Bridged triphenylamine-based dendrimers: Tuning enhanced two-photon absorption performance with locked molecular planarity

Article abstract of DOI:10.1021/ol801238n

Triphenylamine derivatives bridged by methylene units afford a near planar molecular platform in a series of one dimer and two oligomers exhibiting increased structural rigidity compared to that of their parent triphenylamines. This series of dendrimers s

Full text of DOI:10.1021/ol801238n

ORGANIC
LETTERS
2009
Vol. 11, No. 1
1-4
Bridged Triphenylamine-Based
Dendrimers: Tuning Enhanced
Two-Photon Absorption Performance
with Locked Molecular Planarity
Zhen Fang,† Tang-Lin Teo,† Liping Cai,† Yee-Hing Lai,*,† Anna Samoc,‡ and
Marek Samoc*,‡
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Singapore 117543, Singapore, and Laser Physics Centre, Research School of Physical
Sciences and Engineering, Australian National UniVersity,
Canberra ACT 0200, Australia
chmlaiyh@nus.edu.sg; marek.samoc@anu.edu.au
Received February 27, 2008
ABSTRACT
Triphenylamine derivatives bridged by methylene units afford a near planar molecular platform in a series of one dimer and two oligomers
exhibiting increased structural rigidity compared to that of their parent triphenylamines. This series of dendrimers show significantly enhanced
two-photon absorptions that are up to 3-fold that of triphenylamines with similar molecular size and structure.
Organic materials with large two photon absorption (TPA)
cross-sections show optical power limiting properties1 and
have potential applications in scanning excitation micros-
copy,2 three-dimensional optical data storage,3 and photo-
dynamic therapy.4 TPA performance in these applications
depends greatly on large TPA coefficients. Design of
multipolar5 and dendritic molecular structures is among
several strategies6 in achieving large TPA cross-sections. This
has led to extensive work on building π-conjugated dendritic
molecules with π centers and functional groups that possess
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† National University of Singapore.
‡ Australian National University.
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10.1021/ol801238n CCC: $40.75
Published on Web 12/09/2008
2009 American Chemical Society

electron-donating and/or electron-withdrawing properties on
terminal sites.7
The molecules 7-9 were synthesized14 by Sonogashira
couplings. A single crystal of 5 (Figure 1) was grown from
Triphenylamine-based molecules 1-4 exhibit enhanced
nonlinear responses,8 the three N-C bonds in triphenylamine
and its derivatives having been shown9,10 to be essentially
in one common plane. This supports π conjugation through
a lone pair of electrons on the nitrogen atom.11 Extension
of π conjugation over the entire molecular framework may,
however, be poor as a result of relatively large dihedral
angles between the phenyl ring plane and the plane of the
N-bonded carbon atoms.8 Recent research has verified that
molecular planarity is an important positive factor for
enhancing TPA cross-sections.12 The methylene units in
bridged triphenylamine 513 are expected to hold the three
phenyl rings in a locked, planar manner compared to those
in triphenylamine. In this work a series of a dimer 7 and
two dendrimers 8 and 9 incorporating two to six units of
bridged 5 were synthesized. They show TPA cross-sections
remarkably higher than their triphenylamine counterparts and
hence are more promising models in the field of nonlinear
optics.
Figure 1. X-ray crystallographic structure of 5 (C ) gray; N )
dark gray; H atoms are omitted for clarity).
hexane. The crystal belongs to the monoclinic system with
a space group of P21/c. There are two independent molecules
per asymmetric unit with irregular propeller-like geometries.
The central nitrogen atoms are found to have small deviations
from the plane of the surrounding carbon atoms with
distances of 0.15 and 0.29 Å, respectively. Mean values of
the C-N-C bond angle in the two molecules are, however,
118.5° and 119.0°, respectively, corresponding to an sp2-
hybridized nitrogen favoring intramolecular p-π interactions
between nitrogen and its adjacent phenyl rings. In addition
the dihedral angles between phenyl rings and the plane of
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(14) Synthesis of monomers 5 and 10 is described in Supporting
Information.
2
Org. Lett., Vol. 11, No. 1, 2009

three central carbon atoms remain small with mean values
of 5.9° and 7.6°, respectively, which are much smaller than
those in triphenylamine (>43°).8 The above results clearly
show that a bridged triphenylamine molecular framework is
a significantly more planar structure compard with that of
parent triphenylamine, which is expected to facilitate TPA.
The one-photon absorption and emission spectra of
compounds 7, 8, and 9 are illustrated in Figure 2. It was
Table 1. Comparison of TPA Properties of Triphenylamine
(TPhA) and a Series of Its Derivatives 1-9a
b
d
compd
MW
Nπ cross-section σ2 (GM)c σ2/MW σ2/Nπ
TPhA
245.32
514.66
1053.34
1592.02 130
3746.74 306
365.51
977.45
20
42
86
5 (2)
320 (40)
1900 (500)
2700e
0.02
0.62
1.23
1.70
1.20
0.08
1.28
2.58
2.22
0.03
1.0
2.9
2.8
2.0
0.2
3.9
6.8
5.7
1
2
3
4
5
7
8
9
4500e
20
44
92
30 (5)
1300 (300)
4800 (500)
6100 (1000)
1864.69
2751.94 140
a TPA cross-sections were calculated from dependences of the nonlinear
absorption of solutions on the concentration.18 b Nπ: the number of π
c
electrons. 1 GM ) 1 × 10-5 cm4·s·photon-1
.
d Relative values with
reference to the value for molecule 1. e The peak values for molecules 3
and 4 are cited from the literature.18
A significant obervation is that for similar numbers of π
electron (Nπ) in a conjugated molecular framework, the
bridged-triphenylamine series always exhibit significantly
larger TPA cross-sections (σ2) than the triphenylamine series.
For example, the cross-section of 5 is about 6 times that of
parent triphenylamine, and the figures for 7, 8, and 9 are
about 4, 2.5, and 2.3 times that of 1, 2, and 3, respectively.
Although both dendrimers 3 and 4 have relatively more
extended conjugated molecular frameworks, their two-photon
absorptions are less efficient than those of 8 and 9. In fact a
comparison of their relative efficiency in terms of σ2/Nπ
clearly indicate that the 8/9 pair is two to three times better.
While there is evidence that an ethenyl link may be a
relatively better conjugation spacer group than an ethynyl
link,19 our results suggest that improved molecular planarity
in the triphenylamine moiety is key to enhanced extended
electronic interaction in the entire molecular framework of
7, 8, and 9. This in turn leads to their improved TPA
properties. The molecule 8 is the most promising model in
our work. Although its σ2 value is lower than that of 9, it is
synthetically more accessible. A σ2 value of 4800 GM for a
first generation dendrimer such as 8 is considered large for
an intrinsic TPA cross-section of molecules of similar sizes.
In addition the σ2/Nπ and σ2/MW values for 8 are most
superior among all molecules studied in Table 1.
Figure 2. Absorption and emission spectra of 7 (ε ) 75 000
L·mol-1·cm-1), 8 (ε ) 120 000 L·mol-1·cm-1), and 9 (ε ) 220 000
L·mol-1·cm-1) in chloroform (10-6 M).
observed that an increase in the number of branches from
one in compound 7 to three in dendrimer 8 resulted in a red
shift from 399 nm for 7 to 411 nm for 8. The absorptions at
the 300-350 nm range correspond to the core bridged
triphenylamine moiety in both the dimer and the dendrimer.
There is no further red shift observed for λmax at about 400
nm in going from 8 to the larger, more branched, and more
conjugated molecule 9. The onset to absorption is, however,
red-shifted for the latter due to its relatively more extended
conjugation. All three molecules 7-9 emit blue light with
wavelengths of 428, 438, and 441 nm, respectively. An
extension of conjugation going from 7 to 8 results in a 10-
nm red shift in λmax. There is again no appreciable differences
in their fluorescence spectra going from 8 to 9, similar to
what was observed in their UV-vis spectra. This indicates
that there is little change in conjugation effect in extending
the conjugated molelucar framework after the second genera-
tion. Fluorescent quantum yield was observed to decrease
from 64% for 7 and 53% for 8 to 40% for 9,15 possibly
because of an increasing probability of intermolecular
interaction.
(16) (a) Ahn, T. K.; Kwon, J. H.; Kim, D. Y.; Cho, D. W.; Jeong, D. H.;
Kim, S. K.; Suzuki, M.; Shimizu, S.; Osuka, A.; Kim, D. J. Am. Chem.
Soc. 2005, 127, 12856–12861. (b) Ahn, T. K.; Kim, K. S.; Kim, D. Y.;
Noh, S. B.; Aratani, N.; Ikeda, C.; Osuka, A.; Kim, D. J. Am. Chem. Soc.
2006, 128, 1700–1704. (c) Yoon, Z. S.; Kwon, J. H.; Yoon, M.-C.; Koh,
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N.; Osuka, A.; Shigeiwa, M.; Maeda, S. J. Phys. Chem. A 2005, 109, 2996–
2999.
In TPA studies of conformationally mobile porphrins16
and related examples,17 the magnitude of TPA cross-section
depended on favorable molecular structures that optimize
intramolecular charge transfer. The enhancement in electronic
interactions derived from extended molecular planarity is
strongly correlated to larger TPA cross-sections. The TPA
properties of triphenylamine and its derivatives 1-9 are
summarized in Table 1.
(17) (a) Zhou, X.; Ren, A.-M.; Feng, J.-K.; Liu, X.-J.; Yang, G.-B.;
Tian, W.-J. J. Opt. Mater. 2004, 27, 315–321. (b) Xu, G.; Zhao, X.; Hu,
D.; Shao, Z.; Liu, H.; Tian, Y. Opt. Commun. 2006, 260, 292–297.
(18) Samoc, M.; Samoc, A.; Luther-Davies, B.; Bao, Z.; Yu, L.; Hsieh,
B.; Scherf, U. J. Opt. Soc. Am. B 1998, 15, 817–825.
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45, 5685–5688. (b) Rogers, D. W.; Matsunaga, N.; Andreas, A.; Zavitsas,
A. A.; J. McLafferty, F. J.; Liebman, J. F. Org. Lett. 2003, 5, 2373–2375.
(15) Fluorescent quantum yields observed for 7, 8, and 9 at 5 × 10-7
M were 80%, 75%, and 56%, respectively.
Org. Lett., Vol. 11, No. 1, 2009
3

To demonstrate that the enhanced TPA properties of 7-9
were unlikely to derive from aggregate formation19 or solvent
effects,20 local molecular environment influences21 on the
optical responses (Figure 3) of 8 were studied.
degree of molecular planarity is further enhanced by replac-
ing ethynyl with alkenyl linkages, significantly larger σ2
values could be expected even from low generation den-
drimers in the series. This means more desired nonlinear
Scheme 1
.
Synthesis of Bridged Triphenylamine-Based Dimer 7
and Dendrimers 8 and 9
Figure 3. Linear photoluminescence spectra of 8 in (a) various
solvents (10-6 M) and (b) hexane/THF mixtures.
The fluorescence efficiency (Figure 3a) of 8 in the aprotic
solvents studied remained relatively constant (∼65%).23
A
continuous and relatively regular change in red shift and
vibronic structure observed in Figure 3b is indicative of
nonspecific interactions between 8 and hexane/THF. The
relatively constant fluorescence efficiency associated with
the above changes indicates that the local molecular environ-
ment surrounding 8 involves relatively homogeneous mixture
of hexane and THF molecules. The above observations may
suggest that there is no significant intermolecular interaction
(aggregation) between molecules of 8 in these solvents. This
supports our argument that the enhanced TPA performance
observed for 7-9 was mainly due to their locked molecular
planarity.
The molecules 7-9 combine high fluorescence quantum
yields with large TPA cross-sections (1300-6100 GM).
Their locked molecular planarity facilitates TPA. If the
optical properties from relatively smaller dendrimers that are
synthetically more accessible.
Acknowledgment. This work was supported by the
Singapore Ministry of Education AcRF Tier 1 Grant R-143-
000-248-112.
(20) (a) Ray, R. C.; Sainudeen, Z. J. Phys. Chem. A 2006, 110, 12342–
12347. (b) D’Avino, G.; Terenziani, F.; Painelli, A. J. Phys. Chem. B 2006,
110, 25590–25592. (c) Belfield, K. D.; Bondar, M. V.; Hernandez, F. E.;
Przhonska, O. V.; Yao, S. Chem. Phys. 2006, 320, 118–124. (d) Kim, S.;
Zheng, Q.; He, G. S.; Bharali, D. J.; Pudavar, H. E.; Baev, A.; Prasad,
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Supporting Information Available: Crystallographic data
of 5 and experimental and spectroscopic/elemental data of
all compounds prepared. This material is available free of
charge via the Internet at http://pubs.acs.org.
(21) Woo, Y. H.; Liu, B.; Kohler, B.; Korystov, D.; Mikhailovsky, A.;
Bazan, G. C. J. Am. Chem. Soc. 2005, 127, 14721–14729.
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Reinhardt, B. A.; Vaia, R. A. Chem. Mater. 1999, 11, 2899–2906.
(23) Compound 8 has poor solubility in protic solvents (e.g., MeOH,
EtOH).
OL801238N
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Org. Lett., Vol. 11, No. 1, 2009