Cryptoannulenic Behavior of Monoazacyclacenes

Abstract

Within the framework of Hückel molecular orbital theory, it has been found that similar as cyclacenes monoazacyclacenes exhibit some cryptoannulenic behavior, and their angle of total π-electron energies fluctuates within the series, depending on the size and type of the peripheral circuit (4m or 4m+2 type). However, this effect in monoazacyclacenes is less pronounced as compared to cyclacenes.     

FRONTIER-ORBITAL RING CURRENTS AND THE ANNULENE ANALOGY

Perimeter and ring currents induced in planar monocycles and polycycles by perpendicular magnetic fields are commonly taken as diagnostic of aromaticity and antiaromaticity in these systems. Diatropic π currents are associated with aromaticity, paratropic with its opposite. The ipsocentric method is an accurate and economical way of calculating ab initio current-density maps. In π systems it provides a natural interpretation of ring currents in terms of nonredundant orbital contributions, governed by simple symmetry rules for π–π* virtual excitations and dominated by the frontier electrons. Thus, in planar [4n + 2] monocycles, the product of π HOMO and π* LUMO symmetries includes that of the in-plane translations and leads to a 4-electron diatropic current. In planar [4n] monocycles, this product includes the symmetry of the in-plane rotation and leads to a 2-electron paratropic current. Perturbation arguments based on the monocycle explain the opposite senses of the π ring currents in naphthalene (diatropic) and pentalene (paratropic) as consequences of their different frontier-orbital symmetry products. In a generalization to heterocycles, ring current maps for benzotriazole (BtH), its conjugate base (Bt^?) and the cation (Bt⁺) are calculated at an ab initio level. The diatropic current of the two 10π systems and the paratropic current of the 8π system are rationalized in terms of the perturbed-annulene orbital model, giving an explanation of the applicability of simple electron counting in these cases: where the frontier-orbital structure remains close to that of the [4n+ 2]/[4n] annulene, so does the current-density map.     

π-ELECTRON CONTENTS OF RINGS IN THE DOUBLE-HEXAGONAL-CHAIN HOMOLOGOUS SERIES (PYRENE,ANTHANTHRENE AND OTHER ACENOACENES)

Abstract

Recently three methods for calculating the π-electron content of rings of benzenoid hydrocarbons were put forward: one based on the consideration of Kekuléstructural formulas, and the other two based on an analogous treatment of the Clar aromatic sextet formulas. These three methods are applied to the homologous series consisting of two condensed acene chains (whose first members are pyrene, anthanthrene, peri-naphthacenonaphthacene, …), leading to basically identical results. In contrast to acenes (in which the partition of π-electrons into rings is uniform), in the double-hexagonal-chain species the partition of π-electrons is highly non-uniform. The electron content monotonically decreases, in opposite directions, along the two acene chains, being maximal in the least annelated rings. Some other generally valid regularities in the π-electron properties of the double–hexagonal–chain benzenoids are also pointed out.     

The Ambident Nucleophilic Center. Stereochemical Consequences of HOMO-LUMO and HOMO-HOMO Dominant Processes

The nitrogen centers of pyridine, quinoline, pyrrole anion, carbazole anion, and acetonitrile possess, in each case, two energetically non-equivalent electron pairs: one, a non-bonding orbital, lies in the molecular plane or on the bond axis; the other is part of a π-type orbital, and is perpendicular to the molecular plane or the bond axis. Simple molecular orbital considerations predict that the π-type orbital will always be the higher lying and, therefore, the preferred donor orbital in a HOMO-LUMO controlled frontier molecular orbital interaction with the HOMO and LUMO of an electrophilic reagent. This stereoelectronic effect seems to have been observed experimentally in the case of carbazole anion, but not in the cases of quinoline or acetonitrile (present work). The reasons for the different behavior of these nucleophiles have been analyzed using the semi-empirical procedure AMI. It is pointed out that the lower lying non-bonding orbital can control the stereochemical course of the reaction when HOMO-HOMO interactions dominate. This will occur when the non-equivalent electron pairs of the nucleophile lie close to the HOMO of the electrophilic reagent, as is the case in the reactions of pyridine, quinoline, and acetonitrile with methyl choride or trimethyloxonium cation. Transition structures for N-methylation of acetonitrile and pyrrole anion have been located and are, respectively, linear and planar. However, a 45° bend of the pyrrole transition structure leads to only a 29-fold rate retardation at 25°C, compared to the 300-fold retardation calculated for acetonitrile.     

Chemistry at the Dirac point: Diels-Alder reactivity of graphene

Most of the interesting physics of graphene results from the singular electronic band structure at the so-called Dirac point, where the conduction and valence bands cross in momentum space. Although graphene is very stable thermodynamically, the electronic structure at the Dirac point facilitates basal plane chemistry including pericyclic reactions such as the Diels-Alder reaction. We have discovered a series of facile Diels-Alder reactions in which graphene can function either as a diene when paired with tetracyanoethylene and maleic anhydride or as a dienophile when paired with 2,3-dimethoxybutadiene and 9-methylanthracene. In this Account, we seek to rationalize these findings using simple arguments based on considerations of orbital symmetry and the frontier molecular orbital theory. The graphene conduction and valence bands (HOMO and LUMO) cross at the Dirac point, which defines the work function (W = 4.6 eV). Thus, the HOMO and LUMO form a degenerate pair of orbitals at this point in momentum space with the same ionization potential (IP) and electron affinity (EA). Based on the importance of the energies of the HOMO (-IP) and LUMO (-EA) in frontier molecular orbital (FMO) theory, graphene should be a reactive partner in Diels-Alder reactions due to the very high-lying HOMO and low-lying LUMO (energies of -4.6 eV). Inspection of the orbital symmetries of the degenerate pair of half-occupied band orbitals at the Dirac point confirms that with the appropriate orbital occupancies, both diene and dienophile reaction partners should undergo concerted Diels-Alder reactions with graphene that are allowed based on the Woodward-Hoffmann principles of orbital symmetry.     

Quantum chemical studies on corrosion inhibition for series of thio compounds on mild steel in hydrochloric acid

Quantum chemical calculations were performed on ten thio compounds using semi-empirical method PM3 within program package of Material Studio 5.5. The effect of molecular structure on the corrosion inhibition efficiency was investigated using the quantum chemical calculations. The electronic properties such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, (LUMO–HOMO) energy gap, dipole moment (λ) and fraction of electron transfer (ΔN) were calculated and discussed. A relationship between the corrosion inhibition efficiency and several quantum parameters was established with coefficient correlation (R²) of 0.8894.     

Filiations between the Electronic Spectra of C60 and Its Adduct Derivatives: A Rationalization

Considerable similarities are observed between the absorption spectra in the 200–800-nm region of C₆₀ and its n-adduct derivatives (n = 1 − 6), in spite of the fact that two π-electrons are lost for each adduct. The persistence in the adduct derivatives of the main energy and transition strength characteristics of the allowed and forbidden electronic transitions of C₆₀ is rationalized with a relatively simple model. The filial relation between these spectra is discussed on three levels of increasing physical refinement: (i) the electronic states and transitions resulting from a free-electron, spherical shell model of C₆₀; (ii) the effects of an icosahedral potential on such a model; and (iii) the effects of π-electron loss and symmetry lowering due to the addends. The results show that similar, and related, orbital transitions are involved in the absorption spectra in the 200–800-nm region of C₆₀ and its adduct derivatives, albeit mildly affected by changes in molecular symmetry and in the size of the π-conjugation region.     

THE STRENGTH OF MUTAGENICITY, 13C NMR CHEMICAL SHIFTS,AND ELECTRONIC STATES OF NITROBENZANTHRONES

Mutagenic activities of nitrated benzanthrones (NBAs) vary largely with the position and the number of the nitro group. To investigate the structure-activity correlations for NBAs, we have performed nuclear magnetic resonance (NMR) measurements and molecular orbital calculations for the three nitrobenzanthrones, 2-NBA, 3-NBA, 11-NBA; the three dinitrobenzanthrones, 1,9-DNBA, 3,11-DNBA, 3,9-DNBA; and the trinitrobenzanthrone, 3,9,11-TNBA. It was confirmed that the ¹³ C chemical shifts (δ) of the ortho carbon atoms with respect to the nitro group of the compounds tend to be more upfield with decreasing mutagenic activities. The molecular orbital calculations revealed that the LUMO energies of the compounds decrease with mutagenic activities, and that the HOMO and LUMO densities tend to decrease and increase, respectively, with decreasing mutagenic activities. These results indicate that reduction is very important in the metabolism of nitrobenzanthrones.     

Fine tuning of frontier orbital energy levels in dithieno[3,2-b:2′,3′-d]silole-based copolymers based on the substituent effect of phenyl pendants

A series of dithieno[3,2-b:2′,3′-d]silole-based π-conjugated copolymers containing thieno[3,4-c]pyrrole-4,6-dione or thieno[3,4-b]thiophene units bearing 4-substituted phenyl pendants were synthesized and their thermal stability, optical properties and frontier orbital energy levels were systematically investigated. The introduction of electron-withdrawing substituents on the phenyl rings lowered their frontier orbital energy levels without deteriorating their thermal and optical properties. By replacing an electron-donating methoxy group with an electron-withdrawing trifluoromethyl group, both the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital energy levels of the polymers were deepened by more than 0.3 eV. A relatively linear relationship was observed between the HOMO energy levels and the Hammett substituent constants.     

EFFECT OF HETEROATOMS ON PARTITIONING OF π-ELECTRONS IN RINGS OF CATAFUSENES

An earlier proposed method for assessing the π-electron content (EC) of rings of heteroatom–containing polycyclic conjugated molecules [Balaban et al., Polycyclic Aromatic Compounds/27 (2007) 51] is modified so that the effects of heteroatoms become additive. By means of this, relatively simple rules for the changes of the EC-values could be formulated in the case of catafusenes.     

Synthesis and characterization of organosoluble luminescent poly(amide-imide)s

Poly(amide-imide)s (PAIs) were synthesized from 9,9’-bis(4-aminophenyl)fluorene (APF) and different diimide-dicarboxylic acids (DIDA). The aromatic fluorene group is isolated by DIDA. The structure of the polymer was confirmed by ¹H NMR, ¹³C NMR, and FTIR. These PAIs were soluble in polar aprotic solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, and dimethyl sulfoxide. The crystallinity of the polymers is estimated by means of wide-angle X-ray diffraction. The resultant PAIs exhibited nearly amorphous nature. Thermal stability of the PAIs was characterized by differential scanning calorimetry and thermogravimetric analysis. No obvious glass transition temperatures were detected and the 5% weight loss temperatures of the PAIs were in the range 296–453 °C in nitrogen. The optical properties were studied by Ultraviolet–visible (UV/VIS) spectra and photoluminescent (PL) in solution and film states. The results show similar absorbance and emission to those of the constituting APF monomer. The highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) were estimated from cyclic voltammograms. The values of HOMO and LUMO are -6.67 to -6.72, and -3.54 to -3.58, respectively. Their optical and electrochemical properties are not changed significantly by different DIDA.     

Synthesis and luminescence properties of new blue‐light‐emitting polyconjugated poly{1,1′‐biphenyl‐4,4′‐diyl‐[1‐(4‐t‐butylphenyl)]vinylene} polymers and copolymers

Three new soluble polyconjugated polymers, all of which emitted blue light in photoluminescence and electroluminescence, were synthesized, and their luminescence properties were studied. The polymers were poly{1,1′‐biphenyl‐4,4′‐diyl‐[1‐(4‐t‐butylphenyl)]vinylene}, poly((9,9‐dioctylfluorene‐2,7‐diyl)‐alt‐{1,4‐phenylene‐[1‐(4‐t‐butylphenyl)vinylene‐1,4‐phenylene]}) [P(DOF‐PVP)], and poly([N‐(2‐ethyl) hexylcarbazole‐3,6‐diyl]‐alt‐{1,4‐phenylene‐[1‐(4‐t‐butylphenyl)]vinylene‐1,4‐phenylene}). The last two polymers had alternating sequences of the two structural units. Among the three polymers, P(DOF‐PVP) performed best in the light‐emitting diode devices of indium–tin oxide/poly(ethylenedioxythiophene) doped with poly(styrene sulfonate) (30 nm)/polymer (150 nm)/Li:Al (100 nm). This might have been correlated with the balance in and magnitude of the mobility of the charge carriers, that is, positive holes and electrons, and also the electronic structure, that is, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, of the polymers. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 307–317, 2006     

Synthesis and characterization of poly(fluorene vinylene) copolymers containing thienylene–vinylene units

Narrow‐band‐gap 2,5‐thienylene‐divinylene (ThV) units were incorporated into the poly(fluorene vinylene) backbone via a Gilch reaction as an energy trap with various feed ratios; this yielded pronounced changes in the electrochemical and optical properties of the material. The energy levels of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the polymers {poly(9,9‐di‐iso‐octylfluorene vinylene) [poly(fluorene vinylene‐co‐thiophene vinylene (FV))], C1, and C2 } were estimated to be ?5.53 to ?5.10 eV and ?2.98 to ?2.84 eV, respectively, by cyclic voltammetry measurements. In comparison with poly(FV), the HOMO energy levels of polymers poly(fluorene vinylene‐co‐thiophene vinylene (FV) (90 : 10) ( C1 ) and poly(fluorene vinylene‐co‐thiophene vinylene (FV) (80 : 20) ( C2 ) were significantly increased, but their LUMO energy levels were slightly decreased. The optical properties were investigated by absorption and emission spectra of the polymers. The good spectral overlap between the emission of poly(FV) and the absorption of polymers C1 and C2 revealed a sufficient energy transfer from the majority of 9,9‐di‐iso‐octylfluorene vinylene units to the minority of ThV units. The reduction of self‐absorption losses of polymers C1 and C2 due to spectral separation caused by the incorporation of ThV units could be indirectly confirmed by nonlinear optical (NLO) properties. The result of the NLO properties of the polymers showed that the third‐order NLO coefficients of poly(FV), C1, and C2 were 8.1 × 10^?10, 1.35 × 10^?9, and 1.51 × 10^?9 esu, respectively. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Fischer aminocarbene complexes of chromium and iron: Anomalous electrochemical reduction of p-carbonyl substituted derivatives

This contribution shows how a small change of a remote substituent (COOCH₃ for e.g. OCH₃) on Fischer aminocarbene complexes can change totally the LUMO location, and thus electron distribution, extent of π-electron delocalization and, consequently, redox properties of these potential catalysts. During investigation of redox properties of extended series of title compounds, an exceptionally positive reduction potential was observed (non fitting the LFER plot) for p-COOR substituents. This effect is caused by a strong intramolecular electronic interaction, which is specific for p-phenylene dicarbonyl compounds. In this context, the CN bond in aminocarbene moiety has a double bond character and resembles carbonyl function. The interpretation was proved by DFT calculations.     

Molecular Orbital Based Design Guideline for Hypergolic Ionic Liquids

Currently, monomethyl hydrazine is the most widely used hypergolic rocket fuel. Due to its high toxic vapor, there is a thrust towards developing low‐toxic hypergolic fuels. Ultra‐low vapor pressure ionic liquids are one such potential category of fuels. However, designing ionic liquid with ignition delay comparable to monomethyl hydrazine is a challenge, because fundamental understanding of the hypergolic nature of ionic liquids is far from clear. This work used the computed energy gap values between the highest occupied molecular orbitals (HOMO) of the anions for a series of ionic liquids and the lowest occupied molecular orbital (LUMO) of HNO₃, and variation in the computed relative heats of formation, ΔH_f, of these anions to develop correlations to predict hypergol activity between an ionic liquid fuel and nitric acid as the oxidizer. The observed trends in HOMO LUMO energy gap and ΔH_f values can be used successfully to verify not only hypergolicity of known systems but also the lack of this phenomenon in OH⁻ and BF₄⁻ based ionic liquids. It was shown that through suitable substitution of electron withdrawing or electron donating groups in the anion, the energy gap and the ΔH_f values could be tailored into an optimal range that would have a high probability for the new system to exhibit hypergolic reactivity. To validate our method, we suggest herein new ionic liquid structures for synthesis and experimental screening.     

PARTITIONING OF π-ELECTRONS IN RINGS OF POLYCYCLIC conjugated HYDROCARBONS. PART 1: CATACONDENSED BENZENOIDS

Recently a novel view on Kekulé valence structures (or resonance structures) was reported in which their standard geometrical representation was replaced by a numerical representation obtained by assigning π-electrons associated with CC double bonds to individual benzenoid rings. In the present article, we examine in more detail the partitioning of π-electrons to benzenoid rings for cata-condensed benzenoid hydrocarbons. For special families of cata-condensed benzenoids, we offer formulas which allow one to obtain the average π-electron ring content for individual benzenoid rings of polycyclic conjugated hydrocarbons. We also show that the average π-electron ring content for individual benzenoid rings can be calculated from Pauling bond orders without a need to examine all Kekulé resonance structures of a molecule.     

含N杂环化合物吸收光谱理论研究

本文运用量子化学中的密度泛函理论在B3LYP/6-31+G(d)水平下首先对含N杂环的四种化合物嘧啶、咪唑、吡唑和吡咯的电子几何结构、振动分析和化合物的分子能量进行的理论计算,根据振动分析结果显示均能稳定存在,后对稳定存在的结构利用含时密度泛函理论在B3LYP/6-31+G(d)水平下对嘧啶、咪唑、吡唑和吡咯的分子最大吸收光谱和前线分子轨道进行了理论数值比较研究,研究发现四种含N杂环芳香化合物中吡咯的最大吸收波长对应的跃迁为HOMO-1→LUMO跃迁,嘧啶、咪唑和吡唑的最大吸收波长均为HOMO→LUMO跃迁,所有化合物最大吸收波长对应的轨道跃迁均为π→π*。     

Electronic structures and photophysical properties of carbazole- and thiophene-based organic compounds used as hole-injecting layer for organic light-emitting diodes (OLEDs)

In this study, we have carried out a theoretical study on six organic compounds based on thiophene and carbazole, with the aim of using them as a hole-injecting layer of organic light-emitting diodes (OLEDs). In this study, we have tested two types of structures: D-π-D for MO1, MO2, MO3, and MO4 compounds and D-π-A for MO5 and MO6 compounds. The correlation structure-properties of these studied compounds have been proceeded and discussed by analyzing highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), energy gap, polarization effect, atom transition density matrix, absorption, and photoluminescence (PL). This theoretical study, based on density functional theory (DFT)/TPSSTPSS/aug-cc-pVQZ and the integral-equation-formalism polarizable continuum model/Coulomb attenuated method-Becke, 3-parameter, Lee–Yang–Parr (IEFPCM/TD-CAM-B3LYP)/6-31++G(d,p) is consolidated by experimental data for MO1, MO2, and MO4 compounds, allowing the determination of their structural and optoelectronic properties (HOMO, LUMO, gap [Eg], absorption, and emission parameters). The obtained results appear very conclusive and show that the performance of these compounds in terms of luminescence, absorption, and current–voltage (I–V) characteristics of OLED devices make them a promising candidate for the realization of light-emitting diodes.     

Synthesis,structures, and density functional theory computational study of thiophene‐containing and fluorodecorated imine‐linked polymers

A new series of extended–conjugated and thermally stable thiophene‐containing imine‐linked polymers were synthesized via a Schiff‐base condensation reaction between aryl aldehydes and 2,6‐diaminopyridine building blocks. The backbones of the polymers were functionalized with phenyl, fluorosubstituted phenyl, thienyl, and pyridyl aromatic rings. The successful synthesis was confirmed with spectrochemical characterization techniques, including IR, ¹H‐NMR, ¹³C‐NMR, and elemental analyses. The electronic properties of the polymers were investigated with ultraviolet–visible (UV–vis) absorption spectroscopy; the properties were collected experimentally and calculated with density functional theory (DFT) in the gas phase. The maximum absorption calculated from DFT was higher than the experimental values by about 60 nm; this was attributed to the absence of the solvent effect in the DFT case. The frontier molecular orbital ((HOMO) highest occupied molecular orbital and (LUMO) lowest unoccupied molecular orbital), optical band gap (E_g), and total energy (E_T) values of the optimized structures were calculated. Apparently, there was a significant relation between the number of thiophene rings and the resulting E_g and E_T values. As the number of thiophene rings in the polymer chain increased, E_g and E_T decreased, and the thermal stability of the polymers increased. E_g and the absorption band edges were determined experimentally from the UV–vis and transmittance spectra, respectively. Poly(terthienyl–azomethine–pyridine–azomethine), with the highest thiophene content, had the lowest experimental and calculated E_g values (2.10 and 2.63 eV, respectively). In contrast, upon fluorination, poly[(2,5‐dithienyl–1,4‐difluorobenzene)–azomethine–pyridine–azomethine] exhibited the highest E_g (2.81 eV) and absorption band edges (2.94 eV), whereas the thermal stability decreased to 250 °C. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 44331.     

Relationship between Lieb and Wu Approach and Standard Configuration Interaction Method for the B−2u State of the Hubbard Model of Benzene

It is shown that starting from Lieb and Wu equations, written explicitly for the B⁻_2u state of the Hubbard model of benzene π-electron system, one can extract the corresponding configuration interaction matrix in which the states associated with the “hidden” symmetry are already factored out.