Synthesis, photophysical properties of tribranched chromophores based on 1,3,5-triazine core and different electro- donating end-groups

Article abstract of DOI:10.1002/cjoc.201180369

Three two-photon absorption (TPA) tribranched chromophores were successfully prepared, in which 1,3,5-triazine is been as electron deficient core, 1,4-phenylenedivinylene as conjugated bridge, 3,4-ethylenedioxythiophene (EDOT) (T1), N-methylpyrrole (T2) or triphenylamine (T3) as electron-donating end-groups. Their photophysical properties were studied by absorption, one- and two-photon fluorescence and TPA cross-section determination. The nonlinear transmission (NLT) measurement in femtoseconds (fs) regime at 800 nm indicates that TPA cross-section σ₂ values of T1, T2 and T3 with extended π-conjugated bridge are much larger than the corresponding chromophore T4 with a short length bridge, and TPA cross-section of T1 with end-groups EDOT exhibits a remarkable enhancement compared with T2 and T3 having the same length π-system. The chromophores T1, T2 and T3 show also remarkable up-converted luminescence and optical limiting activity.

Full text of DOI:10.1002/cjoc.201180369

FULL PAPER
Synthesis, Photophysical Properties of Tribranched Chromo-
phores Based on 1,3,5-Triazine Core and Different Elec-
tro-donating End-groups
Liu, Lia(刘莉)
Huang, Weib(黄维)
Shi, Jiepinga(史界平)
Lü, Changguib(吕昌贵)
Cui, Yipingb(崔一平)
Lu, Guo-Yuan*,a(陆国元)
a Department of Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing, jiangsu
210093, China
b Advanced Photonics Center, Southeast University, Nanjing, Jiangsu 210096, China
Three two-photon absorption (TPA) tribranched chromophores were successfully prepared, in which
1,3,5-triazine is been as electron deficient core, 1,4-phenylenedivinylene as conjugated bridge, 3,4-ethylenedioxy-
thiophene (EDOT) (T1), N-methylpyrrole (T2) or triphenylamine (T3) as electron-donating end-groups. Their pho-
tophysical properties were studied by absorption, one- and two-photon fluorescence and TPA cross-section deter-
mination. The nonlinear transmission (NLT) measurement in femtoseconds (fs) regime at 800 nm indicates that
TPA cross-section σ2 values of T1, T2 and T3 with extended π-conjugated bridge are much larger than the corre-
sponding chromophore T4 with a short length bridge, and TPA cross-section of T1 with end-groups EDOT exhibits
a remarkable enhancement compared with T2 and T3 having the same length π-system. The chromophores T1, T2
and T3 show also remarkable up-converted luminescence and optical limiting activity.
Keywords chromophore, triazine, 3,4-ethylenedioxythiophene, two-photon absorption, photophysics, Horner-
Wadsworth-Emmons reaction
Introduction
approaches to obtain large intrinsic TPA cross sections,
whether for one-dimensional or multi-dimensional
chromophores, prolonging the π-conjugated length, in-
creasing the planarity and strengthening the donor
and/or acceptor can increase TPA cross-section value.
However, the design of TPA cross-section of chromo-
phores must be increased and comprehensive properties
such as solubility improved for practical application.
Therefore, the design of chromophores with high TPA
cross-section and required other properties is still a ma-
jor challenge.
1,3,5-triazine-based chromophores have been shown
excellent TPA properties because of the good coplanar-
ity of the conjugated system, strong intramolecular
charge-transfer (ICT), and additional cooperative en-
hancement between the branches.13 Recently, Tian and
Qian14 reported tribranched chromophores with fluorene
or phenyl aromatic bridge attaching to the s-triazine
core, diphenylamino groups as the electro-donating
end-groups, and Fang15 and Gong16 reported tribranched
compounds with vinylene attaching to the s-triazine
core, N,N-disubstituted amines as end-groups, respec-
tively. Their studies have shown that TPA cross-section
of their tribranched chromophores with the s-triazine
core increases significantly, and the latter were consid-
The materials with intense two-photon absorption
(TPA) have attracted increasing attention owing to their
potential applications, such as optical power limiting,1
fluorescence imaging,2 two-photon microscopy,3
photodynamic therapy,4 optical data storage5 and new
nanobiophotonics applications.6 In order to realize the
commercial applications, major improvements are nec-
essary in the design and synthesis of large TPA
cross-section (σ) organic chromophores together with
requirements of desirable TPA absorption wavelength
region, high photostability and good solubility.7 The
several design strategies have been proposed to improve
TPA cross-section.8 The π-electron donor (D) and
π-electron acceptor (A) are symmetrically or asymmet-
rically connected by a polarizable π-bridge to form
D-π-D/A-π-A type quadrupole chromophores or D-π-A
type dipole chromophores. Furthermore, certain num-
bers of these D and A units connected by a π-core, such
as 4,4',4"-triphenylamine,9 1,3,5-benzene10 and 1,3,5-
tricyanobenzene,11 result in multibranched molecules
(octupolar compounds), which possess a non-additive
increase in TPA cross-section because of the coopera-
tive interactions between the TPA units.12 In further
*
E-mail: lugyuan@nju.edu.cn; Tel.: 0086-025-83685613
Received March 18, 2011; revised May 23, 2011; accepted June 9, 2011.
Project supported by the National Natural Science Foundation of China (No. 20774039) and the National Basic Research (No. 2007CB925103).
Chin. J. Chem. 2011, 29, 2129— 2133 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 2129

Liu et al.
FULL PAPER
ered more enhancements because of the better planar
structure and less steric hindrance.17 Therefore, in order
to study the relationship of two-photon absorption and
molecular structure in the triazine chromophores, we
employed 1,4-phenylenedivinylene as extended π con-
jugated-bridge and connected it by two C＝C bonds to
s-triazine core and different electro-donating end-groups.
The 3,4-ethylenedioxythiophene (EDOT),18 N-methyl-
pyrrole19 or triphenylamine20 groups, well known strong
electron-pushing groups, were employed as the electro-
donating end-groups. Especially, EDOT is a unique
electrical active group, which has emerged in the con-
ducting polymers and in organic light emitting diodes
(OLED).21 Thus tribranched octupolar molecules (T1,
T2 and T3) with strong ICT were constructed (Scheme
1). Their photophysical properties were studied by ab-
sorption, one- and two-photon fluorescence, NMR
spectroscopy and TPA cross-section determination.
Experimental
Melting points were measured with an SGW X-4
micromelting point apparatus (uncorrected). Fluores-
cence spectra were measured with Perkin Elmer LS55,
Scheme 1 Synthesis of target chromophores T1— T4
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Chin. J. Chem. 2011, 29, 2129— 2133

Synthesis, Photophysical Properties of Tribranched Chromophores
the absorption spectra were recorded with UV-3600
2,4,6-Tris[2-(1-methyl-1H-pyrrol-2yl)-(E)-vinyl]-
1,3,5-triazine (T4) Potassium tert-butoxide (1.1 g,
9.78 mmol) and 1-methylpyrrole-2-carboxaldehyde (2)
(0.87 g, 80 mmol) were added into a solution of
2,4,6-trimethyl-1,3,5-triazine (1) (0.20 g, 16 mmol) in
dry THF (15 mL) under N2 atmosphere, the mixture was
stirred and refluxed for 16 h. The reaction mixture was
poured into methanol (50 mL) and stirred for an addi-
tional 2 h. The obtained solid after filtration was washed
with methanol (20 mL×3) and dried in vacuum to af-
ford T4 (0.61 g, 94.8%) as pure yellow powder. m.p.
222— 224 ℃; 1H NMR (CDCl3, 500 MHz) δ: 8.16 (d, J
＝15.5 Hz, 3H, olefinic H), 6.85 (d, J＝15.5 Hz, 3H,
olefinic H), 6.78— 6.81 (m, 6H, pyrrole H), 6.22 (t, J＝
3.0 Hz, 3H, pyrrole H), 3.85 (s, 9H, NCH3); 13C NMR
(CDCl3, 125 MHz) δ: 171.1, 130.1, 128.5, 126.7, 121.5,
111.6, 109.5, 34.7. Anal. calcd for C24H24N6: C 72.70, H
6.10, N 21.20; found C 72.71, H 6.17, N 21.27.
1
UV-VIS-NIR spectrophotometer. H NMR and 13C
NMR data were recorded on a Brucker AM 300 and 500
MHz spectrometer (Germany). Elemental analysis was
performed with Elementary Vario MICRO. TPA
cross-section was measured at 800 nm using the nonlin-
ear transmission (NLT) method. The excitation source
used for TPA studies was a Ti: sapphire oscillator/am-
plifier system (model Mira900-F, Legend-F from Co-
herent Inc.) producing ca. 150 fs duration, 800-nm-
wavelength laser output with a repetition rate of 1 kHz.
General procedure for the synthesis of the title com-
pounds T1-T3
Compounds 1, 2, 3 and 4 were prepared according to
reported procedure.22 A solution of corresponding alde-
hyde (4 mmol) was added using a syringe to a mixture
of NaH (60%, 10 mmol ) and 4 (0.48 mmol) in anhy-
drous THF (15 mL) under N2 atmosphere. The mixture
was stirred at room temperature in dark for 16 h. The
color of the reaction turned blue and then later dark red.
Methanol (50 mL) was then added, red or orange pre-
cipitate was formed and the mixture was stirred for an-
other 2 h. The target compound was collected by filtra-
tion, washed with methanol carefully, and purified by
column chromatography on silica gel using hexane and
chloroform (V/V＝1∶2).
Results and discussion
Synthesis of chromophores
The target chromophores T1, T2 and T3 were syn-
thesized according to Scheme 1. Compounds 1— 4
and their detailed synthesis are reported elsewhere.22 T1,
T2 and T3 are reported here for the first time and were
synthesized by Horner-Wadsworth-Emmons reaction of
4 with the excess aldehyde [3,4-ethylenedioxythio-
phene-2-carboxaldehyde, 1-methylpyrrole-2-carbox-
aldehyde and 4-(diphenylamino)benzaldehyde]. The
chromophores were purified by column chromatography
on silica gel using hexane and chloroform (V∶V＝1∶2)
as eluent to obtain corresponding target tribranched
compounds T1, T2 and T3. The yields of these com-
pounds are moderate (30.5%— 39.5%) in spite of using
a 100% excess quantity of aldehyde in the reaction. In
addition, tribranched triazine chromophore T4 with only
vinylene as π-bridge (short π-system length) and
N-methylpyrrole as end-groups was also prepared by the
condensation of 1,3,5-trimethyl-s-triazine and N-methyl-
pyrrole-2-carboxaldehyde. All tribranched triazine
chromophores show good solubility in ordinary organic
2,4,6-Tris{4-[2-(3,4-ethylenedioxythiophene-2-yl)-
vinyl]phenyl-(E)-vinyl}-1,3,5-triazine (T1) Dark-red
1
solid, yield 30.5%, m.p.＞300 ℃; H NMR (CDCl3,
300 MHz) δ: 8.28 (d, J＝15.9 Hz, 3H), 7.63, 7.51
(AA'BB', 12H), 7.18— 7.13 (m, 6H), 6.88 (d, J＝15.9
Hz, 3H)), 6.26 (s, 3H), 4.28— 4.24 (m, 12H); 13C NMR
(DMSO, 75 MHz,) δ: 171.2, 142.4, 139.7, 140.5, 134.3,
129.3, 127.0, 125.4, 119.9, 116.2, 99.6, 65.3, 64.9. Anal.
calcd for C51H39N3O6S3: C 69.13, H 4.44, N 4.74; found
C 68.87, H 4.72, N 5.02.
2,4,6-Tris{4-[2-(N-methyl-1H-pyrrol-2-yl)vinyl]-
phenyl-(E)-vinyl}-1,3,5-triazine (T2) Orange solid,
1
yield 39.5%, m.p. 113— 115 ℃; H NMR (CDCl3, 300
MHz) δ: 8.29 (d, J＝15.6 Hz, 3H), 7.66, 7.51 (AA'BB',
12H), 7.18 (d, J＝15.6 Hz, 3H), 7.09— 6.86 (m, 6H),
6.67— 6.55 (m, 6H), 6.19 (t, J＝3.0 Hz, 3H), 3.73 (s,
9H); 13C NMR (CDCl3, 75 MHz ) δ: 171.6, 141.1, 139.6,
134.1, 131.8, 129.3, 126.3, 124.9, 124.2, 118.0, 108.6,
107.4, 34.2. Anal. calcd for C48H42N6: C 82.02, H 6.02,
N 11.96; found C 81.78, H 6.28, N 11.71.
solvents such as THF, CHCl
3, CH2Cl2, and toluene with
1
a strong orange or green fluorescence. H NMR, 13C
NMR of all chromophores and their elementary analyti-
cal data are in accord with the assigned structures.
2,4,6-Tris{4-[2-(4-diphenylamino-yl)vinyl]phenyl-
(E)-vinyl}-1,3,5-triazine (T3) Brilliant orange solid,
Absorption and fluorescence spectra of chromopho-
res
1
yield 32.8%, m.p. 139— 141 ℃; H NMR (CDCl3, 300
The linear single-photon absorption and excited fluo-
rescence (SPEF) spectra of T1— T4 in THF are shown
in Figure 1. The corresponding spectroscopic parameters
are summarized in Table 1.
As shown in Figure 1, each compound shows two
strong absorption (290— 315 nm and 406— 429 nm) and
no significant NIR absorption. The former absorption is
localized in the s-triazine rings and the latter one
MHz) δ: 8.29 (d, J＝15.9 Hz, 3H), 7.65, 7.55 (AA'BB',
12H), 7.41 (d, J＝8.4 Hz, 6H), 7.29— 7.24 (m, 12H),
7.19— 7.16 (m, 3H), 7.13— 7.10 (m, 18H), 7.06 (d, J＝
7.5 Hz, 6H), 7.03— 6.97 (m, 6H); 13C NMR (CDCl3, 75
MHz) δ: 171.2, 147.3, 139.2, 134.3, 130.9, 129.2, 128.6,
127.5, 126.7, 126.0, 124.6, 123.2. Anal. calcd for
C87H66N6: C 87.41, H 5.56, N 7.03; found C 87.15, H
5.71, N 7.31.
Chin. J. Chem. 2011, 29, 2129— 2133
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Liu et al.
FULL PAPER
accord with SPEF.
Figure 1 Linear one-photon absorption (left) and emission
spectra (right) for compounds T1— T4 in 1.0× 10－ mol•L－
6
1
THF (λex＝ 400 nm).
Figure 2 Two-photon excited fluorescence spectra of T1— T4
in 1.0× 10－ 6 mol•L－ 1 THF (λex＝ 800 nm).
can be assigned to an ICT transition. In addition, all
these chromophores show strong fluorescence emission,
which is needed in TPA-based important applications
such as fluorescence imaging and upconverted lasing.
Comparing chromophore T4 with a short π-bridge
length, the λ of single-photon absorption and the λ
Two-photon absorption of chromophores
Two-photon absorption cross-sections were meas-
ured by the nonlinear transmission method (NLT).24 To
avoid possible complications due to the excited-state
excitation, we have used fs laser pulses. Measured in-
max
max
of SPEF of T2 in THF are red-shifted. The λ
of ab-
max
put-output intensity relation of chromophore T1 in THF
－
solution (c＝1.0×10－ 3 mol•L ) at 800 nm is shown in
1
sorption shifts from 401 nm to 428 nm and λmax of emis-
sion (λ ＝400 nm) increases from 486 nm to 546 nm.
ex
Figure 3. TPA cross-sections of chromophores in THF
'
2
The red-shifts can be ascribed to the introduction of
phenylenedivinylene units in the backbones, which re-
sults in charge extended delocalization.
at 800 nm are reported in Table 1. Analyzing the σ
data of the compounds T2 and T4, T2 (86.7 GM) is
more than 5 times of T4 (15.5 GM), this dramatic en-
hancement in TPA cross-section can be attributed to the
incorporation of additional 1,4-phenylenedivinylene unit
as the π-bridge. Among tribranched chromophores, TPA
cross-section of T1 with end-groups EDOT is the largest
(247.8 GM), and it is almost 3 times larger than T2
(86.7 GM) although both T1 and T2 have the same length
π-systems. Figure 3 shows also the optical limiting prop-
erties of chromophore T1 obtained in the fs regime. At
low input energies (namely, at low light intensity) 87%
of the light is transmitted (T＝0.87), but as the input
energy is increasing, the output energy remains very
Table 1 Photophysical properties for chromophores T1— T4
b
SPEF
TPEF
/
d/ β/
λ
λ
'
2
max
Compd.
a/nm (ε)
σ
e/GM
max
abs
λ
－
max
1
nm (Φc) nm
(cm•GW )
T1 409
(0.95)
T2 428
(0.58)
T3 427
(1.17)
T4 401
546
546
598
605
487
0.006
247.8
86.7
(0.19)
596
0.021
0.059
0.0038
(0.32)
600
243.7
(0.53)
486
15.7
a Measured in a 1.0× 10－ 6 mol•L－ 1 THF solution. b Measured in a
1.0× 10－ mol•L－ THF solution (λex＝ 400 nm). Fluorescent
quantum yields were determined in toluene with N-methyl ANBD
in CH3CN (Φ＝ 0.38, λex＝ 458 nm) as a reference.23 d Measured
6
1
c
with T1— T4 in 1.0× 10－ mol•L－ THF at 800 nm. e Measure-
6
1
ments with T1 at 1.0× 10－ mol•L－ and T2— T4 in 1.0× 10－
3
1
2
mol•L－ 1 THF using NLT method at 800 nm. 1 GM＝ 10－ 50 cm4•s.
The two-photon excited fluorescence spectra (TPEF)
of T1—T4 are shown in Figure 2. The frequency-
upconverted fluorescence emission can be easily ob-
served from T1—T3 in THF solution excited with the
near IR laser beam at the wavelength 800 nm. This ob-
servation indicates that a two-photon absorption process
occurs within the T1—T3. The λmax of the two-photon
excited fluorescence for T1—T4 are 546, 598, 605 and
487 nm, respectively. These peaks of fluorescence are in
Figure 3 Input-output intensity profile of chromophore T1 in
－
THF solution (c＝ 1.0× 10－ mol•L ) at 800 nm for a fs laser
source. The straight line corresponds to the linear transmission of
the solution.
3
1
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Chin. J. Chem. 2011, 29, 2129— 2133

Synthesis, Photophysical Properties of Tribranched Chromophores
low, and most of the light is absorbed. Therefore, the
new tribranched triazine chromophores T1, T2 and T3
may be useful in optical limiting materials.
J.; Wasielewski, M. R.; Kim, D. J. Phys. Chem. A 2008, 112,
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Conclusions
In summary, a series of 1,3,5-triazine TPA chromo-
phores with 1,4-phenylenedivinylene as extended π con-
jugated-bridge, EDOT, N-methylpyrrole and triphenyl
amine as the electron-donating end-groups were success-
fully synthesized. They are soluble in ordinary organic
solvents. NLT measurement in femtoseconds (fs) regime
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at 800 nm indicates that TPA σ values of T1, T2 and
2
T3 with extended π-conjugated bridge are much larger
than the corresponding chromophore T4 with a short
length bridge, and TPA cross-section of T1 with
end-groups EDOT exhibits a remarkable enhancement
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remarkable up-converted luminescence and optical lim-
iting activity.
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www.cjc.wiley-vch.de
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