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Title: 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•+‐TMS12and 2•+‐TMS8. Closed‐shell neutral double helices, 1‐TMS12and 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 lowg‐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•+‐TMS12has 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.

 
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Award ID(s):
1955349 1665256
NSF-PAR ID:
10446368
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
Wiley-Blackwell
Date Published:
Journal Name:
Photochemistry and Photobiology
Volume:
97
Issue:
6
ISSN:
0031-8655
Page Range / eLocation ID:
p. 1376-1390
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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