Chemistry Reference
In-Depth Information
PTM units. Computer spectral simulation gave an isotropic
g
-value of 2.0028 and the following hyperfine
coupling constants: a(
1
H)
88 G (2H), a(
13
C
α
)
≈
0G, a(
13
C
bridge
3G, and a(
13
C
ortho
1G,
which are approximately half the value of those found for related monoradical PTM species.
76
It is then
possible to conclude that the two unpaired electrons of
14
are magnetically coupled with an exchange cou-
pling constant,
J
, that fulfils the following condition:
J
≈
0
.
13
.
)
≈
6
.
)
≈
5
.
a
i
. Furthermore, the radical
14
does not exhibit
the forbidden
2 transition characteristic of a triplet species, which is in accordance with the rapid
fall down of the magnetic dipolar interaction with the effective distance of the two unpaired electrons, which
for
14
is extremely large.
77
Despite the presence of two electronically active units, electrochemical studies
(dichloromethane, nBu
4
NPF
6
(0,1 M) vs SCE) show only one reversible two-electron reduction process
at
m
s
=±
0,13 V (vs SCE) due to the conjugation of the bridge. Spectroelectrochemical experiments performed
during the reduction of diradical
14
to the radical anion
14
−
•
evidenced the lack of any IVT band originated
by an IET. Isotropic hyperfine coupling constants of
14
−
•
were approximately double than those found for
diradical
14
, demonstrating that the unpaired extra electron is strongly localized on one side of the molecule
and the rate of transfer, if any, is very slow. An increase of temperature neither leads to a thermally activated
IET process since any change in the spectrum is observed. This result, together with the lack of an IVT
band, suggests that there is a strong localization of the extra electron in the radical anion
14
−
•
species. In
conclusion, in this rigid nano-sized M-V system there is a subtle interplay between the acceptor ability of the
PTM unit and the size and nature of the bridge that makes feasible or not the electron transfer phenomena.
The influence of the bridge topology on the IET phenomena was also studied with the PTM dirad-
icals
15
and
16
and their corresponding radical anions
15
−
•
and
16
−
•
using magnetic measurements,
spectroelectrochemistry and variable temperature EPR.
78
−
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
16
15
Magnetic susceptibility data of powder samples of diradicals
15
and
16
both follow the Curie - Weiss
law with
0.4 K for
15
and
16
, respectively, indicating the presence of very weak intra
or intermolecular antiferromagnetic interactions. The magnetic moments at room temperature for both
diradicals are 2
θ
=−
4K and
−
µ
B
, indicating the existence of non-interacting pair of doublets at this temperature.
Experimental electronic coupling parameters,
H
AB
, were determined for both diradicals by means of spec-
troelectrochemistry experiments. Thus, during the reduction of both diradicals the evolution of the resulting
spectra was similar, although only for the radical anion
15
−
•
an IVT band, centred on 1400 nm, was
observed. From the position and line width of this band it is possible to calculate the effective electronic
coupling,
H
AB
, between the two PTM sites of
15
, which is found to be
H
AB
=
.
42
±
1
121 cm
−
1
. The absence of
any significant IVT band for radical anion
16
−
•
is an indication that its effective electronic coupling is at
least one order of magnitude smaller than in
15
. The thermally activated IET was also studied by variable
temperature EPR for radical anions
15
−
•
and
16
−
•
. Thus, the EPR spectrum of
15
−
•
at 200 K displays two
symmetrical lines arising from the hyperfine coupling of the unpaired electron with one hydrogen atom of
the ethylene moiety and the resulting
1
H hyperfine coupling constant is very close to that of related mono-
radicals. This result clearly demonstrates that at this temperature the unpaired electron of radical anion
15
−
•
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