离, 得到了π型氢键体系的π电子的密度等值线和拉普拉斯量等值线图以及各原子的π电子积分, 形象地说明了π型氢键的作用本质.     

Donor-acceptor molecular figures-of-eight

The intermolecular template-directed synthesis, separation and characterisation of two constitutional isomers that are self-complexing donor-acceptor [1]rotaxanes has been achieved by click chemistry, starting from a π-electron deficient tetracationic cyclophane containing two azide functions and a π-electron rich 1,5-dioxynaphthalene-containing polyether chain terminated by propargyl groups.     

Donor/acceptor complex of triphenylene and trinitrotoluene on Au(111): a scanning tunneling microscopy study

The co-adsorption of trinitrotoluene (TNT), a typical π-electron acceptor, and triphenylene (TP), a typical π-electron donor, on a Au(111) surface was investigated by in situ Electrochemical Scanning Tunneling Microscopy (ECSTM). DFT calculations proved that parallelly stacked and well-overlapped TP and TNT molecules can form Donor-Acceptor dyads through intermolecular π-π charge transfer, which agree well with the experimental results in the present work.     

Planarity of ethylene/linear polyene analogues focused on π-electron holding ability of the components

Ethylene/polyene analogues composed of heavier group 14 elements, such as silicon and germanium, do not prefer a planar structure. In the repulsion dominant (RD) model of our previous study mainly focusing on the planarity of hexasilabenzene, it was demonstrated that electron repulsion promotes nonplanarization of heavy benzene analogues. In this study, we have investigated a correlation between intramolecular π-electron transfers (polarization effect) and planarity in various linear unsaturated compounds in order to deepen the RD model. Herein, it was revealed that the ability to hold π-electrons in the planar molecular structure is characteristic of each element. For example, carbon can hold more than one π-electron, whereas silicon and germanium cannot tolerate even one π-electron to keep the planar structure. Thus, π-accepting substituents on the heavy atom were found to make the heavy ethylenes and linear polyenes planar by controlling the number of π-electrons on each skeletal atom.     

Bond orders in 1,5-benzodiazepinium cations compared to those in biphenylenes. Another example of bonds of low bond order which isolated π-electron systems.

The bonds which link separate stablishing π-electron systems in 1,5-benzodiazepinium cations are shown from X-ray analysis and MNDO calculations to be of very low bond order, like the bridging bonds in biphenylenes, thereby inhibiting destabilising interaction between the π-electron systems.     

Graph theory and molecular orbitals. The loop rule

Graph theory is applied to the study of the dependence of total π-electron energy, π-electron charge distribution and free valency indices of conjugated hydrocarbons on molecular topology. It is shown that the number of loops in the molecular graph determines these quantities.     

Synthesis and properties of azobenzocrown ethers with π-electron donor, or π-electron donor and π-electron acceptor group(s) on benzene ring(s)

New azobenzocrown ethers of differentiated size and with substituted benzene residues have been synthesized. These crown ethers possess π-electron donor, or π-electron donor-π-electron acceptor pair of functional group(s) in benzene ring(s) in the para position to azo-grouping. Their metal ion complexation abilities in solution were studied using UV-vis spectrophotometry. The X-ray structure of a 19-membered crown ether with 4-dimethylamino-4′-nitroazobenzene fragment has been solved.     

Mndo study of bond orders in some conjugated BI- and tri-cyclic hydrocarbons

MNDO calculations have been carried out on a number of conjugated bicyclic and tricyclic hydrocarbons. For reference purposes some acenes and monocyclic non-benzenoid hydrocarbons are also mentioned. While the influence of 2 or 6 π-electron systems in stabilising, and of 4 π-electron systems in destabilising, structures is readily evident, it is clear that arrays of 10 or more π-electrons have markedly less effect, and the balance between fixation of bonds and delocalisation in many compounds possessing such systems is subtle.     

Silicon-containing formal 4π-electron four-membered ring systems: antiaromatic, aromatic, or nonaromatic?

Density functional theory calculations (B3LYP) have been carried out to investigate the 4π-electron systems of 2,4-disila-1,3-diphosphacyclobutadiene (compound 1) and the tetrasilacyclobutadiene dication (compound 2). The calculated nucleus-independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4π-electron four-membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4π-electron system. The monoanionic bidentate ligand interacts with the conjugated π?system to cause π-orbital splitting. This ligand-induced π-orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied π?orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated π?ring system and does not fulfill the requirements of a Hückel system.     

Electron-Deficient Benzo[de]isoquinolino[1,8-gh]quinoline Diamide π-Electron Systems

Synthetically versatile electron-deficient π-electron systems are urgently needed for organic electronics, yet their design and synthesis are challenging due to the low reactivity from large electron affinities. In this work, we report a benzo[de]isoquinolino[1,8-gh]quinoline diamide (BQQDA) π-electron system. The electron-rich condensed amide as opposed to the generally-employed imide provides a suitable electronic feature for chemical versatility to tailor the BQQDA π-electron system for various electronic applications. We demonstrate an effective synthetic method to furnish the target BQQDA parent structure, and highly selective functionalization can be performed on bay positions of the π-skeleton. In addition, thionation of BQQDA can be accomplished under mild conditions. Fine-tuning of fundamental properties and supramolecular packing motifs are achieved via chemical modifications, and the cyanated BQQDA organic semiconductor demonstrates a high air-stable electron-carrier mobility.     

A Triply [5]Helicene-Bridged (1,3,5)Cyclophane

A rigid propeller-shaped conjugated triple macrocycle consisting of two nearly perfectly stacked benzene rings and three linking [5]helicene moieties has been synthesized using a glyoxylic Perkin approach. Analysis of the electron delocalization in this atypical aromatic molecule revealed global aromaticity and a 78 π-electron circuit along the edge of its triple loop, to the detriment of the two 6 π-electron circuits in the two stacked benzene rings.     

Towards the self-assembly of polyrotaxanes

The self-assembly of a number of rotaxanes, pseudorotaxanes, and a pseudopolyrotaxane based on the π-electron deficient cyclobis(paraquat-p-phenylene)^d̊ and threads and dumbbell components composed of π-electron rich hydroquinone rings incorporated symmetrically within polyether chains terminated in the case of the rotaxanes by adamantoyl stoppers.     

Effect of methyl and hydroxyl substituents on the π-electronic spectra and ionization potential of p-benzoquinone

SCF -LCAO -MO -CI semi-empirical π-electron calculations have been made of the first two singlet transitions and ionization potentials of hydroxy-, methyl- and hydroxy-methyl substituted p-benzoquinone, using a modified Pariser-Parr-Pople method. The various molecules could be divided into two classes on the basis of their first π-electron transition, namely, those in which the 3-position was occupied and others where this position was vacant.     

Graph theoretical approach to the topological spin hamiltonian applied to conjugated molecules

Approximate formulae for the total π-electron energy calculated within the topological spin Hamiltonian are derived using the connected moments expansion (CMX) technique up to the third order. This provides the first reported graph theoretical approach to the topological Hamiltonian other than the Hückel one. The formulae, valid for alternant, singlet ground state hydrocarbons, are used to estimate the total π-electron energy of some large molecules     

新型缺电子联吡啶大环化合物的模板合成

通过富电子对苯二酚醚链衍生物和缺电子联吡啶间的模板效应, 成功地合成了一种新型联吡啶大环化合物, 利用元素分析、核磁共振和质谱等手段对新化合物进行了表征。     

Electron delocalization contribution to single crystal thermal expansion of a polydiacetylene

The thermal expansion contribution due to temperature-dependent π-electron delocalization is evaluated from spectral measurements on a single crystal polydiacetylene (poly-2,4-hexadiyne-1,6-diol bisphenylurethane). The observed temperature independence of backbone associated vibrations (less than ±1 cm^?1 change in ν_C?C and ν_C?C between 25 and 90°C) implies that thermal conformational fluctuations and equilibrium defect formation (which produce a negative thermal expansion coefficient) do not measurably affect π-electron delocalization. The separation of equilibrium defects is either much longer than that of nonequilibrium defects or much longer than required to appreciably limit π-electron delocalization in an effectively defect-free polymer. Arguments presented indicate that, in the experimental temperature interval, the observed thermal expansion coefficient in the chain direction is over an order of magnitude larger than the delocalization-associated contribution.