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Review
. 2022 Jul 8;13(34):9833-9847.
doi: 10.1039/d2sc02480b. eCollection 2022 Aug 31.

Organic radicals with inversion of SOMO and HOMO energies and potential applications in optoelectronics

Affiliations
Review

Organic radicals with inversion of SOMO and HOMO energies and potential applications in optoelectronics

Sitthichok Kasemthaveechok et al. Chem Sci. .

Abstract

Organic radicals possessing an electronic configuration in which the energy of the singly occupied molecular orbital (SOMO) is below the highest doubly occupied molecular orbital (HOMO) level have recently attracted significant interest, both theoretically and experimentally. The peculiar orbital energetics of these SOMO-HOMO inversion (SHI) organic radicals set their electronic properties apart from the more common situation where the SOMO is the highest occupied orbital of the system. This review gives a general perspective on SHI, with key fundamental aspects regarding the electronic and structural factors that govern this particular electronic configuration in organic radicals. Selected examples of reported compounds with SHI are highlighted to establish molecular guidelines for designing this type of radical, and to showcase the potential of SHI radicals in organic spintronics as well as for the development of more stable luminescent radicals for OLED applications.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. (a) Schematic comparison of the frontier orbitals of a neutral closed-shell vs. an open-shell compound. The latter is obtained from the former by a one electron oxidation process and shows the SOMO as the highest energy level and its unoccupied (SUMO) counterpart, illustrated here with the triphenylmethane and corresponding triphenylmethyl radical, TPM˙. (b) Key optoelectronic properties of an organic radical.
Fig. 2
Fig. 2. Comparison of the emission process of the organic radical emitter per-chlorotriphenylmethyl PTM (left) and the TADF emitter 2,3,5,6-tetra(9H-carbazol-9-yl)terephthalonitrile, 4CzTPN (right).
Fig. 3
Fig. 3. Schematic representation of the electronic configuration found in organic radicals with SOMO–HOMO inversion (SHI). Figure adapted with permission from ref. . Copyright 2021 American Chemical Society.
Fig. 4
Fig. 4. (a) Schematic illustration of the Sugawara spin-polarized donor structure. (b) Nitronyl nitroxide radical 1, 2, and 3 with corresponding torsion angles between the radical and donor fragments in degrees with schematic illustration of the corresponding electronic configuration. Figures adapted with permission from ref. . Copyright 2000 American Chemical Society.
Fig. 5
Fig. 5. Chemical structures of (a), the bicarbazole radical cations 4˙+ and 5˙+ and (b), the cationic radical aza-thia[7]helicene 6˙+ and neutral aza-thia[7]helicene 6˙, with a schematic illustration of the steps leading to SHI.
Fig. 6
Fig. 6. Orbital energies (in eV) and isosurfaces (0.030 a.u.) of selected MOs of the TTM–PPTA, 7 and TTM–3NCz, 8 systems. See the text for a description of the CS//CS and R//CS labelling. The vertical dimension of the figure is associated with the orbital energy; however, as in other figures herein, the energy scales are drawn qualitatively and are not aligned. The table provides selected Coulomb and exchange electron repulsion integrals (ERIs, in eV) for the corresponding closed-shell parent systems. aCoulomb repulsion between the α- and β-spin component of the MO. Figures adapted with permission from ref. . Copyright 2021 American Chemical Society.
Fig. 7
Fig. 7. (a) Example of spin-polarized donors 9 where both magnetic and spin conductivity properties co-exist. (b) Magnetic switching of resistance of 9b at constant voltage. (c) Temperature-dependence of the ratio of magnetoresistance of 9b at a bias voltage of 7 V. Reprinted figures with permission from ref. . Copyright 2008 by the American Physical Society.
Fig. 8
Fig. 8. (a) Molecular structures of radical ligand tempodt 10a, platinum–tempodt complex 10, with the formation of the dimerized complex 1022+ and (b) the UV-vis spectra of 10 and 1022+; (c) EPR spectrum of 1022+. Figures adapted with permission from ref. . Copyright 2008 American Chemical Society.
Fig. 9
Fig. 9. Molecular orbital configurations of the carboxy-aminoxyl radical in both anionic 11˙ and neutral 11H+ forms.
Fig. 10
Fig. 10. Molecular structures of tris(2,4,6-trichlorophenyl)-methyl radical, PyBTM 12, and [TPA–RH+]˙, 12H+.
Fig. 11
Fig. 11. Molecular structure of triplet ground state cyclopentane-1,3-diyl diradical 13 within a π-conjugated macrocycle, exhibiting partial SHI in the sense that the SOMO − 1 is below a doubly occupied MO while the SOMO remains the highest energy level.
Fig. 12
Fig. 12. Chemical structure of galvinoxyl radical 14 and its simplified orbital energy diagram.
Fig. 13
Fig. 13. (a) Axially and helically chiral SHI cationic bicarbazole monoradicals 4˙+ and 5˙+, and orbital comparison with their common non-SHI cationic carbazole radical precursor, with the near IR circular dichroism of (+) and (−)-4˙+ (counter anions are omitted for clarity); (b) calculated orbital energies (in eV) and isosurfaces (±0.030) of frontier molecular orbitals computed for monoradicals 4˙+ (left) and 5˙+ (right). The HOMO–SOMO energy differences are averaged over the highest occupied spin orbital energies. Oxidation potentials and separation of the two first oxidation potentials in V vs. SCE, theoretical (calc.) and experimental (exp.) dihedral angles between the carbazole fragments; (c) schematic illustration of the electronic configuration of diradical 152˙2+ obtained upon oxidation of SHI radical 15˙+, near IR circular dichroism of (+) and (−)-152˙2+ in acetonitrile at 298 K, and its magnetic susceptibility (χMT vs. T). Figures adapted with permission from ref. . Copyright 2020 American Chemical Society.
Fig. 14
Fig. 14. Chemical structures of thiophene-based double helix radicals 16 and 17˙+ (TMS = trimethylsilyl). The molecular fragment in red highlights the predominant localization of the SOMO within the molecular structure.
Fig. 15
Fig. 15. Azulenes 18–19 and dimers of azulene 20–21 (top), and spiroconjugated systems 22–23 (bottom).
Fig. 16
Fig. 16. Chemical structures of cationic radicals thia[7]helicene 24˙+[PF6] and aza-thia[7]helicene 6˙+[PF6], and neutral aza-thia[7]helicene 6˙. Orbital energies (in eV) and isosurfaces (±0.030 a.u.) of selected MOs of 7˙. Figures adapted with permission from ref. . Copyright 2021 American Chemical Society.
Fig. 17
Fig. 17. Top: triplet carbenes included in a cycloparaphenylene (CPP) scaffold 25 and 2,2-difluorocyclopentane diradical 26 (left) showing SHI; with corresponding molecular orbital diagrams for 26a and 26b. Figure reprinted with permission from ref. . Copyright 2021 by the American Chemical Society.

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