Synthesis and properties of tribranched chromophores with triazine and fluorene units

Article abstract of DOI:10.1016/j.cclet.2011.05.045

Two tribranched chromophores, 1,3,5-triazine as electron deficient core, 9,9-dimethylfluorene as π-conjugated bridge, diphenylamino (4a) and naphthylamino (4b) as electron-donating end-groups, were successfully prepared via Buchwald-Hartwig coupling react

Full text of DOI:10.1016/j.cclet.2011.05.045

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 1147–1150
www.elsevier.com/locate/cclet
Synthesis and properties of tribranched chromophores
with triazine and fluorene units
Li Liu a, Zhi Qiang Zhou b, Jie Ping Shi a, Chang Gui Lu b,
Yi Ping Cui b, Guo Yuan Lu a,
*
a Department of Chemistry, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China
b Advanced Photonics Center, Southeast University, Nanjing 210096, China
Received 2 April 2011
Available online 18 July 2011
Abstract
Two tribranched chromophores, 1,3,5-triazine as electron deficient core, 9,9-dimethylfluorene as p-conjugated bridge,
diphenylamino (4a) and naphthylamino (4b) as electron-donating end-groups, were successfully prepared via Buchwald–Hartwig
coupling reaction. Their linear photophysical and two-photon absorption (TPA) properties were investigated by absorption,
fluorescence and nonlinear transmission method, respectively. The absorption cut-offs of the chromophores are below 550 nm and
both chromophores have strong fluorescence emission. The compound 4a (206.3 GM) exhibits larger TPA cross-section than 4b
(57.8 GM) in the femtosecond regime at 800 nm.
# 2011 Guo Yuan Lu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Octupolar molecules; Buchwald–Hartwig cross-coupling; Photophysical properties; TPA cross-section
The materials with intense two-photon absorption (TPA) have attracted increasing attention due to their potential
applications in the areas of optical power limiting, two-photon microscopy, two-photon up-conversion laser, data
storage, photodynamic therapy and new nanobiophotonics applications [1–4]. To meet market criteria for the
applications, molecules with large TPA cross-section are required. Triazine-cored octopolar materials have been
extensively investigated in nonlinear optical (NLO) materials for their good coplanarity of the conjugated system,
strong intramolecular charge-transfer (ICT), and additional cooperative enhancement between the branches [5].
Accordingly, coplanarity of the conjugation bridges plays an important role in enhancing the molecular optical
properties. Fluorene has extended conjugation length through the planar structure [6], excellent thermo- and
photostability [7,8], which has good stability in both solid and solution state [9] and is used widely in electronic and
photonic fields [10]. In this work, 9,9-dimethylfluorene was applied as p-conjugated bridge, it was connected to S-
triazine through vinylene at one end and electron donating group at the other one. As a result, tribranched octopolar
molecules 4a and 4b with strong ICT were constructed.
The chromophores 4a and 4b were synthesized from 2,4,6-trimethyl-1,3,5-triazine (Scheme 1). The 2,4,6-
trimethyl-1,3,5-triazine 1 was condensed with 7-bromo-9, 9-dimethyl-2-formylfluorene 2 under alkaline condition to
* Corresponding author.
E-mail address: lugyuan@nju.edu.cn (G.Y. Lu).
1001-8417/$ – see front matter # 2011 Guo Yuan Lu. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2011.05.045

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L. Liu et al. / Chinese Chemical Letters 22 (2011) 1147–1150
1
R
Br
N
2
R
N
N
2
R
N
1
Br
H
R
N
N
N
N
N
N
t-BuOK/THF
1
2
R
R
CHO
Br
N
N
reflux, 12h
91.1%
Cs CO , P(t-Bu)
3
2
3
pd(OAc) , toluene
2
1
2
3
1
2
4 a
R
= R
=
=
2
(58.3%)
R
Br
N
1
2
1
R
R
=
4 b R
(68.5%)
Scheme 1. Synthetic route of chromophores.
give compound 3 in high yield (91%). Compound 1 and 2 were synthesized according to literature [11,12]. Buchwald–
Hartwig cross-coupling reaction was used between 6 equivalent aryl amine and 1 equivalent 3 in the presence of Pd
(OAc)2/P (t-Bu)3 in toluene at 110 8C to give 4a and 4b in moderate yield, respectively. General synthetic procedure
for Buchwald–Hartwig cross-coupling reaction: Under N2 atmosphere, a mixture of 3 (0.19 g, 1 equiv mmol), aryl
amine (6.0 equiv mmol), Pd (OAc)2 (6 mg, 0.027 mmol), Cs2CO3 (0.44 g, 1.35 mmol), P (t-Bu)3 (10 wt% in pentane
0.9 mL) and toluene (50 mL) was refluxed for 6 h. After cooling to room temperature, saturated aqueous ammonium
chloride solution (30 mL) was added to the reaction solution. The solution was extracted with ethyl acetate
(30 mL ꢀ 2), and then the organic phase was dried over Na2SO4. After solvent evaporation, the crude product was
purified by column chromatography on silica gel using petroleum ether and chloroform (v/v = 2:1). 4a yield: 58.3%,
mp 173–174 8C. 4b yield: 68.5%, mp 131–132 8C. 4a and 4b are soluble in common organic solvents and their
1
structures were characterized with H NMR, 13C NMR and elementary analysis [13].
The single-photon absorption and excited fluorescence (SPEF) spectra of 4a and 4b in THF are shown in Fig. 1. The
corresponding spectroscopic parameters are summarized in Table 1. Both chromophores have absorption cut-offs
below 550 nm, indicating that there will be no linear absorption when laser source of 800 nm is used for TPA
applications. As shown in Fig. 1, both of them exhibit two strong absorptions (300–310 nm and 400–420 nm). The
former absorption is localized in the S-triazine rings [14] and the latter one can be assigned to an ICT. As compared
with 4a, which absorption band was centered at 429 nm, 4b (403 nm) showed shorter absorption. The influence of
solvent polarity on UV–vis spectra has also been studied. It was found that the effect on UV–vis absorption could be
ignored when solvents of increasing polarity were used. This phenomenon was similar to that of some other star-
shaped molecules [15]. In addition, both of them have good fluorescence emission, which is needed in TPA-based
important applications such as fluorescence imaging and upconverted lasing. The fluorescent quantum yields (Ff) of
0.8
0.4
0.0
1000
800
600
400
200
0
4a
4b
4a
4a
4b
4b
300
400
500
600
700
800
Wavelength/nm
Fig. 1. Linear one-photon absorption (left) and emission spectra (right) for compounds 4a and 4b in 1.0 ꢀ 10ꢁ6 mol/L THF.

L. Liu et al. / Chinese Chemical Letters 22 (2011) 1147–1150
1149
Table 1
Single- and two-photon properties of 4a and 4b.
e/molꢁ1 cmꢁ1
F
toluene
b (cm/GW)
s02 (GM)d
c
b
lambasx/nma
l
/nm
SPEF
max
Compound
4a
4b
429
403
6.4 ꢀ 103
555
440, 554
0.36
0.25
0.050
0.014
206.3
57.8
3.1 ꢀ 103
a
Measured in a 1.0 ꢀ 10ꢁ6 mol/L THF solution.
b
c
d
Measured in a 1.0 ꢀ 10ꢁ6 mol/L THF solution (lex = 400 nm).
Fluorescent quantum yield were determined in toluene with NBD-NHMe in CH3CN (F = 0.38, lex = 458 nm) as a reference.
Measurements in a 1.0 ꢀ 10ꢁ3 mol/LTHF using NLT method at 800 nm. 1 GM = 10ꢁ50 cm4 s. The error in the values of smax are estimated to be
approximately (15%).
4a and 4b in toluene are 0.36 and 0.25, respectively, as determined with NBD-NHMe as the reference in acetonitrile
[16].
TPA cross-sections were measured at 800 nm using the nonlinear transmission (NLT) method [17]. The excitation
source used for TPA studies was a Ti: sapphire oscillator/amplifier system (model Mira900-F, Legend-F from
Coherent Inc.) producing ꢂ150 fs duration, 800 nm-wavelength laser output with a repetition rate of 1 kHz. The
nonlinear absorption coefficient b was experimentally measured, and the molecular two-photon cross-section s02 can
be determined using the following expression:
bðlÞ ¼ s02ðlÞN0 ¼ s02ðlÞNAd0 ꢀ 10ꢁ3
Here s02 is the molecular TPA cross-section (in units of cm4/GW), N0 is the molecular density (in units of 1/cm3), NA
is Avogadro’s number, and the d0 is the molar concentration of the absorbing molecules (in units of M). Measured
input–output intensity relation of chromophore 4a in THF solution (c = 1.0 ꢀ 10ꢁ2 mol/L) at 800 nm is shown in
Fig. 2. Analysis of the s20 data of the chromophore 4a and 4b, TPA cross-section of 4a (206.3 GM) is almost 4 times
that of 4b (57.8 GM). This dramatic decreasement of TPA cross-section of 4b compared with 4a may be attributed to
that naphthalene substitution on the nitrogen atom of pyramidalization decreases the N donor ability [18,19].
In summary, 1,3,5-triazine chromophores with 9,9-dimethylfluorene as p-conjugated-bridge, aryl amine as the
electron-donating end-groups were successfully synthesized and characterized. The absorption cut-off of the
chromophores is below 550 nm and both of them have stronger fluorescence emission. TPA measurements by
nonlinear transmission method indicate that chromophores have larger TPA cross-section in the femtosecond regime,
and TPA cross-section of 4a (206.3 GM) is 3 times larger than that of 4b (57.8 GM).
Fig. 2. Input–output intensity profile of chromophore 4a in THF solution (c = 1.0 ꢀ 10ꢁ2 mol/L) at 800 nm for a fs laser source. The straight line
corresponds to the linear transmission of the solution.

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L. Liu et al. / Chinese Chemical Letters 22 (2011) 1147–1150
Acknowledgments
This work was supported financially by National Natural Science Foundation of China (Nos. 20774039 and
20872061) and the National Basic Research (No. 2007CB925103).
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