Synthesis, characterization, two-photon absorption, and optical limiting properties of triphenylamine-based dendrimers

Article abstract of DOI:10.1039/b9nj00393b

Three π-conjugated dendrimers (Ph-G0, Ph-G1 and Ph-G2) bearing triphenylamine moieties have been synthesized through a convergent synthetic strategy without any protection-deprotection chemistry. The linear photophysical properties, two-photon absorption (TPA), and optical limiting behavior of the dendrimers were investigated in solution at room temperature. Linear absorption and emission spectra revealed a bathochromic shift and decreased fluorescence quantum yields with increasing dendrimer generation. A strong cooperative effect in the TPA absorption of these dendrimers was observed. The TPA cross-sections increase gradually with the proportion of triphenylamine units and the maximum value of the TPA cross-section can reach 5690 GM for Ph-G2. These triphenylamine-based dendrimers exhibited efficient two-photon optical limiting under femtosecond excitation.

Full text of DOI:10.1039/b9nj00393b

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis, characterization, two-photon absorption, and optical limiting
properties of triphenylamine-based dendrimersw
a
b
a
a
a
Bin Xu, Honghua Fang, Feipeng Chen, Hongguang Lu, Jiating He,
a
b
b
a
Yaowen Li, Qidai Chen, Hongbo Sun* and Wenjing Tian*
Received (in Victoria, Australia) 7th August 2009, Accepted 8th September 2009
First published as an Advance Article on the web 6th October 2009
DOI: 10.1039/b9nj00393b
Three p-conjugated dendrimers (Ph-G0, Ph-G1 and Ph-G2) bearing triphenylamine moieties have
been synthesized through a convergent synthetic strategy without any protection–deprotection
chemistry. The linear photophysical properties, two-photon absorption (TPA), and optical
limiting behavior of the dendrimers were investigated in solution at room temperature. Linear
absorption and emission spectra revealed a bathochromic shift and decreased fluorescence
quantum yields with increasing dendrimer generation. A strong cooperative effect in the TPA
absorption of these dendrimers was observed. The TPA cross-sections increase gradually with the
proportion of triphenylamine units and the maximum value of the TPA cross-section can reach
5
690 GM for Ph-G2. These triphenylamine-based dendrimers exhibited efficient two-photon
optical limiting under femtosecond excitation.
Introduction
synthesized and their TPA cross-sections have shown cooperative
2
enhancement with increasing dendrimer generation.
0–22,36
p-Conjugated organic molecules with large two-photon
absorption (TPA) cross sections have attracted increasing
attention due to their versatile applications in optical power
limiting, 3D microfabrication, three-dimensional optical data
storage, multi-photon pumped lasing, two-photon fluorescent
microscopy, two-photon cellular imaging and photodynamic
Among them, triphenylamine has been widely used as an
electron donating moiety, and has shown promise as a frame-
work for the design of TPA chromophores. Since the central
nitrogen atom and the three adjacent carbon atoms are highly
coplanar, the triphenylamine unit can maintain uninterrupted
conjugation between the central nitrogen lone pair electrons
38–44
1
–16
therapy.
The realization of these technological applications
and the dendrimer arms.
Dendritic molecules or hyper-
relies greatly on the development of novel organic molecules
with large TPA cross-sections. It is known that a large TPA
response can be attributed to an extended p-conjugated system
branched polymers containing such moieties are thus
45–48
anticipated to show efficient TPA response.
Meanwhile,
the triphenylamine derivatives connected by CQC double
bonds can be synthesized by Heck and Wittig–Horner reactions,
1
7–19
and increased charge transfer character within a molecule.
Recently, dendrimers have been introduced into the field of
organic nonlinear optics and have shown great promise, due
and integrated into p-phenylene vinylene structures which also
49–50
show interesting nonlinear optical properties.
On the
2
0–27
to their well-defined and controllable architectures.
A
other hand, most p-conjugated dendrimers containing large
numbers of strongly coupled conjugated blocks, which are
two-branched systems (dimers) or three-branched systems
fundamental advantage of dendrimers over conventional
linear polymers in optical applications is mostly associated
with the concentration of a large number of chromophores in
an ordered and confined geometry which grows exponentially
(
trimers), demonstrate the enhanced TPA response beyond
34–37
simple additive behavior.
When the branch of the
2
8–31
with the generation number.
In addition, strong intra-
dendrimer are larger than trimer, the enhancement effect of
TPA has been reported in stilbenyl dendrimer which can be
interpreted in terms of coherent domains extending beyond the
chromophore interactions, efficient energy transfer, and migration
through the structural design of dendritic architecture and
formation of delocalized excited states render them to have
enhanced nonlinear optical properties in comparison to their
22
trimer configuration. Therefore, with a useful design strategy
of dendrimers with larger branches architecture than trimer, a
more efficient TPA response may be expected.
3
2–35
linear counterparts.
With these advantages in mind, several p-conjugated phenyl
cored, nitrogen centered dendritic chromophores have been
In this paper, we report the synthesis, characterization
and nonlinear optical properties of novel, strongly coupled,
conjugated four-branched dendrimers (Chart 1) based on
triphenylamine branches connected by alkene linkers.
Fixation of a rigid and planar core, bis(styryl)benzene, into
these p-conjugated dendrimers not only enhances the
p-conjugation, but also greatly facilitates p-delocalization,
a
State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, 2699 Qianjin Avenue, Changchun 130012,
P. R. China. E-mail: wjtian@jlu.edu.cn; Fax: +86 431 85193421;
Tel: +86 431 85155359
State Key Laboratory on Integrated Optoelectronics,
Jilin University, 2699 Qianjin Avenue, Changchun 130012,
P. R. China. E-mail: hbsun@jlu.edu.cn
b
since such
relaxations from the excited states.
the triphenylamine-based branches involved an iterative,
a structure can severely restrict geometric
4
,51
The synthetic of
w Electronic supplementary information (ESI) available: Additional
images (Fig. S1–S3). See DOI: 10.1039/b9nj00393b
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Chart 1 Molecular structures of the dendrimers containing triphenylamine branches (Ph-G0, Ph-G1 and Ph-G2).
convergent procedure without protection–deprotection chemistry.
The linear and two-photon absorption properties, as well as
the one- and two-photon excited fluorescence properties of
these dendrimers are reported herein. By systematically tuning
the generation of the dendrimers, an attempt to correlate and
understand the property-generation relationship is made. The
effect of TPA-based optical limiting for dendrimers was
demonstrated under femtosecond excitation.
7.63 (4H, d, J = 8.5 Hz, Ar), 7.05 (2H, d, J = 8.5 Hz, Ar),
6.89 (4H, d, J = 8.5 Hz, Ar).
Preparation of N,N-diphenyl-4-vinylaniline (3). 4-(N,N-
Diphenylamino)benzaldehyde 1 (5.46 g, 20 mmol) and methyl-
triphenylphosphonium bromide (8.57 g, 24 mmol) were
dissolved in 100 mL of dry THF. t-BuOK (3.36 g, 30 mmol)
in 30 mL of dry THF was added slowly dropwise to the
resulting solution at 0 1C, then the reaction mixture was
warmed to room temperature and stirred under N for 12 h.
2
Experimental
The reaction mixture was poured into water and extracted
General procedures
with CH Cl (3 ꢁ 60 mL). The combined organic extracts were
2
2
4
washed with brine, dried with MgSO , and concentrated to
All reagents were purchased from Aldrich Chemical Co. and
used as received without further purification except for the
following. Tetrahydrofuran (THF) was refluxed with sodium
and benzophenone, and distilled. N,N-Dimethylacetamide
dryness under vacuum. The crude product was purified by
flash column chromatography (petroleum ether–CH Cl = 4 : 1)
to give (4.12 g, 76%) of product 3 as a white solid. H NMR
(500 MHz CDCl ): d = 7.28 (2H, d, J = 8.5 Hz, Ar),
.23–7.26 (4H, m, Ar), 7.09 (4H, d, J = 8.0 Hz, Ar),
.98–7.03 (4H, m, Ar), 6.63–6.69 (1H, m, CH), 6.63 (1H, d,
2
2
1
1
13
3
(
2
DMAc) was dried with CaH . The H NMR and C NMR
7
6
spectra were recorded on AVANCZ 500 spectrometers and
Varian Mercury-300 NMR at 298K by utilizing deuterated
J = 16.5 Hz, CHQCH ), 5.15 (1H, d, J = 8.5 Hz, CH = CH ).
2
2
CDCl
Matrix assisted laser desorption ionization time-of-flight
MALDI-TOF) mass spectrometry experiments were performed
3
as solvent and tetramethylsilane (TMS) as standard.
Preparation of 4-(bis(4-(4-(diphenylamino)styryl)phenyl)-
(
amino)benzaldehyde (4). A round-bottomed flask (250 mL)
on a Kratos MALDI-TOF mass system, and the spectrum was
recorded in the linear or reflect mode with anthracene-1,8,9-
triol as the matrices.
was oven dried and cooled under an N
2
atmosphere.
Compound 2 (5.25 g, 10 mmol), compound 3 (7.0 g, 26 mmol),
K PO (6.5 g, 30 mmol) and Pd(OAc) (10 mg) were dissolved
3
4
2
Preparation of 4-(bis(4-iodophenyl)amino)benzaldehyde (2).
in dry DMAc (50 mL). The reaction mixture was heated to
110 1C in an oil bath and stirred for 24 h at this temperature.
After being cooled to room temperature, the reaction mixture
4
-(N,N-Diphenylamino)benzaldehyde 1 (14.00 g, 51.28 mmol),
potassium iodide (11.43 g, 68.85 mmol), and acetic acid
210 mL) were heated to 85 1C, and the solution was allowed
to cool. Then potassium iodate (10.97 g, 51.26 mmol) was
added and the reaction was heated at 85 1C for 5 h. The
solution was allowed to cool to room temperature and poured
into ice-water under stirring. The yellow precipitated solid was
collected by filtration, and the collected solid was poured into
(
was poured into the water and extracted with CH
(3 ꢁ 50 mL). The combined organic extracts were washed
with brine, dried with MgSO , and concentrated to dryness
under vacuum. The crude product was purified by flash
column chromatography (petroleum ether–CH Cl = 1 : 1)
2 2
Cl
4
2
2
1
to give (4.47 g, 55%) of product 4 as a yellow solid. H NMR
5
% NaHSO (100 mL) to clear I and KIO . The final filtrated
3
(500 MHz CDCl ): d = 9.83 (1H, s, CHO), 7.71 (2H, d,
J = 8.5 Hz, Ar), 7.45 (4H, d, J = 8.5 Hz, Ar), 7.38 (4H, d, J =
2
3
3
1
yellow solid was pure compound 2. H NMR (500 MHz
CDCl ): d = 9.85 (1H, s, CHO), 7.71 (2H, d, J = 8.5 Hz, Ar),
8.5 Hz, Ar), 7.25–7.28 (12H, m, Ar), 7.10–7.15 (12H, m, Ar),
3
2
458 | New J. Chem., 2009, 33, 2457–2464
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7
.00–7.06 (8H, m, Ar), 6.96 (2H, d, J = 16.5 Hz, Ar),
MALDI-TOF MS: Calcd for C59 O 812.0; found:
12.9. Anal. calcd for C H N O: C, 87.27; H, 5.59; N,
Preparation of Ph-G0. The resulting dendrimer Ph-G0 was
H
45
N
3
obtained as a yellow–green powder in a yield of 86%.
1
8
5
H NMR (500 MHz CDCl ) d 7.47 (4H, s, Ar), 7.39 (4H, d,
5
9
45
3
3
.17; found: C, 87.14; H, 5.72; N, 5.06%.
J = 8.5 Hz, Ar), 7.26 (8H, t, J = 8.0 Hz, Ar), 7.11 (8H, d,
J = 7.5 Hz, Ar), 7.02–7.08 (10H, m, Ar), 6.99 (2H, d,
J = 16.5 Hz, CH = CH). MALDI-TOF MS: Calcd for
Preparation of 4-(4-(diphenylamino)styryl)-N-(4-(4-(diphenyl-
amino)styryl)phenyl)-N-(4-vinylphenyl)aniline (5). Compound
(2.02 g, 2.5 mmol) and methyltriphenylphosphonium
46 36 2 36 2
C H N : 616.3; found: 616.3. Anal. calcd for C46H N : C,
4
89.58; H, 5.88; N, 4.54%; found: C, 89.46; H, 5.94; N, 4.48%.
bromide (1.1 g, 3 mmol) were dissolved in 50 ml of dry
THF. t-BuOK (0.45 mg, 4 mmol) in 10 ml of dry THF was
slowly added dropwise to the resulting solution at 0 1C, then
the reaction mixture was warmed to room temperature and
Preparation of Ph-G1. The resulting dendrimer Ph-G1 was
1
obtained as a yellow–green powder in a yield of 53%. H NMR
(
500 MHz CDCl ) d 7.49 (4H, s, Ar), 7.37–7.43 (20H, m, Ar),
3
7
6
.25–7.28 (12H, m, Ar), 7.11 (28H, d, J = 7.5 Hz, Ar),
stirred under N for 12 h. The reaction mixture was poured
2
.97–7.07 (32H, m, Ar). MALDI-TOF MS: Calcd for
into water and extracted with CH Cl2 (3 ꢁ 50 mL). The
2
C
C
126
126
H
H
96
N
N
6
:
1692.7; found: 1693.3. Anal. calcd for
combined organic extracts were washed with brine, dried with
96
6
: C, 89.33; H, 5.71; N, 4.96%; found: C, 89.21; H,
MgSO , and concentrated to dryness under vacuum. The
4
5
.85; N, 4.87%.
crude product was purified by flash column chromatography
(
petroleum ether–CH
product 5 as a yellow solid. H NMR (500 MHz CDCl
d = 7.31–7.39 (12H, m, Ar), 7.10–7.27 (12H, m, Ar), 7.01–7.09
20H, m, Ar), 6.65–6.71 (1H, m, CH), 6.70 (1H, d, J =
7.5 Hz, CH = CH ), 5.18 (1H, d, J = 11.0 Hz, CHQCH ),
MALDI-TOF MS: Calcd for C60 809.4; found: 811.4.
Anal. calcd for C60 : C, 88.96; H, 5.85; N, 5.19%; found:
2 2
Cl = 4 : 1) to give (1.36 g, 67%) of
Preparation of Ph-G2. The resulting dendrimer Ph-G2 was
1
obtained as a yellow–green powder in a yield of 56%. H NMR
1
3
):
(
500 MHz CDCl ): d = 7.48 (4H, s, Ar), 7.36–7.42 (48H, m,
13
3
(
Ar), 7.24–7.27 (36H, m, Ar), 6.96–7.12 (128H, m, Ar).
C
1
2
2
NMR (75 MHz CDCl ): 122.95, 123.66, 124.24, 124.41,
3
47 3
H N
1
1
26.43, 126.66, 126.86, 127.16, 129.45, 131.77, 132.25,
32.44, 146.21, 146.40, 147.10, 147.54. MALDI-TOF MS:
47 3
H N
C, 88.85; H, 6.01; N, 5.07%.
Calcd for C286
216
H N14: 3845.7; found: 3846.6. Anal. calcd
for C286 14: C, 89.25; H, 5.66; N, 5.09%; found: C, 89.19;
H
216
N
Preparation of 4-(bis(4-(4-(bis(4-(4-(diphenylamino)styryl)-
phenyl)amino)styryl)phenyl)amino)benzaldehyde (6). A round-
bottomed flask (50 mL) was oven dried and cooled under an
H, 5.72; N, 5.03%.
Linear absorption and emission measurements
N
2
atmosphere. Compound 5 (1.6 g, 2 mmol), compound 2
0.48 g , 0.9 mmol), K PO (0.58 g, 2.7 mmol), and Pd(OAc)
5 mg) were dissolved in dry DMAc (20 mL). The reaction
Linear absorption spectra for dendrimers in dilute solution
were recorded on a UV-3100 spectrophotometer. One-photon
excited fluorescence was measured in dilute solutions using an
RF-5301PC and 10 mm path-length cuvettes. Fluorescence
quantum efficiency at room temperature was determined by a
standard method, with quinine sulfate as a reference.
(
(
3
4
2
mixture was heated to 110 1C in an oil bath and stirred for 24 h
at this temperature. After being cooled to room temperature,
the reaction mixture was poured into the water and extracted
with CH Cl . The combined organic extracts were washed
2
2
with brine, dried with MgSO , and concentrated to dryness
4
under vacuum. The crude product was purified by flash
Results and discussion
column chromatography (petroleum ether–CH
2
Cl
to give compound 6 as a yellow solid. H NMR (500 MHz
CDCl ): d = 9.83 (1H, s, CHO), 7.71 (2H, d, J = 8.5 Hz, Ar),
.36–7.41 (20H, m, Ar), 7.24–7.27 (12H, m, Ar), 7.14 (2H, d,
J = 8.5 Hz, Ar), 7.08–7.12 (30H, m, Ar), 6.96–7.05 (32H, m,
Ar). MALDI-TOF MS: Calcd for C139 O 1887.8;
found: 1889.0. Anal. calcd for C139 O: C, 88.36; H,
.60; N, 5.19%; found: C, 88.24; H, 5.77; N, 5.15%.
2
= 1 : 1)
1
Synthesis and characterization of dendrimers
3
The iterative strategy developed by our group used for the
4
aldehyde-focused dendron synthesis is shown in Scheme 1.
9,50
7
As illustrated, the two iterative steps for dendron formation
5
involve a simple Wittig reaction followed by a Heck
2
H
105
N
7
5
coupling to give a dendron with an aldehyde at its center.
3
H
105
N
7
5
The first step in the iterative procedure was the formation of
5
N,N-diphenyl-4-vinylbenzenamine 3 which was prepared in
4
General reaction procedure for entries Ph-G0, Ph-G1 and
76% yield from the reaction of 4-(diphenylamino)benzaldehyde
5
5
Ph-G2. In an oven-dried flask with a stirrer bar, corresponding
dendrons with formyl and biphosphonate 7 were dissolved in
1
with methyltriphenylphosphonium bromide. The second
step in the first cycle of the iterative procedure was the
2
0 mL dry THF. The resulting solution was slowly added
coupling of compound 3 with 4-(bis(4-iodophenyl)amino)-
5
6
dropwise to t-BuOK (84 mg, 0.75 mmol) in 20 mL of dry THF
benzaldehyde 2 to give the first generation aldehyde-focused
dendron 4 in 55% yield. To form the next generation dendron
the same two-step procedure was used. Aldehyde 4 was reacted
with the methyltriphenylphosphonium bromide to give
compound 5 in 67% yield. Compound 3 was then coupled
with 4-(bis(4-iodophenyl)amino)benzaldehyde under the
above Heck conditions to form the second generation
dendron aldehyde 6 in a 51% yield. The targeted dendrimers
at 0 1C, then the reaction mixture was warmed to room
temperature and stirred under N overnight. The mixture
2
was poured into water and extracted with dichloromethane.
The organic phase was washed with water, brine and dried
over MgSO . After removing the solvent, the product was
4
purified by column chromatography using dichloromethane–
petroleum ether (1 : 2), giving Ph-G0, Ph-G1 and Ph-G2.
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5
7
Ph-G0, Ph-G1 and Ph-G2 were synthesized via the typical
Wittig–Horner reaction between the core biphosphonate 7 and
aldehydes 1, 4 and 6 in dry tetrahydrofuran in 86%, 53%,
Linear absorption and emission
The normalized linear absorption and fluorescence spectra for
dendrimers Ph-G0, Ph-G1 and Ph-G2 in toluene are shown in
Fig. 1. Linear measurements were performed in dilute toluene
5
6% yields, respectively, using potassium tert-butoxide as base
(
Scheme 2).
ꢂ6
solutions (B 10 M). The position of the main peak in both
As we anticipated, the targeted dendrimers are soluble in
absorption and emission shows a bathochromic shift with
increasing dendrimer generation number. All dendrimers show
two major absorption bands: the peak at 305 nm is attributed
to the absorption of the triphenylamine moiety and remains
almost unchanged among the three dendrimers, while the
other at long wavelength shows a shift from 411 nm (Ph-G0)
to 422 nm (Ph-G2) which can be assigned to a p–p* transition.
A small bathochromic shift in the linear absorption can be
observed from Ph-G0 to Ph-G1, and further to Ph-G2, which
is due to the incremental extension of the conjugation length
with increasing dendrimer generation. The crystallographic
data on triphenylamine as well as quantum chemical calculations
indicate that the three N–C bonds of the triphenylamine group
lie in one common plane and further confirm continuity of the
p-conjugation all the way through the nitrogen atom within
most common organic solvents such as toluene, chloroform,
THF, etc. All dendrons and dendrimers were characterized by
1
13
FT-IR, elemental analysis, H NMR spectroscopy, C NMR
spectroscopy and MALDI-TOF mass spectrometry. Dendrons 4
and 6 both show a peak at 9.83 ppm in the H NMR spectrum,
which confirms the presence of the formyl hydrogen. The
disappearance of the aldehyde peak and increased integration
1
values of aromatic protons in the H NMR spectra of the
Ph-G0, Ph-G1 and Ph-G2 dendrimers prove the successful
coupling reaction between the respective aldehyde focused
dendrons (4 or 6) and the core reagents (7). The coupling
constant (J B 16.5 Hz) of the olefinic protons in the dendrimers
indicates that the Heck and Wittig–Horner reaction sequence
afforded pure all-trans isomers. The all-trans isomers of
3
8,42
dendrimers Ph-G0, Ph-G1 and Ph-G2 were further confirmed
ꢂ1
triphenylamine and related molecules.
Therefore, the
by the presence of characteristic vibration bands at 958 cm
ꢂ1 ꢂ1
,
58
systematic shift supports the notion that the triphenylamine
moiety serves as a p-electron bridge for extended electron
conjugation within the dendrimers. However, after the initial
shift of the linear absorption spectra from Ph-G0 to Ph-G1,
the absorption maxima remain almost the same, up to the
highest dendrimer generation, which indicates that there is
9
60 cm and 960 cm in the FT-IR spectra, respectively.
Additional definitive evidence for the dendrimer molecular
structures was obtained from MALDI-TOF mass spectra,
which revealed only a single intense signal corresponding to
the calculated mass of dendrimers Ph-G1 and Ph-G2. (see ESIw)
Scheme 1 Synthetic route to compounds 2–7.
2
460 | New J. Chem., 2009, 33, 2457–2464
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Scheme 2 Synthesis of the dendrimers Ph-G0, Ph-G1 and Ph-G2.
little change in the conjugation effect in extending the
molecular framework beyond the second generation.
fluorescence quantum yields (Z) of 0.78, 0.56 and 0.54 in
toluene solutions, respectively, measured against a quinine
sulfate standard. There is a gradual decrease of fluorescence
quantum yields with an increasing dendrimer generation.
From the point of view of molecular structure, due to the
large dihedral angle between the phenyl ring plane and the
plane of the N-bonded C atoms, higher generation triphenyl-
amine based dendrimers have a greater tendency to adopt
twisted configurations than those of lower generation, which
might reduce the fluorescence quantum yield due to a higher
The emission spectra of Ph-G0, Ph-G1 and Ph-G2 in dilute
toluene solutions all displayed similar behaviors. They
exhibited emission maxima at 460 nm, 480 nm and 478 nm,
respectively. The red shift from Ph-G0 to Ph-G1 and Ph-G2 in
the fluorescence spectra is more pronounced than that in the
absorption spectra demonstrated that the effective conjugation
length significantly improved with increasing generation of the
dendrimers. This red shift of fluorescence spectrum with
increasing generation number may also suggest that the
energy of relaxed electronic states decreases with the size of
dendrimer. One explanation for this could be that larger
molecules can have a wider distribution of states due to
increased branching, allowing greater relaxation of the excited
6
0
tendency to nonradiatively decay by geometrical relaxation.
Two-photon absorption
The TPA spectra were determined by using pulsed laser light
82 MHz, 120 fs) from 700 nm to 850 nm generated by a mode
(
5
9
state prior to emission. On the other hand, the emission
spectra of dendrimers of a higher generation do not exhibit
any further detectable shift, consistent with the absorption
spectra. It seems that the saturation value for the energy
of the lowest excited state is reached for the number of
branch segments in the dendrimer Ph-G1. The photophysical
characteristics of dendrimers Ph-G0, Ph-G1 and Ph-G2 are
summarized in Table 1.
locked Ti:sapphire laser (Tsunami, Spectra Physics). A spectrum
analyzer (AvaSpec-2048) was used to monitor the excitation
wavelengths. All data were obtained by the two-photon-excited
fluorescence method with fluorescein in pH 11 water as in
ref. 12. It is noted that all the results presented here correspond
to intrinsic TPA cross-section values measured with the
fluorescence method in the femtosecond regime, and thereby
prevent contributions from linear nonresonant absorption or
from excited-state absorption known to lead to artificially
enhanced TPA cross sections when conducted in the nano-
second regime. The measured TPA spectra for the dendrimers
are shown in Fig. 2. The TPA cross-section peak values (d) for
the dendrimers Ph-G0, Ph-G1 and Ph-G2 are 970, 3160, and
In general, all of the dendrimers show large Stokes shifts
ꢂ1
ꢂ1
ranging from 2592 cm for Ph-G0 to 2863 cm for Ph-G1,
indicating that the energy of the emitting states are lower than
the Franck–Condon singlet states. The Stokes shifts of the
dendrimers increase with the conjugation length, but that of
Ph-G2 is slightly smaller than that of Ph-G1. A further
increase in the branching of Ph-G2 has little influence on the
Stokes shift, indicating that there is little difference in the
excited-state energies between Ph-G1 and Ph-G2. Also, all of
the dendrimers Ph-G0, Ph-G1 and Ph-G2 have relatively high
ꢂ50 ꢂ1
4
5
690 GM (1 GM = 1 ꢁ 10
cm s photon ), respectively.
The TPA cross-section peak value increases by a factor of 3.24
and 5.82 from Ph-G0 to Ph-G1 and to Ph-G2. The maximum
TPA
of TPA spectra (lmax
Fig. 2 is clearly red-shifted with respect to Ph-G0.This shift, as
) of dendrimers Ph-G1 and Ph-G2 in
well as a similar red-shift in the linear absorption, suggests
either dipole–dipole-type or conjugation-type interaction
TPA
between the dendrimer’s branches. Otherwise, the lmax
/2
value of the dendrimers is located at a shorter wavelength than
the lowest energy of linear absorption maximum (lmax), which
is consistent with the prediction that the two-photon allowed
states are at a higher energy than the Franck–Condon states.
There is a strong enhancement over simple additive behavior
when moving from Ph-G0 to higher generations (Ph-G1 or
Ph-G2). A theoretical study for two-photon absorption of
multi-branch structure has revealed that such an enhancement
is mainly caused by vibronic coupling. The electronic coupling
in the multi-branch molecules, which contain triphenylamine
units, is weak, probably because the central amino group is
used as the connecting unit, which breaks the conjugation of
6
1
Fig. 1 Linear absorption and emission spectra for dendrimers Ph-G0,
Ph-G1 and Ph-G2 in dilute toluene solution.
the whole network. On the other hand, when the extended
p-conjugation and the size of the system increase at higher
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Table 1 Photophysical characteristics of dendrimers Ph-G0, Ph-G1 and Ph-G2
a
max
b
ꢂ1
c
Z
d
e
f
g
Dendrimer
l
abs/nm
e
lem/nm
D /cm
t /ns
d
max /GM
Zdmax
dmax/MW
Ph-G0
Ph-G1
Ph-G2
411
422
422
87 000
182 500
490 000
460
480
478
2592
2863
2776
0.78
0.56
0.54
0.79
0.94
0.62
970
3160
5690
760
1770
3080
1.58
1.87
1.50
a
ꢂ1
ꢂ1
b
c
Stokes shift. Fluorescence quantum yield in dilute toluene solution, determined by a standard method with quinine sulfate as a
In M cm
d
.
reference. Fluorescence lifetime obtained from time-correlated single photon counting (TCSPC). Peak TPA cross-section in 10
e
ꢂ50
4
cm s
ꢂ1
f
photon (GM). The two-photon excited fluorescence (TPEF) action cross section. Maximum TPA cross section per molecular weight.
g
generations, the density of states will increase, providing
more effective coupling channels between the ground and
two-photon allowed states, which would in turn increase the
Two-photon excited (TPE) fluorescence
The TPE fluorescence spectra of dendrimers Ph-G0, Ph-G1
and Ph-G2 in toluene are shown in Fig. 3. The pump source
for two-photon excitation as well as TPA cross-section
measurements was a focused ultra-short pulsed laser beam
from a Ti:sapphire laser oscillator/amplifier system. This beam
was focused by an f = 12 cm lens with the solution sample in a
fluorimeter cuvette with four optically clear windows placed at
a fixed distance of B12 cm from the focusing lens. Under these
conditions, the intensity for two-photon excitation was in an
excitation regime where the fluorescence signal showed a
quadratic dependence on the intensity of the excitation beam,
as expected for two-photon induced emission. For example, as
shown in Fig. 4, the log–log plots show slopes of 1.91, 1.93,
and 1.95 for dendrimers Ph-G0, Ph-G1 and Ph-G2. The
collection of the TPE fluorescence signal was performed by
using a CCD-array spectrometer (iDus CCD Spectroscopic
CCD Camera, Andor) in conjunction with a fiber coupler
head. Similarly to their linear emission in toluene, Ph-G1 and
Ph-G2 exhibit a red-shifted emission compared to that of
Ph-G0. From Fig. 1 and 3, one may find that the TPE
fluorescence spectra for the three generation dendrimers in
toluene are essentially the same as their linear fluorescence
spectra, confirming that both emissions are from the same
excited state.
2
6,62
TPA cross-section.
To understand how the TPA cross section correlates with
the chromophores density, the TPA cross-section per unit
triphenylamine is calculated and plotted as a function of
dendrimer generation. as shown in the inset of Fig. 2. It
can be seen from the calculated result that the d value of
dendrimers from Ph-G0 to Ph-G2 raise in a proportion of
1
: 3 : 6 by increasing the number of triphenylamine units. This
behavior is close to d p N scaling, namely 1 : 3 : 7 proportion,
where N is the number of triphenylamine units. This phenomenon
is reminiscent of what has been observed in the case of
cooperative enhancement of the TPA cross-section with an
increase in branching chromophores, as well as an increase of
cross-section in nitrogen centered dendrimers. Furthermore,
0
compared with the 4,4 -bis(diphenylamino)stilbene (BDPAS)
series dendrimers which are well-known triphenylamine-based
2
1–22
TPA dyes,
the dendrimers with a bis(styryl)benzene core
always exhibit significantly larger TPA cross-sections when the
numbers of triphenylamine units in a conjugated molecular
framework are similar. This enhancement of TPA cross-
section is likely due to the large initial superiority of the
bis(styryl)benzene core over a simple ethylene bridge, which
could be due to its longer conjugation length and/or the
The values of Zdmax, the two-photon excited fluorescence
TPEF) action cross section, increase in the order of
4
electron accepting ability of the central phenylene ring.
(
Thus, the use of a bis(styryl)benzene unit as the core of the
dendrimer may provide a higher TPA response with enhanced
emission.
Ph-G0 o Ph-G1 o Ph-G2. The smaller Zd
value for
max
Ph-G0 is attributed to the much smaller dmax value. For
applications that require strong TPA such as optical limiting,
3D micro-fabrication or strong TPEF for bioimaging, molecules
with large TPA cross section per molecular weight (dmax/MW)
1
,19
are needed.
To compare relative TPA properties per unit
mass, we calculate dmax/MW values of these dendrimers. The
results show that dmax/MW values of these dendrimers are
larger than 1.5, indicating that these dendrimers may be useful
1
9
for such practical applications.
TPA-based optical limiting
For some laser-based applications such as optical communication,
optical fabrication, and manufacturing, the intensity or energy
stability of the utilized laser beam is very important because
random intensity or power fluctuation may be troublesome for
these applications. Optical power limiting, the optical analog
to an electrical surge protector, is important for the protection
of optical sensors and eyes from sudden exposure to high
Fig. 2 TPA spectra for dendrimers Ph-G0, Ph-G1 and Ph-G2 in
toluene. The TPA cross-sections were determined by using the
two-photon excited fluorescence method relative to fluorescein.
1
8
incident intensities.
However, materials with a strong
and efficient nonlinear absorption response as a function of
2
462 | New J. Chem., 2009, 33, 2457–2464
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Fig. 3 Two-photon excited fluorescence (TPEF) spectra for dendrimers
Ph-G0, Ph-G1 and Ph-G2 in toluene.
Fig. 5 Output energy as a function of the input energy for Ph-G1 and
Ph-G2 in CH
2
Cl
2
under femtosecond excitation at 800 nm, in 10 mm
ꢂ3
quartz cuvettes with concentrations B10 M.
are comparable to the efficiency of multibranched triphenyl-
6
amine-based chromophores in the femtosecond regime.
3,64
Thus, if we use dendrimers as a two-photon absorbing
medium to stabilize the input laser, we would expect that
more than a two-fold reduction in laser fluctuation could be
achievable, which suggests dendrimers Ph-G1 and Ph-G2 may
be useful for the development of new multifunctional organic
materials for two-photon based optical limiting.
Conclusions
A novel series of triphenylamine-based dendrimers Ph-G0,
Ph-G1 and Ph-G2 have been synthesized and characterized.
Their linear and nonlinear optical properties were investigated.
Linear absorption and emission spectra revealed a bathochromic
shift with increasing dendrimer generation number, whereas
their fluorescence quantum yields decrease with increasing
generation. Investigation of the TPA properties of these
dendrimers reveals a strong cooperative effect and their TPA
cross section values increase with increasing the generation, as
well as the p-conjugation lengths. Moreover, the TPA cross-
sections values increase gradually with the proportion of
triphenylamine units and the maximum value of the TPA
cross-section can reach 5690 GM for Ph-G2. The effect of
two-photon optical limiting behaviors for these dendrimers
was demonstrated in CH Cl under femtosecond excitation.
Fig. 4 Dependence of the peak intensity of TPE fluorescence intensity
versus the input laser intensity at 800 nm for all three dendrimers in
toluene.
incident intensity could be one of the approaches to reducing
such fluctuation and stabilizing the pulsed laser signals, and
are thus of great interest for optical power limiting. Two
photon optical power limiting was investigated for Ph-G1
and Ph-G2 in CH
2
Cl
2
under femtosecond excitation.
Nonlinear optical transmission measurements were performed
using a 10 mm quartz cuvette with molecular concentrations
ꢂ3
B10 M, with a Ti:sapphire laser oscillator/amplifier system
generating output pulses of 120 fs duration and 800 nm
wavelength. The measured input and output intensity data
for Ph-G1 and Ph-G2 are plotted in Fig. 5. The plots display a
typical optical limiting curvature in which the output energy
levels off at higher input energies because of the nonlinear
optical properties of the dendrimers. When the input energy
increases from 0.51 to 14.95 mJ (an increase of about 29.3
times), the transmitted energy changed from 0.45 to 5.85 (an
increase of 13.0 times) for Ph-G1 and from 0.44 to 4.5 (an
increase of 10.2 times) for Ph-G2. These are typical optical
limiting behaviors based on a TPA mechanism. The results
suggest that this series of dendrimers would provide better
optical limiting performance with increasing generation due
to an enhanced two photon response. The optical limiting
properties of Ph-G1 and Ph-G2 under femtosecond excitation
2
2
The high TPA cross section and two-photon optical limiting
abilities of Ph-G1 and Ph-G2 suggest that triphenylamine-
based dendrimers are a highly suitable class of two-photon
absorbing materials in a number of nonlinear optical applications.
Acknowledgements
This work was supported by the National Basic Research
Program of China (973 program, 2009CB623605), the Natural
Science Foundation of China (Grant No. 50673035), the
Program for New Century Excellent Talents in Universities
of China Ministry of Education, the 111 Project (Grant No.
B06009) and the Graduated Innovation Fund of Jilin
University (No. 20080109).
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