Polaron delocalization in ladder macromolecular systems

Article abstract of DOI:10.1021/ja050184n

Organic macromolecules with conjugated building blocks have been the focus of extensive research that is motivated, in part, by the potential to create optical and electronic devices. We have shown that palladium-catalyzed amination can assemble triarylam

Full text of DOI:10.1021/ja050184n

Published on Web 06/04/2005
Polaron Delocalization in Ladder Macromolecular Systems
X. Z. Yan,‡ J. Pawlas,† T. Goodson, III,*,‡ and J. F. Hartwig*,†
Contribution from the Department of Chemistry, Yale UniVersity,
New HaVen, Connecticut 06520-8107, and Department of Chemistry,
UniVersity of Michigan, Ann Arbor, Michigan 48109
Received January 31, 2005; E-mail: tgoodson@umich.edu; john.hartwig@yale.edu
Abstract: Organic macromolecules with conjugated building blocks have been the focus of extensive
research that is motivated, in part, by the potential to create optical and electronic devices. We have shown
that palladium-catalyzed amination can assemble triarylamine ladder materials with extended structures.
Two ladder macromolecules have been prepared in high yields by a series of twelve or sixteen C-N coupling
reactions. Studies of the electronic and optical properties of neutral and oxidized forms of the ladder
structures were conducted. The optical and electronic properties of the ladder systems are compared to
those of the linear tetra-phenyl-p-phenylenediamine as well as the tetra-p-anisyl-p-tetraazacyclophane.
The electrochemistry of the ladder systems consists of a multiwave voltammogram with a relatively low
first oxidation potential. Electron paramagnetic resonance spectroscopy of the ladder systems suggests
the presence of a large density of delocalized polarons. Linear absorption measurements of the chemically
oxidized ladders revealed both polaron and intervalence absorption bands. Steady-state and time-resolved
fluorescence measurements were also carried out to characterize the dynamics in these novel systems.
Introduction
Polymeric arylamines are promising candidates for the
generation of polaronic ferromagnets.9-12 Polaronic states of
macromolecules result from charge separation after excitation
and are stabilized by deformation of the molecular conforma-
tion.54 There is a significant interaction between the polarons
and phonons. While polaronic ferromagnets have attracted
attention over the last two decades,10c,13,14 the majority of such
materials are spinless bipolarons (quinonoid-type dication), or
π-dimers with low spin concentrations in doped conjugated
polymers. This lack of spin has limited the applications of these
structures as magnetic materials.15 A notable exception is the
emeraldine salt of polyaniline. In this material, polarons are
favored over bipolarons, and the presence of polarons in
polyaniline leads to a significant spin concentration.16,17 Because
n-doped polymeric arylamines are similar to the aryl-substituted
leucomeraldine form of polyaniline, interesting magnetic prop-
erties have also been observed from n-doping of polymeric
arylamines.10
Organic macromolecules containing triarylamine building
blocks have been extensively investigated due to their interesting
electronic1-8 and magnetic9-12 properties. A common hurdle
confronting the synthesis of macromolecular and polycyclic
arylamines has been the low yields that result from multiple
aminations of aryl halides, historically conducted by copper-
mediated chemistry. Improved methods to prepare arylamines
would, therefore, allow one to prepare new architectures with
increasing complexity.
† Yale University.
‡ University of Michigan.
(1) Selby, T. D.; Blackstock, S. C. J. Am. Soc. 1998, 120, 12155.
(2) Selby, T. D.; Kim, K.-Y.; Blackstock, S. C. Chem. Mater. 2002 14, 1685.
(3) Kim, K.-Y.; Hassenzahl, J. D.; Selby, T. D.; Szulczewski, G. J.; Blackstock,
S. C. Chem. Mater. 2002, 14, 1691.
(4) Bonvoisin, J.; Launay, J.-P.; Verbouwe, W.; der Auweraer, M. V.; de
Schryver, F. C. J. Phys. Chem. 1996, 100, 17079.
(5) Bonvoisin, J.; Launay, J.-P.; der Auweraer, M. V.; de Schryver, F. C. J.
Phys. Chem. 1994, 98, 5052.
(6) Thelakkat, M. Macromol. Mater. Eng. 2002, 287, 442.
(7) Redecker, M.; Bradley, D. D. C.; Inbasekaran, M.; Wu, W. W.; Woo, E.
P. AdV. Mater. 1999, 11, 241.
Cyclophanes have been the subject of recent interest in
materials science because of their defined three-dimensional
structure.11,18,19 To exploit the favorable electronic and magnetic
(8) Forrest, S. R. Chem. ReV. 1997, 97, 1793.
(9) Small molecules: (a) Sato, K.; Yano, M.; Furuichi, M.; Shiomi, D.; Takui,
T.; Abe, K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota, Y. J. Am. Chem.
Soc. 1997, 119, 6607-6613. (b) Yoshizawa, K.; Chano, A.; Ito, A.; Tanaka,
K.; Yamabe, T.; Fujita, H.; Yamauchi, J.; Shiro, M. J. Am. Chem. Soc.
1992, 114, 5994. (c) Stickley, K. R.; Blackstock, S. C. Tetrahedron Lett.
1995, 36, 1585. (d) Ito, A.; Urabe, M.; Tanaka, K. Angew. Chem., Int. Ed.
2003, 42, 921.
(13) (a) Anderson, K. K.; Dougherty, D. A. AdV. Mater. 1998, 10, 688. (b)
Murray, M. M.; Kaszynski, P.; Kaisaki, D. A.; Chang, W.; Dougherty, D.
A. J. Am. Chem. Soc. 1994, 116, 8152. (c) Kaisaki, D. A.; Chang, W.;
Dougherty, D. A. J. Am. Chem. Soc. 1991, 113, 2764.
(14) (a) Fukutome, H.; Takahashi, A.; Ozaki, M. Chem. Phys. Lett. 1987, 133,
34. (b) Bushby, R. J.; Ng, K. M. Chem. Commun. 1996, 659.
(15) (a) Hill, M. G.; Mann, K. R.; Penneau, J.-P. J. Am. Chem. Soc. 1992, 114,
2728. (b) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. ReV. 1988, 88, 183.
(16) (a) Wudl, F.; Angus, R. O.; Lu, F. L.; Allemand, P. M.; Vachon, D. J.;
Nowak, M.; Liu, Z. X.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 3677.
(b) Stafstrom, S.; Bredas, J. L.; Epstein, A. J.; Woo, H. S.; Tanner, D. B.;
Huang, W. S.; MacDiarmid, A. G. Phys. ReV. Lett. 1987, 59, 1464.
(17) Frisch, H. L.; Song, H.; Ma, J.; Rafailovich, M.; Zhu, S.; Yang, N.-L.;
Yan, X. J. Phys. Chem. B. 2001, 105, 11901.
(10) Polymers and dendrimers: (a) Wienk, M. M.; Janssen, R. A. J. J. Am.
Chem. Soc. 1997, 119, 4492. (b) Takahashi, M.; Nakazawa, T.; Tsuchida,
E.; Nishide, H. Macromolecules 1999, 32, 6383. (c) Bushby, R. J.; Gooding,
D.; Vale, M. E. Philos. Trans. R. Soc. London A 1999, 357, 2939.
(11) Macrocyclic molecules: (a) Hauck, S. I.; Lakshmi, K. V.; Hartwig, J. F.
Org. Lett. 1999, 1, 2057. (b) Ito, A.; Ono, Y.; Tanaka, K. Angew. Chem.,
Int. Ed. 2000, 39, 1072. (c) Selby, T. D.; Blackstock, S. C. Org. Lett. 1999,
1, 2053.
(12) Cho, J. S.; Uchida, K.; Yoshioka, N.; Yamamoto, K. Sci., Technol. AdV.
Mater. 2004, 5, 697.
9
10.1021/ja050184n CCC: $30.25 © 2005 American Chemical Society
J. AM. CHEM. SOC. 2005, 127, 9105-9116
9105

A R T I C L E S
Yan et al.
(144 mg, 0.251 mmol, 0.050 equiv), and tri-tert butylphosphine (40
mg, 0.20 mmol, 0.04 equiv) in 30 mL of toluene to give a purple slurry.
The mixture was removed from the box and stirred at room temperature.
After 17 h, the thick brown mixture was poured into a saturated aqueous
solution of ammonium chloride and extracted two times with toluene.
The combined organics were washed with brine, dried over magnesium
sulfate, and concentrated under vacuum to give an oil. The crude
material was crystallized from toluene-ethanol to give 2.45 g (89%)
of 4 as a tan solid. 1H NMR (500 MHz, C6D6) δ 6.65-6.69 (m, 12 H),
6.78-6.81 (m, 4 H), 7.09-7.10 (m, 4 H). 13C NMR (125 MHz, C6D6)
δ 122.16, 123.49, 124.10, 125.95, 130.68, 135.44, 142.75, 148.74. Anal.
Calcd for C30H20N2Cl4: C, 65.48; H, 3.66; N, 5.09. Found: C, 65.77;
H, 3.64; N, 4.86.
properties of arylamine materials in combination with a defined
architecture, we and others have studied cyclophane structures
composed of triarylamine units. Cyclophanes that contain
alternating p-phenylene and m-phenylene or related aromatic
units have been prepared as means to place stable radicals in a
configuration that could impart ferromagnetic interactions
between spins.20-22 Extension of the cyclophane structure to
oligomeric ladder molecules built of cyclophane units would
place a series of alternating m- and p-phenylenediamines in an
equally defined geometry in larger structures.23
Polycyclic arylamines, such as ladders built from cyclophane
units, could possess energy traps and electron traps at the
junctions between individual macrocycles.11 These electron traps
could increase the density of polarons and enhance spin-spin
interaction between polaronic segments in certain arylamine
systems.11 The large number of redox sites on macromolecular
arylamines with stable polarons may also be used in the
development of charge-capture and storage applications.1-3
Thus, the preparation of such ladder structures has been
discussed as a target for materials synthesis.11a,b
Although polymeric and oligomeric arylamines might possess
useful electronic and magnetic properties, the fundamental
optical and electronic properties of these materials are not well
understood. Further investigations of the polaronic behavior of
arylamines should improve the electronic and magnetic perfor-
mances of new materials and should elucidate the fundamental
physical chemistry of these materials. We report in this
manuscript the synthesis of unusual triarylamine ladder struc-
tures by a series of palladium-catalyzed C-N coupling reactions
and the use of these materials as a platform to study the
fundamental electronic, magnetic, and photophysical properties
of triarylamine materials of defined architectures.
N,N,N′,N′-Tetrakis(3-(4-hexyloxyanilino))-1,4-benzenediamine (5).
In a drybox, 4 (275 mg, 0.500 mmol, 1.00 equiv) was combined with
4-hexyloxyaniline (502 mg, 2.60 mmol), sodium tert-butoxide (260 mg,
2.71 mmol), Pd(dba)2 (14 mg, 0.025 mmol, 0.050 equiv), and tri-tert
butylphosphine (4 mg, 0.02 mmol, 0.04 equiv) in 6 mL of toluene.
The reaction was heated at 110 °C. After 40 h, the crude mixture was
adsorbed onto a silica gel plug (5 g) and vacuum-filtered using toluene/
1
hexanes (4/1) to give 538 mg (91%) of 5 as tan solid. H NMR (500
MHz, C6D6) δ 0.86 (t, J ) 7.0 Hz, 12 H), 1.19-1.38 (m, 16 H), 1.59-
1.66 (m, 8 H), 0.3.68 (t, J ) 6.5 Hz, 8 H), 4.82 (s, 4 H), 6.62 (dd, J
) 8.0, 1.5 Hz, 4 H), 6.72-6.84 (m, 16 H), 6.91 (d, J ) 6.8 Hz, 8 H),
7.04 (t, J ) 8.0 Hz, 4 H), 7.17 (s, 4 H). 13C NMR (125 MHz, C6D6)
δ 14.60, 23.34, 26.49, 30.08, 32.30, 68.64, 109.99, 112.52, 115.87,
116.01, 122.70, 126.59, 136.26, 143.93, 146.90, 150.06, 155.63.
MALDI-TOF m/z 1176.47, requires 1177.60. Anal. Calcd for
C78H92N6O4: C, 79.55; H, 7.87; N, 7.14. Found: C, 79.68; H, 7.92; N
7.15.
4-Hexyloxy-bis-tetraazacyclophane (2). In a drybox, 5 (150 mg,
0.13 mmol) was combined with 1,4-dibromobenzene (63 mg, 0.27
mmol), sodium tert-butoxide (62 mg, 0.65 mmol), Pd(dba)2 (3.8 mg,
0.0065 mmol), and tri-tert butylphosphine (1.1 mg, 0.0052 mmol) in
15 mL of toluene. The sealed flask was removed from the drybox, and
the reaction was stirred at room temperature. After 42 h, the crude
mixture was concentrated, redissolved in CH2Cl2 (1 mL), applied onto
a silica gel plug (5 g), and filtered using EtOAc/hexanes (1/20) to give
148 mg (88%) of 2 (>95% 1H NMR pure). Crystallization from EtOAc/
Experimental Section
All palladium-catalyzed amination reactions were assembled in
sealed reaction vessels in a drybox and were run either at room
temperature or heated to 110 °C in an oil bath. 1H NMR spectra were
acquired at 400 or 500 MHz, and proton-decoupled 13C NMR spectra
were obtained at 100 or 125 MHz. Mass spectrometry analyses were
performed by the Mass Spectrometry Facility at the University of
Illinois at Urbana-Champaign. Elemental analytical data were obtained
from either Robertson Microlit Laboratories, Inc or Atlantic Microlab
Inc. Toluene solvent was distilled from sodium/benzophenone ketyl.
Dichloromethane was distilled from calcium hydride under a nitrogen
atmosphere. Palladium bis(dibenzylideneacetone) (Pd(dba)2) was pre-
pared according to literature procedures.24 Other chemicals were used
as received from commercial suppliers.
Preparation of Intermediates and the Ladder Compounds. The
synthesis routes are shown in Schemes 1-3. The square molecule,
tetraazacyclophane (1) has been reported previously.11a The protocols
for these preparations are described below.
N,N,N′,N′-Tetrakis(3-chlorophenyl)-1,4-benzenediamine (4). In a
drybox, 1,4-phenylenediamine (541 mg, 5.00 mmol, 1.00 equiv) was
combined with 1,3-bromochlorobenzene (4.02 g, 21.0 mmol, 4.20
equiv), sodium tert-butoxide (2.40 g, 25.0 mmol, 5.00 equiv), Pd(dba)2,
1
hexanes gave tan solid. H NMR (500 MHz, C6D6) δ 0.89 (t, J ) 7.2
Hz, 12 H), 1.13-1.40 (m, 16 H), 1.58-1.70 (m, 8 H), 3.65 (t, J ) 6.3
Hz, 8 H), 6.68-6.80 (m, 24 H), 6.82 (s, 4 H), 7.02 (t, J ) 8.1 Hz, 4
H), 7.22 (d, J ) 8.5 Hz, 8 H), 7.31 (dd, J ) 7.8, 1.6 Hz, 4 H). 13C
NMR (100 MHz, C6D6) δ 14.61, 23.34, 26.46, 29.99, 32.24, 68.44,
114.22, 115.73, 116.02, 117.44, 125.80, 127.64, 129.42, 130.12, 140.73,
143.68, 144.36, 148.27, 150.75, 155.56. MALDI-TOF m/z 1325.48,
requires 1325.76. Anal. Calcd for C90H96N6O4: C, 81.54; H, 7.30; N,
6.34. Found: C, 81.15; H, 7.36; N, 6.25.
N,N′-Bis(3-chlorophenyl)-1,4-benzenediamine (6). In a drybox,
3-chloroaniline (3.83 g, 30.0 mmol, 3.00 equiv) was combined with
1,4-dibromobenzene (2.36 g, 10.0 mmol, 1.00 equiv), sodium tert-
butoxide (2.88 g, 30.0 mmol, 3.00 equiv), Pd(dba)2 (288 mg, 0.502
mmol, 0.050 equiv) and tri-tert butyl phosphine (81 mg, 0.40 mmol,
0.040 equiv) in 100 mL of toluene. The sealed flask was removed from
the drybox, and the reaction was stirred at room temperature. After 45
h, the crude mixture was concentrated, applied onto a silica gel plug
(5 g) and vacuum-filtered using EtOAc/hexanes (1/10) to give 1.96 g
1
(60%) of 6 as a yellow solid. H NMR (400 MHz, C6D6) δ 4.80 (s, 2
H), 6.48-6.55 (m, 2 H), 6.70 (s, 4 H), 6.75-6.82 (m, 6 H). 13C NMR
(100 MHz, C6D6) δ 114.51, 116.53, 120.27, 122.08, 130.91, 135.79,
137.27,146.53. Anal. Calcd for C18H14Cl2N2: C, 65.67; H, 4.29; N,
8.51. Found: C, 66.05; H, 4.35; N 8.40.
(18) (a) Vo¨gtle, F. Cyclophane Chemistry; Wiley: Chichester, 1993. (b)
Deiderich, F. Cyclophanes; The Royal Society of Chemistry: Cambridge,
UK, 1991.
(19) (a) Ito, A.; Ono, Y.; Tanaka, K. New J. Chem. 1998, 22, 779. (b) Ito, Y.;
Ono, Y.; Tanaka, K. J. Org. Chem. 1999, 64, 8236.
(20) Borden, W. T.; Iwamura, H.; Berson, J. A. Acc. Chem. Res. 1994, 27, 109.
(21) Rajca, A. Chem. ReV. 1994, 94, 871.
(22) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88.
(23) Struijk, M. P.; Janssen, R. A. Synth. Met. 1999, 103, 2287.
3-Chlorophenyl-tetraazacyclophane (7). In a drybox, 6 (400 mg,
1.22 mmol, 1.00 equiv) was combined with 1,3-dibromobenzene (287
mg, 1.22 mmol, 1.00 equiv), sodium tert-butoxide (352 g, 3.66 mmol,
3.00 equiv), Pd(dba)2 (35 mg, 0.061 mmol, 0.051 equiv), and tri-tert
9
9106 J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005

Polaron Delocalization
A R T I C L E S
butylphosphine (10 mg, 0.40 mmol, 0.040 equiv) in 85 mL of toluene.
The reaction was stirred at room temperature. After 50 h, the crude
mixture was concentrated, applied onto a silica gel plug (5 g), and
vacuum-filtered using toluene/hexanes (1/5) to give 215 mg (44%) of
Steady-State Absorption and Emission. UV-visible absorption
spectra were recorded with a Hewlett-Packard 8452A diode array
spectrophotometer. The fluorescence spectra were measured with a
Shimadzu RF-1501 spectrofluorometer. The concentrations of the square
and the ladder CH2Cl2 solutions were 0.1 mM. For UV-vis absorption
measurements, a 1 mm quartz cell was used. The oxidized solutions
(∼0.3 mL) described above were transferred to the cell by a syringe
under nitrogen flow. For the emission measurements, the oxidized
samples were prepared in the 1 cm quartz cuvette using 0.1 mM CH2-
Cl2 solutions under a nitrogen atmosphere. To obtain the UV-vis-
NIR spectra 0.05 mM ladder solutions and 1 cm quartz cuvette were
used.
EPR Measurements. The X-band EPR spectra were recorded on a
Bruker EMX spectrometer, equipped with a variable temperature
accessory (an ER4131VT system) and an ER041XG Microwave bridge.
0.2 to 0.3 mL 0.1 mM ladder solutions oxidized by Pb(OAc)4 were
cooled to ∼120 K in a sealed 4 mm diameter quartz EPR tube under
a nitrogen atmosphere. The g-values were calculated based on the peak
positions of DPPH solid (g ) 2.0036). Spin concentrations were
estimated by a reported method,17 which was based on the intensity of
known concentrations of aqueous solutions of 2,2,6,6-tetramethyl-1-
piperidinyloxy, TEMPO. Appropriate conditions, including data resolu-
tion, sweep time, and modulation amplitude, were ascertained for all
EPR measurements.
Time-Resolved Fluorescence Decay. Time-resolved polarized
fluorescence was studied by femtosecond upconversion spectroscopy.
The upconversion system used in our experiments has been previously
described.26 Briefly, the sample solution was excited with frequency-
doubled light from a mode-locked Ti-sapphire laser pulse that has a
pulse width of ∼55 fs at a wavelength of 820 nm (Tsunami, Spectra
Physics). The polarization of the excitation beam for the anisotropy
measurements was controlled with a Berek compensator. The sample
cell was 1 mm thick and was held in a rotating holder to avoid possible
photodegradation and other accumulative effects. The fluorescence
emitted from the sample was collected with achromatic lens and directed
to a nonlinear crystal of â-barium borate. The rest of the fundamental
light first passed through a motorized optical delay and was then mixed
with the sample emission in another nonlinear crystal to generate sum
frequency. This sum frequency light, i.e., the upconversion signal, was
dispersed using a monochromator and detected by a photomultiplier
tube (Hamamatsu R1527P).
1
7 as a white solid. H NMR (500 MHz, C6D6) δ 6.30(d, J ) 3.0 Hz,
2 H), 6.60 (s, 4 H), 6.60-6.65 (m, 8 H), 6.70-6.80 (m, 6 H), 6.83 (d,
J ) 7.8 Hz, 4 H), 6.91 (d, J ) 8.1 Hz, 4 H), 7.35 (d, J ) 2.0 Hz, 4
H),. 13C NMR (125 MHz, C6D6 δ 117.20, 118.60, 122.81, 123.49,
124.61, 130.63, 130.99, 135.70, 143.36, 148.94, 149.14. MALDI-TOF
m/z 805.55, requires (M+-1) 805.61. Anal. Calcd for C48H32Cl4N4: C,
71.47; H, 4.00; N, 6.95. Found: C, 70.82; H, 4.26; N, 6.65.
3-(4-Hexyloxyanilino)-tetraazacyclophane (8). In a drybox, 7 (240
mg, 0.300 mmol, 1.00 equiv) was combined with 4-hexyloxyaniline
(259 mg, 1.34 mmol, 4.50 equiv), sodium tert-butoxide (159 mg, 1.65
mmol, 5.50 equiv), Pd(dba)2 (8.6 mg, 0.015 mmol, 0.050 equiv), and
tri-tert butylphosphine (2.5 mg, 0.012 mmol, 0.040 equiv) in 15 mL
of toluene. The sealed flask was heated at 110 °C. After 44 h, the crude
product was vacuum-filtered through a silica pad (5 g) using toluene
1
to give 361 mg (85%) of 8 as a yellow solid. H NMR (500 MHz,
C6D6) δ 0.88 (t, J ) 7.0 Hz, 12 H), 1.11-1.29 (m, 24 H), 1.62 (t, J )
8.0 Hz, 8 H), 3.68 (t, J ) 6.5 Hz, 8 H) (m, 6 H), 4.81 (s, 4 H), 6.60-
7.10 (m, 48 H). 13C NMR (125 MHz, C6D6) δ 14.63, 23.36, 26.50,
29.24, 30.06, 32.29, 68.59, 110.22, 113.11, 115.82, 116.53, 116.80,
118.00, 122.92, 126.69, 130.12, 130.67, 136.12, 147.05, 149.18, 150.06,
155.71
4-Hexyloxy-tris-tetraazacyclophane (3). In a drybox, 8 (320 mg,
0.220 mmol, 1.00 equiv) was combined with 1,4-dibromobenzene (105
mg, 0.441 mmol, 2.00 equiv), sodium tert-butoxide (106 mg, 1.10
mmol, 5.00 equiv), Pd(dba)2 (6.3 mg, 0.011 mmol, 0.050 equiv), and
tri-tert butylphosphine (1.8 mg, 0.0089 mmol, 0.040 equiv) in 100 mL
of toluene. The reaction was stirred at room temperature. After 41 h,
the crude mixture was concentrated, applied onto a silica gel plug (5
g) and vacuum-filtered using EtOAc/hexanes (1/6) to give 290 mg
(82%) of 3 as a tan solid. Analytically pure sample was obtained by
1
recrystallization from tetrahydrofuran (as a 3‚THF adduct). H NMR
(400 MHz, C6D6) δ 0.88 (t, J ) 6.8 Hz, 12 H), 1.11-1.40 (m, 24 H),
1.59 (t, J ) 6.8 Hz, 8 H), 3.60 (m, 8 H), 6.40-7.42 (m, 56 H). 13C
NMR (100 MHz, C6D6) δ 14.60, 23.33, 26.46, 29.98, 32.24, 68.43,
114.00, 115.37, 115.41, 116.01, 116.50, 119.75, 125.56, 125.81, 126.20,
129.83, 130.19, 140.67, 143.57, 144.29, 148.13, 148,72, 150.69, 156.63.
MALDI-TOF m/z 1581.96, requires 1582.06. Anal. Calcd for
C
108H108N8O4‚C4H8O: C, 81.32; H, 7,07; N, 6.77. Found: C, 81.41;
Results and Discussion
H, 6.79; N 6.87.
A. Synthesis. 1. Overall Synthetic Strategy. Our synthetic
approach to the ladder structures based on tetraazacyclophanes
relies on recent developments in the palladium-catalyzed
amination of aryl halides to form triarylamines.27-33 We have
Preparation of Oxidized Materials. A 0.050 mM solution of the
ladder molecule in CH2Cl2 was sequentially treated with portions of a
0.010 M solution of bis(trifluoroacetoxy)iodobenzene (PIFA) under
nitrogen atmosphere. To avoid background fluorescence from PIFA,
lead acetate (Pb(OAc)4) replaced PIFA as the oxidant for the preparation
of the samples for emission measurements. A concentration of 10 mM
of tetra-n-butylammonium tetrafluoroborate (Bu4NBF4) and a small
amount of trifluoroacetic anhydride ((CF3CO)2O) were also added to
stabilize the polyradicals.9a-b Increments of between 1 and 20 µL of a
0.010 M fresh solution of Pb(OAc)4 (10 mM) in CH2Cl2 were added
to the portions of 1 mL of an 0.1 mM solution of the ladders in CH2-
Cl2 with the ammonium salt and (CF3CO)2O. Clear solutions with a
light green to blue color were obtained upon oxidation.
(24) Ukai, T.; Kawazura, H.; Ishii, Y. J. Organomet. Chem. 1974, 65, 253.
(25) Murata, H.; Takahashi, M.; Namba, K.; Takahashi, N.; Nishide, H. J. Org.
Chem. 2004, 69 (3), 631.
(26) (a) Ranasinghe, M. I.; Wang, Y.; Goodson, T., III. J. Am. Chem. Soc. 2003,
125 (18), 5258. (b) Ranasinghe, M. I.; Varnavski, O. P.; Pawlas, J.; Hauck,
S. I.; Louie, J.; Hartwig, J. F.; Goodson, T., III. J. Am. Chem. Soc. 2002,
124 (23), 6520. (c) Varnavski, O. P.; Ostrowski, J. C.; Sukhomlinova, L.;
Twieg, R. J.; Bazan, G. C.; Goodson, T., III. J. Am. Chem. Soc. 2002, 124
(8), 1736. (d) Varnavski, O. P.; Goodson, T., III. Chem. Phys. Lett. 2000,
320, 688. (e) Varnavski, O. P.; Leanov, A.; Liu, L.; Takacs, J.; Goodson,
T., III. Phys. ReV. B 2000, 61, 12732. (f) Varnavski, O. P.; Samuel, I. D.
W.; Palsson, L. O.; Beavington, R.; Burn, P. L.; Goodson, T., III. J. Chem.
Phys. 2002, 116 (20), 8893.
(27) Hartwig, J. F. Angew. Chem., Intl. Ed. Engl. 1998, 33, 385.
(28) Hartwig, J. F. In Modern Aminaton Methods; Ricci, A., Ed.; Wiley-VCH:
Weinheim, 2000.
(29) Wolfe, J. P.; Wagaw, S.; Marcoux, J.-F.; Buchwald, S. L. Acc. Chem. Res.
1998, 31, 805.
(30) Yang, B. H.; Buchwald, S. L. J. Organomet. Chem. 1999, 576, 125.
(31) Muci, A. R.; Buchwald, S. L. Top. Curr. Chem. 2002, 219, 131.
(32) Hartwig, J. F. In Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E. I., Ed.; Wiley-Interscience: New York, 2002; Vol.
1; pp 1051-1096.
(33) Hartwig, J. F. In Modern Arene Chemistry; Astruc, C., Ed.; Wiley-VCH:
Weinheim, 2002; pp 107-168.
Electrochemistry Measurements. Electrochemical studies of the
arylamine ladder macromolecules were conducted in anhydrous dichlo-
romethane, with 0.1 M Bu4NBF4 as the supporting electrolyte using a
BAS Model 100A electrochemical workstation at a scan rate of 100
mV/s. Solution concentrations in the range of 0.1 and 0.6 mM of the
square and the ladder systems were used. A glassy carbon electrode
was used as the working electrode, which was polished with an alumina
paste before analysis. Counter and reference electrodes were composed
of platinum wire and Ag/AgCl, respectively. The ferrocenium/ferro-
cence couple occurred at E0′ ) +0.58 V vs Ag/AgCl (satd)).25
9
J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005 9107

A R T I C L E S
Scheme 1
Yan et al.
Scheme 2
shown that catalysts generated from a 1:0.8 ratio of Pd(dba)2
34,35
and P(t-Bu)3
are highly efficient for the amination of aryl
bromides and chlorides. This catalyst couples bromo and
chloroarenes with diarylamines under mild conditions in high
yields.
In addition, we devised a synthetic route to the cyclophanes
that avoids the use of any protective groups. This route would
avoid protective groups by taking advantage of the selectivity
of the catalyst for reactions of aryl bromides over aryl chlorides.
Our strategy assembles the diarylamine precursors to the
cyclophanes by coupling of a primary arylamine with the aryl
bromide portion of a bromochloroarene. Although the palladium
catalysts used in this study containing P(t-Bu)3 as ligand react
with aryl chlorides, high selectivity for reaction of aryl bromides
over chlorides has been observed previously with palladium
catalysts containing this ligand.36 Coupling of amines, either
intermolecularly or intramolecularly with the remaining chlo-
rarene function, then generates the macrocycle structures.
2. Synthesis of Tetraazacyclophane Rectangles. The syn-
thesis of tetraazacyclophanes with identical N-aryl groups at
each position of the periphery and with two different N-aryl
groups at the periphery have been reported previously.11a In
brief, reaction 1,4-dibromobenzene with anisidine in the pres-
ence of Pd(dba)2 and P(t-Bu)3 as catalyst generated an N,N′-
anisyl-substituted phenylenediamine, and reaction of this ma-
terial with 1,3-dibromobenzene generated the tetraazacyclophane
1 in 31% yield (Scheme 1). This last step forms four C-N bonds
and closes the 18-membered cyclophane ring in yields high
enough to isolate substantial quantities of material.
3. Synthesis of Three-Runged Ladder 2. Ladder 2 was
prepared by the three synthetic steps consisting of palladium-
catalyzed amination (Scheme 2). Phenylenediamine 4 served
as the center rung. Reaction of 1,4-phenylenediamine with 4.2
equiv of 1,3-bromochlorobenzene in the presence of 5 mol %
Pd(dba)2 and P(t-Bu)3 per amine NH bond generated the four
new C-N bonds of diamine 4. As anticipated, the palladium
catalyst showed complete selectivity for reaction with the
aromatic carbon-bromine bonds and therefore generated little
if any polymer. Reaction of benzenediamine 4 with 4-hexy-
loxyaniline in the presence of 5% Pd(dba)2 and 4% P(t-Bu)3
(1.25% versus each bromide) in toluene at 110 °C afforded
hexamine 5 in 91% yield. The hexyloxy groups on the aniline
were used to ensure high solubility of the ladder materials.
Hexamine 5 was then treated with 1,4-dibromobenzene in the
presence of the palladium catalyst in toluene at room temperature
to close the two rings and provide a three-runged ladder 2. This
reaction generates four C-N bonds and closes two macrocycles
in a high overall yield of 89%. The reaction was run at a low
9 mM concentration to favor the two cyclizations over polym-
erization.
Isolation and characterization of ladder 2 proved to be
straightforward. Compound 2 was the fastest eluting reaction
component on silica gel. Therefore, we isolated the desired
product by simple vacuum filtration of the crude reaction
mixture through a silica plug, eluting with 5% EtOAc in
hexanes. The resulting product was analytically pure. The 13C
NMR spectrum of this material was fully resolved, and the
MALDI mass spectrum contained the appropriate molecular ion.
The hexyloxy side chains were required for formation of the
ladder material. Precursors analogous to 5 that contained
peripheral methyl, methoxy, or ethoxy groups all failed to
produce the ladder materials in good yield when treated with
1,4-dibromobenzene. Because each of these derivatives is
electronically and sterically similar to 5, we presume that either
partially coupled intermediates with the methyl, methoxy, or
ethoxy substituents on the periphery are too insoluble to allow
complete reaction in toluene or the final product was too
insoluble to allow extraction from the solid salt byproducts.
4. Synthesis of Four-Runged Ladder 3. Ladder 3 was
prepared by the four coupling reactions in Scheme 3 with
tetraazacyclophanes serving as the center two rungs. The
precursor to the tetraazacyclophane, diamine 6, was generated
from dibromobenzene and 3-chloroaniline in acceptable yield
with 5 mol % palladium, with respect to the dibromoarene. The
core cyclophane 7 was then prepared in 44% yield from
dichlorodiamine 7 and 1,3-dibromobenzene, by a reaction
analogous to those used to prepare related tetraazacyclophanes
bearing methyl, methoxy, and tert-butyl ester groups on the
peripheral aryl rings.11a The two outer rings containing the first
and fourth rung of the ladder were assembled by the strategy
(34) Hartwig, J. F.; Kawatsura, M.; Hauck, S. I.; Shaughnessy, K. H.; Alcazar-
Roman, L. M. J. Org. Chem. 1999, 64, 5575.
(35) Nishiyama, M.; Yamamoto, T.; Koie, Y. Tetrahedron Lett. 1998, 39, 617.
(36) Littke, A.; Dai, C.; Fu, G. J. Am. Chem. Soc. 2000, 122, 4020.
9
9108 J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005

Polaron Delocalization
A R T I C L E S
Scheme 3
Figure 1. ORTEP drawing of the core of ladder 2. Only one carbon of the
hexyl side chains is shown.
Table 1. Formal Oxidation Potentials (E0′ in Volts)a
b
E0
′
E0
′
E0
′
E0
′
E0
′
5
1
2
3
4
TPPD
1
2
0.66
0.52
0.49
-
0.76
0.60
1.16
1.09
0.97
1.23
1.06
1.38
a [1] ) 0.6 mM, [2] ) 0.5 mM, and [3] ) 0.1 mM; the ferrocenium/
ferrocence (Fc+/Fc) couple occurs at E0′ ) +0.58 V vs Ag/AgCl (satd).25
b The second oxidation of TPPD is listed as E0′3 instead of E0′2 because
this oxidation is analogous to the E0′3 and E0′4 waves of the ladder
materials.
the ladder is 9.68 Å. The distance between the two nitrogens
across a single cyclophane is 7.49 Å and between two nitrogens
across the two cyclophanes is 11.27 Å. The two distances from
the center of one external aromatic ring to the center of another
are 8.63 Å and 14.29 Å along the sides of the rectangle and
16.82 Å across the diagonal of the full structure.
B. Electrochemical Investigations. Electrochemical mea-
surements have been carried out on materials 1-3. Listed in
Table 1 are oxidation potentials for the systems investigated
and oxidation potentials for a model compound (tetraphenyl-
p-phenylenediamine, TPPD), which contains a single phe-
nylenediamine unit. Two formal oxidation waves were observed
for TPPD, one at E0′ ) 0.660V and another at E0′ ) 1.162V.
The first wave corresponds to a one-electron oxidation process
and yields a stable and delocalized p-phenylenediamine radical
cation.10c The second wave corresponds to the generation of a
quinonoid dication.10c The electrochemical results obtained with
TPPD are similar to those reported for the analogous tetra-p-
anisyl-p-phenylenediamine (TAPD).1-3 However, significantly
lower oxidation potentials (E0′ ) 0.46 and 0.92 V) were
observed for oxidation of the more electron-rich TAPD.1-3
Figure 2 shows the cyclic voltammograms of 1 and the two
different ladder (2 and 3) molecules. The cyclic voltammogram
of 1 consisted of four reversible waves with potentials at E0′ )
0.516, 0.757, 1.087, and 1.238 V. This multiwave electrochemi-
cal behavior has been reported for other similar organic
macrocycles.11 The first two oxidation processes are a result of
the removal of an electron from each of the two p-phenylene-
diamine units. Because these p-phenylenediamine units may be
oxidized twice, the third and fourth oxidation potentials (1.087
and 1.238 V), most likely, correspond to the second oxidation
of each the two p-phenylenediamine units.
used to prepare bicyclic 2. The tetrachloro tetraazacyclophane
7 was treated with 4-hexyloxyaniline under the palladium-
catalyzed conditions. In the absence of any aryl bromide units,
the aryl chlorides reacted in high yield to form octamine 8 in
71% yield, again by the construction of four new C-N bonds.
Octamine 8 was then treated with 1,4-dibromobenzene to
generate ladder 3 in 82% yield by formation of four new C-N
bonds and closure of the two outer rings to install the first and
fourth rungs. Again, ladder 3 was the fastest eluting material
on silica gel and was isolated by filtration of the crude reaction
through a silica plug. This material was then crystallized to
generate analytically pure ladder. The 13C NMR spectrum of 3
was fully resolved, and the MALDI mass spectrum contained
the appropriate molecular ion.
5. Structure of Ladder 2. A single crystal of the three-runged
ladder 2 was obtained by slow evaporation of a benzene solution,
and the structure was determined by X-ray crystallography
(Figure 1). The molecule possesses a crystallographic mirror
plane. The six nitrogens lie in a plane, and the three 1,4-
phenylenediamine units are canted out of this plane. Each
nitrogen shows the propeller arrangement of the three aryl
groups of a conventional triarylamine. The distances between
proximal nitrogens of a single cyclophane unit are 4.84 Å for
the meta linkages and 5.65 Å for the para linkages. The distance
between the two nitrogens at the ends of the support piece of
The cyclic voltammogram of the three-step ladder 2 consisted
of a number of oxidation waves with potentials given in Table
1. The potentials resulting from the first two oxidations of the
9
J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005 9109

A R T I C L E S
Yan et al.
ylenediamine unit. With 3, oxidation of three phenylenediamine
units would again require that two contiguous phenylenediamine
units be oxidized, and the coplanarity of the phenylenediamine
units of these two would then lead to a steric interaction that
would be similar to that encountered in oxidized 2. Because of
the requirement of contiguous charges and the steric constraint,
oxidation of a third phenylenediamine unit in 3 would be
expected to be less favorable than oxidation of the second unit
in 3, regardless of the assignment of the oxidation wave.
C. Absorption Measurements. Absorption measurements of
the neutral and oxidized forms of 1-3 were conducted. Figure
3a shows the UV-vis spectra of the neutral samples. The
general characteristics of the spectra of the neutral forms of
1-3 were similar. Two major absorption peaks, one near ∼220
nm and the other at ∼320 nm, were observed for all of the
neutral samples. A similar absorption spectrum was also
obtained for the model compound TPPD (also shown in Figure
3a). The lower energy absorptions of 2 and 3 were red-shifted
by about 10 nm from the absorption maximum of TPPD. The
red shift is likely to correlate with the extent of conjugation in
the ladder molecules.
Figure 2. Cyclicvoltammogrametry curves of 1-3 molecules. ([1] ) 0.6
mM, [2] ) 0.5 mM, and [3] ) 0.1 mM; the ferrocenium/ferrocence (Fc+/
Fc) couple occurs at E0′ ) +0.58 V vs Ag/AgCl (satd)).25
The absorption spectra of 1, 2, and 3 in the presence of Bu4-
NBF4 and (CF3CO)2O are shown in Figure 3B. The combination
of Bu4NBF4 and (CF3CO)2O has been used previously to oxidize
molecules containing phenylenediamine units and to facilitate
oxidation with lead acetate.9a-b An immediate change in color
of the solution from clear to green was observed after the
addition of the salt and the anhydride. This change of color
suggests that the combination of salt and anhydride functions
as a weak oxidant of ladders 2 and 3. The absorptions of the
four different systems at the lower wavelengths, illustrated in
Figure 3B, were unchanged by addition of the salt and
anhydride. However, absorptions at higher wavelengths near
430 and 580 nm were observed in the spectra of 1, 2, and 3
after treatment of the amines with the additives. Addition of
the salt and anhydride to the model compound TPPD generated
only a single low energy band at ∼420 nm.
phenylenediamine units are close to each other, and correspond
to one pair of one-electron oxidations. Similarly, the two
potentials for the second two oxidations (between 0.9 and 1.2
V) are close to each other, and correspond to a second pair of
one-electron oxidations. A third irreversible oxidation at even
higher potential is also observed. It is difficult to provide a
detailed assignment of these oxidation waves, but it appears
that two of the three rungs of the ladder, perhaps the outer rungs,
are participating in the two pairs of redox processes that occur
at potentials close to those of 1. The first oxidation process (E0′
) 0.49 and 0.6 V) would then correspond to loss of an electron
from each of the two phenylenediamine units that comprise the
first and third rungs of the ladder. The next highest oxidation
potential of 2 would then generate quinonoid structures from
the phenylenediamines oxidized at the lower potential. Alter-
natively, E0′5 could correspond to the loss of an electron from
the center phenylenediamine unit of 2 with a potential that is
higher than the potential required to oxidize the two outer
phenylenediamine rings.
The absorption spectra of the samples oxidized with Pb(OAc)4
in the presence of Bu4NBF4 and (CF3CO)2O are shown in Figure
4A. The addition of the lead oxidant, in addition to the salt and
anhydride, resulted in a decrease in the absorption band at 320
nm and an increase in the intensity of the absorption bands in
the visible spectral wavelengths near 430 and 580 nm. In
addition to these bands, an additional absorption band in the
near-IR with a maximum greater than 800 nm was generated
by addition of the oxidant.
The intense green color of the solution from the oxidized 2
and 3 persisted when the solutions were kept under a nitrogen
atmosphere at room temperature. When the oxidized form of 2
was incorporated into the polymer matrix poly(vinyl butyral-
co-vinyl alcohol-co-vinyl acetate) (PVB), the green color
persisted for more than eight months. Absorption measurements
of the thin film after eight months (Figure 4B) indicated that
the oxidized form of 2 was still present.
In either case, the potentials for oxidation of the first two
phenylenediamine units are lower than the potential to oxidize
the third unit, and this difference in potential could result from
at least two effects. First, the oxidation of the center phenylene-
diamine unit would create charges on three contiguous rungs
of the ladder. Second, the geometry required for generation of
three contiguous phenylenediamine units would create signifi-
cant steric interactions from the generation of three coplanar
aryl groups in the three rungs of the ladder. Consistent with
this assertion, X-ray crystallography (vide infra) showed that
the central phenylene unit of the neutral ladder 2 is significantly
more canted from the plane of the six nitrogens than are the
outer phenylene.
D. Characterization of Polarons. We have characterized the
ladder and monocyclic systems by absorption and EPR spec-
troscopy. As referred to above, a polaron band in polyaniline
has been measured at 440 nm by linear absorption.37 Likewise,
The cyclic voltammogram of the four-step ladder 3, contained
two oxidation waves. Again, the first oxidation most likely
results from loss of electrons from the exterior p-phenylenedi-
amine units. The second wave would then correspond to
oxidations of the same phenylenediamines to generate quinonoid
structures, or, less likely, to oxidation of a contiguous phen-
(37) (a) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G. Chem. Mater. 1995, 7,
443. (b) Stilwell, D. E.; Park, S. M. J. Electrochem. Soc. 1989, 136, 427.
9
9110 J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005

Polaron Delocalization
A R T I C L E S
Figure 3. Absorption spectra of 1-3 and TPPD in the absence (A) and in the presence of Bu4NBF4 and (CF3CO)2O (B).
Figure 4. Absorption of oxidized forms of 1-3. (A) The absorption spectra of 1-3 in the presence of the Pb(OAc)4. (B) The absorption spectrum of the
oxidized 2 incorporated in a PVB film, obtained after 8 months.
Figure 5. Relative intensities of the EPR signals (left y-axis) and the molar extinction coefficient of the polaron band at ∼430 nm (right y-axis) as a
function of the concentration of oxidant Pb(OAc)4 in solutions of 2 and 3 with the presence of Bu4NBF4 and (CF3CO)2O). (a) 2; (b) 3; Insert: EPR spectra
near g ∼ 2 at the molar ratio Pb(OAc)4/ladder of 1.
we observed a band near 430 nm in the absorption spectra of
both ladder compounds 2 and 3. The molar extinction coefficient
of this polaronic band was found to be ∼103 M-1‚cm-1 (right
y-axes of Figures 5a and 5b). The amplitude of this band
depended on the concentration of oxidant added to the ladder
and monocyclic compounds. As shown in Figure 5b, the
absorption of the four-step ladder system increased with
increasing amounts of added oxidant until it reached a maximum
value at a molar ratio of Pb(OAc)4:3 of 2 and levels off
afterward.
Oxidation of 2 and 3 by PIFA (Figure 6) generated an
absorption band at ∼1050 nm. This absorption at long wave-
lengths originates from an intervalence (IV) transition.38 This
IV transition of the monocationic and dicationic forms of 1
generated in CH2Cl2 with PIFA as oxidant had an absorption
(38) (a) Nelsen, S. F.; Ismagilov, R. F.; Trieber, D. A., II Science 1997, 278
(5339), 846. (b) Coropceanu, V.; Lambert, C.; Noll, G.; Bredas, J. L. Chem.
Phys. Lett. 2003, 373, 153. (c) Coropceanu, V.; Malagoli, M.; Andre, J.
M.; Bredas, J. L. J. Am. Chem. Soc. 2002, 124, 10519. (d) Lambert, C.;
Noll, G. Angew. Chem., Int. Ed. 1998, 37, 2107. (e) Lambert, C.; No¨ll, G.
J. Am. Chem. Soc. 1999, 121, 8434.
9
J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005 9111

A R T I C L E S
Yan et al.
Figure 6. UV-vis-NIR spectra of neutral and oxidized 2 (left) and 3 (right) using 0.010 M PIFA in CH2Cl2.
Scheme 4
Scheme 5
maximum near 900 nm.11a A similar absorption at long
wavelengths was also observed in a triarylamine based poly-
mer.39 The observation of IV bands may be understood by
considering the interactions between polarons and redox centers.
In this context, the IV band may be attributed to coupling
between delocalized polarons (p-phenylenediamine cations) and
redox centers (neutral p-phenylenediamines) (Schemes 4 and
5).
The linking of phenylenediamine units in the ladder systems
at mutually meta positions on an aromatic unit apparently allows
the charge of the oxidized ladder system to transfer between
the phenylenediamine steps of the ladder structure that are
separated by 4.84 Å. The intensity of the IV band of the
monocations of the ladders results from a mixture of the two
different intervalence transitions, P•+PP (or PPP•+)fPP•+P and
PP•+PfP•+PP (or PPP•+) for 2•+ as outlined in Scheme 4, and
P•+PPP (or PPPP•+)fPP•+PP (or PPP•+P) and PP•+PP (or
PPP•+P)fP•+PPP (or PPPP•+) + PPP•+P (or PP•+PP) for 3•+,
as illustrated in Scheme 5. Although both of these types of
transitions may occur, only one relatively broad absorption band
at ∼1050 nm was observed. The difference in energy between
these transitions is apparently too small to distinguish by the
steady-state absorption measurements.
To analyze the effective electronic coupling between the
redox sites in the radical cations one may apply Hush theory.40
The coupling parameter (V) can be estimated by the expres-
sion:40
0.0206
V )
(ꢀmaxν
max∆ν1/2)1/2
d
in which V is in units of cm-1, d is the site-to-site separation of
the redox positions in angstroms, ꢀmax is the maximum extinction
coefficient, Vmax (in cm-1) is the transition energy, and ∆V1/2 is
the full width at half-maximum (fwhm, in cm-1) of the IV
band.40 The absorption maximum of the IV band was found to
be at ∼9547 cm-1 (1.19 eV) and ∼9872 cm-1 (1.23 eV) for 2
and 3, respectively. The fwhm of the IV band of both ladder
systems was ∼4700 cm-1. From these values, the coupling
parameter between the delocalized phenylenediamine cations
and the neutral phenylenediamines in both ladder systems was
estimated to be in the range of 2250-2450 cm-1 (0.28-0.30
eV). The adiabatic charge-transfer energy [∆G*, ∆G* ) (λ/4
- V)+V2/λ] was estimated to be ∼0.08 eV for the IV charge
transfer in both ladder systems. It is interesting to note that the
through-bond distance is 10.49 Å, and for this distance a
(39) Lambert, C.; No¨ll, G. Synth. Met. 2003, 139, 57.
(40) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391.
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9112 J. AM. CHEM. SOC. VOL. 127, NO. 25, 2005

Polaron Delocalization
A R T I C L E S
coupling constant in the range of 1060-1146 cm-1 (0.13-0.14
eV) results. This coupling constant leads to a adiabatic charge-
transfer energy of 0.18 eV. Thus both charge-transfer energy
calculations (through space and through bond) lead to an energy
which is comparable in magnitude to the values obtained for
weak polaron bands (∼0.1 eV) in conjugated polymers.41 The
transition energy at the maximum absorption of the IV band of
the monocationic form of the ladders is less than 14 kcal/mol.
This energy is lower than the transition energy (34.7 kcal/mol)
of the TPPD monocation radical.42 Thus, the polaronic coupling
in these macrocyclic ladder arylamines appears to be larger than
that in the linear, tetraarylphenylenediamine component of the
ladders, TPPD. This larger coupling may suggest that the
polarons of the cyclophanes in this study are more delocalized
than the polarons of the single phenylenediamines.4,5,38,42
EPR measurements provided further information about the
degree of delocalization of the polarons in the ladder systems.
EPR signals of oxidized 2 and 3 were observed to have a
Lorentzian line shape and a g value of 2.003. This g value is
similar to those of organic radicals and highly electronically
delocalized polarons in polyaniline.17 The magnitude of the
maximum slope line widths (∆Hmsl) of the signals for 2 and 3,
11-12 G at a sweep width of 4500 G, were also similar to
each other. The line shape and the g value for these single-
peak EPR spectra reflected the delocalized character of the
polarons generated in these ladder systems in comparison to
the EPR spectra of the polarons in polyaniline.17,43
Addition of Pb(OAc)4 as oxidant generated intense signals
from the oxidized ladders. The intensity of the EPR signal
increased with an increase in the molar ratio of oxidant to 2 or
3 (Figure 5). The largest intensity was observed at a molar ratio
of approximately 1:1. EPR signals of samples containing a ratio
of oxidant to ladder greater than 1:1 were less intense than those
of samples with a 1:1 ratio of oxidant to ladder. Using a known
method to calculate the spin concentration from EPR data,17
we found the spin concentration of both oxidized ladders to be
∼1 spin/molecule when the molar ratio of oxidant to 2 or 3
was 1:1. The maximum spin concentration was observed when
the ladders had undergone a single oxidation because the
dications of the ladders are singlet ground states (vide infra).
The decrease in spin concentration when the ratio of oxidant to
ladder is greater than 1:1 indicates that bipolarons are generated.10c
EPR measurements of the oxidized ladders were also
performed at low temperature (120 K) to determine the ground
spin state of the dications. The ladders 2 and 3 were oxidized
by Pb(OAc)4 with molar ratios of Pb(OAc)4 to ladders ranging
from 0.5 to 3. A forbidden transition near g ) 4 at ∆m ) (2,
which would indicate population of a triplet spin state was not
observed. Thus, the ground state of the dications is most likely
a singlet in the ladder systems. In contrast, a forbidden transition
due to a triplet state was observed in the low-temperature
spectrum of 1 oxidized with two equiv of PIFA. This difference
in ground-state further suggests that the polarons in the ladder
systems are more delocalized than in the square system 1.11a
E. Steady-State Emission Spectra. The emission spectra of
TPPD, cyclophane 1, and ladders 2 and 3 were obtained with
an excitation wavelength of 410 nm under a variety of
experimental conditions. These spectra are shown in Figure 7.
A clear emission band at 440 nm for the neutral forms of TPPD,
1, 2, and 3 was observed. This emission originates from a π*
f π transition. The emission spectra of all the neutral molecules
in pure CH2Cl2 (Figure 7 marked by arrows) also contained a
shoulder at 466 nm. The emission spectra of these materials in
a mixture of Bu4NBF4 and (CF3CO)2O exhibited two maxima
at 440 and 466 nm, and the emission spectra obtained after
addition of the oxidant Pb(OAc)4 were similar to those obtained
after addition of only Bu4NBF4 and (CF3CO)2O. This similarity
between the emission spectra with and without the oxidant in
the presence of Bu4NBF4 and (CF3CO)2O is consistent with
partial oxidation of the ladders by these additives in CH2Cl2
solutions. The oxidation may be due to small amounts of
trifluoroacetic acid in the anhydride.
The emission spectra in Figure 7 also reveal different
quenching behavior for each of the square and ladder molecules
in the presence of Bu4NBF4 and (CF3CO)2O. In the presence
of Bu4NBF4 and (CF3CO)2O, quenching of the emission at 440
nm was approximately 20% for TPPD, 35% for 1, 50% for 3,
and over 60% for 2. This high degree of quenching for 2 may
result from strong ion (dynamic) quenching that can occur with
charged materials.44 The quenching of the emission of 1 at 466
nm in the presence of these additives was weaker than that of
2 and 3 (Figure 7a,b). The observation that TPPD has the lowest
degree of quenching was surprising because this molecule has
the smallest diameter and the highest diffusion controlled
collisional frequency.45
Upon addition of the Pb(OAc)4 oxidant to the solutions
containing Bu4NBF4 and (CF3CO)2O, the ladder systems
exhibited both dynamic and static quenching. This combination
of quenching mechanisms was revealed by the parabolic shape
of the Stern-Volmer plots of the quenching of 2 and 3 (Figure
8).45 Thus, it appears that the ladder systems, which have a
higher density of polarons, exhibit emission quenching behavior
that is different from that of 1 and TPPD. The oxidation of
ladders 2 and 3 gives rise to a broad vibronic (IV) transition in
the near-IR, which is related to the delocalization of polarons
(Section C). The polaron delocalization and polaron-phonon
interactions help to dissipate the energy of the emission and
thereby accelerate the relaxation of the excited states through
nonradiative pathways.46,47 This pathway for relaxation of the
excited states may explain why a strong static quenching is
observed for both oxidized ladder systems in the presence of
Bu4NBF4 and (CF3CO)2O.
F. Time-Resolved Fluorescence Measurements. The fluo-
rescence decays of 1-3 in CH2Cl2 measured by fluorescence
upconversion are shown in Figure 9. A fast component, followed
by a slower component, was observed in the fluorescence decay
of the square and ladder materials. The decay time of the fast
process was comparable to the duration of the fwhm of our
instrument response function (IRF, ∼200 fs). Thus, the decay
(41) (a)Wohlgenannt, M.; An, C. P.; Vardeny, Z. V. J. Phys. Chem. B 2000,
104, 3846. (b) Heeger, A. J.; Kievelson, S.; Schrieffer, J. R.; Su, W. P.
ReV. Mod. Phys. 1988, 60, 781.
(42) Nelsen, S. F.; Tran, H. Q.; Nagy, M. A. J. Am. Chem. Soc. 1998, 120,
298.
(44) MacDiarmid, A. G.; Epstein, A. J. Synth. Met. 1995, 69, 85.
(45) Lakowicz, J. R. Principles of Fluorescence Spectroscopy; Kluwer Academic/
Plenum: New York, 1999.
(43) Poole, C. P. Electron Spin Resonance: A ComprehensiVe Treatise on
Experimental Techniques, 2nd ed.; Wiley-Interscience Publication: New
York, 1996; pp 459-539.
(46) Kim, J.; Unterreiner, A. N.; Rane, S.; Park, S.; Jureller, J.; Book, L.; Liau,
Y.-H.; Scherer, N. F. J. Phys. Chem. B 2002, 106, 12866.
(47) Ben-Nun, M.; Martinez, T. J. Chem. Phys. Lett. 1998, 298, 57.
9
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A R T I C L E S
Yan et al.
Figure 7. Emission spectra of 0.1 mM CH2Cl2 solutions of neutral and oxidized TPPD, 1-3 with an excitation of 410 nm. The oxidation level is indicated
by the molar ratio of Pb(OAc)4/neutral-molecule (Pb/neutral), and the arrows indicated the emission bands in the neutral systems.
Figure 8. The Stern-Volmer plots for the ladder samples at the emission
Figure 9. Fluorescence decay of the emission at 480 nm in 0.1 mM CH2-
of 466 nm in the presence of Bu4NBF4/(CF3CO)2O (10 mM/1 mM).
Cl2 solutions of 1-3 with an excitation at 410 nm (black, 1; green, 2; blue,
3).
time of the fast component of the fluorescence quenching of
the square and ladder systems was estimated to be less than
∼100 fs with an amplitude of ∼0.78 for the ladders and ∼0.82
for 1, while the decay time of the slower decay component was
found to be about ∼6 ps with an amplitude of 0.22 for the
ladders and 0.18 for 1.
decrease in the residual value with the increasing number of
p-phenylenediamine units in 1-3 was observed.
Figure 11 shows the fluorescence dynamics of solutions of
the square and ladder systems containing Bu4NBF4 and (CF3-
CO)2O. As described in section C, a polaron absorption band
was observed upon addition of Bu4NBF4 and (CF3CO)2O to a
solution of the ladders in CH2Cl2. In general, the fluorescence
decay of square 1 in the presence of Bu4NBF4 and (CF3CO)2O
was similar to the decay in the absence of these additives. This
similarity agrees with the similar steady-state emission of 1 at
466 nm in the presence and absence of the same additives
(Figure 7). The fluorescence decay curves obtained after addition
of Pb(AcO)4 to solutions of 1-3 in CH2Cl2 containing Bu4-
NBF4 and (CF3CO)2O contained only a small amplitude from
Although the fluorescence decays were similar for the ladder
and square molecules, the rates of depolarization were signifi-
cantly different (Figure 10). The decay of fluorescence of the
linear molecule TPPD did not contain an obvious initial fast
anisotropic decay component. However, the fluorescence of the
square and the ladder molecules did occur with an ultrafast
anisotropic decay process with a time constant of ∼70 fs. The
residual value of the decay of the fluorescence anisotropy of 1
was 0.23, while the analogous value of 3 was 0.05. A systemic
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Polaron Delocalization
A R T I C L E S
which is induced by self-trapping of photogenerated carriers)
may be generated around the nitrogen center by photoexcitation
of the neutral phenylenediamine units.6,49-51 The overlap of the
emission and the absorption spectra suggest that the electron-
exciton coherence induces exciton delocalization around the
frame of the tetraazacyclophane ring.25 This exciton delocal-
ization was suggested by the fast depolarization of the fluores-
cence anisotropy. For the oxidized ladder structures, a polaron
transfer rate ∼1010-1012 s-1 (1-100 ps) was obtained. The
pinning frequency of a polaron is 9 × 1012 or 3.6 × 1013 Hz.52,53
This thermal electron (hole) transfer process is much slower
than the delocalization of the photoexcited polarons, as shown
by the ultrafast decay of the fluorescence dynamics within the
first ∼100 fs. This observation agrees with the fast dephasing
time observed in polyaniline, which is connected with the
delocalization of photoexcited polarons.46
Figure 10. Fluorescence anisotropy decay of the emission at 480 nm for
0.1 mM CH2Cl2 solutions of 1 and 3 with an excitation at 410 nm.
Conclusions
In summary, we have shown that palladium-catalyzed ami-
nation can assemble triarylamine materials with a complex, but
well-defined, architecture. In each operation, four C-N bonds
are formed, and in the final two steps of each synthesis two
rings are closed in yields between 71 and 89%. During the
preparation of 2, twelve C-N bonds are constructed in three
synthetic operations, with an overall yield that corresponds to
over 97% for each C-N coupling. During the preparation of 3,
sixteen C-N bonds are formed in four operations, with a yield
that corresponds to over 89% for each coupling. A crystal
structure analysis of one ladder reveals that the aryl groups that
serve as the ladder rungs are canted from the plane containing
the nitrogens and m-phenylene units that link the nitrogens on
the ladder sidepieces.
Figure 11. Fluorescence decay of the emission of 0.1 mM CH2Cl2 solutions
of 1-3 with the presence of Bu4NBF4/(CF3CO)2O (10 mM/1 mM) (black,
1; green, 2; blue, 3; excitation at 410 nm; emission at 480 nm).
We have utilized electrochemical and EPR measurements,
as well as steady-state and time-resolved spectroscopy, to probe
the properties of these materials. Both ladder systems demon-
strated significant polaronic behavior. A multiwave oxidation
process for both ladder systems was observed and was concluded
to result from consecutive oxidation processes. Absorption
measurements of oxidized forms of the ladder systems revealed
polaron and intervalence bands at long wavelengths. An
electronic coupling parameter for the coupling between the
phenylenediamine cation radicals and the neutral phenylenedi-
amine units of ∼0.1 eV was deduced for the ladder systems.
The spin concentration obtained by EPR measurements in-
creased with the amount of oxidant added, and the g values
and line shape of the EPR signals suggested that the polarons
have a delocalized character. The quenching of the steady-state
emission of the ladder systems depended on the amount of
oxidant added. Also, an ultrafast decay of the fluorescence
anisotropy for the square and the ladder molecules was observed
and may be connected with the interactions of polarons. These
results provide a direct comparison of the electronic and optical
properties of the macromolecular ladder architecture with the
Figure 12. Fluorescence decay of the emission of 0.1 mM oxidized CH2-
Cl2 solutions of 1-3 with the presence of Bu4NBF4/(CF3CO)2O (10 mM/1
mM) (excitation at 410 nm; emission at 480 nm).
the slower decay component. Over 90% of the intensity of the
total emission could be attributed to the fast component (Figure
12).
The mechanism of the fast fluorescence decay may be
connected to polaronic-exciton delocalization and polaron-
polaron interactions.46,48 Because the oxidation potential of the
triarylamine units is low, a polaronic exciton (quasiparticle
(49) No¨ll, G.; Lambert, C. Chem. Eur. J. 2002, 8, 3476.
(50) Heun, S.; Biissler, H.; Borsenberger, P. Chem. Phys. 1995, 200, 265.
(51) Koslowski, T.; Jurjiu, A.; Blumen, A. J. Phys. Chem. B 2004, 108, 3283.
(52) Nishiumi, T.; Nomura, Y.; Chimoto, Y.; Higuchi, M.; Yamamoto, K. J.
Phys. Chem. B 2004, 108, 7992.
(48) (a) Maciel, G. C.; de Araujo, C. B.; Correia, R. R. B.; de Azevedo, W. M.
Opt. Comm. 1998, 157, 187. (b) Wang, H.; Lai, T.; Lin, W.; Mo, D.; Meng,
O.; Gong, K. Chin. Phys. Lett. 1994, 11, 491. (c) Book, L. D.; Ostafin, A.
E.; Ponomarenko, N.; Norris, J. R.; Scherer, N. F. J. Phys. Chem. B 2000,
104, 8295.
(53) Pinto, N. J.; Shah, P. D.; Kahol, P. K.; McCormick, B. J. Solid State
Commun. 1996, 97, 1029.
(54) For the purpose of this manuscript, the term polaron is used to describe
the excitation associated with charge-transfer localized on a part of the
molecule as a result of conformational (phonon assisted) changes
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A R T I C L E S
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properties of the related linear system containing the same
building blocks. This comparison draws analogies to previous
reports that compare branched dendritic structures with linear
chromophores that are the building blocks of the macromolecule.
of Naval Research and National Science Foundation (NIRT and
CAREER to TG) and the DOE (to J.F.H.).
Supporting Information Available: Experimental procedures,
metrical parameters. and .cif files for the structure of 2
determined by X-ray diffraction. This material is available free
of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank B. Lucht for measuring the
UV-vis-NIR spectra of the oxidized ladder systems. We thank
S. Hauck for preliminary experiments to help outline the strategy
for the synthesis of ladder 2. This work is supported by Office
JA050184N
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