A series of stable
and genuinely organic open-shell systems, π-conjugated
phenoxyl–nitroxide free radicals (hybrid phenoxyl–nitroxide
radicals), have been synthesized and their magnetic properties in
the crystalline state investigated, revealing their usefulness as
new building blocks for molecular magnetic materials. The salient
electronic structure of the hybrid phenoxyl–nitroxide radicals
is extended π-spin delocalization from the nitroxide moiety,
mediating the localization effect intrinsic to nitroxide radicals.
Five representative hybrid radicals containing an aliphatic, aromatic,
and heteroaromatic substituent in the side part of the compact hybrid
radical centers were synthesized, and their molecular/crystal structures
in the crystalline state were determined by X-ray diffraction analyses.
CW X-band ESR, 1H-ENDOR spectroscopy, and DFT calculations
for the hybrid radicals confirmed that an unpaired spin delocalizes
over the whole molecular frame including the nonconjugated fragments,
suggesting the possibility of tuning their electronic properties through
substituent effects in the crystalline state. Significant influence
of the phenoxyl moiety on the electronic structure was analyzed in
terms of the g-tensor calculations. The SQUID magnetization
measurements revealed that the nitroxides bearing alkyl or aromatic
substituents behave as 3D Curie–Weiss paramagnets with weak
antiferromagnetic (AFM) (Θ = −1 to −2.6 K) or
ferromagnetic (FM) (Θ = +0.33 K) spin–spin exchange interactions.
On the other hand, heteroaromatically substituted hybrid phenoxyl–nitroxide
showed significant AFM interactions with J/k
B = −25.6 K. The analysis of the bulk magnetic properties
based on the crystallographic data and DFT calculations revealed competition
between the intermolecular AFM and FM interactions which originate
from the C–O(phenoxyl)···Me(nitroxide) or (N)O–C(arom)
infinite 1D head-to-tail chains and the C(arom)–C(arom) head-over-tail
dimers forming 3D networks in their crystal lattices.