Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 108

SOMO levels of the OPO systems compared to that of the PLY system imply high electron accepting
abilities of the OPO systems. The energy gaps between the SOMO and LUMO in the OPO systems are
significantly smaller than that of the PLY system, indicating multistage electron accepting abilities which
may be key factors for applications as electrode active materials in secondary batteries ( vide infra ). As
a matter of fact, cyclic voltammetry measurements of 3OPO, 4OPO ,and 6OPO systems show two-stage
one-electron reduction behaviors ( vide infra ). Importantly, the coefficients of the SOMOs of the resulting
radical dianions (LUMO in the neutral radical systems) mainly exist on the

positions of the phenalenyl
skeletons, implying the phenalenyl-type electronic structure (Figure 3.30). This electronic feature is also
supported by quantum chemical calculations (Figure 3.31). Such a phenomenon - showing a remarkable
change in the electronic structures depending on the redox states - is a rare event, and is termed “redox-
based spin diversity”. 36,37a,38 Considering all of these significant electronic features, OPO s are viewed as
completely novel neutral radical systems, unlike the PLY system, and have a high potentiality for creating
a qualitatively different molecular functionality from that obtained by conventional phenoxyl radical.

α

3.8.2 3-Oxophenalenoxyl (3OPO) system

The synthetic method for 3OPO is depicted in Scheme 3.9. Condensations of 1,8-naphthalic anhydride with
arylacetic acid derivatives gives 2-aryl-substituted 3-hydroxyphenalenones as 3OPO precursors. Oxidation
of these hydroxyl compounds affords the closed shell dimers of 3OPO derivatives. 39 These experimental

observations are well understood by the major contribution of the spin density distribution on the 2-position

σ

(a) (b) (c)

Figure3.30 Molecularorbitalpicturesandenergydiagramsof(a) PLY ,(b) 4OPO ,and(c) 6OPO . Calculations
carriedoutattheROBLYP/6-31G**//ROBLYP/6-31G** leveloftheory.