Synthesis and two-photon properties of a multipolar chromophore containing indenofluorenyl units

Article abstract of DOI:10.1016/j.tetlet.2008.10.114

A new multipolar fluorophore derived from triphenylamine as the core with diphenylaminoindenofluorenyl moieties incorporated at the peripheral positions has been synthesized and experimentally shown to possess strong two-photon absorptivities in near-IR region and intense upconverted visible emission under the irradiation of femtosecond laser pulses.

Full text of DOI:10.1016/j.tetlet.2008.10.114

Tetrahedron Letters 50 (2009) 182–185
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and two-photon properties of a multipolar
chromophore containing indenofluorenyl units
*
Tzu-Chau Lin , Cheng-Sheng Hsu, Chia-Ling Hu, Yong-Fu Chen, Wei-Je Huang
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 multipolar fluorophore derived from triphenylamine as the core with diphenylaminoindenofluo-
renyl moieties incorporated at the peripheral positions has been synthesized and experimentally shown
to possess strong two-photon absorptivities in near-IR region and intense upconverted visible emission
under the irradiation of femtosecond laser pulses.
Received 26 August 2008
Revised 20 October 2008
Accepted 23 October 2008
Available online 29 October 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Two-photon absorption
Two-photon-excited fluorescence
Indenofluorene
Heck reaction
Over the past fifteen years, two-photon absorption (2PA) has at-
tracted increasing attention due to its variety of potential applica-
tions in photonics and biophotonics including optical power
limiting, 3-D data storage, frequency upconverted lasing, 3-D
microfabrication, noninvasive bio-imaging/tracking, and two-
photon photodynamic therapy.1 To be of use in these applications,
organic compounds that display strong 2PA within specific spectral
region are consequently in great demand. The accumulated knowl-
edge and experience based on enormous efforts devoted in under-
standing the connections between molecular structure and 2PA
property has revealed that large 2PA can be achieved by judicious
control of the intramolecular charge-transfer efficiency and/or
tion properties in conjugated systems, we have been interested
in exploring the impact brought by potential 2PA-enhancing aro-
matic units with various structural/electronic properties when
they are incorporated into the framework of a multi-branched
chromophore. In this Letter, we present the synthesis of a new
two-photon-active model chromophore (2) derived from triphen-
ylamine and indenofluorene moieties as well as the initial investi-
gations of its 2PA properties in the femtosecond time domain.
The chemical structure and the synthetic route of the studied
model compound are illustrated in Scheme 1. The trigonal skeleton
of this octupolar fluorophore is derived from the symmetric func-
tionalization of an electron-donating core (i.e., triphenylamine)
with three conjugated diphenylaminoindenofluorenyl branches
extended outward from the center. The structure of indenofluorene
can be virtually treated as an elongated version of fluorene that in-
volves two fluorenyl units sharing one central benzene ring. Com-
pared to fluorene, the scaffold of indenofluorene not only keeps the
effective size of
P
-conjugation domain within a molecule.2–7
Among the investigated structures, it has been experimentally
shown that branched structures could lead to highly efficient mul-
ti-photon absorption3d,e,4,5d,6a,b,d,f,7e–g,8 while maintaining linear
transparency over wide spectral range.9 On the other hand,
branched skeleton offers an access to incorporate numbers of
2PA-enhancing parameters into a single dye system and this could
be very desirable for material chemists from the aspect of molecu-
lar design since it is possible to optimize a single molecule that
simultaneously combines various expected characteristics for
different specific applications. Besides, branched skeletons are po-
tential building blocks for constructing dendritic and/or supermo-
lecular structures which are suggested as probable approaches to
pursue fundamental limits of molecular 2PA.10 In searching the
correlation between structural parameters and nonlinear absorp-
coplanar character but also extends the p-conjugation length, and
additionally it provides extra sites to attach alkyl chains for the
solubility improvement, which are the features desirable for 2PA-
active dyes. On the other hand, it has been reported that organic
structures comprising indenofluorenyl units either in polymeric11
or small molecular12 forms are highly fluorescent and this is very
beneficial for the nonlinear optical property characterizations
based on two-photon-excited fluorescence (2PEF) technique. All
the above considerations lead to the strategy to incorporate
indenofluorene moieties into the
P-framework of our model dye
system. We have utilized vinylene units as the connecting spacers
between the central core and the peripheral groups in order to
ensure effective electronic conjugation between end groups and
* Corresponding author. Tel.: +886 3 4227402; fax: +886 3 4227664.
E-mail address: tclin@ncu.edu.tw (T.-C. Lin).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.10.114

T.-C. Lin et al. / Tetrahedron Letters 50 (2009) 182–185
183
COOH
CH3
CH3
Br
(b)
(a)
(d)
B(OH)2
+
2
Br
H3C
HOOC
H3C
4
3
O
C6H13
C6H13
(c)
(e)
C6H13
C6H13
O
6
7
5
(f)
C6H13
C6H13
HN
C6H13
C6H13
C6H13
N
2
Br
Br
Br
(g)
C6H13
C6H13
C6H13
9
8
C6H13
N
C6H13
N
C6H13
C6H13
C6H13
N
C6H13
3
C6H13
C6H13
(h)
N
N
C6H13
C6H13
N
C6H13
C6H13
N
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
N
N
reference
compound
1
2
Scheme 1. Synthetic procedure for the studied model compound 2. Reagents and conditions: (a) Suzuki reaction: 2,5-dibromo-xylene (1 equiv), PhB(OH)2 (2.2 equiv), K2CO3
(5 equiv), Bu4NBr (2 equiv) in H2O, 70 °C, 2 h (98%); (b) 3 (1 equiv), KMnO4 (4.7 equiv) in H2O–pyridine, reflux, 24 h (88%); (c) 4 (1 equiv) in H2SO4, rt, 4 h (82%); (d) 5
(1 equiv), KOH (24.8 equiv), N2H4ꢃH2O (7 equiv) in diethylene glycol, 180 °C, 48 h (76%); (e) 6 (1 equiv), n-BuLi (6 equiv), C6H13Br (6 equiv) in THF, ꢂ78 °C, 12 h (70%); (f) 7
(1 equiv), BTMABr3 (2.1 equiv), ZnCl2 (2.3 equiv) in CH3COOH, 80 °C, 12 h (73%); (g) 8 (1 equiv), Ph2NH (1.1 equiv), NaOtBu (4 equiv), Pd2(dba)3 (0.01 equiv), BINAP
(0.03 equiv) in toluene, 80 °C, 24 h (54%); (h) Heck reaction: 9 (3.3 equiv), tris(4-vinylphenyl)amine (1 equiv), Pd(OAc)2 (0.06 equiv), P(o-tolyl)3 in NEt3–MeCN, 110 °C, 48 h
(80%). (See ref. 17 for the chemical abbreviation.)
core moiety and allow multi-dimensional intramolecular charge
transfer to take place within the chromophore. Hence, the overall
molecular structure of the model compound (2) is expected to
simultaneously possess several potential 2PA-enhancing charac-
ters including multi-directional intermolecular charge transfer be-
Figure 1 presents the linear absorption and fluorescence spectra
of the studied dye molecule (2) and the reference compound (1) in
THF. Intense one-photon absorption (1PA) of both compounds was
found around 400 nm (with
e
ꢀ1.17 ꢁ 105 cmꢂ1 Mꢂ1 for 1 and
e
ꢀ1.95 ꢁ 105 cmꢂ1 Mꢂ1 for 2). The model chromophore (2) also
exhibits strong two-photon-excited upconversion emission which
can be readily observed through naked eyes even under the illumi-
nation of an ꢀ790 nm unfocused femtosecond laser beam. Figure 2
illustrates the 2PA-induced fluorescence spectrum of compound 2
and the inset curve validates that the 2PA process is responsible for
the detected upconversion emission. In order to explore and com-
pare the dispersion of 2PA behaviors of these two dye molecules as
a function of wavelength, we have conducted the degenerate two-
photon-excited fluorescence (2PEF) measurement in the near-IR
regime (700–900 nm) based on an experimental setup very similar
to that reported by Xu and Webb16 as depicted in Supplementary
tween core and molecular termini, increased
P-electron number,
elongated coplanarity of the conjugation system, and improved
solubilities. This model fluorophore was synthesized in 80% yield
via a triple Heck-type coupling between 2-bromo-8-diphenylamin-
oindenofluorene 9 and tris(4-vinylphenyl)amine (Scheme 1). The
framework of the key intermediate 9 was accomplished by prepar-
ing the indenofluorene unit (compound 6) first12 followed by a ser-
ies of functionalization processes at the corresponding positions
including alkylation at C6 and C12 positions to obtain 7, bromina-
tion at C2 and C8 positions to prepare 8,13 and finally a mono-ami-
nation to afford compound 9.14 For the purpose of comparison, a
previously reported analogous dye molecule (1) bearing three
diphenylaminofluorenyl units at the peripheral positions has been
chosen as the reference compound in this work.15
data (see Fig. S1). We have used Fluorescein (ꢀ80
lM in pH 11,
NaOH solution) as the standard for this experiment. Figure 3 shows
the measured degenerate two-photon absorption spectra of com-

184
T.-C. Lin et al. / Tetrahedron Letters 50 (2009) 182–185
2.4
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
6000
5000
4000
3000
2000
1000
0
compound 1
compound 2
compound 1
compound 2
7
4.0x10
7
3.5x10
7
3.0x10
7
2.5x10
7
2.0x10
7
1.5x10
7
1.0x10
6
5.0x10
0.0
450
550
650
700
750
50w0 avelen6g0t0h (nm)
300
400
500
600
700
800
900 1000 1100
700 720 740 760 780 800 820 840 860 880 900
wavelength (nm)
,
λ
Excitation Wavelength (
nm)
exc
Figure 1. Linear absorption and fluorescence spectra (see the inset) of compounds
1 and 2 in solution phase (1 ꢁ 10ꢂ5 M in THF).
Figure 3. Measured degenerate two-photon absorption spectra of compounds 1
and 2 by 2PEF method in THF solution at 1 ꢁ 10ꢂ4 M (with experimental error
ꢀ 15%).
pound 1 and 2 in THF and the combined photophysical data are
summarized in Table 1.
It is notable that the reference compound (1) exhibits relatively
strong 2PA (r2 P 500 GM) from 700 nm to 770 nm accompanied
Table 1
Photophysical properties of compounds 1 and 2 in THFa
by a local maximum (r2 ꢀ 1620 GM) around 720 nm while the
model fluorophore 2 possesses salient increment in the overall
two-photon absorption within the entire investigated spectral re-
gion compared to compound 1, indicating that the expanded
i
Compound 1PA-related optical properties
2PA-related optical
propertiesg
Neff
k
logeMAX
k
UF d7220nm
d22k
k
MAX
1PA b,c
MAX
(nm)
d
1PA-FL
MAX
(nm)
e
f
2PA-Fl
e
1PA
p-delocalization domain has positive contribution to the increased
(GM)h
(GM)h
(nm)
molecular 2PA. Also, indenofluorene appears to be a superior
2PA-promoting unit to fluorene in this triphenylamine-derived
dye system. The dispersions of 2PA activities for these two chro-
mophores are very similar within our current spectral detection
range, that is, the 2PA cross-section values for both compounds ap-
proached to higher values at short excitation wavelength range
and leveled down monotonically in the long excitation wavelength
direction. The observed rising two-photon absorptivities in the
short wavelength region implies that the two-photon allowed
states with larger transition probabilities of these dye molecules
are energetically positioned above their lowest one-photon al-
lowed states.2a,b The dispersion of large two-photon activities in
relatively wide near-IR spectral regime suggests that this model
chromophore can be a potential broad-band optical power limiter
when against ultra-short pulses. On the other hand, the medium-
1
2
408
419
5.07
5.29
493
479
0.76 ꢀ1620
0.72 ꢀ5270
ꢀ180
ꢀ510
494
482
37.6
47.4
a
b
c
d
e
f
All photophysical data were obtained in THF solution.
1PA stands for one-photon absorption.
The concentration was 1 ꢁ 10ꢂ5 M.
e
MAX: molar absorption coefficient.
Spectral positions of 1PA-induced fluorescence emission maxima.
Fluorescence quantum efficiency was determined with coumarin 153 as a
reference.
The concentration was 1 ꢁ 10ꢂ4 M for 2PEF measurements.
g
2PA cross-section values at various wavelengths; 1GM = 1 ꢁ 10ꢂ50 cm4 s/pho-
h
ton-molecule.
Effective p
-electron number.10
i
high quantum yield observed from this dye molecule also indicates
that it could be an efficient frequency upconverter for the biopho-
tonic applications such as two-photon-excited fluorescence
microscopy. Additionally, from the viewpoint of practical applica-
tions, it might be interesting to explore the basic nonlinear photo-
physical properties of the model compound 2 in its solid state (e.g.,
in film configuration) and such work is under taken.
In summary, we have synthesized and investigated a novel mul-
tipolar chromophore with triphenylamine as the central core and
diphenylaminoindenofluorene units as the peripheries. The large
enhancement of 2PA observed from this fluorophore compared to
the reference compound (1) implies that indenofluorene unit could
be a useful 2PA-enhancer when correctly incorporated into a dye
system. Furthermore, the strong 2PA that covers relatively wide
spectral range in near-IR regime and intense 2PA-induced upcon-
verted emission observed from this model compound make it a po-
tential candidate for both broad-band optical limiter and frequency
upconverter for various photonic and biophotonic applications.
1.5
8.6
8.4
= 790 nm
λpump
8.2
8.0
7.8
7.6
7.4
7.2
1.0
0.5
0.0
Slope = 2.04
-0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0
Log (input intensity)/a.u.
400
450
500
550
600
650
700
750
Wavelength (nm)
Acknowledgments
Figure 2. Two-photon excited upconversion emission spectra of fluorophore 2 in
THF at 1 ꢁ 10ꢂ4 M. Inset, power dependence of the 2PA-induced upconversion
emission intensity on the input intensity.
We acknowledge the financial support from National Science
Council (NSC), Taiwan (Grant No. NSC 95-2113-M-008-013-MY2).

T.-C. Lin et al. / Tetrahedron Letters 50 (2009) 182–185
185
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We also thank Professor Chao-Yi Tai and Dr. Sheng-Hsung Chang in
the Department of Optics and Photonics, National Central Univer-
sity, Taiwan for their helpful assistance in part of photophysical
property measurements.
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Supplementary data
The supplementary data includes detailed experimental proce-
dures for optical property characterizations for model compounds
and synthetic processes/full characterization data for compounds
2, 7, 8, and 9. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/
j.tetlet.2008.10.114.
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