SOMO–HOMO Conversion in Triplet Carbenes

Abstract

In this study, the SOMO−HOMO conversion has been shown for the first time in triplet carbenes embedded in cycloparaphenylene units. The high-lying HOMO originating from the curved π-conjugated system and the low-lying SOMO−1 originating due to the small carbene angle are the key to endowing this interesting electronic configuration. Furthermore, simple planar triplet carbenes such as fluorenylidene were found to possess SOMO− HOMO energy-converted electronic configurations.

SOMO–HOMO Conversion in Triplet Cyclopentane-1,3-diyl Diradicals

“…Recently, SOMO−HOMOconverted triplet carbenes have been found by this laboratory. 15 The search for molecules that undergo SOMO−HOMO conversion will advance the field of radical chemistry.…”

“…Although the reactivity of SOMO–HOMO-converted molecules has attracted considerable attention in radical chemistry, only a limited number of organic molecules have been reported to date. Recently, SOMO–HOMO-converted triplet carbenes have been found by this laboratory . The search for molecules that undergo SOMO–HOMO conversion will advance the field of radical chemistry.…”

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

“…In 2021, SHC in triplet carbenes were reported by Abe, Antol, and their co-workers. [35] The SHC in nC, triplet carbenes embedded in cycloparaphenylene (CPP), and simple triplet diaryl carbenes was confirmed computationally (Figs 14 and 15) at the UB3LYP/6-31G(d) level of theory. CPP is known to have a higher HOMO level with a smaller ring size due to the increase in quinoidal character (Fig.…”

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

“…In 2021, Abe, Antol, et al reported another example of this type of molecular design, based on carbene derivatives displaying SHI. 54 Based on theoretical calculations, it was shown that triplet carbenes included in a cycloparaphenylene (CPP) scaffold have SOMO energy levels below the HOMO of the CPP ring (Figure 17). This electronic configuration is found for the ring composed of four phenyl units (25), whereas in larger carbene-CPP derivatives (5 to 10 phenyl units), the HOMO lies energetically in between the two SOMOs.…”

“…This SOMO-HOMO inversion (SHI), alternatively referred to as SOMO-HOMO conversion or a quasi-closed-shell configuration, has generated widespread interest, both theoretically and experimentally, to rationalize and understand the consequences of the peculiar electronic structure of this class of open-shell systems. 23,50,[53][54][55][56][57] SHI has been investigated in organic spin-polarized donor systems as a route toward high-spin diradicals, 18,[58][59][60][61][62] and also in the case of luminescent radicals where SHI was associated with a significant increase of photostability. 38 Coote, Corminboeuf et al summarized various theoretical aspects related to the characterization of this phenomenon with molecular examples of SHI in 2015.…”