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N
O
N
O
N
O
O
N
O
N
O
N
(A)
(B)
(C)
Scheme 5.13
the N-O nitrogen atoms are significantly pyramidalized and the N-O bonds are elongated. More recently
Ishida
et al
.
58
succeeded to obtain a new crystallographic allotrope of
41
(
β
-
41
). X-ray data show that
for
N-O distances are very long, and, as expected, magnetic susceptibility
measurements revealed strong intramolecular ferromagnetic coupling.
β
-
41
the intermolecular N-O
...
5.3.2 Miscellaneous aspects of di- and polynitroxides
EPR studies have been devoted to numerous dinitroxides
59
; some of the studied species are shown in
Scheme 5.14.
The impact of electron - electron spin interaction on electron spin relaxation of a series of nitroxide dirad-
icals and a nitroxide tetraradical in glassy solvents between 10 and 300 K has been studied.
60
Relaxivity
studies on dinitroxides and dendritic nitroxide multiradicals (
46
, Scheme 5.15) have been performed.
61
Dendritic polynitroxides exhibit higher per-nitroxide-based water relaxivity compared to a mononitroxide
model. The higher generation polynitroxide dendrimers exhibit relaxivities exceeding those for gadolin-
ium(III) chloride.
Prefluorescent nitroxides possess a fluorophore tethered by a short covalent link to a nitroxide moiety. The
fluorophores exhibits low fluorescence due to enhanced intersystem crossing from the first excited singlet
state of the fluorophore to its triplet state via electron exchange interactions with the nitroxide moiety.
Upon radical trapping to form a diamagnetic alkoxyamine normal fluorophore emission is restored, and thus
these prefluorescent nitroxides are extremely responsive sensors for the detection of free radical species.
Recently Bottle
et al
.
62
prepared a new prefluorescent dinitroxide based on a 9,10 diphenylanthracene core
O
O
N
O
N
N
O
N
O
N
O
43
59b
42
59a
O
O
N
O
O
N
O
O
44
59c
H
O
N
(CH
2
)
n
N
N
N
O
H
n = 1,2,3,4,6
45
59d
Scheme 5.14
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