X-ray structural characterization of charge delocalization onto the three equivalent benzenoid rings in hexamethoxytriptycene cation radical

Article abstract of DOI:10.1021/ol900558s

Definitive X-ray crystallographic evidence is obtained for a single hole (or a polaron) to be uniformly distributed on the three equivalent 1,2-dimethoxybenzenoid (or veratrole) rings in the hexamethoxytriptycene cation radical. This conclusion is further

Full text of DOI:10.1021/ol900558s

ORGANIC
LETTERS
2
009
Vol. 11, No. 11
253-2256
X-ray Structural Characterization of
Charge Delocalization onto the Three
Equivalent Benzenoid Rings in
2
Hexamethoxytriptycene Cation Radical
Vincent J. Chebny, Tushar S. Navale, Ruchi Shukla, Sergey V. Lindeman, and
Rajendra Rathore*
Department of Chemistry, Marquette UniVersity, P.O. Box 1881,
Milwaukee, Wisconsin 53201
rajendra.rathore@marquette.edu
Received March 17, 2009
ABSTRACT
Definitive X-ray crystallographic evidence is obtained for a single hole (or a polaron) to be uniformly distributed on the three equivalent
,2-dimethoxybenzenoid (or veratrole) rings in the hexamethoxytriptycene cation radical. This conclusion is further supported by electrochemical
1
analysis and by the observation of an intense near-IR transition in its electronic spectrum, as well as by comparison of the spectral and
electrochemical characteristics with the model compounds containing one and two dimethoxybenzene rings.
3
Interactions between aromatic rings via π-stacking are at the
origin of many phenomena of organic material science and
sandwich-like geometry. For example, we and others have
1
4
shown that a hole (formed by the removal of a single
biological chemistry including the electron transport in DNA
through stacked π-bases. It has been noted in a number of
recent studies that electronic coupling among the cofacially
oriented aryl moieties does not necessarily require a perfect
electron) can hop among multiple cofacially oriented aryl
moieties in various hexaarylbenzene derivatives as judged
by the appearance of a characteristic near-infrared (NIR)
intervalence transition in the absorption spectra of their cation
radicals. From this standpoint, triptycene and its derivatives
are fundamentally important molecules in which the three
equivalent benzenoid rings are cofacially oriented at an angle
of ∼120°. It has been shown with the aid of ESR and
photoelectron spectroscopy that there is significant electronic
coupling among the three benzenoid rings in triptycene and
2
(
1) (a) Wu, J.; Pisula, W.; M u¨ llen, K. Chem. ReV. 2007, 107, 718. (b)
Haddon, R. C.; Chi, X.; Itkis, M. E.; Anthony, J. E.; Eaton, D. L.; Siegrist,
T.; Mattheus, C. C.; Palstra, T. T. M. J. Phys. Chem. B 2002, 106, 8288.
(
2
c) Plass, K. E.; Grzesiak, A. L.; Matzger, A. J. Acc. Chem. Res. 2007, 40,
87. (d) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.; Br e´ das,
J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436. (e) Rathore,
R.; Le Magu e` res, P.; Kochi, J. K. J. Org. Chem. 2000, 65, 6826, and
references cited therein.
5
its derivatives. It is also important to note that the triptycene
(
2) (a) Lewis, F. D.; Letsinger, R. L.; Wasielewski, M. R. Acc. Chem.
6
Res. 2001, 34, 159. (b) Dekker, C.; Ratner, M. A. Phys. World 2001, 14,
scaffold is being extensively explored for the preparation
2
9. (c) Grinstaff, M. W. Angew. Chem., Int. Ed. 1999, 38, 3629.
(
3) (a) Rathore, R.; Burns, C. L.; Abdelwahed, S. H. Org. Lett. 2004,
6
1
9
, 1689. (b) Chebny, V. J.; Shukla, R.; Rathore, R. J. Phys. Chem. B 2006,
10, 13003. (c) Shukla, R.; Lindeman, S. V.; Rathore, R. Org. Lett. 2007,
, 1291. (d) Chebny, V. J.; Dhar, D.; Lindeman, S. V.; Rathore, R. Org.
(4) (a) Sun, D.; Rosokha, S. V.; Kochi, J. K. Angew. Chem., Int. Ed.
2005, 44, 5133. (b) Rosokha, S. V.; Neretin, I. S.; Kochi, J. K. J. Am.
Chem. Soc. 2006, 128, 9394. (c) Lambert, C. Angew. Chem., Int. Ed. 2005,
44, 7337, and references cited therein.
Lett. 2006, 8, 5401.
10.1021/ol900558s CCC: $40.75
Published on Web 05/04/2009
 2009 American Chemical Society

of modern materials for the potential use in the emerging
areas of molecular electronics and nanotechnology.
transfer of one electron) at Eox1 ) 1.22, 1.11, and 1.00 (V
7
vs SCE), respectively. The second oxidation in 2 (Eox2
1.39 V) and the second and third oxidations in 3 (Eox2
)
)
Our continued interest in the design, syntheses, and
exploration of electronic coupling in the cofacially arrayed
1.29 and Eox3 ) 1.78 V) occur at relatively higher potentials
8
polybenzenoid structures led us to isolate a crystalline
(see Figure 1) and thus are indicative of the electronic
10
cation-radical salt of a hexamethoxytriptycene derivative (3),
which provides definitive (X-ray) structural evidence that a
single hole (or a polaron) is distributed evenly on the three
equivalent 1,2-dimethoxybenzene (or veratrole) rings. Fur-
thermore, the effective electronic coupling among the three
cofacially oriented veratrole rings in 3 is further corroborated
by comparison of the spectral and electrochemical charac-
teristics with the model compounds containing one and two
dimethoxybenzene rings (i.e., 1 and 2).
coupling among the cofacially oriented veratrole moieties.
As such, the lowered first oxidation potentials of 2 and 3 by
∼110 and ∼220 mV, respectively, as compared to the model
donor 1 attest to the significant stabilization of the cationic
charge by the cofacially arrayed veratrole rings in 2 and 3
owing to the electronic coupling among the cofacially
1
0
oriented veratrole rings in 2 and 3.
The electrochemical reversibility and relatively low oxida-
tion potentials of 1-3 allow the generation of their cation
radicals in solution using a hydroquinone ether cation radical
•
+
(
7
(
3
CRET , Ered ) 1.14 V vs SCE; λmax ) 518 nm; ε518 )
-
1
-1 11
300 M cm ) or a hindered naphthalene cation radical
•
+
NAP , Ered ) 1.34 V vs SCE; λmax ) 672, 616, 503, and
-
1
-1 12
96 nm; ε672 ) 9300 M cm ) as stable (aromatic) one-
electron oxidants.
Thus, Figure 2A shows the spectral changes observed upon
an incremental addition of substoichiometric amounts of 3
The syntheses of 1-3 were easily accomplished by the
6
c,9
adaptation of standard literature procedures,
and the
1
13
relevant details along with the H and C NMR spectral data
are summarized in the Supporting Information.
The electrochemical oxidations of 1-3 at a platinum
electrode in CH
first electron at varying scan rates of 25-400 mV s ; all
anodic/cathodic peak current ratios were I /I ) 1.0 (theoreti-
2 2
Cl were reversible for the removal of the
-
1
a
c
cal) at room temperature (Figure 1). A quantitative evaluation
of the CV peaks and peak currents with added ferrocene (as
an internal standard, Eox ) 0.45 V vs SCE) revealed that
the first wave in the reversible cyclic voltammograms of 1-3
correspond to the production of the mono cation radical (by
Figure 2. (A) Spectral changes observed upon the reduction of 6.5
-5
•+
×
10 M CRET by an incremental addition of substoichiometric
amounts of 3 in CH Cl at 22 °C. (B) Plot of depletion of
absorbance of CRET (red squares, at 518 nm) and an increase of
2
2
•
+
•
+
the absorbance of 3 (green circles, at 1510 nm) against the
equivalent of added neutral 3 (data points from plot 2A). (C)
Comparison of the molar absorptivity of the 1-3 cation radicals
2 2
in CH Cl at 22 °C.
to a solution of CRET• in dichloromethane at 22 °C. A plot
+
of formation of the 3• (i.e., increase in the absorbance at
+
1
510 nm) against the increments of added neutral 3 (see
•+
Figure 2B) established that CRET was completely con-
sumed after the addition of 1 equiv of 3, and the resulting
•
+
absorption spectrum of 3 (λmax ) 408, 1510 nm; ε1510
)
-1
-1
1
8300 M cm ) remained unchanged upon further addition
1
3
of neutral 3 (i.e., eq 1).
(
5) (a) Quast, H.; Fuchsbauer, H. L. Chem. Ber. 1986, 119, 1016. (b)
Russell, G. A.; Suleman, N. K.; Iwamura, H.; Webster, O. W. J. Am. Chem.
Soc. 1981, 103, 1560. (c) Kobayashi, T.; Kubota, T.; Ezumi, K. J. Am.
Chem. Soc. 1983, 105, 2172.
(
6) (a) Swager, T. M. Acc. Chem. Res. 2008, 41, 1181. (b) Thomas,
S. W., III; Joly, G. D.; Swager, T. M. Chem. ReV. 2007, 107, 1339. (c)
Zhu, X.-Z.; Chen, C.-F. J. Am. Chem. Soc. 2005, 127, 13158. (d) Zhu,
X.-Z.; Chen, C.-F. J. Org. Chem. 2005, 70, 917.
Figure 1
containing 0.2 M nBu
at a scan rate of ν ) 200 mV s at 22 °C.
.
Cyclic voltammograms of 1, 2, and 3 (5 mM) in CH
2 2
Cl
(
4
NPF as the supporting electrolyte) measured
6
(
7) (a) Introduction to Molecular Electronics; Petty, M. C., Bryce, M. R.,
-1
Bloor, D., Eds.; Oxford University Press: New York, 1995. (b) Maiya, B. G.;
Ramasarma, T. Curr. Sci. 2001, 80, 1523.
2254
Org. Lett., Vol. 11, No. 11, 2009

Similarly, the cation radical spectra of 1 and 2 were generated
•+
•+
using NAP and/or CRET and are compared with the cation
radical spectrum of triptycene 3 in Figure 2C. The observation
of intense NIR transitions in the absorption spectra of bichro-
•
+
-1
-1
mophoric 2 (λmax ) 420, 1560 nm; ε1560 ) 8400 M cm )
•+
and trichromophoric 3 (λmax ) 1510 nm) should be contrasted
with a singular lack of any absorption beyond 500 nm in the
absorption spectrum of the monochromophoric cation radical
•+
-1
-1
1
(λmax ) 461 nm; ε461 ) 5200 M cm ) (Figure 2C).
•+
Moreover, the fact that the NIR transition in 3 (λmax ) 1510
nm) is roughly twice as intense as in 2• (λmax ) 1560 nm) is
most likely due to the increased probability of the hole migration
+
•+
over the three veratrole rings in 3 as opposed to only the two
•
+
14
rings in 2 .
The cation radicals of donors 1-3, obtained according to eq
, are highly persistent at ambient temperatures and did not
1
show any decomposition during a 12 h period at ∼22 °C, as
confirmed by UV-vis spectral analysis. The single crystals of
•+
the 3 , suitable for X-ray crystallography, were obtained by a
slow diffusion of toluene into the dichloromethane solutions
•+
-
+
-
-
6 6
of 3 SbCl , prepared using equimolar NO SbCl as a 1-e
oxidant, at -10 °C during the course of 2 days (see the
Supporting Information for the experimental details). Note that
•
+
-
the repeated attempts to obtain single crystals of 1 SbCl
6
•+
-
6
and 2 SbCl , suitable for X-ray crystallography, have been
thus far unsuccessful.
The crystallographic analysis of the highly colored crystals
of 3• SbCl
+
-
6
revealed that cationic triptycenes pack in layers
•
+
-
Figure 3
the toluenes and SbCl
.
(A) Packing diagram of 3 SbCl
6
cation radical showing
in the crystallographic plane ac with embedded toluene
molecules, and in between these layers lie the hexachloro-
antimonate anions and additional solvent (i.e., disordered
toluene) molecules (Figure 3A). Within a layer, each
-
6
counteranions embedded between the layers
of cationic triptycenes and additional toluene molecules. (B)
Arrangement of cationic triptycenes and toluene molecules within
•
+
-
a single layer. (C) ORTEP diagram of 3 SbCl
6
showing the
closest (2.4 Å) and farthest (6.5 Å) separation between the veratrole
1
5
moieties in the cationic triptycene.
(
8) (a) Rathore, R.; Chebny, V. J.; Kopatz, E. J.; Guzei, I. A. Angew.
Chem., Int. Ed. 2005, 44, 2771. (b) Rathore, R.; Abdelwahed, S. H.; Guzei,
I. A. J. Am. Chem. Soc. 2003, 125, 8712. (c) Stevenson, C. D.; Kiesewetter,
M. K.; Reiter, R. C.; Abdelwahed, S. H.; Rathore, R. J. Am. Chem. Soc.
•
+
2
005, 127, 5282. (d) Rathore, R.; Abdelwahed, S. H.; Kiesewetter, M. K.;
independent molecule of the cationic triptycene (3 ) makes
a centrosymmetric dimer over the centers of symmetry [0 0
0] and [0 0 1/2] by forming a pseudo-1/2-translation along
the z-axis (i.e., Figure 3B). These dimeric units, in turn, form
infinite chains along the z-axis in which two out of the three
veratrole rings of each cationic triptycene moiety is involved
in face-to-face interactions with the neighboring molecules
with a center to center distance of ∼3.3-3.4 Å. The
remaining third veratrole ring of the cationic triptycene makes
Reiter, R. C.; Stevenson, C. D. J. Phys. Chem. B 2006, 110, 1536. (e)
Chebny, V. J.; Rathore, R. J. Am. Chem. Soc. 2007, 129, 8458. (f) Debroy,
P.; Lindeman, S. V.; Rathore, R Org. Lett. 2007, 9, 4091, and references
cited therein.
(
(
9) Davidson, I. M.; Musgrave, O. C. J. Chem. Soc. 1963, 3154.
10) Note that the removal of the first electron from 2 and 3 results in
the corresponding cation radicals where a charge is delocalized over two
pre-organized) cofacial veratrole moieties in 2 and over three veratrole
(
moieties in 3, and thereby rendering the ejection of the second electron
•
+
•+
from both 2 and 3 difficult by roughly ∼280 and 290 mV, respectively.
It is also noted that owing to the delocalization of charges in the dicationic
2
+
3
4
makes the ejection of the third electron much more difficult (i.e., by
90 mV).
(
11) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Synth. 2005, 82,
(15) Although, the geometrical parameter of all three benzenoid rings
in the triptycene cation radical are similar (within experimental precision),
each ring is crystallographically unique, and therefore, this structure does
not suffer from the problem of static crystalline disorder. Furthermore, the
X-ray crystallography together with the observation of the intense NIR
intervalence transitions in 2 and 3 cation radicals suggests that these
intervalence system belongs to Robin Day borderline class II/III systems.
Compare: Sun, D.-L.; Lindeman, S. V.; Rathore, R.; Kochi, J. K. J. Chem.
Soc., Perkin Trans. 2 2001, 1585.
1
.
(
(
12) Rathore, R.; Burns, C. L.; Deselnicu, M. I. Org. Lett. 2001, 3, 2887.
13) It should be noted that the addition of up to 10 equiv of neutral 3
•
+
•+
to the solution of 3 or increasing the concentration of 3 by 10-fold did
not show any change in its absorption spectrum.
(
14) (a) Nelsen, S. F. Chem.sEur. J. 2000, 6, 581. (b) Badger, B;
Brocklehurst, B. Nature 1968, 219, 263. (c) Kochi, J. K.; Rathore, R.; Le
Magueres, P. J. Org. Chem. 2000, 65, 6826.
Org. Lett., Vol. 11, No. 11, 2009
2255

a somewhat limited contact between the triptycene moieties
of the neighboring chains, as well as more distinct face-to-
face contacts with the toluene molecules (see Figure 3B).
A closer look at the structural parameters of the
veratrole groups in 3• suggests that the three crystallo-
graphically unique benzenoid rings undergo similar struc-
tural changes (within the experimental precision of ∼0.006
Å for various C-C/C-O bond lengths) and thus suggests
that the cationic charge is evenly distributed over all three
It is noteworthy that the experimental observations of
the similar bond length changes in all three veratrole
moieties together with a significant shortening of the bonds
labeled “d” in 3• were found to be in reasonable
agreement with the calculated values obtained using DFT
+
+
1
6
calculations at the B3LYP/6-31G* level (see Table 1).
Furthermore, it is noted that the bonds which undergo the
most dramatic lengthening (i.e., bonds e and f) and shortening
•+
(i.e., bonds a-d) in 3 are the bonds on which the HOMO
shows the largest bonding and antibonding character, re-
spectively (see Table 1 and Figure 4).
1
5
veratrole moieties. Furthermore, the bond length changes
•
+
in the cation radical 3 , together with a comparison of
its neutral form, the structure of which was established
by X-ray crystallography, correspond to the predominant
contributions from p-quinoidal resonance structures A/B,
as judged by the significant shortening of bonds labeled
“
a” and elongation of bonds labeled “e” and “f”, along
with only marginal shortening of the bonds labeled “b”
and “c” (see Scheme 1 and Table 1). It is also noted that
Scheme 1
.
Resonance Structures for the Stabilization of the
Figure 4. Two views of the HOMO of 3 obtained by DFT
calculations at the B3LYP/6-31G* level.
Cationic Charge in 3•
+
In summary, we have demonstrated that hexamethox-
ytriptycene (3) and the model compounds containing one
and two veratrole moieties (i.e., 1 and 2) undergo
reversible electrochemical oxidation and form stable
cation-radical salts. The significantly lowered first oxida-
tion potentials of bichromophoric 2 and triptycene 3 by
Table 1. Experimental (X-ray) and Theoretical (DFT- B3LYP/
-31G* level) Bond Lengths of the Neutral and Cation Radical
of 3 in Picometers (pm) (Two Views of the HOMO of 3
∼
110 and ∼220 mV, respectively, as compared to the
6
monochromophoric donor 1, as well as the observation
Obtained by DFT Calculations at the B3LYP/6-31G* Level)
•+
of intense NIR transitions in 3 (λmax ) 1510 nm) and
•
+
2
(λmax ) 1560 nm), and a complete absence of the NIR
•
+
transition in monochromophoric 1 , attest to the effective
electronic coupling among the cofacially arrayed veratrole
rings in 2 and 3. The isolation and X-ray crystal structure
determination of the neutral and cation radical of 3, as
well as the DFT calculations now provide unequivocal
evidence that a single charge (or polaron) is evenly
distributed over all three veratrole moieties. Efforts are
now underway to construct polyaromatic arrays based on
the triptycene scaffold to explore their conducting proper-
ties as well as potential applications in the emerging areas
B3LYP/6-31G*
3•+ ∆ (3•+-3)
X-ray data
3•+ ∆ (3•+-3)
a
bond
3
3
7
of molecular electronics and nanotechnology.
a
b
c
d
e
f
g
σ
a
136.6 134.3
139.8 139.6
139.6 139.5
154.2 153.4
141.3 143.4
139.6 141.2
141.5 142.7
-2.3
-0.2
-0.1
-0.8
+2.1
+1.6
+1.2
137.2 135.0
139.2 138.4
139.0 138.9
154.0 153.0
141.1 142.3
138.8 140.5
142.7 143.8
-2.2
-0.8
-0.1
-1.0
+1.2
+1.7
+1.1
Acknowledgment. We thank the National Science
Foundation (CAREER Award) and Marquette University
for financial support.
Supporting Information Available: Synthetic details,
H/ C NMR data, and X-ray data of various compounds.
1
13
0.4
0.6
This material is available free of charge via the Internet
at http://pubs.acs.org.
Average of equivalent bonds.
OL900558S
the alternate resonance structures C-E do not seem to
contribute to the stabilization of the cationic charge in
any significant way.
(
16) Compare: Banerjee, M.; Vyas, V. S.; Lindeman, S. V.; Rathore,
R. Chem. Commun. 2008, 1889.
(17) Davidson, I. M.; Musgrave, O. C. J. Chem. Soc. 1963, 3154.
2256
Org. Lett., Vol. 11, No. 11, 2009