Zwitterionic ladder bis(arylethenyl)benzenes with large two-photon absorption cross sections

Article abstract of DOI:10.1002/asia.200900517

Various phosphonium- and borate-bridged bis(arylethenyl)benzenes were synthesized. The large two-photon absorption (TPA) cross sections in these zwitterionic ladder π-electron systems revealed that the introduction of zwitterionic bridges in the coplanar π-conjugated framework is a powerful strategy for designing excellent TPA materials. 2010 Wiley-VCH Verlag GmbH&Co. KGaA.

Full text of DOI:10.1002/asia.200900517

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DOI: 10.1002/asia.200900517
Zwitterionic Ladder Bis(arylethenyl)benzenes with Large Two-Photon
Absorption Cross Sections
Aiko Fukazawa,[a] Hiroshi Yamada,[a] Yutaka Sasaki,[b] Seiji Akiyama,[b] and
Shigehiro Yamaguchi*[a]
Dedicated to the 150th anniversary of Japan–UK diplomatic relations
The two-photon absorption (TPA) properties of organic
chromophores have recently attracted much attention owing
to a variety of potential applications, such as 3D microfabri-
cation, ultra-high-density optical data storage, optical-limit-
ing devices, up-converted lasing, and photodynamic thera-
py.[1] Considerable effort has been devoted to developing an
excellent organic TPA material with a large TPA cross sec-
tion (d). Several TPA materials with d values of several hun-
dreds to thousands GM (1 GM=10À50 cm4 sphotonÀ1) have
already been reported, including dipolar[2] or quadrupo-
lar[2a,3] p-conjugated chromophores, oligomeric porphyrin
derivatives,[4,5] and giant macrocycles.[6] These studies have
provided important guidelines for the molecular design of
excellent TPA materials. Namely, 1) a sufficiently long p-
conjugation length, 2) introduction of strong donor and ac-
ceptor moieties into a p-conjugated skeleton, and 3) a
Figure 1. Zwitterion-bridged ladder p-conjugated compounds.
planar and rigid molecular structure are important structural
requirements for large d values. Regarding these guidelines,
one promising class of TPA materials may be the ladder-
type p-conjugated compounds with polar electronic struc-
tures. We have recently succeeded in synthesizing such com-
pounds categorized into this class, i.e., phosphonium- and
fashion, but also provide a highly polar electronic structure.
The phosphonium and borate functionalities serve as strong
electron-accepting and -donating moieties, respectively. All
these features seem to well satisfy the foregoing require-
ments for excellent TPA properties. Whereas it has already
been reported that several p-electron systems with zwitter-
ionic groups have excellent nonlinear optical properties,[8–13]
our current interest lies in the effects of the zwitterionic
bridges in the completely planar p skeleton. We now report
the TPA characteristics of a series of phosphonium- and
borate-bridged ladder p-electron systems. In particular, we
newly synthesized dipolar P+BÀP+BÀ-DSB 3 (A-D-A-D
type, Figure 1) and compared its photophysical properties
with those of the quadrupolar P+BÀBÀP+-DSB 1 (A-D-A
type) and BÀP+P+BÀ-DSB 2 (D-A-D type) to address the
effect of the bridging patterns (D and A denote the donor
and acceptor, respectively). Furthermore, we designed and
synthesized a thiophene-terminated analogue, the BÀP+P+
borate-bridged distyrylbenzenes (DSBs)
1
and
2
(Figure 1).[7] In these compounds, the zwitterionic bridges
not only fix the DSB framework in a rigid and coplanar
[a] Dr. A. Fukazawa, Dr. H. Yamada, Prof. Dr. S. Yamaguchi
Department of Chemistry, Graduate School of Science
Nagoya University
Furo, Chikusa, Nagoya 464-8602 (Japan)
Fax : (+81)52-789-5947
E-mail: yamaguchi.shigehiro@b.mbox.nagoya-u.ac.jp
[b] Y. Sasaki, Dr. S. Akiyama
Mitsubishi Chemical Group
Science and Technology Research Center, Inc.
1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900517.
466
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Chem. Asian J. 2010, 5, 466 – 469

BÀ-bridged bis(thienylethenyl)benzene (BTB) 4, as a poten-
tial TPA material in the near-infrared (NIR) region.
Thus, the monolithiation of 7 with excess nBuLi in the pres-
ence of TMEDA, followed by trapping with Cy2PCl, pro-
duced a mono-phosphanyl intermediate, which smoothly un-
derwent the cyclization to give the partially cyclized product
8. The mixture was again subsequently treated with an
excess amount of nBuLi and then with Cy2PCl. This one-pot
protocol successfully produced the fully fused compound 4
in 39% yield as deep blue solids. Notably, both compounds
3 and 4 were sufficiently stable in the air and moisture.
As summarized in Table 1, the photophysical properties
of the phosphonium- and borate-bridged DSBs significantly
depend on their bridging patterns. In the absorption spectra,
The syntheses of P+BÀP+BÀ-DSB 3 and BÀP+P+BÀ-BTB
4 were accomplished by intramolecular cyclization protocols,
as shown in Scheme 1 and 2. For the synthesis of 3, an un-
Table 1. Photophysical data for a series of bis(arylethenyl)benzenes 1–5
and 9.[a]
Absorption
Fluorescence
TPA
Cmpd
labs [nm][b]
log e
lem [nm][c]
FF
d [GM][d]
1
2
3
522
531
480
4.30
4.06
4.06
4.24
4.63[h]
599
636
601
0.52[e]
0.47[e]
0.35[e]
0.021[f]
0.31[h]
349[i]
469[i]
476[i]
784[j]
216[i]
Scheme 1. Synthesis of compound 3 (Cy=cyclohexyl, Mes=mesityl).
4
620
734
9[g]
439[h]
528[h]
[a] In benzene unless stated otherwise. [b] Only longest absorption
maxima are shown. [c] Emission maxima upon excitation at the absorp-
tion maximum wavelengths. [d] Two-photon absorption cross sections;
1 GM=10À50 cm4 sphotonÀ1
. [e] Absolute fluorescence quantum yields
determined by a calibrated integrating sphere system within Æ3% error.
[f] Relative fluorescence quantum yields determined with cresyl violet
perchlorate[14] as a standard. [g] Reference [17]. [h] In toluene. [i] Excited
at 900 nm. [j] Excited at 1200 nm.
among the three positional isomers 1–3, the dipolar P+BÀP+
BÀ-DSB 3 has the shortest absorption maximum (labs
=
480 nm in benzene), which is 40–50 nm shorter than those of
quadrupolar 1 and 2. In the fluorescence spectra, all the
DSB derivatives showed intense emissions with quantum
yields of 0.3–0.5 in benzene. The D-A-D type BÀP+P+BÀ-
DSB 2 has the longest emission maximum wavelength (lem),
while those of 1 and 3 are comparable to each other. In
terms of the Stokes shift, the dipolar A-D-A-D type 3 has
the largest value of 4200 cmÀ1, while those of the quadrupo-
lar 1 and 2 are 2500 and 3100 cmÀ1, respectively. These re-
sults indicate that the dipolar compound 3 has a significant
intramolecular charge-transfer (CT) character in the lowest-
energy transition.
Scheme 2. Synthesis of compound 4.
symmetrically diborylated bis(phenylethynyl)benzene 5 was
employed as a starting material. This compound was ob-
tained from 1,4-dibromo-2,5-bis(trimethylsilylethynyl)ben-
zene in seven steps with a total yield of 17% (see the Sup-
porting Information). The dilithiation of 5 with tBuLi in
THF followed by treatment with Cy2PCl produced a bis-
A
To elucidate the effect of the bridging patterns on the
electronic structures, we conducted DFT calculations on the
model compounds 1’–3’ at the B3LYP/6-31G(d) level of
theory, in which the cyclohexyl and mesityl groups in 1–3
are replaced with methyl and phenyl groups, respectively.[15]
Dipolar 3’ has its HOMO and LUMO energy levels in the
middle of those of quadrupolar 1’ and 2’, as shown in
Figure 2. This suggests that, in the A-D-A-D system, the
electronic effects of the electron-donating borate and elec-
underwent nucleophilic double cyclization to give P+BÀP+
BÀ-DSB 3 as bright orange solids in 60% yield. On the
other hand, for the synthesis of 4, we employed borylthien-
yl-terminated diethynylbenzene 7 as a starting material and
first tried the dilithiation using various bases. However, all
attempts were unsuccessful, presumably owing to the insta-
bility of the resulting dilithiated species. Therefore, we con-
ducted the intramolecular cyclization in a stepwise manner.
Chem. Asian J. 2010, 5, 466 – 469
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467

S. Yamaguchi et al.
COMMUNICATION
tion at 900 nm.[19] These
d
values are approximately twice
as high as that of the reference
compound 9 (d=216 GM at
900 nm). These results demon-
strate that the zwitterionic
modulation of the distyrylben-
zene chromophore is useful for
the design of high-performance
TPA materials, while the bridg-
ing pattern does not signifi-
cantly affect the TPA proper-
ties, and dipolar 3 shows a
slightly larger d value among
the three DSB derivatives. Be-
sides, we found that the incre-
ment of the electron-donating
ability of the terminal borate
moieties in the BÀP+P+BÀ-
DSB 2 by replacing the ben-
zene rings with thiophene rings
resulted in a much larger TPA
cross section. The d value of
Figure 2. Energy diagrams and pictorial representations of the frontier MOs for the phosphonium- and borate-
bridged DSBs 1’–3’ and BTB 4’ calculated at the B3LYP/6-31G(d) level, and their lowest-energy transitions es-
timated by TD-DFT calculations at the B3PW91/6-31+G(d)//B3LYP/6-31G(d) level. Methyl and phenyl
groups were used instead of the cyclohexyl and mesityl groups in the corresponding compounds 1–4, respec-
tively. The trimethylsilyl groups were used instead of the triisopropylsilyl groups in 4.
tron-accepting phosphonium located at the inner positions
cancel the electronic effects of each other. Indeed, the
HOMO and LUMO of 3’ are mostly localized on the termi-
nal benzoborole and the terminal benzophosphole moieties,
respectively. These rather localized frontier MOs may en-
hance the intramolecular CT character and thus result in the
large Stokes shift.
the BÀP+P+BÀ-BTB 4 was 784 GM even upon excitation at
1200 nm. It is worth noting that there are only a few exam-
ples of excellent TPA materials that can be excited with a
NIR light, such as the highly polarized D-p-A type p-conju-
gated compounds[2b] and the oligomeric porphyrin deriva-
tives with inherently long-wavelength absorption bands.[4]
These facts clearly demonstrate that the present zwitterionic
modulation is also useful for attaining a large d value in the
NIR region.
In summary, we investigated the linear absorption, fluo-
rescence, and two-photon absorption properties of a series
of phosphonium- and borate-bridged distyrylbenzenes 1–3.
All these compounds showed larger d values compared to
the previously known TPA-active distyrylbenzene derivative,
regardless of the bridging patterns. Moreover, a large TPA
cross section, even upon excitation at 1200 nm, was also at-
tained in the thienyl-terminated derivative 4. These results
clearly demonstrate that the introduction of zwitterionic
bridges into the ladder p-conjugated framework is a power-
ful strategy for the molecular design of excellent TPA mate-
rials.
The replacement of the terminal benzene rings in the
BÀP+P+BÀ-DSB 2 with thiophene rings significantly affects
the linear absorption and fluorescence properties. The BÀP+
P+BÀ-BTB 4 has an absorption maximum at 620 nm in ben-
zene, which is nearly 90 nm longer than that of the DSB an-
alogue 2. Moreover, 4 exhibits an emission in the red to
NIR region with a lem value of 734 nm, although the fluores-
cence quantum yield is low (FF =0.021). As shown in
Figure 2, the DFT calculations of their model compounds 2’
and 4’ at the B3LYP/6-31G(d) level revealed that the
HOMO level of 4’ (À4.81 eV) is higher than that of 2’
(À5.21 eV), while their LUMO levels are comparable to
each other (À2.82 eV for 2’, and À2.81 eV for 4’). This fact
clearly demonstrates that the enhancement of the electron-
donating character of the terminal borate moieties by re-
placing the benzene rings with thiophene rings is responsible
for the significantly red-shifted absorption and fluorescence
bands.
Two-photon absorption properties for a series of zwitter-
ionic ladder bis(arylethenyl)benzenes 1–4 were evaluated by
the Z-scan method using a femtosecond optical parametric
amplifier.[16] The TPA cross sections d are also collected in
Table 1, together with the data of a representative quadru-
polar distyrylbenzene derivative 9[17] for comparison.[18] The
obtained d values of quadrupolar P+BÀBÀP+-DSB 1 and
BÀP+P+BÀ-DSB 2, and dipolar P+BÀP+BÀ-DSB 3 in ben-
zene, were 349, 469, and 476 GM, respectively, upon excita-
Acknowledgements
This work was partially supported by Grants-in-Aid (Nos. 17069011,
19675001, and 20750029) from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan. H.Y. thanks the Research Fel-
lowships of the Japan Society for the Promotion of Science (JSPS) for
Young Scientists.
Keywords: boron · phosphorus · pi interactions · two-
photon absorption · zwitterions
468
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Chem. Asian J. 2010, 5, 466 – 469

Zwitterionic Ladder Bis(arylethenyl)benzenes with Large Two-Photon Absorption Cross Sections
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Received: October 1, 2009
Published online: January 14, 2010
Chem. Asian J. 2010, 5, 466 – 469
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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