Nonalternant Systems Research Articles

Understanding the luminescent nature of organic radicals for efficient doublet emitters and pure-red light-emitting diodes.

The doublet-spin nature of radical emitters is advantageous for applications in organic light-emitting diodes, as it avoids the formation of triplet excitons that limit the electroluminescence efficiency of non-radical emitters. However, radicals generally show low optical absorption and photoluminescence yields. Here we explain the poor optical properties of radicals based on alternant hydrocarbons, and establish design rules to increase the absorption and luminescence yields for donor-acceptor-type radicals. We show that non-alternant systems are necessary to lift the degeneracy of the lowest energy orbital excitations; moreover, intensity borrowing from an intense high-lying transition by the low-energy charge-transfer excitation enhances the oscillator strength of the emitter. We apply these rules to design tris(2,4,6-trichlorophenyl)methyl-pyridoindolyl derivatives with a high photoluminescence quantum yield (>90%). Organic light-emitting diodes based on these molecules showed a pure-red emission with an over 12% external quantum efficiency. These insights may be beneficial for the rational design and discovery of highly luminescent doublet emitters.

EPR studies on carboxylic esters, 23 [1]. Preparation of new dialkyl azulenedicarboxylates and EPR-spectroscopic study of their radical anions

AbstractSelected dialkyl azulenedicarboxylates were prepared by carbo-bromination of alkyl azulenemonocarboxylates and subsequent alcoholysis or by alkoxycarbonylcarbene insertion of ethyl indane-2-carboxylate. The electrochemical behavior of the diesters was studied by means of differential pulse polarography and cyclovoltammetry.In-situelectroreduction of the azulenedicarboxylic esters led to the corresponding radical anions, the EPR spectra of which were recorded. For certain radical anions a rearrangement under migration of the functional group from the 5- into the 6-position was observed. The spin density distribution in these non-alternantπ-electron systems as determined from the proton hyperfine structure (hfs) coupling constants was compared with the results of MO calculations and discussed with respect to the influence of substituents.

Molecular Nonlinear Optical Parameters of π-Conjugated Nonalternant Hydrocarbons Obtained in Semiempirical Local Coupled-Cluster Theory

The π-electron dipole polarizabilities and hyperpolarizabilities of organic conjugated hydrocarbons with odd-membered carbon cycles (nonalternant systems) are calculated using local semiempirical coupled-cluster theory. The peculiarity of the approach is the use of local ethylene π-molecular orbitals to represent double bonds in the molecule (cue-CCSD approach). The size-limit values of the (hyper)polarizabilities for different types of nonalternant oligomers obtained in the calculations show a significant dependence on the molecular structure. The effect of the electron correlation in the hyperpolarizability calculations is investigated using different levels of the coupled-cluster method including the simplest CCSD model and other models up to the model where the correlation effects due to all four electron excitations are accounted for, as well as with the full configuration interaction model.

EPR Studies on Carboxylic Esters, 22 [1, 2]. Preparation of New Alkyl Azulenecarboxylates and EPR-spectroscopic Study of Their Radical Anions

Five regio-isomeric alkyl azulenecarboxylates were prepared. Additional substituents such as tertbutyl groups or deuterium were introduced in certain positions of the azulene skeleton in order to facilitate the assignment of the proton hyperfine structure (hfs) coupling constants of the EPR spectra. The electrochemical behavior of the esters was studied by means of differential pulse polarography and cyclovoltammetry. In-situ electroreduction of the azulenecarboxylic esters led to the corresponding radical anions, the EPR spectra of which were recorded. The spin density distribution in these non-alternant systems as determined from hfs coupling constants was compared with the results of MO calculations and discussed with respect to the influence of substituents.

Electronic and Vibrational Spectra of Fullerenes in Contact with Oxygen

The high-resolution electronic and vibrational spectra of fullerenes C60, C70, and C78 were analyzed. The absorption spectrum of fullerene C60 contains a series of seven well-resolved bands in the range 200-280 nm, the strongest of them being at 240 nm, as well as two broadened bands of lower intensity at 280 and 330 nm. The former seven bands were assigned to vibrationally excited π-π* transitions in the molecular system of conjugated π bonds, and the latter two, to intermolecular interactions in a fullerene-oxygen contact complex. As the molecular weight of the fullerene increases, the short-wave absorption band suffers a hypsochromic shift, which is characteristic of nonalternant aromatic systems. The IR spectra provide evidence for the formation of a contact charge-transfer complex with oxygen by displaying emission and absorption in the region of fundamental frequencies of the triplet, singlet, radical-ion, and ionic states of molecular oxygen.

Variational resonance valence bond study on the ground state of C60 using the Heisenberg model

A detailed variational resonance valence bond (RVB) study is performed for the S=0 ground state of the C60 molecule in the framework of the Heisenberg model. It is shown that the 12 500-dimensional Kekulé space can be divided into two subspaces of respective dimensions 5828 and 6672, of which the first one recovers 99.82% of the energy of the full Kekulé space. This 5828-dimensional subspace is derived from the main Kekulé function, which is formed from spin pairs on hexagon–hexagon bonds only, by simple rotations of the three spin pairs in disjoint sets of hexagonal rings of C60 in all possible ways. This indicates that the concept of the stability of the aromatic sextet still plays an important role even in this nonalternant system. Further, the inclusion of some longer range RVB functions like Dewar-type functions and functions involving Claus structures is investigated, and the effect on the ground-state energy as well as on the nearest neighbor correlation functions is examined.

Electron Correlation and Magnetism: A Simple Molecular Orbital Approach for Predicting Ground-State Spins of Conjugated Hydrocarbons

A simple molecular orbital (MO) method, the unrestricted Hartree−Fock (UHF) Hubbard method, is proposed to predict the spin preferences of conjugated hydrocarbons. It is shown in this work that this method can account well for ground-state spin multiplicities of not only neutral alternant and nonalternant systems but also ion species. More importantly, the underlying relation between the electron correlation and magnetism in various conjugated systems with different topological characters is revealed by using the present UHF approach. For alternant systems, the magnitude of electron correlation is found to be less important in determining their ground-state spins. Whereas for some nonalternant and ion systems, their ground-state spin multiplicities are shown to depend strongly on the magnitude of electron correlation. In addition, the applicability and limitations of some currently used semiempirical models are analyzed in a unified way.

Radical cations of non-alternant systems as probes of the Shaik–Pross VB configuration mixing model

L. Eberson, R. González-Luque, M. Merchán, F. Radner, B. O. Roos and S. Shaik, J. Chem. Soc., Perkin Trans. 2, 1997, 463 DOI: 10.1039/A606702F

Analysis of coal tar pitch: relations between thermal behaviour and composition

This paper discusses the influence of pitch composition on its thermal behaviour. The value of the approach consisting in the characterization of these materials by analysis of their volatile fraction is demonstrated. The pyrolysis of nine coal tar pitches was studied by thermogravimetric analysis (t.g.a.) and derivative thermogravimetry (d.t.g.). The coke yield was closely related to both the rate of weight loss and the temperatures at which the d.t.g. curve reached maxima. To obtain information about the influence of the structures present in pitches on their thermal behaviour, the composition of the volatile fraction (evaluated by gas chromatography) was considered. The degree of condensation and the relative proportions of the different kinds of polycyclic aromatic hydrocarbons in this fraction were related to important thermal parameters deduced through t.g.a. and d.t.g. High coke yields are associated with a highly condensed volatile fraction in which alternant peri-condensed polycyclic aromatic hydrocarbons predominate over non-alternant systems.

Chromatographic evaluation of some selected polycyclic aromatic hydrocarbons of coal tars produced under different coking conditions and pitches derived from them

The volatile fractions of carbon disulfide extracts in tars and pitches have been studied by gas chromatography (GC), using the stationary phase OV-1701. Major polycyclic aromatic hydrocarbons (PAHs) in the fractions were selected as a basis for assessment of differences in the compositions of tars associated with their production. Several coal tars produced by wet charging carbonization in industrial- and semi-industrial-scale ovens with different mean flue heating temperatures were analyzed. The effects of preheating coal at about 200°C prior to the coking process were also studied. From chromatographic data, ratios of different individual compounds or classes of compounds were calculated. They revealed that an increase in the carbonization temperature and coal preheating produced similar effects on tar and pitch composition, leading to a reduction in methyl-substituted PAHs, non-alternant PAH systems and highly reactive compounds such as acenaphthylene. A higher proportion of peri-condensed PAHs and PAHs with a higher degree of condensation were also observed. GC analysis provides an adequate method for estimating the degree of pyrolysis of tars in coke ovens and their thermal stability in pitch production.

Maximum overlap symmetry molecular orbitals in conjugated systems

The application of the maximum overlap symmetry orbital (MOSO) method to conjugated systems is discussed briefly. The MOSO method can be employed to construct not only the symmetry orbitals and the molecular orbitais in alternant conjugated systems, but also the symmetry orbitais in non-alternant systems. It is shown that under the Hückel approximation the matrix MM + can be written out directly from the molecular skeleton and may be treated by using chemical graph theory. Because the matrix MM + of a system can be regarded as the Huckel matrix of a smaller system, the maximum overlap symmetry molecular orbital (MOSMO) calculation results can be easily obtained from those of the smaller systems. For homonuclear conjugated systems and for systems in which two kinds of atom appear altemantly, the MOSMOs and the corresponding molecular orbital (MO) energies obtained by the MOSMO calculation are the same as the MOs and the MO energies calculated by the Hückel molecular orbital method.

Aromaticity: A new Multilinear Approach

AbstractSix equations obtained by multiple regression analysis were developed to characterize aromaticity of alternant and nonalternant hydrocarbons. Three equations are applicable to alternant systems, three to nonalternant systems. The quantities used in the equations are the various experimental and theoretical (HMO and SCF‐MO) data reflecting aromatic behavior of delocalized π–electron systems. The aromaticity index, AI, is given as a numerical value and the equations can be used to predict aromaticity of various compounds on a quantitative basis.

The Hubbard model for alternant and nonalternant systems

In this paper we consider the effects of two-electron, one-center interactions when added to the one-electron, two-center molecular orbital model. There are, therefore, two parameters considered: The standard Hückel β (two-center, one-electron) term and the Hubbard one-center, two-electron term U. It is shown how the change in the ground state as one changes the U/β ratio is highly dependent on the presence or absence of odd member rings.

The Hubbard transition and unsaturated hydrocarbons

We contrast a simple molecular orbital theory (Hückel) with a simple valence bond theory (Heisenberg–Dirac). We find for alternant systems in which both models have nondegenerate ground states that both models have ground states belonging to the most symmetrical irreducible representation of the molecular point group. We also find there exist nonalternant systems which have ground states with different irreducible representations. In these latter systems neither the Hückel nor the Heisenberg–Dirac model is sufficient to give a qualitative picture of the molecule. Instead a combined Hückel–Heisenberg–Dirac model (the Hubbard model) must be used. Finally we list some organic unsaturated hydrocarbons, whose ground state changes from one irreducible representation to another as the Hubbard parameters vary.

An extension of the Coulson-Rushbrooke-Longuet-Higgins theorem

A novel topological method which predicts the number of Huckel nonbonding molecular orbitals and the set of atomic orbitals of which they are composed is put forward. It may be applied to a large class of homo-, heteronuclear, alternant, and nonalternant systems.

Selection rules in alternant systems

AbstractThe CI space Xn generated by n electrons moving over 2n spin orbitals is considered. It is shown that the transitions between different eigenstates ψi ϵ Xn of alternant and weakly alternant Hamiltonians are governed by some special selection rules. These selection rules are characteristic to alternant systems, and they do not apply to nonalternant systems. The set of all such selection rules can be easily derived from the splitting theorem. In particular, the selection rules associated with spin independent alternant systems are considered. As an example, the PPP Hamiltonian Ĥp describing netural alternant hydrocarbons is treated. In the case of electron dipole transitions between eigenstates ψi ϵ Xn of the Hamiltonian Ĥp, the selection rules obtained are in agreement with the selection rules derived previously by Pariser and McLachlan.

ChemInform Abstract: NONALTERNANT SYSTEMS. 2. ELECTRONIC EFFECTS OF SUBSTITUENTS IN 6‐METHYL‐6‐PHENYLFULVENES: CARBON‐13 NMR AND THEORETICAL STUDIES

Chemischer InformationsdienstVolume 15, Issue 49 Physical Organic Chemistry ChemInform Abstract: NONALTERNANT SYSTEMS. 2. ELECTRONIC EFFECTS OF SUBSTITUENTS IN 6-METHYL-6-PHENYLFULVENES: CARBON-13 NMR AND THEORETICAL STUDIES D. J. SARDELLA, D. J. SARDELLASearch for more papers by this authorC. M. KEANE, C. M. KEANESearch for more papers by this authorP. LEMONIAS, P. LEMONIASSearch for more papers by this author D. J. SARDELLA, D. J. SARDELLASearch for more papers by this authorC. M. KEANE, C. M. KEANESearch for more papers by this authorP. LEMONIAS, P. LEMONIASSearch for more papers by this author First published: December 4, 1984 https://doi.org/10.1002/chin.198449045AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume15, Issue49December 4, 1984 RelatedInformation

Nonalternant systems. 2. Electronic effects of substituents in 6-methyl-6-phenylfulvenes: carbon-13 NMR and theoretical studies

Nonalternant systems. 2. Electronic effects of substituents in 6-methyl-6-phenylfulvenes: carbon-13 NMR and theoretical studies

Some Features of Excited States Density Matrix Calculation and Their Pairing Relations in Conjugated Systems

Direct PPP-type calculations of self-consistent (SC) density matrices for excited states are described and the corresponding “thawn” molecular orbitals (MO) are discussed. Special atten­tion is addressed to particular solutions arising in conjugated systems of a certain symmetry, and to their chemical implications. The U(2) and U(3) algebras are applied, respectively, to the 4- electron and 6-electron cases; a natural separation of excited states in different cases follows. A simple approach to the convergence problem for excited states is given. The complementarity relations, an alternative formulation of the pairing theorem valid for heteromolecules and non-alternant systems, allow some fruitful experimental applications. Together with the extended pairing relations shown here, they may help to rationalize general trends.

On evaluation of the characteristic polynomial for large molecules

AbstractExisting schemes for evaluation of the characteristic polynomial of a graph suffer from limited practicality. Their application to large molecules inordinately increases the amount of labor. Here a procedure is outlined which is useful even for large molecules. It is based on a not widely known property of the collection of characteristic polynomials for Ulam's subgraphs, which, when added, give the derivative of the characteristic polynomial of the initial graph. The characteristic polynomials for Ulam's subgraphs are, as a rule, easier to derive due to the presence of many pending bonds in graphs of chemical interest. The last step requires an integration of a polynomial (which is a straightforward step) and determining the constant of integration, which represents the determinant of the adjacency matrix. The available methods for determing the additive constant (the determinant) are combinatorially much simpler than the initial task of finding all of the coefficients of the characteristic polynomial. The approach is illustrated on selected benzenoid hydrocarbons, nonbenzenoid, and nonalternant systems. Construction of the characteristic polynomials can be accelerated by considering auxiliary fragments and irreducible subgraphs separately and combining them in the final expression. Many auxiliary fragments allow their characteristic polynomials to be expressed in a closed form using recursive relations. The results for complex molecules can thus be written in a relatively compact form. Finally, the derivative of the characteristic polynomial, expressed in terms of selected auxiliary functions and irreducible components, can be integrated directly to give the result in terms of contributions signifying various fragments rather than as an explicit function of x.