SOMO–HOMO Conversion in Triplet Cyclopentane-1,3-diyl Diradicals

Abstract

According to the Aufbau principle, singly occupied molecular orbitals (SOMOs) are energetically higher lying than a highest doubly occupied molecular orbital (HOMO) in the electronically ground state of radicals. However, in the last decade, SOMO–HOMO-converted species have been reported in a limited group of radicals, such as distonic anion radicals and nitroxides. In this study, SOMO–HOMO conversion was observed in triplet 2,2-difluorocyclopentane-1,3-diyl diradicals DR3F1, DR4F1, and 2-fluorocyclopentante-1,3-diyl diradical DR3HF1, which contain the anthracyl unit at the remote position. The high HOMO energy in the anthracyl moiety and the low-lying SOMO–1 due to the fluoro-substituent effect are the key to the SOMO–HOMO conversion phenomenon. Furthermore, the cation radical DR3F1 + generated through the one-electron oxidation of DR3F1 was found to be a SOMO–HOMO-converted monoradical.

Singly Occupied Molecular Orbital−Highest Occupied Molecular Orbital (SOMO−HOMO) Conversion

“…[3,20,26,[28][29][30] Also, some researchers have studied the functional dependency of SHC in DFT calculations and they reported that the unusual electronic configuration is maintained with various functions. [4,37] Thus, DFT is considered to give reliable results about SHC. Further studies on SHC species may provide opportunities for their application in the new field of materials science, such as magnetic materials and OLEDs.…”

“…Recently, the Abe group reported computational studies on SHC in cyclopentane-1,3-diyl diradicals DR1F1 (X 1 ¼ X 2 ¼ F, n ¼ 1) and DR1HF1 (X 1 ¼ H, X 2 ¼ F, n ¼ 1) at the UB3LYP, UvB97xD or UM06-2X/6-31G(d) level of theory. [37] Moreover, the dihydro-substituted DR1H1…”

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

“…triggers an intramolecular electron transfer from the trisarylamine to the radical center, affording a possibility to switch between TPA- More recently, Abe, et al investigated theoretically the electron configurations of diradical compounds based on a macrocyle composed of an anthracene and a 2,2difluorocyclopentane diradical unit, linked together by two diphenylethyne fragments (13, Figure 11). 114,115 In this molecule, the HOMO is mainly localized on the anthracyl unit, with little electronic interaction with the SOMO and SOMO-1 found on the 1,3-diphenylcyclopentane. Both the presence of of the anthracene fragment and fluorine atoms within the cyclopentane unit were shown to be important factors to establish SHI, by raising the energy of the HOMO and stabilizing the SOMO and SOMO-1, respectively.…”

“…11). 114,115 In this molecule, the HOMO is mainly localized on the anthracyl unit, with little electronic interaction with the SOMO and SOMO − 1 found on the 1,3-diphenylcyclopentane. Both the presence of the anthracene fragment and fluorine atoms within the cyclopentane unit were shown to be important factors to establish SHI, by raising the energy of the HOMO and stabilizing the SOMO and SOMO − 1, respectively.…”

Carbazole Isomerism in Helical Radical Cations: Spin Delocalization and SOMO–HOMO Level Inversion in the Diradical State

“…Irrespective of the spin multiplicity of the present diradicals, the two SOMOs are separated energetically from the highest occupied level by other sets of spin-paired orbitals (Figures 5 and S62 and S63). While a recent theoretical study of Abe et al has proposed a diradical system showing SHI between HOMO and SOMO-1 levels, 94 the electronic configuration of SHI diradical 2 2•2+ appears particularly intriguing. Experiments and calculations suggest that the occurrence of a spatially disjoint pair of MOs energetically below the highest occupied level may contribute to the diradical stability.…”