Effect of Benzo-Annelation on Triplet State Energies in Polycyclic Conjugated Hydrocarbons.

In a series of earlier studies, the effect of benzo-annelation was found to be a useful tool for tuning the aromaticity in polycyclic conjugated compounds to desired level. In this work we studied the (anti)aromaticity of benzo-annelated derivatives of three conjugated hydrocarbons (anthracene, fluoranthene and biphenylene) in their lowest lying singlet (S0) and triplet (T1) states by means of the energy effect (ef), harmonic oscillator model of aromaticity (HOMA), multicentre delocalization indices (MCI), magnetically induced current densities (MICDs) and nucleus independent chemical shifts (NICS). We showed that benzo-annelation is a topology-based effect which can be used to modify the T1 state excitation energies (E(T1)). A quantitative model was established being able to accurately predict the E(T1) based only on the numbers of angularly, linearly and geminally annelated benzene rings. In addition, it was demonstrated that the E(T1) can be directly related to the (anti)aromatic character of the central ring in the studied molecules in their S0 state.

3,4-Dimethylenecyclobutene: A Building Block for Design of Macrocycles with Excited State Aromatic Low-Lying High-Spin States.

3,4-Dimethylenecyclobutene (DMCB) is an unusual isomer of benzene. Motivated by recent synthetic progress to substituted derivatives of this scaffold, we carried out a theoretical and computational analysis with a particular focus on the extent of (anti)aromatic character in the lowest excited states of different multiplicities. We found that the parent DMCB is non-aromatic in its singlet ground state (S0), lowest triplet state (T1), and lowest singlet excited state (S1), while it is aromatic in its lowest quintet state (Q1) as this state is represented by a triplet multiplicity cyclobutadiene (CBD) ring and two uncoupled same-spin methylene radicals. Interestingly, the Q1 state, despite having four unpaired electrons, is placed merely 4.8 eV above S0, and there is a corresponding singlet tetraradical 0.16 eV above. The DMCB is potentially a highly useful structural motif for the design of larger molecular entities with interesting optoelectronic properties. Here, we designed macrocycles composed of fused DMCB units, and according to our computations, two of these have low-lying nonet states (i. e., octaradical states) at energies merely 2.40 and 0.37 eV above their S0 states as a result of local Hückel- and Baird-aromatic character of individual 6π- and 4π-electron monocycles.

On the Unusual Global Aromaticity of Two Cyclopenta-Ring-Fused Oligo(m-Phenylenes).

Some years ago, Jishan Wu reported the synthesis of 8MC and 10MC, two homologues of the cyclopenta-ring-fused oligo(m-phenylene) macrocycles mMC, each behaving as an annulene-within-an-annulene (AWA). This was a surprising result as the AWA behavior is rare. Both molecules have a partial polyradical character, enforced by the quest for restoring some aromatic character of benzene rings. However, that restoration brings back some coupling between the two annulenes. Indeed, we found that the geometry and the magnetically induced currents indicate that, while 8MC does have an AWA character, this is not the case of the larger 10MC. Limitations of the design strategy of AWA molecules should be taken into account in future attempts to prepare novel large coronenes.

The n,π* States of Heteroaromatics: When are They the Lowest Excited States and in What Way Can They Be Aromatic or Antiaromatic?

Heteroaromatic molecules are found in areas ranging from biochemistry to photovoltaics. We analyze the n,π* excited states of 6π-electron heteroaromatics with in-plane lone pairs (nσ, herein n) and use qualitative theory and quantum chemical computations, starting at Mandado's 2n + 1 rule for aromaticity of separate spins. After excitation of an electron from n to π*, a (4n + 2)π-electron species has 2n + 2 πα-electrons and 2n + 1 πß-electrons (or vice versa) and becomes πα-antiaromatic and πß-aromatic. Yet, the antiaromatic πα- and aromatic πß-components seldom cancel, leading to residuals with aromatic or antiaromatic character. We explore vertically excited triplet n,π* states (3n,π*), which are most readily analyzed, but also singlet n,π* states (1n,π*), and explain which compounds have n,π* states with aromatic residuals as their lowest excited states (e.g., pyrazine and the phenyl anion). If the πß-electron population becomes more (less) uniformly distributed upon excitation, the system will have an (anti)aromatic residual. Among isomers, the one that has the most aromatic residual in 3n,π* is often of the lowest energy in this state. Five-membered ring heteroaromatics with one or two N, O, and/or S atoms never have n,π* states as their first excited states (T1 and S1), while this is nearly always the case for six-membered ring heteroaromatics with electropositive heteroatoms and/or highly symmetric (D2h) diheteroaromatics. For the complete compound set, there is a modest correlation between the (anti)aromatic character of the n,π* state and the energy gap between the lowest n,π* and π,π* states (R2 = 0.42), while it is stronger for monosubstituted pyrazines (R2 = 0.84).

Dichotomy of Delocalization/Localization and Charge-Shift Bonding in Germanazene and its Heavier Group 14 Analogues: a Valence Bond Study.

We present here a valence bond analysis of structure and π-delocalization in Ge3 (NH)3 , which models germanazene that was prepared by Power et al. To get a broader perspective, we explore the entire E3 (NH)3 series (E=C, Si, Ge, Sn, Pb). Thus, while (4n+2)π systems of carbon rings are aromatic with cyclic π-delocalization, the E3 (NH)3 rings are dominated by a nonbonded structure, wherein π-lone pairs are localized on the N atoms. Nevertheless, these molecules enjoy large covalent-ionic resonance energies of 153.0, 86.6, 74.2, 61.2, and 58.9 kcal/mol, respectively, for E=C, Si, Ge, Sn, Pb. The covalent-ionic mixing in E3 (NH)3 creates π-systems, which are stabilized by charge-shift bonding. Thus, unlike in benzene, in Ge3 (NH)3 delocalization of π-electron pairs of the N atoms is primarily confined to the domains of their adjacent Ge atoms. These features carry over to the substituted germanazene, Ge3 (NAr)3 (Ar=Ph).

Aromaticity of Osmaacenes in Their Lowest-Lying Singlet and Triplet States.

The aromatic character of a series of osmaacenes in their lowest-lying singlet and triplet states was thoroughly examined by means of the magnetically induced current densities and multicentre delocalization indices (MCIs). Both employed approaches agree that the osmabenzene molecule (OsB) in the S0 state exhibits dominant π-Hückel-type aromatic character, with a small but nonnegligible amount of π-Craig-Möbius aromaticity. Contrary to benzene, which is antiaromatic in the T1 state, OsB preserves some of its aromaticity in the T1 state. In higher members of the osmaacene series in the S0 and T1 states, the central Os-containing ring becomes nonaromatic, acting as a barrier between the two side polyacenic units which, on the other hand, exhibit a significant extent of π-electron delocalization.

Tuning the Structure and Properties of N-doped Positively Charged Polycyclic Aromatic Hydrocarbons.

A detailed study of the geometry, aromatic character, electronic and magnetic properties for a series of positively charged N-doped polycyclic aromatic hydrocarbons (PAHs) was performed. Magnetic properties of the examined molecules were analyzed by means of the magnetically induced current density calculated using the diamagnetic-zero version of the continuous transformation of origin of current density (CTOCD-DZ) method. The comparative study of the local aromaticity of the studied molecules was performed using several different indices: energy effect (ef), harmonic oscillator model of aromaticity (HOMA) index, six centre delocalization index (SCI) and nucleus independent chemical shifts (NICS). The presence of N-atoms in the inner rings was found to cause a planarity distortion in the studied N-doped systems. The geometric changes and charged nature of the studied N-doped systems do not significantly influence the current density and the local aromaticity distribution in comparison with the corresponding parent benzenoid hydrocarbons. The present study demonstrates how quantum chemical calculations can be used for rational design of novel PAHs and for fine tuning of their properties.

Spatial and Electronic Structures of BeB8 and MgB8 : How far Does the Analogy Go?

Doping boron clusters with Be and its heavier alkaline-earth congener, Mg, usually leads to complexes of different geometry and electronic structure. In this work we show that both neutral BeB8 and MgB8 exhibit a singlet ground state umbrella-like form. In addition, the stability, electronic structure, and aromaticity of the target molecules are compared. The magnetically induced current densities show that BeB8 and MgB8 are double aromatic systems: π and σ electrons induce strong diatropic currents. The current densities induced in the studied complexes are of very similar intensity, but with a different spatial distribution. The differences in the current density patterns observed for BeB8 and MgB8 arise from the very nature of the bonding interactions between the M atom and B8 fragment, as demonstrated through the energy decomposition analysis.

Aromaticity of Singlet and Triplet Boron Disk-like Clusters: A Test for Electron Counting Aromaticity Rules.

Boron clusters are polyhedral boron-containing structures that have unique features and properties. The disk-like boron clusters are among the most fascinating boron cluster forms. These clusters have a molecular orbital (MO) distribution similar to the one derived from the simple particle-on-a-disk model. In this model, the MOs come in pairs except for m = 0. Disk-like boron clusters in their singlet ground state are aromatic when they reach a closed-shell structure. One could expect that disk-like aromatic boron clusters in the singlet state, when acquiring or releasing two electrons, may also be aromatic in the lowest-lying triplet state. We use magnetically induced current densities and bond current strengths to analyze the aromatic character of a series of disk-like boron clusters. Our results show that, with the exception of triplet 3B19-, the disk-like boron clusters follow Hückel and Baird's rules if one considers the different MOs grouped by their symmetry. We also found that if the lowest-lying triplet state in disk-like boron clusters is aromatic, this triplet state is the ground state for this species.

Electronic structure, stability, and aromaticity of M2B6 (M = Mg, Ca, Sr, and Ba): an interplay between spin pairing and electron delocalization.

It has been shown in previous studies that the Be2B6 complex exhibits a triplet ground state with double aromaticity. In this work, the stability, electronic structure, and aromaticity of the homologous series M2B6 (M = Mg, Ca, Sr and Ba) were examined and compared to those of Be2B6. At the CCSD(T)/def2-TZVP//B3LYP/def2-TZVP level of theory, the target molecules were found to be more stable in the singlet than in the triplet spin state. Magnetically induced current densities and multicentre delocalization index (MCI) were employed to assess the aromatic character of the studied complexes. Both employed methods agree that M2B6 (M = Mg, Ca, Sr and Ba) are π aromatic and σ nonaromatic in the singlet ground state, and double aromatic in the triplet state. It was demonstrated that the electron counting rules of aromaticity cannot be used to correctly predict the aromaticity and relative stability of the examined molecules in different spin states.