Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 89

(a) (b) (c)

Figure3.7 Crystal structure of TBPLY . (a) Top view, (b) side view, and (c) a herringbone motif of the crystal.
(cReprintedwithpermissionfrom[13a].Copyright1999AmericanChemicalSociety.)

being similar to that of the pristine PLY (2.0027). 8i,11b The experimental and theoretical spin density
distributions (Figure 3.6c) indicate that TBPLY possesses a characteristic spin delocalized nature and spin
polarized structure, in which large positive and small negative spin densities locate at the six α carbons

and carbons with a C 3 -symmetry, respectively.

The first X-ray crystal structure analysis of an odd-alternant hydrocarbon neutral radical with a fused
polycyclic

β

-conjugated system was carried out on a single crystal of TBPLY obtained by recrystallization
from hexane at

π

30 ◦ C (Figure 3.7). 13a The molecule has D 3 h symmetry, and the phenalenyl skeleton shows
a nearly planar geometry. The C-C bond lengths in the phenalenyl ring range from 1.374 to 1.421 A.
The molecule forms a

−

-dimeric pair in a staggered arrangement of the tert -butyl groups to avoid their
steric repulsion. In such an arrangement, the maximum overlaps of the radical SOMOs are expected. The
interplanar distances in the

π

-dimeric pair range from 3.201 to 3.323 A, which is shorter than the sum
of van der Waals radius of carbon atom (3.4 A). The

π

-dimeric pair is in a herringbone motif in the
crystal, and no effective interdimeric interaction is found. The magnetic susceptibility measurement of
a polycrystalline sample of TBPLY (2 - 350 K) shows a large antiferromagnetic intermolecular exchange
interaction (2 J

π

-dimeric pair. 13 In the solid
state electronic spectrum, a broad and strong absorption band attributable to the strong intermolecular
transition in the

/ k B =− 2000 K), indicating a ground state spin singlet in the π

-dimeric pair is observed in the low energy region (510 - 700 nm).
Interestingly, the strong interaction between TBPLY s observed in the solid state is also found in solution
at low temperature, leading to a thermochromic phenomenon. 14,15 The red purple color of the hexane
solution in TBPLY at room temperature gradually changes to blue at low temperature (

π

200 K), as seen
in the increment of the absorbance of 530 - 670 nm with a lowering of temperature (Figure 3.8a), and this
color change occurs reversibly. The chemical species exhibiting each color are assigned as the neutral
radical monomer at room temperature and the

∼

-dimeric pair at low temperature. 14,15 The temperature
dependence of the electronic structure originating from the neutral radical was also observed by EPR
measurements. 14,15

π

dimer formed at low temperature was unequivocally
detected by cold-spray ionization mass spectrometry (CSI-MS), by which substances can be ionized at
much lower temperatures than conventional ESI-MS (Figure 3.8b). 15 Notably, this is the first experimental
observation of the molecular weight of the open shell molecule aggregates formed in solution.

The molecular weight of the π