Thiophene‐Based Double Helices: Radical Cations with SOMO–HOMO Energy Level Inversion^†

Abstract

We report relatively persistent, open-shell thiophene-based double helices, radical cations 1 •+ -TMS 12 and 2 •+ -TMS 8 . Closed-shell neutral double helices, 1-TMS 12 and 2-TMS 8 , have nearly identical first oxidation potentials, E +/ 0 ≈ +1.33 V, corresponding to reversible oxidation to their radical cations. The radical cations are generated, using tungsten hexachloride in dichloromethane (DCM) as an oxidant, E +/0 ≈ +1.56 V. EPR spectra consist of a relatively sharp singlet peak with an unusually low g-value of 2.001-2.002, thus suggesting exclusive delocalization of spin density over π-conjugated system consisting of carbon atoms only. DFT computations confirm these findings, as only negligible fraction of spin density is found on sulfur and silicon atoms and the spin density is delocalized over a single tetrathiophene moiety. For radical cation, 1 •+ -TMS 12 , energy level of the singly occupied molecular orbital (SOMO) lies below the four highest occupied molecular orbitals (HOMOs), thus indicating the SOMO-HOMO inversion (SHI) and therefore, violating the Aufbau principle. 1 •+ -TMS 12 has a half-life of the order of only 5 min at room temperature. EPR peak intensity of 2 •+ -TMS 8 , which does not show SHI, is practically unchanged over at least 2 h.

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

“…62 Interestingly, our findings revealed that this electronic configuration contributes to stabilizing the unprotected chiral carbazole monoradical, while providing, upon oxidation, an interesting approach for isolating one of the rare persistent organic chiral diradicals. 31,46 Our results also showed that SHI is not a sufficient prerequisite to promote radical stability, as also recently illustrated by Rajca et al 63 Indeed, helical bicarbazole monoradical 1 •+ (Figure 1b) remains highly reactive despite SHI and the use of steric t-Bu groups (1a •+ ), in contrast to its axial counterpart. Whereas this difference has been explained in terms of electronic coupling between the SOMO and HOMO levels, it appears crucial to understand better how and why such subtle structural factors lead to a dramatic change of stability.…”

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

“…10). [27] DFT calculations suggested the presence of SHC in the radical cation 2 þ and a normal electronic configuration in the radical cation 3 þ at the UvB97xD/6-31G(d,p)/PCM (water or DCM) level of theory. 2 þ and 3 þ were experimentally generated, and their half-lives were estimated by EPR measurements.…”

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

“…136 Although no chiroptical properties were reported for the radicals thiophene-based double helices 14). 139,140 The cationic monoradicals 16 ). This result is in contradiction with the aforementioned findings for SHI vs. non-SHI radicals in terms of stability.…”

“…14). 139,140 The cationic monoradicals 16c + and 17c + showed high persistence at 193 K (half-lives exceeding 24 hours). However, at room temperature in dichloromethane solution, radical 17c + showed much less stability than 16c + (t 1/2 < 5 minutes vs. >12 hours in 17c + ).…”

“…38,74,110 The authors of ref. 140 therefore suggested that the steric hindrance brought by the peripheral TMS-thiophene units in 16˙ + plays a decisive role in stabilizing the unpaired electron. In fact, the spin density in 16˙ + is mostly localized in the center of the molecule whereas for 17˙ + , it is also delocalized in the peripherical thiophene fragments (highlighted in red, Fig.…”