Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2024 Jul 17;146(28):19168-19176.
doi: 10.1021/jacs.4c04191. Epub 2024 Jul 2.

Redox-Switchable Aromaticity in a Helically Extended Indeno[2,1- c]fluorene

Eric Sidler  1 , Robert Hein  1 , Daniel Doellerer  1 , Ben L Feringa  1
Affiliations

Redox-Switchable Aromaticity in a Helically Extended Indeno[2,1- c]fluorene

Eric Sidler et al. J Am Chem Soc. .

Abstract

Molecular switches have received major attention to enable the reversible modulation of various molecular properties and have been extensively used as trigger elements in diverse fields, including molecular machines, responsive materials, and photopharmacology. Antiaromaticity is a fascinating property that has attracted not only significant fundamental interest but is also increasingly relevant in different applications, in particular organic (opto)electronics. However, designing systems in which (anti)aromaticity can be judiciously and reversibly switched ON and OFF remains challenging. Herein, we report a helicene featuring an indenofluorene-bridged bisthioxanthylidene as a novel switch wherein a simultaneous two-electron (electro)chemical redox process allows highly reversible modulation of its (anti)aromatic character. Specifically, the two thioxanthylidene rotors, attached to the initially aromatic indenofluorene scaffold via overcrowded alkenes, adopt an anti-folded structure, which upon oxidation convert to singly bonded, twisted conformations. This is not only associated with significant (chir)optical changes but importantly also results in formation of the fully conjugated, formally antiaromatic as-indacene motif in the helical core of the switch. This process proceeds without the buildup of radical cation intermediates and thus enables highly reversible switching of molecular geometry, aromaticity, absorbance, and chiral expression under ambient conditions, as evidenced by NMR, UV-vis, CD, and (spectro)electrochemical analyses, supported by DFT calculations. We expect this concept to be extendable to a wide range of robust antiaromatic-aromatic switches and to provide a basis for modulation of the structure and properties of these fascinating inherently chiral polycyclic π-scaffolds.

PubMed Disclaimer

Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Helical extension of the BTX redox switch affords the novel, chiral switch 1 in which a formally antiaromatic as-indacene core can be conveniently, and highly reversibly, generated by simultaneous two-electron oxidation. This process is also associated with significant conformational rearrangements of the thioxanthylium rotors along with (chir)optical changes.
Figure 2
Figure 2
Synthetic scheme and crystal structure of rac-1. Conditions: (a) (1) Ag2CO3, [RhCl(PPh3)3], THF, mw, 180 °C, 1.5 h and (2) pyridinium chlorochromate, Celite, CH2Cl2, rt, 3 h. (b) Lawesson’s reagent, toluene, reflux, 1.5 h. (c) Ag2O, KOH (sat. in methanol), MgSO4, diethyl ether, 0 °C, 45 min. (d) HMPT, toluene, diethyl ether, rt, 15 min.
Figure 3
Figure 3
Top and side view of the DFT-optimized geometries of (P)-1 (top) and (P)-12+ (bottom), as well as illustration of the Newman-type projection, indicating an anti-folded and twisted structure for both rotors in (P)-1 and (P)-12+, respectively. The calculations were performed at the r2SCAN-3c/CPCM(CH2Cl2) level of theory.
Figure 4
Figure 4
(a) UV–vis spectrum of rac-1 in CH2Cl2 (c ∼ 10–6 M). (b) CD spectra of (M)-1 (purple) and (P)-1 (blue) in CH2Cl2 (c ∼ 10–6 M). Enantiomers were assigned by comparison of the TD-DFT-calculated CD spectrum of (P)-1.
Figure 5
Figure 5
NICS1.7πzz-XY scan of (P)-1 (blue) and (P)-12+ (purple). The scan was performed from the edge of ring A to the center of ring D.
Figure 6
Figure 6
Stacked 1H NMR (600 MHz, 298 K) spectra of rac-1 (top), rac-12+ obtained by oxidation with Fe(ClO4)3 (middle), and rereduced rac-1 obtained by reduction with Zn (bottom) in CD2Cl2. The black dotted lines illustrate the large shift of the central protons Ha, and the gray dotted lines illustrate the remaining starting material. Side products are marked with an asterisk. The full spectra including proton assignments are shown in Figure S13.
Figure 7
Figure 7
(a) CV of 0.5 mM rac-1 in CH2Cl2, 100 mM TBAPF6 at ν = 100 mV/s. The black arrow indicates the starting point and initial direction of the first scan. (b) UV–vis spectra and (c) time traces of spectroelectrochemical interconversion of rac-1/rac-12+ in CH2Cl2, 200 mM TBAPF6. The areas shaded in gray represent the reductive cycle (E = −0.45 V), while the blue areas represent oxidation (E = +0.70 V). (d) CD spectra of spectroelectrochemical conversion of (P)-1 to (P)-12+ in CH2Cl2, 200 mM TBAPF6. The corresponding time traces are shown in Figure S27.
See this image and copyright information in PMC

References

    1. Tran Ngoc T.; Grabicki N.; Irran E.; Dumele O.; Teichert J. F. Photoswitching Neutral Homoaromatic Hydrocarbons. Nat. Chem. 2023, 15 (3), 377–385. 10.1038/s41557-022-01121-w. - DOI - PMC - PubMed
    1. Merino G.; Solà M.; Fernández I.; Foroutan-Nejad C.; Lazzeretti P.; Frenking G.; Anderson H. L.; Sundholm D.; Cossío F. P.; Petrukhina M. A.; Wu J.; Wu J. I.; Restrepo A. Aromaticity: Quo Vadis. Chem. Sci. 2023, 14 (21), 5569–5576. 10.1039/D2SC04998H. - DOI - PMC - PubMed
    1. Solà M. Forty Years of Clar’s Aromatic π-Sextet Rule. Front. Chem. 2013, 1, 22. 10.3389/fchem.2013.00022. - DOI - PMC - PubMed
    1. Boronski J. T.; Seed J. A.; Hunger D.; Woodward A. W.; van Slageren J.; Wooles A. J.; Natrajan L. S.; Kaltsoyannis N.; Liddle S. T. A Crystalline Tri-Thorium Cluster with σ-Aromatic Metal-Metal Bonding. Nature 2021, 598 (7879), 72–75. 10.1038/s41586-021-03888-3. - DOI - PubMed
    1. Rickhaus M.; Jirasek M.; Tejerina L.; Gotfredsen H.; Peeks M. D.; Haver R.; Jiang H.-W.; Claridge T. D. W.; Anderson H. L. Global Aromaticity at the Nanoscale. Nat. Chem. 2020, 12 (3), 236–241. 10.1038/s41557-019-0398-3. - DOI - PMC - PubMed