Generation and spectroscopic profiles of stable multiarylaminium radical cations bridged by fluorenes

Article abstract of DOI:10.1021/ol2007988

A series of arylaminofluorene derivatives (DTFA-1, TTFA-2, TAFB-3, and TAFA-4) were synthesized, and the generation of their corresponding arylaminium cation radicals was readily achieved by Cu(ClO₄)₂ in CH₃CN. Moreover, t

Full text of DOI:10.1021/ol2007988

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2702–2705
Generation and Spectroscopic Profiles of
Stable Multiarylaminium Radical Cations
Bridged by Fluorenes
Chao-Che Chang, Han Yueh, and Chao-Tsen Chen*
Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, R.O.C.
chenct@ntu.edu.tw
Received March 26, 2011
ABSTRACT
A series of arylaminofluorene derivatives (DTFA-1, TTFA-2, TAFB-3, and TAFA-4) were synthesized, and the generation of their corresponding
arylaminium cation radicals was readily achieved by Cu(ClO4)2 in CH3CN. Moreover, the cation radicals were stable at ambient temperature with
substantially long life times and exhibited distinct colors. The oxidation mechanism and spectroscopic features of the resulting cation radicals
were probed by UVꢀvisꢀNIR spectroscopy and electron spin resonance experiments.
Triarylamine-based compounds have been found in
many applications such as organic optoelectronics,1
electrochromism,2 organic magnets,3 intervalence charge-
transfer (IVCT),4 metal ion sensors,5 and synthetic
utilities.6 Most of these applications stem from the un-
shared lone pair of electrons at the nitrogen, which can be
readily converted to cation radicals at a moderate oxidiz-
ing potential. Organic magnets and organic mixed-valence
(MV) compounds all possess two or more triarylamino
centers connected by extended π-conjugated molecular
bridges.4a,c,g,h Arylaminium cation radicals warrant a sig-
nificant spin concentration for spin-mediated organic mo-
lecular spintronics as well as an intense and well-separated
IVCT band to allow accurate estimation of electronic
coupling and valence delocalization. The bridging struc-
tures have a strong influence on the chemical stability and
spin alignment of cation radicals as well as the efficiency of
intersite electron coupling.
(1) (a) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953. (b) Chi,
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Kinnibrugh, T.; Timofeeva, T.; Bredas, J.-L.; Marder, S. R. J. Am.
The generation of arylaminium cation radicals can be
achieved either by electrochemical oxidation or chemical oxi-
dants such as certain metal ions,7 zeolite,8 and protic acids.9
Although electrochemical anodic oxidation12 warrants
ꢀ
Chem. Soc. 2004, 126, 11782. (g) Barlow, S.; Risko, C.; Chung, S.-J.;
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Tucker, N. M.; Coropceanu, V.; Jones, S. C.; Levi, Z.; Bredas, J.-L.;
Marder, S. R. J. Am. Chem. Soc. 2005, 127, 16900. (h) Coropceanu, V.;
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Gruhn, N. E.; Barlow, S.; Lambert, C.; Durivage, J. C.; Bill, T. G.; Noll,
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G.; Marder, S. R.; Bredas, J.-L. J. Am. Chem. Soc. 2004, 126, 2727. (i)
Lancaster, K.; Odom, S. A.; Jones, S. C.; Thayumanavan, S.; Marder,
(6) (a) Sumalekshmy, S.; Gopidas, K. R. Chem. Phys. Lett. 2005, 413,
294. (b) Sreenath, K.; Suneesh, C. V.; Ratheesh Kumar, V. K.; Gopidas,
K. R. J. Org. Chem. 2008, 73, 3245.
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S. R.; Bredas, J.-L.; Coropceanu, V.; Barlow, S. J. Am. Chem. Soc. 2009,
131, 1717 and references therein. (j) Odom, S. A.; Lancaster, K.;
(7) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
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Chem. Chem. Phys. 2001, 3, 2955.
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Beverina, L.; Lefler, K. M.; Thompson, N. J.; Coropceanu, V.; Bredas,
J.-L.; Marder, S. R.; Barlow, S. Chem.;Eur. J. 2007, 13, 9637.
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r
10.1021/ol2007988
Published on Web 04/25/2011
2011 American Chemical Society

complete oxidation, transient-oxidized species are difficult
to isolate and structurally elucidate. Moreover, the cation
radicals are formed in very high concentrations at the
electrode surface, possibly leading to dimerization. Several
concerns also arise when using chemical oxidants, which
are rather expensive, environmentally unfriendly, and give
low oxidization yields depending on the substance. Re-
cently, Cu(ClO4)2, an inexpensive and mild chemical oxi-
dant, was reported to effectively generate stable
monoarylaminium cation radicals and tris(4-anisyl)amine
dications.6,10 The general applicability of this oxidant in
generating multiple arylaminium cation radicals remains
to be explored.
fluorene (1), dialkylation proceeded smoothly in the pre-
sence of excess KOH to afford 2 in 92% yield. The
subsequent BuchwaldꢀHartwig cross-coupling reaction14
of 2 with di-p-tolylamine yielded 83% of the desired
Scheme 1. Synthesis of DTFA-1, TTFA-2, and TAFB-3
A wide variety of bridge structures such as phenylene,
biphenyl, phenylene-ethynylene, vinylene, and various
lengths of phenylene-vinylene have been extensively inves-
tigated for their roles in mediating electron coupling and
valence delocalization.4a,g,h Fluorene, which possesses two
benzene rings coplanar with the central carbon and ismuch
investigated for use as a luminophore in organic light-
emitting diodes,1b is surprisingly absent among these
mixed valence systems. To decipher and understand how
DTFA-1. Both TTFA-2 and TAFB-3 started with 2,7-
dibromo-9,9-dibutyl-9H-fluorene (3) but produced either
di- or mono-cross-coupling products depending on the
amination conditions. Using Pd(OAc)2 and PtBu3 as the
catalyst and 3 equiv of di-p-tolylamine to react with 3, the
di-cross-coupling product-TTFA-2 was afforded in almost
quantitative yield. The mono-cross-coupling product 4
was obtained in a reasonable yield when only 1 equiv each
of di-p-tolylamine, Pd(dba)2, and DPPF were used. Addi-
tion of n-BuLi to 4 and quenching with DMF yielded the
Vilsmeier formylation product 5 in 46% yield. Treatment
of 5with {3,5-bis[(diethoxyphosphoryl)methyl]phenyl}methyl
diethyl phosphonate (6) via the HornerꢀWadsworthꢀ
Emmons reaction gave TAFB-3 in 68% yield. TAFA-4
was obtained by a combination of dialkylation of 7 with
1-iodobutane followed subsequently by coupling with di-
p-tolylamine via palladium catalysis, hydrogenation, and
repeated BuchwaldꢀHartwig cross-coupling of 4.
Figure 1. Chemical structures of tolylaminofluorenes DTFA-1,
TTFA-2, TAFB-3, and TAFA-4.
fluorene-bridged redox centers interact, a new series of
tolylaminofluorene derivatives, denoted as DTFA-1,
TTFA-2, TAFB-3, and TAFA-4 (Figure 1) are designed
and synthesized. These derivatives consist of one to four
tritolylamino centers bridged by fluorenes; the 9-position
of fluorene is alkylated with two n-butyl groups to prevent
it from being oxidized or deprotonated and to increase
solubility inorganicsolvents. Inaddition, a convenientand
cheap chemical oxidant capable of generating polycation
radicals is also addressed.
Scheme 2. Synthesis of TAFA-4
Syntheses of tolylaminofluorenes are depicted in
Schemes 1 and 2. Details of the procedures and structural
characterization can be found in the Supporting Informa-
tion. Compounds 3,11 6,12 and 713 were synthesized as
described in the literature. Starting from 2-bromo-9H-
(10) Sreenath, K.; Thomas, T. G.; Gopidas, K. R. Org. Lett. 2011, 13,
1134.
(11) Low, P. J.; Paterson, M. A. J.; Yufit, D. S.; Howard, J. A. K.;
Cherryman, J. C.; Tackley, D. R.; Brook, R.; Brown, B. J. Mater. Chem.
2005, 15, 2304.
DTFA-1, TTFA-2, TAFB-3, and TAFA-4 weresubjected
to cyclic voltammetry (CV) and differential pulse
(12) Mizoshita, N.; Ikai, M.; Tani, T.; Inagaki, S. J. Am. Chem. Soc.
2009, 131, 14225.
(13) Rotzler, J.; Vonlanthen, D.; Barsella, A.; Boeglin, A.; Fort, A.;
Mayor, M. Eur. J. Org. Chem. 2010, 1096.
(14) (a) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36, 3609. (b)
Guram, A. S.; Rennels, R. A.; Buchwald, S. L. Angew. Chem., Int. Ed.
1995, 34, 1348.
Org. Lett., Vol. 13, No. 10, 2011
2703

voltammetry (DPV) analyses. Cyclic voltammograms
showed that four arylaminofluorene derivatives under-
went reversible oxidizations without any subsequent
chemical side reactions such as dimerization or dispropor-
tionation, indicating that the tolyl group is electrochemi-
cally inert after the formation of tolylaminium cation
radicals (Figure 2).11 Furthermore, the first oxidation vol-
tages of four compounds were all shifted to lower values. In
the case of DTFA-1, the oxidation potential is shifted to 0.37
V (vs Fc/Fcþ) relative to tritolylamine (0.47 V vs Fc/Fcþ).
This decrease in redox potential11 can be attributed to
charge delocalization of the cation radical to the extended
π-system of fluorene.11 Although TAFB-3 contains three
redox centers, its redox feature is very similar to that of
DTFA in that only one oxidation process at 0.36 V is
observed. This reflects the fact that there is no charge
delocalization among the three redox centers via the trivi-
nylbenzene core. TTFA-2 exhibited two chemically rever-
sible waves at 0.14 and 0.36 V ascribed to one- and two-
electron processes, respectively. It is worth noting that the
first redox potential is shifted to a much lower voltage,
strongly suggesting the delocalization of the first radical
cation between two nitrogen atoms. TAFA-4, which can be
regarded as a three-interconnected TTFA-2 compound shar-
ing the same nitrogen atom, displayed only three (instead of
four) redox potentials at 0.30, 0.44, and 0.60 V, respectively.
It was speculated that the lowest oxidation wave was buried
in the Fc/Fcþ wave. In fact, when the solvent was THF in lieu
of CH3CN, four oxidation waves were clearly observed
(Figure S1 in the Supporting Information).
Figure 3. Overlaid UVꢀvisꢀNIR absorption spectra of (a)
DTFA-1 (1.25 ꢁ 10ꢀ5 M) and (b) TAFB-3 (10ꢀ5 M) in CH3CN
with different equivalents of Cu(ClO4)2. Insets show the corre-
lation of the absorbance with the amount of Cu(ClO4)2.
no absorption in UVꢀvisꢀNIR spectra. Moreover, the
reduction potential of Cu(ClO4)2 in CH3CN is 0.7 V vs Fc/
Fcþ (Figure S6 in the Supporting Information), which is
sufficiently high to readily oxidize all four tolylaminofluor-
ene derivatives to form the corresponding polycation
radicals. Other metal-ion perchlorate or chloride salts in
different organic solvents were also examined for the
feasibility of generating tolylaminium radical cations of
TTFA-2 and DTFA-1 (Figures S3ꢀS5 in the Supporting
Information). Fluorescence intensity changes were used
for the quick assessment of cation radical formation
since the presence of the unpaired electrons quenched
the fluorescence. The results indicated that the oxidiz-
ing power of copper salts can be modulated by counter-
ions as well as solvents, and that Cu(ClO4)2 in CH3CN
is the best combination for effectively generating ary-
laminium cation radicals.
The cation radicals ofthe tolylaminofluorenederivatives
are thus generated using Cu(ClO4)2 in CH3CN solution
and characterized by UVꢀvisꢀNIR spectroscopy. DTFA-
1 exhibits one absorption band at 338 nm (ε = 17000 Mꢀ1
cmꢀ1). Upon titration with Cu(ClO4)2, three new absorp-
tion peaks gradually appear at 405, 640, 827, and a
shoulder emerges at 450 nm, with a concomitant decrease
of primordial absorption at 338 nm (Figure 3a). The new
absorption peaks are ascribed to the formation of the
tolylaminium radical cation. Monitoring the absorbance
increment at 827 nm as a function of the amount of
Cu(ClO4)2 reveals that saturation is reached when a stoi-
chiometric amount of Cu(ClO4)2 is added (inset in
Figure 3a). In the case of TAFB-3, with an increasing
amount of Cu(ClO4)2, four new absorption peaks appear
in the visibleand near-IR regionsat348, 520, 602, and 1046
nm, corresponding to the tolylaminium cation radicals
(Figure 3b). These absorption signals are saturated when
3 equiv of Cu(ClO4)2 is added (inset in Figure 3b), indicat-
ing that three neutral tritolylamines are converted to three
monocation radicals. Moreover, no IVCT band is ob-
served, strongly suggesting that there is no interaction
among these monocation radicals, consistent with the
observation of only one oxidation wave in the cyclic
voltammogram.
To search suitable oxidants for generating the cation
radicals of tritolylaminofluorenes, several chemical oxi-
dants including (4-BrC6H4)3NSbCl6, Pb(OAc)4 and Cu-
(ClO4)2 in CH3CN were evaluated. Among them,
Cu(ClO4)2 and (4-BrC6H4)3NSbCl6 gave complete con-
version of the cation radical with dosimetric quantities,
whereas a large excess amount of Pb(OAc)4 failed to give
satisfactory cation radicals (Figure S2 in the Supporting
Figure 2. (a) CV and (b) DPV of tolylaminofluorene derivatives
(10ꢀ3 M) in CH3CN solution: working electrode, platinum wire;
supporting electrolyte, tetrabutylammonium hexafluoropho-
sphate (0.1 M); scan rate, 0.1 V/s; reference against ferrocene
with ferrocene coupling at 0 V.
Information). One advantage of using Cu(ClO4)2 over (4-
BrC6H4)3NSbCl6 as the oxidant is that the former shows
2704
Org. Lett., Vol. 13, No. 10, 2011

The overlaid UVꢀvisꢀNIR absorption spectra of
TTFA-2 titrated with Cu(ClO4)2 in CH3CN are shown in
Figure 4a. Two new bands appearing at 482 and 1305
nm,4a characteristic of the tolylaminium monocation ra-
dicals, linearly increase with the amount of Cu(ClO4)2
before the addition of one equivalent. When further Cu-
(ClO4)2 is added, the IVCT band at 1305 nm starts to
diminish and a new band at 739 nm, assigned to dication
radicals where each tritolylamine is converted to a tritoly-
laminium radical, gradually forms. After addition of 2.4
equiv, saturation is reached and two isosbestic points are
observed in the spectra obtained from 1.2 to 2.4 equiv,
indicating where the cation and dication radicals are in
equilibrium during the period. The inset in Figure 4a
clearly depicts the correlation of changes in the cation
radical and dication radical concentrations during the
oxidation process. The symmetric bell-shaped curve
with an inversion point around 1.2 equiv of Cu(ClO4)2
is observed by monitoring the absorbance change at
1305 nm, whereas a sigmoidal curve with a midpoint at
2.0 equiv and an end point at 2.4 equiv is observed for
the dication radical. As a result, the solution changed
from colorless to yellow, then to green, and finally to
dark blue during the titration process.
Figure 5. ESR spectra of TTFA-2 (10ꢀ3 M) in CH3CN solution
with different equivalents of Cu(ClO4)2 at 77 K.
The formation of cation radicals and the oxidation
states of copper ions during the oxidation processes are
further confirmed by electron spin resonance (ESR)
spectroscopy. The titration of TTFA-2 with Cu(ClO4)2
is used for purposes of illustration. A broad band is
observed for Cu2þ, whereas TTFA-2 shows no ESR
signal (Figure 5). However, in the presence of 0.5ꢀ2.0
equiv of Cu(ClO4)2, a unique ESR signal centered at g =
2.0028 appears. This g value and the lack of hyperfine
splitting features are characteristic of arylaminium
cation radicals with extensive electron delocalization.4j
Therefore, the signal can be attributed to the one-
electron oxidation product of TTFA-2 in concomitance
with a reduction of Cu2þ to Cuþ. When 2.5 equiv of
Cu(ClO4)2 is used for the oxidation, a broad signal at g =
In the case of TAFA-4, the titration profile is very similar to
that of TTFA-2, except that more oxidant is consumed to
oxidize the four redox centers (Figure 4b). Three absorption
bands at 442, 700, and 1712 nm steadily increase before the
addition of 3 equiv of Cu(ClO4)2, indicating that three
peripheral tritolylaminofluorenes are oxidized to the monoca-
tion radical. The absorption band in the near-IR region is
2.0028 is observed for TTFA-22þ and another broad
3
signal due to excess Cu2þ reappears.
In conclusion, four tritolylaminofluorene deriva-
tives are successfully synthesized, and the corre-
sponding arylaminium cation radicals are dosi-
metrically generated by Cu(ClO4)2. Their structural
features and oxidation processes are characterized
and deciphered by UVꢀvisꢀNIR spectroscopy in
combination with ESR spectroscopy. Studies utiliz-
ing these compounds to further investigate mixed-
valence charge transfer (MVCT) in detail and to
characterize spin states are underway and will be
reported in due course.
Figure 4. Overlaid UVꢀvisꢀNIR absorption spectra of (a)
TTFA-2 (1.25 ꢁ 10ꢀ5 M) and (b) TAFB-4 (10ꢀ5 M) in CH3CN
with different equivalents of Cu(ClO4)2. Insets show the corre-
lation of the absorbance with the amount of Cu(ClO4)2.
Acknowledgment. We thank the National Science Coun-
cil of the Republic of China, Taiwan, for financial support,
Ms. Ya-Cheng Fang (National Taiwan University) for
the ESR measurements, and Dr. Yuki Shibano (Kyoto
University) for helpful discussions.
believed to be the IVCT band as TAFA-4 is oxidized to trication
radicals. When more than 3 equiv are added, the IVCT band
decreases and two new bands at 857 and 990 nm emerge from
the formation of tetracation radicals. Moreover, isosbestic
points start to appear in the spectra taken from 3 equiv or
more. Collectively, these observations provide solid and
convincing evidence that the oxidation processes occur in a
stepwise fashion, starting with the oxidation of peripheral
redox centers first, followed by that of the central one later.
Supporting Information Available. Synthetic proce-
dures, structural characterization, photospectroscopic,
and electrochemical data of DTFA-1, TTFA-2, TAFB-3,
and TAFA-4. This material is available free of charge via
the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 10, 2011
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