Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 113

1 H-ENDOR/TRIPLE and diluted solution IR spectra, with the aid of quantum chemical
calculations, reveal the dominant contribution of the 6OPO -type electronic structure with an intramolecular
hydrogen bond between the 1- and 9-positions (Figure 3.36a), although the radical might exist as a
mixture of tautomers. An intramolecular proton transfer gives unique dynamic dielectric properties, as
is well demonstrated by a 9-hydroxyphenalenone system (closed shell molecule). 46 Thus, the possible
interconversion between structures b and c may contribute to the control of magnetic properties in the
solid state, as well as to the realization of dynamic spin nature.
7-Hydroxy- TB4OPO , a topological isomer of 7-hydroxy- TB6OPO , was also designed and prepared
(Figure 3.37a). 47 This neutral radical also has several possible tautomeric structures based on the 4OPO
electronic structure (Figure 37b - d). Solution EPR measurements and quantum chemical calculation confirm
this electronic structure. Interestingly, the radical has a hydrogen bonding capability directed outwards from
the molecular skeleton, raising the possibility of the construction of inter molecular hydrogen bonding
interactions.
To investigate a heteroatomic chemical modification in 6OPO , a 7,9-diaza derivative of 6OPO system
( DAOPO ) was designed and generated by a treatment of a corresponding anion with DDQ oxidation
in a toluene solution (Figure 3.38). 48 The position of the two oxygen and nitrogen atoms raises the
possibility that this radical could be used as a bridging ligand for intermolecular networks. EPR

The solution EPR /

1 H-,
14 N-ENDOR/TRIPLE spectroscopies and quantum chemical calculations reveal that DAOPO possesses
the 6OPO -type electronic structure. Interestingly, exposure of the EPR tube of this radical degassed
solution to sunlight at

/

78 ◦ C induces a rapid decomposition of the radical within 10 minutes, although
in the dark the neutral radical is stable at

−

30 ◦ C for a few weeks. Thin layer chromatography indicates

−

(a) (b) (c) (d) (e)

Figure 3.37 (a) Local spin structure, (b-d) possible tautomers, and (e) spin density distribution of 7-hydroxy-
TB4OPO .

M M

N N N N

O O O O

DAOPO

(a) (b) (c)

Figure3.38 (a)Alocalspinstructure,(b)spindensitydistribution,and(c)ligationmotifof DAOPO .