Verdazyls and Related Radicals Containing the Hydrazyl [R2N–NR] Group - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 256

Ph

N

N

N

N t Bu

Ph

Ph Ph

N
NN

N

N
NN

N

Ph

t Bu

t Bu

29 30 31

The localized electronic structure attenuates the “communication” of verdazyl radicals with other
covalently-linked chromophores. Experimental and computational 52,73,80 - 82 studies on a variety of
C3-linked verdazyl di- (or tri- or tetra-) radicals 32 or their N-linked 19,27,83 counterparts 33 show the
two radicals to invariably be fairly weakly electronically coupled, irrespective of whether the diradical
in question is based on a Kuhn 19,24 - 26,28,84 or 6-oxoverdazyl 42,49 - 52 structure. Even the 6-oxoverdazyl
diradical, in which the two radicals are directly linked, remains an open shell singlet diradical in its
ground state, with the triplet excited state some 760 cm − 1 higher in energy. 52 Electronic and magnetic
communication has been investigated in polyradicals of general structure 34 , in which a verdazyl is linked
by a -conjugated spacer “X” to a different open shell chromophore, for example, nitronyl nitroxide, 82

tetrathiafulvalene radical cation, 81 triplet nitrene (generated by photolysis of the corresponding azide), 63,85
or triplet excited states of organic compounds, such as pyrene or anthracene. 74,86 In all of these cases,
where spin - spin interactions were probed quantitatively (using EPR spectroscopy and/or computational
studies) the exchange coupling energies remain on the order of a few hundred wavenumbers.

π

R R

N N N N

X X X

NN NN

R

R R N N

32

X X Rad

R' R

NN

N N

R

34

N N X N N

N N

R R'

33

7.2.3 Verdazyl radical reactivity
Verdazyls are stable radicals but are also species with a substantial and varied chemistry. The general
stability of verdazyls is sufficient for chemical transformations on other parts of the molecule to take
place without affecting the radical itself. For example, amino-substituted verdazyl 35 can be converted
into amide 36 or imine 37 derivatives (Scheme 7.10), 21 and 1,3,5,-triphenylverdazyl 38 can be sulfonated
to give a water soluble derivative 39 (Scheme 7.11). 87 Many of the verdazyl-containing polymers shown
in Figure 7.1 were made via multistep syntheses in which the pre-assembled verdazyl is carried through
reactions which do not affect the radical center.