Synthesis and Structure-Linear and Structure-Nonlinear Optical Properties of Multi-Dipolar Zigzag Oligoaryleneethynylenes

Full text of DOI:10.1021/jo070607z

Synthesis and Structure-Linear and Structure-Nonlinear Optical
Properties of Multi-Dipolar Zigzag Oligoaryleneethynylenes
Pik Kwan Lo,† King Fai Li,‡ Man Shing Wong,*,† and Kok Wai Cheah*,‡
Department of Chemistry, Department of Physics, and Centre for AdVanced Luminescence Materials,
Hong Kong Baptist UniVersity, Kowloon Tong, Hong Kong SAR, China
mswong@hkbu.edu.hk; kwcheah@hkbu.edu.hk
ReceiVed March 28, 2007
A novel series of monodisperse, multi-dipolar zigzag oligoaryleneethynylenes DA(n) and D-Ar-A(n),
bearing electron-donating dibenzothiophene and electron-accepting dibenzothiophene dioxide as arenes,
with up to six charge-transfer (dipolar) units have been designed and synthesized by palladium-catalyzed
Sonogashira coupling reactions. The linear and nonlinear optical properties of these multi-dipolar
oligoaryleneethynylenes can easily be modified or enhanced by incorporating/extending with various
central aryleneethynyl moieties such as phenylethynyl, oligo(9,9-dibutylfluorenyl)ethynyl, and oligo-
thienylethynyl within the donor-acceptor units. Interestingly, the absorption and emission of these zigzag
oligoaryleneethynylenes are not dependent on the number of covalently linked dipolar chromophores;
however, the fluorescence quantum efficiencies consistently decrease with increased number of covalently
linked dipolar units. These zigzag oligoaryleneethynylenes exhibit a linear increase in the two-photon
absorption (TPA) cross-sections with increased number of covalently linked dipolar units without red-
shifting the absorption and emission spectra. In addition, very large TPA cross-sections in the femtosecond
regime (σ800 ) 1306 GM in DMF or σ750 ) 1522 GM in CH2Cl2) were obtained for D-TF-A(4) despite
the moderate strength of the donor-acceptor pair. Our results suggest that the TPA properties of these
zigzag oligoaryleneethynylenes including TPA wavelength and TPA cross-section can easily be tuned
by means of modifying the central aryleneethynylene units and increasing the number of dipolar units,
respectively. This approach provides an alternative means to tune or enhance the TPA cross-section at
a specific wavelength.
Introduction
excited fluorescence (TPEF) microscopy,3 photodynamic therapy,4
and two-photon microfabrication.5 There has been significant
progress in designing and synthesizing one-dimensional chro-
mophores such as dipolar and quadrupolar molecules with large
TPA cross-sections, which generally increases with the donor/
Nonlinear optical materials that exhibit large two-photon
absorption (TPA) cross-sections (σ) have drawn considerable
attention recently as they show potential applications in various
emerging technologies, which include three-dimensional optical
data storage,1 two-photon optical power limiting,2 two-photon
(3) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73.
(4) Bhawalker, J. D.; Kumar, N. D.; Zhao, C. F.; Prasad, P. N. J. Clin.
Laser, Med. Surg. 1997, 15, 201.
(5) Cumpston, B. H.; Ananthavel, S. P.; Barlow, S.; Dyer, D. L.; Ehrlich,
J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I. Y. S.; McCord-
Maughon, D.; Qin, J.; Ro˜ckel, H.; Rumi, M.; Wu, X. L.; Marder, S. R.;
Perry, J. W. Nature 1999, 398, 51.
† Department of Chemistry and Centre for Advanced Luminescence Materials.
‡ Department of Physics and Centre for Advanced Luminescence Materials.
(1) Parthenopoulos, D. A.; Rentzepis, P. M. Science 1989, 245, 843.
(2) He, G. S.; Xu, G. C.; Prasad, P. N.; Reinhardt, B. A.; Bhatt, J. C.;
Dillard, A. G. Opt. Lett. 1995, 20, 435.
10.1021/jo070607z CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/04/2007
6672
J. Org. Chem. 2007, 72, 6672-6679

Multi-Dipolar Zigzag Oligoaryleneethynylenes
acceptor strength, conjugated length, and planarity of the
π-conjugated system.6 However, such structural modifications
often shift the two-photon absorption maximum to a longer
wavelength because of the enhanced π-conjugation/electron
delocalization. On the other hand, strategy/guideline that can
enhance the TPA cross-section at a specific wavelength, i.e.,
800 nm, is rather limited. TPA-active molecules that can exhibit
large σ800 at 800 nm excitation wavelength are particularly
important as it is the most accessible and useful laser wavelength
for practical applications, i.e., TPEF microscopy. More recently,
the introduction of multidimensional π-conjugated systems, in
which the active chromophores extend into two or three
dimensions, offers a new opportunity to enhance the nonlinear
optical properties in different aspects. This has led to the
development of highly TPA-active multibranched chromophores,
octupolar molecules, and dendrimers.7 The TPA enhancement
of multidimensional π-conjugated systems arises either from
the increase in the number of chromophore density or from the
π-π interactions among π-conjugated moieties or chro-
mophores. As a result, studies of multidimensional π-conjugated
chromophoric systems may also provide insight into the
influence of chromophoric interaction on the functional proper-
ties of a material as the bulk (or material) performance of
chromophores depends greatly on the molecular arrangement
in the solid state and the morphology of a material.
Oligoaryleneethynylenes, in which arene moieties are con-
nected by the ethynyl bridge, constitute one of the widely
investigated π-conjugated molecules. Substantial work has been
done to modulate or enhance a specific functional property of
linear oligoaryleneethynylenes8 such as electrical conductivity,
optical nonlinearity, and sensing property by means of applying
different types of arene moieties such as benzene, thiophene,
anthracene, pyridine, or pyrimidine to the system. The merit of
using an ethynyl unit as a conjugated bridge is that it possesses
an excellent photochemical stability,9 which is particularly
important for nonlinear optical materials. In contrast to linear
oligoaryleneethynylenes, zigzag oligomers are not widely
explored for use as a functional material.10
We describe herein the synthesis and structure-linear and
-nonlinear optical investigation of a novel series of multi-dipolar
chromophores, namely multi-dipolar zigzag oligoarylene-
ethynylenes, DA(n) and D-Ar-A(n), which exhibit tunable TPA
wavelength and TPA cross-sections based on electron-donating
dibenzothienylethynyl and electron-accepting dibenzothienyl-
ethynyl dioxide as arene units, respectively. We have shown
that the linear/nonlinear optical properties of these zigzag
oligoaryleneethynylenes can easily be modified by incorporating/
extending with various central aryleneethynyl units such as
phenylethynyl, oligo(9,9-dibutylfluorenyl)ethynyl, and oligo-
thienylethynyl within the charge transfer (dipolar) units. We
have also demonstrated that the TPA cross-sections of these
zigzag oligoaryleneethynylenes can be enhanced by the number
of dipolar chromophores incorporated without causing the red-
shift of the TPA band.
Results and Discussion
To enhance the solubility of the longer homologues of the
zigzag oligomers, a polyalkyleneoxy donating group was
employed as solubilizing electron-donating endcaps. Alkylation
of 4-iodophenol with 1-[2-(2-chloroethoxy)ethoxyl]butane in the
presence of K2CO3 in DMSO yielded the alkylated iodophenol
1. Palladium-catalyzed Sonogashira coupling of 1 with TMS-
acetylene followed by deprotection of the TMS group in a basic
medium afforded terminal alkyne 2 in excellent yields (Scheme
1). Double Sonogashira coupling of 2 and 3,6-diiododiben-
zothiophene S,S′-dioxide 5, which was prepared by iodination
of dibenzothiophene 3 with periodic acid/I2 yielding 4 followed
by mCPBA oxidation,11 afforded bis-dipolar molecule DA(2)
in 50% yield. On the other hand, with the use of an excess of
5 over 2 (3:1), the monocoupled compound 9 would become
the major product with 45-47% yield and DA(2) in 22-23%
yield (Scheme 2). The monocoupled compound 9 was used to
synthesize tetrakis-dipolar chromophore DA(4) in which double
Sonogashira coupling of 9 and 6, which was prepared by double
Sonogashira coupling of 4 with TMS-acetylene followed by
deprotection of the TMS group, affording 68% yield. Because
of the reactivity and solubility problems of intermediates, among
various possible routes, the slightly soluble hexakis-dipolar
molecule DA(6) could only be synthesized by the double
Sonogashira coupling of 9 and 10, which was obtained by the
coupling of intermediates 7 and 8 as shown in Scheme 2. The
monocoupled compound 7 was prepared by the coupling of 4
and TMS-acetylene (3:1) giving the desired product in
50-60% and the dicoupled product in 25-30% yields; on the
other hand, double Sonogashira coupling of 4 and TMS-
acetylene, followed by mCPBA oxidation and deprotection
afforded 8 in 92% yield.
(6) (a) Reinhardt, B. A.; Brott, L. L.; Clarson, S. J.; Dillard, A. G.; Bhatt,
J. C.; Kannan, R.; Yan, L.; He, G. H.; Prasad, P. N. Chem. Mater. 1998,
10, 1863. (b) Albota, M.; Beljonne, D.; Bredas, J. L.; Ehrlich, J. E.; Fu, J.
Y.; Heikal, A. A.; Hess, S. E.; Kogej, T.; Levin, M. D.; Marder, S. R.;
McCord-Maughon, D.; Perry, J. W.; Rockel, H.; Rumi, M.; Subramanian,
G.; Webb, W. W.; Wu, X. L.; Xu, C. Science 1998, 281, 1653. (c) Belfield,
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9500. (e) Kannan, R.; He, G. S.; Yuan, L.; Xu, F.; Prasad, P. N. Chem.
Mater. 2001, 13, 1896. (f) Mongin, O.; Porre`s, L.; Moreaux, L.; Mertz, J.;
Blanchard-Desce, M. Org. Lett. 2002, 4, 719. (g) Mongin, O.; Porre`s, L.;
Charlot, M.; Katan, C.; Blanchard-Desce, M. Chem. Eur. J. 2007, 13, 1481.
(7) (a) Chung, S. J.; Kim, K. S.; Lin, T. C.; He, G. S.; Swiatkiewicz, J.;
Prasad, P. N. J. Phys. Chem. B 1999, 103, 10741. (b) Adronov, A.; Frechet,
J. M. J.; He, G. S.; Kim, K. S.; Chung, S. J.; Swiatkiewicz, J.; Prasad, P.
N. Chem. Mater. 2000, 12, 2838. (c) Yang, W. J.; Kim, D. Y.; Kim, C. H.;
Jeong, M. Y.; Lee, S. K.; Jeon, S. J.; Cho, B. R. Org. Lett. 2004, 6, 1389.
(d) Bartholomew, G. P.; Rumi, M.; Pond, S. J. K.; Perry, J. W.; Tretiak,
S.; Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 11529. (e) Belfield, K. D.;
Morales, A. R.; Hales, J. M.; Hagan, D. J.; Van Stryland, E. W.; Chapela,
V. M.; Percino, J. Chem. Mater. 2004, 16, 2267. (f) Wei, P.; Bi, X.; Wu,
Z.; Xu, Z. Org. Lett. 2005, 7, 3199. (g) Zheng, Q.; He, G. S.; Prasad, P. N.
Chem. Mater. 2005, 17, 6004.
To enhance the intramolecular charge transfer of zigzag
chromophores, various central aryleneethynyl units including
phenylethynyl, 9,9-dibutylfluorenylethynyl, bis(9,9-dibutylfluo-
renyl)ethynyl, ter(9,9-dibutylfluorenyl)ethynyl, bithienylethynyl,
and terthienylethynyl, which are denoted by MP, MF, DF, TF,
DTP, and TTP, respectively, were incorporated to extend the
(8) (a) Bunz, U. H. F. Chem. ReV. 2000, 100, 1805. (b) McQuade, D.
T.; Pullen, A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537. (c) Tour, J.
M. Acc. Chem. Res. 2000, 33, 791. (d) Bunz, U. H. F. Acc. Chem. Res.
2001, 34, 988.
(9) (a) Porres, L.; Mongin, O.; Katan, C.; Charlot, M.; Pons, T.; Mertz,
J.; Blanchard-Desce, M. Org. Lett. 2004, 6, 47. (b) Yang, W. J.; Kim, C.
H.; Jeong, M. Y.; Lee, S. K.; Piao, M. J.; Jeon, S. J.; Cho, B. R. Chem.
Mater. 2004, 16, 2789.
(10) (a) Wong, M. S.; Nicoud, J. F. Tetrahedron Lett. 1994, 35, 6113.
(b) Anderson, S. Chem. Eur. J. 2001, 7, 4706. (c) Shotwell, S.; Windscheif,
P. M.; Simth, M. D.; Bunz, U. H. F. Org. Lett. 2004, 6, 4151.
(11) Li, Z. H.; Wong, M. S.; Fukutani, H.; Tao, Y. Chem. Mater. 2005,
17, 5032.
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Lo et al.
CHART 1. Molecular Structures of Multi-Dipolar Zigzag Oligoaryleneethynylenes DA(n) and D-Ar-A(n)
SCHEME 1. Synthesis of Intermediates for Multi-Dipolar Zigzag Oligoaryleneethynylenes
π-conjugated path. Adopting the same synthetic strategy of
DA(2), the extended bis-dipolar zigzag chromophores
D-Ar-A(2), where Ar ) MP, MF, DF, TF, DTP, and TTP, were
synthesized accordingly by means of the double Sonogashira
coupling of 3,6-diiododibenzothiophene S,S′-dioxide 5 with the
extended terminal alkynes 12a-f. These extended alkynes were
generally prepared by the monocoupling of diiodo-(oligo)arylene
with 2 and then with TMS-acetylene followed by TMS-
deprotection affording the desired alkynes in 66-97% yields
as shown in Scheme 3. As the preparation of the extended
analogues of 6 was not so trivial and efficient, an alternative
approach was developed for the extended tetrakis-dipolar zigzag
chromophore series, D-Ar-A(4), rather than adopting the
synthetic strategy of DA(4). Double Sonogashira coupling of 8
and 1,4-diiodophenylene or diiodooligo-9,9-dibutylfluorenes15
afforded 13a-d in moderate to excellent yields. Monocoupling
of 13a-d and 2 (3:1) gave 14a-d, respectively, in 45-65%
yield. Again, double Sonogashira coupling of 6 and the
monocoupled 14a-d yielded the desired products D-Ar-A(4)
where Ar ) MP, MF, DF, and TF, respectively, in moderate
(12) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem.
1965, 11, 371.
(13) Albota, M. A.; Xu, C.; Webb, W. W. Appl. Opt. 1998, 37,
7352.
6674 J. Org. Chem., Vol. 72, No. 18, 2007

Multi-Dipolar Zigzag Oligoaryleneethynylenes
SCHEME 2. Synthesis of Multi-Dipolar Zigzag Oligoaryleneethynylenes DA(2), DA(4), and DA(6)
yields (Scheme 3). All the newly synthesized multi-dipolar
zigzag oligoaryleneethynylenes were fully characterized by
1H NMR, 13C NMR, and high-resolution mass spectroscopy
and elemental analysis and found to be in good agreement with
the expected structures.
as phenylethynyl, oligo(9,9-dibutylfluorenyl)ethynyl, and oligo-
thienylethynyl varying from 341 nm for DA(2) to 373 nm for
D-DF-A(2) and to 425 nm for D-TTP-A(2) in CH2Cl2 (Figure
2) which is attributed to the enhanced π-conjugation within the
dipolar unit.
In view of the absorption spectra of these zigzag oligomers,
All the emission spectra of these multi-dipolar zigzag oligo-
abs
there is no significant shift in the absorption maxima (λ
)
aryleneethynylenes show moderate to strong positive solvato-
max
em
max
upon an increase in the number of charge-transfer (dipolar) units
within a zigzag oligoaryleneethynylene even though the absorp-
chromic effect (e.g., D-MP-A(n): λ
) 466 nm in CH2Cl2
em
to 510 nm in DMF; and D-TF-A(n): λ
) 498 nm in
max
tion bands are broadened and the molecular absorptivities are
CH2Cl2 to 544 nm in DMF) suggesting a strong charge-transfer
character in the excited state. Generally, the emission spectra
of these oligoaryleneethynylenes are also independent of the
number of dipolar units linked (Figure 1 and Table 1). Con-
sistently, the emission maxima (λemmax) are greatly red-shifted
upon extending with various central aryleneethynyl cores from
440 nm for DA(2) to 483 nm for D-DF-A(2) and to 553 nm
for D-TTP-A(2) in CH2Cl2 (Figure 3). On the other hand, the
fluorescence quantum efficiencies (φFL) of these oligoarylene-
ethynylenes decrease slightly when the dipolar units increase
from two to four within a molecule (i.e., µFL of D-MF-A(2) )
0.86 vs µFL of D-MF-A(4) ) 0.72 and µFL of D-TF-A(2) )
0.40 vs µFL of D-TF-A(4) ) 0.32), which is attributed to the
neighboring quenching. Interestingly, the φFL values can be
greatly enhanced by incorporation of phenylethynyl, 9,9-dibutyl-
abs
max
enhanced, for example, λ
) 345 nm for D-MP-A(2) and
345 nm for D-MP-A(4) in CH2Cl2 and λabs ) 344 nm for
max
D-MP-A(2) and 342 nm for D-MP-A(4) in DMF (Figure 1 and
Table 1).
On the other hand, the λambasx shift to longer wavelengths upon
an incorporation of various central aryleneethynyl cores such
(14) Examples of large TPA cross-sections measured using the femto-
second laser pulse as an excitation source at 800 nm in the literature. N,N,N-
Tris(4-(1-cyano-1-(4-cyanophenyl)vinyl)phenyl)amine: σ800 ) 790 GM
(Z-scan method).7c 4,7,12,15-Tetrakis((4′′′-(4′′,4′-dihexylaminostyryl)sty-
ryl)styryl)[2.2]paracyclophane: σ800 ) 3200 GM (TPEF).7e 4,4′-[(9,9-
Dinonyl-9H-fluorene-2,7-diyl)bis[(1E)-2,1-ethenediyl-4,1-phenylene-(1E)-
2,1-ethenediyl]]bis(N,N-dihexylbenzenamine): σ800 ) 1200 GM (TPEF).6g
(15) Li, H. Z.; Wong, M. S. Org. Lett. 2006, 8, 1499. (b) Li, H. Z.;
Wong, M. S.; Tao, Y.; Lu, J. Chem. Eur. J. 2005, 11, 3285.
J. Org. Chem, Vol. 72, No. 18, 2007 6675

Lo et al.
SCHEME 3. Synthesis of Extended Multi-Dipolar Zigzag Oligoaryleneethynylenes D-Ar-A(n)
fluorenylethynyl, and bis(9,9-dibutylfluorenyl)ethynyl as a
central extending bridge but are moderately reduced with a
bithienylethynyl and a terthienylethynyl bridge. The φFL values
of DA(n) compounds show no significant solvent effect due to
their moderate charge-transfer character; however, those of the
extended oligoaryleneethynylenes, D-Ar-A(n) compounds, are
greatly reduced in polar solvent as shown in Table 1. The strong
charge-transfer character of D-Ar-A(n) compounds in polar
DMF solvent could lead to excitonic coupling, which is
detrimental to the φFL.12
The thermal properties of the zigzag oligoaryleneethynylenes
were examined by thermogravimetric analysis (TGA). All the
zigzag oligomers showed very good thermal stabilities with
decomposition temperatures (Td) in the range of 329-429 °C
except for DA(6).
pulsed laser as an excitation source in both CH2Cl2 and DMF.
The results of two-photon absorption cross-sections measured
at 800 nm are given in Table 1. Compared with the one-photon
emission spectra, the two-photon excited fluorescence (TPEF)
spectra consistently show a small red-shift likely due to the
reabsorption effect with a typical example shown in Figure 4a
suggesting that their emissive states might be very similar. In
addition to the one-photon excited fluorescence spectra, the two-
photon excited fluorescence spectra do not depend on the
number of linked dipolar units.
Generally, there is a progressive increase in the TPA cross-
sections as the central aryleneethynyl core within the dipolar
unit of these zigzag oligoaryleneethynylenes changes from
phenylethynyl, 9,9-dibutylfluorenylethynyl, bithienylethynyl, bi-
(9,9-dibutylfluorenyl)ethynyl, terthienylethynyl to ter(9,9-dibu-
tylfluorenyl)ethynyl with σ800 of D-TF-A(4) up to 1214 GM in
The two-photon cross-sections (δ) were determined by the
two-photon induced fluorescence method,13 using a femtosecond
6676 J. Org. Chem., Vol. 72, No. 18, 2007

Multi-Dipolar Zigzag Oligoaryleneethynylenes
FIGURE 1. Absorption spectra (10-6 M) and normalized emission spectra (10-7 M) excited at 341 and 345 nm, respectively, of (a) DA(n) and
(b) D-MP-A(n) in CH2Cl2 and DMF, respectively.
TABLE 1. Summaries of Physical Measurements of DA(n) and D-Ar-A(n) Series
abs
max
em
max
abs
max
em
max
a/nm
λ
a,c/nm
ΦFL
σ800a,g/GM
λ
b/nm
λ
b/nm
ΦFL
σ800b,g/GM
Th/°C
a
b
λ
DA(2)
DA(4)
DA(6)
302, 341
302, 341
299, 341
345
345
364
367
373
374
377
440
0.45d
0.37d
0.22d
0.86d
0.80d
0.86e
0.72e
0.74e
0.53e
0.40e
0.32e
0.28f
0.30f
301, 331
300, 331
299, 334
344
342
365
364
378
374
380
478
477
476
510
509
415, 503
505
427, 507
511
544
544
574
590
0.42d
0.37d
0.19d
0.18d
0.17d
0.13e
0.09e
0.17e
0.15e
0.04e
0.05e
0.12f
0.05f
360
371
194
375
354
391
429
399
416
409
329
391
400
443
446
466
466
478
476
483
485
498
499
534
553
D-MP-A(2)
D-MP-A(4)
D-MF-A(2)
D-MF-A(4)
D-DF-A(2)
D-DF-A(4)
D-TF-A(2)
D-TF-A(4)
D-DTP-A(2)
D-TTP-A(2)
24.5
23.3
212
433
378
723
609
1214
264
465
18.6
37.1
233
558
449
795
784
1306
326
702
378
405
425
378
404
427
a Measured in CH2Cl2. b Measured in DMF. c Excited at the absorption maxima. d With quinine in 1.0 M H2SO4 (µ334 ) 0.56) as a standard. e With
9,10-diphenylanthrancene in cyclohexane (µ360 ) 0.90) as a standard. f With fluorescein in 0.1 M NaOH (µ436 ) 0.92) as a standard. g Determined by the
two-photon-induced fluorescence method, using 800 nm femtosecond laser pulses with rhodamine 6G as a standard. h Determined by thermal gravimetric
analyzer with a heating rate of 10 deg/min under N2.
increase in the number of dipolar units within a molecule, TPA
cross-sections of these zigzag oligomers increase linearly (σ800
in CH2Cl2: D-TF-A(2) ) 609 GM and D-TF-A(4) ) 1214
GM) without causing the red-shift of absorption and emission.
This result suggests that the inter- and intraelectronic couplings
of dipolar units are not significant within this zigzag skeleton.
This result also implies that varying/increasing the number of
dipolar units within a zigzag molecule can provide a means to
tune/enhance the TPA cross-section at a specific wavelength
such as at 800 nm. Figure 4b shows TPA spectra of D-Ar-A(4)
measured in CH2Cl2 in which the maximum of the TPA cross-
section seems to be close to twice that of the λambasx, suggesting
that the lowest energy excited state is both one-photon and two-
photon allowed as observed in the typical donor-acceptor
FIGURE 2. Absorption spectra of DA(2) and D-Ar-A(2)s in CH2Cl2
molecules. Even though dibenzothiophene and dibenzothio-
CH2Cl2 or 1306 GM in DMF (Table 1), which are attributed to
phene dioxide are the moderate electron donor and electron
the enhanced electron delocalization and hyperpolarizability of
acceptor, respectively, very large TPA cross-sections, σ800
)
the dipolar unit and the better match of the two-photon
absorption wavelength and excitation wavelength. These results
indicate that the 9,9-dibutylfluorenylene moiety is more useful
than the thienylene moiety to enhance the TPA cross-section.
Even though φFL values are much smaller in DMF, TPA values
measured in DMF are slightly larger than those measured in
CH2Cl2, which is likely due to the enhanced π-conjugation/
charge delocalization in polar solvent. Importantly, upon an
1306 GM in DMF or σ750 ) 1522 GM in CH2Cl2, were obtained
for D-TF-A(4).14 The power-squared dependence of the two-
photon excited fluorescence for these zigzag oligomers was also
investigated. In all cases, the power-squared dependence of
TPEF was followed with the slope in the range of 1.99-2.04,
which directly gives experimental evidence of two-photon
excitation nature as shown in Figure 5.
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Lo et al.
FIGURE 3. (a) Luminescence colors upon illumination and (b) emission spectra of DA(2) and D-Ar-A(2) in CH2Cl2.
FIGURE 4. (a) Comparisons of two-photon excited emission spectra with one-photon emission spectra of D-DF-A(n). (b) TPA spectra of D-Ar-
A(4) (Ar ) MF, DF, TF) in CH2Cl2.
to six charge-transfer (dipolar) units by palladium-catalyzed
Sonogashira coupling reactions. The linear/nonlinear optical
properties of these multi-dipolar oligoaryleneethynylenes can
easily be modified/enhanced by incorporating/ extending with
various central aryleneethynyl moieties such as phenylethynyl,
oligo(9,9-dibutylfluorenyl)ethynyl, and oligothienylethynyl within
the donor-acceptor units. Interestingly, the absorption and
emission of these zigzag oligoaryleneethynylenes are not
dependent on the number of dipolar chromophores linked;
however, the fluorescence quantum efficiencies consistently
decrease with increased number of covalently linked dipolar
units. Because of the negligible inter- and intrachromophoric
interactions, these zigzag oligoaryleneethynylenes exhibit a
linear increase in the TPA cross-sections with increased number
of covalently linked dipolar units without red-shifting the
FIGURE 5. Logarithmic plots of the power dependence of relative
two-photon induced fluorescence on pulse intensity, using an 800 nm
femtosecond laser as an exciting source.
absorption and emission spectra. Our results suggest that the
TPA cross-section can easily be enhanced at a specific
wavelength by means of increasing the number of dipolar units
within a zigzag molecule. Furthermore, very large TPA cross-
sections in the femtosecond regime (σ800 ) 1306 GM in DMF
or σ750 ) 1522 GM in CH2Cl2) were obtained for D-TF-A(4),
which shows potential for practical applications. This approach
provides an alternative means to tune or enhance the TPA cross-
section at a specific wavelength.
Conclusion
In summary, we have designed and synthesized a novel series
of multi-dipolar zigzag oligoaryleneethynylenes DA(n) and
D-Ar-A(n), bearing electron-donating dibenzothiophene and
electron-accepting dibenzothiophene dioxide as arenes, with up
6678 J. Org. Chem., Vol. 72, No. 18, 2007

Multi-Dipolar Zigzag Oligoaryleneethynylenes
(100 MHz, CDCl3) δ 159.5, 136.4, 133.3, 133.1, 131.3, 129.8,
124.2, 122.1, 114.8, 114.2, 93.8, 86.6, 71.2, 70.9, 70.1, 69.5, 67.5,
31.6, 19.2, 13.9. MS (FAB) m/z 736.9 (M+). HRMS (MALDI-
TOF) m/z calcd for C44H48O8SNa 759.2968, found 759.3004 [M
+ Na]+. Anal. Calcd for C44H48O8S: C, 71.71; H, 6.57. Found:
C, 71.62; H, 6.35.
Experimental Section
General Coupling Procedure: 3,6-Bis(4-{1-[2-(2-butoxyethoxy)-
ethoxy]}phenylethynyl)dibenzothiophene Sulfone DA-(2). To a
stirred solution of 5 (0.20 g, 0.43 mmol), Pd(PPh3)2Cl2 (12.0 mg,
0.02 mmol), and CuI (1.6 mg, 0.009 mmol) in triethylamine (5
mL) under N2 was added 2 (0.30 g, 1.14 mmol) in dry THF. The
reaction mixture was stirred overnight at room temperature under
N2. The reaction mixture was extracted with CH2Cl2 (3 × 30 mL).
The combined organic layer was washed with water three times,
dried over anhydrous Na2SO4, and evaporated to dryness. The crude
product was purified by silica gel column chromatography with
CH2Cl2/EtOAc (v/v 100:1) as eluent. The pure product was obtained
by precipitation from CH2Cl2/methanol to afford DA-(2) as a
brownish-orange solid of 0.16 g yield (50%). 1H NMR (270 MHz,
CDCl3) δ 7.89 (s, 2H), 7.76 (d, J ) 8.1 Hz, 2H), 7.61 (d, J ) 9.2
Hz, 2H), 7.47 (d, J ) 8.9 Hz, 4H), 6.91 (d, J ) 8.7 Hz, 4H), 4.16
(t, J ) 4.1 Hz, 4H), 3.87 (t, J ) 5.4 Hz, 4H), 3.71 (t, J ) 5.4 Hz,
4H), 3.59 (t, J ) 5.4 Hz, 4H), 3.46 (t, J ) 6.8 Hz, 4H), 1.51-1.66
(m, 4H), 1.31-1.39 (m, 4H), 0.90 (t, J ) 7.3 Hz, 6H). 13C NMR
Acknowledgment. We are grateful to the Hong Kong
Research Grants Council (HKBU 2020/06P and HKBU/2058/
01P) and a Faculty Research Grant (HKBU FRG/04-05/II-11)
for financial support of this work.
Supporting Information Available: General experimental
1
details, synthetic procedures and spectral data, and copies of H
NMR and 13C NMR spectra of DA(n) and D-Ar-A(n) compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO070607Z
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