Electronic decoupling in ground and excited states of asymmetric biaryls

Article abstract of DOI:10.1021/jp9932315

New asymmetric biaryls have been synthesized in order to clarify the conditions necessary for charge-transfer transitions in photoexcited neutral compounds and biradical formation in doubly charged ground-state species. A parallel behavior for both types of approaches is observed and explained with a simple coupling model allowing prediction of the intermoiety coupling strength. It is shown that for weakly coupled biaryls the monoions are connected with charge localization and the dications and dianions form thermally excited biradicals while the fluorescence data indicate biradicaloid excited states connected with partial charge separation. More strongly coupled biaryls, on the other hand, result in diamagnetic species for the doubly charged ground-state ions, and a charge-transfer contribution in the fluorescence spectra of the neutral compound is absent.

Full text of DOI:10.1021/jp9932315

1130
J. Phys. Chem. A 2000, 104, 1130-1140
Electronic Decoupling in Ground and Excited States of Asymmetric Biaryls
Martin Baumgarten,*,†,‡ Lileta Gherghel,† Jan Friedrich,† Martin Jurczok,§ and
Wolfgang Rettig*,§
Max-Planck-Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, and Institute for
Physical and Theoretical Chemistry, Humboldt UniVersity Berlin, Bunsenstrasse 1, 10117 Berlin, Germany
ReceiVed: September 13, 1999; In Final Form: NoVember 22, 1999
New asymmetric biaryls have been synthesized in order to clarify the conditions necessary for charge-transfer
transitions in photoexcited neutral compounds and biradical formation in doubly charged ground-state species.
A parallel behavior for both types of approaches is observed and explained with a simple coupling model
allowing prediction of the intermoiety coupling strength. It is shown that for weakly coupled biaryls the
monoions are connected with charge localization and the dications and dianions form thermally excited
biradicals while the fluorescence data indicate biradicaloid excited states connected with partial charge
separation. More strongly coupled biaryls, on the other hand, result in diamagnetic species for the doubly
charged ground-state ions, and a charge-transfer contribution in the fluorescence spectra of the neutral compound
is absent.
1. Introduction
independent. In the case of 1-, generated by potassium
reduction, a spin delocalization is observed for small anion
concentrations.5a A further increase of the concentration of
monocharged species 1- leads to a transition from a delocalized
spin distribution over both subunits to a localization of the spin
density on one subunit. In this mixture alternating line width
phenomena in the EPR spectra sometimes have been erroneously
interpreted as a kinetic process,15a although no temperature
dependence can be found and this effect is independent of
counterions as demonstrated also by electrochemical gener-
ation.15b,c In accord with these findings are descriptions by
Gerson et al.16a,b for other symmetric bridged binaphthyls where
a transition from a spin delocalized to a localized form upon
increasing reduction was observed.16a,b This signifies that the
increasing concentration of monoradicals hinders the spin
exchange process. For 2-, the electron hopping is independent
of the concentration of charge-carrying molecules.9,15b In the
case of the dianions, 22- is diamagnetic and only 12- exists as
a biradicaloid species with paramagnetic properties. With the
assumption that the magnitude of the bridgehead MO coef-
ficients and the torsional angle controls the interaction between
the two π-subunits, a consequent step is to investigate new
asymmetric biaryl compounds with similar redox potentials to
better understand the nature of the interactions between the
coupled π-systems. A large progress would be possible if a
general rule for the structural requirements could be found,
which determines whether a strong or weak electronic coupling
between the subunits is to be expected. In the case of the
asymmetric biaryls, one has to deal with directly connected
π-systems possessing different MO coefficients at the bridge-
head carbon centers. Furthermore, the asymmetric biaryls differ
from their symmetric analogues by unequal redox potentials and
optical absorptions of the submoieties. In this respect, the central
point deals with the question of whether an electron-hopping
process can still be observed in the monoradical species.
Moreover, it is of interest to compare the doubly charged
symmetric and asymmetric biaryls with respect to the formation
of biradicaloid states and with respect to a possible photoinduced
There has been much interest in the design of organic
materials that are suitable for electron transfer, electron storage,
and the formation of high-spin states.1-4 In this context, biaryls,
oligoarylenes, and polyarylenes serve as important chro-
mophores and electrophores.5-8 In the case of biaryl compounds
the main question deals with the magnitude of the interaction
between the connected aromatic π-systems. The electronic
coupling in bi- and oligoarylenes depends sensitively on the
topology and geometry of the connected molecules, including
the angle of torsion between them. If the coupling between the
connected π-systems is strong, there will be resonance, and one
can expect an extended π-system that can be described by a
single minimum of the potential hypersurface as a result of
strong overlap of the two minima for nearly planar conforma-
tions. When the molecule is singly charged, a complete charge
delocalization in such biaryls and extended delocalization over
several subunits in the higher oligomers are found as expected,
and a second charging process leads to diamagnetic diions.
Medium and weakly coupled biaryls, on the other hand, can
be described by double minima of the potential hypersurface
with a conformation closer to perpendicular and with differently
sized activation barriers between them. Here, the process of
singly charging may lead either to delocalization of the charge/
spin, induced by a fast self-exchange processes or to a charge
and spin localization on one subunit. Further charging can lead
to the generation of biradicals for the doubly charged ions.
Earlier investigations of symmetric biaryls such as 9,9′-bianthryl
(1)5 or 1,1′-bipyrenyl (2)8 have established that an electron
exchange between the two subunits takes place in the singly
charged species.1,9 Similar to barrierless electron-transfer pro-
cesses that can be described in terms of Marcus theory,10-14 it
is found that the electron transfer between the subunits of the
above-mentioned symmetric biaryls often is temperature-
* To whom correspondence should be addressed.
† E-mail: baumgart@mpip-mainz.mpg.de.
‡ Max-Planck-Institute for Polymer Research.
§ Humboldt University Berlin.
10.1021/jp9932315 CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/22/2000

Asymmetric Biaryls
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1131
SCHEME 1: Molecular Structure of Symmetric and
Asymmetric Biaryls
SCHEME 2: General Procedure for the Synthesis of
Asymmetric Biaryls 3-5
charge separation in the neutral species, which leads to
oppositely charged monoradical subunits. This work therefore
deals with the spectroscopic properties of different asymmetric
biaryls (Scheme 1) and investigates if it is possible to predict
the magnitude of the electronic coupling between the subunits.
TABLE 1: Redox Potentials (vs SCE) of the Biaryls 3-5
Measured in THF with [TBA]PF6 as Conducting Salta
compound
E11/2/V
E21/2/V
E31/2/V
E41/2/V
3
4
5
-2.13
-1.93
-1.90
-2.30
-2.22
-2.18
-2.40
-2.48
-2.40
-2.85
-2.73
2. Synthesis of Asymmetric Biaryls
The Pd(0)-catalyzed aryl-aryl cross-coupling according to
Suzuki17 can be used as a general procedure for the synthesis
of asymmetric biaryls. The proposed route is examplified as
outlined in Scheme 2, with the monobromo derivatives as
starting material for the synthesis of 1-(9′-anthryl)pyrene (3).
From the bromoanthryl, the boronic acid derivative of the
lithiated intermediate is prepared and the final product results
in high yield after Pd(0)-induced coupling and chromatographic
separation. Compounds 4 and 5 are prepared in a similar manner.
The detailed preparation procedures are described in sections
6.2, 6.3, and 6.4.
a Ferrocene was used as an internal standard (310 mV vs SCE) for
calibration.
TABLE 2: Optical Absorption of the Neutral Biaryls 3-5 in
Comparison with the Parent Compounds Perylene,
Anthracene, and Pyrenea
λmax,perylene unit
λmax,anthracene unit
λmax,pyrene unit
compound
(nm)
(nm)
(nm)
3
4
5
386
383
340
444
444
436
340
perylene
anthracene
pyrene
3. Results
376
334
3.1. Characterization of the Neutral Compounds by Cyclic
Voltammetry and Optical Absorption. The cyclic voltam-
mograms of 1-(9′-anthryl)pyrene (3), 3-(9′-anthryl)-8,11-di(tert-
butyl)-perylene (4), and 3-(1′-pyrenyl)-8,11-di(tert-butyl)-
perylene (5) demonstrate that the first two reduction potentials
of each compound are reversible (Table 1), and it should be
noted that the first reduction potentials of the three asymmetric
compounds are very similar to those of the parent molecules
anthracene, perylene, and pyrene, or their phenyl-substituted
derivatives. Thus, there are negligible conjugative interactions
and the biaryls consist of two electronically almost separated
π-units. These findings are also reflected by the results of the
UV/vis measurements (Table 2), since the spectra of the neutral
dimers show only a small bathochromic shift compared to the
model monomers and appear as superposition of the absorption
a All the experiments were performed in THF at room temperature.
spectra of the isolated chromophores. This points toward a
hindered conjugation between the extended π-units.
3.2. Radicals in Solution. Reduction of the asymmetric
biaryls 3, 4, and 5 with potassium in THF under high vacuum
as well as oxidation by means of 2,3-dichloro-5,6-dicyanoben-
zoquinone (DDQ) or antimony pentachloride in dichloromethane
yields strongly colored solutions of the charged ions, which were
characterized by UV/vis and EPR/ENDOR spectroscopy in the
same sample tube after one another at a given redox stage. Back
and forth control of the two methods showed no further change
of the generated species between the experiments and that the

1132 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Baumgarten et al.
developed cell.15c The observed ENDOR spectra of the mono-
anions 3- were identical with those obtained upon chemical
reduction.
The EPR spectrum of 4- shows eight sets of lines caused by
the hyperfine interaction of the unpaired electron with seven
hydrogen atoms at positions of high spin density (Figure 2).
The corresponding ENDOR spectrum at 240 K consists of four
large coupling constants. Computer simulation of the EPR
spectrum allows an unambiguous assignment to the seven
protons mentioned above (Table 3 and Figure 2). By comparison
with the hfc constants of the monoanion of 3,3′-bis(8,11-di-
(tert-butyl)perylenyl) (6), which has been investigated earlier,19
it can be concluded that in this case the perylenyl unit is charged.
The treatment of 4 with small amounts of DDQ in dichlo-
romethane leads to the formation of the stable monocation again
localized on the perylene unit. The spectral width of the EPR
spectrum of 4+ is larger than that of the monoanion, which is
caused by the slightly larger hfc constants at the peri positions
of the perylenyl unit in the case of the cations.22 The evaluation
of the ENDOR spectrum of 4+, pictured in Figure 2, leads to
four hfc constants (Table 3), and the computer simulation with
the given multiplicities is in good agreement with the experi-
mental spectrum.
The reduction of 5 leads to the formation of the monoanion,
which is again observable in the EPR spectrum by the
appearance of eight broad lines. In contrast to 4-, the ENDOR
spectrum of 5- consists of an additional large coupling and
shows that the odd electron is localized on the perylene unit
and that the bay positions of perylene possess different spin
densities. Table 3 shows the coupling constants determined from
the ENDOR experiment. By using the given multiplicities, one
obtains a good simulation of the experimental EPR spectrum
(Figure 3).
The oxidation of 5 performed using DDQ and trifluoroacetic
acid in dichloromethane at 240 K facilitates the formation of a
monocation that is stable for about 70 min and enables the
measurement of EPR and ENDOR spectra.
The ENDOR spectrum of 5+ (Figure 3) reveals five large
hfc constants corresponding to seven hydrogen atoms, similar
to the results for the monoanion 5- (Table 3).
A different behavior of these three compounds is also
discernible by comparing the optical absorptions of the mono-
charged species of the asymmetric biaryls (Figure 4). In the
vis/IR spectrum of 3-, an absorption typical of the substituted
anthracene anion at 671 nm is observable, as well as a broad
but very weak charge-transfer band in the NIR domain at around
1800 nm. For the other two monoanions 4- and 5-, the same
intensive absorption in the visible region at around 588 nm is
observed, characteristic of the negatively charged perylene. This
confirms the results and the assignments from the EPR/ENDOR
measurements to the charged subspecies. In the vis/NIR
spectrum of 5-, an additional well-developed charge-transfer
band appears at 1650 nm (Figure 4) while for 4- no IR
absorption is found at all. Although the optical absorption spectra
(∼10-11 s) operate in a different time scale than the CW EPR
spectra (∼10-7 s), they give additional proof of the generated
species. The indicated charge-transfer (CT) bands for 3- and
5- in the absorption, on the other hand, is not paralled by a
fast exchange (delocalization) in the slower EPR regime, as is
sometimes found for symmetric biaryls as 2-.
Figure 1. EPR (a) and ENDOR spectra (b) of monoanion 3•- in THF
solution at 240 K.
generated anionic species have a long lifetime (months). The
identification of the different radical ions generated rests upon
comparison with the well-established spectroscopic data of the
parent compounds anthracene, perylene, and pyrene.
One question to deal with is whether the unpaired electron
of the monoanions of the asymmetric biaryls is localized on
one subunit or whether it is delocalized over both electrophoric
units as can be observed in the case of the related symmetric
biaryls 9,9′-bianthryl (1) and 1,1′-bipyrenyl (2). Another point
of interest represents the possibility of the formation of biradicals
in the doubly charged species (dication or dianions).
3.2.1. Monoanions and Monocations. The contact of a
solution of 1-(9′-anthryl)pyrene (3) with a potassium mirror
leads to the formation of the monoanion. The intensive EPR
spectrum of 3- in solution (Figure 1) shows relatively high
resolution, but the signals are too complex for an unambiguous
interpretation. The ENDOR spectrum of 3-, taken at a central
saturated line of the EPR spectrum, yields much better resolution
and exhibits seven pairs of lines yielding one large, four
medium, and two small hyperfine coupling (hfc) constants
(Table 3 and Figure 1). Since the redox potential of anthracene
is slightly less negative than that of pyrene and the coupling
constants of 3- are in good agreement with those of the
monoanion of 9-phenylanthracene,18 it is obvious that the
unpaired electron is localized on the anthracene unit. Therefore,
the largest hfc constant is assigned to the 10-position of anthryl
and the other four smaller ones to the other four pairs of
equivalent protons of the anthryl unit, while two very small
coupling constants (0.024 and 0.010 mT) are derived from
protons on pyrene but cannot be assigned properly to defined
positions. To account for counterion influences, 3 was also
charged electrochemically (THF, TBAPF6) in a specially
The above results demonstrate that opposite to their sym-
metric counterparts the radical ions of the asymmetric biaryls
3-5 do not exhibit an electron self-exchange process at ambient
temperatures leading to complete charge and spin delocalization

Asymmetric Biaryls
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1133
TABLE 3: Hyperfine Coupling Constants (HFCs) aH and the Corresponding Number of Equivalent Protons of the Mono- and
Triply Charged Species of Compounds 3-5 (Numbering According to Scheme 1)c
compounds and
charged states
aH/mT, (equiv protons), [assignment to molecular position where possible]
3, monoanion 3-, A•-Py
0.559, (1), [10′]; 0.266, (2), [4′,5′]; 0.217, (2), [1′,8′]; 0.172, (2), [2′,7′]; 0.109, (2), [3′,6′]; 0.024, 0.010, [Pya]
0.475, (3), [3,6,8]; 0.289, (1); 0.219, (2); 0.132, (1); 0.100, (2)
trianion 33-, A2-Py•-
4, monoanion 4-, A-Per•-
monocation 4+, A-Per•+
trianion 43-, A•-Per2-
0.335, (2), [perib]; 0.315, (1), [perib]; 0.304, (2), [bayb]; 0.272, (2), [bayb]; 0.019, (2)
0.432, (2), [perib]; 0.374, (1), [perib], 0.287, (2), [bayb]; 0.272, (2), [bayb], 0.04, 0.01a
0.475, (1), [10]; 0.290, (2), [1,8]; 0.271, (2), [4,5]; 0.158, (2); 0.139, (2)
5, monoanion 5-, Py-Per•-
monocation 5+, Py-Per•+
trianion 53-, Py•-Per2-
0.330, (2), [peri]; 0.321, (1), [peri]; 0.295, (2), [bay]; 0.250, (1), [bay]; 0.235, (1), [bay]; 0.04-0.02a
0.431, (1),[peri]; 0.403, (1), [peri]; 0.345, (1); 0.283, (2), [bay]; 0.262, (2), [bay]; 0.03 a
0.460, (3), [3′, 6′, 8′]; 0.239, (2), 0.172, (1); 0.159, (1); 0.101, (1); 0.095, (1)
a Not further assigned. b Positions 4, 9, 10 called peri, and positions 1, 6, 7, 12 called bay of perylene (Scheme 1). c A ) anthryl. Per ) perylenyl.
Py ) propenyl. bay peri ) assignment.
Figure 3. (a) ENDOR spectra of monoanion 5•- and monocation 5•+
in THF solution at 240 K and (b) EPR spectrum of monoanion 5•- at
240 K and the corresponding computer simulation.
Figure 2. (a) ENDOR spectra of monoanion 4•- (upper) and
monocation 4•+ (lower spectra) in THF and dichloromethane solution,
respectively, at 240 K and (b) EPR spectrum of the monoanion 4•- at
240 K and the corresponding computer simulation.
where higher order terms from hyperfine Hhf, nuclear Zeeman
HnZ, and quadrupole interactions HQ have been neglected.20,21
over the dimers. Instead, charge and spin are localized, and the
spin-carrying site is always given by the unit possessing the
slightly lower reduction or oxidation potential.
H ) HeZ + HSS + (Hhf + HnZ + HQ) )
âBgS + SDS + ... (1)
3.2.2. Dianions and Dications. The dianions are easily formed
upon further contact with the potassium mirror, while for the
dications generally fresh samples were prepared, with a larger
amount of suitable oxidant. As a first example, successive
reduction of 3•- is described, leading to a new paramagnetic
species, with characteristic optical absorptions (Table 4) and
an EPR spectrum of increasing intensity in liquid solution. This
where g and D are 3 × 3 tensors and B and S are vectors.
The additional fine structure or zero-field splitting term SDS
leads to characteristic further splittings of the (ms) energy levels
as given in eq 2.
new species can be assigned to the biradical 32-
.
SDS ) -(DxSx2 + DySy2 + DzSz2)
(2)
Here, the doubly charged species possess two unpaired
electrons either with parallel spins, corresponding to a triplet
ground state, or with antiparallel spins in the case of a singlet
ground state. For the characterization of spin-spin interaction
one needs the spin Hamiltonian HSpin for quantum numbers of
the spin angular momentum S larger than /2. HSpin can be
described in first order by the electronic Zeeman contribution
HeZ together with the fine structure contributions HSS (eq 1)
where Dx, Dy, and Dz are the eigenvalues of the fine structure
tensor (Dz is the largest component in absolute value) and x, y,
and z are the eigenvectors. Because of the traceless property of
D, these three principal values can be reduced to two compo-
nents,21 called zero-field splitting (zfs) parameters, which have
been denoted as D ) -(3/2)Dz and E ) (1/2)(Dx - Dy). They
characterize the strength (D) and the symmetry (E) of the dipolar
1

1134 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Baumgarten et al.
Figure 4. Vis/IR spectra at room temperature of the monoanions (a)
3-, (b) 4-, and (c) 5-.
TABLE 4: Vis/NIR Optical Absorption Maxima of the
Negatively Charged Species of the Biaryls 3-5 in THF at
Room Temperature
λ/nm
Figure 5. (a) EPR spectrum of the triplet state (only the ∆ms ) 1
field region is shown) and the corresponding computer simulation of
biradical 32.2- in frozen THF/2K+ solution (135 K). lD′| ) 16.1 mT
and lE′| ) 1.8 mT are used for the simulation. (b) ∆mS ) 1 field region
of the EPR spectrum of dication 32+ in CH2Cl2/SbCl5 solid state (T )
130 K) and the corresponding ∆mS ) 2 transition.
compound
monoanions
dianions
trianions
3
452, 560, 609,
671, 1800a
452, 1300
491, 603, 723
4
5
586, 678, 822
454, 581, 690
588, 815, 1650b 1400
388, 459, 501, 603
a Weak. b Strong.
yields additional evidence that the detected species is in a triplet
state at this temperature (135 K).
interaction. It is convenient to express D and E in units of
magnetic field (mT) by defining the quantities D′ ) D/(gâ) and
E′ ) E/(gâ), where D′ and E′ are the experimental values
determined from the magnetic field differences between the
resonances. Since the dipolar components average to zero, the
determination of D′ and E′ is only possible in anisotropic
systems; i.e., the solutions of the radical ions have to be frozen.
The EPR spectrum of the frozen solution of 32- consists of
one central signal for the monoanion 3- and six symmetrically
placed spectral peaks corresponding to the X, Y, and Z
components of the biradical, which can be assigned to a triplet
species with rhombic symmetry (Figure 5).21
Since the EPR spectral width is approximated by 2D′,20 D′
was determined to be 16.1 mT. From the small splitting of the
Y and X components, one can deduce the asymmetry parameter
E′ ) 1.8 mT. The assignment is confirmed by the computer
simulation of the triplet EPR spectrum (see Figure 5).
With the assumption of a point-dipole approximation (eq
3) an average mean distance of the radical sites R ) 0.56 nm
is found for 32:
If the biradical 32+ is generated by oxidation with antimony
pentachloride, the EPR spectrum in frozen solution again shows
the typical structure for a triplet state with rhombic symmetry.
The evaluation of the zfs parameter D′ leads to 15.0 mT, but
the asymmetry parameter E′ is difficult to determine. The
measured ∆mS ) 2 transition further confirms the biradical
nature of the dication (Figure 5).
The doubly charged species of 4 are characterized by UV-
vis (Table 4) and EPR spectroscopy in liquid solution, respec-
tively. The paramagnetic character of the dianion led to
additional investigations in frozen solution. EPR measurements
of the paramagnetic dianion 42- at 135 K in THF delivers a
spectrum with six lines symmetrically placed with respect to
the Zeeman resonance (e.g., the signal of the monoanion 4-)
belonging to a triplet state with rhombic symmetry, since Dx *
Dy (Figure 6). The evaluation of the six signals of the ∆mS )
1 field region yields the zfs parameters D′ ) 12.4 mT and E′ )
0.7 mT, as well as the average distance between the interacting
spin centers of R ) 0.61 nm, which is slightly larger than for
3
2- and indicates a weaker dipole interaction. The temperature
dependence of the signal intensities in the range 10-80 K
reveals a thermally activated triplet state, with maximum
intensity around 30 K, corresponding to ∆EST of about 90 cal/
mol (375 J/mol). In comparison with earlier findings for 12-
(∆EST ) 60 cal/mol)5 and 2,2′-bipyrenyl (∆EST ) 120 cal/mol
3ge2â2
4R3
D′ [0.1 mT] =
(3)
The ∆mS ) 2 transition observed at the half-field resonance

Asymmetric Biaryls
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1135
the anthryl unit, indicated by a small hfc constant (0.010 mT)
that cannot be assigned safely to molecular positions.
By continuing the reduction of 42-, the shape of the EPR
spectrum at room temperature changes and six hfc constants
can be determined from the corresponding ENDOR experiment.
The values of the hfc constants for 43- as summarized in Table
3 lead to the conclusion that the unpaired electron is probably
localized on the anthracene unit, although the size of the largest
coupling constant is smaller (0.475 instead of 0.540 mT) than
for a singly charged substituted anthracene as in 1- or 3-. The
computer simulation of the resolved EPR spectrum confirms
this assignment and the conclusion that the unpaired electron
is localized on the anthracene unit.
Also further reduction of the diamagnetic dianion of 5 leads
to the formation of a new paramagnetic species. Six hyperfine
coupling constants are detected by means of ENDOR spectros-
copy, and the largest one is on the order of the largest hfc
constant of a monocharged pyrene (Table 3). It follows that
two negative charges reside on the perylene unit, whereas the
third unpaired electron is localized on the pyrene unit. The
comparison with a monocharged pyrene allows the assignment
of the coupling constants to different positions within the pyrene
unit and results in a good simulation of the experimental EPR
spectrum.
4. Discussion and Comparison to Photophysical
Properties
4.1. Ground-State Properties of the Ions. The investigations
of the monoanions of the asymmetric biaryl compounds
demonstrate that no ion exchange processes take place and that
the unpaired electron is always located on the π subunit with
the lower redox potential, the perylene unit in the case of 4-
and 5- and the anthrylene unit in the case of 3-. The same
behavior has been found for the monocations 4+ and 5+, where
the unpaired electron is located on the perylene subunit that
possesses the lower oxidation potential. This is contrary to the
findings for monoanions of symmetric biaryls, e.g., of 1- and
2-, where a complete delocalization of charge and spin density,
at least at the beginning of the reduction, has been observed
experimentally. The results for 3-5 are in good agreement with
the corresponding AM1 calculations, where the HOMO and
LUMO are also located on the π-subunit with the lower redox
potential. Thus, the monocharged biaryls 3-5 are comparable
with phenyl- or 2-methylphenyl-substituted π-systems where
just a small amount of spin density is transferred into the second
aryl. While often phenyl hyperconjugation18 has been used to
explain the spreading of spin density in phenyl-substituted
aromatics, we may speak more generally of aryl hyperconju-
gation in the case of asymmetric biaryls.23
An essential difference between the asymmetric compounds
can be shown by the comparison of the corresponding optical
absorption spectra (Figure 4). The Vis/NIR spectrum of 5-
exhibits a fairly intense absorption in the NIR region (1650 nm)
characteristic of a charge-transfer band. Also for 3-, a NIR band
at around 1800 nm is found, but its intensity is weak, and for
4- no band in the NIR region could be found. Because of the
reduced steric hindrance caused by the interactions of the
hydrogen atoms between the subunits of 5, the equilibrium twist
angle (θ = 70°, AM1) is lower than for 3 (θ = 90°, AM1) and
4 (θ = 90°, AM1) and one can correlate the appearance of the
strong NIR absorption for 5- with the better overlap of the
π-orbitals (Figure 4).
Figure 6. (a) EPR spectrum (only the ∆ms ) 1 field region) of biradical
42•2- in frozen THF/2K+ solution. (b) ∆mS ) 1 field region of the
EPR spectrum of dication 42•2+ in frozen CH2Cl2/SbCl5 (130 K) and
the corresponding ∆mS ) 2 transition in the inset.
(0.5 kJ/mol))8 the stability of the triplet state of 42- is
intermediate.
The triplet state is also accessible by a quick oxidation of 4
with antimony pentachloride under anaerobic conditions. The
EPR spectrum of 42+ in frozen solution (135 K) reveals a central
signal assigned to the monocation and four additional resonances
that can be assigned to a triplet state with nearly axial symmetry
(Figure 6). A slightly larger zero-field splitting (D′ ) 13.7 mT)
for the dication is found compared to the dianion because of
the larger spin density at the bridgehead positions.22
In contrast to the increased paramagnetic contributions in the
case of the dianions and dications of 3 and 4, further charging
of 5 leads to a decrease of the paramagnetic contributions, and
the doubly charged species 52- and 52+ are diamagnetic. The
UV-vis spectra recorded for these species represent another
useful reference for identifying the intact dianion, since the
distinct absorptions of perylene and pyrene anions are still
present together with a hypsochromically shifted IR band (Table
4).
3.2.3. Trianions. The triply charged species were only
accessible through anionic charging, while cationic charging
ended with the formation of doubly charged states. The
spectroscopic evidence of the formation of trianions are again
provided by UV-vis and EPR measurements. Successive
reduction of the dianion of 3 reveals a drastic change in the
EPR and visible spectra at room temperature, already indicating
the formation of the trianion. From the ENDOR spectrum of
3
3-, one can extract six hfc constants (Table 3), and the
multiplicity and size of the largest five constants leads to the
conclusion that the unpaired electron is localized on the pyrene
unit with a minor amount of spin density being distributed into
Indeed, a simple description of the wave function of the CT
state by eq 4 is able to relate the intensity of the CT band with

1136 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Baumgarten et al.
the coupling matrix element V between ground-state GS and
zero-order electron-transfer state ET(0).
ΨCT ) aΨET(0) + bΨGS + cΨLE
(4)
The admixture of locally excited (LE) states is important for
CT states of neutral species that are situated energetically close
to the LE states in the vis/UV region.24 In the case of the radicals
studied here with very low-lying CT states that are far apart
from the LE states, the contribution of LE to the total wave
function of the CT state can be neglected (c ≈ 0). Following
Verhoeven et al.25 in this case the absolute intensity or transition
moment f of the CT band is directly determined by the coupling
matrix element V:
f ) 10-5µ2V2/ν
(5)
Figure 7. Normalized absorption and fluorescence spectra of 3 in
solvents of different polarity at room temperature (MCH, methyl-
cyclohexane; EtOH, ethanol; ACN, acetonitrile).
In this case, µ and ν are the dipole moment and energy of
the CT state, respectively. V, on the other hand, is related to
the coupling of CT to the ground state, i.e., to the overlap of
the π-orbitals of the donor and acceptor moieties. For a
perpendicular arrangement of donor and acceptor (D and A), V
) 0 and the HOMO and LUMO of the combined system are
localized on D and A, respectively. For θ * 90°, these orbitals
mix, and we can write for the general case
ψHOMO ) (1 - λH)ψHOMO(D) + λHψHOMO(A)
ψLUMO ) λLψLUMO(D) + (1 - λL)ψLUMO(A)
(6)
(7)
where only the most important mixing of pairs of energetically
close-lying orbitals has been considered.
At 90°, where V ) λL ) λH ) b ) 0, optical excitation from
HOMO to LUMO leads to the pure one-electron ET(0) state
Figure 8. Normalized absorption and fluorescence spectra of 4 in
solvents of different polarity at room temperature (MCH, methyl-
cyclohexane; EtOH, ethanol; ACN, acetonitrile).
ΨCT ) ΨET(0)
(8)
The transition moment is then given by eq 9,
MCT(θ)90) ) ψGS|er|ψET(0)
)
ψHOMO(D)|er|ψLUMO(A) ) 0 (9)
due to the vanishing overlap between ψHOMO(D) and ψLUMO(A)
.
For θ * 90°, λH and λL differ from 0, and MCT is then nonzero
because of the admixture of ground and possibly locally excited
states. The twist angle θ can in principle be determined from
the oscillator strength by eq 5 if the usual angular dependence
V ) V(0) cos θ is assumed. However, because of the unknown
concentration of the ions studied, it was not possible to
determine the oscillator strengths quantitatively.
4.2. Excited-State Properties of the Neutral Species. To
find a possible relation between the formation of paramagnetic
biradicals for the doubly charged species in the ground state
and the formation of charge-transfer type biradical excited states
for the uncharged asymmetric biaryls, photophysical experiments
were carried out. For this reason, the solvent polarity dependence
of the fluorescence spectra and that of the photophysical rate
constants were determined. Bianthryl (1) has been thoroughly
studied in this context, and the solvent-induced excited-state
charge separation, accompanied in this symmetric case by a
symmetry-breaking process,26 manifests itself (i) by a solvato-
chromic red shift of the fluorescence spectra27-30,32 and (ii) by
a reduction of the rate constant kf of the emissive transition29,30
explainable by the near-perpendicular arrangement of the
subunits in the charge-transfer (CT) excited state26,32 as well as
in the ground state.34 In other symmetric systems such as
Figure 9. Normalized absorption and fluorescence spectra of 5 in
solvents of different polarity at room temperature (MCH, methyl-
cyclohexane; EtOH, ethanol; ACN, acetonitrile).
bipyrenyl 2 and biperylenyl 6, these effects were predicted34b
to be weaker, and this was tentatively correlated with the smaller
steric hindrance and therefore less perpendicular ground-state
structure compared to 1.33
The absorption and fluorescence spectra for 3-5 as recorded
in solvents of different polarity are shown in Figures 7-9. Only
in the case of 3 a sizable solvatochromic red shift with increasing
solvent polarity is observed, while the spectra shift very little
to the red in the case of 4. A solvatochromic red shift is

Asymmetric Biaryls
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1137
TABLE 5: Photophysical Properties of 3-5 in Solvents of
electrons separated on different moieties for S1(3,4) and
S0(32-,42-) but with the two odd electrons being spin-paired
on the same moiety for S1(5) and S0(52-).
Different Polarity at Room Temperature
compound/solvent
Φf τf/ns kf/107 s-1 knr/107 s-1 Mf [D]
The following characteristic differences can be extracted from
the EPR, ENDOR, UV, and fluorescence spectroscopic data of
the asymmetric biaryls 3-5.
(1) So far, only in symmetric biaryls a delocalization of the
ground-state spin density over the complete molecule could be
found, while asymmetric biaryls tend to localize the spin density
on one subunit with some small transfer of spin density into
the other unit (aryl hyperconjugation).
3/methylcyclohexane 0.71 3.7
19
14
10
22
22
15
24
23
22
7.9
6.1
7.0
8.4
5.8
8.6
14
11
13
4.5
4.0
3.6
5.6
5.6
4.9
6.1
6.1
6.1
3/ethanol
3/acetonitrile
0.70 4.9
0.58 6.0
4/methylcyclohexane 0.73 3.3
4/ethanol
4/acetonitrile
0.79 3.7
0.64 4.2
5/methylcyclohexane 0.64 2.7
5/ethanol
5/acetonitrile
0.68 2.9
0.64 2.9
(2) Only the monoanions of 5 show an intense NIR absorption
band (Table 4) that can be assigned to a charge-transfer
transition, while it is very weak for 3-.
(3) Further charging of the monoradicals leads to the
formation of paramagnetic biradicals in the case of 3 and 4,
while the doubly charged biaryl 5 is diamagnetic.
completely absent for 5. If the wave function of the emitting
state is assumed to be a superposition of a nonpolar locally
excited (LE) and a strongly polar charge transfer (CT) contribu-
tion,35 then the different solvent dependence of the fluorescence
spectra can be interpreted by different weights of charge-transfer
contributions to the total fluorescence spectrum. By use of the
fluorescence quantum yields and lifetimes, the rate constants
of the emissive (kf) and nonemissive (knr) excited-state decay
as well as the transition moment for fluorescence decay of the
excited state (Mf) can be determined using eq 10 (Table 5).34c
The data for 3 and 4 demonstrate that the radiative decay rate
constant kf and the transition moment Mf decrease with
increasing solvent polarity, consistent with an increased CT
contribution in polar solvents, whereas the nonradiative decay
rate constant is more or less independent of solvent polarity in
all cases. In contrast to 3 and 4, the results for 5 show a solvent-
polarity independent behavior for both radiative and nonradiative
decay rate constants as well as for the transition moment Mf,
indicative that CT contributions are absent.
(4) The excited states of the neutral species can also exhibit
charge separation linked with “electronic decoupling”. In parallel
with charge separation in the excited states observed for 3 and
4, the dianions of 3 and 4 are found to be paramagnetic, whereas
5 shows diamagnetic behavior compared with that in the absence
of excited-state charge separation.
From these characteristics, it is possible to classify the
symmetric and asymmetric biaryls into weakly and medium
strongly electronically coupled compounds. A strong coupling
leads to the formation of a diamagnetic dianion or dication (5)
without charge separation in the excited state (5), while a weak
coupling between the subunits results in the formation of diionic
or uncharged biradicals (3, 4), where the subunits carry either
the same charge (in the ionic ground-state species) or opposite
charges (in the charge-separated excited state of the neutral
species).
ESR/ENDOR measurements on mono- and dianions or
-cations of 3-5 yield evidence that the (odd) electron in the
monoradicals is localized on the moiety with the highest HOMO
(for cation radicals) or with the lowest LUMO energy (anion
radicals). In the doubly charged species (both dianion and
dication), either diamagnetic spin-paired ground states are
formed or paramagnetic biradicals with uncoupled spins (one
electron or hole on each moiety) can occur.
64π4
3h
kf )
n3νjf3Mf2
(10)
The photophysical results suggest that the emitting excited
state of 3 and 4 possesses some charge-transfer character, the
contribution of which depends on the solvent polarity. Although
only 3 reveals a solvatochromic shift in the fluorescence spectra,
the decreasing transition moments for fluorescence Mf in both
3 and 4 indicate that also in the case of 4 a contribution from
a state with less allowed and more polar character is observable
in high-polarity solvents. Since the fluorescence decays observed
are strictly monoexponential on the time scale used (g0.1 ns),
we can infer either that a rapid excited-state equilibrium between
an allowed, polarity-independent LE state and a solvent-sensitive
less allowed CT state is established within times less than 0.1
ns or that the wave function of the emissive state is a true,
vibronically induced mixture of LE and CT contributions with
increasing weight of CT for highly polar solvents. Since 5 does
not exhibit a decrease of kf and Mf, the CT contribution for this
compound is obviously absent, and the excited state can be better
described by the LE contribution alone with both unpaired
electrons residing on the same fragment (perylene) and a
stronger electronic interaction between them compared to the
CT state of 3 and 4, where the two unpaired electrons reside
on different moieties. These results are directly comparable with
the results from EPR spectroscopy of the doubly charged
ground-state species, i.e., CT contributions leading to partial
charge separation as (D•+A•-) are only found in those molecules
that are able to form biradicals (D•-A•- or D•+A•+) upon doubly
charging.
The interaction between the two subunits is determined by
the magnitude of the MO coefficients of the carbon centers at
the bridgehead position and by the twist angle resulting from
the steric hindrance.
By means of the McConnell equation36
aH ) QF(x)
(11)
the spin density F and thus the corresponding MO coefficients
can be extracted from the measured coupling constants aH.
Starting from the well-investigated aromatic π-systems an-
thracene, perylene, and pyrene37 and with the assumption that
the connection of these molecules to form a biaryl does not
dramatically change the magnitude of spin density, it is possible
to compare the experimental results with theoretical data.
Table 6 shows that the expression consisting of the MO
coefficients of the bridgehead carbon atoms Fi and the dihedral
angle θ (AM1 calculation) as F1F2 cos(θ) immediately allows
a classification of biaryls 1-6. The data of Table 6 are in accord
with the experimental results of the symmetric and asymmetric
biaryls and subdivide the compounds into weakly coupled
systems (1, 3, and 4) and into more strongy coupled ones (2
and 5) based on spin densities and dihedral angle. The
expression F1F2 cos(θ) can thus be used to quantify the
5. Conclusions
In summary, it has been found that ground- and excited-state
properties show a strikingly parallel behavior with two odd

1138 J. Phys. Chem. A, Vol. 104, No. 6, 2000
Baumgarten et al.
TABLE 6: Calculated Spin Densities and MO Coefficients
of the Carbon Centers at the Bridgehead by Position
Determined by Using the Experimental Coupling Constants
of the Monoradicals of the Parent Compounds Anthracene,
Perylene, and Pyrene as Well as the McConnell Equation
(See Text)
The optical absorption measurements were performed at
ambient temperature in a quartz tube using a special sample
holder and typical transmission detection with a Perkin-Elmer
Lambda 9 spectrometer.
Quantum-corrected fluorescence spectra (concentrations less
than 10-4 mol/L) were measured on an SLM-Aminco Bowman
2 fluorometer. Fluorescence quantum yields were determined
relative to a solution of quinine bisulfate in 0.1 N H2SO4 (φf )
0.52)40 and corrected for the refractive index of the solvents.
Decay times of aerated solutions were determined with the time-
correlated single photon counting (SPC) technique41 using
equipment described elsewhere.42 Synchrotron radiation from
the Berlin electron storage ring BESSY operating in the single
bunch mode was used for excitation.42 The excitation wave-
length was adjusted to the longest wavelength absorption band.
θ (deg) aH1 (mT) aH2 (mT)
F1
F2
F1F2 cos(θ)
1
2
3
4
5
6
88
70
88
88
70
70
0.534
0.475
0.534
0.534
0.475
0.353
0.534
0.475
0.475
0.353
0.353
0.353
0.227 0.227
0.202 0.202
0.227 0.202
0.227 0.150
0.202 0.150
0.150 0.150
0.0009
0.0139
0.0008
0.0006
0.0103
0.0077
magnitude of the electronic coupling between the two subunits
of a biaryl and allows us to make theoretical predictions about
the electronic properties of biaryls and their corresponding
oligomers. It is also possible to use calculated spin densities or
MO coefficients.
The electronic decoupling behavior of 1, 3, and 4 in ground
and excited states is related to the formation of high-spin states
in compounds with more than two aryl units. Examples are
known in the series of oligoanthrylenes where both high-spin
states5 and giant excited-state dipole moments38 were detected.
A general strategy for the development of such compounds is,
according to the above considerations, to use weakly coupled
systems.
6.2. Synthesis of 1-(9-Anthryl)pyrene (3). An amount of
1.5 mL n-butyllithium (1.6 M/hexane) was added to a stirred
solution of 0.6 g (2.13 mmol) of 1-bromopyrene in THF at -78
°C. After 15 min the temperature was increased to 0 °C followed
by the addition of 0.24 mL of boronic acid trimethyl ester, which
results in the formation of the 1-pyrenylboronic acid. After 20
min of stirring, the mixture was quenched with 0.01 M HCl.
The solvent was removed under reduced pressure, and the
residue was recrystallized from water and dried. An amount of
195 mg (0.79 mmol) of 1-pyrenylboronic acid was dissolved
in ethanol and added to a mixture of 10 mmol of sodium
carbonate in 20 mL of water and 204 mg (0.79 mmol) of
9-bromoanthracene in 40 mL of toluene. After addition of 40
mg of tetrakis(triphenylphosphine)palladium(0), degassing of
the solution, and the exclusion of light, the mixture was heated
at 115 °C for 30 h. Then the solvent was evaporated under
reduced pressure and the crude product was purified by column
chromatography (silica gel/n-pentane), which results in a yield
of 65% of 3.
The molecules studied here can also serve as a guideline for
the development of new extended π-systems with electric field
or light-induced switching properties39 desirable for molecular
electronic devices.
6. Experimental Section
6.1. Instrumentation and Methods. 1H and 13C NMR spectra
were recorded on a Bruker AMX 500 spectrometer. Mass spectra
were obtained on a FD-MS, VG-Instruments ZAB-2 mass
spectrometer. Melting points were measured on a Bu¨chi Dr.
Tottolie apparatus (uncorrected). Elemental analyses were
determined by the Department of Chemistry and Pharmacy of
the University of Mainz. Column chromatography was per-
formed employing a glass column packed with silica gel (70-
320 mesh) with n-pentane as eluant. The commercial solvents
were purified according to standard procedures.
The anions described here were prepared under high vacuum
in tetrahydrofuran (THF) by contact with a potassium mirror
in sealed quartz tubes with two side chambers. The cations were
prepared in quartz sample tubes under argon in freshly distilled
air and oxygen-free dichloromethane by means of 2,3-dichloro-
5,6-dicyanobenzoquinone (DDQ) or antimony pentachloride. To
account for counterion influences, anthrylpyrene (3) was also
charged electrochemically in a specially developed cell.15b,c The
observed ENDOR spectra of the monoanions 3- were identical
with those obtained upon chemical reduction.
The EPR and ENDOR spectra were recorded on Bruker ESP
300 and ESP 300 E spectrometers at X-band (9.4 GHz) with
100 (EPR cavity) and 12.5 (ENDOR cavity) kHz modulation
frequency. The temperature was controlled by a Bruker continu-
ous flow N2 cryostat and the very low temperature by an Oxford
ESR 910 helium continuous flow cryostat.
The computer program for the simulation of biradicals has
been written by Dr. Stoyan Karabunarliev for arbitrary spin
states (S ) 0.5-5.5) and direct diagonalization of the spin
Hamiltonian.
1H NMR (500 MHz, CDCl3), δ: 8.58 (s, 1H, H10); 8.32 (d,
3J(H, H) ) 7.7 Hz, 1H), 7.98 (d, 3J(H, H) ) 7.6 Hz, 1H); (H16,
3
H17 o. H18, H19 o. H23, H24); 8.18 (d, J(H, H) ) 8.0 Hz, 1H),
3
3
8.04 (d, J(H, H) ) 7.5 Hz, 1H); (H20, H22); 8.13 (d, J(H, H)
3
) 9.2 Hz, 1H), 8.13 (d, J(H, H) ) 9.1 Hz, 1H); (H16, H17 o.
H18, H19 o. H23, H24); 8.07 (d, 3J(H, H) ) 8.8 Hz, 2H), 7.30 (d,
3J(H, H) ) 9.0 Hz, 2H); (H4, H5, H1, H8); 7.94 (dd, J1(H, H)
3
) 7.9 Hz, 3J2(H, H) ) 7.6 Hz, 1H); (H21); 7.72 (d, 3J(H, H) )
9.2 Hz, 1H), 7.27 (d, 3J(H, H) ) 9.2 Hz, 1H); (H16, H17 o. H18,
H
19 o. H23, H24); 7.39 (dd, 3J1(H, H) ) 7.5 Hz, 3J2(H, H) ) 7.4
3
3
Hz, 2H), 7.15 (dd, J1(H, H) ) 7.6 Hz, J2(H, H) ) 7.4 Hz,
2H); (H6, H3, H7, H2). 13C NMR (500 MHz, CDCl3), δ: 135.4,
134.0 (C9, C15); 125.0, 124.9 (C27, C29); 131.6, 131.5, 131.4,
131.2, 131.1, 130.8 (C11, C12, C13, C14, C25, C26, C28, C30); 128.5,
127.1, 125.7, 125.3 (C1, C8, C2, C7, C3, C6, C4, C5); 129.4, 127.8,
127.7, 127.6, 127.1, 126.2, 125.8, 125.3, 125.2, 124.7 (C10, C16,
C17, C18, C19, C20, C21, C22, C23, C24). FD-MS: m/e 378 (100,
M+). Anal. Calcd for C30H18: C, 95.20; H, 4.80. Found: C,
94.71; H, 4.98.
6.3. Synthesis of 3-(9-Anthryl)-8,11-di(tert-butyl)perylene
(4). An amount of 2.7 mL of n-butyllithium (1.6 M/hexane)
was added to a solution of 1 g (3.90 mmol) of 9-bromo-
anthracene in tetrahydrofuran at -78 °C. After 15 min, the
reaction mixture was stirrred at 0 °C for 5 min. Then 0.44 mL
of boronic acid trimethyl ester was added to the solution to form
the 9-anthrylboronic acid. After 20 min, the mixture was
quenched with 0.01 M HCl. After the solvent was evaporated
under reduced pressure, the crude product was purified by
recrystallization from water. An amount of 100 mg (0.45 mol)
of the 9-anthrylboronic acid obtained was dissolved in 20 mL
Cyclic voltammograms were measured with a Princeton
Applied Research model PAR 272, and ferrocene was used as
an internal standard for calibration.

Asymmetric Biaryls
J. Phys. Chem. A, Vol. 104, No. 6, 2000 1139
of ethanol and added to a mixture of 10 mmol of sodium
carbonate in 20 mL of water and 200 mg (0.45 mmol) of
3-bromo-8,11-di(tert-butyl)perylene in toluene. The last com-
pound was prepared by methods analogous to those already
described earlier.19 After the addition of 40 mg of tetrakis-
(triphenylphosphine)palladium(0), degassing of the solution with
argon, and exclusion of light, the reaction mixture was heated
to 115 °C for 30 h. After removal of the solvent under reduced
pressure, the resulting residue was chromatographed on silica
gel (n-pentane) to obtain 0.194 g of 4 in 80% yield.
131.4, 131.1, 130.9, 130.6, 130.5, 129.9, 129.3, 125.4 (C8, C11,
C13, C14, C15, C16, C18, C19, C20, C39, C40, C42, C44); 125.2, 125.2
(C41, C43); 37.4, 37.4 (C21, C25); 129.1, 128.3, 127.4, 126.5,
126.4, 126.0, 125.8, 125.4, 125.2, 124.6, 123.6, 123.6, 120.0,
119.5, 118.6, 118.4 (C1, C2, C4, C5, C6, C7, C12, C30, C31, C32,
C33, C34, C35, C36, C37, C38). FD-MS: m/e 564 (100, M+). Anal.
Calcd for C44H36: C, 93.57; H, 6.43. Found: C, 93.49; H, 6.89.
mp: 255 °C.
Acknowledgment. M.B. and W.R. gratefully acknowledge
support from Stiftung Volkswagenwerk, Project VW 69641.
M.B. also thanks the Fond der chemischen Industrie.
1H NMR (500 MHz, CDCl3), δ: 1.45 (s, 9H), 1.43 (s, 9H);
(3H23, 3H24, 3H25 o. 3H28, 3H29, 3H30); 8.51 (s, 1H; H10); 8.32
(s, 1H), 8.23 (s, 1H); (H18 o. H19); 7.64 (s, 1H), 7.62 (s, 1H);
(H17 o. H20); 8.36 (d, 3J(H, H) ) 7.6 Hz, 1H), 7.46 (d, 3J(H, H)
References and Notes
3
) 7.6 Hz, 1H); (H11 o. H12); 8.15 (d, J(H, H) ) 7.4 Hz, 1H),
3
3
(1) Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel
Dekker: New York, 1986. Handbook of Conducting Polymers; Skotheim,
T. A., Ed.; Marcel Dekker: New York, 1997.
(2) Conjugated Polymers; Bre´das, J. L., Silbey, R., Eds.; Kluwer
Academic Publishers: Dordrecht, 1991.
(3) Baumgarten, M.; Mu¨llen, K. Top. Curr. Chem. 1994, 169, 1.
(4) Baumgarten, M.; Huber, W.; Mu¨llen, K. AdV. Phys. Org. Chem.
1993, 28, 1.
(5) (a) Mu¨ller, U.; Baumgarten, M. J. Am. Chem. Soc. 1995, 117, 5840.
(b) Baumgarten, M.; Mu¨ller, U.; Bohnen, A.; Mu¨llen, K. Angew. Chem.
1992, 104, 482.
(6) Naarmann, H. Angew. Makromol. Chem. 1982, 109, 295.
(7) Mu¨llen, K. Pure Appl. Chem. 1993, 65, 89.
(8) (a) Kreyenschmidt, M.; Baumgarten, M.; Tyutyulkov, N.; Mu¨llen,
K. Angew. Chem. 1994, 106, 2062 (Angew. Chem., Int. Ed. Engl. 1994,
33, 1957). (b) Kreyenschmidt, M. Ph.D. Thesis, Johannes Gutenberg-
Universita¨t Mainz, Mainz, Germany, 1995.
6.85 (d, J(H, H) ) 8.3 Hz, 1H); (H6 o. H8); 8.02 (d, J(H, H)
) 8.5 Hz, 2H), 7.49 (d, 3J(H, H) ) 8.8 Hz, 2H); (H5, H4 o. H8,
3
3
H1); 7.39 (dd, J1(H, H) ) 7.2 Hz, J2(H, H) ) 7.5 Hz, 2H),
7.21 (dd, 3J1(H, H) ) 7.6 Hz, 3J2(H, H) ) 7.7 Hz, 2H); (H3, H6
3
3
o. H2, H7); 7.12 (dd, J1(H, H) ) 7.9 Hz, J2(H, H) ) 7.8 Hz,
1H); (H15). 13C NMR (500 MHz, CDCl3), δ: 26.0, 26.1 (C23,
C24, C25, C28, C29, C30); 119.9, 120.3, 121.6, 123,1 (C1, C8, C2,
C7, C3, C6, C4, C5); 113.1, 113.2, 114.3, 114.7, 118.2, 118.3,
121.0, 121.4, 124.4, 124.5 (C10, C11, C12, C14, C15, C16, C17,
C18, C19, C20); 29.6, 29.7 (C22, C27); 125.6, 126.2 (C39, C41, C40,
C42); 143.9 (C13, C9); 120.1, 124.1 (C32, C37); 125.1, 125.2,
126.3, 126.5, 129.5, 129.6, 129.8, 130.7 (C21, C26, C31, C33, C34,
C35, C36, C38). FD-MS: m/e 540 (100, M+). Anal. Calcd for
C42H36: C, 93.28; H, 6.27. Found: C, 93.64; H, 6.68.
(9) Mazur, S.; Dixit, V. M.; Gerson, F. J. Am. Chem. Soc. 1980, 102,
5343.
6.4. Synthesis of 3-(1-Pyrene)-8,11-di(tert-butyl)perylene
(5). Similar to the synthesis of 3 and 4, the formation of the
boronic acid was performed. An amount of 1.4 mL of n-
butyllithium (1.6 M/hexane) was added to 570 mg (2.03 mmol)
of 1-bromopyrene in THF at -78 °C. After being stirred for
15 min, the mixture was allowed to warm to 0 °C. Then 0.19
mL of boronic acid trimethyl ester was added to form the
1-pyrenylboronic acid. After 20 min, the mixture was quenched
with 0.01 M HCl. The solvents were removed under reduced
pressure, and the crude product was recrystallized from water.
An amount of 200 mg (0.81 mmol) of the resulting product
dissolved in 40 mL of ethanol, and this was added to a mixture
of 10 mmol of sodium carbonate in 20 mL of water and 360
mg (0.81 mmol) of 3-bromo-8,11-di(tert-butyl)perylene in 40
mL of toluene. The last compound was prepared by methods
described in the literature.17 After the addition of 40 mg of
tetrakis(triphenylphosphine)palladium(0), degassing of the sol-
vent with argon, and the exclusion of light, the reaction mixture
was heated to 115 °C for 30 h. The solvent was removed under
reduced pressure, and the resulting residue was purified by
chromatography (silica gel/n-pentane). 0.390 g of the product
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