Triethynylmethane: A molecular unit inducing excimer-like emission in aggregated states of hydrocarbon fluorophores

Article abstract of DOI:10.1016/j.tetlet.2012.10.093

We report the synthesis of triethynylmethane (TEM) derivatives substituted with fluorescent aromatic hydrocarbons and the fluorescence properties in dilute aqueous solution and in spin-coated film. Pyrene-tethered TEM derivatives showed excimer-like fluorescence in 80% water in THF and spin-coated film. UV and ¹H NMR analyses showed the aggregation of TEM derivatives. Phenanthrene-tethered TEM also showed excimer-like fluorescence in high water content in THF. These data showed that TEM is a novel molecular unit inducing excimer-like emission in the aggregated states of aromatic hydrocarbon fluorophores.

Full text of DOI:10.1016/j.tetlet.2012.10.093

Tetrahedron Letters 54 (2013) 143–146
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Triethynylmethane: a molecular unit inducing excimer-like emission
in aggregated states of hydrocarbon fluorophores
Takahiro Otabe, Saki Matsumoto, Hiroki Nakagawa, Changfeng Hong, Chikara Dohno,
⇑
Kazuhiko Nakatani
Department of Regulatory Bioorganic Chemistry and SILOU Laboratory, The Institute of Scientific and Industrial Research (ISIR), Osaka University, Ibaraki 567-0047, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis of triethynylmethane (TEM) derivatives substituted with fluorescent aromatic
hydrocarbons and the fluorescence properties in dilute aqueous solution and in spin-coated film.
Pyrene-tethered TEM derivatives showed excimer-like fluorescence in 80% water in THF and spin-coated
film. UV and 1H NMR analyses showed the aggregation of TEM derivatives. Phenanthrene-tethered TEM
also showed excimer-like fluorescence in high water content in THF. These data showed that TEM is a
novel molecular unit inducing excimer-like emission in the aggregated states of aromatic hydrocarbon
fluorophores.
Received 26 September 2012
Revised 19 October 2012
Accepted 22 October 2012
Available online 5 November 2012
Keywords:
Fluorescent
Triethynylmethane
Excimer emission
Pyrene
Ó 2012 Elsevier Ltd. All rights reserved.
Pyrene is a typical aromatic hydrocarbon showing concentration
dependent fluorescence and aggregation caused quenching (ACQ)
in condensed states and emitting excimer fluorescence with much
decreased quantum yield than that of monomer emission due to the
OPiv
tendency of p–p
stacking of these planar molecules.1–4 However, it
has been described in literatures that pyrenes incorporated into the
polymer material and other organized media in an aqueous envi-
ronment often produced the excimer-like fluorescence due to
the ground state aggregation of pyrenes.5–17 In contrast to ACQ-
fluorophores, molecules exhibiting a totally opposite phenomenon
termed as aggregation induced emission (AIE) show only weak or a
little fluorescence in a dilute solution but became highly fluorescent
in the aggregated or solid states.18–20 It was recently demonstrated
that ACQ-fluorophores could be transformed into the AIE-active
fluorophores by tethering to the AIE unit tetraphenylethene21–24
and other molecular systems.25–27 The success of transformation
of ACQ-fluorophores into AIE-active fluorophores in addition to
the fact that ground state aggregates of pyrenes sometime emit
excimer-like emission prompted us to explore the auxiliary
molecular units that can induce the excimer-like emission in the
aggregation state of ACQ-fluorophores.
1 : TEM unit
2a
2b
OPiv
OPiv
2a
2b
OPiv
PivO
Toward this end, we were interested in a triethynylmethane
(TEM) unit involving the sp3-hybridized carbon center that fixes
three ethynyl groups extruding from the quaternary carbon center
at the non-planar orientation. (Fig. 1) Structural characteristics of
TEM are as follows; (1) various aromatic chromophores could be
3
4
Figure 1. Chemical formulae of TEM unit 1, TEMs tethered with 1-pyrene 2a,
4-pyrene 2b, and 4-phenanthrene 3, and a reference pyrene monomer 4. 2a and 2b
were also shown by the space-filling model (top right, the pivaloyl group was
omitted for clarity).
⇑
Corresponding author. Tel.: 81 6 6879 8455; fax: +81 6 6879 8459.
E-mail address: nakatani@sanken.osaka-u.ac.jp (K. Nakatani).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.093

144
T. Otabe et al. / Tetrahedron Letters 54 (2013) 143–146
introduced into the terminal carbon of the ethynyl group by metal-
catalyzed cross coupling, (2) due to the sp3-hybridized carbon cen-
ter, three aromatic chromophores are not on the same plane and
have limited rotational freedom along the sp–sp carbon axis to al-
low little intramolecular stacking, (3) due to a concaved shape of
TEM with three aromatic chromophores suggested by molecular
modeling simulation, intermolecular stacking by hydrophobic inter-
actions was anticipated. We here report that TEM is a novel molec-
ular unit to which aromatic fluorescent hydrocarbons tethered
produced excimer-like fluorescence in aggregated states with much
higher quantum yield than that of excimer fluorescence. Since mol-
ecules that emit fluorescence at a different wavelength depending
on the degree of molecular aggregation are potentially useful envi-
ronment-sensitive ratiometric fluorescent sensors,1,28–31 TEM unit
may be useful for the novel design of environment-sensitive fluo-
rescent sensors.
Due to the facile availability of triethynylmethanol, we first
investigated the triethynylmethanol derivatives (Ar3COH and
Ar3COR, where R = CH2CH2OPiv) substituted with pyrene and phen-
anthrene, but these molecules were found labile under photochemical
conditions. Komatsu et al. have reported that triphenylethynyl-
methanol produced triphenylethynylmethyl carbocation under
photoirradiation conditions.32 The chemical instability of pyrene-
and phenanthrene-substituted triethynlymethanol derivatives is
most likely due to a facile formation of triarylethynylmethyl carbo-
cation, suggesting the necessity of the quaternary carbon center for
the chemically stable molecular unit. The TEM unit 1 we then inves-
tigated consisted of the sp3-hybridized quaternary carbon center,
and was synthesized from 3,3,3-tris(triethylsilylethynyl)propene,33
of which synthetic application remained to be addressed. Hydrobo-
ration of the vinyl group and subsequent oxidative work-up
followed by protection of the resulting primary alcohol as a pivalate
provided TEM 1. The pivalate is not essential but substantially
decreased the handling difficulty caused by a low molecular weight
of the unprotected alcohol.
properties of TEM possessing hydrocarbon fluorophores 2a, 2b,
and 3. In THF, the absorption maximum was observed at 366 nm.
In a mixed solvent of THF and water, the absorption was found
to decrease steadily as the water fraction increased, accompanied
by the red shift of the absorption maximum by 6 nm and signifi-
cant signal broadening. (Fig. 2a, For enlarged spectra, please see
Fig. S1) An index PA, the peak-to-valley intensity ratio obtained
from the absorption intensity of the red-edge band (1La) to that
of the adjacent minimum at shorter wavelength was used for eval-
uating the association of pyrenes in the ground state,34 and was
2.72 in THF and 1.27 in 80% water in THF. Responding to the
absorption change, fluorescence spectra of 2a were changed from
its monomer emission to the excimer-like emission. (Fig. 2b) Thus,
in THF solution, the fluorescence spectra of 2a presented the typi-
cal spectra of the monomeric pyrene, with the (0,0) band at
385 nm. The peak intensity was first increased as the water frac-
tion increased from 0% to 40%, and then decreased to the original
level with the water fraction of 60%. Further increase of the water
fraction to 80% resulted in a complete loss of the monomer emis-
sion and, instead, produced broad fluorescence centered at
502 nm, which resembled the emission from the pyrene excimer.
The absolute quantum yield of excimer-like fluorescence reached
as high as 0.27 in 80% water in THF. (Fig. 2c) The fluorescence
change of 2a solution (10 lM) from blue to greenish color depend-
ing on the water fraction in THF was clearly detected by naked
eyes. (Fig. 2d) Excimer-like emission of 2a was also clearly
observed in the spin-coated film prepared from 10 mM toluene
solution, whereas the spin-coated film of reference pyrene mono-
mer 4 did not show any detectable fluorescence under the same
conditions. (Fig. 2e) The fluorescence quantum yield of the 2a film
was 0.15 with spectra showing a peak top at 504 nm. (Fig. S2) To
compare the excimer-like emission of 2a in 80% water in THF, exci-
mer fluorescence of 2a was measured in 10 mM THF solution. The
broad excimer emission of 2a was observed at 493 nm accompanied
by still intense monomer emission. (Fig. 2f, For non-normalized
spectra, see Fig. S2c) The signal intensity of excimer emission in
10 mM THF was much weaker than that of excimer-like emission
With the TEM unit 1 in hand, we first tethered the pyrene chro-
mophore to the unit. Under the standard conditions, the coupling
of 1 and 1-iodopyrene proceeded smoothly to give 2a having three
pyrene chromophores in 66% yield. Likewise, the coupling with
4-iodopyrene and 4-iodophenanthrene produced 2b and 3 in 48%
and 75% yields, respectively. Heterocycles having potential metal
binding sites and aromatic molecules with electron withdrawing
groups can be tethered to the TEM unit, demonstrating the avail-
ability of a variety of TEM derivatives with different chemical prop-
erties and functionalities (Table 1).
in 10 lM 80% water in THF.
Another pyrene derivative 2b tethered to a TEM unit with pyr-
ene substitution at the C4 position showed a broad excimer-like
emission without monomer fluorescence at 493 nm in 80% water
in THF. The UV spectra of 2b also showed a significant peak broad-
ening as the water fraction in THF increased. The PA values changed
from 1.83 in THF to 1.52 in 70% water in THF. In 80% water in THF
(Fig. S3), the absorption spectrum of 2b was too broad to identify
the peaks for the PA calculation. In separate experiments, we con-
firmed that reference compound 4 having only one ethynylpyrene
Having completed the synthesis of TEM derivatives with three
aromatic chromophores, we then investigated the fluorescent
Table 1
Synthesis of TEM derivatives
OPiv
OPiv
Pd(PPh3)4, CuI
Ar
X
Ar
Ar
Ar
DMF
Ar–X
Yield (%)
74
Ar–X
Yield (%)
Ar–X
Yield (%)
37
O
O
I
N
Br
N
51
N
Br
O
79
63
Br
N
I

T. Otabe et al. / Tetrahedron Letters 54 (2013) 143–146
145
THF-d8 / D2O
100/0 50/50 30/70 20/80
(a)
(b)
water fraction (%)
1.5
Hb
8.43
8.26
9.10
8.77
8.42
8.19
8.44
8.30*
8.28*
8.43
8.16
8.95
8.63
8.36
8.20
8.44
8.29*
8.29*
8.41
8.13
8.94
8.59
8.34
8.19
8.43
8.28*
8.28*
8.40
8.11
8.93
8.57
8.33
8.19
8.43
8.28*
8.28*
Ha
Hb
Hc
Hd
He
Hf
Hg
Hh
Hi
OPiv
water fraction
0%
40
80
Hc
Ha
Hi
Hh
Hg
H
H
800
80
60
40
20
0
C
H
H 3
1
0.5
0
600
400
200
0
0
80
Hd
He
80%
0
440 480 520 560 600
Hf
Figure 3. Chemical shifts (ppm) of hydrogens of 2b (10 lM) in THF-d8 and D2O in
250
300
350
400
450
400
450
500
550
600
different fractions. Hydrogens that exhibited apparent up-field shift were shown in
red, whereas other aromatic hydrogens were shown in blue. ⁄These hydrogens were
not identified.
wavelength (nm)
wavelength (nm)
(c)
0.4
(d)
(e)
0.3
0.2
0.1
0
(a)
(b)
0.5
60
50
40
30
20
10
0
0.4
0.3
0.2
0.1
0
0
20
40
60
80
100
water fraction (%)
(f)
300
350
350
400
450
500
1
wavelength (nm)
wavelength (nm)
0.8
0.6
0.4
0.2
0
Figure 4. (a) UV–Vis and (b) fluorescence spectra of 3 (10 lM) in THF (black) and
80% water in THF (red).
moiety, the aggregation inducing the absorption, fluorescence,
and 1H NMR changes is most likely due to the intermolecular
stacking of these pyrene-tethered TEM derivatives.
To know if the TEM unit could induce the emission of excimer-
like fluorescence of other aromatic hydrocarbons in the aggregated
state, 4-phenanthrene was introduced to the TEM unit, because
excimer fluorescence was not observed for phenanthrene in solu-
tion at room temperature but does for phenanthrenophane, where
two phenanthrene rings were fixed rigidly by covalent link-
ages.35,36 Upon increasing the water fraction of the THF solution
400 450 500 550 600 650
wavelength (nm)
Figure 2. (a) UV–Vis spectra of 2a (10 lM) in THF with the water fraction of 0%,
20%, 40%, 60%, 70%, and 80%. (b) Fluorescence spectra of 2a in THF with the water
fraction of 0%, 20%, 40%, 60%, 70%, and 80%. The excitation wavelength for
fluorescence measurements was 366 nm. Inset: expanded spectra in the region of
excimer-like emission. (c) The plots of quantum yield of 2a in different water
fractions in THF for monomer emission (calculated from 380–470 nm) (filled circle)
and for excimer-like emission (calculated from 410–650 nm) (filled square). (d) A
captured image of fluorescence change of 2a (10 lM) in THF (left) and in 80% water
in THF (right). (e) Fluorescence image of spin-coated films of 10 mM toluene
solution of 2a (top) and 4 (bottom). (f) Normalized fluorescence spectra of 2a in
10 mM THF (black) and 10 lM in 80% water in THF (red).
of 3 (10 lM), red shifting of the absorption maximum, decrease
of PA value, and signal broadening were observed (Fig. 4), suggest-
ing the aggregation of 3 even in a dilute solution. The typical fluo-
rescence of phenanthrene observed in THF disappeared in 80%
water in THF, instead broad emission peaks centered at about
385, 425, and 440 nm appeared. It is reported that the wavelength
of excimer emission in phenanthrenophane was sensitively depen-
dent on the orientation of two phenanthrene rings and a degree of
unit did not show excimer-like fluorescence in 10 lM THF solution
when the water fraction in THF was increased to 80%.
p-p
stacking, ranging from 380 to 430 nm.36 The observed broad
peaks in fluorescent spectra of 3 in 80% water in THF are suggesting
to involve different stacking orientations of phenanthrenes in the
aggregation of 3.
The absorption changes of 2a and 2b accompanied by the emis-
sion of excimer-like fluorescence in high water fraction in THF
indicated the aggregation of these pyrene derivatives tethered to
the TEM unit at a low concentration. To gain any insight into the
aggregation of the pyrene-tethered TEM derivatives, 1H NMR spec-
In conclusion, the hydrocarbon fluorophores tethered to TEM
tra of 2b (10 lM) were measured in THF-d8 with different D2O frac-
unit 1 aggregated even in a dilute THF solution of 10 lM concen-
tions. (Fig. 3 and Fig. S4) As the D2O fraction increased, an apparent
up-field shift of the signals was observed for Ha, Hb, Hc, Hd, and He.
Other four aromatic hydrogens Hf, Hg, Hi, and Hj showed a small
up-field shift. The hydrogens showing apparent up-field shift were
located on the molecular edge proximal to the TEM center,
whereas the other four hydrogens are in the opposite edge. Since
the intramolecular stacking of pyrene moieties in 2b is not con-
ceivable due to the restricted conformational freedom of pyrene
tration with high water content. The aggregates prepared in
solution for pyrenes and phenanthrene and in spin-coated film of
pyrene derivative 2a emit excimer-like fluorescence. The exci-
mer-like emission of phenanthrene chromophore could be also ob-
served when the chromophore was tethered to the TEM unit. These
results suggested that the TEM unit 1 is a novel molecular unit
inducing excimer-like emission in aggregated states of hydrocar-
bon fluorophores.

146
T. Otabe et al. / Tetrahedron Letters 54 (2013) 143–146
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Supplementary data (synthetic protocols of all compounds, a
plot of fluorescent quantum yield of 2a in different water fractions
in THF, fluorescence spectra of 2a film, UV, and Fluorescence Spec-
tra of 2b in different water fractions in THF, 1D, and 2D 1H NMR of
2b) associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.tetlet.2012.10.093.
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