Ab initio calculations of the spectra and lifetimes of the lead dimer.

Owing to the key role of the lead dimer (Pb2) as a heavy element benchmark for the Group IV-A dimers the assignment of its spectroscopic properties and chemical bonding is an important undertaking. To meet this demand, the present work provides comprehensive and detailed information on electronic structure and properties comprising a wide set of Pb2 states. Calculations are performed by a high-level ab initio approach. Firstly, the potential energy curves (PECs) of 19 Λ-S states as well as those of 24 ungerade Ω states are calculated by utilizing the multi-reference configuration interaction plus Davidson correction (MRCI + Q) method taking into account core-valence correlation (CV) and spin-orbit coupling (SOC) effect, where Ω is a quantum number of the total (Λ + S) angular momentum projection. Secondly, interactions between the bound F3Σ-u, 23Σ+u states and repulsive 15Πu state induced by strong SOC are discussed based on the PECs analysis and calculated SOC matrix, which also indicates that the F3Σ-u state is predissociative. Thirdly, based on the calculated electric dipole transition moments and energy gaps between the 0+u(III), F0+u(II), C0+u(I) and X0+g states, the intense absorption bands of Pb2 due to these transitions are interpreted. Our results indicate that the trends in intensity of absorption spectra (F0+u(II), C0+u(I) ← X0+g) in the range of 12 600-13 600 and 22 200-23 800 cm-1 are consistent with the previously observed spectra of Pb2 in the qualitatively similar regions (15 200-16 200 and 19 800-21 800 cm-1). Finally, the calculated intensity of the weak magnetic-dipole transitions from the singlet excited b1Σ+g and a1Δg states to the triplet ground X3Σ-g state and their electric quadrupole components are presented for the Pb2 molecule in terms of SOC perturbations for the calculated Ω states expressed in Λ-S state notation. Based on our theoretical assignment, we predict that the weak emission a1Δg2 → X3Σ-g1 bands could be observed experimentally. The present work provides comprehensive electronic structure information and sheds new light on the absorption and emission spectra of the Pb2 dimer.

MRCI Study of the Electronic Structure and Transition Properties of a Tin Dimer.

The ground and excited states of Sn2 are calculated using the multireference configuration interaction method combined with Davidson correction (MRCI+Q). The influence of the spin-orbit coupling (SOC) effect on the electronic structure is also considered by the state interaction method of Breit-Pauli Hamiltonian. In the calculations, the potential energy curves and spectroscopic constants of 23 Λ-S states and 31 Ω states of Sn2 are obtained. The prominent spectral features in the visible region, new constants, and potential energy curves are discussed. The intensity of weak magnetic and quadrupole transitions in the near IR spectra is also calculated. From a computational point of view, we predict that the weak v'(0-2)-v″(0-5) bands of the magnetic b1Σg,0++-X3Σg,1(Ms=±1)- transition may be detected experimentally; the sub-bands (0, 0), (1, 0), and (2, 0) of the a1Δg,2-X3Σg,1(Ms=±1)- transition also may be observed in experiments since they are not overlapped by the strong electric dipole transition in the same IR region. According to the SOC matrix elements and contributions of the 15Πu0+, 15Πu1 (|Σ| = 0), and 15Πu1 (|Σ| = 2) states to the predissociation line width of the 13Σu- -X3Σg1- transition, the broading and other predissociation features of the 13Σu- state are analyzed. From our calculations, it follows that the strong coupling between the bound 13Σu- state and the repulsive 15Πu state causes the predissociation of the 13Σu- state at the vibrational levels v' ≥ 8. In addition, our results suggest that the previously observed bands of Sn2 in the visible range of 19000-20000 cm-1 should be reassigned into the mixing transitions among the X3Σg,1--23Σu,0-+ and X3Σg,0+--23Σu,1+ manifold. The results are expected to provide new comprehensive information for better understanding the spectra and dynamics of the electronic excited states of the Sn2 molecule.

Theoretical study of the spectroscopy and radiative transition probabilities of Si2 from visible to infrared.

High level ab initio calculations on the electronic states of a silicon dimer (Si2) have been carried out by employing a multi-reference configuration interaction plus Davidson correction (MRCI + Q) approach with the aug-cc-pVQZ basis set. The scalar relativistic correction is taken into consideration by the second-order Douglas-Kroll-Hess approximation. In the present work, the transition properties (oscillator strength, Einstein spontaneous emission coefficient and radiative lifetime) of the singlet-singlet, triplet-triplet, and quintet-quintet transitions of Si2 are discussed. We emphasize the triplet-triplet emission bands H3Σ-u-X3Σ-g, K3Σ-u-X3Σ-g and D3Πu-L3Πg which are dominant for 0-11 (18 822 cm-1), 0-0 (30 672 cm-1), and 0-0 (28 881 cm-1) transitions, respectively. In addition, the strong experimentally observed b1Πu-d1Σ+g band around 4184 cm-1 corresponds to the second 1Σ+g-b1Πu combination in the infrared region. The calculated oscillator strengths of the singlet-singlet transitions (f1Πg-e1Σ-u, 21Πg-b1Πu, b1Πu-d1Σ+g and g1Δu-a1Δg) are in the order of 10-3. From a theoretical point of view, the 0-0 sub-band for the f1Πg-e1Σ-u transition, 0-7 for 21Πg-b1Πu, 0-0 for b1Πu-d1Σ+g and the 0-7 vibronic bands for the g1Δu-a1Δg transition may be observed experimentally. It is expected that the present results could provide theoretical support for a deeper understanding of the experimental Si2 spectra providing further applications in astrophysics.

Prediction of new spin-forbidden transitions in the N2 molecule-the electric dipole A'5Σg + → A3Σu + and magnetic dipole a'1Σu -← A3Σu + transitions.

On the ground of multi-reference configuration interaction calculations with an account of spin-orbit coupling, we have predicted the probability of two unknown spin-forbidden transitions in the spectrum of the N2 molecule: the electric dipole A'5Σg + → A3Σu + emission system and the magnetic dipole a'1Σu - ← A3Σu + transition. The radiative lifetime of the lowest A'5Σg + sublevel is less than a microsecond; the magnetic transition induced by the spin current in the triplet state is predicted with relatively low oscillator strength (f = 10-10), which still could be detectable.

Synthesis and Spectroscopic Characterization of Selected Phenothiazines and Phenazines Rationalized Based on DFT Calculation.

Two unique structures were isolated from the phosphorylation reaction of 10H-phenothiazine. The 5,5-dimethyl-2-(10H-phenothiazin-10-yl)-1,3,2-dioxaphosphinane 2-oxide (2a) illustrates the product of N-phosphorylation of phenothiazine. Moreover, a potential product of 2a instability, a thiophosphoric acid 2b, was successfully isolated and structurally characterized. Molecule 2a, similarly to sulfoxide derivative 3, possesses interesting phosphorescence properties due to the presence of d-pπ bonds. The X-ray, NMR, and DFT computational studies indicate that compound 2a exhibits an anomeric effect. Additionally, the syntheses of selected symmetrical and unsymmetrical pyridine-embedded phenazines were elaborated. To compare the influence of phosphorus and sulfur atoms on the structural characteristics of 10H-phenothiazine derivatives, the high-quality crystals of (4a,12a-dihydro-12H-benzo[5,6][1,4]thiazino[2,3-b]quinoxalin-12-yl)(phenyl)methanone (1) and selected phenazines 5,12-diisopropyl-3,10-dimethyldipyrido[3,2-a:3',2'-h]phenazine (5) and 5-isopropyl-N,N,3-trimethylpyrido[3,2-a]phenazin-10-amine (6a) were obtained. The structures of molecules 1, 2a, 2-mercapto-5,5-dimethyl-1,3,2-dioxaphosphinane 2-oxide (2b), 3,7-dinitro-10H-phenothiazine 5-oxide (3), 5 and 6a were determined by single-crystal X-ray diffraction measurements.

Dianthracenylazatrioxa[8]circulene: Synthesis, Characterization and Application in OLEDs.

A soluble, green-blue fluorescent, π-extended azatrioxa[8]circulene was synthesized by oxidative condensation of a 3,6-dihydroxycarbazole and 1,4-anthraquinone by using benzofuran scaffolding. This is the first circulene to incorporate anthracene within its carbon framework. Solvent-dependent fluorescence and bright green electroluminescence accompanied by excimer emission are the key optical properties of this material. The presence of sliding π-stacked columns in the single crystal of dianthracenylazatrioxa[8]circulene is found to cause a very high electron-hopping rate, thus making this material a promising n-type organic semiconductor with an electron mobility predicted to be around 2.26 cm2 V-1 s-1 . The best organic light-emitting diode (OLED) device based on the dianthracenylazatrioxa[8]circulene fluorescent emitter has a brightness of around 16 000 Cd m-2 and an external quantum efficiency of 3.3 %. Quantum dot-based OLEDs were fabricated by using dianthracenylazatrioxa[8]circulene as a host matrix material.

A Fully Conjugated Planar Heterocyclic [9]Circulene.

Fully aromatic [n]circulenes have only been known to encompass up to eight aromatic rings (n = 8), with no reports of endeavors in the synthesis of higher-order analogues (n > 8). Herein we present the first [9]circulene, formally a diazatrioxa[9]circulene, along with a tetrahydro-diazatetraoxa[10]circulene. The key transformation, for construction of the macrocyclic framework, is a simple high-yielding dimerizing condensation between 3,6-dihydroxycarbazole and glyoxal. Single crystal X-ray analysis reveals the [9]circulene to be perfectly planar and containing elongated benzene rings, which is induced by strain to accommodate planarity. Alternating bond lengths in the solid state indicate contribution from a [9]radialene resonance structure in the [9]circulene π-system. The central nonaromatic rings of both circulenes have paratropic ring currents, as evident by nucleus independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations, which can be attributed to induced paratropicity from the surrounding aromatic rings.

Compressing a Non-Planar Aromatic Heterocyclic [7]Helicene to a Planar Hetero[8]Circulene.

This work describes a synthetic approach where a non-planar aromatic heterocyclic [7]helicene is compressed to yield a hetero[8]circulene containing an inner antiaromatic cyclooctatetraene (COT) core. This [8]circulene consists of four benzene rings and four heterocyclic rings, and it is the first heterocyclic [8]circulene containing three different heteroatoms. The synthetic pathway proceeds via a the flattened dehydro-hetero[7]helicene, which is partially a helicene and partially a circulene: it is non-planar and helically chiral as helicenes, and contains a COT motif like [8]circulenes. The antiaromaticity of the COT core is confirmed by nucleus independent chemical shift (NICS) calculations. The planarization from a helically π-conjugated [7]helicene to a fully planar heterocyclic [8]circulene significantly alters the spectroscopic properties of the molecules. Post-functionalization of the [7]helicenes and the [8]circulenes by oxygenation of the thiophene rings to the corresponding thiophene-sulfones allows an almost complete fluorescence emission coverage of the visible region of the optical spectrum (400-700 nm).

Anti-Aromatic versus Induced Paratropicity: Synthesis and Interrogation of a Dihydro-diazatrioxa[9]circulene with a Proton Placed Directly above the Central Ring.

We present a high-yielding intramolecular oxidative coupling within a diazadioxa[10]helicene to give a dihydro-diazatrioxa[9]circulene. This is the first [n]circulene containing more than eight ortho-annulated rings (n>8). The single-crystal X-ray structure reveals a tight columnar packing, with a proton from a pendant naphthalene moiety centred directly above the central nine-membered ring. This distinct environment induces a significant magnetic deshielding effect on that particular proton as determined by 1 H NMR spectroscopy. The origin of the deshielding effect was investigated computationally in terms of the NICS values. It is established that the deshielding effect originates from an induced paratropic ring current from the seven aromatic rings of the [9]circulene structure, and is not due to the nine-membered ring being antiaromatic. UV/Vis spectroscopy reveals more efficient conjugation in the prepared diazatrioxa[9]circulene compared to the parent helical azaoxa[10]helicenes, and DFT calculations, including energy levels, confirm the experimental observations.

Theory and Calculation of the Phosphorescence Phenomenon.

Phosphorescence is a phenomenon of delayed luminescence that corresponds to the radiative decay of the molecular triplet state. As a general property of molecules, phosphorescence represents a cornerstone problem of chemical physics due to the spin prohibition of the underlying triplet-singlet emission and because its analysis embraces a deep knowledge of electronic molecular structure. Phosphorescence is the simplest physical process which provides an example of spin-forbidden transformation with a characteristic spin selectivity and magnetic field dependence, being the model also for more complicated chemical reactions and for spin catalysis applications. The bridging of the spin prohibition in phosphorescence is commonly analyzed by perturbation theory, which considers the intensity borrowing from spin-allowed electronic transitions. In this review, we highlight the basic theoretical principles and computational aspects for the estimation of various phosphorescence parameters, like intensity, radiative rate constant, lifetime, polarization, zero-field splitting, and spin sublevel population. Qualitative aspects of the phosphorescence phenomenon are discussed in terms of concepts like structure-activity relationships, donor-acceptor interactions, vibronic activity, and the role of spin-orbit coupling under charge-transfer perturbations. We illustrate the theory and principles of computational phosphorescence by highlighting studies of classical examples like molecular nitrogen and oxygen, benzene, naphthalene and their azaderivatives, porphyrins, as well as by reviewing current research on systems like electrophosphorescent transition metal complexes, nucleobases, and amino acids. We furthermore discuss modern studies of phosphorescence that cover topics of applied relevance, like the design of novel photofunctional materials for organic light-emitting diodes (OLEDs), photovoltaic cells, chemical sensors, and bioimaging.

Singlet Oxygen Photophysics in Liquid Solvents: Converging on a Unified Picture.

Singlet oxygen, O2(a1Δg), the lowest excited electronic state of molecular oxygen, is an omnipresent part of life on earth. It is readily formed through a variety of chemical and photochemical processes, and its unique reactions are important not just as a tool in chemical syntheses but also in processes that range from polymer degradation to signaling in biological cells. For these reasons, O2(a1Δg) has been the subject of intense activity in a broad distribution of scientific fields for the past ∼50 years. The characteristic reactions of O2(a1Δg) kinetically compete with processes that deactivate this excited state to the ground state of oxygen, O2(X3Σg-). Moreover, O2(a1Δg) is ideally monitored using one of these deactivation channels: O2(a1Δg) → O2(X3Σg-) phosphorescence at 1270 nm. Thus, there is ample justification to study and control these competing processes, including those mediated by solvents, and the chemistry community has likewise actively tackled this issue. In themselves, the solvent-mediated radiative and nonradiative transitions between the three lowest-lying electronic states of oxygen [O2(X3Σg-), O2(a1Δg), and O2(b1Σg+)] are relevant to issues at the core of modern chemistry. In the isolated oxygen molecule, these transitions are forbidden by quantum-mechanical selection rules. However, solvent molecules perturb oxygen in such a way as to make these transitions more probable. Most interestingly, the effect of a series of solvents on the O2(X3Σg-)-O2(b1Σg+) transition, for example, can be totally different from the effect of the same series of solvents on the O2(X3Σg-)-O2(a1Δg) transition. Moreover, a given solvent that appreciably increases the probability of a radiative transition generally does not provide a correspondingly viable pathway for nonradiative energy loss, and vice versa. The ∼50 years of experimental work leading to these conclusions were not easy; spectroscopically monitoring such weak and low-energy transitions in time-resolved experiments is challenging. Consequently, results obtained from different laboratories often were not consistent. In turn, attempts to interpret molecular events were often simplistic and/or misguided. However, over the recent past, increasingly accurate experiments have converged on a base of credible data, finally forming a consistent picture of this system that is resonant with theoretical models. The concepts involved encompass a large fraction of chemistry's fundamental lexicon, e.g., spin-orbit coupling, state mixing, quantum tunneling, electronic-to-vibrational energy transfer, activation barriers, collision complexes, and charge-transfer interactions. In this Account, we provide an explanatory overview of the ways in which a given solvent will perturb the radiative and nonradiative transitions between the O2(X3Σg-), O2(a1Δg), and O2(b1Σg+) states.

Super high-energy density single-bonded trigonal nitrogen allotrope-a chemical twin of the cubic gauche form of nitrogen.

A new ambient-pressure metastable single-bonded 3D nitrogen allotrope (TrigN) of trigonal symmetry (space group R3[combining macron]) was calculated using density functional theory (DFT). A comprehensive characterization of this material, comprising thermodynamic, elastic, and spectral (vibrational, UV-vis absorption, and nuclear magnetic resonance) properties, was performed. Using high-throughput band structure calculation, the TrigN phase was characterized as an insulator with an indirect band gap of 2.977 eV. Phonon dispersion calculations justified that this structure is vibrationally stable at ambient pressure. The calculated Raman activities at the Γ-point demonstrated a rich pattern, whereas no relatively intense transitions were observed in its IR absorption spectrum. The TrigN material is almost transparent to visible light as well as to ultraviolet A and B. The main absorption peaks appeared within the range of 50-200 nm. The electron arrangement of the nitrogen nuclei in the studied nitrogen allotrope is much denser compared to that of the molecular nitrogen, which is in agreement with the calculated magnetic shielding tensor values. Robust mechanical stability is revealed from the elastic constants calculation. Due to strong anisotropy, the values of the Young's moduli vary from 281 to 786 GPa. A huge amount of internal energy is enclosed in the TrigN material. Upon decomposition to molecular nitrogen, the energy release is expected to be 11.01 kJ g-1 compared to the value of 10.22 kJ g-1 for the cubic gauche form of nitrogen. The TrigN allotrope possesses unique detonation characteristics with a detonation pressure of 146.06 GPa and velocity of 15.86 km s-1.

Aromaticity of the doubly charged [8]circulenes.

Magnetically induced current densities and current pathways have been calculated for a series of fully annelated dicationic and dianionic tetraphenylenes, which are also named [8]circulenes. The gauge including magnetically induced current (GIMIC) method has been employed for calculating the current density susceptibilities. The aromatic character and current pathways are deduced from the calculated current density susceptibilities showing that the neutral [8]circulenes have two concentric pathways with aromatic and antiaromatic character, respectively. The inner octatetraene core (the hub) is found to sustain a paratropic (antiaromatic) ring current, whereas the ring current along the outer part of the macrocycle (the rim) is diatropic (aromatic). The neutral [8]circulenes can be considered nonaromatic, because the sum of the ring-current strengths of the hub and the rim almost vanishes. The aromatic character of the doubly charged [8]circulenes is completely different: the dianionic [8]circulenes and the OC-, CH-, CH2-, SiH-, GeH-, SiH2-, and GeH2-containing dicationic species sustain net diatropic ring currents i.e., they are aromatic, whereas the O-, S-, Se-, NH-, PH- and AsH-containing dicationic [8]circulenes are strongly antiaromatic. The present study also shows that GIMIC calculations on the [8]circulenes provide more accurate information about the aromatic character than that obtained using local indices such as nuclear-independent chemical shifts (NICSs) and (1)H NMR chemical shifts.

Benzoannelated aza-, oxa- and azaoxa[8]circulenes as promising blue organic emitters.

In the present work, we studied the synergetic effect of benzoannelation and NH/O-substitution for enhancing the absorption intensity in a series of novel designed benzoannelated aza- and oxa[8]circulenes. Semi-empirical estimations of the fluorescence rate constants allowed us to determine the most promising fluorophores among all the possible benzoannelated aza-, oxa- and mixed azaoza[8]circulenes. Among them, para-dibenzoannelated [8]circulenes demonstrated the most intense light absorption and emission due to the prevailing role of the linear acene chromophore. Calculated φfl values are in complete agreement with experimental data for a number of already synthesized circulenes. Thus, we believe that the most promising circulenes designed in this study can demonstrate an intensive fluorescence in the case of their successful synthesis, which in turn could be extremely useful for the fabrication of future blue OLEDs. Special attention is devoted to the aromaticity features and peculiarities of the absorption spectra for the two highly-symmetrical (D4h ground state symmetry) π-isoelectronic species as well as the so-called tetrabenzotetraaza[8]circulene and tetrabenzotetraoxa[8]circulene molecules. Both of them are characterized by rich electronic spectra, which can be assigned only by taking into account the vibronic coarse structure of the first electronic absorption band; the 0-1 and 0-2 transitions were found to be active in the absorption spectrum in complete agreement with experimental data obtained for both energy and intensity. The corresponding promotive vibrational modes have been determined and their vibronic activity estimated using the Franck-Condon approximation.

Computational and experimental investigation of the optical properties of the chromene dyes.

Absorption and fluorescence spectra of the chromene 3 and chromene 13 dyes are studied experimentally and by density functional theory (DFT) including vibronic structure analysis. Vertical electronic absorption spectra are also calculated with the ab initio multiconfiguration method XMC-QDTP2. The vibronic progression for the S0 → S1 electronic transition is calculated within the Franck-Condon approximation including Dushinsky effect and promoting modes are analyzed. The laser-active solid-state media with high efficiency and long operation time are created implementing the studied dyes. The results of investigation indicate that the studied compounds can be used as effective laser dyes in the red range of visible light.

Principles of phosphorescent organic light emitting devices.

Organic light-emitting device (OLED) technology has found numerous applications in the development of solid state lighting, flat panel displays and flexible screens. These applications are already commercialized in mobile phones and TV sets. White OLEDs are of especial importance for lighting; they now use multilayer combinations of organic and elementoorganic dyes which emit various colors in the red, green and blue parts of the visible spectrum. At the same time the stability of phosphorescent blue emitters is still a major challenge for OLED applications. In this review we highlight the basic principles and the main mechanisms behind phosphorescent light emission of various classes of photofunctional OLED materials, like organic polymers and oligomers, electron and hole transport molecules, elementoorganic complexes with heavy metal central ions, and clarify connections between the main features of electronic structure and the photo-physical properties of the phosphorescent OLED materials.

Design of nanoscaled materials based on tetraoxa[8]circulene.

Nanotubes and two-dimensional sheet-like compounds containing tetraoxa[8]circulene core units are theoretically predicted. The large cavities with a diameter of 6 Å in the 2D sheets could be useful for hydrogen storage. The designed compounds are predicted to adsorb strongly in the visible region and to be a promising material for nanophotonics because of their electron-donor properties (p-type semiconductors).

Theoretical study of relationships between structural, optical, energetic, and magnetic properties and reactivity parameters of benzidine and its oxidized forms.

Structural, topological, optical, energetic, and magnetic properties and reactivity parameters of benzidine, its radical cation, and its dication as well as molecular complexes of the benzidine dication with the F(-), Cl(-), Br(-), I(-), NO3(-), HSO4(-), and H2PO4(-) anions were calculated at the B3LYP/6-311++G(2d,2p) level of theory in the CH2Cl2 medium. The CAM-B3LYP functional (as the most reliable one) and the 6-311++G(3df,3pd) basis set were used for the UV-vis absorption spectra prediction. The obtained spectral results are in a good agreement with available experimental data. A number of the calculated global and local molecular properties, including several recently developed ones, (in general, more than 20 parameters), namely, λmax, the bond lengths and orders (l and LA,B), adiabatic ionization energy (IEad), global electrophilicity index (ω), condensed electrophilic Fukui functions (f(+)) and dual descriptor (ΔfA), van der Waals molecular volume, nuclear independent chemical shifts (NICS) and QTAIM topological parameters were estimated in the critical points of the C(1)-C(1'), C(2)-C(3), and C(4)-N bonds as well as at the ring critical point. These quantities were found to be in a strong linear dependence (R(2) > 0.99 in most cases) with the number of detached electrons (Nel) from the benzidine molecule up to formation of the dication (Nel = 2). On one hand, a position of the long-wave absorption band (λCT) corresponding to the anion-to-cation charge transfer in the neutral complexes of the benzidine dication with anions, correlates with the Mulliken electronegativity of the anion (R(2) = 0.8646) and its adiabatic ionization energy (R(2) = 0.8054). On the other hand, the correlations with the anion charge in the complexes and the anion isotropic polarizability are rather poor (R(2) = 0.6392 and 0.3470, respectively). On the ground of the obtained strong relationships, one may recommend the calculated molecular properties as potentially preferable descriptors for the benzidine-based compounds in terms of the QSAR methodology.

State-dependent global and local electrophilicity of the aryl cations.

Two alternative approaches­vertical and adiabatic­are used to estimate global and local electrophilicity (ω and ωk+) indexes for a series of aryl cations in both the ground and first excited electronic states using the well-known Parr scheme. The energy parameters used in these methods are obtained by the B3LYP/6-311++G(2d,2p) calculations of the aryl cations and of their oxidized and reduced forms in acetonitrile medium. The ground state ω values are lower than those for the excited state, which is in accord with the maximum hardness principle. Analysis of the ω indexes calculated with more reliable adiabatic approach reveals a dependence of the ground and first excited state ω indexes on the singlet­triplet energy gap of the aryl cations. A plot of the above dependence has a hyperbola-like shape; thus, the maximum (ground state) and minimum (first excited state) ω indexes correspond to the aryl cation, for which the singlet­triplet splitting is close to zero. Moreover, the ωk+ index distribution at the ipso-carbon atoms does not obey the maximum hardness principle, since it depends on spin multiplicity, not on the electronic state spatial type. For many singlet ground state aryl cations, the ωk+ indexes at the ipso-carbon atom are lower when calculated in the excited triplet state; that is due to a strong ω delocalization onto two electrophilic centers. This explains a higher chemoselectivity of the triplet aryl cations in reactions with the π-nucleophiles compared to the corresponding singlet arylium species. Applicability of the adiabatic approach for calculation of the ω and ωk+ indexes is supported by the experimental data on the nucleophile-independent parameter E for the singlet and triplet state of the p-Me2NC6H4+ cation.

Diazadioxa[8]circulenes: planar antiaromatic cyclooctatetraenes.

In this paper we describe a new class of antiaromatic planar cyclooctatetraenes: the diazadioxa[8]circulenes. The synthesis was achieved by means of a new acid-mediated oxidative dimerization of 3,6-dihydroxycarbazoles to yield the diazadioxa[8]circulenes in high yields. The synthetic protocol appears to be general, and is a one-pot transformation in which two C-C bonds and two C-O bonds are formed with the loss of two molecules of water. We also present a detailed characterization of the optical and electrochemical properties of this new class of stable planar cyclooctatetraenes. The properties of the diazadioxa[8]circulenes are compared with the properties of isoelectronic tetraoxa[8]circulenes and azatrioxa[8]circulenes. We discuss the antiaromatic nature of the planar central cyclooctatetraene moiety. The antiaromatic nature of the planar cyclooctatetraenes was studied by using computational methods (NICS calculations), and these calculations reveal that the central eight-membered ring has antiaromatic character.