Thiophene-Based Double Helices: Radical Cations with SOMO-HOMO Energy Level Inversion(dagger)

We report relatively persistent, open-shell thiophene-based double helices, radical cations 1(center dot+)-TMS12 and 2(center dot+)-TMS8. Closed-shell neutral double helices, 1-TMS12 and 2-TMS8, have nearly identical first oxidation potentials, E+/0 approximate to +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 approximate to +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 pi-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(center dot+)-TMS12, 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(center dot+)-TMS12 has a half-life of the order of only 5 min at room temperature. EPR peak intensity of 2(center dot+)-TMS8, which does not show SHI, is practically unchanged over at least 2 h.