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. 2024 Nov 18;15(48):20215-20222.
doi: 10.1039/d4sc03996c. eCollection 2024 Dec 11.

6,6'-Biindeno[1,2- b]fluorene: an open-shell indenofluorene dimer

Himanshu Sharma  1 , Palash Jana  2 , Dibyendu Mallick  3 , Subhajit Bandyopadhyay  2 , Soumyajit Das  1
Affiliations

6,6'-Biindeno[1,2- b]fluorene: an open-shell indenofluorene dimer

Himanshu Sharma et al. Chem Sci. .

Abstract

Nakano et al. reported that the antiaromatic indenofluorene (IF) isomers are diradicaloid molecules having varying degrees of open-shell character, with indeno[1,2-b]fluorene displaying a weaker diradical character index (y 0 = 0.072). Unlike 6,12-trimethylsilylethynyl disubstituted [1,2-b]IF, the 6,12-aryl disubstituted [1,2-b]IF derivatives did not show any experimental evidence of diradical properties. This raised the question of whether a [1,2-b]IF dimer would prefer a closed-shell or an open-shell ground state. To address this, herein we report the synthesis of a 6,6'-biindeno[1,2-b]fluorene derivative, which is a [1,2-b]IF dimer, constructed by linking two [1,2-b]IF units with a C-C single bond at carbons 6 and 6' bearing the largest orbital coefficients for the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO). The C6-C6' linkage effectively narrowed the HOMO-LUMO gap while the strong desire to avoid s-indacene antiaromaticity restored two Clar sextets in two proaromatic para-quinodimethane subunits, resulting in an open-shell bifluorenylidene-type diradicaloid (y 0 = 0.268) ground state with minor tetraradical character index (y 1 = 0.007). The open-shell nature was confirmed by single crystal X-ray and electron paramagnetic resonance analyses, and supported by theoretical calculations.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. (a) Antiaromatic indeno[1,2-b]fluorene derivatives 1-M, 1-MF and 2; (b) Zimmerman and Stępień's tetrafluorenofulvalene (TFF) 3; (c) our reported 3,3′-biindeno[1,2-b]fluorene derivative 4; (d) closed-shell structure of targeted mesityl disubstituted 6,6′-biindeno[1,2-b]fluorene 5 and its representative open-shell (OS) diradicaloid bifluorenylidene-type structure 5-OS1 and tetraradicaloid biindenofluorene-type structure 5-OS2.
Fig. 2
Fig. 2. (a) Spin density map of 5. Isovalue for surfaces: MO = 0.02, density = 0.005; frontier molecular orbital profiles for the (b) α-spin and (c) β-spin of 5.
Scheme 1
Scheme 1. Synthesis of BIF 5.
Fig. 3
Fig. 3. (a) X-ray crystallographic structure of 5 with the ellipsoids drawn at 30% probability level (hydrogens omitted), including NICS(1)zz (blue) and HOMA (green) indices; (b) side-view of 5; (c) EPR spectra of 5 recorded at variable temperatures for the powder form.
Fig. 4
Fig. 4. Current–density vectors plotted onto the ACID isosurface of 0.02 for the π system of 5.
Fig. 5
Fig. 5. (a) UV-vis-NIR spectrum of 5 in chloroform; (b) CV and DPV of 5.
See this image and copyright information in PMC

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