Hopping of a single hole in hexakis-[4-(1,1,2-triphenyl-ethenyl)phenyl] benzene cation radical through the hexaphenylbenzene propeller

Article abstract of DOI:10.1021/ol036037g

Matrix presented. A versatile synthesis of a dendritic structure (5) is described in which six tetraphenylethylene moieties are connected to a central benzene ring in such a way that one of the phenyl rings of each tetraphenylethylene is also part of the

Full text of DOI:10.1021/ol036037g

ORGANIC
LETTERS
2004
Vol. 6, No. 11
1689-1692
Hopping of a Single Hole in hexakis-
[4-(1,1,2-Triphenyl-ethenyl)phenyl]benzene
Cation Radical through the
Hexaphenylbenzene Propeller
Rajendra Rathore,* Carrie L. Burns, and Sameh A. Abdelwahed
Department of Chemistry, Marquette UniVersity, P.O. Box 1881,
Milwaukee, Wisconsin 53201
rajendra.rathore@marquette.edu
Received October 17, 2003
ABSTRACT
A versatile synthesis of a dendritic structure (5) is described in which six tetraphenylethylene moieties are connected to a central benzene
ring in such a way that one of the phenyl rings of each tetraphenylethylene is also part of the propeller of the hexaphenylbenzene core.
Observation of multiple oxidation waves in its cyclovoltammogram as well as an intense charge-resonance transition in the near-IR region in
its cation radical spectrum suggests that a single hole delocalizes via electron transfer over six identical redox-active centers.
The ready availability of hexaphenylbenzene (HPB) and
tetraphenylmethane (TPM) cores, and the ease of modifica-
tion of their vertexes to incorporate suitable electro-active
functionalities, makes them attractive starting points (or
platforms) for the construction of nanometer-sized materials
with novel light-emitting and charge-transport properties.1
A key to successful design of such structures requires that
the relative spatial organization of the electro-active units
should allow effective electronic coupling among them.
Accordingly, our interest in the hexaphenylbenzene core
arises from the fact that it arranges its (six) functionalizable
vertexes in a circular array around the central benzene ring
in a propeller-shaped structure. Conceivably, such a cofacial
arrangement of phenyl groups may allow an effective
electronic coupling among the six aryl rings via an overlap
of orbitals on carbon 1 of phenyls that lie cofacially at a
close distance of 2.9 Å from each other (see structures A
and B below).2 The origin of such an interaction has its roots
in our recent observations3 of the effective electronic
coupling between cofacially oriented aryl rings in a number
of polyaromatic derivatives. For example, we have noted that
a hole (formed by removal of a single electron) can hop
between two cofacially oriented aryl moieties even at an
angle of 120° in various ethanoanthracene derivatives as
readily judged by the appearance of characteristic near-
infrared (NIR) charge-resonance absorption bands in the
absorption spectra of their cation radicals.3
(1) (a) Zimmermann, T. J.; Freundel, O.; Gompper, R.; Muller, T. J. J.
Eur. J. Org. Chem. 2000, 3305. (b) Keegstra, M. A.; De Feyter, S.; De
Schryver, F. C.; Mullen, K. Angew. Chem., Int. Ed. Engl. 1996, 35, 774.
(c) Wang, S.; Oldham, W. J., Jr.; Hudock, R. A., Jr.; Bazan, G. C. J. Am.
Chem. Soc. 2000, 124, 5695 and references therein.
(2) Compare: Larson, E. M.; Von Dreele, R. B.; Hanson, P.; Gust, J. D.
Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1990, C46, 784.
(3) (a) Rathore, R.; Kochi, J. K. Can. J. Chem. 1999, 121, 913. (b)
Rathore, R.; Abdelwhaed, S. H.; Guzei, I. A. J. Am. Chem. Soc. 2003, 125,
8712. (c) Rathore, R.; Johnson, R. S. Manuscript under preparation.
10.1021/ol036037g CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/23/2004

Scheme 1. Synthesis of Polychromophoric Donor 5 and a
Model Tetraphenylethylene Donor 8a
Figure 1. Structures showing the propeller arrangement (A, left)
and orbital overlap among six phenyl groups (B, right) in hexaphe-
nylbenzene.
Interestingly, however, we4 and others5 have recently
shown that the incorporation of electro-active aryl groups
on the vertexes of the HPB core [such as hexakis(4-methyl-
2,5-dimethoxy-biphenyl)-benzene, 1] did not show any
significant electronic interaction among each other in their
cation radicals.4 Such a conclusion was supported by the fact
that (i) the cyclic voltammogram of 1 showed a single
reversible (six-electron) wave and (ii) the UV-vis absorption
spectra of 1 at various oxidation states (i.e., 1+• to 16+•) were
identical to the absorption spectrum of the cation radical of
4,4′-dimethyl-2,5-dimethoxybiphenyl (2, a model com-
pound). The lack of electronic coupling between various aryl
groups in 1 was attributed to the fact that the hole is
completely localized on the dimethoxytolyl (p-hydroquinone
ether) moiety.4 It was thus envisioned that incorporation of
an electro-active group that allows the hole to reside over
the phenyl groups that constitute the propeller of the
hexaphenylbenzene core may allow an effective electronic
coupling among the electro-active groups.
a Conditions: (a) (PPh3)PdCl2, CuI, CH3CN/piperdine (2:1),
reflux; (b) Co2(CO)8, dioxane, reflux; (c) diphenylmethyl lithium
(generated from n-BuLi and diphenyl-methane in ether); (d)
p-toluenesulfonic acid in toluene; (e) tetraphenylcyclopentadienone,
Ph2O, reflux.
phenyl)acetylene (3), obtained by a Pd-catalyzed coupling
of commercially available 4-bromobenzophenone with gas-
eous acetylene in 65% yield, was trimerized in refluxing
dioxane in the presence of a cobalt catalyst to the previously
unknown hexakis(4-benzoylphenyl)benzene (4).
We now report that such a criteria could be met in a
polychromophoric molecule 5 in which the six tetraphenyl-
ethylene moieties are connected to a central benzene ring.
The choice of this electro-active moiety stems from the fact
that one of the phenyl rings of each tetraphenylethylene unit
is also part of the propeller of hexaphenylbenzene core (see
structure 5, Scheme 1) as well as the fact that it has rich
electrochemical and excited-state properties; furthermore, it
has been extensively utilized as an electron-transfer catalyst
in a variety of polymerization and coupling reactions.6
A reaction of freshly prepared diphenylmethyllithium
(obtained by a reaction of n-butyllithium with diphenyl-
methane in ether at 0 °C) with 4 afforded the hexaalcohol,
which was dehydrated directly by refluxing in toluene in the
presence of a catalytic amount of p-toluenesulfonic acid, to
afford a highly soluble hexakis-tetraphenylethylene derivative
5 in >90% yield. The mono-tetraphenylethylene analogue
8 was prepared similarly from 7, which in turn was
synthesized by a Diels-Alder reaction of 4-benzoyltolan with
tetraphenylcyclopentadienone in refluxing diphenyl ether (see
Supporting Information for experimental details). The struc-
tures of these highly symmetrical macromolecular donors 5
and 8 were readily established by 1H/13C NMR spectroscopy
as well as by correct elemental analysis and FAB mass
spectrometry.
Accordingly, this report constitutes the synthesis of
dendritic donor 5 (Scheme 1) as well as a model compound
8, containing only one tetraphenylethylene moiety (Scheme
1), and delineation of the hopping of a single hole over six
tetraphenylethylene moieties in its monocation radical (5+•)
by electrochemical and optical measurements as follows.
As shown in Scheme 1, the synthesis of 5 was ac-
complished via a three-step route. Thus, bis(4-benzoyl-
The electronic absorption spectra of the parent TPE, model
donor 8, and the hexaethylenic donor 5 obtained as dichlo-
romethane solutions are compared in Figure S1 (see Sup-
porting Information). The UV-vis absorption spectra of 5
and 8 are highly characteristic to that of the parent TPE with
(4) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett. 2001, 3, 2887.
(5) Lambert, C.; Noll, G. Angew. Chem., Int. Ed. 1998, 37, 2107 and
references therein.
(6) (a) Salem, L. Science 1976, 191, 822. (b) Salem, L.; Bruckmann, P.
Nature 1975, 258, 526. (c) Wolf, M. O.; Fox, H. H.; Fox, M. A. J. Org.
Chem. 1996, 61, 287.
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Org. Lett., Vol. 6, No. 11, 2004

The presence of multiple oxidation waves in the CV of 5
(in contrast to the single six-electron oxidation wave previ-
ously observed for 1)4 suggests that the removal of the first
electron from 5 affects the removal of further electrons and
thus bears the earmarks that various tetraphenylethylene (D)
moieties in 5 are electronically coupled via the propeller
formed by six phenyl rings. A quantitative evaluation of the
CV peak currents of various waves in 5 (with ferrocene as
an internal standard) was hampered due to the overlapping
of the waves in its cyclic voltammogram (see Figure 2A and
S2).
To determine how many electrons were involved in the
various CV peaks of 5 as well as the spectral characteristics
of various oxidized species, we carried out chemical cou-
lometry using a hindered naphthalene cation radical salt4 (9+•)
as a stable one-electron aromatic oxidant (Ered ) 1.34 V vs
SCE) in dichloromethane as follows. [It is noteworthy that
species with redox potential greater than ∼1.35 V cannot
be oxidized to any appreciable amounts using 9+•.] Thus,
treatment of a blue solution of 9+• (λmax ) 672 nm, log ꢀ672
) 3.97 M-1 cm-1, see green spectrum in Figure 3A) with
incremental amounts of 5 showed that naphthalene cation
radical 9+• was completely consumed after the addition of
1/3 equiv of 5 as judged by the presence of well-defined
isosbestic points at 704 and 420 nm (see Figure 3A, red
spectra). Moreover, reverse addition of a solution of 9+• (0.3
mmol, 0.01 M) to a solution containing 1/3 equiv of 5 (0.1
mmol, 0.1 M) in dichloromethane completely consumed the
naphthalene cation radical, as shown by UV-vis spectros-
copy (see Figure 3A). Moreover, both modes of mixing
oxidant 9+• and 5 produced an identical red-colored species
with characteristic twin absorption bands7 at 490 and 965
nm (together with a very weak band at 1240 nm). Interest-
ingly, reduction of the red-colored solution obtained above
with zinc dust afforded the neutral 5 in quantitative yield as
shown by NMR spectroscopy and by comparison with an
authentic sample. Thus, a clean spectral coulometry in Figure
3A as well as quantitative regeneration of 5 from the oxidized
solution (i.e., by reduction with zinc dust) suggests that the
Figure 2. Cyclic voltammograms of 2 mM hexakis(4-tetraphen-
ylethylene)benzene 5 (A) and model donor 8 (B) in CH2Cl2
containing 0.2 M tetra-n-butylammonium hexafluorophosphate at
a scan rate of 200 mV s-1 and at 22 °C.
the slight red shift of the absorption maxima in the spectrum
of 5 as compared to model compound 8 and parent TPE.
Moreover, the measured extinction coefficient at the maxi-
mum of 5 (λmax ) 324 nm, ꢀmax ) 103 200 M-1 cm-1) is
found to be slightly higher than the expected 6-fold increase
in comparison to the model 8 (λmax ) 319 nm, ꢀmax ) 16 300
M-1 cm-1) and parent tetraphenylethylene (λmax ) 306 nm,
ꢀmax ) 14 100 M-1 cm-1). These observations suggest a weak
electronic coupling among the tetraphenylethylene moieties
in ground state of 5 through the hexaphenylbenzene propeller.
The electron donor strength of 5 in comparison with its
model compound 8 was evaluated by electrochemical oxida-
tion at a platinum electrode as a 2 × 10-3 M solution in
dichloromethane containing 0.2 M n-Bu4NPF6 as the sup-
porting electrolyte. The cyclic voltammogram of 5 in Figure
2A showed three overlapped oxidation waves at potentials
of 1.22, 1.34, and 1.47 V vs SCE, whereas the model donor
8 showed only one reversible wave at a potential of 1.28 V
vs SCE (Figure 2B). [Note that due to the extensive overlap
of the oxidation waves in the CV of 5, its three oxidation
potentials were further confirmed by square wave voltam-
metry, as shown in Figure S2 in Supporting Information.]
Figure 3. A. Spectral changes upon the reduction of 1.1 × 10-4 M 9+• (green) in dichloromethane at 22 °C by incremental addition of 4.7
× 10-3 M 5 to give first 53+• (red) and then 5+• (black) upon further additions of neutral 5. B. Plot of deconvoluted spectra of 5+•. C.
Similarity in the UV-vis region of the spectrum of the cation radical 5+• (blue, partial spectrum obtained by deconvolution) with that of
the model 8+• (magenta).
Org. Lett., Vol. 6, No. 11, 2004
1691

oxidation of 5 to its trication radical 53+• indeed occurs
according to the stoichiometry in Scheme 2.
hexaphenylbenzene propeller by utilizing adjacent TPE
donor (D1-D6) moieties, as pictorially shown below.
The thesis that the hole hops over all six TPE donor (D)
moieties in 5+• is based on ample literature precedence.3,8
For example, the cofacial orientation of phenylene moieties,
even at 120°, in various ethanoanthracene derivatives allows
extensive electronic coupling, as established by the observa-
tion of intense charge-resonance bands in the NIR region.3
It is also important to mention here that the hole in 5+• cannot
hop on various TPE moieties via the central benzene ring
because they lie perpendicular to each other.8 Moreover, the
symmetrical nature of the molecule suggests that the hole
can hop via electron transfer from one of the two adjacent
donor moieties with equal probability, and occurrence of
enough rapid repetitions of this electron-transfer process in
5+• ensures that the hole migrates on all TPE moieties (see
the structures above in Scheme 3).
Scheme 2 Redox Titration of 5 with Naphthalene Cation
Radical 9+•
The exact nature of the 53+• in Scheme 2 cannot be
delineated with confidence at this point; however, a treatment
of the red-colored solution obtained in Scheme 2 with 3 equiv
of octamethylbiphenylene4 (OMB, an electron donor with
Eox ) 0.80 V vs SCE) produced 3 equiv of highly robust
OMB+• (λmax ) 600 nm, log ꢀ672 ) 3.97 M-1 cm-1),4 as
confirmed by UV-vis spectroscopy.
Scheme 3. Schematic Representation of the Hole Hopping in
5+•
Interestingly, a further incremental addition of hexaeth-
ylenic donor 5 (∼2 equiv) to the above solution of 53+•
showed a steady increase in the absorbance at 490 and ∼1000
nm as well as an appearance of a new (broad) overlapped
absorption band extending in the near-IR region (Figure 3A,
black spectra). Furthermore, the final spectrum in Figure 3A
remained unchanged upon addition of a large excess of 5
and was thus assigned to the monocation radical 5+•. The
spectral identity of 5+• was further confirmed by its genera-
tion using different oxidants4 (such as DDQ/CF3COOH,
SbCl5, NOSbCl6, etc.) in the presence of an excess of 5, as
well as by complete recovery of the neutral 5 by the reduction
of the above solutions using zinc dust and octamethylbi-
phenylene (compare Scheme 2).
In summary, we have synthesized a novel (circular)
dendritic structure in which the observation of an intense
charge-resonance transition (1450 nm) is suggestive of the
fact that a single hole is mobilized via electron transfer over
six identical redox-active centers arranged cofacially in a
circular array.10 Such an observation, coupled with ready
preparation of these structures, should spur theoretical
exploration of this new class of intervalence materials in
which a hole can hop over multiple redox centers. We are
currently exploring the potential applications of these materi-
als and a variety of other hexaarylbenzene derivatives for
the preparation of photonic devices.
A careful deconvolution of the spectrum of 5+• in Figure
3B (using a standard software) revealed that it consisted of
the characteristic twin absorption bands (λmax ) 496, 568
(sh), and 1010 nm) due to the cationic tetraphenyl-ethylenic
moiety (D+•),7 as confirmed by a spectral comparison with
the model 8+• (λmax ) 495, 570(sh), and 1000 nm; magenta
spectrum) in Figure 3C, as well as an additional broad
absorption band at 1450 nm (WAHM ) 5608 cm-1), see
Figure 3B. It is important to note that the position and
intensity of the near-IR band in 5+• remained unchanged upon
changing the solvent polarity, i.e., from dichloromethane to
acetonitrile.
Acknowledgment. We are grateful to Professor F. A.
Khan (State University of West Georgia at Carrolton) for
the mass spectral data of 5 and 8 and the donors of the
Petroleum Research Fund (AC12345), administered by the
American Chemical Society, and National Science Founda-
tion (Career Award) for financial support; C.L.B. thanks the
Department of Education for a GAANN fellowship.
Supporting Information Available: Synthetic details and
1H/13C NMR data for 5 and 8 and Figure S1 and S2. This
material is available free of charge via the Internet at
http://pubs.acs.org.
The new NIR transition, which is largely absent in the
trication radical 53+• and the model cation radical 8+•, is
believed to arise due to the rapid hopping of a single hole
over six tetraphenylethylene (TPE) moieties in 5+• via the
OL036037G
(8) See: Nelsen, S. F. Chem. Eur. J. 2000, 6, 581.
(9) Also note that the stability of C6I6 dication is attributed to a similar
electronic coupling between six iodo groups in C6I6; see: Sagl, D. J.; Martin,
J. C. J. Am. Chem. Soc. 1988, 110, 5827.
(7) For tetraphenylethylene cation radical, see: Barbosa, F.; Peron, V.;
Gescheidt, G.; Fu¨rstner, A. J. Org. Chem. 1998, 63, 8806.
(10) Photophysical characteristics of 5 will be reported separately.
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