Neutral organic mixed-valence compounds: Synthesis and all-optical evaluation of electron-transfer parameters

Article abstract of DOI:10.1021/ja068235j

In this paper we present the synthesis as well as a detailed study of the electrochemical and photophysical properties of a series of neutral organic mixed-valence (MV) compounds, 1-7, in which different amine donor centers are connected to perchlorinated

Full text of DOI:10.1021/ja068235j

Published on Web 04/04/2007
Neutral Organic Mixed-Valence Compounds: Synthesis and
All-Optical Evaluation of Electron-Transfer Parameters
Alexander Heckmann and Christoph Lambert*
Contribution from the Institut fu¨r Organische Chemie, UniVersita¨t Wu¨rzburg, Am Hubland,
D-97074 Wu¨rzburg, Germany
Received November 17, 2006; E-mail: lambert@chemie.uni-wuerzburg.de
Abstract: In this paper we present the synthesis as well as a detailed study of the electrochemical and
photophysical properties of a series of neutral organic mixed-valence (MV) compounds, 1-7, in which
different amine donor centers are connected to perchlorinated triarylmethyl radical units by various spacers.
We show that this new class of compounds are excellent model systems for the investigation of electron
transfer due to their uncharged character and, consequently, their excellent solubility, particularly in nonpolar
solvents. A detailed band shape analysis of the intervalence charge-transfer (IV-CT) bands in the context
of Jortner’s theory allowed the electron-transfer parameters (inner vibrational reorganization energy λv,
outer solvent reorganization energy λo, and the difference in the free energy between the diabatic ground
and excited states, ∆G°°, as well as the averaged molecular vibrational mode ν˜v) to be extracted
independently. In this way we were able to analyze the solvatochromic behavior of the IV-CT bands by
evaluating the contribution of each parameter. By comparison of different compounds, we were also able
to assign specific molecular moieties to changes in ν˜v. For this class of molecules, we also demonstrate
that the adiabatic dipole moment difference ∆µab and, consequently, the electronic coupling V12 can be
evaluated directly from the absorption spectra by a new variant of the solvatochromic method. Furthermore,
an investigation of the electrochemistry of compounds 1-7 by cyclic voltammetry as well as spectroelec-
trochemistry shows that, not only in the neutral MV compounds but also in their oxidized forms, a charge
transfer can be optically induced but with exchanged donor-acceptor functionalities of the redox centers.
Introduction
is a measure of the electronic communication between two states
in which the charge is located at either redox center. In this
Organic mixed-valence (MV) compounds are the focus of
recent research because they are simple and suitable model sys-
tems for the investigation of electron-transfer (ET) processes.1-5
Usually these MV systems consist of two redox centers in
different redox states, linked by a saturated or unsaturated bridge
unit. Many investigations have focused on the influence of the
type and the length of the bridge unit (spacer)6-25 or the nature
of the redox centers on the electronic coupling V12. This coupling
respect also multidimensional MV compounds16,26-31 (with three
or more redox centers) as well as symmetrical10-14,17,23-25,29,32-42
(17) Coropceanu, V.; Malagoli, M.; Andre´, J. M.; Bre´das, J. L. J. Am. Chem.
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9
10.1021/ja068235j CCC: $37.00 © 2007 American Chemical Society
J. AM. CHEM. SOC. 2007, 129, 5515-5527
5515

A R T I C L E S
Heckmann and Lambert
(with two degenerate redox centers) and asymmetrical43-45 MV
species (with two non-degenerate redox centers) have been
investigated. The influence of temperature15 and solvent polar-
ity46,47 on electron transfer in organic MV compounds was also
studied. All the above-mentioned MV compounds are either
radical cations or radical anions. However, quite recently,
Veciana et al. synthesized organic-inorganic hybrid MV
compounds consisting of an organic perchlorinated triarylmethyl
radical acceptor and a ferrocene donor center.48-50
While the evaluation of ET parameters in MV compounds is
of prime interest, the determination of these parameters often
involves major inaccuracies and, in particular, possibly errone-
ous assumptions about the effective ET distance. In this paper
we outline a procedure that yields reliable ET parameters within
the given framework of the applied theoretical model. These
parameters were evaluated using a specifically designed set of
suitable MV compounds as described below.
The adiabatic potential energy surfaces (PES) of the ground
state and the excited state of a MV system with two redox states
can be calculated in the context of a two-state model by solving
secular eq 1, where harmonic potentials are used for the diabatic
(non-interacting) states along an ET coordinate x (Figure 1).51-54
In MV compounds the optically induced ET from the ground
state to the excited state is associated with a so-called interva-
lence charge-transfer (IV-CT) band in the near-infrared (Figure
1). Within the framework of generalized Mulliken-Hush
(GMH) theory,54-57 the parameters describing the ET can be
evaluated by analyzing the IV-CT band (eqs 2-4).58-63 In this
Figure 1. Diabatic (dashed blue) and adiabatic (solid red) potential energy
surfaces (PES) of a one-dimensional MV system with two non-degenerate
redox centers. V12 is the coupling energy, λ the Marcus reorganization
energy, and ∆G°° the difference in the free energy between the diabatic
states. In the weak coupling regime (V12 f 0), ν˜max corresponds to the sum
of λ and ∆G°°.
context, in the weak coupling regime (V12 f 0),3,52 the
maximum of the IV-CT band ν˜max corresponds to the sum of
the Marcus reorganization energy λ and the difference between
the free energies of the minima of the diabatic states ∆G°°
(Figure 1). We point out here that the PES in Figure 1 does
apply for the situation of the MV systems outlined below. In
these cases, ∆G°° is in fact so strong compared to the
reorganization energy that no double minimum potential is
present in the ground-state adiabatic potential curve.
The reorganization energy λ can be partitioned into two terms
(eq 5): the outer solvent reorganization energy λo, which
characterizes the energy needed for the reorientation of the
solvent after ET has taken place, and the inner vibrational
reorganization energy λv, describing the energy corresponding
to geometrical changes of the molecule during ET. However, it
is difficult to obtain the values for ∆G°°, λo, and λv separately
by a Marcus-Hush analysis.58-63
(33) Nelsen, S. F.; Weaver, M. N.; Zink, J. I.; Telo, J. P. J. Am. Chem. Soc.
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V11 - E V12
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Soc. 1999, 121, 7108 -7114.
) 0
(1)
|
|
V12
V22 - E
(40) Nelsen, S. F.; Ismagilov, R. F.; Teki, Y. J. Am. Chem. Soc. 1998, 120,
2200-2201.
with
V11 ) λx2
and
V22 ) λ(1 - x)2 + ∆G°°
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J.; Launay, J.-P.; Dive, G.; Dehareng, D. J. Am. Chem. Soc. 1995, 117,
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µabν˜max
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Nano Lett. 2001, 1, 117-120.
V12
)
with
ν˜max ) ∆G°° + λ
(2)
∆µ12
(43) Yano, M.; Ishida, Y.; Aoyama, K.; Tatsumi, M.; Sato, K.; Shiomi, D.;
Ichimura, A.; Takui, T. Synth. Met. 2003, 137, 1275-1276.
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3hcꢀ0 ln 10
9n
ꢀ
µab
)
dν˜
(3)
(4)
(46) Nelsen, S. F.; Trieber, D. A.; Ismagilov, R. F.; Teki, Y. J. Am. Chem. Soc.
2001, 123, 5684-5694.
∫
2
2
2
ν˜
x
2000 π N (n + 2)
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Chem., Int. Ed. 2004, 43, 5266-5268.
∆µ12
)
∆µ2ab + 4µ2ab
x
where E is the eigenvector, λ the Marcus reorganization energy,
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Gu¨tlich, P.; Veciana, J. J. Am. Chem. Soc. 2003, 125, 1462-1463.
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λ ) λo + λv
(5)
x the ET coordinate, ∆G°° the free energy difference between
the adiabatic ground and excited states, µab the transition moment
between the adiabatic states, ν˜max the absorption maximum of
the IV-CT band, ∆µ12 the diabatic dipole moment difference, h
Planck’s constant, c the speed of light, ꢀ0 the permittivity of
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9
5516 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

Neutral Organic Mixed-Valence Compounds
A R T I C L E S
e-SSj
j!
2 (n2 + 2)2
9n
the vacuum, N Avogadro’s number, n the refractive index of
solvent, ꢀ the extinction coefficient, ∆µab the adiabatic dipole
moment difference, λo the outer solvent reorganization energy,
and λv the inner vibrational reorganization energy.
∞
2000 Nπ
3ꢀ0 ln10
1
ꢀ/ν˜ )
µ2ab
×
∑
4πhcλ kT
x
j)0
o
hc(jν˜v + λo - ν˜ + ∆G°°)2
Because both the solvent reorganization energy λo and ∆G°°
depend on the polarity of the solvent, IV-CT bands are expected
to show a solvatochromic behavior. However, because of the
charged character of organic MV compounds, their solubility,
especially in nonpolar solvents, usually is not high enough to
allow the study of the solvent dependence of IV-CT bands in a
broader range of solvents.46 While the transition moment µab
of the IV-CT band can be determined easily with eq 3 by
integration of the reduced absorption band,64 it is much more
complicated to obtain a reliable value for the diabatic dipole
moment difference between the two diabatic states ∆µ12, which
is needed for the evaluation of the electronic coupling V12 via
eq 2. However, the diabatic dipole moment difference ∆µ12 can
be calculated by eq 4 from the difference of the adiabatic dipole
moments ∆µab and the transition moment µab. Unfortunately,
∆µab cannot be obtained directly by a Hush analysis of the IV-
CT band but must be either estimated from the geometrical
distance between the two redox centers or calculated by quantum
chemical methods. Both methods are afflicted with major
inaccuracies,65 and the experimental determination of ∆µab by
means of electro-optical absorption measurements (EOAM)66-70
unfortunately is very complicated for charged species because
of ion migration in the electric field.
All the above-mentioned disadvantages can be circumvented
by using neutral MV systems, a class of organic molecules
which has, to our knowledge, never been investigated in detail
before our quite recent communication on the synthesis of the
first neutral organic MV compound 1.71,72 The neutral character
of 1 facilitated the acquisition of ultraviolet/visible/near-infrared
(UV/vis/NIR) spectra in a broad variety of solvents, ranging
from totally nonpolar (n-hexane) to strongly polar (acetonitrile),
and allowed us to accomplish a band-shape analysis of the IV-
CT band of this MV compound with the aid of a golden rule
expression (eq 6)73 in the context of Jortner’s theory.74-76
exp -
(6)
[
]
4λokT
with Huang-Rhys factor
S ) (λv/ν˜v)
This model, which can be applied to the analysis of both
fluorescence emission76,77 and absorption spectra,74,77,78 is based
on a semiclassical approach that considers a classical solvent
mode and a single high-temperature averaged molecular vibra-
tion mode ν˜v, which is treated quantum chemically.3 Thereby,
the values for ∆G°°, ν˜v, and the reorganization energies λo and
λv can be extracted separately from the absorption spectrum by
a least-squares fit of the IV-CT band varying these four
parameters in eq 6. However, while a Hush analysis is only
accurately applicable for nearly Gaussian-shaped bands, a Jortner
fit is generally limited to unsymmetrical bands, since otherwise
the parameters are dependent and too ambiguous. The main
factors that determine the asymmetry of the IV-CT bands are
the averaged molecular vibration mode ν˜v and the inner
reorganization energy λv. The ratio S ) λv/ν˜v (Huang-Rhys
factor) is a measure of the electron-phonon coupling, i.e., the
number of vibrations generated during the vertical electronic
transition. Large factors produce Gaussian-shaped bands, while
small factors lead to asymmetric absorption bands.79 Thus, only
systems with small Huang-Rhys factors may be analyzed with
Jortner’s theory accurately.
In the following we present the synthesis and electrochemical
properties of a series of neutral MV compounds, 1-7, where
perchlorinated triarylmethyl (PCTM) radical centers are linked
by various spacers to nitrogen donor centers. Furthermore, owing
to the neutral character of these MV compounds, we were able
to perform a detailed investigation of the solvatochromic
behavior of these compounds in the context of Jortner’s theory
which will yield insight into the parameters governing the ET
behavior in 1-7. In addition, we investigated the solvatochromic
behavior of these ET parameters in order to obtain reliable
values for the diabatic dipole moment differences ∆µ12. The
latter will allow us to calculate accurately the electronic coupling
V12 by eq 4 of all compounds 1-7.
(64) Birks, J. B., Ed. Photophysics of Aromatic Molecules; Wiley-Interscience:
New York, 1969; Chapter 3.5-3.7.
(65) While ab initio and semiempirical UHF methods tend to overemphasize
charge localization, DFT calculations usually overemphasize charge delo-
calization; see ref 11 in Lent, C. S.; Isaksen, B.; Liebermann, M. J. Am.
Chem. Soc. 2003, 125, 1056. Nelsen et al. recently demonstrated that the
often used N-N distance in bis(triarylamine) radical cations results in too
large effective ET distances; see ref 34.
Results and Discussion
(66) Wortmann, R.; Kra¨mer, P.; Glania, C.; Lebus, S.; Detzer, N. Chem. Phys.
1993, 173, 99-108.
Synthesis. The last step of the syntheses of all PCTM radicals
is the in situ generation of the corresponding carbanions starting
from the R-H compounds and their oxidation to the radicals
with p-chloranil in dimethylsulfoxide (DMSO). The synthesis
of radical 1 was described previously.71 In order to synthesize
a great number of differently substituted triarylmethyl radicals,
the perchlorinated compound 8 is a useful intermediate (Scheme
1). With this compound 8, we were able to synthesize the donor-
substituted triarylmethyl radicals 2, 5, 6, and 7 with an alkynyl
spacer via Stille coupling, compound 4 with a biphenyl spacer
by a Suzuki coupling, and radical 3 via a Buchwald-Hartwig
(67) Oh, D. H.; Sano, M.; Boxer, S. G. J. Am. Chem. Soc. 1991, 113, 6880-
6890.
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(72) See footnote 19 in ref 71.
(73) We derived eq 6 from eq 4a given in ref 74, where we replaced V2∆µ2 by
(Mνhν)2 (given in their notation). Thus, in contrast to ref 74, we assume a
frequency-independent transition moment Mν, which is an observable, rather
than a frequency-independent product V2∆µ2, which consists of diabatic
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9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5517

A R T I C L E S
Heckmann and Lambert
Scheme 2
Scheme 3
Scheme 4
Scheme 1
amination. The synthesis of compound 8 was achieved by
perchlorination reaction of 980 with the so-called BMC method
of Ballester et al.81 in good yield (Scheme 1).
The donor and acceptor units in 2 were linked by a Stille
coupling82 of the terminal alkyne 1083 and the perchlorinated
triarylmethane 8 (Scheme 2). The low yield may be due to the
steric hindrance of 8. Unfortunately, Hagihara-Sonogashira
coupling reactions84,85 of 8 and 10 using different catalyst
systems gave no better yields. Finally, the R-H compound 11
was transformed into the radical 2 as mentioned above (Scheme
2).
The synthesis of radical 3 is shown in Scheme 3. Starting
from 8, 4,4′-dimethoxydiphenylamine was directly connected
to the perchlorinated ring system by a Pd-catalyzed Buchwald-
Hartwig amination.86,87 Also in this case, the low yield could
be traced back to the steric hindrance of 8. After transformation
into the radical, 3 can be isolated as an auburn solid (Scheme
3).
Radical 4, in which the two redox centers are connected by
a biphenyl spacer, was synthesized by a Suzuki coupling
according to the literature.88,89 Starting from pinacole borane
13,13 the radical precursor 14 was synthesized in 31% yield by
a Pd-catalyzed Suzuki coupling with 8. The somewhat better
yield indicates that the Suzuki coupling is not as strongly
affected by steric effects as the Stille, Hagihara-Sonogashira,
or Buchwald-Hartwig coupling (Scheme 4).
Compounds 5 and 6 were both synthesized in a similar
manner (Scheme 5). The terminal alkynes 15a90 and 15b90 were
coupled to the perchlorinated triarylmethyl compond 8 by a Stille
coupling reaction. The R-H compounds 16a and 16b were
transformed to the corresponding radicals 5 and 6 as described
above.
In Scheme 6, the synthesis of the dimethylamino-substituted
perchlorinated radical 7 is sketched. The synthesis is straight-
forward and analogous to those of radicals 5 and 6.
Electrochemistry. In order to investigate the redox properties
of 1-7, cyclic voltammograms (CVs) in dichloromethane/
tetrabutylammonium hexafluorophosphate (TBAH) solution
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(88) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
(89) Suzuki, A. J. Organomet. Chem. 1999, 576, 147-168.
(90) Elangovan, A.; Chen, T.-Y.; Chen, C.-Y.; Ho, T.-I. Chem. Commun. 2003,
2146-2147.
(86) Harris, M. C.; Buchwald, S. L. J. Org. Chem. 2000, 65, 5327-5333.
(87) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaugnessy, K. H.; Alcazar-
Roman, L. M. J. Org. Chem. 1999, 64, 5575-5580.
9
5518 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

Neutral Organic Mixed-Valence Compounds
A R T I C L E S
being a weaker donor than piperidine or dimethylamine,90,91 the
∆E value of 6 is larger than that of 5 or 7.
Scheme 5
UV/Vis/NIR Spectroscopy. The UV/vis/NIR absorption
spectra of 1-7 in dichloromethane are shown in Figure 2. In
all spectra, an intense band A at ca. 25 500 cm-1 can be
observed which is assigned to a localized absorption of the
perchlorinated ring system.81,92,93 Furthermore, all spectra show
one or two weak absorption bands B and C at about 19 000
cm-1, which are characteristic for PCTM radical systems.81,94
The absorption maxima ν˜max and the corresponding extinction
coefficients ꢀ of these bands are summarized in Table 2. It is
noteworthy that, for 3 and 4, the radical bands B and C are
very weak, which is obviously due to the lack of either alkenyl
or alkynyl spacer units.
Scheme 6
The absorption bands in the NIR region at ca. 12 000 cm-1
are the focus of our interest. These bands can be assigned to an
optically induced ET from the donor center to the triarylmethyl
radical unit. As can be seen from Figure 2 and Table 3, the
extinction coefficients of all these IV-CT bands are similar,
except for radical 4, which shows a much weaker IV-CT band
intensity. This indicates a much lower electronic communication
between the two redox centers as a result of a twist of the
aromatic biphenyl spacer unit caused by the di-ortho-substitution
of one benzene subunit.95 In radical 3, a related effect might be
compensated by the shorter distance between the two redox
centers.
The neutral character of compounds 1-7 and their excellent
solubility gave us the chance to investigate the IV-CT bands in
13 solvents of different polarity, ranging from totally nonpolar
(n-hexane) to strongly polar (acetonitrile). All compounds show
a weak but superficially nonsystematic dependence of ν˜max on
the solvent polarity. On the other hand, we observed a systematic
dependence of the band shape on the polarity of the solvent,
i.e., higher extinction coefficients and smaller bandwidths at
half-height ν˜1/2 in nonpolar solvents such as cyclohexane than
in polar solvents like acetonitrile (Figure 3). Furthermore, the
IV-CT bands show a more or less intense vibronic fine structure
in nonpolar solvents like the hexanes.
Table 1. Redox Potentials of Radicals 1-7 in Dichloromethane/
0.1 M TBAH Solution with Fc/Fc+ as Reference
E1 2(Red1)a/mV
E1 2(Ox1)a/mV
∆E/mV
/
/
1
2
3
4
5
6
7
-670
-590
-650
-645
-590
-580
-570
+240
+315
+445
+305
+365b
+455b
+385b,c
910
905
1095
950
955
1035
955
a All values were determined with a maximum error of (5 mV. b Quasi-
reversible under semi-infinite conditions but irreversible in thin-layer
measurements. c Second oxidation at +540 mV.
In order to understand this behavior, we applied Jortner’s
model (eq 6) to fit the IV-CT bands in each solvent by varying
the parameters for the inner reorganization energy λv and the
solvent reorganization energy λo, as well as the parameters of
the difference in the free energy between the diabatic ground
and excited states ∆G°° and the average molecular vibrational
mode ν˜v. In this way, we achieved an excellent least-squares
fit of the IV-CT bands of compounds 1, 2, and 5-7 (see Figure
4 for a typical example).
were measured (Table 1). The chemical reversibility of all
observed redox processes was also checked by thin-layer
measurements. All donor-substituted compounds 1-7 show one
reversible reduction process at about E1/2 ) -600 mV vs
ferrocene, which is assigned to the reduction of the triarylmethyl
radical to the carbanion. Radicals 1-7 also show at least one
oxidation signal, corresponding to the oxidation of the nitrogen
center to the radical cation. This oxidation is reversible for all
radicals 1-4 with dianisylamino moieties. A signal for the
oxidation of the triarylmethyl radical center was not observed
in any case up to +1000 mV potential. For the dialkyl-
substituted radicals 5, 6, and 7, the amine-based oxidation is
reversible under semi-infinite conditions only. We assume that
R-H elimination processes are responsible for the instability of
the generated radical cations. For radical 7 we observed a second
oxidation signal that also is irreversible in thin-layer experi-
ments.
However, we were unable to fit the IV-CT band of radical 4
in polar solvents because the bands are too symmetrical to obtain
a fit with acceptable maximum errors. For the fit of radical 3,
(91) Effenberger, F.; Fischer, P.; Schoeller, W. W.; Stohrer, W.-D. Tetrahedron
1978, 34, 2409-2417.
(92) Veciana, J.; Armet, O.; Rovira, C.; Riera, J.; Castaner, J.; Molins, E.; Rius,
J.; Miravitlles, C.; Olivella, S.; Brichfeus, J. J. Phys. Chem. 1987, 91, 5608-
5616.
(93) Ballester, M.; Riera, J.; Castaner, J.; Badia, C.; Monso, J. M. J. Am. Chem.
Soc. 1971, 93, 2215-2225.
(94) The UV/vis/NIR spectra of 16a, 16b, and 18 show no absorption in this
region, so we can exclude that the bands at ca. 21 500 cm-1 are due to an
absorption associated with the morpholine, piperidine, or dimethylamino
moieties.
The redox potentials of radicals 5-7 with constant spacer
but varying donors show that the donor unit has no influence
on the reduction of the radical center because of the large
distance between the two redox centers. Owing to morpholine
(95) Low, P. J.; Paterson, M. A. J.; Goeta, A. E.; Yufit, D. S.; Howard, J. A.
K.; Cherryman, J. C.; Tackley, D. R.; Brown, B. J. Mater. Chem. 2004,
14, 2516-2523.
9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5519

A R T I C L E S
Heckmann and Lambert
Figure 2. UV/vis/NIR absorption spectra of radicals 1-7 (black) and of the corresponding anions (red) and cations (blue) in dichloromethane generated by
spectroelectrochemistry.
9
5520 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

Neutral Organic Mixed-Valence Compounds
A R T I C L E S
Table 2. Absorption Maxima and Extinction Coefficients of Bands
A, B, and C of 1-7 in Dichloromethane
knowledge, the first time that a difference in averaged molecular
mode ν˜v can directly be assigned to a molecular parameter, i.e.,
CdC double bond vs CtC triple bond.97
1
1
ν
˜/cm-
(ꢀ
/M-1 cm-
)
band A
band B
band C
Estimation of Electronic Coupling V12. As we outlined in
the Introduction, estimation of the electronic coupling V12 via
eqs 2 and 4 requires knowledge of the diabatic dipole moment
difference ∆µab. Quite recently, we evaluated this dipole moment
difference for 1 by EOAM, which yielded 19 ( 1 D.71 Here
we apply a quite different and much less laborious methodology
which is based on the evaluation of band shape parameters
only: according to Lippert and Mataga,98,99 the difference in
absorption and fluorescence energy can be correlated with the
Onsager solvent parameter as given in eq 7, with the assumption
of a spherical chromophore geometry with radius a0 (for a
derivation of eq 7, see the Appendix in the Supporting
Information). Based on a purely classical model (with parabolic
potentials for the ground and the excited states), the total
reorganization energy λ is given by eq 8.100
1
2
3
4
5
6
7
25700 (35800)
25600 (25300)
25800 (21300)
25600 (27000)
25600 (44700)
25900 (33900)
25600 (47600)
20000 (7700)
17800 (4600)
17800 (4200)a
21200 (11500)a
19800 (3800)a
19400 (2700)
21400 (16000)
21700 (15400)
21500 (10500)
18200 (6700)a
18200 (6100)a
a Shoulder.
the same problems appeared but in an alleviated form. The
symmetry and, as a consequence thereof, the problems in fitting
the IV-CT bands of 3 and 4 can be traced back to a high
Huang-Rhys factor S. We assume that the lack of an unsatur-
ated C-C bridge unit in 3 and 4 causes a lower average
vibrational mode ν˜v, which makes S high and the IV-CT bands
symmetrical. The vibronic fine structure of the IV-CT bands in
nonpolar solvents mentioned above is a consequence of the low
solvent reorganization energy λo compared to the average
molecular vibrational mode ν˜v. The data of all fits are sum-
marized in Table 3.
2(λv + λo) ) ν˜abs - ν˜fl )
(∆µab)2
(f(D) - f(n2)) + (ν˜ agbass - ν˜ gas) (7)
fl
2πꢀ0hca30
It is apparent from the data in Table 3 that the Marcus
reorganization energy is dominated by the solvent part, except
for the very nonpolar solvents n-hexane and cyclohexane. A
plot of each parameter vs the Onsager solvent polarity function
(D - 1)/(2D + 1) - (n2 - 1)/(2n2 + 1), where D is the
permittivity and n the refractive index, reveals linear correlations
for both ∆G°° and λo with solvent polarity (for comments about
the use of the polarity function, see the Appendix in the
Supporting Information). In Figure 5 the correlations are plotted
and the data points for compound 2 only are shown for clarity.
The scatter of data for the other compounds is similar.
As expected from Marcus theory,96 the solvent reorganization
energies λo increase with solvent polarity and approach zero
for totally apolar environments (Figure 5b). Concomitantly, the
values for ∆G°° decrease due to the fact that the excited states
of 1-7 have zwitterionic character while the ground states
possess a small dipole moment71 (Figure 5a). Thus, the weak
and unsystematic solvent dependence of the IV-CT bands is a
result of these two compensating trends. Consequently, the
positive solvatochromism which is expected for chromophores
with small ground-state dipole moment and large excited-state
dipole moment is not observed. The plots also prove the
independence of the vibrational reorganization energy λv on the
solvent polarity (Figure 5c). However, we also recorded a weak
but systematic dependence of the average molecular vibrational
mode ν˜v on solvent polarity (Figure 5d), which could be a result
of the varying importance of different molecular modes
contributing to the averaged mode ν˜v. However, it might also
be that this shift compensates inaccuracies of the theory in
general. It is noteworthy that for all compounds with an alkynyl
spacer unit, ν˜v is noticeably higher (average 700 cm-1) than
for compound 1, with an alkenyl spacer, and radical 3, with no
unsaturated spacer unit. This energy difference is due to an
alkynyl vibrational mode having typically more energy than an
alkenyl or a C-C single bond vibrational mode. This is, to our
D - 1
2D + 1
n2 - 1
2n2 + 1
with
f(D) )
and
f(n2) )
ν˜abs - ν˜fl
λv + λo )
(8)
2
Consequently, plots of 2(λv + λo) (values taken from the Jortner
fits above) vs the Onsager solvent parameter f(D) - f(n2) for
MV compounds 1-3 and 5-7 in a broad variety of solvents
(Figure 6) and linear fits (Figure 6) of these plots yield the
adiabatic dipole moment differences ∆µab from the slope. The
radii a0 of all compounds, which are necessary input into eq 7,
were determined by calculating the Connolly solvent-excluded
volume of the molecules101 and then assuming that the
compounds have spherical geometry.
With the data for the adiabatic dipole moment differences
obtained in the above-mentioned way, we were able to calculate
the diabatic dipole moment differences ∆µ12 between the ground
and the excited states by eq 4. The corresponding values for
the electronic coupling V12 were determined by using eq 2. The
data for all compounds 1-3 and 5-7 are summarized in Table
4. The good agreement of the dipole moment difference ∆µab
of 1 (17.6 D) derived by the solvatochromic method above with
the value obtained recently by EAOM (19 ( 1 D)71 shows that
our new solvatochromic method described here is well suitable
to yield reliable dipole moment differences, comparable to those
obtained by the much more complicated EAOM method.
It is apparent from the data in Table 4 that the electronic
coupling values V12 of all compounds 1-3 and 5-7 are within
a narrow range of about 2300-3000 cm-1 and the solvent
dependence of V12 is also quite small. The electronic coupling
(97) Kjaer, A. M.; Ulstrup, J. J. Am. Chem. Soc. 1987, 109, 1934-1942.
(98) Mataga, N.; Kaifu, Y.; Koizumi, M. Bull. Chem. Soc. Jpn. 1956, 29, 465-
470.
(99) Lippert, E. Z. Elektrochem. 1957, 61, 962-975.
(100) Brunschwig, B. S.; Ehrenson, S.; Sutin, N. J. Phys. Chem. 1987, 91, 4714-
4723.
(96) Brunschwig, B. S.; Ehrenson, S.; Sutin, N. J. Phys. Chem. 1986, 90, 3657-
3668.
(101) CS Chem3D Pro; CambridgeSoft Corp.: Cambridge, MA, 1999.
9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5521

A R T I C L E S
Heckmann and Lambert
Table 3. Absorption Maxima, Extinction Coefficients, and Resulting ET Parameters of the Least-Squares Fits of the IV-CT Bands of 1-7 in
Solvents of Different Polarity
νmax
˜
/
ꢀ
/
∆
G
°°
cm-
/
a
λo
/
λv
/
ν
˜v/
νmax
˜
/
ꢀ
/
∆
G
°°
cm-
/
λo
/
λv
/
ν˜v/
cm-
M-1 cm-
cm-
cm-
cm-
cm-
M-1 cm-
cm-
cm-
cm-
1
1
1
1
a
1
a
1
a
1
1
1
a
1
a
1
a
1 a
Radical 1
Radical 5
n-hexane
12400
12250
12200
11950
12000
12000
11900
11750
12150
11650
11800
11950
12200
4750
4600
3900
4150
4050
3650
3850
3550
3600
3600
3900
3550
3400
10250
10200
9450
9250
8850
8750
8050
7500
8100
7300
8100
1000
1050
1850
1700
2050
2200
2900
3350
3000
3400
2750
1250
1100
1650
1200
1350
1300
1250
1100
1400
1150
1200
1150
1200
1400
1400
1450
1450
1650
1750
1550
1700
1650
2000b
1750
n-hexane
13550
13450
13100
13050
13000
12950
12650
12650
5300
5100
5000
4850
5250
5150
5250
5300
4900
5700
5100
5050
4550
11550
11350
10050
10050
9600
9450
8100
8250
8900
8550
8550
7750
7900
1250
1300
2400
2350
2700
2900
4150
3750
3300
3500
3500
4350
4400
1050 1700
1050 1700
950 2000
950 2050
900 2100
900 2150
850 2550
800 2350
900 2200
850 2300
850 2250
800 2500
800 2500
cyclohexane
1,4-dioxane
dibutyl ether
diethyl ether
MTBE
ethyl acetate
THF
dichloromethane
benzonitrile
2-propanol
acetone
cyclohexane
1,4-dioxane
dibutyl ether
diethyl ether
MTBE
ethyl acetate
THF
dichloromethane 12800
benzonitrile
2-propanol
acetone
12650
12750
12600
12750
6550b 4750b 1050
7250b 4150b 1350
acetonitrile
acetonitrile
Radical 2
Radical 6
n-hexane
12650
12400
12400
12200
12200
12200
12200
12000
12300
11850
12000
12200
12450
8150
7250
5800
7050
6400
6400
6000
5550
5000
5250
5800
5400
5400
10900
10600
9400
9500
8800
8850
8150
7650
8300
6800
8250
7550
7900
1050
1150
2550
2200
2900
2900
3650
3950
3600
4750
3350
4300
4200
650
650
650
600
600
600
600
650
650
650
550
600
650
1650
1650
2250
2100
2400
2350
2650
3050
2600
3950b
2550
2950
2800
n-hexane
14200
13950
13600
13600
13550
13500
13300
13200
3800
3800
3500
4300
4100
3950
4250
3650
3509
3650
3900
3900
4050
12350
1050
1150
2000
1900
2250
2300
3250
3450
2450
3850
3050
3700
1200 1650
1200 1700
1100 1950
1100 1950
1050 2000
1100 2000
1050 2300
1200 2550
1100 2050
1050 2800
1000 2200
950 2400
cyclohexane
1,4-dioxane
dibutyl ether
diethyl ether
MTBE
ethyl acetate
THF
dichloromethane
benzonitrile
2-propanol
acetone
cyclohexane
1,4-dioxane
dibutyl ether
diethyl ether
MTBE
ethyl acetate
THF
dichloromethane 13550
12150
11000
11100
10700
10650
9600
9250
10500
8800
benzonitrile
2-propanol
acetone
13100
13300
13250
13400
9750
9100
acetonitrile
acetonitrile
9500b 3450b 1050 2300
Radical 3
Radical 7
n-hexane
12050
11950
11900
11750
11750
11750
11750
11650
12050
11600
11600
11800
11900
5050
4600
4150
4350
4750
4300
4050
3850
3700
3450
3950
3400
3550
9650
9600
8350
8500
7900
8100
7750
7250
7350
7200
1650
1450
2950
2650
3350
3050
3300
3850
4450
3800b
800
850
650
650
550
650
700
500
450
650
1300
1200
1500
1500
1650
1450
1400
1950
2000
1400
n-hexane
13400
13300
12950
12900
12800
12800
12650
12450
4600
4700
3650
4300
4250
3950
3950
3950
3850
3550
3450
3400
3700
11900
800
750
950 1600
900 1550
900 2050
900 2000
900 2150
900 2150
950 2500
800 2450
850 2250
750 2450
850 2450
850 2500
900 2600
cyclohexane
1,4-dioxane
dibutyl ether
diethyl ether
MTBE
cyclohexane
1,4-dioxane
dibutyl ether
diethyl ether
MTBE
ethyl acetate
THF
dichloromethane 12150
11750
10400
10550
9950
9950
9000
8850
9450
8550
9050
8450
8400
1900
1700
2250
2250
3050
3000
2650
3050
2900
3500
3700
ethyl acetate
THF
dichloromethane
benzonitrile
2-propanolc
acetone
benzonitrile
2-propanol
acetone
12250
12550
12500
12650
7450
3450b
900
1200
acetonitrilec
acetonitrile
Radical 4
10350b 2350b
n-hexane
13200
13050
13250
12900
13000
12950
13250
12850
1400
1300
1050
1200
1100
1050
1100
1000
1050
950
700
900
1600b
1250
cyclohexane
1,4-dioxanec
dibutyl ether
diethyl ether
MTBEc
10600
1600
7900
7600
4700b
5000b
500
550
2450b
1950b
ethyl acetatec
THFc
dichloromethanec 13150
benzonitrilec
2-propanolc
acetonec
13000
13050
13200
13450
1000
950
1000
acetonitrilec
a All values were determined with a maximum error of (50 cm-1
possible because of a too large maximum error of >150 cm-1
.
b All values were determined with a maximum error of (150 cm-1
.
c No fit was
.
of 3 is about the same as that of 2, which supports our
assumption made above, that the smaller ET distance (demon-
strated by the smaller ∆µab value) is compensated by a smaller
transition moment which is due to the weakening of electronic
coupling, presumably by torsional effects. Comparison of V12
of 1 and of 2 shows that of the latter to be significantly higher
by ca. 280 cm-1. This is in contrast to the behavior observed in
analogous bis(triarylamine) radical cations with alkynyl and
alkenyl bridges, where the electronic coupling is somewhat
larger in the alkenyl system.22,71,102 This effect might also be
explained by twisting of the alkenyl group due to steric
interactions with ortho-chloro atoms.
UV/Vis/NIR Spectroelectrochemistry. In order to probe the
spectral changes that result upon charging the compounds, the
UV/vis/NIR absorption spectra of 1-7 upon oxidation to the
monocations, as well as upon reduction to the monoanions, were
(102) Heckmann, A. Diploma Thesis, Universita¨t Wu¨rzburg, 2004.
9
5522 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

Neutral Organic Mixed-Valence Compounds
A R T I C L E S
1+ and 2+, which can also be conceived as donor-acceptor
compounds (Figure 7) in which the triarylmethyl and triaryl-
amine moieties of 1 and 2 exchanged their donor and acceptor
properties.
It is noteworthy that only in the absorption spectra of cations
1+ and 2+, with an unsaturated bridge unit, band H can be
observed, while compounds 3+ and 4+ do not show any
comparable absorption band. Instead, a very intense band G is
visible which might hide a potentially blue-shifted IV-CT band
at the red edge. Furthermore, band F is less intense and blue-
shifted in 3+ and 4+ compared to 1+ and 2+.
Conclusions
Figure 3. IV-CT bands of 7 in solvents of different polarity. MTBE )
methyl tert-butyl ether.
Our concept to combine perchlorinated triarylmethyl radical
acceptors with nitrogen donor centers yields the neutral, purely
organic MV compounds 1-7. This new class of organic
molecules, which are isoelectronic to the well-established bis-
(triarylamine) radical cations, show several unique properties
which allow a straightforward and detailed study of the charge-
transfer behavior of these compounds by all-optical measure-
ments. The neutral character and, consequently, the excellent
solubility of the MV compounds 1-7 allowed the investigation
of the ET behavior in both polar and nonpolar solvents and
offered the unique chance to study the solvatochromic behavior
of these compounds by UV/vis/NIR spectroscopy. All radicals
1-7 show a characteristic IV-CT band at 11 000-13 000 cm-1
with a weak but nonsystematic solvatochromic behavior.
However, by applying Jortner’s model, we achieved excellent
least-squares fits of the IV-CT bands of all MV compounds in
each solvent except for compound 4 and, in an alleviated form,
for radical 3. In these compounds, the lack of an unsaturated
bridge unit finally causes the Huang-Rhys factor S to be too
high, which in turn is the reason for symmetrical IV-CT bands.
With the aid of the Jortner fits, we were able to evaluate reliable
values for the inner reorganization energy λv and the solvent
reorganization energy λo, as well as values for the free energy
∆G°° and the averaged molecular vibrational modes ν˜v in each
solvent separately. Plots of each parameter vs the Onsager
solvent parameter revealed that the missing visible solvatochro-
mic behavior is the result of two opposing effects, a positive
solvatochromism of ∆G°° and a negative solvatochromism of
λo. The plots also proved the independence of the inner
reorganization energy λv on the solvent polarity. Furthermore,
the plots show that the values of the averaged vibrational modes
of radicals 1 and 5-7 with a spacer containing a CtC triple
bond are distinctly higher than the values determined for radicals
2 and 3, where the triple bond is absent. This is the first time
that molecular parameters can be directly associated with
differences in averaged molecular modes, and this work
demonstrates the usefulness of the band shape analysis in the
context of Jortner’s model.
Figure 4. Least-squares fits of the IV-CT bands of 2 in solvents of different
polarity.
investigated by spectroelectrochemistry in dichloromethane
solutions. While for all radicals 1-7 the spectra of the
corresponding monoanions 1--7- could be generated, only the
monocations 1+-4+ with dianisyl moieties could be investi-
gated, because 5-7 undergo irreversible oxidation to their
monocations. As can be seen from Figure 2, the IV-CT band
of all radicals disappears upon reduction, and no absorptions
in the NIR region are observed due to the fact that the
monoanions are not mixed valent. This behavior also proves
the NIR bands of the neutral compounds 1-7 to be IV-CT
bands. At the same time, a new intense band D at around 19 500
cm-1 is visible, which we assign to an absorption of the PCTM
carbanion moiety in 1--7-.25,49 For all systems with alkynyl
or alkenyl spacers, this band partially overlaps with a second,
even more intense band E at around 17 500 cm-1. Furthermore,
a decrease in band A can be observed, associated with reduction
of the carbon radical center; additionally, the typical radical
bands B and C decrease, which can be seen in the spectra of
carbanions 5--7- (Figure 2). The data of all the bands discussed
for the monoanions are summarized in Table 5.
The spectra of radical cations 1+-4+ in dichloromethane
generated by spectroelectrochemistry are also displayed in
Figure 2 (blue solid lines). All cations show intense absorption
signals (F and G) in the region at ca. 13 000-14 500 cm-1
which can be assigned to the oxidation of the triarylamine
moiety to the corresponding radical cations (Table 6).103
Furthermore, a relatively intense band (H) at ca. 9600 cm-1
increases upon oxidation of radicals 1 and 2. This band can be
traced back to an optically induced IV-CT process in the cations
The separately determined values for λo and λv allowed a
plot of 2(λo + λv) vs the Onsager solvent parameter according
to Lippert and Mataga. By a linear fit of this plot, we were
able to determine the adiabatic dipole moment difference ∆µ12.
This dipole moment difference is in excellent agreement with
that obtained by an EAOM measurement. This new variant of
dipole moment determination made an all-optical evaluation of
the electronic coupling V12 possible. Moreover, the solvato-
chromic method outlined here is much simpler than the more
(103) Amthor, S.; Noller, B.; Lambert, C. Chem. Phys. 2005, 316, 141-152.
9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5523

A R T I C L E S
Heckmann and Lambert
Figure 5. Linear correlations of (a) ∆G°°, (b) λo, (c) λv, and (d) ν˜v versus the solvent polarity function f(D) - f(n2), with f(D) ) (D - 1)/(2D + 1) and f(n2)
) (n2 - 1)/(2n2 + 1) for compounds 1-3 and 5-7. For simplicity, only the data points for 1 are shown.
absorption spectra of compounds 1+ and 2+, with an unsaturated
bridge unit, show the rise of a new intense band in the NIR
region which can be assigned to intramolecular charge-transfer
processes from the carbon radical centers to the nitrogen radical
cation centers. Thus, compounds 1 and 2 show, both in the
neutral state and in the oxidized state, a charge-transfer behavior
but with exchanged donor and acceptor functionalities. A
detailed study of this remarkable behavior, which until now was
observed only in neutral MV compounds, is the focus of further
investigations in our laboratories.
Experimental Section
Cyclic Voltammetry and Spectroelectrochemistry. The electro-
Figure 6. Plot of 2(λv + λo) vs the Onsager solvent parameter f(D) -
chemical measurements were performed in a 0.1 M solution of TBAH
f(n2) for radical 1 and linear fit. The values for the reorganization energies
as supporting electrolyte in dry, argon-saturated CH2Cl2 under an argon
atmosphere. The concentration of the substrates was 0.1 mM. All
potentials were referenced vs ferrocene (Fc/Fc+). The spectroelectro-
chemical measurements were carried out with the solutions of the CV
experiments that were transferred into a thin-layer cell with a gold
minigrid working electrode. The design of the cell was described
elsewhere;104 the optical path length was 100 µm. All UV/vis/NIR
spectra were recorded with a Jasco V-570 UV/vis/NIR spectropho-
tometer in transmission mode.
Mass Spectroscopy. The ESI-TOF mass spectra were obtained with
a Bruker Daltonics micrOTOF focus mass spectrometer equipped with
an APCI ion source (Agilent G1947-60101). A stainless steel spraying
capillary and, as transfer capillary, a nickel-coated glass capillary with
an inner diameter of 500 µm were used. Due to the isotopic distribution
over a broad m/z region caused by chlorine and/or bromine, the
monoisotopic signals were too small in intensity for some compounds
for an accurate mass measurement. In those cases, typically the most
intense signal (X + n) of this isotopic distribution was taken and
compared with the calculated value. For calculation of the mass values
were taken from Table 3.
complex and time-consuming EOAM method or the methods
based on quantum chemical calculations of the effective charge-
transfer distance, which is often afflicted by major inaccuracies
inherent in the theoretical approaches. While the straightforward
application of Jortner’s theory in combination with the Lippert-
Mataga method to investigate the ET transfer behavior is, in
principle, not restricted to neutral compounds, it is the uncharged
character of neutral MV compounds which makes the analysis
in a broad variety of solvents so simple. Thus, the new class of
organic PCTM-donor molecules presents an almost perfect
model system for the investigation of electron-transfer processes.
We also performed a detailed study of both the UV/vis/NIR
absorption spectra of the carbanions 1--7- and the spectra of
the diradical cations 1+-4+ obtained by spectroelectrochemical
measurements. Thereby, the optically induced charge transfer
of compounds 1-7 was proved by the absence of the IV-CT
band upon reduction to the corresponding carbanions. The
(104) Salbeck, J. Anal. Chem. 1993, 65, 2169-2173.
9
5524 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

Neutral Organic Mixed-Valence Compounds
A R T I C L E S
Table 4. Connolly Solvent-Excluded Volume VCon, Radii of the Molecules a0, Diabatic Dipole Moment Difference ∆µab, Transition Moment
ab, Adiabatic Dipole Moment Difference ∆µ12, and Electron Coupling V12 of Radicals 1-7 in n-Hexane and Acetonitrile
µ
VCon
/
a0/
Å
∆µab
D
/
µab
Da
/
∆µ12
D
/
V12/
1
Å3
cm-
1
2
3
5
6
7
n-hexane
MeCN
n-hexane
MeCN
n-hexane
MeCN
n-hexane
MeCN
n-hexane
MeCN
n-hexane
MeCN
718.4
710.4
698.2
605.6
592.8
564.0
5.56
5.54
5.50
5.25
5.21
5.13
17.6 ( 1.3
17.9 ( 1.4
15.4 ( 1.3
15.4 ( 1.1
14.3 ( 1.0
14.6 ( 0.9
3.6
3.6
4.1
4.1
3.7
3.3
3.7
3.9
2.9
3.4
3.1
3.5
19.0 ( 1.7
19.0 ( 1.7
19.7 ( 1.6
19.7 ( 1.6
17.1 ( 1.6
16.8 ( 1.5
17.1 ( 1.3
17.3 ( 1.3
15.4 ( 1.3
15.8 ( 1.3
15.9 ( 1.1
16.2 ( 1.1
2350 ( 230
2310 ( 230
2630 ( 240
2590 ( 230
2610 ( 270
2340 ( 230
2930 ( 240
2870 ( 240
2670 ( 250
2880 ( 260
2610 ( 200
2730 ( 200
a All values were determined with a maximum error of (0.15 D.
Table 5. Absorption Maxima and Extinction Coefficients of Bands
D and E in the Absorption Spectra of Carbanions 1--7- in
Dichloromethane
was then added, and the reaction mixture was stirred for a further 1.5
h in the dark at room temperature. The deep brown solution was purified
by flash chromatography on silica gel. The crude product was
precipitated twice by adding a concentrated solution in dichloromethane
to methanol to obtain the desired product.
1
1
ν
˜/cm- /M-1 cm-
(ꢀ )
band D
band E
GP2: General Procedure for the Stille Coupling Reactions. The
terminal alkyne (1.00 equiv) was dissolved in dry Et2O (10 mL per
1.00 mmol of alkyne) under a nitrogen atmosphere. The solution was
cooled to -78 °C, and n-BuLi (1.6 M solution in hexanes, 1.13 equiv)
was added via a syringe over a period of 5 min. The reaction mixture
was stirred at -78 °C for 30 min and for a further 30 min at -30 °C
and then was cooled to -78 °C again. Tri-n-butyltin chloride (1.15
equiv) was added dropwise via a syringe over a period of 5 min. The
reaction mixture was stirred at -78 °C for 30 min and for a further 30
min at room temperature. The solution was hydrolyzed with water, the
organic layer was separated and dried over MgSO4, and the solvent
was removed under reduced pressure. The residue was dried in vacuo
for 3 h and then dissolved in dry THF (1 mL per 100 mg of liquid)
under a nitrogen atmosphere. The aryl bromide 8 (1.00 equiv) was
added, and the solution was degassed. Pd(PPh3)4 (0.05 equiv) was
added, and the mixture was heated under reflux for 24 h under a
nitrogen atmosphere. The solvent was removed in vacuo, the residue
was dissolved in dichloromethane, and the organic layer was washed
with water three times. After the organic layer was dried over MgSO4,
the solvent was removed under reduced pressure, and the crude product
was purified by flash chromatography on silica gel. The crude product
was precipitated twice by adding a concentrated solution in dichlo-
romethane dropwise to methanol to obtain the desired tolan.
1-
2-
3-
4-
5-
6-
7-
18900 (25800)a
19500 (16600)a
19300 (23600)
19100 (25700)
19600 (22400)a
19700 (22300)a
19500 (17100)a
18200 (25700)
17300 (26900)
17500 (33200)
17400 (35500)
17500 (26000)
a Shoulder.
Table 6. Absorption Maxima and Extinction Coefficients of Bands
F, G, and H in the Absorption Spectra of Cations 1+-4+ in
Dichloromethane
1
1
ν
˜/cm- /M-1 cm-
(ꢀ )
band F
band G
band H
9600 (15100)
9700 (5900)
1+
2+
3+
4+
14400 (30300)
14500 (16700)
16500 (8500)
16900 (9300)
12900 (10700)
11400 (35200)
13000 (37700)
GP3: General Procedure for the Pd-Catalyzed Amination of Aryl
Bromides and Aryl Iodides with Secondary Amines. The aryl
bromide (aryl iodide) (1.10 equiv) and the secondary amine (1.00 equiv)
were dissolved in absolute toluene (10 mL per 1.00 mmol of aryl
bromide) under a nitrogen atmosphere. Pd2(dba)3‚CHCl3 (0.05 equiv),
P(t-Bu)3 (0.33 M in n-hexane, 0.04 equiv), and NaO-tBu (2.50 equiv)
were added, and the reaction mixture was stirred in the dark under a
nitrogen atmosphere under reflux for 24 h. The solvent was removed
in vacuo, and the residue was dissolved in dichloromethane. The organic
layer was washed with water three times and dried over MgSO4, and
the solvent was removed under reduced pressure. Flash chromatography
on silica gel afforded the desired product.
Figure 7. Optically induced ET of neutral MV compounds to a zwitterionic
excited state (left-hand side) and optically induced hole transfer of the
oxidized MV compounds (right-hand side).
of the isotopic distribution, the software module “Bruker Daltonics
IsotopePattern” of the software Compass 1.1 (Bruker Daltonik GmbH,
Bremen, Germany) was used.
GP1: General Procedure for the Radical Formation of r-H
Compounds. The R-H compound (1.00 equiv) was dissolved in dry
DMSO (1 mL per 100 µmol of R-H compound) under an inert gas
atmosphere. KO-tBu (2.00 equiv per R-H atom) was added, and the
deep purple solution that formed immediately was stirred for 1.5 h in
the dark at room temperature. p-Chloranil (1.00 equiv per R-H atom)
Synthesis of 8. Sulfur monochloride (5.64 g, 41.8 mmol, 0.76 equiv)
and dry aluminum chloride (2.87 g, 21.5 mmol, 0.39 equiv) were
dissolved in freshly distilled sulfuryl chloride (700 mL), and the solution
was stirred under reflux for 5 min. A solution of 9 (17.9 g, 55.0 mmol,
1.00 equiv) in sulfuryl chloride (500 mL) was added dropwise under
heat while the initially yellow-orange solution turned to deep red. The
reaction mixture was stirred for a further 8 h under reflux while the
volume was kept constant by stepwise addition of sulfuryl chloride.
The solution was cooled to room temperature, the solvent was removed
9
J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5525

A R T I C L E S
Heckmann and Lambert
in vacuo, and ice water (600 mL) was added to the yellow-orange
residue. NaHCO3 was added until no gas evolution was recognized,
and the mixture was heated under reflux for 1 h. The solution was
cooled to room temperature, a concentrated HCl solution (14 M, 90
mL) was added, and the mixture was stirred under reflux for a further
1 h. The green crude product was filtered, washed with boiling n-hexane
(3 × 50 mL), and dried in vacuo to afford 37.7 g (46.1 mmol, 84%) of
134.40 (quart.), 134.0 (quart.), 133.94 (quart.), 133.93 (quart.), 133.86
(quart.), 132.90 (quart.), 132.85 (quart.), 122.4 + 122.2 (NCCHCHCO
or NCCHCHCO), 114.89 + 114.86 (NCCHCHCO or NCCHCHCO)
56.9 (R-C), 55.8 (OCH3); IR (KBr) ν˜ ) 3044 (vw), 2995 (w), 2931
(w), 2833 (w), 1505 (vs), 1463 (w), 1439 (w), 1404 (m), 1367 (w),
1341 (w), 1298 (m), 1243 (s), 1172 (w), 1111 (w), 1039 (m), 822 (m),
809 (m), 683 (w), 665 (w), 526 (w) cm-1; MS (EI, 70 eV) m/z (relative
1
a colorless solid: mp >300 °C; H NMR (600 MHz, CDCl3, 20 °C,
intentsity) 953 (100, M+), 938 (27, M+ - CH3), 918 (44, 27, M+
-
TMS) δ ) 6.98 (s, R-H, 1H); 13C NMR (151 MHz, CDCl3, 20 °C,
TMS) δ ) 137.0 (quart.), 136.3 (quart.), 136.2 (quart.), 135.7 (quart.),
135.1 (2C, 2 × quart.), 134.8 (quart.), 134.7 (quart.), 134.1 (2C, 2 ×
quart.), 134.0 (2C, 2 × quart.), 133.8 (quart.), 133.7 (quart.), 132.7
(2C, 2 × quart.), 132.5 (quart.), 125.6 (quart.), 56.8 (R-C); IR (KBr)
ν˜ ) 2924 (w), 1521 (w), 1369 (m), 1336 (s), 1295 (s), 1238 (w), 1190
(w), 1116 (w), 809 (m), 784 (m), 712 (w), 691 (w), 674 (m), 639 (w),
609 (w), 522 (w), 419 (w) cm-1; MS (FAB, 70 eV, 2-(octyloxy)-
Cl); APCI positive (high resolution) calcd for C33H16Cl14NO2, 947.68149,
found 947.68150; ∆/ppm, 0.01. Elemental analysis calculated for
C33H15Cl14NO2: C, 41.56; H, 1.59; N, 1.47. Found: C, 41.88; H, 1.41;
N, 1.47.
Synthesis of Radical 3. Radical 3 was synthesized from 12 (100
mg, 105 µmol) according to general procedure GP1. Flash chroma-
tography with 4:1 CH2Cl2/petroleum ether as eluent yielded 70.0 mg
(73.5 µmol, 70%) of a brown solid: Rf ) 0.91 (CH2Cl2/petroleum ether
) 4:1); mp 280 °C (decomposition); IR (KBr) ν˜ ) 3039 (vw), 2994
(w), 2932 (w), 2905 (w), 2833 (w), 1504 (vs), 1463 (w), 1439 (w),
1398 (w), 1335 (m), 1298 (w), 1244 (s), 1172 (m), 1110 (w), 1038
(m), 821 (m), 737 (w), 709 (w), 682 (vw), 665 (m), 586 (w), 526 (m)
cm-1; MS (EI, 70 eV) m/z (relative intensity) 952 (40, M+), 937 (6,
nitrobenzene) m/z (relative intensity) 1477 (27, M+), 1442 (8, M+
-
Cl); APCI negative (high resolution) calcd for the X + 8 of [M - H]-
) C19BrCl14-, 804.47168, found 804.47097; ∆/ppm, 0.88.
Synthesis of Tolan 11. Compound 11 was synthesized from 10 (420
mg, 1.28 mmol) and 8 (1.03 g, 1.28 mmol) according to general
procedure GP2. Flash chromatography on silica gel with 1:1 CH2Cl2/
petroleum ether as eluent yielded 229 mg (218 µmol, 17%) of an intense
yellow solid: Rf ) 0.59 (CH2Cl2/petroleum ether ) 1:1); mp 178 °C;
1H NMR (600 MHz, acetone-d6, 20 °C, TMS) δ ) 7.43 (AA′, 2H,
NCCHCHCCC), 7.16 (AA′, 4H, OCCHCH), 7.10 (s, 1H, R-H), 6.97
(BB′, 4H, OCCHCHC), 6.80 (BB′, 2H, NCCHCHCCC), 3.81 (s, 6H,
OCH3); 13C NMR (151 MHz, acetone-d6, 20 °C, TMS) δ ) 158.2 (2C,
2 × quart.), 151.6 (quart.), 140.2 (2C, 2 × quart.), 137.6 (quart.), 137.5
(quart.), 137.1 (quart.), 135.9 (quart.), 135.8 (quart.), 135.7 (quart.),
135.2 (quart.), 134.88 (quart.), 134.85 (quart.), 134.6 (quart.), 134.38
(quart.), 134.36 (2C, 2 × quart.), 134.3 (quart.), 134.2 (quart.), 133.9
(NCCHCHCCC), 133.24 (quart.), 133.21 (quart.), 128.8 (OCCHCH),
126.8 (quart.), 118.4 (NCCHCHCCC), 115.9 (OCCHCHC), 111.7
(quart.), 105.5 (quart.), 83.4 (quart.), 57.5 (R-C), 55.8 (OCH3); IR (KBr)
ν˜ ) 2927 (w), 2832 (w), 2198 (m), 1600 (w), 1542 (w), 1504 (vs),
1463 (w), 1440 (w), 1364 (w), 1338 (m), 1332 (m), 1292 (m), 1241
(s), 1174 (w), 1105 (w), 1037 (w), 827 (m), 809 (w), 709 (w), 686
(w), 577 (w), 534 (w) cm-1; MS (EI, 70 eV) m/z (relative intensity)
1053 (100, M+), 1038, (8, M+ - CH3); APCI positive (high resolution)
calcd for [M + H]+ ) C41H20Cl14NO2+, 1047.71279, found 1047.71219;
∆/ppm, 0.57. Elemental analysis calculated for C41H19Cl14NO2: C,
46.72; H, 1.82; N, 1.33. Found: C, 47.09; H, 2.16; N, 1.31.
M+ - CH3), 902 (10, M+ - CH3 - Cl), 882 (100, M+ - Cl2); APCI
+
positive (high resolution) calcd for [M + H]+ ) C33H15Cl14NO2
946.67367, found 946.67184; ∆/ppm, 1.93.
,
Synthesis of 14. Pinacole borane 13 (400 mg, 927 µmol, 1.00 equiv)
and the perchlorinated bromoaryl 8 (747 mg, 927 µmol, 1.00 equiv)
were dissolved in dry toluene (10 mL) under a nitrogen atmosphere.
Sodium carbonate (2.60 mL, 1.00 M in H2O, 2.80 equiv) and Pd(PPh3)4
(21.4 mg, 927 µmol, 0.02 equiv) were added, and after degassing the
solution was stirred under reflux and under an inert gas atmosphere
for 24 h. The solvent was removed in vacuo, and the residue was
dissolved in dichloromethane (100 mL). The organic layer was washed
with water (2 × 100 mL) and dried over MgSO4, and the solvent was
removed under reduced pressure. The residue was purified by flash
chromatography on silica gel (CH2Cl2/petroleum ether ) 1:1). The crude
product was precipitated twice by adding a concentrated solution of
14 in dichloromethane into methanol to obtain 300 mg (291 µmol, 31%)
of a pale yellow solid: Rf ) 0.57 (CH2Cl2/petroleum ether ) 1:1); mp
198 °C; ν˜ ) 3034 (vw), 2999 (vw), 2945 (vw), 2928 (vw), 2900 (vw),
2831 (w), 1604 (m), 1504 (vs), 1463 (w), 1439 (w), 1392 (vw), 1361
(w), 1315 (m), 1294 (m), 1241 (s), 1178 (m), 1105 (w), 1080 (w),
1037 (m), 827 (m), 809 (m), 741 (w), 718 (w), 689 (w), 675 (m) cm-1
;
1H NMR (600 MHz, acetone-d6, 20 °C, TMS) δ ) 7.16 (AA′, 4H,
NCCHCHCO), 7.13 (s, 1H, R-H), 7.09-7.14 (m, 2H, NCCHCHCC),
6.96 (BB′, 4H, NCCHCHCO), 6.87-6.90 (m, 2H, NCCHCHCC), 3.81
(s, OCH3); 13C NMR (151 MHz, acetone-d6, 20 °C, TMS) δ ) 157.7
(quart.), 150.1 (quart.), 143.3 (quart.), 140.8 (quart.), 137.7 (quart.),
137.6 (quart.), 137.2 (quart.), 135.85 (quart.), 135.84 (quart.), 135.4
(quart.), 135.1 (quart.), 134.84 (quart.), 134.79 (quart.), 134.3 (quart.),
134.24 (2 × quart.), 134.20 (2 × quart.), 134.18 (quart.), 133.13 (quart.),
133.11 (quart.), 130.5 (NCCHCHCC), 129.0 (quart.), 128.5 (NCCH-
CHCO), 118.5 (NCCHCHCC), 115.7 (NCCHCHCO), 57.5 (R-C), 55.7
(OCH3); APCI positive (high resolution) calcd for [M + H]+ ) C39H20-
Cl14NO2+, 1023.71279, found 1023.71143; ∆/ppm, 1.33.
Synthesis of Radical 2. Radical 2 was synthesized from 11 (100
mg, 94.4 µmol) according to general procedure GP1. Flash chroma-
tography with 4:1 CH2Cl2/petroleum ether as eluent yielded 80.0 mg
(76.0 µmol, 80%) of a red-brown solid: Rf ) 0.84 (CH2Cl2/petroleum
ether ) 4:1); mp 185 °C (decomposition); IR (KBr) ν˜ ) 3026 (w),
2928 (w), 2833 (w), 2188 (s), 1685 (w), 1598 (m), 1505 (s), 1453 (w),
1336 (m), 1289 (w), 1241 (m), 1175 (w), 1106 (w), 1037 (w), 827
(w), 791 (w), 761 (w), 737 (w), 662 (w), 533 (w) cm-1; MS (FAB, 70
eV, nitrobenzyl alcohol) m/z (relative intensity) 1052 (1, M+); APCI
positive (high resolution) calcd for [M + H]+ ) C41H19Cl14NO2
1046.70497, found 1046.70545; ∆/ppm, 0.46.
,
+
Synthesis of Radical 4. Radical 4 was synthesized from 14 (200
mg, 194 µmol) according to general procedure GP1. Flash chroma-
tography with 1:1 CH2Cl2/ petroleum ether as eluent yielded 140 mg
(136 µmol, 70%) of a red-brown solid: Rf ) 0.94 (CH2Cl2/petroleum
ether ) 1:1); mp 240 °C (decomposition); IR (KBr) ν ) 3039 (vw),
2994 (vw), 2950 (vw), 2932 (vw), 2905 (vw), 2834 (vw), 1603 (w),
1504 (s), 1464 (vw), 1437 (vw), 1320 (m), 1285 (w), 1242 (m), 1178
(w), 1103 (w), 1038 (m), 827 (m) cm-1; APCI positive (high resolution)
calcd for [M + H]+ ) C39H19Cl14NO2+, 1022.70497, found 1022.70641;
∆/ppm, 1.41.
Synthesis of 12. Compound 12 was synthesized from 8 (1.00 g,
1.24 mmol) according to general procedure GP3. Flash chromatography
with 1:1 CH2Cl2/petroleum ether as eluent afforded the crude product
that was precipitated twice by adding a concentrated solution in acetone
dropwise into methanol, to yield 155 mg (163 µmol, 13%) of a pale
yellow solid: Rf ) 0.60 (CH2Cl2/petroleum ether ) 1:1); mp >290
°C; 1H NMR (600 MHz, CD2Cl2, 20 °C, TMS) δ ) 7.06 (s, 1H, R-H),
6.80-6.86 (8H, Aryl-H), 3.77 (2s, 6H, OCH3); 13C NMR (151 MHz,
CD2Cl2, 20 °C, TMS) δ ) 155.8 + 155.7 (2 signals, NCCH or OCCH),
142.7 (quart.), 138.8 (2 signals, NCCH or OCCH), 136.97 (quart.),
136.95 (quart.), 136.8 (quart.), 136.0 (quart.), 135.9 (quart.), 135.7
(quart.), 135.5 (quart.), 135.4 (quart.), 134.9 (quart.), 134.41 (quart.),
Synthesis of 16a. Compound 16a was synthesized from 15a (300
mg, 1.62 mmol) according to general procedure GP2. Flash chroma-
9
5526 J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007

Neutral Organic Mixed-Valence Compounds
A R T I C L E S
tography on silica gel with 1:1 CH2Cl2/petroleum ether as eluent yielded
80.1 mg (87.9 µmol, 80%) of a dark brown solid: Rf ) 0.42 (CH2-
Cl2); mp 180 °C (decomposition); IR (KBr) ν˜ ) 3093 (vw), 3048 (vw),
2933 (m), 2851 (w), 2185 (vs), 1602 (vs), 1522 (vs), 1450 (w), 1384
(m), 1337 (vs), 1233 (s), 1186 (m), 1126 (m), 1022 (m), 937 (w), 918
(vw), 863 (w), 817 (m), 737 (w), 711 (m), 661 (w), 533 (w) cm-1; MS
(70 eV, FAB, 2-(octyloxy)nitrobenzene) m/z (relative intensity) 910
(<1, M+), 840 (<1, M+ - Cl2); APCI positive (high resolution)
calculated for [M + H]+ ) C31H13Cl14NO+, 904.66262, found
904.66345; ∆/ppm, 0.38.
222 mg (244 µmol, 15%) of a yellow solid: Rf ) 0.52 (CH2Cl2/
1
petroleum ether ) 1:1); mp 175 °C; H NMR (600 MHz, CD2Cl2, 20
°C, TMS) δ ) 7.46 (AA′, 2H, NCCHCHCCC), 7.01 (s, 1H, R-H),
3
6.88 (BB′, 2H, NCCHCHCCC), 3.29 (t, JHH ) 5.4 Hz, 4H, NCH2-
CH2), 1.68 (m, 4H, NCH2CH2CH2), 1.63 (m, 2H, NCH2CH2CH2); 13
C
NMR (151 MHz, CD2Cl2, 20 °C, TMS) δ ) 152.8 (quart.), 136.93
(quart.), 136.92 (quart.), 136.2 (quart.), 135.5 (quart.), 135.4 (quart.),
135.3 (quart.), 134.6 (quart.), 134.4 (quart.), 134.3 (quart.), 134.2
(quart.), 134.0 (quart.), 133.93 (quart.), 133.92 (quart.), 133.90 (quart.),
133.8 (quart.), 133.7 (quart.), 133.6 (NCCHCHCCC), 132.9 (quart.),
132.8 (quart.), 126.5 (quart.), 114.9 (NCCHCHCCC), 112.1 (quart.),
109.9 (quart.), 56.9 (R-C), 49.2 (NCH2CH2), 25.8 (NCH2CH2CH2), 24.7
(NCH2CH2CH2); APCI positive (high resolution) calcd for the X + 6
of [M + H]+ ) C32H16Cl14N+, 909.68325, found 909.68318; ∆/ppm,
0.08.
Synthesis of 16b. Compound 16b was synthesized from 15b (300
mg, 1.60 mmol) according to general procedure GP2. Flash chroma-
tography on silica gel with CH2Cl2 as eluent yielded 250 mg (274 µmol,
17%) of a yellow solid: Rf ) 0.33 (CH2Cl2); mp 173 °C (decomposi-
tion); 1H NMR (600 MHz, CD2Cl2, 20 °C, TMS) δ ) 7.52 (AA′,
CCCHCH, 2H), 7.01 (s, R-H, 1H), 6.93 (BB′, NCCHCH, 2H), 3.84
(m, OCH2CH2, 4H), 3.25 (m, NCH2CH2, 4H); 13C NMR (150 MHz,
CD2Cl2, 20 °C, TMS) δ ) 136.90 (quart.), 136.88 (quart.), 136.6
(quart.), 135.52 (quart.), 135.46 (quart.), 135.4 (quart.), 134.7 (quart.),
134.44 (quart.), 134.43 (quart.), 134.36 (quart.), 133.98 (quart.), 133.96
(quart.), 133.92 (quart.), 133.85 (quart.), 133.7 (quart.), 133.6 (CCCHCH),
133.5 (quart.), 133.4 (quart.), 132.9 (quart.), 132.8 (quart.), 126.3
(quart.), 114.9 (NCCHCH), 104.2 (quart.), 83.2 (quart.), 66.9 (OCH2-
CH2), 56.9 (ClCCCH), 48.4 (NCH2CH2); IR (KBr) ν˜ ) 3080 (vw),
3044 (vw), 2959 (m), 2892 (vw), 2852 (w), 2202 (s), 1604 (vs), 1542
(m), 1514 (vs), 1448 (m), 1340 (vs), 1301 (s), 1261 (w), 1236 (vs),
1182 (m), 1124 (s), 1051 (w), 928 (s), 860 (w), 809 (s), 709 (m), 684
(m), 661 (w), 587 (w), 533 (m) cm-1; MS (70 eV, EI) m/z (relative
intensity) 911 (100, M+), 853 (19, M+ - C3H6O), 841 (6, M+- Cl2),
748 (31, M+ - C3H6Cl3O); APCI positive (high resolution) calcd for
[M + H]+ ) C31H12Cl14NO+, 903.65528, found 903.65435; ∆/ppm,
1.03.
Synthesis of Compound 18. Compound 18 was synthesized starting
from 17105 (1.00 g, 6.90 mmol) and 8 (5.56 g, 6.90 mmol) according
to general procedure GP2. Flash chromatography with petroleum ether
as eluent afforded 660 mg (759 µmol, 11%) of an intense yellow
1
solid: Rf ) 0.28 (petroleum ether); mp 183 °C; H NMR (600 MHz,
CD2Cl2, 20 °C, TMS) δ ) 7.46 (AA′, NCCHCH, 2H), 7.01 (s, R-H,
1H), 6.69 (BB′, NCCHCHC, 2H), 3.02 (s, N(CH3)2, 6H), 13C NMR
(150 MHz, CD2Cl2, 20 °C, TMS) δ ) 151.6 (quart.), 137.0 (quart.),
136.0 (quart.), 135.5 (quart.), 135.4 (quart.), 135.1 (quart.), 134.6
(quart.), 134.4 (quart.), 134.3 (quart.), 133.90 (quart.), 133.88 (quart.),
133.6 (NCCHCH), 132.9 (quart.), 132.8 (quart.), 132.0 (quart.), 131.9
(quart.), 126.6 (quart.), 125.4 (quart.), 112.0 (NCCHCHC), 107.9
(quart.), 105.6 (quart.), 105.0 (quart.), 83.0 (quart.), 82.5 (quart.), 56.9
(R-C), 40.2 (N(CH3)3); IR (KBr) ν˜ ) 3093 (vw), 3039 (vw), 2896 (w),
2860 (w), 2806 (w), 2197 (s), 1605 (vs), 1542 (s), 1521 (s), 1482 (vw),
1444 (m), 1362 (s), 1345 (s), 1298 (w), 1234 (w), 1168 (m), 1118 (w),
1064 (w), 946 (w), 860 (w), 813 (s), 709 (m), 530 (m) cm-1; APCI
positive (high resolution) calcd for X + 6 of [M + H]+ ) C29H12-
Cl14N+, 869.65187, found 869.65167; ∆/ppm, 0.23.
Synthesis of Radical 7. Radical 7 was synthesized starting from 18
(150 mg, 172 µmol) according to general procedure GP1. Flash
chromatography with CH2Cl2 as eluent yielded 104 mg, (120 µmol,
70%) of a red-brown solid: Rf ) 0.96 (CH2Cl2); mp 190 °C
(decomposition); IR (KBr) ν˜ ) 3093 (vw), 3039 (vw), 2918 (w), 2855
(w), 2806 (w), 2183 (s), 1604 (vs), 1526 (s), 1443 (m), 1362 (s), 1338
(s), 1258 (w), 1237 (m), 1179 (m), 1139 (vw), 1064 (w), 946 (m), 815
(s), 737 (w), 711 (m), 655 (w), 530 (w) cm-1; APCI positive (high
resolution) calcd for X + 6 of [M + H]+ ) C29H11Cl14N+, 868.64404,
found 868.64287; ∆/ppm, 1.35.
Synthesis of Radical 5. Radical 5 was synthesized from 16a (123
mg, 135 µmol) according to general procedure GP1. Flash chroma-
tography on silica gel with 1:1CH2Cl2/petroleum ether as eluent yielded
75 mg (82.5 µmol, 61%) of a red-brown solid: Rf ) 0.55 (CH2Cl2/
petroleum ether ) 1:1); mp 172-178 °C (decomposition); IR (KBr) ν˜
) 2933 (m), 2852 (w), 2186 (s), 1601 (s), 1522 (s), 1464 (w), 1451
(w), 1384 (m), 1337 (s), 1320 (m), 1259 (w), 1233 (s), 1187 (m), 1126
(m), 1023 (m), 937 (w), 918 (w), 863 (w), 817 (m), 737 (w), 711 (m),
661 (w), 532 (w) cm-1; APCI positive (high resolution) calculated for
[M + H]+ ) C32H15Cl14N+, 902.68348, found 902.68346; ∆/ppm, 0.41.
Synthesis of Radical 6. Starting from tolane 16b (100 mg, 110 µmol,
1.00 equiv), compound 6 was prepared according to general procedure
GP1. Flash chromatography on silica gel with CH2Cl2 as eluent yielded
Acknowledgment. We are grateful to the Deutsche For-
schungsgemeinschaft for financial support, to Heraeus GmbH
for chemicals and to Dr. Bu¨chner for obtaining the mass spectra,
and to Christian Mu¨ller and Andre´ Schuster for synthetic work.
Supporting Information Available: Appendix, with com-
ments about the use of the polarity function and derivation of
eq 7. This material is available free of charge via the Internet
at http://pubs.acs.org.
JA068235J
(105) Miki, Y.; Momotake, A.; Arai, T. Org. Biomol. Chem. 2003, 1, 2655-
2660.
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J. AM. CHEM. SOC. VOL. 129, NO. 17, 2007 5527