Synthesis of triphenylamine-cored dendritic two-photon absorbing chromophores

Article abstract of DOI:10.1021/ol050905n

(Chemical Equation Presented) A new series of dendritic two-photon absorbing chromophores containing triphenylamine moiety as a core or branching points have been synthesized through a convergent synthetic strategy. One-photon and two-photon optical prope

Full text of DOI:10.1021/ol050905n

ORGANIC
LETTERS
2005
Vol. 7, No. 15
3199-3202
Synthesis of Triphenylamine-Cored
Dendritic Two-Photon Absorbing
Chromophores
Peng Wei,† Xiangdong Bi,*,† Zhe Wu,† and Zhi Xu‡
Department of Chemistry, UniVersity of Missouri-Kansas City, Kansas City, Missouri
64110, and Department of Chemistry and Biochemistry, UniVersity of Missouri-St.
Louis, St. Louis, Missouri 63121
xiabi@umich.edu
Received April 24, 2005
ABSTRACT
A new series of dendritic two-photon absorbing chromophores containing triphenylamine moiety as a core or branching points have been
synthesized through a convergent synthetic strategy. One-photon and two-photon optical properties of these molecules were characterized.
-44
In the nanosecond time domain, these molecules exhibited large two-photon absorption (TPA) cross sections up to 7.56
−12.2
×
10
s cm4
at 800 nm, indicating that these molecular structures were viable candidates for various two-photon related applications.
Two-photon absorption (TPA) has potential applications in
many important technological areas including optical power
limiting,1 three-dimensional (3-D) storage media,2 two-
photon dynamic therapy,3 up-converted lasing,4 and two-
photon fluorescence microscopy.5 The realization of these
technological applications relies greatly on the development
of organic molecules with large TPA cross sections. Exten-
sive studies have been conducted, and great progress has
been made on the relationship between molecular structure
and TPA cross section.6-9 Some basic structural motifs have
been revealed to be important regarding TPA molecular
construction.3b,6 Molecular structures containing a π-center
with electron donors or acceptors on the terminal sites of
the conjugation system are expected to exhibit good TPA
* Current address of corresponding author: 200 Zina Pitcher Place,
Center for Biologic Nanotechnology, University of Michigan, MI 48109.
Email: xiabi@umich.edu.
† University of Missouri-Kansas City.
(5) (a) Denk, W.; Strickler, J. H.; Webb, W. W. Science 1990, 248, 73-
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Dombroskie, A. G.; Reinhardt, B. A.; Baur, J. W.; Vaia, R. A.; Tan, L. S.
Chem. Mater. 2001, 13, 1896-1904. (b) Reinhardt, B. A.; Brott, L. L.;
Clarson, S. J. Chem. Mater. 1998, 10, 1863-1874.
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Hu, Z. Y.; McCord-Maughon, D.; Parker, T. C.; Rockel, H.; Thayumanavan,
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9500-9510.
‡ University of Missouri-St. Louis.
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Marder, S. R.; Perry, J. W. Opt. Lett. 1997, 22, 1843-1845. (b) Bhawalkar,
J. D.; He, G. S.; Prasad, P. N. Rep. Prog. Phys. 1996, 59, 1041-1070.
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Ehrlich, J. E.; Erskine, L. L.; Heikal, A. A.; Kuebler, S. M.; Lee, I. Y. S.;
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Rentzepis, P. M. Science 1989, 245, 843-845.
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10.1021/ol050905n CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/24/2005

Figure 1. Structures of dendritic TPA molecules 9, 10, and 16.
response.1a,7 Particularly, the conjugation length, π-electron
center, and chemical functional groups at the end of electron
conjugation are recognized as three important factors for
structure-property optimization.6b,8 Quadrupolar molecules
have received considerable attention in earlier studies,1a,3b,7
whereas many recent studies have focused on octupolar,
multibranched structures whose TPA cross sections have
been greatly enhanced as a result of higher molecular
symmetry and multidimensionality.9 Quantum mechanic
calculations have further elucidated the newly developed
structure-property relationship based on the symmetric and
multibranched π-electron conjugation framework.10
The concept of multidimensionality has further been
extended to the construction of dendritic and polymeric TPA
structures.11 Because of their compact size and exponentially
increased number of branches, dendritic structures possess
effective geometric sizes less than their corresponding linear
polymers and constitute a new class of TPA molecules with
drastic increase of both the conjugation length and the density
of chromophores achieved in a single molecular structure.11c
As such, dendritic TPA molecules are expected to display
much greater TPA cross sections than their linear or branched
counterparts.
three branches extending outward connected by phenylene-
vinylene units (Figure 1). The triphenylamine moiety is
carefully chosen and intensely employed in each structure
as either the core or branching points, since triphenylamine
has been shown to be a good framework in enhancing
nonlinear responses.12 Because triphenylamine displays good
coplanarity of the central nitrogen and the three surrounding
carbon atoms connecting to it, the triphenylamine unit can
maintain uninterrupted conjugation between central nitrogen
lone pair electrons and the arms,13 as well as function as a
strong electron donor to the conjugation system. The
peripherally substituted amino groups further modulate the
solubility of the molecule and provide more electron density
to the structure. Their one-photon optical properties were
characterized by UV-vis and fluorescent emission, and their
TPA cross sections were determined using nonlinear optical
transmission14 and two-photon induced fluorescent emission
measurements.
The synthesis of dendritic structures 9, 10, and 16 was
accomplished using convergent strategy in which the den-
dron/branch molecule 8 was first synthesized and then
coupled with different central units. Central units include
tribromobenzene, tris(4-bromophenyl)amine, or tribromo
species 15 (Scheme 1). Heck coupling reaction and Horner-
Wittig reaction were extensively utilized to construct the trans
double bond in each phenylenevinylene conjugation unit.
Palladium-catalyzed amination was employed for carbon-
nitrogen bond construction. The branch molecule 8 was
prepared from diphenylamine 1, which was protected by
benzyol chloride.15 The resulting species 2 was brominated
In this letter, we report the design, novel synthesis,
characterization, and nonlinear optical properties of three
representative new dendritic molecular structures having
either a phenyl ring or triphenylamine as the π-center with
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Org. Lett., Vol. 7, No. 15, 2005

Scheme 1: Synthesis of Dendritic Molecules 9, 10, and 16
Figure 2. (a) One-Photon absorption and fluorescent emission of
9, 10, and 16 in THF. (b) One-photon (solid line) and two-photon
(660 nm, filled circles; 800 nm, open circles) emission of 9.
in the presence of bromine in chloroform at 40 °C 16 to afford
3 in 95% yield for further Heck coupling reactions. The
4-dibutylaminostyrene 6 was prepared in two steps from
N,N′-dibutylaminobenzene 4 via the Vilsmeier reaction to
yield the aldehyde 5, followed by Wittig reaction with
methyltriphenylphosphine iodide. Heck coupling reaction of
3 and 6 produced 7 in 85% yield. Compound 7 was
hydrolyzed in Claisen’s base17 in THF-H2O to yield the
secondary amine as the branch molecule 8. Finally, the
dendritic structures 9 and 10 were synthesized in high yields
(95% and 91%) through palladium-catalyzed amination18 of
the branch molecule 8 with 1,3,5-tribromobenzene and tris-
(4-bromophenyl)amine, respectively.
reaction with 14 to furnish the core species 15 in 39% yield.
Reaction of 15 with the branch molecule 8 under palladium-
catalyzed conditions afforded the species 16 in 57% yield.
All structures were carefully characterized by 1H NMR and
13C NMR spectroscopy and mass spectrometry.20
One-photon absorption and one-photon fluorescent emis-
sion spectra of molecule 9, 10, and 16 are shown in Figure
2a. The concentrations of 9, 10, and 16 used for optical
measurement were 5 × 10-6 M in THF. The UV-vis peak
absorption maximum occurred at 392, 400, and 420 nm,
respectively, for molecule 9, 10, and 16. This systematic shift
in peak absorbance wavelength can be explained using one-
or two-dimensional quantum well model where longer con-
jugation length usually leads to smaller energy gap between
HOMO and LUMO. This systematic shift supports the notion
that triphenylamine moiety serves as π-electron bridge for
extended electron conjugation within the molecules.11c,13d
The emission peak maximum of molecule 9 appears at
440 nm, while that of molecules 10 and 16 appears at 476
nm. In Figure 2a, the relative emission peak heights of the
three molecules reflect their fluorescent emission quantum
yields (Q), as calibrated using dye molecules with well-
established fluorescent emission quantum yields such as
Rhodamine-6G. The wavelength of one-photon absorption
To construct molecule 16 (Scheme 1b), a more complex
central unit (15) was employed using the same strategy. The
tris(4-formylphenyl)amine 12 was prepared in 35% yield
from triphenylamine 11 under Vilsmeier conditions.19 The
resulting compound 12 further underwent the Horner-Wittig
(15) Horner, L.; Singer, R. J. Tetrahedron Lett. 1969, 10, 1545-1547.
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Org. Chem. 2003, 68, 2861-2873. (c) Meier, H.; Hanold, N.; Kalbitz, H.
Synthesis 1997, 3, 276-8.
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1276.
Org. Lett., Vol. 7, No. 15, 2005
3201

corresponding one-photon emission spectrum (solid curve)
is also shown in Figure 2b for comparison. The two-photon
and one-photon emission spectra resemble each other in both
shape and wavelength, indicating a similar emission relax-
ation path for the two very different excitation mechanisms.22
Table 1. Linear and Nonlinear Optical Properties of Dendritic
Molecules 9, 10, and 16
λabs
λem quantum
σ × 10-44
σ × 10-44
compd (nm) (nm) yield (Q) (s cm4) 660 nm (s cm4) 800 nm
The values of σ at 800 nm are 7.56 × 10-44, 12.2 × 10-44
,
and 9.61 × 10-44 s cm4 for molecule 9, 10, and 16, respec-
tively. These are among the best values ever reported.6a,8a,9a
It is interesting to note that the TPA cross sections have
increased 14-, 38-, and 25-fold for molecules 9, 10, and 16,
respectively, when the excitation wavelength is changed from
660 to 800 nm. The dramatic change in TPA cross sections
cannot be explained solely on the basis of resonance effect,
since the corresponding one-photon absorbance has increased
only by a factor of 3-4 when the wavelength is changed
from 330 to 400 nm. This seems to suggest that the two-
photon transition may have a narrow intrinsic band profile
that does not follow the absorption contour of its one-photon
counterpart. Also, it is unclear at present why molecule 10
has a larger TPA cross-section than 9 and 16. This structure-
function relationship is the subject of our current investiga-
tion.
9
10
16
392 440
400 476
420 476
0.48
0.15
0.28
0.54
0.32
0.38
7.56
12.2
9.61
peak maximum (λabs), one-photon emission peak maximum
(λem), and one-photon emission quantum yield (Q) of the
three molecules are summarized in Table 1.
Also shown in Table 1 are TPA cross sections (σ, (15%
uncertainty) of the three molecules at 660 and 800 nm using
a Quanta-Ray MOPO-730 laser. At 660 nm, the TPA cross
sections are 0.54 × 10-44, 0.32 × 10-44, and 0.38 × 10-44
s cm4 for molecules 9, 10, and 16, respectively, as determined
using transmission two-photon absorption measurements.14
At 800 nm, the TPA cross sections were obtained using two-
photon induced fluorescent emission.21 A typical spectra of
two-photon induced fluorescent emission of molecule 9 is
shown in Figure 2b. The filled circles depict the two-photon
emission spectrum obtained with 660 nm excitation, and open
circles depict that obtained with 800 nm excitation. The
In summary, a new series of dendritic two-photon absorb-
ing molecules have been carefully designed, synthesized, and
characterized. The preliminary studies show that these new
molecules have very large TPA cross sections up to 12.2 ×
10-44 s cm4 (molecule 10). Given the feasible synthetic routes
and superior two-photon absorption properties shown in this
communication, it is conceivable that molecules with even
larger TPA cross sections could be developed based on the
triphenylamine molecular platform. This could further pave
the way for the development of new nonlinear optical devices
based on two-photon absorption.
(20) General Procedure for Palladium-Catalyzed Amination for
Compounds 9, 10, and 16. To a Schlenk tube equipped with a Teflon
valve were added 8 (258.6 mg, 0.412 mmol), tris(4-bromophenyl)amine
(60.3 mg, 0.125 mmol), Pd(OAc)2 (1.7 mg, 0.007 mmol), tri-tert-butyl
phosphine (3 mg, 0.014 mmol), and anhydrous toluene (4 mL). The reaction
mixture was degassed and refilled with nitrogen three times. The tube was
sealed and heated at 95-110 °C for 2 days. The reaction mixture was poured
into water and extracted with ethyl acetate. The crude product was purified
by column chromatography (silica gel, hexane/methylene chloride ) 1:6)
1
to give the pure product as a greenish powder (252.1 mg, yield 95%). H
NMR (CD2Cl2, 500 MHz) δ 7.40-7.35 (m, 24 H), 7.12 (m, 12 H), 6.98
(d, 6 H), 6.88 (d, 6 H), 6.66 (s, 12 H), 6.59 (s, 3 H), 3.35 (t, 24 H), 1.65
(m, 24 H), 1.43 (m, 24 H), 1.02 (t, 36 H); 13C NMR (CD2Cl2, 125 MHz)
δ 149.3, 148.2, 146.0, 133.4, 127.9, 127.3, 125.1, 124.3, 123.3, 112.1, 51.1,
29.9, 20.8, 14.2; m/z 1955.6 (M+, 100%). Anal. Calcd for C138H171N9: C,
84.74; H, 8.81; N, 6.45. Found: C, 84.50; H, 8.86; N, 6.12. Compound 10:
1H NMR (CD2Cl2, 500 MHz) δ 7.43-7.41 (m, 24 H), 7.12-6.93 (m, 36
H), 6.70-6.68 (m, 12 H), 3.36 (t, 24 H), 1.65 (m, 24 H), 1.44 (m, 24 H),
1.05 (t, 36 H); 13C NMR (CDCl3, 125 MHz) δ 148.0, 133.1, 129.6, 127.9,
127.2, 125.8, 124.2, 123.7, 112.2, 51.3, 32.3, 20.8, 14.5; m/z 2122.6 (M+,
100%). Anal. Calcd for C150H180N10: C, 84.86; H, 8.55; N, 6.60. Found:
C, 84.48; H, 8.62; N, 6.51. Compound 16: 1H NMR (CD2Cl2, 500 MHz)
δ 7.47-7.41 (m, 36 H), 7.13-6.89 (m, 42 H), 6.68 (d, 12 H), 3.35 (t, 24
H), 1.62 (m, 24 H), 1.43 (m, 24 H), 1.02 (t, 36 H); 13C NMR (CD2Cl2, 125
MHz) δ 148.3, 146.2, 133.7, 132.9, 132.3, 128.0, 127.6, 127.1, 124.9, 124.1,
12.32, 112.1, 54.1, 29.9, 20.7, 14.2; m/z 2428.9 (M+, 100%). Anal. Calcd
for C174H198N10: C, 86.02; H, 8.21; N, 5.77. Found: C, 85.86; H, 8.30; N,
5.43.
Supporting Information Available: Synthetic procedures
and NMR characterization of all compounds in Scheme 1
(1H and 13C NMR data and spectra). This material is available
free of charge via the Internet at http://pubs.acs.org.
OL050905N
(21) The TPA cross section at 800 nm was evaluated using the following
equation: σ800 ) σ660 × (R800/R660) × (λ660/λ800), where σ660 is the TPA
cross section at 660 nm obtained using transmission measurements, Ri is
the ratio of two-photon emission peak height to the square of the excitation
light intensity at the corresponding wavelength, and the wavelength ratio
(λ660/λ800) is for correction of photon energy at different wavelengths. The
special excitation beam profiles at the two wavelengths were the same.
(22) Yan, Y. S.; Tao, X. T.; Sun, Y. H.; Xu, G. B.; Wang, C. K.; Yu, X.
Q.; Jiang, M. H. Mater. Sci. Eng. B 2004, 113, 170-174.
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