Synthesis and two-photon properties of a multi-polar chromophore based on tetra-quinoxalinylethylene scaffold

Article abstract of DOI:10.1016/j.tetlet.2011.10.019

A new multi-branched fluorophore derived from tetra-quinoxalinylethylene as the core with diphenylaminofluorene units incorporated at the peripheral positions has been synthesized via Pd-catalyzed Suzuki coupling and experimentally shown to possess strong

Full text of DOI:10.1016/j.tetlet.2011.10.019

Tetrahedron Letters 52 (2011) 6748–6753
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and two-photon properties of a multi-polar chromophore
based on tetra-quinoxalinylethylene scaffold
⇑
Tzu-Chau Lin , Yu-Jhen Huang, Bor-Rong Huang, Ying-Hsuan Lee
Photonic Materials Research Laboratory, Department of Chemistry, National Central University, Jhong-Li 32001, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new multi-branched fluorophore derived from tetra-quinoxalinylethylene as the core with diphenyla-
minofluorene units incorporated at the peripheral positions has been synthesized via Pd-catalyzed Suzuki
coupling and experimentally shown to possess strong solvent effect on its fluorescence emission and life-
time behaviors. The initial characterization also found that this dye molecule exhibits two-photon activ-
ities and effective optical power limiting properties in the near-IR region under the irradiation of
nanosecond laser pulses.
Received 26 August 2011
Revised 1 October 2011
Accepted 5 October 2011
Available online 12 October 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Two-photon absorption
Two-photon-excited fluorescence
Optical power limiting
Tetra-quinoxalinylethylene
Heck reaction
Suzuki reaction
Two-photon absorption (2PA) was theoretically predicted by
Maria Göppert-Mayer in 19311 and the first experimental observa-
tion of this nonlinear optical phenomenon was realized about
30 years later when Kaiser and Garret observed two-photon in-
duced up-converted fluorescence emission from a CaF2/Eu2+ inor-
ganic crystal in 1961.2 With the advent of high peak-power laser
systems, numerous 2PA-based photonic and biophotonic applica-
tions have been explored and demonstrated including optical
power limiting, frequency up-converted lasing, 3-D data storage,
3-D microfabrication, nondestructive bio-imaging and tracking,
and two-photon photodynamic therapy.3 This, in turn, has inspired
a great demand in functional chromophores with enhanced 2PA
properties. Molecular design and engineering focused on 2PA opti-
mization has become very active for the past 15 years leading to
ical properties other than strong molecular 2PA may also be
required for the designed molecules because of various practical
needs. In the case of optical power limiting against long laser
pulses in nanosecond regime, compounds that possess extended
excited state lifetime are very desirable because it may benefit
the apparent nonlinear absorption due to two-photon-assisted
excited-state absorption (2PA-assisted ESA)9 so that the effective
optical power restriction performance will be enhanced. In search-
ing for effective strategies toward highly efficient 2PA materials,
we are interested in exploring the 2PA properties of multi-
branched or dendritic organic structures composed of various
types of
p-conjugated frameworks with heterocyclic moieties
incorporated. Following our continuous efforts in developing mul-
ti-branched and dendritic
p-structures based on tri-substituted
the investigation of a range of
ous archetypes including quasi-linear, multi-branched, and den-
dritic geometries. Among these investigated -systems, multi-
p-conjugated molecules with vari-
olefin scaffolds,8e,g,i in this Letter we report the facile synthesis of
a new fluorophore (3) based on the structural motif of tetra-substi-
tuted olefin using functionalized quinoxaline units as the major
building blocks and the initial investigations of its effective 2PA
properties in the nanosecond time domain.
p
branched, and dendritic chromophores have been experimentally
shown to manifest highly efficient multi-photon absorption.4–8
From the aspect of molecular design, branched and dendritic skel-
etons provide an access to incorporate numbers of 2PA-enhancing
parameters into a single dye system so that it is possible for mate-
rial chemists to optimize a single molecule that simultaneously
combines various expected characteristics for different specific
applications. Depending on the application, additional photophys-
The chemical structures and the synthetic routes toward the
studied compounds are illustrated in Figure 1 and Scheme 1,
respectively. The structure of compound (3) is conceptually based
on the skeletal motif of tetraphenylethylene (TPE) but introduces
four identical functionalized quinoxaline units as the aryl substit-
uents. TPE-based molecules have been extensively studied recently
owing to their unique emission behaviors in various physical
states10 as well as their potential usage for fluorescent bio/chemo-
sensors11 and electrochemical switching devices.12 The synthesis
⇑
Corresponding author.
E-mail address: tclin@ncu.edu.tw (T.-C. Lin).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.10.019

T.-C. Lin et al. / Tetrahedron Letters 52 (2011) 6748–6753
6749
N
N
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
N
N
N
N
N
N
2
1
N
N
N
C6H13
C6H13
C6H13
C6H13
N
N
N
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
N
N
N
N
N
N
N
N
N
N
N
N
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
N
N
3
Figure 1. Molecular structures of the studied chromophores.
of chromophore (3) is relatively straightforward as outlined in
Scheme 1. Although we originally did expect that attaching four
bulky 2,3-diarylquinoxaline units to the central ethylene core
could be relatively difficult and either low yield or incompletion
of reaction for this target compound may be encountered but nei-
ther these situations happened in our case and chromophore 3 was
successfully obtained in 72% yield through Pd-catalyzed Suzuki
coupling using commercialized tetraiodoethylene and properly
functionalized quinoxaline (9) as the major synthons. For the pur-
pose of comparison, we have chosen another two analogous struc-
tures (1 and 2) as the reference compounds. Reference compound 1
was prepared in 92% yield by using 1,2-diketone (4)8h and diamine
(5) as the starting materials while reference compound 2 was syn-
thesized in 72% yield under Heck reaction condition using corre-
sponding 6-bromoquinoxaline (7) and 6-vinylquinoxaline (8) as
the key precursors. All the model fluorophores are highly soluble
in common organic solvents such as toluene, ethyl acetate, dichlo-
romethane (DCM), and tetrahydrofuran (THF). The detailed syn-
thetic procedures including the preparation of intermediates and
final coupling reactions toward the targeted compounds are de-
scribed in Supplementary data.
Figure 2 presents the linear absorption and fluorescence spectra
of the studied dye molecules (1–3) in THF. Compound 1 shows two
major absorption bands with a lowest-energy peak located at
388 nm (
e
ꢀ 5.28 ꢁ 104 cmꢂ1 Mꢂ1) while both compounds 2 and
3 exhibit similar linear absorption behaviors with intense and
broad absorption ranging from 300 to 430 nm. These chromoph-
ores also emit strong visible fluorescence in their solution phase
(in THF) under the irradiation of a common UV-lamp with blue-
greenish color for 1 and yellow–orange color for 2 and 3, which
is in agreement with the measured emission spectra (see the inset
of Fig. 2). From the view point of molecular structure, chromoph-
ores 2 and 3 possess symmetric substitution patterns so that these
two compounds would be intuitionally expected to show very lim-
ited solvatochromic characters. Interestingly, these two fluoro-
phores indeed possess strong solvent effect on their fluorescence

6750
T.-C. Lin et al. / Tetrahedron Letters 52 (2011) 6748–6753
H2N
C6H13
C6H13
H2N
O
O
5
N
N
1
(a)
C6H13
C6H13
4
N
N
C6H13
C6H13
C6H13
C6H13
H2N
H2N
Br
6
(c)
(d)
N
N
2
(b)
N
Br
N
C6H13
C6H13
C6H13
7
8
C6H13
N
N
(e)
N
C6H13
C6H13
N
(f)
3
O
N
B
O
C6H13
C6H13
9
N
Scheme 1. Synthetic procedure for the studied chromophores 1–3. Reagents and conditions: (a) 4 (1 equiv), 5 (1.1 equiv) in CH3COOH, reflux, 6 h (90%); (b) 4 (1 equiv), 6
(1.1 equiv) in CH3COOH, reflux, 6 h (92%); (c) 7 (1 equiv), tributyl(vinyl)stannane (1.5 equiv), Pd(OAc)2 (0.5 equiv), PPh3 (0.2 equiv) in NEt3, reflux, 9 h (92%); (d) Heck
reaction: 7 (1.1 equiv), 8 (1 equiv), Pd(OAc)2 (0.02 equiv), P(o-tolyl)3 (0.12 equiv) in NEt3–MeCN, 110 °C, 48 h (72%).; (e) 7 (1 equiv), bis(pinacolato)diboron (1.3 equiv),
potassium acetate (3 equiv), Pd(dppf)Cl2 CH2Cl2 (0.06 equiv) in DMF, reflux, 3 h (92%); (f) Suzuki coupling: 9 (4.4 equiv), tetraiodoethylene (1 equiv), 2 M Na2CO3(aq)
,
Pd(PPh3)4 (0.02 equiv) in dioxane, reflux, 9 h (72%).
properties including band positions, quantum yields, and life-times
as shown and summarized in Figure 313 and Table 1, respectively.
Similar properties have been observed from other multi-polar
chromophore systems14 and the symmetry-breaking phenomenon
caused by electron-vibration coupling and dipolar solvation effects
were proposed to account for this behavior.15 From Figure 3, one
can see that both 2 and 3 show larger bathochromic shift for their
fluorescence bands when compared to that of reference compound
1. This feature indicates that chromophores 2 and 3 may exhibit
strong dipolar character in their (relaxed) excited-states.14,15 The
inset photographs in Figure 3 demonstrate the distinct difference
in the fluorescence color of 3 in toluene and THF when irradiated
by a UV-lamp. The same solvatochromic behavior was also ob-
served from compounds 2 and 3 for their up-converted emission
under two-photon excitation, suggesting that the final emissive
states accessed by two-photon process of these two molecules
are strongly dipolar as well. On the other hand, the excited-state
lifetimes of the studied compounds can be tuned by solvent polar-
ity (e.g., s1 ꢀ 2.5 ns in toluene and s1 ꢀ 5.5 in THF for 3) and this
feature is particularly desirable with regard to optical power atten-
uation in the nanosecond regime based on 2PA-induced excited-
state absorption (2PA-induced ESA) since the effective three-pho-
ton absorption coefficient increases significantly with the ex-
cited-state lifetime.9f,g
It is noted that the excited-state lifetime of the studied chro-
mophores in the present work are in the nanosecond range, which
indicates that significant excited-state population may be retained
and may consequently possess excited-state absorption during the

T.-C. Lin et al. / Tetrahedron Letters 52 (2011) 6748–6753
6751
Table 1
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Photophysical properties of the studied fluorophores 1–3 in solution phasea
compound 1
compound 2
compound 3
e
Lifetimef
(ns)
Em b,d
max
2PꢂExc
k
max
g
Abs b,c
max
Compound
UF (%)
k
(nm)
(nm)
k
(nm)
Toluene THF Toluene THF Toluene THF Toluene THF
1.6x106
1.4x106
1.2x106
1.0x106
8.0x105
6.0x105
4.0x105
1
2
312
394
309
365
436
302
364
425
308 501
388
306 504
364
434
300 498
363
567 50
603 68
49 2.3
53 2.0
4.5 ꢀ800
4.5 ꢀ840
3
601 66
57 2.5
5.5 ꢀ740
ꢀ840
2.0x105
0.0
419
a
b
c
450 500 550 600 650 700 750 800
Wavelength(nm)
All photophysical data were obtained in toluene and THF solutions as indicated.
The concentration was 1 ꢁ 10ꢂ5 M.
Linear absorption maxima.
d
Spectral positions of one-photon-absorption-induced fluorescence emission
300
400
500
600
700
800
900 1000 1100
maxima.
Wavelength(nm)
e
Fluorescence quantum efficiency.
Fluorescence lifetime.
Local maxima of photo-excitation spectra.
f
Figure 2. Linear absorption and fluorescence spectra (see the inset) of compounds
1–3 in solution phase (1 ꢁ 10ꢂ5 M in THF).
g
area of ꢀ25 mm in diameter. Figure 4(a) depicts the measured
average transmittance in the spectral range of 600–1000 nm. It is
noted that under our experimental conditions compounds 2 and
3 manifest effective attenuation of nanosecond laser pulses within
a broad spectral region. The attenuation of the incident laser pulses
based on these sample solutions can be presented in another way
as shown in Figure 4(b). In this experiment, the local intensity
within the tested sample solution was tuned by a set of neutral-
density filter. We have selected laser pulses with wavelengths at
which that each studied compound exhibits larger ability of
power-restriction for this demonstration. One can see that com-
pound 3 displays the superior power restriction property over
the other two congeners. This initial finding suggests the potenti-
ality of using this model fluorophore for broad-band power-sup-
pressing-related applications in the nanosecond time domain.
Additionally, the up-converted fluorescence intensity across dif-
ferent excitation wavelengths for these chromophores was de-
700
650
600
550
500
450
400
700
650
600
550
500
450
400
&
&
&
for compound 1
for compound 2
for compound 3
Toluene
THF
tected
through
two-photon-excited
fluorescence
(2PEF)
technique utilizing the same tunable nanosecond laser system as
the pumping light source. In this photo-excitation experiment,
the incident pulse energy was controlled at 0.5 mJ for each probing
wavelength to avoid any photo-bleaching or saturation of absorp-
tion. To minimize the effect of re-absorption due to the relatively
high concentration of the sample solution used in this measure-
ment, we have focused the excitation beam as close as possible
to the wall of the quartz cell (10 mm ꢁ 10 mm cuvette) so that
only the emission from the surface of the sample was recorded.
The recorded fluorescence intensity was normalized by the quan-
tum yield of each studied chromophore so that we can map out
and compare the dispersion and the magnitude of relative excita-
tion efficiency of these three dye molecules directly. At current
stage, the preliminary experimental results indicate that consecu-
0.00
0.05
0.10
0.15
0.20
)
Orientation Polarizability (
Δ
f
Figure 3. Solvatochromic property study of chromophores 1–3 in toluene–THF
mixed solvent system at 1 ꢁ 10ꢂ5 M (See Ref. 13). Inset, photographs that represent
the color difference in fluorescence of compound 3 in toluene and THF.
excitation by long laser pulses. In order to evaluate the effective
optical power restriction performance of these model molecules,
we have utilized nanosecond laser pulses for this investigation.
In our experiment, the nonlinear absorbing medium was a 1-cm
path-length solution of each model fluorophore in THF with a con-
centration of 0.02 M. As for the excitation laser light source, a tun-
able nanosecond laser system (an integrated Q-switched Nd:YAG
laser and OPO: NT 342/3 from Ekspla) was employed to provide
ꢀ6 ns laser pulses with controlled average pulse energy of ꢀ1 mJ
and repetition rate of 10 Hz for this study. The laser beam was
slightly focused onto the center of the sample solution in order
to obtain a nearly uniform laser beam radius within the whole cell
path-length. The transmitted laser beam from the sample cell was
detected by an optical power (energy) meter with a large detection
tive expansion of the molecular p-structures has led to ascending
overall relative excitation efficiency (from chromophores 1–3) as
shown in Figure 5. It should be interesting to delineate the disper-
sion of entire excited-state absorption (i.e., ESA-spectra) of these
structures and study the correlation between ESA and up-conver-
sion efficiency for the case of long pulse excitation. Such investiga-
tion is undertaken and will be reported in due course.
In summary, a novel dendritic chromophore based on the skel-
eton of tetra-quinoxalinylethylene has been designed and synthe-
sized via Pd-catalyzed Suzuki reaction using tetraiodoethylene and
properly functionalized 2,3-diarylquinoxaline moiety as the essen-
tial synthons. Despite the symmetric nature of its molecular struc-
ture, the fluorescence properties including band positions,

6752
T.-C. Lin et al. / Tetrahedron Letters 52 (2011) 6748–6753
lar nature of its emissive excited-state. On the other hand, the ini-
100
90
80
70
60
50
40
30
20
10
0
(a)
tial results from nonlinear transmission and photo-excitation
experiments in the nanosecond regime have indicated that this
multi-polar fluorophore manifests wide-dispersed two-photon
activities in the near IR-region and this feature may be beneficial
for various photonic and biophotonic applications such as effective
broad-band optical limiting and frequency up-conversion.
Acknowledgment
compound 1
compound 2
compound 3
We thank National Science Council (NSC), Taiwan for the finan-
cial support.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.10.019.
600 640 680 720 760 800 840 880 920 960 1000
Wavelength (nm)
2.0
References and notes
(b)
compound 1@800nm
compound 2@720nm
compound 3@720nm
1.8
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9x105
8x105
7x105
6x105
5x105
4x105
3x105
2x105
compound 1
compound 2
compound 3
1x105
0
600 640 680 720 760 800 840 880 920 960 1000
Wavelength (nm)
Figure 5. Measured photo-excitation spectra of compounds 1–3 by 2PEF method in
THF solution at 1 ꢁ 10ꢂ4 M using nanosecond laser pulses as the probe (with
experimental error ꢀ 15%).
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presentation of Figure 3. Orꢂie1ntation polarizability (
Df) was calculated based
e
n2ꢂ1
on the equation of
D
f ¼ ½ þ1 ꢂ þ1ꢃusing a ‘rule of mixtures’ approach to
2n2
2
e

T.-C. Lin et al. / Tetrahedron Letters 52 (2011) 6748–6753
6753
determine the values of dielectric constant (
mixed solution from the values of the constituent solvents (THF and toluene).
In general, f is a measure of solvent polarity and the development of f is
described in detail in: Onsager, L. J. Am. Chem. Soc. 1936, 58, 1486–1493.
e
) and refractive index (n) for the
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D
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