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.
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