Synthesis and characterization of highly soluble two-photon-absorbing chromophores with multi-branched and dendritic architectures

Article abstract of DOI:10.1002/ejoc.201001165

A set of novel multi-polar compounds composed of four fluorene-based derivatives with systematic alteration of the molecular structure was synthesized and their nonlinear optical properties in the femtosecond and nanosecond time domain were examined. Preliminary experimental results show that the two-photon activities of the model compounds are connected to structural parameters, such as the number of peripheral electron-donating groups and/or the size of the π-domain in a dye molecule. It is also found that these model chromophores possess large nonlinear attenuation under the irradiation of laser pulses working at nanosecond regimes, indicating that these compounds may have strong two-photon-assisted excited-state absorption within the studied spectral region. Effective optical-power-limiting behaviors of the dendritic fluorophores were also demonstrated to show that such dye molecules can be potential materials for use as broadband and rapid-responsive optical limiters, especially against laser lights with longer pulses.

Full text of DOI:10.1002/ejoc.201001165

FULL PAPER
DOI: 10.1002/ejoc.201001165
Synthesis and Characterization of Highly Soluble Two-Photon-Absorbing
Chromophores with Multi-Branched and Dendritic Architectures
Tzu-Chau Lin,*[a] Wei-Lun Lin,[a] Chih-Ming Wang,[a] and Chih-Wei Fu[a]
Keywords: Chromophores / Dendrimers / Two-photon absorption / Optical power limiting
A set of novel multi-polar compounds composed of four fluor-
ene-based derivatives with systematic alteration of the mo-
lecular structure was synthesized and their nonlinear optical
properties in the femtosecond and nanosecond time domain
were examined. Preliminary experimental results show that
the two-photon activities of the model compounds are con-
nected to structural parameters, such as the number of pe-
ripheral electron-donating groups and/or the size of the π-
domain in a dye molecule. It is also found that these model
chromophores possess large nonlinear attenuation under the
irradiation of laser pulses working at nanosecond regimes,
indicating that these compounds may have strong two-pho-
ton-assisted excited-state absorption within the studied spec-
tral region. Effective optical-power-limiting behaviors of the
dendritic fluorophores were also demonstrated to show that
such dye molecules can be potential materials for use as
broadband and rapid-responsive optical limiters, especially
against laser lights with longer pulses.
parameters, such as intramolecular charge-transfer efficiency
and/or the effective size of the π-conjugation domain within
1. Introduction
Two-photon absorption (2PA) processes occur when a a molecule.[4–10] Recently, it has been experimentally shown
molecule is promoted from the ground state to an excited that multipolar chromophores could lead to highly efficient
state through simultaneous absorption of two photons. Al- multi-photon absorption,[5d–5f,5i,6,7d,8a–8b,8f,9c,9e–9h,11,12] while
though the theory of this nonlinear optical phenomenon retaining linear transparency over a wide spectral range;[13]
was predicted by Maria Göppert-Mayer in 1931,[1] experi- this is a desirable feature especially for the broadband op-
mental evidence was obtained about thirty years later when tical limiting applications based on 2PA. In general, fluoro-
up-converted emission from the media was observed under phores with octupolar and/or higher-polar characters are
irradiation of laser light with wavelengths far from the lin- composed of branched or dendritic backbones. From the
ear absorption bands of the studied media.[2] Over the past viewpoint of molecular design, branched architecture not
fifteen years, the availability of high peak power lasers has only provides a way to incorporate several 2PA-enhancing
created the momentum to explore two-photon technologies. parameters into a single molecular system, but also allows
Many promising applications in the emerging field of pho- material chemists to optimize a dye molecule to simulta-
tonics and biophotonics based on 2PA have been proposed, neously combine various expected characteristics for spe-
including optical power limiting, frequency up-converted cific purposes. Furthermore, branched skeletons are poten-
lasing, 3D data storage, 3D microfabrication, nondestruc- tial building blocks for constructing dendritic and/or super-
tive bioimaging and tracking, and two-photon-assisted pho- molecular structures, which are suggested as possible ways
todynamic therapy.[3] To be of use in these applications, or- to probe the fundamental limits of molecular 2PA.[14] In the
ganic compounds that manifest strong 2PA within a specific search for well-defined strategies that can be used to gener-
spectral region are consequently in great demand. The ac- ate highly efficient 2PA materials, we have been interested
cumulated knowledge and experience of many research in exploring the 2PA properties of multi-branched and den-
groups to understand the connections between molecular dritic organic structures. In particular, we wanted to sys-
structure and 2PA properties by testing and modeling vari- tematically vary the molecular structures so that the essen-
ous dye systems since the mid-1990s have revealed that mo- tial structural parameters governing the strength of molecu-
lecular 2PA is related to a combination of several structural lar 2PA could be elucidated. In this paper, we report our
studies on degenerate two-photon absorption, up-converted
emission, and effective optical power limiting properties of
[a] Photonic Materials Research Laboratory, Department of
a series of newly synthesized model chromophores with sys-
tematically varied structures by using high peak power IR
laser pulses working in the femtosecond and nanosecond
regimes as probing tools.
Chemistry, National Central University,
Jhong-Li 32001, Taiwan
E-mail: tclin@ncu.edu.tw
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001165.
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Soluble Dendritic Chromophores
based on a combination of four triphenylamine and twelve
diphenoaminofluorenyl units, using vinyl groups as linkers to
form the largest π-framework in this model compound set.
Many efforts have been made to study large two-photon-
2. Results and Discussion
I. Molecular Structures and Syntheses
The chemical structures of the model chromophores
studied in this work are presented in Scheme 1. This model absorbing molecules with either conjugated or non-conju-
compound set contains four multi-branched congeners and gated dendritic frameworks using fluorene and/or stibenoid
the frameworks of these four members are derived by incor- units as the main building blocks.[15] The molecular design
porating fluorene and triphenylamine as the main building of the model compounds used in the present work origi-
units. Among these models, compounds 1 and 2 are three- nated from the simple idea of constructing a set of multi-
branched chromophores, whereas compounds 3 and 4 have branched and dendritic frameworks with systematically al-
dendritic architectures with six and twelve branches, respec- tered molecular structures based on a combination of fluo-
tively. Compound 1 is the smallest model, with triphenyl- rene and di-/tri-arylamine units so that the resulting fluoro-
amine as the central core surrounded by three fluorenyl phores may possess consecutively expanded π-domains and
arms and vinyl groups as the linkers. The generic structure increased multipolar characteristics. On the other hand, the
of compound 2 is the same as that of 1 but end-capped pendent alkyl chains on the fluorenyl moieties in these
by an electron-donating diphenylamino unit on each arm. model chromophores were expected to enhance their solu-
Compared to compound 2, compound 3 has three ad- bility in common organic solvents, which is another impor-
ditional diphenylaminofluorenyl units attached to the cen- tant parameter to be considered in the molecular design of
tral triphenylamine core to form a six-branched dendritic compounds for use in experimental studies and in potential
structure. The molecular structure of model compound 4 is applications.
Scheme 1. Molecular structures of the studied model fluorophores.
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The synthetic procedures used to prepare the model flu- as the major synthons and were obtained in yields of 50
orophores are outlined in Schemes 2 and 3. To obtain the and 40%, respectively. For the synthesis of compound 3,
target chromophores, various synthetic protocols were fol- instead of following the previously utilized one-pot syn-
lowed to prepare key intermediates, which were sub- thetic strategy,[16]
sequently combined in the final coupling reactions. Com-
pounds 1 and 2 were both synthesized through threefold
Heck reaction procedures using compounds 9, 10, and 14
a two-step catalytic process (i.e.,
Scheme 2. Synthetic procedures for the key intermediates (5–9, 11,
and 13). Reagents and conditions: (a) Triphenylamine (1 equiv.), KI
(3 equiv.), KIO3 (3 equiv.) in HOAc, 80 °C, 15 h (76%); (b) Tri-
phenylamine (1 equiv.), tetrabutylammonium tribromide (TBABr3)
(1 equiv.) in CH2Cl2/MeOH, r.t., 24 h (84%); (c) 6 (1 equiv.), KI
(1.35 equiv.), KIO3 (0.67 equiv.) in HOAc, 85 °C, 12 h (80%); (d)
triphenylamine (1 equiv.), POCl3 (25 equiv.) in DMF (23 equiv.),
95 °C, 24 h (40%); (e) 8 (1 equiv.), NaH (6 equiv.), CH3PPh3+I–
(4 equiv.) in THF, reflux, 1 h (70%); (f) 10 (1 equiv.), CuI
(0.05 equiv.), (CH3)2NC2H4NH2 (0.1 equiv.), NaI (2 equiv.) in diox-
ane, 110 °C, 24 h (98%); (g) 11 (2 equiv.), 12 (1 equiv.), [Pd2(dba)3]
(0.03 equiv.), P(tBut)3 (0.06 equiv.), NEt3 (4 equiv.) in toluene,
90 °C, 12 h (52%).
Scheme 3. Synthetic procedures for model chromophores (1–4) and
compound 15. Reagents and conditions: (a) For the synthesis of
compound 1: 9 (1 equiv.), 14 (3.3 equiv.), [Pd(OAc)2] (0.06 equiv.),
P(o-tolyl)3 (0.36 equiv.) in NEt3/MeCN, 110 °C, 48 h (50%); For
the synthesis of compound 2: 9 (1 equiv.), 10 (3.3 equiv.), [Pd-
(OAc)2] (0.06 equiv.), P(o-tolyl)3 (0.36 equiv.) in NEt3/MeCN,
110 °C, 48 h (40%); (b) For the synthesis of compound 3: 5
(1 equiv.), 13 (3.3 equiv.), Bu4NF (4.5 equiv.), [PdCl2(PhCN)2]
(0.15 equiv.) in toluene, 110 °C, 48 h (70%); For the synthesis of
compound 15:
7 (1 equiv.), 13 (2 equiv.), Bu4NF (2 equiv.),
[PdCl2(PhCN)2] (0.1 equiv.) in toluene, 70 °C, 2 h (50%); (c) 9
(1 equiv.), 15 (3.3 equiv.), [Pd(OAc)2] (0.06 equiv.), P(o-tolyl)3
(0.36 equiv.) in NEt3/MeCN, 110 °C, 48 h (25%).
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Soluble Dendritic Chromophores
11Ǟ13Ǟ3) that combined stepwise Heck and cross-cou- Two-Photon-Excited Fluorescence (2PEF) Emission
pling reactions, was adopted;[17] this model dye structure Properties
was obtained in an overall yield of 35%. The synthesis of
model compound 4 started from the preparation of the key
intermediate (i.e., compound 15) through the same two-step
catalytic protocol (i.e., 11Ǟ13Ǟ15) followed by a three-
fold Heck reaction to construct the final dendritic scaffold.
All the final model fluorophores were highly soluble in
of these model chromophores. The sample solutions were
The studied model chromophores manifest strong two-
photon-excited up-conversion emission, which can be easily
observed by the naked eye even under the illumination of a
ca. 790 nm unfocused femtosecond laser beam. Figure 2 (a)
illustrates the normalized 2PA-induced fluorescence spectra
freshly prepared at concentrations of 1ϫ10–4 m in THF for
this measurement, and the excitation source utilized for this
two-photon induced fluorescence study was from a mode-
locked Ti:Sapphire laser (Tsunami-pumped with a Millen-
nia 10W, Spectra-Physics), which generates ca. 80 fs pulses
with a repetition rate of 80 MHz and a beam diameter of
2 mm. The intensity level of the excitation beam was care-
fully controlled to avoid saturation of the absorption and
photodegradation. To minimize the effect of re-absorption
due to the relatively high concentration of the sample solu-
tion used in this measurement, we focused the excitation
beam as close as possible to the wall of the quartz cell
(5ϫ5 mm cuvette) so that only the emission from the sur-
face of the sample solution was collected. It can be seen in
Figure 2 (a) that, for each model chromophore, the shape
and spectral position of the measured 2PA-induced emis-
sion is basically identical to the corresponding 1PA-induced
fluorescence band shown in Figure 1. This result indicates
that, in our dye system, the radiative relaxation processes
that occurred within the studied samples are from the same
final excited states regardless of the excitation process.
common organic solvents, such as hexane, toluene, dichlo-
romethane and tetrahydrofuran (THF). The detailed syn-
theses, including the preparation of the key intermediates
and the final catalytic coupling reactions toward the tar-
geted model compounds, are described in the Exp. Section.
II. Optical Properties
One-Photon Absorption (1PA) and Fluorescence Spectra
Measurement
Linear absorption and fluorescence spectra of the
studied compounds in THF solutions (with concentrations
of 1ϫ10–5 m) are shown in Figure 1. The 1PA spectra were
recorded with a Shimadzu 3501 PC spectrophotometer and
the 1PA-induced fluorescence spectra were measured
with a Jobin–Yvon FluoroMax-3 spectrometer. All these
compounds were found to show strong 1PA within the spec-
tral range of 400–425 nm with increasing absorption inten-
sity (i.e., ε ≈ 0.82ϫ105, 1.26ϫ105, 2.04ϫ105, and
2.78ϫ105 cm–1 m–1 for 1, 2, 3, and 4, respectively). More-
over, the shape and spectral position of the absorption
bands of compounds 2–4 were almost identical, which may
be useful for the design of molecules that can enhance mo-
lecular nonlinear absorption without shifting the major ab-
sorption band. This feature could be important for applica-
tions for which large multi-photon absorbing strength in
specific spectral regions are required. These compound
solutions also emit intense blue-greenish fluorescence under
the irradiation of a common UV-lamp, which is in agree-
ment with the measured emission spectra (see the inset of
Figure 2).
Figure 2. (a) Normalized two-photon excited up-conversion spectra
of fluorophores 1–4 in THF at 1ϫ10–4 m; (b–e) logarithmic plots
of power-squared dependence of the 2PA-induced fluorescence in-
tensity on the input intensity of the model compounds in THF.
The power-squared dependence of the 2PA-induced fluo-
rescence intensity on the excitation intensity of the studied
fluorophores was also examined. Figure 2 (b–e) show the
log–log plots of the measured data and the results (slopes
ca. 2) confirm that a 2PA process is the major cause of the
observed up-converted fluorescence emissions in all cases.
Figure 1. Linear absorption and fluorescence spectra (see the inset)
of compounds 1–4 in the solution phase (concentration was
1ϫ10–5 m in all cases; solvent: THF). The excitation wavelength
was fixed at 400 nm for the fluorescence spectra measurements.
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Table 1. Photophysical properties of the model chromophores in THF.[a]
1PA-Related properties
2PA-Related properties
τ
[ns][e] λ2max/GM[f] τ2PA–FL [ns][g] Neπff[h]
2PA–FL
abs
em
[d]
λ
[nm][b]
logεmax
λ
[nm][c]
ΦF
δ2max/Neπff
max
max
1
2
3
4
402
408
419
423
4.91
5.10
5.30
5.44
489
497
506
519
0.52 1.58
0.76 1.32
0.48 1.39
0.36 1.60
ca. 725
1.58
1.32
1.39
1.60
34.6
37.6
59.2
87.2
ca. 21
ca. 43
ca. 61
ca. 106
ca. 1620
ca. 3500
ca. 9240
[a] Concentration was 1ϫ10–5 m and 1ϫ10–4 m for 1PA-related and 2PA-related measurements, respectively. [b] One-photon absorption
maximum. [c] 1PA-induced fluorescence emission maximum. [d] Fluorescence quantum efficiency. [e] 1PA-induced fluorescence lifetime.
[f] Maximum 2PA cross-section value (experimental error ca. Ϯ15%); 1 GM = 1ϫ10–50 cm4 s/photon. [g] 2PA-induced fluorescence
lifetime. [h] Effective π-electron number.
The temporal behavior and lifetime of the 1PA- and 2PA- Fluorescein (ca. 80 μm in pH 11 NaOH solution) as the
induced fluorescence of the same sample solutions were standard for this experiment.[18] Figure 4 shows the mea-
also probed by using the time-correlated single photon sured degenerate two-photon absorption spectra of these
counting (TCSPC) technique with a highly sensitive photo- model compounds in THF (at concentrations of 1ϫ10–4 m)
multiplier equipped with an accumulating real-time proces- and the combined photophysical data are summarized in
sor as the detection system (PMA-182 and TimeHarp 200, Table 1. It is notable that all these model chromophores ex-
PicoQuant). The same Ti: sapphire laser system (see above) hibit detectable 2PA within the investigated spectral range
was employed for this experiment. The measured fluores- and possess an overall ascending capability of 2PA from
cence decay curves of the studied compound solutions are compound 1 to compound 4.
depicted in Figure 3 on a 10 ns full scale for one- and two-
photon excitation (in the femtosecond regime). Theoretical
fitting of each decay curve by single-exponential depen-
dence revealed that, for each chromophore solution, iden-
tical 1PA-/2PA-induced fluorescence lifetimes were ob-
served and that this phenomenon was independent of the
excitation process. The measured 1PA/2PA-induced fluores-
cence lifetime values for each model compound solution are
listed in Table 1.
Figure 4. Measured degenerate two-photon absorption spectra of
model chromophores 1–4 generated by the 2PEF method in THF
solution at 1ϫ10–4 m (error ca. Ϯ15%).
III. Discussion of Results
From the measured photophysical properties of the
studied compounds in the present work, some features are
notable:
(1) These chromophores exhibit very similar dispersion
behaviors for the 2PA activities within the detection range,
that is, the 2PA cross-section values for each compound ap-
Figure 3. Normalized fluorescence decay curves of the studied
proach higher values at short excitation wavelength ranges
model compound solutions excited by one-photon absorption (at
and drops down monotonically in the long excitation wave-
395 nm) and two-photon absorption (at 790 nm) in THF.
length direction. The observed rising two-photon absorp-
tivities in the short wavelength region implies that the most
accessible two-photon states are higher in energy than those
Degenerate Two-Photon Absorption Spectra Measurement
Changes in the 2PA behaviors of the studied dye mole- of the lowest one-photon allowed states of these com-
cules as a function of wavelength was explored in the spec- pounds. Similar trends have been observed and reported
tral region 700–900 nm by using a degenerate two-photon- previously for other chromophore systems.[4a–4b,5c,5e,6a,7e]
excited fluorescence (2PEF) technique based on a set-up Furthermore, the widely dispersed two-photon activities in
very similar to that reported by Xu and Webb;[18] we used near-IR spectral regime suggests that these model chromo-
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Soluble Dendritic Chromophores
phores could be potential candidates for broadband optical the molecular 2PA behavior. Efforts towards delineating the
power limiters.
structure–nonlinear absorption properties by using quan-
(2) It is observed that consecutive expansion of the mo- tum-mechanical analysis of this chromophore system are
lecular π-structures has led to ascending overall 2PA (from needed, and such work is underway currently.
model chromophores 1–4) as shown in Figure 3. We tenta-
tively collect and discuss some potential structural param- emission spectra (shown in Figures 1 and 2) and the fluo-
eters that may relate to the molecular 2PA as follows: rescence lifetimes (shown in Figure 3) for each model com-
(3) Based on the fact that the measured fluorescence
(a) Using compound 1 as the reference point, the incor- pound are nearly the same for one- and two-photon exci-
poration of peripheral electron-donating groups seems to tations, it may be concluded that, in these cases, the fluores-
play an essential role in the promotion of two-photon ac- cence emission is predominantly from the same singlet state
tivities. It is clear that the molecular 2PA rises for model of each model chromophore.[19] On the other hand, the me-
compound 2 after the attachment of three additional di- dium-high quantum yields observed from these model com-
phenylamino groups to the terminal sites of model com- pounds also indicate that these dyes could be efficient fre-
pound 1.
quency up-converters for biophotonic applications, such as
(b) The structural motif of two identical electron-donat- two-photon-excited fluorescence microscopy.
ing arms at the geminal positions of each olefinic unit to
Although the dynamic tuning range of the pumping
form a six-branched dendritic scaffold (as in compound 3) source limits the spectral probing range at this stage, the
provides an effective way to expand the π-domain of the studied model compounds may possess either higher magni-
molecule, which also leads to a doubling of the molecular tudes of 2PA or additional 2PA bands in the visible region
two-photon absorptivity [i.e., δ2max(3)/δ2max(2) ≈ 2.2]. Further (i.e., shorter than 700 nm). These are desirable features for
expansion of the π-framework from six-branched structure some specific applications utilizing two-photon technol-
(compound 3) to a twelve-branched dendritic skeleton ogies within the visible regime, and further investigations
(compound 4) led to further enhanced two-photon activities on these aspects are needed.
[i.e., δ2max(4)/δ2max(3) ≈ 2.6]. Additionally, it is worth noting
that when the size of the π-conjugation was expanded
from compound 1 to compound 4, both the effective
π-electron number and multi-polar character of the dye
molecule also increased simultaneously in this model
3. Effective Optical Power-Limiting Behavior in
the Nanosecond Regime
chromophore set. The changes in these two properties
are believed lie behind the enhanced molecular tensity-dependent transmission feature so that it can act as
2PA.[5d–5f,5i,6,7d,8a,8b,8f,9c,9e–9h,11,12,14]
a transparent medium when the incident intensity of light
Ideal optical-limiters would be expected to show an in-
(c) From the molecular structures of the model chromo- stays low and, once the input intensity is increased, the me-
phores 2 and 3, it can be reasonably assumed that the π- dium starts to regulate the transmitted output intensity so
framework of each compound may permit unsymmetrical that it is maintained below a certain maximum value before
charge-transfer/redistribution from the molecular termini to any optical saturation or damage occurs. This feature makes
the central part of each molecule under the excitation of optical-limiters useful for protecting human eyes and sen-
light, because both π-systems combine different electron- sors against hazardous sources of light. Additionally, this
donating units (at the peripheral and central positions) with power-limiting phenomenon is also important for optical
uneven electron-pushing strengths within one molecule. Al- dynamic range compression and noise suppression in signal
though at present there is no clear understanding of how processing, as well as in nonlinear ultra-fast filtering/re-
this imbalance of electron-donating strength combines with shaping of optical fiber signals. Compared to other mecha-
the structural arrangement in the cases of compounds 2 and nisms for optical power limiting, such as reverse saturable
3 to influence the molecular two-photon activities, our pre- absorption and nonlinear scattering,[20] the 2PA-based
liminary results show that such structural combinations mechanism provides several promising features, including:
may help to promote the molecular 2PA and, interestingly, (1) high initial transmission (ca. 100%) for weak optical
both of these two model compounds were also found to signals; (2) rapid response to a change in the intensity or
possess medium to strong three-photon absorption (3PA) the peak power of the incident optical signal; and (3) reten-
in the near-IR region.[12]
(d) As the size of π-system is enlarged further to con- through the nonlinear absorption medium.
tion of the optical quality of the input beam after passing
struct the dendritic model compound 4, no deterioration in
Recently, it has been reported that the intensity-depend-
the molecular 2PA was observed, which may suggest that ent 2PA-induced excited-state absorption (2PA-induced
the combination of the above-mentioned structural param- ESA) plays an essential role in the observed large 2PA in
eters in a dendrimeric framework with early generation various organic systems under the irradiation of nanose-
could be a useful approach toward enhanced molecular 2PA cond laser pulses.[21] For this reason, the term “effective
in our dye system.
2PA” is used to describe the apparent 2PA parameter mea-
(e) Another interesting issue that requires further study sured in the nanosecond time domain.[9b,21b] From the view-
with respect to this multi-branched/dendritic model com- point of applications, any medium that exhibits strong ap-
pound set is the importance of structural coplanarity on parent nonlinear absorption covering a wide spectral range
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could be utilized as optical power-attenuators against laser shows the measured average transmittance in the spectral
beams with long pulse widths.[22]
range 700–1000 nm. Although the entire distribution of
It should be noted that the excited-state lifetimes of the ESA as a function of wavelength (i.e., ESA spectra) has not
model chromophore in the present work are in the nano- yet been determined, it should be noted that, under our
second range, which implies that significant excited-state experimental conditions, these model compounds can atten-
population may be retained and consequently undergo ex- uate nanosecond laser pulses within a broad spectral region,
cited-state absorption during excitation by longer laser and especially around 700 nm.
pulses. We have used nanosecond laser pulses to investigate
The attenuation of the incident laser pulses based on
the effective power-limiting performance of these model these sample solutions can also be presented as shown in
chromophores. In these experiments, the nonlinear absorb- Figure 5 (b). In this experiment, the local intensity within
ing medium was a 1-cm path-length solution of the studied this sample solution was adjusted by a set of neutral-density
model fluorophore in THF with a concentration of 0.02 m. filters. Laser pulses with wavelengths at which each model
For the probing laser light source, a tunable nanosecond compound possesses the greatest power-restriction were se-
laser system (an integrated Q-switched Nd:YAG laser and lected. It can be readily seen that, compared to other mem-
OPO: NT 342/3 from Ekspla) was employed to provide ap- bers of this model compound set, chromophore 4 exhibits
proximately 6 ns laser pulses with controlled average pulse the most rapid deviation from the linear attenuation behav-
energies of approximately 1 mJ and repetition rates of ior [designated by the diagonal line in Figure 5 (b)] and thus
10 Hz. The laser beam was only slightly focused onto the represents the best power-suppressing material. This initial
center of the sample solution to obtain a nearly uniform finding demonstrates the potential for using this model flu-
laser beam radius within the whole cell path length. The orophore in broadband power-suppression applications
transmitted laser beam from the sample cell was detected that operate in the nanosecond time-scale.
by an optical power/energy meter with a large detection
area of approximately 25 mm in diameter. Figure 5 (a)
4. Conclusion
We have synthesized a novel, multi-polar model com-
pound set composed of four fluorene-based chromophores
with systematic structural modifications and characterized
their nonlinear optical properties in the femtosecond and
nanosecond regimes. The experimental results show that
compounds with larger π-frameworks manifest widely dis-
persed and strong two-photon absorption in the near-infra-
red region. It was also found that these model chromo-
phores possess strong nonlinear attenuation when exposed
to irradiation of laser pulses working at the nanosecond
regime, indicating that these model compounds may have
strong two-photon-assisted excited-state absorption within
the studied spectral region. Effective optical-power-limiting
behaviors of the dendritic fluorophores also demonstrated
that such dye molecules have the potential to be used as
broadband, rapid-response optical limiters, especially when
used with laser lights with longer pulses.
Experimental Section
General: All commercially available reagents were purchased from
Aldrich Chemical Co. and were used as received, unless stated
otherwise. 1H NMR and 13C NMR spectra were recorded with
either 200 or 300 MHz spectrometers and were referenced to TMS
or residual CHCl3. High-resolution mass spectroscopy (HRMS)
were conducted with a Waters LCT ESI-TOF mass spectrometer.
MALDI-TOF MS were obtained with a Voyager DE to PRO mass
spectrometer (Applied Biosystem, Houston, USA).
Figure 5. (a) Measured transmission spectra of the modal com-
pounds as 0.02 m solutions in THF (1 cm path-length). The energy
of the input laser pulses was controlled at ca. 1 mJ for every data
point; (b) Measured optical-power-attenuation curves at wave-
lengths with best power-limiting performance based on the same
sample solutions.
Photophysical Methods: All the linear optical properties of the
model compounds were measured with the spectrometers described
above and the detailed experimental conditions as well as the op-
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Eur. J. Org. Chem. 2011, 912–921

Soluble Dendritic Chromophores
tical set-up for nonlinear optical property investigations are de-
scribed in the Supporting Information.
14.04 ppm. HRMS-FAB: calcd. for C37H42IN [M]+ 627.6405;
found 627.6423.
Synthesis: In Schemes 2 and 3, compounds 8–10, 12, and 14 were
synthesized by following the established literature processes,[9a,23–27]
with overall yields of approximately 35, 90, 40, 80, and 65% for
compounds 8, 9, 10, 12, and 14, respectively. The experimental de-
tails for the syntheses of other key intermediates 5–7, 11, 13, and
15 and the model compounds 1–4 are presented here.
7-{1-[2-(Diphenylamino)-9,9-dihexyl-9H-fluoren-7-yl]-2-[dimeth-
yl(pyridin-2-yl)silyl]vinyl}-9,9-dihexyl-N,N-diphenyl-9H-fluoren-2-
amine (13): To a solution of 12 (3.43 g, 0.021 mol) and 11 (26.36 g,
0.042 mol) in toluene (80 mL), was added P(tBu)3 (0.255 g,
0.00126 mmol), [Pd(dba)3] (0.28 g, 0.3 mmol), and NEt3 (4.1 g;
0.042 mol) and the resulting solution was stirred at 90 °C for 12 h.
After cooling to r.t., the reaction mixture was filtered and the solu-
tion phase was collected and concentrated by rotary evaporation.
The crude product was purified by column chromatography on sil-
ica gel (ethyl acetate/hexane, 1:20) to give the purified product as
a transparent oil (12.79 g, 52%). 1H NMR (300 MHz, CDCl3): δ
= 8.83–8.82 (d, J = 4.5 Hz, 1 H), 7.58–7.45 (m, 8 H), 7.35–6.95 (m,
27 H), 6.66 (s, 1 H), 1.84 (br., 8 H), 1.17–1.08 (br., 24 H), 0.86–
0.72 (m, 20 H), 0.22 (s, 6 H) ppm. 13C NMR (75 MHz, CDCl3): δ
= 168.38, 159.85, 152.51, 152.12, 150.30, 150.12, 147.84, 147.05,
141.22, 141, 140.73, 140.31, 136.09, 135.80, 133.81, 129.03, 128.65,
126.72, 124, 123.60, 122.35, 121.41, 120.34, 119.35, 119.23, 118.53,
118.34, 55, 40.01, 31.42, 29.49, 23.66, 22.46, 14.01, –1.72 ppm.
HRMS-FAB: calcd. for C83H95N3Si [M]+ 1161.7295; found
1161.7278.
Tris(4-iodophenyl)amine (5): A mixture of triphenylamine (0.29 g,
0.0012 mol), KI (0.6 g, 0.0036 mol), and KIO3 (0.77 g, 0.0036 mol)
was stirred at 85 °C in HOAc for 15 h. After cooling to r.t., aq.
NaHSO3 (35mL) was added into the reaction mixture and stirred
for 20 min. The solution was extracted with CH2Cl2 (3ϫ 50 mL),
then the organic layer was collected and dried with MgSO4. After
removal of the solvent, the crude product was purified by
recrystallization (CH2Cl2/MeOH) to give the purified product as a
1
white solid (0.58 g, 78%). H NMR (300 MHz, CDCl3): δ = 7.55–
7.52 (d, J = 8.7 Hz, 6 H), 6.82–6.79 (d, J = 8.7 Hz, 6 H) ppm.
HRMS-FAB: calcd. for C18H12NI3 [M]+ 622.8104; found 622.8117.
N-(4-Bromophenyl)-N-phenylbenzenamine (6): To a solution of tri-
phenylamine (1.6 g, 0.0065 mol) in CH2Cl2 (50 mL) was added
TBABr3 (3.13 g, 0.0065 mol) and MeOH (20 mL), and the resulting
solution was stirred at r.t. for 24 h. Another portion of MeOH
(50 mL) was added and the reaction mixture was stirred for 25 min.
The precipitate was filtered and collected, and the final purified
product was obtained through recrystallization from MeOH as
white solid (ca. 2.29 g, 78%). 1H NMR (200 MHz, CDCl3): δ =
7.36–7.27 (m, 3 H), 7.26–7.21 (m, 3 H), 7.09–7.03 (m, 6 H), 6.98–
6.91 (m, 2 H) ppm. HRMS-FAB: calcd. for C18H14BrN [M]+
323.031; found 323.0295.
N,N-Bis{4-[2,2-bis(9,9-dihexyl-N,N-diphenyl-9H-fluoren-7-yl)-
vinyl]phenyl}-4-bromobenzenamine (15): A mixture of 7 (2.13 g,
0.0037 mol), 13 (8.6 g, 0.0074 mol), Bu4NF (1.0 m in THF, 7.4 mL,
0.0074 mol), and [PdCl2(PhCN)2] (0.142 g, 0.00037 mol) was
stirred at 70 °C for 2 h. After cooling to r.t., CH2Cl2 (50 mL) was
added and the reaction mixture was stirred at r.t. for 15 min. The
resulting solution was filtered and the filtrate was concentrated by
rotary evaporation. The crude product was purified by column
chromatography on silica gel (ethyl acetate/hexane, 1:40) to give the
purified product as a yellow solid (4.39 g, 50%). 1H NMR
(300 MHz, CDCl3): δ = 7.59–7.44 (m, 22 H), 7.28–7.20 (m, 16 H),
7.12–7.09 (m, 24 H), 7.00–6.96 (m, 10 H), 6.85–6.83 (m, 2 H), 6.74–
6.71 (m, 4 H), 1.79 (br., 16 H), 1.15–0.98 (br., 48 H), 0.87–0.67 (m,
40 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 152.51, 152.25,
150.96, 150.63, 147.96, 147.14, 147.00, 145.29, 142.26, 141.77,
140.31, 138.97, 136.17, 136.00, 132.54, 132.11, 130.46, 129.12,
126.52, 125.99, 125.63, 124.55, 123.74, 123.31, 122.42, 121.51,
120.35, 119.64, 119.39, 118.64, 55.11, 40.08, 31.50, 29.59, 23.78,
22.53, 14.08 ppm. HRMS-FAB: calcd. for C170H182BrN5 [M +
H]+ 2373.3579; found 2373.3565.
N-(4-Bromophenyl)-4-iodo-N-(4-iodophenyl)benzenamine (7):
A
mixture of compound (1.62 g, 0.005 mol), KI (1.12 g,
6
0.0068 mol), and KIO3 (0.72 g, 0.00335 mol) was stirred at 85 °C
in HOAc for 12 h. After cooling to r.t., aq. NaHSO3 (50 mL) was
added and the reaction mixture was stirred for 20 min. The solution
was extracted with CH2Cl2 (3ϫ 50 mL), and the organic layer was
dried with MgSO4. After removal of the solvent, the crude product
was purified through recrystallization (CH2Cl2/MeOH) to afford
the target product as a white solid (2.29 g, 80%). 1H NMR
(300 MHz, CDCl3): δ = 7.54–7.51 (d, J = 8.7 Hz, 4 H), 7.36–7.33
(d, J = 8.7 Hz, 2 H), 6.94–6.91 (d, J = 8.7 Hz, 2 H), 6.81–6.78 (d,
J = 8.7 Hz, 4 H) ppm. 13C NMR (300 MHz, CDCl3): δ = 146.57,
145.86, 138.39, 132.50, 125.86, 125.70, 116.26, 86.51 ppm. HRMS-
FAB: calcd. for C18H12BrI2N [M]+ 574.8242; found 572.8245.
Model Compound 1:
A mixture of compound 9 (0.47 g,
0.00146 mol), 14 (2 g, 0.00482 mol), [Pd(OAc)2] (0.0196 g,
0.000088 mol), and P(o-tolyl)3 (0.1067 g, 0.0003504 mol) in a mixed
solvent system composed of NEt3/MeCN (5 mL/10mL) was stirred
9,9-Dihexyl-7-iodo-N,N-diphenyl-9H-fluoren-2-amine (11): A mix-
ture of 10 (3.0 g; 5.17 mmol), N,N-dimethylenediamine (0.068 g;
0.77 mmol), CuI (0.049 g; 0.258 mmol), and NaI (1.7 g; in a sealed pressure tube at 110 °C for 48 h. After cooling to r.t.,
0.0137 mmol) in dioxane (10 mL) was heated to 110 °C under ar-
gon for 24 h. After cooling, aq. NH4OH (30 mL) was added to the
reaction mixture, which was stirred for 20 min, poured into water
(50 mL), and extracted with CH2Cl2 (3ϫ 20 mL). The combined
organic phase was dried, concentrated, and the residue was purified
by flash chromatography on silica gel (n-hexane) to provide of the
target product (3.15 g, 98%) as a colorless oil. 1H NMR (200 MHz,
ethyl acetate (40 mL) was added and the reaction mixture was
stirred for 20 min. The solution was extracted with water (50 mL)
then the organic layer was collected and dried with MgSO4. The
resulting solution was filtered and the organic solution was concen-
trated by evaporation. The crude product was purified by column
chromatography on silica gel (ethyl acetate/hexane, 1:20) to give
1
the final purified product as a yellow solid (1.0 g, 52%). H NMR
CDCl3): δ = 7.54–7.50 (m, 1 H), 7.50–7.44 (m, 1 H), 7.45–7.40 (m, (200 MHz, CDCl3): δ = 7.53 (m, 12 H), 7.30–7.22 (m, 24 H), 7.15–
2 H), 7.30–7.22 (m, 4 H), 7.14–7.09 (m, 2 H), 7.14–7.09 (m, 2 H),
7.05–6.98 (m, 4 H), 1.87–1.79 (m, 4 H), 1.17–0.99 (m, 12 H), 0.81
7.02 (m, 30 H), 1.85 (br., 12 H), 1.08 (br., 36 H), 0.81–0.73 (m, 30
H) ppm. 13C NMR (50 MHz, CDCl3): δ = 152.39, 151.17, 150.64,
(t, J = 6.9 Hz, 6 H), 0.64 (m, 4 H) ppm. 13C NMR (300 MHz, 147.99, 147.04, 136.10, 135.77, 129.13, 123.77, 122.44, 121.59,
CDCl3): δ = 152.95, 151.43, 147.79, 147.65, 140.49, 135.77, 135.01, 120.34, 119.39, 118.65, 54.96, 54.79, 40.29, 31.59, 31.51, 29.63,
131.72, 129.12, 123.86, 123.62, 123.33, 122.58, 120.74, 120.49,
119.37, 118.86, 91.56, 55.17, 40.08, 31.54, 29.53, 23.65, 22.52, 1320.9240; found 1320.9264.
23.54, 22.54, 14.08 ppm. HRMS-FAB: calcd. for C99H117N [M]+
Eur. J. Org. Chem. 2011, 912–921
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
919

T.-C. Lin et al.
FULL PAPER
Model Compound 2:
A mixture of compound 9 (0.44 g,
Acknowledgments
0.00137 mol), 10 (2.85 g, 0.00453 mol), [Pd(OAc)2] (0.0185 g,
0.000082 mol), and P(o-tolyl)3 (0.1 g, 0.0003288 mol) in a mixed
solvent system composed of NEt3/MeCN (5 mL/10mL) was stirred
in a sealed pressure tube at 110 °C for 48 h. After cooling to r.t.,
CH2Cl2 (50 mL) was added and the reaction mixture was stirred
for 15 min. The resulting solution was filtered and the filtrate was
concentrated by evaporation. The crude product was purified by
column chromatography on silica gel (ethyl acetate/hexane, 1:100)
to give the final purified product as a yellow solid (1.0 g, 40%). 1H
NMR (200 MHz, CDCl3): δ = 7.70–7.62 (m, 6 H), 7.57–7.46 (m, 6
H), 7.38–7.26 (m, 12 H), 7.24–7.04 (m, 9 H), 7.00–6.9 (m, 6 H),
1.95 (br., 12 H), 1.05 (br., 36 H), 0.76–0.73 (m, 30 H) ppm. 13C
NMR (50 MHz, CDCl3): δ = 151.25, 151.11, 150.96, 140.83,
136.46, 132.43, 127.37, 126.95, 126.74, 125.36, 124.30, 123.43,
122.85, 120.61, 119.62, 119.23, 54.96, 40.47, 29.72, 23.74, 22.57,
12.03 ppm. 13C NMR (200 MHz, CDCl3): δ = 153.16, 150.87,
146.61, 139.84, 139.56, 136.92, 132.30, 132.25, 129.91, 127.82,
126.10, 125.53, 124.21, 55.31, 40.35, 31.51, 29.63, 23.69, 22.56,
13.99 ppm. HRMS-FAB: calcd. for C135H144N4 [M]+ 1821.1391;
found 1821.1343.
We thank the National Science Council (NSC), Taiwan for finan-
cial support.
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Model Compound 3:
A mixture of compound 5 (0.53 g,
0.00086 mol), 13 (3.3 g, 0.00284 mol), Bu4NF (1.0 m in THF,
3.9 mL, 0.00387 mol), and [PdCl2(PhCN)2] (0.05 g, 0.000129 mol)
was stirred at 110 °C for 48 h. After cooling to r.t., CH2Cl2 (50 mL)
was added and the reaction mixture was stirred for 20 min. The
resulting solution was filtered and the organic solution was col-
lected and concentrated by rotary evaporation. The crude product
was purified by column chromatography on silica gel (ethyl acetate/
hexane, 1:40) to give the final purified product as a yellow solid
(1.0 g, 35%). 1H NMR (300 MHz, CDCl3): δ = 7.54–7.49 (m, 12
H), 7.28–7.19 (m, 36 H), 7.12–7.08 (m, 32 H), 7.02–6.97 (m, 28 H),
6.72–6.69 (m, 3 H), 1.78 (br., 24 H), 1.15–0.98 (br., 72 H), 0.82–
0.71 (br., 60 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 152.56,
152.32, 150.97, 150.65, 148.04, 147.16, 141.95, 140.24, 139.08,
136.25, 136.13, 132.26, 130.37, 129.15, 126.56, 124.62, 123.78,
123.04, 122.45, 121.56, 120.34, 119.61, 118.64, 54.98, 40.13, 31.83,
29.62, 23.82, 22.51, 14.09 ppm. MALDI-TOF: calcd. for
C
246H267N7 [M + H]+ 3322.80; found 3322.89.
Model Compound 4: mixture of compound
A
9 (0.18 g,
0.00056 mol), 15 (4.35 g, 0.00185 mol), [Pd(OAc)2] (0.0076 g,
0.000034 mol), and P(o-tolyl)3 (0.04 g, 0.000136 mol) in a mixed
solvent system composed of NEt3/MeCN (5 mL/10mL) was stirred
in a sealed pressure tube at 110 °C for 48 h. After cooling to r.t.,
CH2Cl2 (70 mL) was added and the reaction mixture was stirred
for 25 min. The resulting solution was filtered and the filtrate was
concentrated by evaporation. After removing the solvent, the crude
product was purified by column chromatography on silica gel (ethyl
acetate/hexane, 1:10) to give the final purified product as a yellow
solid (1.0 g, 25%). 1H NMR (300 MHz, CDCl3): δ = 7.57–7.48 (m,
24 H), 7.34–7.19 (m, 84 H), 7.15–6.99 (m, 72 H), 6.97–6.91 (m, 60
H), 6.75 (m, 12 H), 1.759 (br., 48 H), 1.13–0.983 (m, 144 H), 0.818–
0.671 (m, 120 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 152.26,
152.03, 150.71, 150.36, 147.72, 146.84, 146.72, 146.03, 145.27,
141.78, 141.61, 139.98, 138.79, 135.97, 135.79, 132.01, 130.14,
128.85, 126.82, 126.26, 125.94, 124.35, 123.93, 123.48, 123.43,
123.07, 122.14, 121.26, 120.06, 119.37, 119.16, 119.37, 54.86, 40.00,
31.31, 29.33, 23.52, 22.26, 13.81 ppm. MALDI-TOF: calcd. for
C534H564N16 [M + H]+ 7207.30; found 7207.51.
Supporting Information (see also the footnote on the first page of
this article): Photophysical methods for the measurement of linear
and nonlinear optical properties.
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920
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 912–921

Soluble Dendritic Chromophores
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Received: August 18, 2010
Published Online: December 27, 2010
Eur. J. Org. Chem. 2011, 912–921
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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