Thiophene‐Based Double Helices: Radical Cations with SOMO–HOMO Energy Level Inversion. (5th July 2021)

• Record Type: Journal Article
• Authors: Rajca, Andrzej; Shu, Chan; Zhang, Hui; Zhang, Sheng; Wang, Hua; Rajca, Suchada
• Is Part Of: Photochemistry and photobiology. Volume 97:Number 6(2021)
• Page Start: 1376
• Page End: 1390
• Publication Date: 2021-07-05
• Subjects: Photochemistry -- Periodicals; Light -- Physiological effect -- Periodicals; 541.35
• DOI: 10.1111/php.13475
• Languages: English
• ISSNs: 0031-8655

Abstract

Abstract: We report relatively persistent, open‐shell thiophene‐based double helices, radical cations 1 + ‐TMS12 and 2 + ‐TMS8 . Closed‐shell neutral double helices, 1‐TMS12 and 2‐TMS8, 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 + ‐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 + ‐TMS12 has a half‐life of the order of only 5 min at room temperature. EPR peak intensity of 2 + ‐TMS8, which does not show SHI, is practically unchanged over at least 2 h. Abstract : Radical cations of thiophene‐based double helical π‐systems possess their spin densities near exclusively delocalized over the carbon atoms and, for 1 .+ ‐TMS12, the SOMO energy level is below the HOMO energy level.