Intramolecular (electron) delocalization between aromatic donors and their tethered cation-radicals. Application of electrochemical and structural probes

Article abstract of DOI:10.1039/b103139m

To study the mechanism of electronic transduction along (poly)phenylene chains, a series of aromatic donors with general formula D-B-D has been synthesized [where D = 2,5-dimethoxy-4-methylphenyl donor and B = (poly)-phenylene bridge]; and the correspondi

Full text of DOI:10.1039/b103139m

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Intramolecular (electron) delocalization between aromatic donors
and their tethered cation–radicals. Application of electrochemical
and structural probes†
2
Duoli Sun, Sergey V. Lindeman, Rajendra Rathore and Jay K. Kochi*
Department of Chemistry, University of Houston, Houston, TX 77204, USA
Received (in Cambridge, UK) 6th April 2001, Accepted 6th June 2001
First published as an Advance Article on the web 18th July 2001
To study the mechanism of electronic transduction along (poly)phenylene chains, a series of aromatic donors with
general formula D–B–D has been synthesized [where D = 2,5-dimethoxy-4-methylphenyl donor and B = (poly)-
ϩ
ؒ
Ϫ
phenylene bridge]; and the corresponding cation–radical salts D–B–D SbCl6 have been isolated for X-ray
ϩ
ؒ
crystallographic analyses. The magnitude of the electronic interaction between the D and D moieties through
the various B bridges has been measured (i) as the difference between the first and the second oxidation potentials
ϩ
ؒ
of D–B–D donors and (ii) as the structural changes induced in neutral D by the presence of the tethered D group in
ϩ
ؒ
ϩ
ؒ
D–B–D cation–radicals. The intramolecular interaction of D and D groups was found to occur via π-conjugation
of the bridging (poly)phenylene group. As such, the electronic interaction is highly dependent on the planarity of the
(
poly)phenylene bridge, and can be either inhibited or promoted by the deliberate modifications of the molecular
ϩ ϩ ϩ ϩ ϩ ϩ
ؒ
ؒ
ؒ
ؒ
ؒ
ؒ
conformation. Crystal structures of compounds A, B, B
, 1, 1 , 2, 2 , 3 , 8 and 9 are reported.
4
Introduction
chemical oxidants. The resulting cation–radicals are relatively
persistent and can be reversibly reduced back to the original
Structural and electronic aspects of long-range chemical trans-
duction are important to modern supramolecular chemistry;
and chemical systems of the general structure: D –B–D are
considered to be prototypes of nanoscale electronic devices
such as chemical sensors, switches, wires etc. In these molec-
ular systems, the application of some physical/chemical action
to the primary group D (e.g. irradiation, complexation,
protonation, reduction/oxidation) induces changes in its
structure/electronic state. The effect of these changes is sub-
sequently transmitted through the [extended] bridging group
neutral donors. Importantly, p-dimethoxy-substituted aro-
matic groups exhibit relatively large geometrical changes
during oxidation, and this makes them desirable structural
indicators—even for small amounts (<0.1 e) of charge redistri-
bution. Importantly, these groups do not show significant
intermolecular self-association upon oxidation as opposed to
the majority of other aromatic electron donors (naphthalene,
anthracene etc.). D ϩ D
intramolecular effects can be readily differentiated from those
derived from intermolecular interactions.
The three classes of aromatic donors examined in this study
are represented in Chart 1. The Class I donors contain two D
groups linked through a (poly)phenylene chain of different
length. Class II includes donors in which the rotation about the
bridging phenylene moiety is restricted by the presence of one
or more methyl groups. Class III donors contain a modified
1
2
1–3
1
5
ϩ
ؒ
ϩ
ؒ
(D2)
Consequently, the
/
2
B to the [remote] D moiety. The structural/electronic/optical
reply of the D moiety can then produce either a desirable
target effect or begin another chain of physical/chemical
transformations.
2
(
either planarized or an extended) (bis)phenylene bridging
group. We approach this problem by simultaneously applying
electrochemical (cyclic voltammetry and Osteryoung square-
6
wave voltammetry) as well as structural (X-ray crystal-
In this study, we examine the structural and electronic reply
lography) techniques to probe the electronic interaction of D
and D through the agency of various types of polyphenylene
bridges.
2
ϩ
ؒ
of a remote donor group D upon the removal of one electron
1
2
from the primary donor group D that is separated from D
through different bridging (poly)-p-phenylene moieties B. As
such, we initially focus on the simple symmetrical system: D–B–
D, where D is the 2,5-dimethoxy-4-methylphenyl donor group.
Results and discussion
I. Group D as the electrochemical/structural probe
For the electronic transduction process, we first examined the
structural change of the model donors A and B upon electro-
chemical oxidation. The cyclic voltammogram (CV) of the
monobenzenoid donor A showed a characteristic (1-electron)
0
reversible wave at Eox = 1.10 V vs. SCE with peak separation of
0
D is a good electron donor (Eox = 1.1 V vs. SCE) that
Epa Ϫ Epc = 60 mV in dichloromethane solution at a scan rate
Ϫ1
allows its ready oxidation electrochemically and by a variety of
of 100 mV s .
Essentially the same cyclic voltammogram was obtained with
the bis-benzenoid donor B; but calibration of the CV wave
showed that the anodic oxidation was composed of a 2-electron
†
Dedicated in memoriam to Lennart Eberson, friend to whom
oxidation mechanisms were a constant challenge.
DOI: 10.1039/b103139m
J. Chem. Soc., Perkin Trans. 2, 2001, 1585–1594
This journal is © The Royal Society of Chemistry 2001
1585

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Chart 1
ϩ
ؒ
ϩ
ؒ
Table 1 Electrochemical and structural parameters of A, B and B
a
Bond lengths/Å
E /V vs.
SCE
Coulombic
charge qi
ox
Donor
α
β
γ
δ
ε
A
B
D–CH3
1.10
1.10
1.401
1.399
1.447
1.394
1.396
1.407
1.396
1.397
1.372
1.380
1.375
1.327
1.426
1.425
1.457
0
0
ϩ1
D–(CH ) –D
2
3
ϩ
ؒ
ϩ
ؒ
ϩ
ؒ
ϩ
ؒ
B
D–(CH ) –D
2
3
a
ؒ ؒ
ϩ ϩ
Averaged over four and two chemically equivalent values for structures B and B
, respectively.
process. Since such a CV characteristic suggested the identical
behavior of two independent D moieties in the donor B, we
examined the molecular structure of the dication in the follow-
ing way. The dicationic salt of the bis-benzenoid donor B was
successfully isolated as a (black) crystalline solid with stoichio-
2ϩ
Ϫ
metry: D(CH ) D (SbCl ) , as described in the Experimental
2
3
6 2
section. Single crystal X-ray crystallography established the
presence of a crystallographic symmetry (mirror plane) to
demonstrate that both D moieties are identical, and both bear a
unit of positive charge. Therefore, it is easy to conclude that
ϩϩ
ϩ
ؒ ؒ
ϩ
B
is in fact the dication–diradical (Bϩ ). The pertinent aro-
ؒ ؒ
ϩ
matic C–C bond distances in B and B
are listed in Table 1,
7
together with those of the monobenzenoid donor A.
range. Furthermore, the exocyclic C(Ar)–O (δ) and C(Me)–O
(ε) bond lengths of 1.375(1) and 1.424(1) Å are also close to the
Examination of the structural parameters in Table 1 showed
that the neutral D moiety has the usual planar centrosymmetric
benzenoid geometry, as can readily be seen from the pertinent
aromatic bond lengths. Thus the endocyclic C–C bond lengths
α, β and γ are all equal within the standard (1.397–1.400 Å)
7
standard values of 1.370 and 1.426 Å, respectively. However,
upon oxidation, the D moieties in D–(CH ) –D exhibit the
significant geometric changes shown in Table 1. Although the
system retains its local center of symmetry, the C–C bonds α
ϩ
ؒ
ϩ
ؒ
2
3
1
586
J. Chem. Soc., Perkin Trans. 2, 2001, 1585–1594

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a
Table 2 Electrochemical characteristics of aromatic donors 1–13
±0.03 V)
(
b
b
Class
Donor
Eox1
Eox2
∆Eox
I
1
2
3
4
5
1.11 (1e)
1.15 (1e)
1.40 (1e)
1.26 (1e)
0.29
0.11
1.18 (2e)
1.18 (2e)
1.18 (2e)
II
6
7
8
1.16 (1e)
1.13 (1e)
1.25 (1e)
1.19 (1e)
0.09
0.06
1.17–1.24 (2e)
1.21 (2e)
III
9
1
1
1
1
0
1
2
3
1.17 (2e)
1.18 (2e)
1.18 (2e)
1.17 (2e)
a
Experimental conditions: concentration 0.5 mM in 0.1 M supporting
electrolyte in dichloromethane solution at room temperature; scan
Ϫ1
rate 2 V s ; the oxidation potentials are given in V vs. SCE.
b
In parentheses—number of electrons transferred.
and β are elongated by ϩ4.7 and ϩ1.0 pm, respectively, and the
γ bonds are shortened by Ϫ2.5 pm in accord with the major
contribution of the quinonoidal resonance form Q shown
8,9
below. Most remarkably, the exocyclic δ bonds C(Ar)–O
exhibit the greatest change—being shortened by Ϫ4.8 pm. Also,
the ε bonds C(Me)–O are elongated by 3.3 pm. Thus, the qui-
ϩ
ؒ
nonoidal distortion of D is a highly sensitive measure of its
oxidation degree. Indeed, such large magnitudes of the geo-
metrical changes provide confidence in our ability to distinguish
between neutral and cationic oxidation states of the D groups,
Fig. 1 Coalescence of two 1-electron oxidation waves with increasing
length of p-phenylene bridge. From (a) donor 1 directly linked D
moieties together; (b) in donor 2, with a single p-phenylene bridge; to
(c) donor 3 with a (bis)-p-phenylene bridge. (d) The waves are split
again for donor 9 with a planarized bridge.
as indicated by the values of q in Table 1 (last column). For
i
relatively small geometrical changes in conjugated π-systems, a
simple linear regression was found to be adequate for evaluat-
compared with the “standard” geometries (in Table 1) in the
ing the partial (effective) charge q of a particular D moiety, as
i
following way.
10
shown in eqn. (1), where q is the partial charge over the D
ϩ
ؒ
i
(
i) In cation–radical 1 , the structure of both D moieties is
almost identical (though there is no crystallographic symmetry
between them). This structure is essentially intermediate
between geometries of the neutral D and the cationic D
q = (d Ϫ d )/(d Ϫ d )
(1)
i
0
i
0
1
ϩ
ؒ
moiety, d and d are values for a bond length in neutral D and
0
1
ϩ
ؒ
group. From the evaluation of the quinonoidal distortion in
Table 3 with the aid of eqn. (1), we conclude that there is equal
1
-electron oxidized D , respectively (Table 1), and d is the per-
i
tinent bond length in the oxidized donor. For the purpose of
clarity, we will hereafter focus on only the δ bonds for the calcu-
lation of q owing to their largest bond-length change upon
1
2
ϩ
ؒ
charge (ϩ0.5 e) on D and D in the cation–radical 1 .
(
ϩ
ؒ
ii) In cation–radical 2 , the D moieties are structurally dif-
i
1
1
1
ferent. The geometry of D is closer to that of the cationic
oxidation.
2
structure, whereas D has a more neutral geometry. However,
they both differ significantly from the pure neutral and cationic
geometries. From the evaluation of the quinonoidal distortion
in Table 3, we conclude that there is a positive charge corre-
II. Electrochemical and structural properties of Class I aromatic
donors. Effects of the (poly)phenylene chain
1
2
A pair of D moieties linked directly together, as in donor 1,
sponding to the loss of 0.8 e on D and 0.2 e on D in cation–
ϩ
ؒ
shows two well-resolved reversible (one-electron) oxidation
radical 2 .
12
ϩ
ؒ
1
waves at 1.11 and 1.40 V (Table 2, Fig. 1a). When the two D
(iii) In cation–radical 3 , the geometry of D reproduces
2
moieties are tethered through a single p-phenylene bridge
the standard geometry of a cationic D group, whereas the D
(
1
donor 2) they also show clearly two resolved oxidation waves at
.15 and 1.26 V (Fig. 1b). These two 1-electron waves coalesce
moiety does not exhibit any geometric changes relative to the
neutral D group. From the evaluation of the quinonoidal dis-
1
into a single 2-electron wave at 1.18 V in donor 3 having two
p-phenylene groups in the bridge (Fig. 1c). Further incorpor-
ation of p-phenylene units, as in donors 4, 5, does not change
the simple electrochemical pattern of donor 3 (Table 2).
tortion in Table 3, we conclude that D is the full cation–radical
ϩ
ؒ
2
(D ), and D is equivalent to the unperturbed donor.
The magnitude of the interaction between the D moieties can
be also quantified from electrochemical data if we take the
Structurally, the D moieties in the bridged donors 1, 2 and
their cation–radicals 1 , 2 and 3 (Tables 3 and 4) can be
observed splitting of the oxidation waves (∆E ) as a qualitative
indicator of the resonance energy of this interaction. As
ox
ϩ
ؒ
ϩ
ؒ
ϩ
ؒ
13a
J. Chem. Soc., Perkin Trans. 2, 2001, 1585–1594
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ϩ
ؒ
Table 3 Quinonoidal distortion and positive charge distribution between D moieties in D–B–D cation–radicals
a
b
σc
d
Donor
Group
δ /Å
δ /Å
δ /Å
qi
av
1
2
1
2
2
1
D –D
D , D
1.373
1.341
1.344
1.368
1.331
1.363
1.325
1.370
1.351
1.362
1.379
1.356
1.356
1.375
1.344
1.369
1.324
1.385
1.365
1.368
1.376
1.349
1.350
1.371
1.337
1.366
1.325
1.377
1.358
1.365
0.002
0.003
0
ϩ
ؒ
1
2
ϩ
ؒ
1
1
[D –D ]
D
ϩ0.5
ϩ0.5
0
ϩ0.8
ϩ0.2
ϩ1.0
0.0
2
D
1
2
1
2
2
D –(p-C H )–D
D , D
0.004
0.002
6
4
ϩ
ϩ
ϩ
ؒ
ؒ
ؒ
1
2
ϩ
ؒ
1
[D –(p-C H )–D ]
D
6
4
2
D
1
2
ϩ
ؒ
1
3
9
[D –(p-C H ) –D ]
D
0.004
0.003
6
4 2
2
D
1
D
ϩ0.35
ϩ0.2
2
D
a
b
c
d
With ortho-methoxy group. With meta-methoxy group. Precision of structure. Charge estimated from δav values using eqn. (1).
Ϫ1
Fig. 2 Linear relationship between resonance energy ∆Eox (kcal mol
)
2
and amount of positive charge q distributed in the D moiety in cation–
i
ϩ
ؒ
ϩ
ؒ
ϩ
ؒ
radicals 1 , 2 and 3
.
such, the resonance energy increases from 0 to 2.5 and 6.7 kcal
Ϫ1
ϩ
ؒ
ϩ
ؒ
ϩ
ؒ
mol in a series 3 → 2 → 1 and appears to be pro-
portional to the amount of positive charge q transferred
i
13b
between D units (Fig. 2).
To better understand the mechanism of the interaction
ϩ
ؒ
between the D and D units, we examined other structural
changes accompanying the oxidation of the donors 1–3. Most
importantly, there are some remarkable changes in the overall
molecular shapes as follows.
Fig. 3 Planarization of dihedral angle and contraction of bond length
(
i) The essentially nonplanar neutral donor 1 exhibits sub-
stantial planarization as a result of its 1-electron oxidation
Fig. 3). The twist angle φ around the central C(Ar)–C(Ar) bond
1
2
between D and D of donor 1 owing to 1-electron oxidation: (top)
ORTEP diagram of the neutral donor 1, (bottom) ORTEP diagram of
ϩ
ؒ
(
the corresponding cation–radical 1 . Thermal ellipsoids are given at
0% probability level.
5
is reduced from 69.1 to 39.5Њ with an accompanying shortening
of this bond from 1.491 to 1.458 Å.
(
ii) Donor 2 is less sterically hindered than 1 and thus less
twisted around its two symmetrically equivalent C(Ar)–C(Ar)
bonds by 44.9Њ. During 1-electron oxidation, the twist is
reduced even more to φ = 32.0Њ and 28.6Њ, with a progressive
shortening of the C(Ar)–C(Ar) bonds from 1.493 Å to 1.466
and 1.474 Å, respectively (Fig. 4b and c). Noticeably, the D
group with more cationic geometry (see above) gives the shorter
C–C bond with the p-phenylene bridge.
observed shortening of the central C(Ar)–C(Ar) bond in the
molecule 1 during oxidation corresponds to changes in bond
order from 1.0 to 1.15. This means that the bond becomes 30%
ϩ
ؒ
π-conjugated in 1 (when referred to 100% conjugation in
ϩ
ؒ
benzene). In cation–radical 2 , the degree of π-conjugation
ϩ
ؒ
between the central p-phenylene bridge and the terminal D
and D groups is 20% and 10%, respectively. For cation–radical
ϩ
ؒ
ϩ
ؒ
ϩ
ؒ
ϩ
ؒ
(
iii) The cation–radical 3 has dihedral angles similar to 2
3 , conjugation does not spread further than 15% between D
with φ = 36.4 and 31.4Њ between the D planes and p-phenylene
groups. The dihedral angle between the planes of the
p-phenylene rings is 16.9Њ. The intercyclic C(Ar)–C(Ar) bonds
continuously change from 1.469 to 1.485 and to 1.488 Å on
going from the positively charged to the neutral D moiety.
The co-planarization of aromatic π-moieties accompanied by
shortening of the bonds between them indicates an increasing
and its neighboring p-phenylene group. Beyond that the π-
conjugation is barely detectable.
Additional contraction of the ortho-bonds δ (as compared to
meta-bonds δЈ) is another indicator of π-conjugative interaction
ϩ
ؒ
ϩ
ؒ
ϩ
ؒ
between D and D moieties in 1 and 2 (Table 3). A detailed
consideration of geometry of these groups (Table 4) shows loss
of the local symmetry center and significant contribution of the
ortho-quinonoidal resonance structure QЈ. We thus conclude
π–π-conjugation.
A quantitative degree of the π-conju-
ϩ
ؒ
gation can be calculated through the bond orders of the
that interaction between D and D moieties largely occurs via
π-conjugation. This interaction is weakened by a bridging
14
corresponding C–C bonds using Pauling’s theory. Thus, the
1
588 J. Chem. Soc., Perkin Trans. 2, 2001, 1585–1594

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Fig. 4 Successive planarization of molecular geometry going from
ϩ
ؒ
sterically hindered donor 8 to 2 and to its cation–radical 2 .(a)
ORTEP diagram of almost orthogonal donor 8, (b) ORTEP diagram
of the neutral donor 2, (c) ORTEP diagram of planarized structure of
ϩ
ؒ
cation–radical 2
.
p-phenylene group and becomes almost undetectable when the
bridge is comprised of two or more p-phenylene units.
Since π-conjugation is highly dependent structurally on geo-
metric conditions (values of dihedral φ angles etc.), it can be
both suppressed and promoted by deliberate control of the
molecular conformation which affects the interaction between
ϩ
ؒ
D
and D. To probe this point, we examined the Class II
donors derived from the successive methylation of donor 2.
III. Electrochemical and structural properties of Class II
aromatic donors. Effects of steric hindrance
Aromatic donor 2 shows two (1-electron) oxidation waves that
coalesce into one (2-electron) wave upon addition of one more
p-phenylene unit in the bridge in donor 3. The same effect
can be achieved via successive methylation of the central
p-phenylene bridge in Class II donors (Fig. 5, Table 2). The
introduction of one methyl group shifts the pair of anodic
waves together, from two oxidation potentials of 1.15/1.26 V for
the parent donor 2 to 1.16/1.25 V for the monomethyl analog 6.
The presence of two methyl groups (donor 7) makes the two
waves barely distinguishable in the range 1.17–1.24 V (Fig. 5c)
and these waves finally coalesce at 1.21 V for donor 8 with the
permethylated p-phenylene bridge (Fig. 5d).
J. Chem. Soc., Perkin Trans. 2, 2001, 1585–1594
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ϩ
ؒ
Fig. 6 Comparison of molecular geometry of (a) cation–radical 3
ϩ
ؒ
with localized positive charge, and (b) related cation–radical 9 with
delocalized electronic structure.
IV. Electrochemical and structural properties of Class III
aromatic donors. Effects of improved ꢀ-conjugation
ϩ
ؒ
If the existing chain of π-conjugation in cation–radical 2 can
be broken by the introduction of steric hindrance, can the
ϩ
ؒ
broken chain of π-conjugation in cation–radical 3 then be
restored by any deliberate means?
The Class III donors represent analogs of compound 3
where the central (bis)phenylene bridge has been structurally
modified to achieve a better degree of π-conjugation through it.
In particular, the introduction of π-conductive groups with
smaller degrees of steric hindrances at the juncture of the two
p-phenylene bridging units has been considered. However, the
introduction of either ethylene (11) or acetylene (12) bridges as
well as an oxide bridge (13) did not result in any detectable
conductivity of the bridge. All these compounds show the
same electrochemical behavior as the parent donor 3 (and the
model donor 10 with an "insulating" bis-methylene bridge), in
each case a single 2-electron (reversible) oxidation wave was
observed at 1.17–1.18 V vs. SCE (Table 2).
Importantly, an additional –CMe – link introduced between
2
Fig. 5 Progressive coalescence of the two oxidation waves of donor 2
following mono-, di-, and tetramethyl substitution of the single
p-phenylene bridge (a, b, c, and d for donors 2, 6, 7, and 8 respectively).
the two p-phenylene units of donor 3 afforded surprising
results. For the fluorenyl-bridged compound 9 a pair of resolv-
able 1-electron (reversible) oxidation waves is observed at 1.13
and 1.19 V (Table 2, Fig. 1d) instead of the single (2-electron)
wave at 1.18 V (Fig. 1c).
Ϫ1
ϩ
ؒ
ϩ
ؒ
The resonance energy of 2.5 kcal mol in cation–radical 2
The X-ray structural study of cation–radical 9 shows the
geometry of both D moieties to be different, and intermediate
between the neutral and cationic geometries (Tables 3 and 4).
Ϫ1
ϩ
ؒ
Ϫ1
drops to 2.1 kcal mol in cation–radical 6 , 1.6 kcal mol in
ϩ
7
ؒ
ϩ
ؒ
, and to almost zero in 8 . The steady fall of the resonance
ϩ
ؒ
energy with successive methylation of the bridging p-phenylene
can be attributed to increasing steric hindrance. We also see
that the twist between the D moiety and the neighboring
p-phenylene bridge is reduced during oxidation from φ = 45 to
φ = 30Њ (Fig. 4b and 4c) in donor 2 to facilitate the increasing
π-conjugation (cosφ increases from 0.71 to 0.87, respectively).
The structure of donor 8 with the permethylated p-phenylene
bridge reveals an almost orthogonal orientation of the groups
However, unlike cation–radical 2 , both D moieties gravitate
more to the neutral than to the cationic geometry. Interestingly,
ϩ
ؒ
the dihedral angles are not very different in cation–radical 9
ϩ
ؒ
as compared with its parent cation–radical 3 . The angles
between D groups and neighboring benzene rings are φ = 30.4
and 29.6Њ, and the angle between the former p-phenylene units
within the bridging fluorenyl moiety is φ = 5.7Њ (cf. Fig. 6a and
6b). However, the intercyclic C(Ar)–C(Ar) bonds exhibit much
ϩ
ؒ
(
Fig. 4a) with φ = 81.3Њ that prohibits any significant π–π-
more pronounced shortening than in 3 going to 1.464 and
ϩ
ؒ
interaction (cosφ = 0.15). The short intramolecular contacts
between ortho-substituents (Me ؒ ؒ ؒ OMe of 3.45 and 4.00 Å)
do not allow this value to be reduced in the corresponding
1.471 Å (cf. similar values of 1.466 and 1.474 Å in 2 ). More-
over, the most affected central bond within the fluorenyl moiety
is shortened to 1.433 Å. Thus, the introduction of a chemical
bridge between two ortho-centers in the bridging biphenylene
moiety causes dramatic changes in π-conjugation in 9 as
compared with the parent cation–radical 3 . Structural and
ϩ
ؒ
cation–radical 8 .
ϩ
ؒ
Such an effect is expected from the interaction between D
ϩ
ؒ
ϩ
ؒ
and D groups through a π-conjugated chain. The interaction
is optimal for more or less unperturbed (coplanar) molecular
conformations and fades with an increasing molecular twist.
The electrochemical behavior of Class II donors strongly
supports this point.
ϩ
ؒ
electrochemical data suggest that cation–radical 9 possesses
(a) substantial resonance energy of 1.4 kcal mol and (b)
Ϫ1
15–20Њ π-conjugation between the D moieties and the fluorenyl
bridge (the degree of π-conjugation between the former
p-phenylenes can be estimated as almost 60%).
We next examined Class III donors in order to determine the
role of the π-conjugation in (poly)phenylene donors, which
cannot be broken but can be promoted for optimal interaction
It is important to note that the quinonoidal distortion in
ϩ
ؒ
the cation–radical 9 (Table 3) leads to only ϩ0.35 and ϩ0.2
ϩ
ؒ
between D and D groups as described below.
fractional positive charge on the terminal D groups, the
1
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remainder (ϩ0.45 e) being associated with the fluorenyl bridge.
Hewlett-Packard 5890A gas chromatograph equipped with a
ϩ
ؒ
ϩ
ؒ
In neither 2 nor in 3 could we detect any measurable frac-
tion of positive charge associated with bridging p-phenylene
groups. The observed charge distribution shows that in the
fluorenyl moiety the p-phenylene groups lose their identity and
their electron system acts jointly as a better donor than the
parent p-biphenylene. We conclude that a longer (poly)-
phenylene chain with the forced planarity should possess good
electric hole-conductive properties and behave as a single
superdonor entity.
HP 3392 integrator. GC-MS analyses were carried out on a
Hewlett-Packard 5890 gas chromatograph interfaced to a HP
5970 mass spectrometer.
Synthesis
The synthesis of the polyphenylenes in Chart I was carried out
via the coupling of 2,5-dimethoxy-4-methylphenylmagnesium
bromide (or iodide) with the requisite aryl bromide in the
presence of bis(triphenylphosphine)palladium() chloride as
21
a catalyst.
Summary and conclusions
Aromatic donors of general formula D–B–D [where D is a
donor 2,5-dimethoxy-4-methylphenyl group and B is a bridging
General procedure. A solution of 2,5-dimethoxy-4-methyl-
magnesium bromide was prepared from 4-bromo-2,5-
16
dimethoxytoluene (4.62 g, 20 mmol) and magnesium turnings
1.44 g, 60 mmol) in anhydrous tetrahydrofuran (30 mL) under
(
poly)phenylene moiety] have been studied structurally and
(
electrochemically. In the corresponding cation–radicals (D–B–
ϩ
ؒ
ϩ
ؒ
an argon atmosphere. The Grignard reagent was transferred by
a hypodermic syringe to a Schlenk flask containing a mixture of
D ), an electronic interaction occurs between D and D
groups through the chain of π-conjugation involving the bridg-
ing group B.
1
,4-dibromobenzene (2.12 g, 9 mmol) and a catalytic amount
of bis(triphenylphosphine)palladium() chloride (50 mg, 0.08
mmol) in anhydrous tetrahydrofuran (20 mL) under an argon
atmosphere. The pale yellow reaction mixture was refluxed for
A single bridging p-phenylene unit exhibits variable conduc-
tivity that is limited by the conformation (twist) of the molec-
ϩ
ؒ
ular chain. However, the interaction between D and D groups
does not propagate through two or more p-phenylene groups.
To achieve a conductivity through the bis(p-phenylene)
bridge, two p-phenylene units have been brought together in a
1
0–70 h, and then poured into 200 mL of water. The aqueous
phase layer was further extracted with dichloromethane
3 × 100 mL). The combined organic extracts were dried over
(
anhydrous magnesium sulfate, and the solvent was removed
in vacuo. The residue was purified by flash chromatography on
silica gel with hexane, and then hexane–dichloromethane (3 : 1)
as the eluents. The product was further recrystallized from
ethanol to afford the desired polyphenylene 2 (3.0 g, 94%). The
characteristic spectral data for the various polyphenylenes are
given below.
rigid co-planar fashion by a chemical –CMe – link between
2
their ortho-positions. The resulting fluorene-bridged derivative
upon oxidation shows an (almost) uniform distribution of the
positive charge along the π-conjugated molecular chain.
The positive charge distribution in D–B–D donors mimics
electric-hole conductivity in polyphenylene sequences. The
results show that the hole conductivity along polyphenylene
chains is either (a) not possible (beyond the first 1 or 2
elementary units) or (b) requires a relatively high activation
energy. However, the mutual co-planarization of p-phenylene
units using rigid chemical links between them greatly improves
their conductive properties.
4
,4Ј-Dimethyl-2,2Ј,5,5Ј-tetramethoxy-1,1Ј-biphenyl 1. Mp
22 Ϫ1
1
1
6
2
35 ЊC (lit. 133 ЊC); yield: 96%; IR (cm ): 2973, 2367, 2338,
865, 1787, 1460, 1230, 1211, 1072, 975, 926, 842, 757, 715, 703,
36, 612, 539, 412; H NMR (CDCl ): δ 6.82 (s, 2H), 6.78 (s,
H), 3.79 (s, 6H), 3.74 (s, 6H), 2.27 (s, 6H); C NMR (CDCl ):
1
3
13
We hope that UV/Vis spectroscopic and variable temperature
EPR studies presently underway will provide further insight
into the mechanism of such an electronic transduction.
3
δ 151.4, 150.6, 126.5, 125.3, 114.9, 113.8, 56.6, 56.0, 16.4; MS
ϩ
ϩ
(
(
m/z): 303 (M ϩ 1, 17%), 302 (M , 100), 287 (29) 272 (25), 257
22), 256 (40).
Experimental
4
,4Љ-Dimethyl-2,2Љ,5,5Љ-tetramethoxy-1,1Ј:4Ј,1Љ-terphenyl 2.
Ϫ1
Mp 188 ЊC; yield: 94%; IR (cm ): 2943, 2827, 2367, 2337, 1538,
501, 1465, 1393, 1211, 1047, 848, 799, 726, 624, 533, 460, 442,
12; H NMR (CDCl ): δ 7.58 (s, 4H), 6.88 (s, 2H), 6.83 (s, 2H),
.83 (s, 6H), 3.78 (s, 6H), 2.17 (s, 6H); C NMR (CDCl ):
δ 151.8, 150.2, 137.1, 129.1, 128.2, 126.5, 114.9, 113.2, 56.4,
6.0, 16.3; MS (m/z): 379 (M ϩ 1, 26%) 378 (M , 100), 363
12), 349 (11), 348 (45), 333 (12); Anal. Calcd. for C H O : C,
6.19; H, 6.88. Found: C, 76.11; H, 6.91%.
Materials
1
4
3
Methyl-p-hydroquinone, 2,5-dibromo-p-xylene, 4,4Ј-dibromo-
biphenyl, bis(triphenylphosphine)palladium() chloride, tri-
ethyloxonium hexachloroantimonate, and tetra-n-butyl-
ammonium hexafluorophosphate from Aldrich, Acros, or
Alfa were used without further purification. 2,5-Dimethoxy-
1
3
13
3
ϩ
ϩ
5
(
7
24
26
4
1
5
16
toluene,
4-bromo-2,5-dimethoxytoluene,
4-bromo-
1
7
17
17
biphenyl, 4-bromoterphenyl, 4,4Љ-dibromoterphenyl, 9,9-
1
8
17
dimethylfluorene,1
dibromodurene,
4,4Ј-dibromostilbene,
2,7-dibromo-9,9-
4
,4ЉЈ-Dimethyl-2,2ЉЈ,5,5ЉЈ-tetramethoxy-1,1Ј:4Ј,1Љ:4Љ,1ЉЈ-
7
19a
dimethylfluorene,
4,4Ј-dibromo-
Ϫ1
quaterphenyl 3. Mp 192 ЊC; yield: 73%; IR (cm ): 3001, 2924,
840, 2362, 2341, 1497, 1462, 1391, 1216, 1047, 1005, 864, 850,
36, 822, 801, 674; H NMR (CDCl ): δ 7.63–7.72 (m, 8H), 6.90
s, 2H), 6.86 (s, 2H), 3.86 (s, 6H) 3.80 (s, 6H), 2.30 (s, 6H);
NMR(CDCl ): δ 151.9, 150.2, 139.5., 137.6, 129.8, 127.9, 126.8,
26.7, 114.9, 113.1, 56.4, 56.0, 16.3; MS (m/z): 455 (M ϩ 1,
4%), 454 (M , 100), 424 (23), 227 (12), 212 (15), 204 (12);
Anal. Calcd. for C H O : C, 79.30; H, 6.61. Found: C, 79.35;
1
9b
19c
tolan,
and 2,5-dimethyl-1,4-dimethoxybenzene
the
1,3-bis(2,5-dimethoxy-4-methylphenyl)propane
B,
2
8
(
1
9d
A
as well as
1
3
2,3,8,9-tetrahydro-1,1,4,4,7,7,10,10-octamethyltetracene
13
C
ϩ
ؒ
4
radical–cation (OMN ) were prepared according to the liter-
ature procedures. Dichloromethane, toluene, and tetrahydro-
furan were purified according to published procedures. All of
3
ϩ
1
3
20
ϩ
1
the compounds were characterized by melting points, IR, H
30
30
4
13
NMR, C NMR, MS and elemental analysis.
H, 6.62%.
Instrumentation
4
,4ЉЉ-Dimethyl-2,2ЉЉ,5,5ЉЉ-tetramethoxy-1,1Ј:4Ј,1Љ:4Љ,1ЉЈ:4ЉЈ,
1
Ϫ1
The H NMR spectra were recorded in CDCl on a General
1ЉЉ-quinquiphenyl 4. Mp 203 ЊC; yield: 46%; IR (cm ): 2930,
3
Electric QE-300 NMR spectrometer and the chemical shifts are
reported in ppm units downfield from internal tetramethyl-
silane. Infrared spectra were recorded on a Nicolet 10 DX FT
spectrometer. Gas chromatography was performed on a
2840, 2367, 2331, 1495, 1477, 1393, 1217, 1047, 1005, 866, 848,
1
829, 805, 757, 708, 666, 533, 484, 430; H NMR (CDCl ):
3
δ 7.55–7.74 (m, 12H), 6.90 (s, 2H), 6.86 (s, 2H), 3.86 (s, 6H),
13
3.80 (s, 6H), 2.30 (s, 6H); C NMR (CDCl ): δ 151.9, 150.2,
3
J. Chem. Soc., Perkin Trans. 2, 2001, 1585–1594
1591

View Article Online
1
1
6
39.8, 139.1, 137.7, 129.8, 127.9, 127.4, 126.7, 126.6, 114.9,
13.1, 56.4, 56.1, 16.3; Anal. Calcd. for C H O : C, 81.51; H,
.42. Found: C, 81.56; H, 6.42%.
(CDCl ): δ 7.59 (s, 4H), 7.58 (s, 4H), 7.20 (s, 2H), 6.87 (s, 2H),
6.85 (s,2H), 3.86 (s, 6H), 3.79 (s, 6H), 2.32 (s, 6H); C NMR
3
13
36
34
4
(CDCl ): δ 151.9, 150.2, 138.0, 136.0, 129.7, 128.3, 128.1, 126.8,
3
ϩ
1
4
26.2, 115.2, 113.0, 56.5, 56.1, 16.3; MS (m/z): 481 (M ϩ 1,
0%) 480 (M , 100), 240 (34), 239 (20), 233 (15), 225 (60), 218,
ϩ
4
,4ЉЉЈ-Dimethyl-2,2ЉЉЈ,5,5ЉЉЈ-tetramethoxy-1,1Ј:4Ј,1Љ:4Љ,
23
1
ЉЈ:4ЉЈ,1ЉЉ:4ЉЉ,1ЉЉЈ-sexiphenyl 5. Mp 228 ЊC; yield 85%; IR
Ϫ1
(
23); Anal. Calcd. for C H O : C, 80.00; H, 6.67. Found: C,
32 32 4
(
cm ): 3027, 2997, 2943, 2918, 2840, 2827, 2373, 2349, 1489,
7
9.15; H, 6.89%.
1,2-Bis(2Ј,5Ј-dimethoxy-4Ј-methylbiphenyl-4-yl)acetylene 12.
1
5
1
6
471, 1393, 1277, 1211, 1047, 1005, 860, 817, 799, 733, 714, 660,
1
63, 508, 484, 460, 436; H NMR (CDCl ): δ 7.64–7.74 (m,
3
1
6H), 6.89 (s, 2H), 6.85 (s, 2H), 3.86 (s, 6H), 3.80 (s, 6H), 2.31 (s,
Mp 195 ЊC; yield: 52%; H NMR (CDCl ): δ 7.55–7.62 (m, 8H),
3
13
13
H); C NMR (CDCl ): δ 151.9, 150.2, 150.0, 139.9, 139.5,
3
6.85 (s, 4H), 3.86 (s, 6H), 3.78 (s, 6H), 2.31 (s, 6H); C NMR
CDCl ): δ 152.1, 150.3, 138.7, 131.3, 129.4, 127.2, 122.0, 115.3,
1
39.1, 137.8, 129.8, 127.9, 127.4, 127.3, 126.7, 115.0, 113.1,
(
3
ϩ
5
6.4, 56.0, 16.3; Anal. Calcd. for C H O : C, 83.17; H, 6.27.
42 38 4
113.0, 89.9, 56.5, 56.1, 16.3; MS (m/z): 479 (M ϩ 1, 35%) 478
ϩ
Found: C, 82.71; H, 6.25%.
(
M , 100), 239 (20), 233 (15), 225 (60), 218 (23), 207 (40); Anal.
Calcd. for C H O : C, 80.33; H, 6.28. Found: C, 81.05; H,
32
30
4
2
Ј,4,4Љ-Trimethyl-2,2Љ,5,5Љ-tetramethoxy-1,1Ј:4Ј,1Љ:4Љ,1ЉЈ-
1
6
.23%.
terphenyl 6. Mp 190–191 ЊC; yield: 57%; H NMR (CDCl ):
3
δ 7.43–6.83 (m, 7H), 3.86 (s, 3H), 3.82 (s, 6H), 3.76 (s, 3H), 2.32
4
,4Ј-Bis(2Ј,5Ј-dimethoxy-4Ј-methylphenyl)bibenzyl
10.
According to the literature procedure, a mixture of 100 mg
0.2 mmol) of 12 and 100 mL of ethyl acetate was placed in a
13
(
s, 3H), 2.31 (s, 3H), 2.43 (s, 3H); C NMR (CDCl ): δ 151.8,
24
3
1
1
5
51.5, 150.4, 150.2, 137.5, 137.3, 136.5, 130.6, 130.0, 128.5,
26.7, 126.2, 115.0, 114.8, 114.7, 114.2, 113.7, 113.4, 56.4, 56.2,
6.1, 56.0, 20.2, 16.4, 16.3; MS (m/z): 392 (M , 11%), 181 (14),
(
standard Paar bottle along with 20 mg of 10% palladium on
carbon catalyst and the mixture hydrogenated at an initial
pressure of 60 psi. It was recrystallized from ethanol–dichloro-
methane to give 99 mg (98%) of 10 as a white solid: mp
96–197 ЊC; H NMR (CDCl ): δ 7.52 (d, J 7.8 Hz, 4H), 7.34 (d,
J 8.1 Hz, 4H), 6.87 (s, 2H), 6.85 (s, 2H), 3.85 (s, 6H), 3.78 (s,
H), 3.03 (s, 4H), 2.30 (s, 6H); C NMR (CDCl ): δ 151.9,
50.2, 140.5, 136.4, 129.5, 128.5, 128.1, 126.4, 115.1, 113.3,
6.5, 56.1, 37.7, 16.3; MS (m/z): 482 (M , 10%), 242 (16), 241
ϩ
1
73 (10), 147 (22), 74 (9), 73 (100); Anal. Calcd. for C H O :
25
28
4
C, 76.53; H, 7.14. Found: C, 75.97; H, 7.14%.
1
1
3
2
Ј,4,4Љ,5Ј-Tetramethyl-2,2Љ,5,5Љ-tetramethoxy-1,1Ј:4Ј,1Љ:4Љ,
Ϫ1
1
2
1
ЉЈ-terphenyl 7. Mp 196 ЊC; yield: 17%; IR (cm ): 2949, 2910,
852, 2827, 2373, 2337, 1520, 1495, 1459, 1399, 1296, 1205,
181, 1053, 1035, 860, 799, 769, 720, 484; H NMR (CDCl ):
13
6
1
5
(
7
3
1
3
ϩ
δ 7.43 (s, 2H), 6.96 (s, 2H), 6.91 (s, 2H), 3.97 (s, 6H), 3.92 (s,
100), 226 (9), 211 (24); Anal. Calcd. for C H O : C, 79.67; H,
.05. Found: C, 79.59; H, 6.83%.
1
3
32 34 4
6
1
1
H), 2.47 (s, 6H), 2.32 (s, 6H); C NMR (CDCl ): δ 151.4,
3
50.2, 137.4, 133.8, 131.3, 128.4, 126.0, 113.9, 113.7, 56.1, 55.9,
9.5, 16.3; MS (m/z): 407 (M ϩ 1, 26%) 406 (M , 100), 376
ϩ
ϩ
Bis(2Ј,5Ј-dimethoxy-4Ј-methylbiphenyl-4-yl) ether 13. Mp
97 ЊC; yield: 87%; H NMR (CDCl ): δ 7.53 (d, 4H, J 7.8 Hz),
.11 (d, 4H, J 7.8 Hz), 6.83 (s, 4H), 3.84 (s, 6H), 3.77 (s, 6H),
.39 (s, 6H); C NMR (CDCl ): δ 151.8, 150.1, 133.6, 130.7,
(
19); Anal. Calcd. for C H O : C, 76.85; H, 7.39. Found: C,
26 30 4
1
1
7
2
1
3
7
6.81; H, 7.43%.
Ј,3Ј,4,4Љ,5Ј,6Ј-Hexamethyl-2,2Љ,5,5Љ-tetramethoxy-1,1Ј:4Ј,
13
3
2
27.8, 126.5, 118.5, 115.0, 113.1, 56.4, 56.1, 16.3; MS (m/z): 471
1
Љ:4Љ,1ЉЈ-terphenyl 8. Yield 12%; the cis and trans isomers were
ϩ
ϩ
(
M ϩ 1, 35%) 470 (M , 100), 239 (20), 243 (48).
separated by flash chromatography on silica gel with dichloro-
methane–hexane (1:9 v/v) and characterized separately: cis-
isomer: mp 190 ЊC; IR (cm ): 2997, 2949, 2930, 2840, 2367,
Ϫ1
Cyclic voltammetry
2
8
2
6
1
2
1
337, 1508, 1471, 1417, 1393, 1284, 1205, 1181, 1047, 1005, 878,
Cyclic voltammetry (CV) was performed on a BAS 100A
Electrochemical Analyzer with a cell of airtight design with
high vacuum Teflon valves and Viton O-ring seals to allow
an inert atmosphere to be maintained without contamination
by grease. The working electrode consisted of an adjustable
platinum disk embedded in a glass seal to allow periodic polish-
ing (with a fine emery cloth) without significantly changing the
1
05, 755, 714, 690, 672, 490, 448; H NMR (CDCl ): δ 6.81 (s,
3
H), 6.58 (s, 2H), 3.77 (s, 6H), 3.69 (s, 6H), 2.30 (s, 12H), 1.97 (s,
13
H); C NMR (CDCl ): δ 151.6, 150.4, 137.4, 132.2, 129.1,
3
25.6, 114.6, 113.6, 56.3, 56.1, 17.9, 16.4; trans-isomer: mp
Ϫ1
05 ЊC; IR (cm ): 2996, 2842, 2368, 1508, 1471, 1417, 1393,
1
284, 1205, 1181, 1047, 1005, 878, 805, 755, 714, 690; H NMR
2
(
CDCl ): δ 6.82 (s, 2H), 6.62 (s, 2H), 3.78 (s, 6H), 3.74 (s, 6H),
3
surface area (~1 mm ). The saturated calomel electrode (SCE)
13
2
1
.31 (s, 12H), 1.97 (s, 6H); C NMR (CDCl ): δ 151.6, 150.4,
37.1, 132.0, 129.0, 125.4, 114.2, 113.7, 56.4, 56.0, 17.9, 16.4;
3
and its salt bridge were separated from the cathode by a sintered
glass frit. The counter electrode consisted of a platinum gauze
that was separated from the working electrode by ~3 mm. The
measurements were carried out in a solution of 0.1 M support-
ing electrolyte (tetra-n-butylammonium hexafluorophosphate)
ϩ
ϩ
cis–trans mixture MS (m/z): 435 (M ϩ 1, 27%) 434 (M , 100),
4
04 (13), 217 (13); Anal. Calcd. for C H O : C, 77.42; H, 7.83.
28 34 4
Found: C, 77.71; H, 7.83%.
Ϫ4
and 5 × 10 M compound in dry dichloromethane under an
2
,7-Bis(4Ј-methyl-2Ј,5Ј-dimethoxyphenyl)-9,9-dimethyl-
Ϫ1
argon atmosphere. All cyclic voltammograms were measured at
fluorene, 9. Mp 213 ЊC; yield: 69%; IR (cm ): 3051, 2991, 2955,
Ϫ1
the same sweep rate of 2 V s with iR compensation. The
2
1
7
930, 2840, 2834, 2373, 2343, 1514, 1465, 1399, 1374, 1328,
296, 1259, 1211, 1175, 1150, 1047, 1005, 878, 860, 821, 781,
45, 720, 678, 661, 496, 448; H NMR (CDCl ): δ 7.76 (d, J 7.8
potentials were referenced to SCE which was calibrated with
Ϫ4
added ferrocene (5 × 10 M). Controlled-potential coulometry
1
3
was conducted with an EG&G Princeton Applied Research
Hz, 2H), 7.63 (s, 2H), 7.55 (d, J 7.8 Hz, 2H), 6.92 (s, 2H), 6.86
(
PAR) 173 potentiostat and digital coulometer. The number of
13
(
s, 2H), 3.87 (s, 6H), 3.80 (s, 6H), 2.32 (s, 6H), 1.57 (s, 6H);
C
electrons transferred was calculated from the relation n = Q/Fm,
where F is the Faraday constant, m is the moles of the material,
and Q is the coulomb reading at the time the current dropped to
NMR (CDCl ): δ 153.6, 151.9, 150.1, 137.8, 137.4, 128.8, 128.2,
3
1
26.5, 123.7, 119.5, 115.1, 113.3, 56.5, 56.1, 46.9, 27.2, 16.3; MS
ϩ ϩ
(
m/z): 495 (M ϩ 1, 39%) 494 (M , 100), 479 (15), 449 (12), 247,
<3–9% of its original value. The oxidation potentials Eox are
(
19), 232 (20), 217 (17); Anal. Calcd. for C H O : C, 80.16; H,
33
34
4
listed in Table 1.
6
.88. Found: C, 80.13; H, 6.99%.
Chemical oxidation via electron exchange with aromatic cation
radicals
trans-4,4Ј-Bis(2Ј,5Ј-dimethoxy-4Ј-methylphenyl)stilbene 11.
Ϫ1
Mp 197 ЊC; yield: 30%; IR (cm ): 2990, 2828, 2366, 2332, 1521,
1
1
492, 1465, 1393, 1374, 1211, 1046, 835, 702, 593; H NMR
A 1 cm quartz cuvette equipped with a Schlenk adaptor was
1
592 J. Chem. Soc., Perkin Trans. 2, 2001, 1585–1594

View Article Online
charged with a freshly prepared solution of a donor with
17125 reflections (10303 unique) with 2θ ≤ 72.2Њ, 271 variables
ϩ
ؒ
Ϫ
0
15,20
2
OMN SbCl6 (E
= 1.34 V vs. SCE
) generated in situ
refined to R = 0.045 [7229 observed data, I ≥ 2σ(I )], wR(F ) =
red
Ϫ3
in anhydrous dichloromethane from the neutral OMN and
0.070, ∆ρmin/∆ρmax = Ϫ0.99/0.82 e Å . Donor 2. C H O ,
24
26
4
¯
NOSbCl . The solution immediately took on a greenish yellow
M = 378.45, triclinic, space group P1, a = 5.0760(3), b =
13.9623(9), c = 14.3411(9) Å, α = 77.796(2), β = 88.587(2),
γ = 88.641(2)Њ, U = 992.9(1) Å . Z = 2, D = 1.266 g cm ,
6
coloration and the UV-Vis-NIR absorption spectrum was
recorded in a Cary UV-Vis-NIR Spectrometer.
3
Ϫ3
x
Ϫ1
µ = 0.085 mm , 10726 reflections (5993 unique) with 2θ ≤ 62Њ,
Diffuse-reflectance spectroscopy
295 variables refined to R = 0.099 [2535 observed data,
2
Ϫ3
I ≥ 2σ(I )], wR(F ) = 0.233, ∆ρ /∆ρ = Ϫ0.46/0.60 e Å .
min
max
The crystals of cation–radical hexachloroantimonate salts were
mixed with potassium hexafluorophosphate (KPF6) and
ground to a fine powder to form a 20 wt% dispersion, and
stored in a 1 mm quartz cuvette. The diffuse-reflectance spectra
were collected in a Cary 500 UV-Vis-NIR Spectrometer. The
spectra were presented as percentage absorption (%ABS) as
defined by %ABS = 100(1 Ϫ R/R ) with R and R representing
the intensities of the diffuse-reflected probe light and of the
reference light, respectively.
ϩ
ؒ
ϩ
4
Ϫ
Compound 2 . C H O SbCl , M = 712.90, monoclinic,
24
26
6
space group P2 /c, a = 13.4765(2), b = 15.5512(3), c =
1
3
1
1
3.8393(3) Å, β = 105.944(1)Њ, U = 2788.8(1) Å . Z = 4, D =
x
Ϫ3 Ϫ1
.698 g cm , µ = 1.594 mm , 34861 reflections (12599 unique)
with 2θ ≤ 72.5Њ, 322 variables refined to R = 0.037 [9054
2
observed data, I ≥ 2σ(I )], wR(F ) = 0.071, ∆ρmin/∆ρ
=
max
0
0
Ϫ3
ϩ
ؒ
ϩ
Ϫ
Ϫ 1.48/0.70 e Å . Compound 3 . C H O SbCl , M =
30
30
4
6
7
1
88.99, monoclinic, space group P2 , a = 7.2182(1), b =
1
6.6646(3), c = 13.4314(1) Å, β = 103.884(1)Њ, U = 1568.44(4)
3
Ϫ3
Ϫ1
Å , Z = 2, D = 1.671 g cm , µ = 1.426 mm , 23274 reflections
x
General procedure for X-ray crystallography of the cation–
radicals
(
13253 unique) with 2θ ≤ 71.3Њ, 376 variables refined to
2
R = 0.046 [9677 observed data, I ≥ 2σ(I )], wR(F ) = 0.067,
∆ρmin/∆ρmax = Ϫ0.70/0.90 e Å . Donor 8. C H O , M =
Ϫ3
A greenish yellow solution of triethyloxonium hexachloro-
28
34
4
ϩ
Ϫ
antimonate (Et O SbCl ) (1.5 mM) and 1 (1 mM) in dichloro-
434.55, T = 93 (2) K, monoclinic, space group C2/c, a =
22.028(1), b = 7.3676(3), c = 14.2868(6) Å, β = 94.883(1)Њ,
3
6
methane (20 mL) was prepared under an argon atmosphere at
25
3
Ϫ3
Ϫ1
room temperature. The solution was carefully layered with
U = 2310.2(5) Å . Z = 4, D = 1.249 g cm , µ = 0.082 mm ,
x
toluene and refrigerated (Ϫ20 ЊC) to afford dark green single
16277 reflections (5036 unique) with 2θ ≤ 71.2Њ, 225 variables
ϩ
ؒ
Ϫ
2
crystals as of the cation–radical salt (1 ؒSbCl ) suitable for
refined to R = 0.040 [3995 observed data, I ≥ 2σ(I )], wR(F ) =
6
Ϫ3
ϩ
ؒ
the X-ray crystallographic analysis. Single crystals of other
0.116, ∆ρmin/∆ρ = Ϫ 0.23/0.55
e
Å . Compound 9 .
max
Ϫ
ϩ
ؒ
ϩ
ؒ
Ϫ
ϩ
ؒ
Ϫ
ϩ
ؒ
Ϫ
ϩ
cation–radical salts (B
ؒ2SbCl , 2 ؒSbCl , 3 ؒSbCl ,
C H O SbCl ؒ1.76(CH Cl ), M = 978.95, triclinic, space
group P1, a = 13.1454(2), b = 13.4568(3), c = 14.1244(3) Å,
α = 103.375(1), β = 101.858(1), γ = 116.056(1)Њ, U = 2044.97(5)
6 6 6
33 34
4
6
2
2
ϩ
ؒ
Ϫ
¯
and 9 ؒSbCl ) were isolated using the same procedure. Single
6
crystals of the neutral donors, A, B, 1, 2 and 8 were obtained
by slow evaporation of the solvent. The intensity data were
collected with the aid of a Siemens SMART diffractometer
equipped with a CCD detector using Mo-Kα radiation
3
Ϫ3
Ϫ1
Å . Z = 2, D = 1.590 g cm , µ = 1.334 mm , 32329 reflections
x
(16881 unique) with 2θ ≤ 71.2Њ, 491 variables refined to
2
R = 0.043 [11957 observed data, I ≥ 2σ(I )], wR(F ) = 0.089,
Ϫ3
(
λ = 0.71073 Å), at Ϫ150 ЊC unless otherwise specified. The
∆ρmin/∆ρmax = Ϫ1.51/1.61 e Å .
26
structures were solved by direct methods and refined by a full
matrix least-squares procedure with IBM Pentium and SGI O2
computers.‡
Acknowledgements
We thank the R. A. Welch Foundation and the National
Science Foundation for financial support. We also thank
Crystal Data. Donor A. C H O , M = 166.21, monoclinic,
10
14
2
ϩ
ϩ
ؒ ؒ
Dr P. Le Maguères for the X- ray analyses of B and B
.
space group P2 /n, a = 6.4943(4), b = 8.9209(5), c = 8.0240(5) Å,
1
3
Ϫ3
β = 101.82(1)Њ, U = 455.02(5) Å , Z = 2, D = 1.213 g cm , µ =
.083 mm , 4073 reflections (1987 unique) with 2θ ≤ 72.5Њ, 57
x
Ϫ1
0
References
variables refined to R = 0.049 [1593 observed data, I ≥ 2σ(I )],
2
Ϫ3
wR(F ) = 0.120, ∆ρmin/∆ρ = Ϫ0.25/0.53 e Å . Donor B.
max
1 (a) J. M. Tour, Acc. Chem. Res., 2000, 33, 791; (b) J. Roncali,
Acc. Chem. Res., 2000, 33, 147; (c) F. Barigelletti and L. Flamigni,
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Bumm, J. J. Arnold, M. T. Cygan, T. D. Dunbar, T. P. Burgin,
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C H O , M = 344.43, T = 93(2) K, orthorhombic, space
2
1
28
4
group P2 2 2 , a = 7.8130(5), b = 7.9994(5), c = 29.763(2) Å,
1
1 1
3
Ϫ3
Ϫ1
U = 1860.2(2) Å . Z = 4, D = 1.230 g cm , µ = 0.084 mm ,
x
2
3294 reflections (4699 unique) with 2θ ≤ 71.8Њ, 338 variables
2
refined to R = 0.039 [4102 observed data, I ≥ 2σ(I )], wR(F ) =
2
71, 1705; ( f ) A. P. deSilva Ed., Molecular-level Electronics.
Ϫ3
ϩ
ؒ ؒ
ϩ
0
.094, ∆ϩρ ϩm in/∆ρ
= Ϫ0.19/0.40 e Å . Compound B
.
max
In: Electron Transfer in Chemistry, V. Balzani, Ed., vol. 5, Wiley,
New York, 2001.
Ϫ
C H O
2SbCl , M = 1013.33, monoclinic, space group
2
1
28
4
6
P2 /m, a = 8.073(2), b = 26.577(5), c = 8.445(2) Å, β = 98.99(3)Њ,
2 (a) A. S. Lukas, P. J. Bushard and M. R. Wasielewski, J. Am. Chem.
Soc., 2001, 123, 2440; (b) S. Fraysse, C. Coudret and J.-P. Launay,
Eur. J. Inorg. Chem., 2000, 1581; (c) C. P. Collier, G. Mattersteig,
E. W. Wong, Y. Luo, K. Beverly, J. Sampaio, F. M. Raymo,
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de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M. Huxley,
C. P. McCoy, J. T. Rademacher and T. E. Rice, Chem. Rev., 1997, 97,
1515.
1
3
Ϫ3
Ϫ1
U = 1789.6(6) Å . Z = 2, D = 1.881 g cm , µ = 2.432 mm ,
x
2
2003 reflections (8082 unique) with 2θ ≤ 72.1Њ, 190 variables
2
refined to R = 0.047 [5945 observed data, I ≥ 2σ(I )], wR(F ) =
0
Ϫ3
.093, ∆ρmin/∆ρmax = Ϫ2.19/1.37 e Å . Donor 1. C H O ,
18 22 4
M = 302.36, monoclinic, space group C2/c, a = 22.199(1), b =
3
5
.8270(3), c = 12.6965(7) Å, β = 109.85(1)Њ, U = 1544.8(1) Å .
Ϫ3 Ϫ1
3 (a) R. M. Metzger, Acc. Chem. Res., 1999, 32, 950; (b) J. Chen,
Z = 4, D = 1.300 g cm , µ = 0.091 mm , 10207 reflections
x
M. A. Reed, A. M. Rawlett and J. M. Tour, Science, 1999, 286, 1550.
R. Rathore, A. S. Kumar, S. V. Lindeman and J. K. Kochi,
(
3439 unique) with 2θ ≤ 72.2Њ, 103 variables refined to R = 0.058
4
2
[1847 observed data, I ≥ 2σ(I )], wR(F ) = 0.137, ∆ρ /∆ρ
=
min
max
J. Org. Chem., 1998, 63, 5847.
Ϫ3
ϩ
ؒ
ϩ
4
Ϫ
Ϫ0.25/0.39 e Å . Compound 1 . C H O SbCl , M =
6
18
22
6
5 (a) B. Badger and B. Brocklehurst, Trans, Faraday Soc., 1969, 65,
2582 and 2588; (b) B. Badger and B. Brocklehurst, Trans, Faraday
Soc., 1970, 66, 2939; (c) J. K. Kochi, R. Rathore and P. Le Maguéres,
J. Org. Chem., 2000, 65, 6826.
¯
36.81, triclinic, space group P1, a = 7.6596(2), b = 12.8575(3),
c = 13.7700(3) Å, α = 62.369(1), β = 83.190(1), γ = 83.567(1)Њ,
3
Ϫ3
Ϫ1
U = 1190.47(5) Å . Z = 2, D = 1.777 g cm , µ = 1.855 mm ,
x
6
(a) D. K. Kyriacou, Modern Electroorganic Chemistry, Springer,
Berlin, 1994; (b) A. J. Fry, Synthetic Organic Electrochemistry, Wiley,
New York, 1989.
‡
CCDC reference number(s) 162395–162404. See http://www.rsc.org/
suppdata/p2/b1/b103139m/ for crystalographic files in .cif or other
electronic format.
7 F. H. Allen, O. Kennard, D. G. Watson, L. Brammer, A. G. Orpen
and R. Taylor, J. Chem. Soc., Perkin Trans. 2, 1987, S1.
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18 R. G. Harvey, P. P. Fu and P. W. Rabideau, J. Org. Chem., 1976, 41,
2706.
1
1 Although the cation–radical of A was stable in solution (as
19 (a) R. N. McDonald and T. W. Campbell, in Organic Syntheses,
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21 K. Kumada, Pure Appl. Chem., 1980, 52, 669 and references therein.
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established by the reversible CV), it was not sufficiently persistent to
ϩ
ؒ
Ϫ
ϩ
ؒ ؒ
ϩ
allow isolation of the cation–radical salt A SbCl6 . Thus B
ϩ
ؒ
and not A was used to establish the geometrical (bond-length)
change in D upon 1-electron oxidation to the cation–radical D
with q = 1.
ϩ
ؒ
1
1
2 All the aromatic donors in Classes I–III show chemically reversible
cyclic voltammograms with anodic/cathodic peak current ratio in
a
c
the value: 0.9 < i /i < 1.1. The quantitative electrode behavior
p
p
relative to the electrochemical reversibility of these interesting redox
couples will be addressed in a later study.
3 (a) Such a stabilization energy may be described in quantum
mechanical terms, as the electronic coupling element HAB. See, e.g.
C. Creutz, M. D. Newton and N. Sutin, J. Photochem. Photobiol. A:
Chem., 1994, 82, 47; (b) The comproportionation constants (and
hence the free energy changes) between cation–radicals and their
dications will be presented separately.
23 5 was prepared by the coupling between 2,5-dimethoxy-4-methyl-4Љ-
bromo-1,1Ј:4Ј,1Љ-terphenyl and its Grignard derivative.
24 P. G. Gassman and K. T. Mansfield, Org. Synth., 1973, Coll. Vol. V,
96.
25 Note that the UV-Vis-NIR absorption of various polyphenylene
cation–radicals generated by triethyloxonium hexachloro-
ϩ
1
4 (a) L. Pauling, Nature of the Chemical Bond,Cornell University
Press, Ithaca, New York, 1960, p. 280; (b) S. M. Hubig, S. V.
Lindeman and J. K. Kochi, Coord. Chem. Rev., 2000, 200–202, 831.
5 G. S Hiers and F. D. Hager, Org. Synth., 1932, Coll. Vol. I, 58.
antimonate (Et O SbCl ) were identical in all respect to those
3
6
؉
ؒ
generated by OMN SbCl in anhydrous dichloromethane.
6
26 G. M. Sheldrick, SHELXS-86, Program for Structure Solution,
University of Göttingen, Germany, 1986.
1
1
594
J. Chem. Soc., Perkin Trans. 2, 2001, 1585–1594