Ph2N-susbtituted ethylene-bridged p-phenylene oligomers: Synthesis and photophysical and redox properties

Article abstract of DOI:10.1021/jo1020163

For a series of p-phenylene-based oligomers terminated with two triphenylamines, their absorption, photoluminescence, and band gaps show a pattern of extensive π-conjugation with increasing array size. Oligomers with large central arrays have greater quan

Full text of DOI:10.1021/jo1020163

pubs.acs.org/joc
Ph2N-Susbtituted Ethylene-Bridged p-Phenylene Oligomers: Synthesis
and Photophysical and Redox Properties
Balagopal Shainamma Shaibu,† Sheng-Hsun Lin,† Chi-Yen Lin,‡ Ken-Tsung Wong,*,‡ and
Rai-Shung Liu*,†
†Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan, ROC, and
‡Department of Chemistry, National Taiwan University, Taipei, Taiwan, ROC
kenwong@ntu.edu.tw; rsliu@mx.nthu.edu.tw
Received October 14, 2010
For a series of p-phenylene-based oligomers terminated with two triphenylamines, their absorption,
photoluminescence, and band gaps show a pattern of extensive π-conjugation with increasing array
size. Oligomers with large central arrays have greater quantum yields than their small analogues.
Cyclic voltammetric (CV) measurements indicated two-step oxidations of the two diphenylamino
groups for compounds 1-5 and one-step oxidations for the two amines of large oligomers 6 and 7.
Introductions
fluorene (B),5 tetrahydropyrene (C),6 and ladder-typed
phenylene (D).7 For these oligomers, each repeated unit is
linked by a σ C-C bond, which impedes the π-electron
delocalization by the nonplanarity of benzene arrays.
p-Phenylene-based οligomers have diverse applications in
optoelectronic applications including organic light-emitting
diodes (OLED),1 field-effect transistors,2 and photovoltaic
devices.3 Scheme 1 shows representative classes of p-phenylene
oligomers bearing repeated units including p-phenylene (A),4
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Published on Web 01/14/2011
DOI: 10.1021/jo1020163
r
2011 American Chemical Society

Shaibu et al.
JOCArticle
SCHEME 1
based array terminated with two triarylamines is an idealized
model to test the degree of charge delocalization through
sequential oxidations of two triaryamines.12,13 In this work,
we prepared new ethene-bridged p-phenylenes 1-7 termi-
nated with two triphenylamines to examine their photophy-
sical and redox properties (Scheme 2). Compounds 1, 3, 4,
and 7 are the standard forms of ethene-bridged p-phenylenes
with spacers of varied lengths. We also prepared compound 2
that serves as an intermediate between 1 and 3. Compounds 5
and 6 are related to oligomer 4 with the replacement of the
two terminal benzenes with naphthalenes and fluorenes.
Results and Discussion
Schemes 3-4,5,6,7,8,9 show the synthetic protocols for
compounds 1-7 with platinum and ruthenium catalyzed
aromatizations of 1-alkynyl-2-arylbenzenes. We used these
two catalytic reactions for the stereocontrolled synthesis of
ethene-bridged p-phenylenes.8 In the platinum catalysis, an
addition of the 2-naphthyl or phenanthryl group to the
adjacent internal alkyne occurs at the more hindered carbon,
as exemplified by species 2d, 3a, and 5c. An opposite regios-
electivity was observed for ruthenium catalysis in that the
less hindered aryl carbon is the preferable site for addition to
the terminal alkyne,8 as shown by species 6b. Preparation of
compound 1 relies on the initial coupling of borate 1a and 1b,
giving the resulting product 1c in 92% yield. Treatment of 1c
with TpRuPPh3(CH3CN)2SbF6 (10 mol %) in hot DCE
(80 °C, 12 h) delivered phenanthrene species 1d that was
subsequently converted to 1e through sequential treatment
with BBr3 and Tf2O/pyridine. A final Buchwald coupling14
of triflate derivative 1e with diphenylamine gave desired
compound 1 in 61% yield. Toward the synthesis of com-
pound 2, we performed two Pd-catalyzed coupling reactions
of initial 2a to obtain key intermediate 2d. A subsequent
PtCl2-catalyzed aromatization of species 2d gave compound
2e that was ultimately transformed into desired 2 following
repeated procedures. Such a platinum catalysis served again
for the stereocontrolled synthesis of compound 3 through the
aromatization of key species 3a, which was readily obtained
from the Suzuki-coupling15 of phenanthryl triflate 1e with
borate 2c. We designed also a 2-fold aromatization of species
4b to obtain a distinct oligomer 4, as depicted in Scheme 6.
Similar p-phenylene oligomers 5 and 7 were synthesized
smoothly with platinum-catalyzed 2-fold aromatization of
key species 5c and 7b, as depicted in Schemes 7 and 9. Such a
2-fold aromatization is also applicable to the stereoselective
synthesis of a large oligomer 6; the protocol is shown in
Scheme 8. Compounds 1-7 are stable in air and fully
We reported8 the stereocontrolled synthesis of ethene-
bridged p-phenylenes of three series, denoted E, E0, and E00
as depicted in Scheme 1. These oligomers fail to show
efficient electron-delocalization, and a moderate energy
gap of 2.75 eV was obtained for the seven-benzene array
E0-7. Variable-temperature NMR spectra revealed8 small
energy barriers for the distortion of planarity of the large
benzene arrays.9 We propose that steric interactions of their
“bay” hydrogen pairs are the primary factor of such a
distortion.
Triarylamines are useful materials as the host layer in
OLED devices because they accelerate the transport of
holes.10,11 Aromatic hydrocarbons bearing triarylamines
serve as dopants in OLED devices because of their high
quantum yields and decreased energy gaps. A p-phenylene-
(7) (a) Scherf, U. J. Mater. Chem. 1999, 9, 1853. (b) Wu, Y.; Zhang, J.; Bo,
Z. Org. Lett. 2007, 9, 4435. (c) Jacob, J.; Sax, S.; Piok, T.; List, E. J. W.;
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Grimsdale, A. C.; Mullen, K. J. Am. Chem. Soc. 2004, 126, 6987. (d) Grem,
G.; Paar, C.; Stampfl, J.; Leising, G.; Huber, J.; Scherf, U. Chem. Mater.
1995, 7, 2.
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Lush, S. F.; Liu, R. S. Chem.;Eur. J. 2010, 16, 1826.
(9) (a) Pascal, R. A., Jr. Chem. Rev. 2006, 106, 4809. (b) Wang, Y.;
Stretton, A. D.; McConnel, M. C.; Wood, P. A.; Parsons, S.; Henry, J. B.;
Mount, A. R.; Galow, T. J. Am. Chem. Soc. 2007, 129, 13193. (c) Baird, T.;
Gall, J. H.; MacNicol, D. D.; Malliinson, P. R.; Michie, C. R. Chem.
Commun. 1988, 1471. (d) Downing, G. A.; Frampton, C. S.; MacNicol,
D. D.; Malliinson, P. R. Angew. Chem. Int. Ed. 1994, 33, 1587.
1
characterized with H and 13C NMR, MALDI, or high-
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(10) (a) Lambert, C.; Noll, G. J. Chem. Soc., Perkin Trans. 2 2002, 2039.
resolution mass and elemental analysis.
(b) Scherf, U.; List, E. J. W. Adv. Mater. 2002, 14, 477. (c) Lambert, C.;
Schelter, J.; Fiebig, T.; Mank, D.; Trifonov, A. J. Am. Chem. Soc. 2005, 127,
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K. Angew. Chem., Int. Ed. 2004, 43, 5331.
(11) (a) Dapperheld, S.; Steckhan, E.; Brinkhaus, K.-H. G.; Esch, T.
Figures 1 and 2 show the UV and PL spectra of compounds
1-7, and their corresponding photophysical properties are
summarized in Table 1. In Figure 1 (left), the absorption
spectra indicate compounds 1, 2, 3, and 7 have similar patterns
in several electronic absorptions whereas species 4 belongs to a
ꢀ
Chem. Ber 1991, 124, 2557. (b) Coropceanu, V.; Malagoli, M.; Andre, J. M.;
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Wienk, M. M.; Janssen, R. A. J. Chem. Mater. 2006, 18, 5832. (d) Szeghalmi,
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J. Am. Chem. Soc. 2004, 126, 7834.
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(12) (a) Lambert, C.; Noll, G. J. Am. Chem. Soc. 1999, 121, 8434.
(b) Bonvoisin, J.; Launay, J.-P.; Auweraer, M. V.; Schryver, F. C .D.
129, 12211. (b) Bonvoisin, J.; Launay, J.-P.; Verbouwe, W.; Auweraer, M. V.;
Schryver, F. C. D. J. Phys. Chem. 1996, 100, 17079. (c) Lambert, C.;
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Gaschler, W.; Schmalzlin, E.; Meerholz, K.; Brauchle, C. J. Chem. Soc.,
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SCHEME 2
SCHEME 3
distinct family due to its different absorption pattern. For the
former family, the absorption maxima at the 360-400 nm
regions gradually move to long wavelengths with increas-
ing benzene arrays. We observed three absorption bands
for compounds 4, 5, and 6, but their intensity patterns are
different from each other. Notably, compound 6 bearing a
9-diphenylaminofluoryl group has a strong absorption at
the low electronic states. The observed wavelengths of this
low electronic state follow the lengths of their benzene
arrays: 4 (388 nm) < 5 (394 nm) < 6 (410 nm).
Figure 2 (left) shows the fluorescent emission of com-
pounds 1, 2, 3, 4, and 7; their wavelengths reflect the
extensive conjugation, with 434 nm for species 1 and 471
nm for species 7. Figure 2 (right) shows the emission spectra
of the remaining species, of which the emission wavelengths
follow the order 5 (470 nm) > 6 (458 nm) > 4 (451 nm).
Diamines 1-7were strongly fluorescent in CH2Cl2 (1ꢀ 10 -5 M);
their quantum yields in Table 1 were measured carefully
and reproducibly. For small arrays 1-3, their quantum
yields were similar to the 24.2-26.9% in CH2Cl2, but
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SCHEME 4
SCHEME 5
SCHEME 6
SCHEME 7
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SCHEME 8
SCHEME 9
increased rapidly to 47.2%, 56.6%, and 56.2% for species
4, 5, and 7, and up to 96.3% for diamine 6. At a dense con-
centration (10-4 M), all of these diamine compounds 1-7
showed decreased quantum yields, presumably due to a
high extent of molecular aggregations; their absorption
and emission spectra remained unchanged under such
conditions. We compared the charge transfer properties
for representative species 2, 4, 5, and 6 on measuring their
PL spectra in toluene and CH2Cl2. The emission maxima of
2, 4, 5, and 6 in toluene appeared at 429, 439, 443, and
449 nm, respectively. Relative to those maxima in CH2Cl2,
we observed the magnitude of red-shifts 12, 13, 27, and 9
nm for 2, 4, 5, and 6, respectively. Accordingly, charge
transfer properties are not the primary factor to rationa-
lize the large quantum yields of oligomers 4-7.
As a transition to the lowest electronic state (S1) is
forbidden by symmetry for compounds 1-7, most of them
are not recognizable.8 We determined the optical gaps, listed
in Table 1, from the intersection of the absorption and
fluorescence spectrum. Compound 1 has a band gap of ca.
3.10 eV that gradually decreases to 3.04 eV for compound 2,
2.96 eV for species 3, and 2.89 eV for 7. The small decrease in
energy gap, as shown by the five-benzene array 7, is not
surprising because its benzene array is easily twisted from
FIGURE 1. Absorption spectra of compounds 1-4 and 7 (left) and 4-6 (right) in CH2Cl2 (1 ꢀ 10-5 M).
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FIGURE 2. Emission spectra of compounds 1-4 and 7 (left) and 4-6 (right) in CH2Cl2 (1 ꢀ 10-5 M).
TABLE 1. Photopysical Properties of Compounds 1-7
compd
Absmaxa (nm)
PLmaxa (nm)
Q.Y.a,b (%)
HOMO/LUMOd (eV)
band gape (eV)
1
2
3
4
5
6
7
286, 298, 364
320, 366
434
441
446
451
470
458
446,471
24.2 (21.8)c
26.2 (23.2)
26.9 (24.1)
47.2 (41.9)
56.6 (51.8)
96.3 (93.5)
56.2 (51.5)
5.07/1.97
5.17/2.13
5.22/2.26
5.20/2.28
5.21/2.45
5.15/2.33
5.24/2.35
3.10
3.04
2.96
2.92
2.76
2.82
2.89
324, 340, 378
284, 332,388
302, 364, 394
310, 364, 410
290, 370, 392
aIn CH2C12, 1 ꢀ 10-5 M. bCoumarin I as a standard. cThe data in parentheses correspond to 10-4 M. dThe energy levels of HOMO were determined
from CV. eBand gaps were calculated from UV absorption.
TABLE 2. Oxidation/Reduction Potentials of Compounds 1-7
oxi a
oxi a
red b
compd
1st E1/2
2nd E1/2
E1/2
1
2
3
4
5
6
7
0.27
0.37
0.42
0.40
0.41
0.35
0.44
0.58
0.60
0.56
0.54
0.51
0.92
-1.37
-1.41
-1.35
-1.35
aMeasured in CH2C12. bMeasured in DMF.
planarity.8 Notably, regioisomers 3 and 4 have comparable
values 2.96-2.92 eV. For the three-benzene arrays, com-
pound 5 has a band gap of 2.76 eV, smaller than those of 4
(2.92 eV) and of 6 (2.82 eV). Compounds 1-7 show one- or
two-step reversible oxidations in CH2Cl2 according to cyclic
voltammetry measurements. We estimated their HOMO
-LUMO gaps from their oxidation potential and band gaps,
as depicted in Table 1.
The electrochemical properties of ethylene-bridged p-
phenylenes (PAHs) 1-7 terminated with two amines were
probed by cyclic voltammetry (CV) (Table 2). All reduction
and oxidation potentials recorded here are relative to the
redox couple of ferrocene/ferrocenium (Fc/Fcþ). The oxida-
tion CV were measured in CH2Cl2, whereas reduction CV
were conducted in DMF. Most compounds tested here
exhibited two reversible oxidation potentials and one rever-
sible reduction feature (Figures 3 and 4). Among these com-
pounds, molecule 1 exhibited the smallest oxidation poten-
tials (0.27 V), compared to those of compounds 2-5 that
showed their first oxidation potentials at about 0.40 V. The
second oxidation potentials of 1-5 are around 0.5-0.6 V.
For species 1-5, we assign the first and second oxidations to
the removal of the first and second electrons of the two amine
FIGURE 3. Oxidation CV of compounds 1-7 in CH2Cl2.
groups. For compound 1, the triphenylamine renders the
other amine readily oxidizable because of a small phenan-
threne spacer; this hypothesis rationalizes the first oxidation
potential of species 1 at 0.27 V. Once the amine radical cation
is formed, the other amine becomes more difficult to oxidize
for a small spacer. This assessment is demonstrated by a large
difference, 0.31 V, between the two oxidation potentials, 0.27
and 0.58 V, for species 1; the difference became small with
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FIGURE 4. Reduction CV of compounds 3-6 in DMF.
increasing size of the spacer as in species 3 (0.14 V), 4 (0.14 V),
and 5 (0.10 V). The lack of reduction potentials of species 1
and 2 reflects their electron-rich phenanthrene and chrysene
spacers donated by two amines. Regioisomers 3 and 4 have
nearly identical oxidation and reduction potentials. For
species 6, we assign the first oxidation potential, 0.35 V, to
simultaneous oxidations of two amines at one stage, which
behave like two independent groups because of a large
spacer. The second oxidation at the high potential (0.92 V)
of species 6 is attributed to formation of a trication species
through the oxidation of the central core. For the largest
oligomer 7, we observed a single-step oxidation, at 0.44 V, of
the two amines at 0.44 V, without formation of a trication
species, likely because of its poor solubility in CH3CN
(5 ꢀ 10-4 M); this low solubility accounts also for the
absence of reduction potentials in DMF.
(EA:hexane = 5:95) to afford coupling product (2.54 g, 76%) as
a yellow oil. To a solution of this silyl compound (1.8 g, 4.02
mmol) in THF (20 mL) was added n-tetrabutylammonium
fluoride (1.0 M in THF, 6.03 mL, 6.03 mmol) at 0 °C, and the
resulting mixture was stirred at 0 °C for 2 h. Column chroma-
tography of the crude material on silica gel (EA/hexane = 5:95)
afforded compound 1c (1.38 g, 92%) as a yellow oil. 1H NMR
(400 MHz, CDCl3) δ 7.48 (d, J = 8.4 Hz, 2 H), 7.34-7.27 (m, 5
H), 7.19-7.12 (m, 7 H), 7.05 (t, J = 7.2 Hz, 2 H), 6.99-6.96 (m,
1 H), 3.85 (s, 3 H), 3.1(s, 1 H); 13C NMR (100 MHz, CDCl3) δ
157.9, 147.6 (2 ꢀ C), 146.8, 133.7, 130.5, 129.8 (2 ꢀ CH), 129.2
(4 ꢀ CH), 124.5 (4 ꢀ CH), 122.8 (2 ꢀ CH), 122.7 (2 ꢀ CH),
120.8, 118.2, 115.8, 83.3, 79.9, 55.4; HRMS calcd for C27H21NO
375.1623, found 375.1629.
b. Synthesis of 7-Methoxy-N,N-diphenylphenanthren-2-amine
(1d). A flask containing TpRuPPh3(CH3CN)2SbF6 (71.4 mg,
0.08 mmol) was dried in vacuo for 2 h before it was charged
with compound 1c (300 mg, 0.79 mmol) and 1,2-dichloroethane
(70 mL). The mixture was heated at 80 °C for 24 h before cooling
to room temperature. After removal of the solvent in vacuo, the
crude material was purified by flash column chromatography on
silica gel (CH2Cl2:hexane = 1:20) to afford compound 1d (243
Conclusions
In summary, we prepared a series of p-phenylene-based
oligomers terminated with two triphenylamines. The absor-
tion spectra, photoluminescence, and band gaps of these
oligomers show a pattern of extensive π-conjugation with
increasing array sizes. We observed fluorescent behaviors
such that large arrays have greater quantum yields than their
small analogues; this phenomenon is attributed to the easy
distortion from planarity of large arrays. Cyclic voltam-
metric (CV) measurements indicated two-step oxidations
of the two amino groups for compounds 1-5; the two poten-
tials approach each other with increasing sizes of spacer. The
two amines of large arrays 6 and 7 show one-step oxidation,
implying that the large spacers impede an extensive delocali-
zation of cation charge.
1
mg, 81%) as a yellow solid. H NMR (400 MHz, CDCl3) δ
8.47-8.42 (m, 2 H), 7.58 (d, J = 8.8 Hz, 1 H), 7.50-7.48 (m, 2
H), 7.39 (dd, J = 2, 6.8 Hz, 1 H), 7.29-7.20 (m, 6 H), 7.15 (d,
J = 7.6 Hz, 4 H), 7.06-7.02 (m, 2 H), 3.95 (s, 3 H); 13C NMR
(100 MHz, CDCl3) δ 157.8, 147.8 (2 ꢀ C), 145.4, 132.7, 132.1,
129.3 (4 ꢀ CH), 127.0, 126.8, 126.2, 124.7, 124.2 (4 ꢀ CH), 123.9,
123.2, 122.8 (2 ꢀ CH), 121.8, 117.2, 108.5, 55.4; HRMS calcd for
C27H21NO 375.1623, found 375.1615.
c. Synthesis of 7-(Diphenylamino)phenanthren-2-yltrifluoro-
methanesulfonate (1e). To a solution of 1d (200 mg 0.53 mmol)
in CH2Cl2 was added BBr3 (160 mg, 0.64 mmol) dropwise at
0 °C, then the resulting mixture was stirred for 2 h. After quenc-
hing with saturated K2CO3 solution, the organic layer was
extracted with dry CH2Cl2, dried over MgSO4, and concen-
trated under reduced pressure. The crude material was purified
by column chromatography on silica gel (CH2Cl2:hexane = 1:6)
to afford 7-(diphenylamino)phenanthren-2-ol (175 mg, 91%) as
a yellow solid. 7-(Diphenylamino)phenanthren-2-ol (300 mg,
0.83 mmol) and DMAP (10.1 mg, 0.08 mmol) were dissolved in
dry CH2Cl2 (25 mL) at room temperature, and to this mixture
was added pyridine (328 mg, 4.15 mmol) and then trifluoro-
methanesulfonic anhydride (351 mg, 1.25 mmol). The mixture
was stirred for 2 h at room temperature before it was quenched
with water. The organic layer was extracted with CH2Cl2,
washed with diluted HCl and brine, and dried over MgSO4.
Rotary evaporation of the solvent gave a brown residue that was
purified by flash column chromatography on silica gel (EA:
hexane = 1:20) to afford compound 1e (1.29 g, 87%) as a yellow
Experimental Section
1. Experimental Procedures for the Synthesis of Oligomer 1. a.
Synthesis of 20-Ethynyl-40-methoxy-N,N-diphenylbiphenyl-4-
amine (1c). A solution of toluene (80 mL), ethanol (20 mL),
and water (20 mL) was degassed for over 30 min, and to this
solution was added compound 1b8 (2.00 g, 7.48 mmol), Pd-
(PPh3)4 (864 mg, 0.748 mmol), Na2CO3 (7.9 g 74.84 mmol), and
borate 1a8 (3.33 g, 8.98 mmol). The mixture was stirred at 80 °C
for 12 h. The solution was cooled to room temperature and
quenched with water; the organic layer was extracted with
CH2Cl2, dried over MgSO4, and concentrated under reduced
pressure. The crude product was eluted through a silica column
1060 J. Org. Chem. Vol. 76, No. 4, 2011

Shaibu et al.
JOCArticle
oil. 1H NMR (400 MHz, CDCl3) δ 8.57 (d, J = 8.8 Hz, 1 H), 8.44
(d, J = 8.8 Hz, 1 H), 7.71 (d, J = 2.4 Hz, 1 H), 7.62-7.55 (m,
2 H), 7.48-7.43 (m, 3 H), 7.32-7.28 (m, 4 H), 7.17 (d, J = 7.6
Hz, 4 H), 7.10-7.07 (m, 2 H); 13C NMR (100 MHz, CDCl3) δ
147.4 (2 ꢀ C), 147.1, 147.0, 133.4, 131.9, 129.8, 129.4 (4 ꢀ CH),
128.5, 126.3, 124.8, 124.6 (4 ꢀ CH) 124.5, 123.8, 123.7 (2 ꢀ CH),
123.5, 120.2, 119.9, 119.5 (one carbon merged to others); HRMS
calcd for C27H18F3NO3S 493.0959, found 493.0965.
chromatography on silica gel (CH2Cl2:hexane = 2:30) afforded
compound 1 (126 mg, 61%) as a yellow solid. Mp 306.9-
308.9 °C; 1H NMR (400 MHz, CDCl3) δ 8.39 (d, J = 8.7 Hz,
1 H), 7.41(s, 1H), 7.28-7.24 (m, 5 H), 7.14-7.12 (m, 4 H),
7.05-7.01 (m, 2 H); 13C NMR (150 MHz, C2D2Cl4) δ 147.8 (2 ꢀ
C), 145.9, 132.7, 129.4 (4 ꢀ CH), 126.9, 126.0, 124.4 (4 ꢀ CH),
123.5, 123.1 (2 ꢀ CH), 121.2, 118.1; HRMS calcd for C38H28N2
512.2252, found 512.2250. Anal. Calcd for C38H28N2: C 89.03,
H 5.51, N 5.46. Found: C 89.09, H 5.63, N 5.38.
d. Synthesis of Oligomer 1. A flask was charged with NaOtBu
(58 mg, 0.60 mmol), Pd(OAc)2 (4.9 mg, 0.02 mmol), and BINAP
(18.8 mg, 0.03 mmol), then to this solution was added a toluene
(5 mL) soluion of compound 1e (200 mg, 0.40 mmol) and
diphenylamine (102 mg, 0.60 mmol). The resulting mixture
was stirred under argon at room temperature for 30 min before
it was heated to 110 °C to stir for 32 h. The reaction mixture was
cooled to room temperature, diluted with CH2Cl2 (10 mL),
and filtered through a Celite pad. Concentration and flash
Acknowledgment. We thank National Science Council,
Taiwan, for financial support of this work.
Supporting Information Available: Detailed synthesis pro-
cedure, spectral data, NMR spectra, and Maldi-Mass of new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 76, No. 4, 2011 1061