Multibranched triarylamine end-capped triazines with aggregation-induced emission and large two-photon absorption cross-sections

Article abstract of DOI:10.1039/c0cc00803f

Multibranched triarylamine derivatives with a 1,3,5-triazine core have been synthesized and exhibit aggregation-induced emission and a large two-photon absorption cross section (8629 GM).

Full text of DOI:10.1039/c0cc00803f

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Multibranched triarylamine end-capped triazines with aggregation-
induced emission and large two-photon absorption cross-sectionsw
Yihua Jiang,a Yaochuan Wang,b Jianli Hua,*a Jin Tang,a Bo Li,c
Shixiong Qianb and He Tian*a
Received 7th April 2010, Accepted 21st May 2010
First published as an Advance Article on the web 1st June 2010
DOI: 10.1039/c0cc00803f
Multibranched triarylamine derivatives with a 1,3,5-triazine
core have been synthesized and exhibit aggregation-induced
emission and a large two-photon absorption cross section
(8629 GM).
phenomenon was found by Tang and coworkers5 and named
aggregation-induced emission (AIE), which is caused by
restricted intramolecular vibrational and rotational motions
in the aggregated solid. It offers a new path to the solution of
the thorny problem of aggregation-caused quenching. Materials
with AIE properties has been an active area of research for a
wide variety of promising applications, including 2PA, fluorescent
switching or sensing, electroluminescence devices and biological
probes.6 Recently, Prasad and coworkers have reported
aggregation-enhanced fluorescence and two photon absorption
in nanoaggregates due to the hindering of molecular internal
rotation.4,7
The promising applications of organic two-photon absorption
(2PA) materials in optical limiting, up-converted lasing,
microfabrication, 3-dimensional optical data storage, bioimaging
and photodynamic therapy1 have attracted considerable
attention in the last decade and stimulated substantial research
on structure–property relationships.2 Some efficient molecular
design strategies were put forward to provide guidelines for the
development of materials with large two-photon absorption
cross sections (s), including donor–acceptor–donor (D–A–D)-
type molecules, donor–p-bridge–acceptor (D–p–A)-type
molecules, donor–p-bridge–donor (D–p–D)-type molecules,
macrocycles, dendrimers, polymers and multibranched molecules.
These studies reveal that the s value of a compound will be
increased by elongating the molecular conjugated system for
charge transfer or incorporating multi-dipole or quadrupole
chromophores into one molecular structure, in which multi-
branched architectures can substantially enhance 2PA cross
section due to the increase of chromophore density.3
Triarylamine has been widely used in opto- and electro-
active materials for its good electron donating and transporting
capability, as well as its special propeller starburst molecular
structure.8 It would be an ideal unit for construction of
tribranched non-planar functional materials. Recently, 2PA
materials with triarylamine as electron donor have aroused
great interest and become the focus of intensive research in the
field of 2PA materials.2c,f Also, the introduction of a multi-
branched triarylamine moiety is expected to result in the AIE
phenomenon.9 On the other hand, 1,3,5-triazine-based
compounds show good optical and electrical properties due
to their high electron affinity and symmetrical structure. In
particular, octupolar molecules consisting of a strong triazine
electron-accepting center and an electron-donating end group
linked through a p-conjugated bridge have been shown to
be excellent 2PA materials.10 In this work, we connect six
electron-donating diphenylamine or phenothiazine groups to
the 2,4,6-positions of the triazine via 4-(4-(4-diphenylamino)-
styryl)phenylene conjugation bridges to form two novel multi-
branched type 2PA derivatives (TAPA-a and TAPA-b,
Scheme 1). It could be expected that the molecules with such
multibranched structure containing multi-electron donating
end-cappers should have excellent 2PA value as well as AIE
properties. TAPA-based compounds were synthesized and
characterized by 1H NMR, 13C NMR and MALDI-TOF
(ESIw).
For biophotonic applications, it is necessary that 2PA
materials are water soluble or dispersable and remain highly
fluorescent in aqueous media. Many 2PA materials, however,
are hydrophobic and their fluorescence quantum yields are
considerably reduced in water owing to the aggregation of
molecules, which generally leads to fluorescence quenching.4
Consequently a special molecular design for a 2PA material is
required not only to ensure a large two-photon activity, but
more importantly, to overcome fluorescence quenching at high
concentration. Indeed, a few compounds have been shown
recently to exhibit significant enhancement of their light emission
upon aggregation or in the solid state. This intriguing
a Key Laboratory for Advanced Materials and Institute of Fine
Chemicals, East China University of Science & Technology,
Shanghai 200237, P.R. China. E-mail: jlhua@ecust.edu.cn,
tianhe@ecust.edu.cn; Fax: +86-21-64252758;
Tel: +86-21-64250940
Fig. 1a shows the corresponding emission spectra of
compound TAPA-a in aqueous tetrahydrofuran (THF) with
different water–THF ratios at 1 Â 10À5 M. The emission from
the THF solution of TAPA-a was so weak that almost no PL
signal was recorded. However, a dramatic enhancement of
luminescence was observed for the 20% (v/v) water–THF
mixture (the PL intensity was boosted to 464 in 90% (v/v)
water–THF from original value 11). Scanning electron
microscopy (SEM) image analysis revealed that a nanoscopic
b Department of Physics, Fudan University, Shanghai 200243,
P.R. China
c Key Laboratory of Polar Materials and Devices,
Ministry of Education, East China Normal University,
Shanghai 200241, P.R. China
w Electronic supplementary information (ESI) available: Experimental
details for the synthesis and photophysical studies; additional figures.
See DOI: 10.1039/c0cc00803f
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4689–4691 | 4689

View Article Online
Scheme 1 Structures of TAPA-based compounds.
aggregate of the dye was formed in the mixtures with high
water contents (Fig. S1w). Evidently, the emission of the dye is
induced by aggregation formation, or in other words, TPAP-a
is AIE-active. Fig. 1b shows absorption and photoluminescence
spectra of TPAP-a in solution (THF) and in dispersion of the
nanoaggregate form (90% water) at 1 Â 10À5 M. In contrast to
fluorescence enhancement, the absorption spectra of TAPA-a
was broadened, implying that molecular stacking is well
ordered within the nanoaggregated structure. The fluorescence
quenching in solution can be understood as dominant non-
radiative decay, enhanced by free intramolecular torsional
motion in solution, as in the case of flexible molecules showing
viscosity dependent fluorescence.6 Moreover, the strong and
blue-shifted emission of the aggregation, compared to that
of the solution, is possibly due to the dye crystallization in
the solid films.11 Similar effects were observed for TAPA-b
(Fig. S2w).
To have a quantitative picture of the AIE process, we
estimated the PL quantum yields (Ff) of TAPA-a and
TAPA-b in water–THF mixtures, using rhodamine B as the
reference. As for TAPA-a and TAPA-b, the Ff of the THF
solution was very low (Ff = 0.009), and was almost
unchanged when water was added up to 10% and 30%
(v/v), respectively, but started to increase swiftly upon addition
of water to 20% and 40% (v/v), respectively (Fig. 1c). When
the volume fraction of water in the water–THF mixture is
increased to 90%, Ff rises to 0.4, which is about 45 times
higher than that of the THF solution. The trajectory of the Ff
change for TAPA-b suggests that the molecules start to
aggregate at a water fraction of 440% and that the size and
population of the aggregates continue to increase as the water
fraction is increased. The missing data for TAPA-a at water
fraction 30–60% is a result of bulk precipitation. It is presumable
that bulk precipitates were formed because the water amount
in those samples is suitable for lowering solubility, but not
enough yet for stabilizing colloidal dispersion by promoting
surface charge.6,12 The fluorescence behaviors of TAPA-a and
TAPA-b are well visualized through fluorescence images of
solution and nanoaggregate dispersions in Fig. 1d.
Fig. 1 (a) Corresponding emission spectra of compound TAPA-a in
aqueous THF with different water–THF ratios at 1Â10À5 M. (b)
Absorption and photoluminescence spectra of TPAP-a in solution
(THF) and in dispersion of the nanoaggregate form (90% water) at
1 Â 10À5 M. (c) PL quantum yields (Ff) of TAPA-a and TAPA-b at
1 Â 10À5 M, as a function of water fraction in a water–THF
mixture. The missing data for TAPA-a at 30–60% is a result of bulk
precipitation. (d) Fluorescence emission image of TAPA-a and
TAPA-b (0 and 90% water) under 365 nm illumination.
TAPA-a thin layer developed on a TLC silica plate emits
bright and deep orange light under illumination with a UV
lamp at room temperature; it may be that solvation of the
vapor of a good solvent releases the interaction of solid
molecules to a greater extent and causes free rotation of single
bonds of the AIE molecules, leading to non-emission. The
fluorescence turns off reversibly in the atmosphere of dichloro-
methane vapor. The same behavior was found for TAPA-b
[Fig. S3(a)w]. Because AIE materials possess the on/off
fluorescence switching property in some organic vapors, it is
suggested that this could be one of their most important
potential applications as a chemical vapor sensor.13
2PA cross sections of the TAPA-a and TAPA-b were
determined by the femtosecond open-aperture Z-scan
technique according to a previously described method.10b,14
Fig. 2b and Fig. S3(b)w show the open-aperture Z-scan data of
TAPA-a and TAPA-b and 2PA coefficient obtained by data
fitting. The values of 2PA cross section (s) for TAPA-a and
TAPA-b are 8629 GM and 4500 GM, respectively.
These values are much larger than those for most
representative materials.2c,15 The high 2PA properties of these
molecules are attributed to the extended p-system and
enhanced intramolecular charge transfer from multibranched
triarylamine to triazine. It should be noted that the s value of
TAPA-a is larger than that of TAPA-b because the triphenyl-
amine group of TAPA-a is more strongly electron-donating
than the phenothiazine group of TAPA-b. This suggests that
the strategy of increasing electron-donating strength of the end
group and extending the conjugation length of the system is
an effective approach for enhancing 2PA cross section of the
star-shaped molecules.
Although the intramolecular rotation of TAPA-a is
restricted in the solid state, the aggregate on a surface might
still be re-organized by solvent vapors, causing fluorescence
quenching like that in solutions. As shown in Fig. 2a, a
ꢀc
This journal is The Royal Society of Chemistry 2010
4690 | Chem. Commun., 2010, 46, 4689–4691

View Article Online
This work was supported by NSFC/China (20772031),
National Basic Research 973 Program (2006CB806200), the
Fundamental Research Funds for the Central Universities
(WJ0913001) and Scientific Committee of Shanghai.
Notes and references
1 (a) B. H. Cumpston, S. P. Ananthavel, S. Barlow, D. L. Dyer,
J. E. Ehrlich, L. L. Erskine, A. A. Heikal, S. M. Kuebler,
I.-Y. S. Lee, D. McCord-Maughon, J. Qin, H. Rockel, M. Rumi,
X.-L. Wu, S. R. Marder and J. W. Perry, Nature, 1999, 398, 51;
(b) D. A. Parthenopoulos and P. M. Rentzepis, Science, 1989, 245,
843; (c) W. Denk, J. H. Strickler and W. W. Webb, Science, 1990,
248, 73; (d) S. Kawata, H. B. Sun, T. Tanaka and K. Takada,
Nature, 2001, 412, 697; (e) H. M. Kim, B. H. Jeong, J. Y. Hyon,
M. J. An, M. S. Seo, J. H. Hong, K. J. Lee, C. H. Kim, T. Joo,
S. C. Hong and B. R. Cho, J. Am. Chem. Soc., 2008, 130, 4246;
(f) H. M. Kim and B. R. Cho, Acc. Chem. Res., 2009, 42, 863;
(g) W. H. Zhou, S. M. Kuebler, K. L. Braun, T. Y. Yu,
J. K. Cammack, C. K. Ober, J. W. Perry and S. R. Marder,
Science, 2002, 296, 1106.
2 (a) B. Strehmel and V. Strehmel, Adv. Photochem., 2006, 29, 111;
(b) M. Rumi, S. Barlow, J. Wang, J. W. Perry and S. R. Marder,
Adv. Polym. Sci., 2008, 213, 1; (c) G. S. He, L. S. Tan, Q. Zheng
and P. N. Prasad, Chem. Rev., 2008, 108, 1245; (d) F. Terenziani,
C. Katan, E. Badaeva, S. Tretiak and M. Blanchard-Desce, Adv.
Mater., 2008, 20, 4641; (e) H. M. Kim and B. R. Cho, Chem.
Commun., 2009, 153; (f) M. Pawlicki, H. A. Collins, R. G. Denning
and H. L. Anderson, Angew. Chem., Int. Ed., 2009, 48, 3244;
(g) Z. J. Liu, P. Shao, Z. L. Huang, B. Liu, T. Chen and J. G. Qin,
Chem. Commun., 2008, 2260.
3 G. S. He, J. Swiatkiewicz, Y. Jiang, P. N. Prasad, B. A. Reinhardt,
L. S. Tan and R. Kannan, J. Phys. Chem. A, 2000, 104, 4805.
4 S. Kim, Q. D. Zheng, G. S. He, D. J. Bharali, H. E. Pudavar,
A. Baev and P. N. Prasad, Adv. Funct. Mater., 2006, 16, 2317.
5 J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu,
H. S. Kwok, X. Zhan, Y. Liu, D. Zhu and B. Z. Tang, Chem.
Commun., 2001, 1740.
6 (a) Y. Hong, J. W. Y. Lam and B. Z. Tang, Chem. Commun., 2009,
4332; (b) H. Tong, Y. Hong, Y. Dong, Y. Ren, M. Haeussler, J. W.
Y. Lam, K. S. Wong and B. Z. Tang, J. Phys. Chem. B, 2007, 111,
2000; (c) Z. Zhao, S. Chen, X. Shen, F. Mahtab, Y. Yu, P. Lu,
J. W. Y. Lam, H. S. Kwok and B. Z. Tang, Chem. Commun., 2010,
46, 686.
7 S. Kim, T. Y. Ohulchanskyy, H. E. Pudavar, R. K. Pandey and
P. N. Prasad, J. Am. Chem. Soc., 2007, 129, 2669.
8 Z. J. Ning and H. Tian, Chem. Commun., 2009, 5483.
9 Z. J. Ning, Z. Chen, Q. Zhang, Y. L. Yan, S. X. Qian, Y. Cao and
H. Tian, Adv. Funct. Mater., 2007, 17, 3799.
10 (a) R. Kannan, G. S. He, T. C. Lin, P. N. Prasad, R. A. Vaia and
L. S. Tan, Chem. Mater., 2004, 16, 185; (b) B. Li, R. Tong, R. Zhu,
F. Meng, H. Tian and S. Qian, J. Phys. Chem. B, 2005, 109, 10705.
11 Q. Zeng, Z. Li, Y. Q. Dong, C. A. Di, A. J. Qin, Y. N. Hong, L. Ji,
Z. C. Zhu, C. K. W. Jim, G. Yu, Q. Q. Li, Z. G. Li, Y. Q. Liu,
J. G. Qin and B. Z. Tang, Chem. Commun., 2007, 70.
12 H. Kasai, H. S. Nalwa, S. Okada, H. Oikawa and H. Nakanish,
Handbook of Nanostructured Materials and Nanotechnology,
Academic, New York, 2000, ch. 8, vol. 5.
13 (a) B. K. An, S. K. Kwon, S. D. Jung and S. Y. Park, J. Am. Chem.
Soc., 2002, 124, 14410; (b) J. Chen, B. Xu, X. Ouyang, B. Z. Tang
and Y. Cao, J. Phys. Chem. A, 2004, 108, 7522.
14 Y. H. Jiang, Y. C. Wang, J. L. Hua, S. Y. Qu, S.Q. Qian and
H. Tian, J. Polym. Sci., Part A: Polym. Chem., 2009, 47, 4400.
15 F. Meng, B. Li, S. Qian, K. Chen and H. Tian, Chem. Lett., 2004,
33, 470.
Fig. 2 (a) On/off fluorescence switching of TAPA-a on TLC plates in
chloroform vapor (left) and without vapor (right) under UV light
(365 nm) illumination at room temperature. (b) Open-aperture Z-scan
trace of TAPA-a (scattered circles experimental data, straight line
theoretical fitted data). (c) Two-photon fluorescence emission spectra
for TAPA-a in solution (THF) and in dispersion of the nano-
aggregate form (90% water) at 1 Â 10À5 M, excited at 800 nm.
(d) TPF emissions image of TAPA-a in the THF and water mixture
(0 and 90% water).
Under the excitation of an 80 fs, 800 nm pulse, TAPA-a
and TAPA-b in a mixture of THF and water emit intense
fluorescence with the peaks located at 593 nm and 567 nm,
respectively. The good match between one- and two-photon
excitation fluorescence (Fig. 1a and Fig. 2c) indicates that the
emissions resulted from the same excited state, regardless of
the different mode of excitation. As shown in Fig. 2d,
two-photon fluorescence (TPF) was remarkably intensified
by nanoaggregation. Similar properties were observed for
TAPA-b [Fig. S3(c) and Fig. S3(d) in ESIw].
In conclusion, two new AIE-active multibranched molecules
containing
a
2,4,6-tri(p-tolyl)-1,3,5-triazine centre were
synthesized and characterized. They are AIE-active molecules:
virtually invisible in common organic solvents while highly
emissive in the aggregation state. The on–off fluorescence
switching behavior of TAPA-a and TAPA-b thin film layer
can be utilized for the sensing of organic solvent vapors. The
introduction of multibranched donors can enrich the electron
density of the system, and the change in the end group can
modify the optical properties. Preliminary studies show that
these TAPA-based multibranched molecules exhibit very large
2PA cross-sections (4500–8629 GM). This result provides a
useful guideline for the design of efficient 2PA materials with
AIE properties.
ꢀc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4689–4691 | 4691