Wu and colleagues describe the challenging synthesis and physical characterization of a macrocyclic octaradicaloid and decaradicaloid. They exhibit the long-sought-after ''annulene-within-an-annulene'' super-ring structure and unusual global aromaticity with individual inner and outer rings following Hü ckel's rule in the singlet state and Baird's rule in the triplet state, respectively.

The description of low-lying ππ* states of linear acenes by standard electronic structure methods is known to be challenging. Here, we broaden the framework of this problem by considering a set of fused heteroaromatic rings and demonstrate that standard electronic structure methods do not provide a balanced description of the two (typically) lowest singlet state (La and Lb) excitations. While the Lb state is highly sensitive to correlation effects, La suffers from the same drawbacks as charge transfer excitations. We show that the comparison between CIS/CIS(D) can serve as a diagnostic for detecting the two problematic excited states. Standard TD-DFT and even its spin-flip variant lead to inaccurate excitation energies and interstate gaps, with only a double hybrid functional performing somewhat better. The complication inherent to a balanced description of these states is so important that even CC2 and ADC(2) do not necessarily match the ADC(3) reference.

Synthesis of stable open-shell polyradicaloids including control of intramolecular spin-spin interactions is a challenging topic in organic chemistry and materials science. Herein, we report the synthesis and physical characterization of two series of fluorenyl based macrocyclic polyradicaloids. In one series (FR-MCn, n = 4-6), the fluorenyl radicals are directly linked at 3,6-positions; whereas in the other series (MC-FnAn, n = 3-5), an additional ethynylene moiety is inserted between the neighboring fluorenyl units. To access stable macrocyclic polyradicaloids, three synthetic methods were developed. All of these stable macrocycles can be purified by normal silica gel column chromatography under ambient conditions. In all cases, moderate polyradical characters were calculated by restricted active space spin-flip method due to the moderate intramolecular antiferromagnetic spin-spin interactions. The excitation energies from the low-spin ground state to the lowest high-spin excited state were evaluated by superconducting quantum interference device measurements. Their physical properties were also compared with the respective linear fluorenyl radical oligomers (FR-n, n = 3-6). It is found that the geometry, i.e., the distortional angle and spacer (w or w/o ethynylene) between the neighboring fluorenyl units, has significant effect on their polyradical character, excitation energy, one-photon absorption, two-photon absorption and electrochemical properties. In addition, the macrocyclic tetramers FR-MC4 and MC-F4A4 showed global antiaromatic character due to cyclic π-conjugation with 36 and 44 π-electrons, respectively.

We discuss the nature of electron-correlation effects in carbon nanorings and nanobelts using an analysis tool known as fractional occupation number weighted electron density (ρ) and the RAS-SF method, revealing for the first time significant differences in static correlation effects depending on how the rings (i.e. chemical units) are fused and/or connected until closing the loop. We choose to study in detail linear and cyclic oligoacene molecules of increasing size, and relate the emerging differences with the difficulties for the synthesis of the latter due to their radicaloid character. We finally explore how minor structural modifications of the cyclic forms can alter these results, showing the potential use of these systems as molecular templates for the growth of well-shaped carbon nanotubes as well as the usefulness of theoretical tools for molecular design.

Spiro conjugation has been proposed to dictate the efficiency of charge transfer,w hich could directly affect the spin-orbit charge transfer intersystem crossing (SOCT-ISC) process.H owever,t his process has yet to be exemplified. Herein, we prepared three spirobis[anthracene]diones,i n which two benzophenone moieties are locked in close proximity and differentially functionalized to fine-tune the charge transfer state.I ts feasibility for SOCT-ISC was theoretically predicted, then experimentally evaluated. Through fine-tuning the spiro conjugation coupling and varying the solvent dielectric constants,ISC rate constants were engineered to vary in ad ynamic range of three orders of magnitude,f rom 7.8 10 8 s À1 to 1.0 10 11 s À1 ,w hich is the highest ISC rate reported for SOCT-ISC system to our knowledge.O ur findings substantiate the key factors for effective SOCT-ISC and offer anew avenue for the rational design of heavy atom free triplet sensitizers.

In this work we study the intricacies of the electronic structure properties of triangular graphene nanofragments (TGNFs) in their ground and low-lying excited states by means ofab initioquantum chemistry calculations.

In recent years, a variety of studies have investigated in great depth the details of singlet fission mechanisms in terms of electronic energies and couplings. The nature and properties of the triplet‐pair intermediate state or the role of charge separated configurations (CT states) have been extensively explored with the help of theoretical models, ab initio calculations, and experimental characterization. On the other hand, much less is known about the involvement of the localized double electron excitation, that is the D state. In organic conjugated molecules, the D state can be typically described as the HOMO‐to‐LUMO promotion of two electrons, which makes it inactive to photoabsorption (dark state). In the present study, we tackle the possible roles of the D state by means of electronic structure calculations and the simulation of singlet fission dynamics. Our results suggest that, indeed, the D state can have a significant impact on singlet fission. On one hand, if it lies energetically below the optical exciton it is able to drive the system to singlet fission through direct coupling or via a CT‐mediated path. The D state can also act as a mediator in the formation of the triplet‐pair state from initial photoactivation. These results should motivate researchers in the field to include this state in their analysis, and help to clarify electronic mechanisms in singlet fission materials.

Wu and colleagues describe the synthesis of a series of stable [n]cyclo-parabiphenylmethine polyradicaloids, which can be regarded as analogs of [n] annulenes. They experimentally observed the valence tautomerization process by VT NMR technique for a macrocyclic tetramer and determined the interconversion energy barrier. The slow valence tautomerization process can be attributed to the global anti-aromatic character of the transition state.