Synthesis and two-photon property studies of symmetrically substituted bisarylacetylene structures using functionalized quinoxalinoid units as the aryl substituents

Article abstract of DOI:10.1016/j.dyepig.2014.04.035

A series of symmetrically substituted multi-polar chromophores (1-3) based on the skeleton of bisarylacetylene using functionalized quinoxaline, indenoquinoxaline, and pyridopyrazine moieties as the aryl substituents has been synthesized and characterized for their two-photon absorption properties using femtosecond laser pulses as the probing tool. Under our experimental conditions, these model fluorophores are found to manifest strong and wide dispersed two-photon absorption in the near infrared (NIR) region. It is demonstrated that molecular structures with multi-branched π-frameworks incorporating functionalized quinoxalinoid units would possess large molecular nonlinear absorptivities within the studied spectral range. Optical power-limiting behavior in the femtosecond time domain of the indenoquinoxaline-derived dye molecule (2) from this model compound set was also investigated and the result indicates that such structural motif could be a useful approach for the molecular design towards strong two-photon absorbing material system for quick-responsive and broadband optical-control related applications.

Full text of DOI:10.1016/j.dyepig.2014.04.035

Accepted Manuscript
Synthesis and Two-photon Property Studies of Symmetrically Substituted
Bisarylacetylene Structures Using Functionalized Quinoxaliniod Units as the Aryl
Substituents
Tzu-Chau Lin , Yi-You Liu , May-Hui Li , Ying-Hsuan Lee
PII:
S0143-7208(14)00171-5
10.1016/j.dyepig.2014.04.035
DYPI 4363
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 26 February 2014
Revised Date: 17 April 2014
Accepted Date: 25 April 2014
Please cite this article as: Lin T-C, Liu Y-Y, Li M-H, Lee Y-H, Synthesis and Two-photon Property
Studies of Symmetrically Substituted Bisarylacetylene Structures Using Functionalized Quinoxaliniod
Units as the Aryl Substituents, Dyes and Pigments (2014), doi: 10.1016/j.dyepig.2014.04.035.
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Graphical Abstract for paper

ACCEPTED MANUSCRIPT
Synthesis and Two-photon Property Studies of Symmetrically
Substituted Bisarylacetylene Structures Using Functionalized
Quinoxaliniod Units as the Aryl Substituents
Tzu-Chau Lin,* a Yi-You Liu,a May-Hui Li,a and Ying-Hsuan Leea
aPhotonic Materials Research Laboratory, Department of Chemistry, National Central
University, Jhong-Li 32001, Taiwan. E-mail: tclin@ncu.edu.tw
Abstract
A series of symmetrically substituted multi-polar chromophores (1-3) based on the
skeleton of bisarylacetylene using functionalized quinoxaline, indenoquinoxaline, and
pyridopyrazine moieties as the aryl substituents has been synthesized and characterized for
their two-photon absorption properties using femtosecond laser pulses as the probing tool.
Under our experimental conditions, these model fluorophores are found to manifest strong
and wide dispersed two-photon absorption in the near infrared (NIR) region. It is
demonstrated that molecular structures with multi-branched frameworks incorporating
functionalized quinoxalinoid units would possess large molecular nonlinear absorptivities
within the studied spectral range. Optical-power-limiting behavior in the femtosecond
time domain of the indenoquinoxaline-derived dye molecule (2) from this model
compound set was also investigated and the result indicates that such structural motif
could be a useful approach for the molecular design towards strong two-photon absorbing
material system for quick-responsive and broadband optical-control related applications.
Keywords: Two-photon absorption, Sonogashira reaction, quinoxaline, indenoquinoxaline,
pyridopyrazine, optical power-limiting.
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1. Introduction
Similar to the development path of many other science subjects, the theoretical prediction
of two-photon absorption (2PA) made by Maria Göppert-Mayer in 1931 is far ahead of its
experimental evidence.[1] After the advent of lasers in 1960, scientists started to have
appropriate light sources to practically observe and study this nonlinear optical
phenomenon.[2, 3] For the past two decades, the availability of high peak power lasers has
triggered the momentum to explore the two-photon technologies and many potential
applications in the emerging field of photonics and biophotonics based on 2PA have been
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.[4-9] In the course to develop two-photon technologies, the
exploration of new materials with strong 2PA plays an equally important counterpart as the
advancement of laser systems. Through rational molecular design, it is possible to
construct organic chromophores that exhibit several orders of intensified 2PA with other
desired molecular characteristics simultaneously incorporated, which greatly compensates
for the relatively poor performance of commercialized dyes for the aforementioned
applications. So far, it has been realized that the combination of several structural
parameters such as intramolecular charge-transfer efficiency, effective size of
-conjugation domain, and molecular dimensionality of a molecule is closely related to
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the molecular 2PA.[10-32] In other words, the arrangement of the selected building units
within a molecule is a hinge for the molecular design toward highly active
2PA-chromophores. In searching new strategies toward highly efficient 2PA materials, we
have been interested in the exploration of effective building units and structural
arrangement that may enhance the molecular 2PA. In this paper, we report our recent
studies of degenerate two-photon absorption, up-converted emission and optical
power-limiting properties of a series of newly synthesized quinoxalinoid chromophores by
using ultafast IR laser pulses working in the femtosecond regime as the major probing
tools.
2. Results and discussion
2.1 Molecular structures and syntheses
The chemical structures of the studied model compounds in this work are illustrated in
Figure 1. These three structural congeners are constructed based on the scaffold of a
bisarylacetylene by using electron-donor-functionalized quinoxalinoid units as the aryl
substituents. We have selected pyrazine-containing ring complexes as the heterocyclic
parts in this model compound system since the involvement of electron-withdrawing
hetero-atoms is expected to alter the electronic nature of the resulting chromophores
compared to the all-carbon analogues. From the standpoint of molecular structure, each
functionalzed quinoxalinoind moiety of the studied model compound can be assumed to
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be dipolar because of its push-pull nature and skeletal geometry. Therefore, the connection
between the presented structural arrangement and molecular 2PA would be an interesting
subject to study. Furthermore, we have attached alkyl chains at C9 positions of all the
fluorenyl units in order to improve the molecular solubility in common organic solvents,
which is another important issue to be considered in the molecular design from both the
aspect of experiments and applications. The syntheses of the target model compounds are
relatively straightforward as illustrated in Scheme 1, which mainly include the preparation
of various precursors with appropriate functional groups for either conventional or
modified Pd-catalyzed Sonogashira reactions to accomplish final chromophores. In brief,
compound 1 was synthesized in one-step fashion by using a more reactive aryl iodide (5)
and bis(methylsilyl)acetylene as the synthons while compound 2 was accomplished
through another recently developed one-step coupling method that involves
decarboxylative processes.[33] Differently, the typical multi-step Sonogashira reaction
was finally found to be the most effective protocol to construct compound 3 instead of
using either the aforementioned one-step cross-coupling conditions after several attempts.
All the new dyes were obtained in acceptable yields and the details for the syntheses of
these model compounds are presented in the Experimental section.
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2.2 Optical properties characterization
2.2.1 One-photon absorption (1PA) and fluorescence-related measurements
Linear absorption and fluorescence spectra of the studied compounds in toluene solutions
(with concentration of 1 10-6 M) are shown in Fig. 2 and the related photophysical data
are collected in Table 1. The 1PA spectra were recorded through a Shimadzu 3501 PC
spectrophotometer and the 1PA-induced fluorescence spectra were measured utilizing a
Jobin-Yvon FluoroMax-4 spectrometer. All these chromophores exhibit intense linear
absorption in the UV-Vis region with the lowest-energy peaks located at 428 nm for 1 (
1
1
1
1
~6.11 104 cm M ), 445 nm for 2 ( ~1.50 105 cm M ), and 466 nm for 3 (
1
1
~6.83 104cm M ), respectively. These compound solutions also emit intense
fluorescence under the irradiation of a common UV lamp with blue-greenish color for 1
and 2 and green-yellowish color for 3, which is in agreement with the measured emission
spectra as illustrated in Fig. 2(b).
The photostability of the studied dye compounds in solution phase was tentatively
evaluated by continuously monitoring the decay of the fluorescence intensity during the
course of UV-irradiation. We have utilized a set of UV-lamps as the excitation light source
to provide ~350 nm radiation with total output of 120 W for this experiment. The detailed
experimental arrangement is described in the Supporting Information. A commercialized
laser dye, Coumarin 307, was selected as the reference and the results are illustrated in Fig
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3. It can be seen that during the first 1-hour consecutive exposure of UV-light, both
compounds 1 and 2 retain >97% of their original emission intensity while compound 3
exhibits an observable decline (~15%) of the original fluorescence intensity. Prolonged
irradiation further deteriorates the emissive property of these model chromophore
solutions and it is found that compound 3 shows a larger fluorescence intensity decrease
(~55%) whilst those of 1 and 2 are kept within ~12% after 3 hours of UV-light exposure.
On the other hand, it is also found that compounds 1 and 2 possess better photostability
compared to Coumarin 307. Overall, model compounds 1 and 2 manifest fairly good
resistance of photo-damage which is comparable to that of Coumarin 307 and is superior
to that of compound 3 under our experimental condition. We suspect that the weak
photo-resistance of compound 3 may be originated from the pyridine unit but the
clarification of this issue is currently beyond the scope of this work.
2.2.2 Two-photon-excited fluorescence (2PEF) emission properties
The studied model chromophores manifest strong two-photon-excited upconversion
emission which can be easily sighted even under low power of excitation by an unfocused
femtosecond laser beam at ~ 800 nm. Fig. 4(a) illustrates the 2PA-induced fluorescence
spectra of the model chromophores 1-3. The sample solutions were freshly prepared at
concentration of 1 10-4 M in toluene for this measurement and the excitation source
utilized for this two-photon induced fluorescence study is from a mode-locked Ti:Sapphire
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laser (Chameleon Ultra II, Coherent) which delivers ~140 fs pulses with the repetition rate
of 80 MHz and the beam diameter of 2 mm. The intensity level of the excitation beam was
carefully controlled to avoid the saturation of absorption and photodegradation. Besides,
the relative position of the excitation beam was adjusted to be as close as possible to the
wall of the quartz cell (10mm 10mm cuvette) so that only the emission from the
front-surface of the sample was recorded in order to minimize the re-absorption or
inner-filter effect. It is noted from Fig. 4(a) that for each model chromophore the shape
and spectral position of the measured 2PA-induced emission is basically identical to its
corresponding 1PA-induced fluorescence band in Fig. 2(b), which implies that in our dye
system the radiative relaxation processes occurred within the studied samples are from the
same final excited states regardless of the excitation method.
The power-squared dependence of the 2PA-induced fluorescence intensity on the
excitation intensity of the studied fluorophores was also examined. Figs. 4(b)-(d) are
logarithmic plots of the measured data and the results (i.e. the fitting slope 2) validate
that 2PA process is responsible for the observed up-converted fluorescence emissions in
all cases.
The temporal behavior and lifetime of 2PA-induced fluorescence of the same sample
solutions were also probed through time-correlated single photon counting (TCSPC)
technique and the same femtosecond laser system (vide supra) was employed as the
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excitation light source for this experiment. The measured fluorescence decay curves of the
studied compound solutions are depicted in Fig. 4(e). Theoretical fitting of each decay
curve by single-exponential dependence has revealed that these model chromophores
possess fluorescence lifetime ranging from 1.2 ns to 3.0 ns and these data are collected in
Table 1.
2.2.3 Degenerate two-photon absorption spectra measurement
The 2PA spectra of the studied model chromophores was delineated in the spectral region
of 700 1040 nm through degenerate two-photon-excited fluorescence (2PEF) technique
using the aforementioned femtosecond laser system as the excitation light source.[34, 35]
Fig. 5 shows the measured degenerate 2PA spectra of these three model compounds in
toluene (at the concentration of 1 10-4 M) and the combined photophysical data are
summarized in Table 1. It is notable that all the studied compounds exhibit detectable 2PA
within the entire dynamic tuning range of the employed Ti:sapphire oscillator. To be more
specific, compound 1 exhibits a broad 2PA band with two local maxima located around
730 nm and 830 nm while compound 2 possesses very similar dispersion of 2PA but with
prominently elevated magnitude of 2PA particularly around 730 nm in comparison to 1,
which indicates that the incorporation of the indenoquinoxaline moiety into this dye
system has a positive contribution on the molecular 2PA especially at shorter wavelength
region. On the other hand, compound 3 also manifests widely dispersed 2PA and the local
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maximum is located around 930 nm.
2.3 Discussion of results
From the measured photophysical properties of the studied model chromophores in the
present work, some features can be noted:
(1) Based on the observed linear absorption behaviour of the studied model
chromophores and use compound 1 as a reference, it is noted that compound 2 exhibits
conspicuously promoted linear absorption with nearly identical spectral dispersion to
those of compound 1 whereas compound 3 possesses relatively smaller enhancement of
the linear absorption and a red-shifted lowest-energy band. These features imply that using
a fluorene unit as a part of heterocyclic ring complex in this molecular system (as in the
case of compound 2) provides an effective access to enhance the linear absorptivity
without shifting of the absorption band while the utilization of a pyridine moiety for the
construction of the ring complex (as in the case of compound 3) is useful for expanding
the spectral range of linear absorption by pressing the lowest-energy band
bathochromically.
(2) We originally expected that the acetylene unit utilized in this model
chromophore system as the linkage could be photo-sensitive and consequently weaken the
photo-resistance of these compounds but interestingly, the experimental results suggest
that this functional group is fairly inert toward UV-radiation at ~350 nm. Among these
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three model compounds, 1 and 2 exhibit good photostability which is comparable to that
of Coumarin 307 and this indicates that the structural units employed in these two
compounds are suitable for the development of fluorescence probes that are required for
long-term light-exposure.
(3) Compared to compound 1, the distinctly hyperchromic 2PA of 2 and saliently
red-shifted 2PA of 3 indicate that fluorene can be an effective unit to enhance molecular
2PA with roughly immobilized spectral position of 2PA band whereas the pyridine scaffold
can work as an efficient red-shifter of 2PA without dramatic decrease of 2PA magnitude.
These features are particularly useful for the molecular design when either large 2PA at a
specific spectral range or a red-shifted 2PA band with a fixed cross-section level is
required for various applications.
(4) It has been theoretically revealed that the molecular 2PA is correlated with the
square of effective -electron number of certain studied structures.[36] Although this
approximation rule is not verified yet to be suitable for every class of molecules, we still
can tentatively analyze our experimental results based on this scaling. It is found that the
ratio between the maximum 2PA cross-section values of the studied model compounds
(
max (1) : max (2) : max (3) ~ 1 : ~1.5 : ~1) is close to the ratio between the square of their
2
2
2
2 ). This result implies that the -electrons
N
calculated effective -electron numbers (i.e.
eff
on these model dye compounds should adopt the same mode to contribute to the nonlinear
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response. Since the major structural difference between these model compounds is the
heterocyclic part of molecule, the compound with a ring complex that provides a larger
number of -electrons can be expected to exhibit higher maximum molecular 2PA. On the
other hand, these chromophores are also found to possess a comparable magnitude of
molecular 2PA among the reported highly two-photon active dyes with similar numbers of
-electrons.[36]
(5) Combining the medium-high fluorescence quantum yields and 2PA cross-section
values, chromophores 1-3 exhibit comparable maximum two-photon action cross-section
(
max )[4-6, 10] and this property suggests that such structural motif could be a useful
F
2
approach to build efficient frequency upconverter for imaging-related applications such as
two-photon-excited fluorescence microscopy.
3. Optical power-limiting properties in femtosecond regime
An ideal optical-limiter is expected to possess an intensity-dependent transmission feature
so that the output intensity is always below a certain maximum value, which makes
optical-limiters useful for protecting human eyes and sensors against hazardous sources of
light. This power-control phenomenon is also important for the optical dynamic range
compression in optical-signal processing and nonlinear ultra-fast filtering/reshaping of
optical fiber signals. To evaluate the power-limiting performance of the studied
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fluorophores, we have selected compound 2 as a representative and utilized femtosecond
laser pulses from a regenerative amplifier to probe its optical power-limiting property at ~
800 nm. The sample solution for this study was prepared in toluene with a concentration
of 0.06 M. Figure 6(a) shows the measured data (in open diamond) for nonlinear
transmissivity, Ti, as a function of the laser input power. The influences from the windows
of the cuvette and from the solvent were eliminated by subtracting the transmission data
from a pure toluene-filled cuvette sample. One can see from figure 6(a) that the
intensity-dependent nonlinear transmission of the sample dropped from ~0.91 to ~0.37 as
the input intensity increased from ~24 mW to ~800 mW. In Figure 6(a), the solid-line
represents the theoretical curve predicted by the basic 2PA theory[37, 38] with a best
fitting parameter of = 1.6 cm/GW, where is the 2PA coefficient in the femtosecond
regime of the sample solution. It will be more conceptually clear if this optical
power-limiting behavior is presented in a different way based on the same set of
experimental data as shown in Figure 6(b). In this figure the measured transmitted
intensity data are presented by open circles, and the solid-line is the theoretical curve, with
the same fitting parameter of = 1.6 cm/GW. The diagonal dot-line shows the behavior
for a medium without nonlinear absorption for comparison and one can see that the
measured input-output curve starts to deviate from the linear transmission (diagonal
dot-line) at low pumping power, and rapidly approach larger values of this deviation as the
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excitation power levels up. Moreover, the 2PA cross-section value of this model
compound was calculated to be ~1100 GM from the performed optical-power-limiting
experiment, which is very close to the result obtained from 2PEF method within
experimental uncertainty and this validates that 2PA should be the major cause for the
observed upconverted emission and optical-power restriction in this chromophore system.
4. Conclusion
A series of bisarylacetylene-based chromophores with functionalized quinoxalinoid units as
the aryl substituents were synthesized and characterized for their two-photon-related
properties in the femtosecond regime by using both two-photon-excited fluorescence and
nonlinear transmission techniques as the probing tools. The experimental results have shown
that these model molecules manifest strong and widely dispersed 2PA in the near-IR region.
Tentative analysis of the structure-property relationship based on the structure of compound
1 has revealed that the involvement of the fluorene unit in the quinoxalinoid ring complex
will enhance both linear absorptivity and molecular 2PA while the incorporation of a
pyridine moiety will result in bathochromic shift of 2PA band without deleterious effect on
the magnitude of 2PA. Combined with the intrinsic medium-high quantum yields of
fluorescence, these model fluorophores are found to possess comparable two-photon action
cross-section values with chromophores of similar size. It is also observed that compounds 1
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and 2 possess superior photostability against UV-radiation at a wavelength of ~350 nm.
Furthermore, it is demonstrated that model compound 2 could be a potential optical
power-limiter working in the near-IR region to confront ultra-short laser pulses.
4. Experimental section
4.1 General
All commercially available reagents for the preparation of the intermediates and targeted
chromophores were obtained from Acros Organics or Alfa Aesar and were used as received,
1
unless stated otherwise. THF was distilled from sodium benzophenone ketyl. H-NMR and
13C-NMR spectra were recorded either at 200 or 300 MHz spectrometers and referenced to
TMS or residual CHCl3. The representative numbering of carbon and hydrogen atoms on
each intermediate and model chromohopres for the NMR signal assignment are
systematized and illustrated in the Supporting Information. High-resolution mass
spectroscopy (HRMS) was conducted by using a Waters LCT ESI-TOF mass spectrometer.
MALDI-TOF MS spectrum was obtained on a Voyager DE-PRO mass spectrometer
(Applied Biosystem, Houston, USA).
4.2 Synthesis
In Schemes 1, compounds 4, 6 and 7 were obtained by following established
procedures.[39] For the synthesis of other key intermediates (compounds 5, 8 and 9) and the
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targeted model compounds (1-3), a series of functionalization steps starting from
compounds 4, 6, and 7 have been conducted and are presented as the following:
4.2.1
7,7'-(6-Iodoquinoxaline-2,3-diyl)bis(9,9-dihexyl-N,N-diphenyl-9H-fluoren-2-amine)
(5)
To a mixture of compound 4 (2 g; 1.05 mmol) in dry dioxane (8 mL) was added CuI
(0.045 g; 0.25 mmol), NaI (0.54g; 3.64 mmol), and N,N-dimethylethylenediamine (0.54g;
0.74 mmol). The resulting mixture was stirred at 110 C under Ar atmosphere for 24 hours.
After cooling to the room temperature, NH4OH(aq) (~50mL) was added to the reaction
mixture. The above solution was then extracted with ethyl acetate (30 mL 3) and the
organic layer was collected and dried over MgSO4(S). After removing the solvent, the
crude product was purified through column chromatography on silica gel using
THF-hexane (1:10) as the eluent to give the final purified product as gray power with
yield of 91.2% (1.89 g). 1H-NMR (300 MHz, CDCl3) δ: 8.64~8.63 (d, J = 1.8 Hz, 1H, HF),
8.00~7.99 (dd, J1 = 8.7Hz, J2 = 2.1 Hz, 1H, HB), 7.92~7.89 (d, J = 8.7 Hz, 1H, HC),
7.57~7.50 (m, 8H, H9, H12, H13, H15), 7.29~7.24 (m, 8H, H2), 7.14~7.08 (m, 10H, H3, H6),
7.03~7.00 (m, 6H, H1, H8), 1.75~1.60 (m, 8H, Hf), 1.14~1.03 (m, 24H, Hc, Hd, He),
13
0.83~0.77( m, 12H, Ha), 0.64-0.62 (s, 8H, Hb) C-NMR (75 MHz, CDCl3, tentative
assignments based on calculated values) δ: 154.44 (CG), 154.34 (CH), 152.55 (C5), 150.52
(C16), 147.82 (C4), 147.51(C11), 141.80 (C7), 141.53 (CD), 141.04 (CE), 140.15 (C10),
136.90 (CF), 136.82 (CC), 135.43 (C15) 135.27 (C15'), 130.26 (C9) , 129.49 (C12), 129.08
(C3), 124.26 (C13), 123.83 (C2), 123.29 (C14), 122.53 (C1), 120.84 (C6), 118.94 (C8, CB),
95.09 (CA), 55.01 (Cg), 40.03 (Cf), 31.46 (Ce), 29.50 (Cd), 23.77 (Cc), 22.53 (Cb), 14.01
(Ca) HRMS-FAB (m/z): M+ calcd for C82H87IN4 1254.5975, found, 1254.5979.
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4.2.2
7,7'-(7-((Trimethylsilyl)ethynyl)pyrido[2,3-b]pyrazine-2,3-diyl)bis(9,9-dihexyl-N,N-di
phenyl-9H-fluoren-2-amine) (8)
To a mixture of compound 7 (1 g; 0.826 mmol) in dry THF (10 mL) was added
PdCl2(PPh3)2 (0.03 g; 0.049 mmol), CuI (0.02g; 0.083 mmol), trimethylsilyl acetylene
(0.12 g; 1.24 mmole), and i-Pr2NH (2.5 mL). The resulting solution was stirred at 90 C
under Ar atmosphere for 12 hours. After cooling to the room temperature, H2O (~100mL)
was added to the reaction mixture. The above solution was then extracted with ethyl
acetate (30 mL 3) and the organic layer was collected and dried over MgSO4(S). After
removing the solvent, the crude product was purified through column chromatography on
silica gel using THF-hexane (1:15) as the eluent to give the final purified product as
orange powder with yield of 91.1% (0.92 g). 1H-NMR (300 MHz, CDCl3) δ: 9.18~9.17 (d,
J = 2.1 Hz, 1H, HB), 8.61~8.60 (d, J = 2.1 Hz, 1H, HE), 7.84 (s, 1H, H15), 7.74~7.71 (d, J
= 7.8 Hz, 1H, H12), 7.65 (s, 1H, H12’), 7.62~7.47 (m, 5H, H13, H13’, H9, H9’, H15’),
7.31~7.26 (m, 8H, H2), 7.17~7.12 (m, 10H, H3, H6), 7.07~7.03 (m, 6H, H1, H8), 1.88~1.73
(m, 8H, Hf, Hf’), 1.09~1.07 (m, 24H, Hc, Hc’, Hd, Hd’, He, He’), 0.85~0.82(m, 12H, Ha, Ha’),
0.68~0.67 (m, 8H, Hb, Hb’), 0.38 (s, 9H, hydrogens of the TMS group); 13C-NMR (75
MHz, CDCl3, tentative assignments based on calculated values) δ: 156.89 (CG), 155.90
(CF), 155.69 (CB), 152.71 (C16), 152.58 (C16’), 150.67 (C5), 150.51 (C5’), 148.72 (CC),
147.79 (C4), 142.51 (C11), 142.29 (C11’), 139.93 (C7), 136.45 (C10), 136.05 (C10’), 135.16
(CD), 135.10 (C14), 134.93 (C14’), 129.70 (C15), 129.12(C2), 128.89(CE), 124.43 (C12) ,
124.29 (C12’) , 123.89 (C3), 123.25 (C9), 122.60 (C1), 121.16 (CA), 120.98 (C6), 120.91
(C6’), 119.11 (C13), 118.83 (C8), 118.54 (C13’), 100.97 (CH), 100.78 (CI), 55.20 (Cg), 55.02
(Cg’), 40.17 (Cf), 40.03 (Cf’), 31.56 (Ce), 31.50 (Ce’), 29.52 (Cd, Cd’), 23.82 (Cc), 23.75
(Cc’), 22.57 (Cb, Cb’), 14.04 (Ca, Ca’), -0.27 (carbons of the TMS group). MALDI-TOF
(m/z): M+ calcd for C86H95N5Si 1226.7935, found, 1226.8293.
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4.2.3
7,7'-(7-Ethynylpyrido[2,3-b]pyrazine-2,3-diyl)bis(9,9-dihexyl-N,N-diphenyl-9H-fluor
en-2-amine) (9)
To a mixture of compound 8 (0.9 g; 0.798 mmol) in THF/MeOH (10 mL/4 mL) was added
KOH (0.08 g; 1.48 mmol), and the resulting solution was stirred at room temperature
under Ar atmosphere for 4 hours. After cooling to the room temperature, H2O (~100mL)
was added to the reaction mixture. The above solution was then extracted with ethyl
acetate (30mL 3) and the organic layer was collected and dried over MgSO4(S). After
removing the solvent, the crude product was purified through column chromatography on
silica gel using THF-hexane (1:10) as the eluent to give the final purified product as
orange powder with yield of 94.8% (0.79 g). 1H-NMR (300 MHz, CDCl3) δ: 9.16~9.15 (d,
J = 2.1 Hz, 1H, HB), 8.61~8.60 (d, J = 2.1 Hz, 1H, HE), 7.81 (s, 1H, H15), 7.71~7.69 (d, J
= 7.8 Hz, 1H, H12), 7.62~7.59 (d, J = 7.8 Hz, 1H, H12’), 7.56~7.43 (m, 5H, H13, H13’, H9,
H9’, H15’), 7.27~7.22 (m, 8H, H2), 7.13~7.07 (m, 10H, H3, H6), 7.04~6.99 (m, 6H, H1, H8),
3.42 (s, 1H, hydrogen on terminal alkyne; -C≡H), 1.84~1.68 (m, 8H, Hf, Hf’), 1.12~1.02
(m, 24H, Hc, Hc’, Hd, Hd’, He, He’), 0.81~0.78( t, J = 7.2 Hz, 12H, Ha, Ha’), 0.64-0.62 (m,
8H, Hb, Hb’); 13C-NMR (75 MHz, CDCl3, tentative assignments based on calculated values)
δ: 157.23 (CG), 156.07 (CF), 155.53 (CB), 152.72 (C16), 152.58 (C16’), 150.72 (C5), 150.53
(C5’), 148.95 (CC), 147.78 (C4), 147.71 (C4’), 142.60 (C11), 142.37 (C11’), 140.58 (C7),
136.33 (C10), 135.93 (C10’), 135.09 (C14), 134.80 (CD), 129.71 (C15), 129.13(C2),
128.85(CE), 124.42 (C12) , 124.26 (C12’) , 123.90 (C3), 123.25 (C9), 122.61 (C1), 121.00
(C6), 120.93 (C6’), 120.07 (CA), 119.18 (C13), 118.82 (C8), 118.54 (C13’), 82.57(CH), 79.94
(CI), 55.21 (Cg), 55.02 (Cg’), 40.15 (Cf), 40.01 (Cf’), 31.55 (Ce), 31.49 (Ce’), 29.52 (Cd, Cd’),
23.82 (Cc), 23.73 (Cc’), 22.57 (Cb, Cb’), 14.04 (Ca, Ca’). HRMS-FAB (m/z): M+ calcd for
C83H87N5 1153.6961, found, 1153.6965
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4.2.4 Compound 1
To a mixture of compound 5 (1.58 g; 1.256 mmol) and 1,2-bis(trimethylsilyl)ethyne (0.1 g;
0.59 mmol) in dry DMF (8 mL) was added CuCl (0.03 g; 0.299 mmol), and Pd(PPh3)4
(0.034 g; 0.0293 mmol). The resulting mixture was stirred at 110 C under Ar atmosphere
for 8 hours. After cooling to the room temperature, H2O (~100mL) was added to the
reaction mixture. The above solution was then extracted with ethyl acetate (30 mL 3) and
the organic layer was collected and dried over MgSO4(S). After removing the solvent, the
crude product was purified through column chromatography on silica gel using
THF-hexane (1:20) as the eluent to give the final purified product as orange powder with
yield of ~50.7% (0.69 g). 1H-NMR (300 MHz, CDCl3) δ: 8.48-8.47 (d , J = 3.0 Hz,2H, HF),
8.23~8.20 (d, J = 9.0 Hz, 2H, HC), 7.97~7.93 (dd, J1 = 3.0 Hz, J2 = 9.0 Hz, 2H, HB),
7.62~7.54 (m, 16H, H9, H12, H13, H15), 7.30~7.25 (m, 16H, H2), 7.15~7.11 (m, 20H, H3,
H6), 7.06~7.01(m, 12H, H1, H8), 1.78~1.76 (m, 16H, Hf), 1.14~1.06 (m, 48H, Hc, Hd, He),
0.84~0.80( m, 24H, Ha), 0.66(s, 16H, Hb); 13C-NMR (75 MHz, CDCl3, tentative
assignments based on calculated values) δ: 154.93 (CG), 154.49 (CH), 152.76 (C5), 150.68
(C16), 148.00 (C4), 147.68(C11), 141.99 (C7), 141.10 (CD), 140.97 (CE), 140.97 (C10),
137.17 (CF), 137.01 (CC), 135.85 (C15), 135.50 (C15'), 132.56 (C9) , 131.36 (C12), 130.39
(CA), 129.60 (C3), 124.51 (C13), 124.01 (C2), 123.47 (C14), 122.71 (C1), 121.04 (C6),
119.12 (CB C8), 91.61 (acetylene carbons), 55.20 (Cg), 40.24 (Cf), 31.66 (Ce), 30.43 (Cd),
23.97 (Cc), 22.73 (Cb), 14.22 (Ca); MALDI-TOF(m/z): M+ calcd for C166H174N8 2281.2732,
found, 2281.2795.
4.2.5 Compound 2
To a mixture of compound 6 (1 g; 0.683 mmol) and acetylenedicarboxylic acid (0.04 g;
0.341 mmol) in DMSO (8 mL) was added 1,4-bis(diphenylphosphino) butane (2.9 mg;
0.068 mmol), and PdCl2(PPh3)2 (2.4 mg; 0.034 mmol). The resulting mixture was stirred at
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110 C under Ar atmosphere for 8 hours. After cooling to the room temperature, H2O
(~100mL) was added to the reaction mixture. The above solution was then extracted with
ethyl acetate (30 mL 3) and the organic layer was collected and dried over MgSO4(S)
.
After removing the solvent, the crude product was purified through column
chromatography on silica gel using THF-hexane (1:15) as the eluent to give the final
purified product as orange powder with yield of ~67% (0.88 g). 1H-NMR (300 MHz,
CDCl3) δ: 8.46 (s, 2H, HH), 8.12(s, 2H, HK), 7.95~7.90 (m , 2H, HD), 7.67~7.51 (m, 16H,
H9, H12, H13, H15, HE, HB), 7.27~7.24 (m, 16H, H2), 7.13~7.09 (m, 20H, H3, H6),
7.03~6.98 (m, 12H, H1, H8), 2.19~2.16 (m, 8H, Hf), 1.76~1.74 (m, 16H, Hf’), 1.11~1.05
(m, 72H, Hc, Hc’, Hd, Hd’, He, He’), 0.82~0.68(m, 60H, Ha, Ha’, Hb, Hb’); 13C-NMR(75 MHz,
CDCl3, tentative assignments based on calculated values) δ: 153.49 (C5, CN), 153.05 (CM),
152.59 (C16), 151.48 (CL), 150.59 (CA), 150.51(CF), 147.90 (C4), 147.41 (C11), 143.21
(CG), 141.54 (CJ), 141.50 (CI), 139.99 (C7), 137.52 (CK), 137.43(CH), 135.53 (C10), 131.12
(CB),129.13 (C2), 128.93(C12), 126.25(C15), 125.49 (CE), 124.37 (C14), 123.84 (C3), 123.37
(C13), 122.52 (C1), 122.32 (CD), 121.13(C8), 120.83 (C6), 119.09 (C9), 118.69 (CC), 91.24
(acetylene carbons), 55.29(Cg), 55.06 (Cg’), 40.12 (Cf), 31.55 (Ce), 29.77 (Cd), 29.59(Cd’),
23.84 (Cc, Cc’), 22.62 (Cb, Cb’), 14.09 (Ca), 13.97(Ca’); MALDI-TOF(m/z): [M+1]+ calcd
for C204H230N8 2795.1472, found, 2795.1548
4.2.6 Compound 3
To a mixture of compound 7 (0.57 g; 0.476 mmol) and compound 9 (0.55 g; 0.476 mmol)
in dry THF (10 mL) was added PdCl2(PPh3)2 (0.02 g; 0.029 mmol), CuI (9.1 mg; 0.048
mmol), and i-Pr2NH (2.5 mL). The resulting solution was stirred at 90 C under Ar
atmosphere for 12 hours. After cooling to the room temperature, H2O (~100mL) was added
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to the reaction mixture. The above solution was then extracted with ethyl acetate (30 mL 3)
and the organic layer was collected and dried over MgSO4(S). After removing the solvent,
the crude product was purified through column chromatography on silica gel using
THF-hexane (1:5) as the eluent to give the final purified product as orange powder with
yield of 72.2% (0.78 g). 1H-NMR (300 MHz, CDCl3) δ: 9.33~9.32 (d, J = 2.1 Hz, 2H, HB),
8.75~8.75 (d, J = 2.1 Hz, 2H, HE), 7.84 (s, 2H, H15), 7.74~7.72 (dd, J1 = 7.8 Hz, J2 = 1.2Hz,
2H, H12), 7.63~7.46 (m, 12H, H13, H13’, H9, H9’, H12’, H15’), 7.27~7.23 (m, 16H, H2),
7.14~7.11 (m, 20H, H3, H6), 7.04~6.98 (m, 12H, H1, H8), 1.83~1.72 (m, 16H, Hf, Hf’),
1.11~1.01 (m, 48H, Hc, Hc’, Hd, Hd’, He, He’), 0.80~0.77( m, 24H, Ha, Ha’), 0.64 (s, 16H, Hb,
13
Hb’); C-NMR (75 MHz, CDCl3, tentative assignments based on calculated values) δ:
157.46 (CG), 156.36 (CF), 155.31 (CB), 152.87 (C16), 152.73 (C16’), 150.86 (C5), 150.71
(C5’), 149.23 (CC), 147.90 (C4), 142.82 (C11), 142.56 (C11’), 140.15 (C7), 136.46 (C10),
136.06 (C10’), 135.21 (CD), 135.16 (C14), 135.07 (C14’), 129.87 (C15), 129.24 (C2), 125.56
(CE), 124.58 (C12) , 124.41 (C12’) , 124.04 (C3), 123.36 (C9), 122.75 (C1), 121.07 (C6),
120.31 (CA), 119.29 (C13), 118.92 (C8), 118.71 (C13’) , 90.99 (acetylene carbons), 55.34
(Cg), 55.17 (Cg’), 40.28 (Cf), 40.16 (Cf’), 31.67 (Ce), 31.61 (Ce’), 29.64 (Cd, Cd’), 23.95 (Cc),
23.88 (Cc’), 22.68 (Cb, Cb’), 14.15 (Ca, Ca’); MALDI-TOF(m/z): [M+1]+ calcd for
C165H172N10 2284.2556, found, 2284.2629.
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Acknowledgements
We acknowledge the financial support from National Science Council (NSC), Taiwan.
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Captions for Figures, Schemes, and Tables
Figure 1. Molecular structures of the studied model chromophores.
Scheme 1. Synthetic procedures for the precursors and final model chromophores.
Figure 2. Linear absorption and fluorescence spectra of the studied model compounds.
Figure 3. Tentative evaluation of photostability: Fluorescence intensity change of the
studied chromophores and Coumarin 307 under consecutive irradiation of UV-light at
~350 nm.
Figure 4. (a) Normalized two-photon excited emission spectra of fluorophores 1-3 in
toluene at 1 10-5M; (b), (c), and (d) are the logarithmic plots of power-squared
dependence of the 2PA-induced fluorescence intensity on the input intensity of these
compounds in toluene; (f) Normalized two-photon excited fluorescence decay curves of
these chromophores in toluene.
Figure 5. Measured degenerate two-photon absorption spectra of model chromophores
1-3 by 2PEF method in toluene solution at 1 10-4M (with experimental error ~15%).
Figure 6. (a) Measured nonlinear transmission of a 1-cm path-length compound 2 solution
in toluene (0.04 M) as a function of the input intensity of the ~800 nm laser beam; (b)
Measured output energy as a function of the input energy based on the same sample
solution. The solid curves are the theoretical curve with a best fitting parameter of = 1.6
cm/GW. ( is the 2PA coefficient)
Table 1. Photophysical properties of the studied model chromophores 1-3 in toluene.
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Table 1. Photophysical properties of the studied model chromophores 1-3 in toluene.a
h
Neπff
abs
max
em
max
/nmb
λ
/nmd
e
ΦF
δ2max/GMg
Φ
Fδ2max/GMi
logεc
τ 2PA-FL /nsf
λ
305
~2000
363
428
4.79
507
0.56
0.63
0.70
1.8
37.95
46.48
37.95
~1120
1
2
3
(at 730 nm)
310
361
445
~3100
5.18
4.83
487
569
1.2
3.2
~1950
~1330
(at 750 nm)
299
367
466
~1900
(at 930 nm)
aConcentration was 1×10-6M and 1×10-4M for 1PA-related and 2PA-related measurements,
b
c
respectively; One-photon absorption maximum; Molar absorption coefficient of the lowest-energy
absorption peak; d1PA-induced fluorescence emission maximum; eFluorescence quantum efficiency; f
g
2PA-induced fluorescence lifetime; Maximum 2PA cross-section value (with experimental error
h
~±15%); 1GM = 1 × 10-50 cm4 s/photon-molecule; Effective π-electron numbers (ref. 36);
hTwo-photon action cross-section value.

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N
C6H13
C6H13
N
C6H13
N
N
C6H13
Ar
N
Ar
C6H13
C6H13
N
N
C6H13
C6H13
N
N
N
N
N
N
N
N
;
Ar
;
N
N
C6H13
C6H13
1
2
3
Figure 1. Molecular structures of the studied model chromophores.

ACCEPTED MANUSCRIPT
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
8.0
(a)
(b)
1
2
3
1
2
3
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.0
300 400 500 600 700 800
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Figure 2. Linear absorption and fluorescence spectra of the studied model
compounds.

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1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Compound 1
Compound 2
Compound 3
Coumarin 307
0
20
40
60
80
100 120 140 160 180
Time (min)
Figure 3. Tentative evaluation of photostability: Fluorescence intensity change of the
studied chromophores and Coumarin 307 under consecutive irradiation of UV-light at
~350 nm.