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.     

Theoretical studies on the diazacyclopentadienylidenes: An analysis of aromatic versus nonaromatic systems and singlet versus triplet reactivity

Ab initio molecular orbital calculations have been carried out on the various electronic states of 2,3- ( 6 ), 2,4- ( 7 ), 2,5- ( 8 ), and 3,4-diazacyclopentadienylidene ( 9 ) at the fully geometry optimized 6–31G* level, with single point calculations being carried out at the MP2/6–31G* level. The calculated geometries are interpreted in terms of the degree of occupancy and the nature of the π and σ-nonbonded MO's. At the 6–31G* level the five π-electron, π,σ-triplet states were calculated to be considerably lower in energy. At the MP2/6–31G* level, however, with 7 the six π-electron singlet state is calculated to lie only slightly above the five π-electron triplet (0.4 kcal mole⁻¹), whereas with 8 and 9 the aromatic six π-electron singlet states are calculated to be lower in energy (9.0 and 8.1 kcal mole⁻¹). With 3 and 9 the aromatic six π-electron σ-triplet states lie only 3.6 and 5.5 kcal mole⁻¹ above the lowest energy states. It is concluded that, in general, the energy gained by having an electron in a lower energy σ-type MO instead of a higher energy π MO effectively offsets the energy gained by having an aromatic π system. The results are discussed in terms of the observed chemistry of 6–9 and their substituted systems.     

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.     

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.     

Free-electron model for hyperpolarizability of the π-electron system in benzene

The hyperpolarizabilities of benzene have been calculated using a free-electron model for the π-electron system. The first hyperpolarizability is identically zero. For the second hyperpolarizability only the components γ_zzzz; γ_xxxx; γ_yyyy; γ_zzxx; γ_zzyy; γ_xxyy are non-zero. Some of these γ quantities show dispersion near the characteristic absorption band of the benzene molecule. The polarizability α of the π-electron system also shows similer dispersion.     

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.     

Preferential site of solvation of the furan ring in acidic solvents

A study of the correlation of solvent effects on the chemical shifts of the methyl protons of sylvan, toluene and α-picoline led to the conclusion that the preferential site of solvation of the furan ring in acidic solvents is the π-electron cloud and not the n-electron orbital of the oxygen atom.     

A multi-configuration LCAO–MO study for complex unsaturated molecules. I. General theory and its application to the benzene anion

A multi-configuration LCAO –MO approach using a π-bond order–bond length linear relation is introduced to predict the geometrical structures for the electronic ground and excited states of unsaturated hydrocarbons. The procedure is designed to include configuration interaction in each iterative computation where the π-electron approximation is employed under the Pariser–Parr type semi-empirical treatment. The π-bond order–bond length relation is determined as r_pq = 1.523 – 0.193P_pq, when the bond lengths of ethylene, benzene and naphthalene are used and the groundstate functions including the singly and doubly excited configurations are taken into account to obtain the bond orders P_pq. The iterative calculation is applied to the ground state and the two lowest excited states of the benzene anion in both D_6h and D_2h molecular geometries. The geometrical structures and the π-electron energies are computed for the ground and excited states of the anion; for the latter, two types of configuration species are used. It is found that the first lowest excited state is not subjected to the Jahn–Teller effect and the calculated excited state energies do not agree with the observed values (c. 1.0 ～ 2.5 eV higher than the observed values). The latter point is discussed in detail. It is also found that the resultant ground state energy depression due to configuration mixing is not very large and the two types of configuration species used give different CI effects on the energy levels of the two lowest excited states of the anion. Finally, the stabilization energy due to the Jahn–Teller distortion is estimated for the ground state of the anion.     

2,8(4,6)-二取代半瞬烯及其BCO衍生物的Cope重排

运用从头计算量化方法计算了一系列Cope重排的2,8(或4,6)-二取代半瞬烯(1和2), 以及2,8:4,6-四取代半瞬烯(3和4)的过渡态结构、活化能、能量差和核独立化学位移. 在2,8(或4,6)-二取代半瞬烯体系里, π电子基均使得Cope重排向2进行, 这与Hoffmann早年对此的定性预测相反. 它们的过渡态是离域双同芳香性的. 本文重点讨论了π电子基与BCO共同取代的2,8:4,6-四取代半瞬烯体系, 由于BCO的稳定效应和π电子基共轭效应, π电子基均使得结构4具有优先稳定性, 其Cope重排活化能也大大降低甚至消除. 随着卤素主量子数的变化, 它们的取代半瞬烯Cope重排相关参数也有相应的规律性变化.     

Carbon-13 nuclear magnetic resonance spectroscopy—VII:para-substituted fluorobenzenes

C-13 and F-19 NMR spectra of seventeen para-substituted fluorobenzenes were measured and the chemical shifts as well as coupling constants with respect to substituents were analysed. The chemical shifts of the fluorine, the C₁ and the C₂ atoms were found to depend on the total electron densities. In the case of the C₃ atom, the chemical shifts seem to depend on π-electron densities rather than the total electron densities. The present calculations also indicate that the chemical shift of the C₄ atom depends mainly on σ-electron densities due to the inductive effects of substituents. The strongest factor influencing the coupling constant, ⁿJ(C? F), is also considered to be the π-electron densities on the carbon atoms. In the case of the direct couplings, ¹J(C? F), the π-bond orders are important.     

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     

Macrocyclic Tetraethynylethene Molecular Scaffolding: Perethynylated aromatic dodecadehydro[18]annulenes,antiaromatic octadehydro[12]annulenes,and expanded radialenes

Tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne) molecular scaffolding provided access to novel macrocyclic nanometer-sized C-rich molecules with unusual structural and electronic properties. Starting from cis-bis-deprotected cis-bis(trialkylsilyl)protected tetraethynylethenes, the per(silylethynyl)ated octadehydro[12]annulenes 1 and 2 and the corresponding dodecadehydro[18]annulenes 4 and 5 were prepared by oxidative Hay coupling. X-Ray crystal-structure analyses of (i-Pr)₃Si-protected 2 and Me₃Si-protected 4 showed that both annulene perimeters are perfectly planar. Electronic absorption spectral comparisons provided strong evidence that the macro rings in the deep-purple-colored 1 and 2 are antiaromatic (4n π-electrons), whereas those in yellow 4 and 5 are aromatic ((4n + 2) π-electrons). Although unstable in solution, the antiaromatic compound 2 gave high-melting crystals in which the individual octadehydro[12]annulene chromophores are isolated and stabilized in a matrix-type environment formed by the bulky (i-Pr)₃Si groups. Electrochemical studies demonstrated that the antiaromatic octadehydro[12]annulene 2 undergoes two stepwise one-electron reductions more readily that the aromatic chromophore 5 . This redox behavior is best explained by the formation of an aromatic (4n + 2) π-electron dianion from 2 , whereas 5 loses its aromaticity upon reduction. The Me₃Si derivative 4 was deprotected with borax in MeOH/THF to give the highly unstable hexaethynyl-dodecadehydro[18]annulene 6 , a C₃₀H₆ isomer and macrocyclic precursor to a two-dimensional all-C-network. Deprotection of 2 did not give isolable amounts of tetraethynyl-octadehydro[12]annulene 3 due to the extreme instability of the latter. Starting from dimeric and trimeric acyclic tetraethynylethene oligomers, a series of expanded radialenes were obtained. They possess large C-cores with silylethynyl-protected peripheral valences and can be viewed as persilylated C₄₀ ( 7 ), C₅₀ ( 8 ), and C₆₀ ( 9 ) isomers. These expanded C-sheets are high-melting, highly stable, soluble materials which were readily characterized by laser-desorption time-of-flight (LD-TOF) mass spectrometry. Due to inefficient macrocyclic cross-conjugation and/or non-planarity, the extent of π-electron delocalization in 7 – 9 is limited to the longest linearly conjugated π-electron fragment. In agreement with these properties, all three expanded radialenes exhibited similar redox behavior; they are difficult to oxidize but undergo several reversible one-electron reductions in similar potential ranges. Presumably, the reduced π-electron delocalization is also at the origin of the particularly high stability of 7 – 9 .     

Resonance energies of fullerenes with 4-membered rings

We report the resonance energies (REs ) of several fullerenes with 4-membered rings and their isomers with only 5- and 6-membered rings computed using the conjugated-circuit model [RE (CC )] and the topological resonance energy (TRE ) model. Both aromaticity indices were normalized by dividing by the size of the considered fullerene [RE (CC )/e and TRE /e]. The results parallel the predictions by Gao and Herndon using the much more advanced SCF –UHF π-electron approach. A good linear correlation is found between the topologically defined indices [RE (CC )/e and TRE /e] and normalized SCF –UHF π-electron energy. © 1995 John Wiley & Sons, Inc.     

Flexibility versus Rigidity: How Conformational Constraint,Steric Hindrance and Solvent Impact Binding between π-Electron Poor Tweezer-Type Hosts and π-Electron Rich Guests

Tweezer-type receptors that form π−π stacked supramolecular complexes are important components in functional polymeric materials and molecular machines. Herein, we study how varying specific structural components of tweezer-receptors impacts their binding. A library of tweezer receptors, each containing two π-electron poor receptor residues and differing by the nature of the linking unit which was either a flexible 2,2′-(ethylenedioxy)bis(ethylamine) residue or a rigid 3,3’’-diamino-m-terphenyl diamine structure, were synthesised. Each tweezer formed 1 : 1 supramolecular complexes with π-electron rich residues (1,5-dihydroxynapthalene and pyrene) as confirmed by UV/Vis and ¹H NMR spectroscopic studies. Binding constants were determined to be between 2.3×10⁻⁵ and 71 M⁻¹ in organic solvents and were one magnitude greater in aqueous solvents for water soluble systems. The nature of the linker had variable effects on the binding constants, showing the design of tweezer type supramolecular receptors with targeted K_a values is non-trivial and requires structural optimisation supported by binding constant determination studies.     

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.     

Two π-Electrons Make the Difference: From BODIPY to BODIIM Switchable Fluorescent Dyes

(aza-)BODIPY dyes (boron dipyrromethene dyes) are well-established fluorophores due to their large quantum yields, stability, and diversity, which led to promising applications including imaging techniques, sensors, organic (opto)electronic materials, or biomedical applications. Although the control of the optical properties in (aza-)BODIPY dyes by peripheral functional groups is well studied, we herein present a novel approach to modify the 12 π-electron core of the dipyrromethene scaffold. The replacement of two carbon atoms in the β-position of a BODIPY dye by two nitrogen atoms afforded a 14 π-electron system, which was termed BODIIM (boron diimidazolylmethene) in systematic analogy to the BODIPY dyes. Remarkably, the BODIIM dye was obtained with a BH₂-rigidifying entity, which is currently elusive and highly sought after for the BODIPY dye class. DFT-Calculations confirm the [12+2] π-electron relationship between BODIPY and BODIIM and reveal a strong shape correlation between LUMO in the BODIPY and the HOMO of the BODIIM. The modification of the π-system leads to a dramatic shift of the optical properties, of which the fluorescent emission is most noteworthy and occurs at much larger Stokes shift, that is, ≈500 cm⁻¹ in BODIPY versus >4170 cm⁻¹ in BODIIM system in all solvents investigated. Nucleophilic reactivity was found at the meso-carbon atom in the formation of stable borane adducts with a significant shift of the fluorescent emission, and this behavior contrasts the reactivity of conventional BODIPY systems. In addition, the reverse decomplexation of the borane adducts was demonstrated in reactions with a representative N-heterocyclic carbene to retain the strongly fluorescent BODIIM compound, which suggests applications as fully reversible fluorescent switch.     

The effects of conformation on the noncovalent bonding interactions in a bistable donor-acceptor [3]catenane

A switchable donor-acceptor bistable [3]catenane, composed of a crown ether containing a pair of alternating π-electron rich tetrathiafulvalene and 1,5-dioxynaphthalene units, encircled by two π-electron deficient cyclobis(paraquat-p-phenylene) rings, has been synthesised and the redox-activated switching it undergoes investigated.     

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.     

π-Orbital electron–energy contributions to chemical reaction heats,I. Reactions involving only linear molecules containing up to three first–row atoms

Ab initio calculations of various expectation energies have been made for the reactant and product species in six reactions that involve only small linear molecules. The reactions include fission by hydrogen, addition of hydrogen, exchange of triply bonded atoms, fluorination, and oxygen atom transfer. The change in total electronic energy is not invariably the result of changes in inner shell energy and outer shell σ- and π-electron energies simply augmenting each other, but in several cases there is a complex interplay of opposing effects. This approach gives a different insight into the energetic aspects of changes in bonding from that derived from the concept of shared electron pairs in σ and π bonds together with lone pairs in valence shells. Changes in π-electron energy are shown to be important in a reaction in which neither reactant nor product molecules contain π bonds in the usual chemical sense. While in a reaction in which there is a complete change in the nature of the triple bonds, and hence the π bonding, the change in π-electron energy makes a smaller contribution than either the change in inner shell or the outer shell σ-electron energies.