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.