Cavitands bearing four fluorophores

Article abstract of DOI:10.1016/j.tetlet.2003.09.070

The synthesis of novel cavitands containing four fluorophores [tert-butoxycarbonyl protected 2,2′-bis(furyl)benzidine (t-BOC FurylBz) or 5,5′-bis(4-aminophenyl)-2,2′-bifuryl (t-BOC PFDA)] and ionophoric functional groups on the upper rim is reported. The cavitands bearing the four fluorophores emit blue light photoluminescence. In particular, the cavitand containing PFDA moieties exhibits a high photoluminescence quantum yield.

Full text of DOI:10.1016/j.tetlet.2003.09.070

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8211–8215
Cavitands bearing four fluorophores
Moonhor Ree,a,* Jong-Seong Kim,a Jae Jung Kim,a Byeang Hyean Kim,a Juyoung Yoonb and
Heesoo Kimc
aDepartment of Chemistry, Center for Integrated Molecular Systems, Division of Molecular and Life Sciences,
and BK21 Program, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
bDepartment of Chemistry, Ewha Womans University, Daeshing-dong, Seodaemoon-gu, Seoul 120-750, Republic of Korea
cDepartment of Microbiology, Dongguk University College of Medicine, 707 Seokjang-dong, Gyeongju 780-714,
Republic of Korea
Received 20 August 2003; revised 9 September 2003; accepted 11 September 2003
Abstract—The synthesis of novel cavitands containing four fluorophores [tert-butoxycarbonyl protected 2,2%-bis(furyl)benzidine
(t-BOC FurylBz) or 5,5%-bis(4-aminophenyl)-2,2%-bifuryl (t-BOC PFDA)] and ionophoric functional groups on the upper rim is
reported. The cavitands bearing the four fluorophores emit blue light photoluminescence. In particular, the cavitand containing
PFDA moieties exhibits a high photoluminescence quantum yield.
© 2003 Elsevier Ltd. All rights reserved.
Cavitands, molecules that contain a rigid enforced cav-
ity, have attracted considerable attention in
supramolecular chemistry as building blocks for the
construction of carcerands, hemicarcerands, and other
hosts.1–6 On the other hand, luminescent organic
molecules and their design and synthesis have recently
become an important area of research for the develop-
ment of molecular photonic devices such as light-emit-
ting diodes7–9 and optical sensors.10–12 It is indeed an
interesting research subject to meld molecular recogni-
tion and luminescent characteristics into one molecular
system, producing a new type of supramolecule having
dual functionalties as receptor and fluorophore.13,14 We
report here the synthesis of cavitands bearing four
fluorophores (see an example shown in Figure 1), and
their photoluminescent properties including quantum
yields.
order to connect the upper rim of cavitands and
fluorophores, we have used spacer containing
a
–CH2C(O)–, which can play as complexing moieties for
metal ions.17,18 Treatment of 1 with four equivalents of
BrCH2C(O)OEt gave tetraester 3. Tetraester 3 was
hydrolyzed under basic conditions to give tetraacid 519
which, upon treatment with oxalyl chloride (20 equiva-
lents) gave the corresponding tetraacyl chloride 7.
Compound 7 was isolated in essentially quantitative
yield and used for next steps without further purifica-
tion or characterization. Analogously, tetraol 2 was
converted to the corresponding tetraacyl chloride 8 in
high yield. The coupling of tetraacyl chloride 7 with
t-BOC protected FurylBz was performed in CH2Cl2
using Et3N as a base to give TetraFurylBz 9 in 48%
yield. TetraFurylBz 10 was also prepared in 35% yield
from tetraacyl chloride 8 under the similar conditions.
Treatment of t-BOC protected PFDA with tetraacyl
chloride 7 likewise provided TetraPFDA 11 in 56%
yield. The products were identified by infrared (IR),
proton and carbon nuclear magnetic resonance (1H and
13C NMR), and mass spectroscopy.20
Syntheses of tert-butoxycarbonyl protected 5,5%-bis(4-
aminophenyl)-2,2%-bifuryl (t-BOC PFDA) and 2,2%-bis-
(furyl)benzidine (t-BOC FurylBz) are based on the
modification of a known procedure.15,16 The desired
cavitands having four fluorophores (9, 10, and 11) on
the upper rim of cavitands (1 and 2) were prepared by
using amide coupling as a key reaction (Scheme 1). In
The photoluminescent properties of the fluorophores
and the fluorescent cavitands 9, 10 and 11 were investi-
gated. Photoluminescence (PL) spectra were measured
at room temperature using a fluorescence spectrophoto-
meter (PTI Fluorescence System) with a Xenon Lamp.
For both excitation and emission monochromators,
band-passes were 2 nm, respectively. The PL behavior
Keywords: cavitands; fluorophores; absorption; photoluminescence;
blue light emit; quantum yield; receptor; dual functionalities.
* Corresponding author. Tel.: +82-54-279-2120; fax: +82-54-279-3399;
e-mail: ree@postech.edu
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.070

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M. Ree et al. / Tetrahedron Letters 44 (2003) 8211–8215
of the samples was investigated for their solutions with
a concentration of 1.5×10−5 g/mL in 1,4-dioxane. The
obtained UV-vis absorption (Abs) and PL spectra are
shown in Figure 2 and 3. FurylBz in 1,4-dioxane has an
absorption maximum at 328 nm which is considered to
be the p–p* transition, leading to the formation of a
singlet exciton. With this UV-vis absorbance character-
istic, the solution of FurylBz was excited at a wave-
length of 328 nm, exhibiting a PL emission peak at 434
nm. In addition, PFDA exhibited a broad featureless
absorption with its low energy edge lying at ca. 376 nm.
PFDA exhibited two emission peaks at 415 and 440 nm
when excited at a wavelength of 380 nm. Both
fluorophores emit a very intense blue fluorescence.
On the other hand, TetraFurylBz 9 and TetraFurylBz
10 had absorption peaks at 258 and 256 nm, respec-
tively. When TetraFurylBz 9 in the solution was excited
at 288 nm, it had the best fluorescence spectrum with
respect to intensity. TetraFurylBZ 9 showed a peak
maximum at 404 nm in the emission spectrum, which
was substantially blue-shifted in comparison to FurylBz
with a PL peak at 434 nm. Similarly, TetraFurylBz 10
was excited at 287 nm, which provided the best fluores-
cence spectrum with respect to intensity. It had a PL
peak maximum at 405 nm.
Figure 1. Molecular structure of a cavitand bearing four
fluorophores (TetraPFDA 11); the blue colored ellipsoids
indicate four fluorophore bulbs emitting blue light. The atoms
are distinguished as O, red; C, grey; N, blue.
Here, one can see two peculiar phenomena for Tetra-
FurylBz 9 and 10. First, the absorption at ca. 328 nm
due to the p–p* transition is decreased, compared to
Scheme 1.

M. Ree et al. / Tetrahedron Letters 44 (2003) 8211–8215
8213
induces such weakening in the absorption at 328 nm
and such blue-shift in the PL peaks for TetraFurylBz 9
and 10.
TetraPFDA 11 solution had an absorption peak at 378
nm and two PL peaks at 413 and 437 nm which were
excited at 377 nm. The absorption and PL spectra of
TetraPFDA 11 are very similar to those of PFDA.
There is only a little shifting of absorption and PL
peaks in the case of TetraPFDA 11. This observation
might be understood as follows. PFDA has furyl
groups between two phenyl rings, so it is less bulky
than FurylBz. Accordingly, when four t-BOC protected
PFDA molecules are connected to the upper rim of a
cavitand, the steric hindrance is relatively small and
thus the conformational change in the structure of
PFDA is little, compared to those of FurylBz. Conse-
quently, there is only a little change of the electronic
structure of PFDA due to the coupling with 7, so that
the absorption and PL peaks of TetraPFDA 11 remain
nearly unchanged, compared to those of PFDA.
We also measured the PL quantum yields of the
fluorescent cavitands. Relative PL quantum yield was
determined by comparing the ratio of the fluorescence
emission intensity maximum to UV-vis absorbance at
excitation wavelength used for a sample with that of a
standard. Absorbances of all sample solutions were
kept between 0.05 and 0.08 by changing concentrations,
in order to avoid any inner filter effect. Here, quinine
sulfate was employed as a standard: quinine sulfate in
1.0 N H2SO4 at 23 1oC was reported to have Ff=0.55
when excited at 365 nm.21–24
Figure 2. UV-Visible absorption (Abs) and photolumines-
cence (PL) spectra of FurylBz and TetraFurylBz 9, 10.
that of FurylBz. Second, the PL peaks are blue-shifted.
We believe that these phenomena are attributed to the
effect of molecular conformations on the ground-state
and excited-state electronic structures of the FurylBz
attached to the upper rim of cavitands. FurylBz has
two furyl side groups, so that it is bulky by itself. If
four t-BOC protected FurylBz molecules are coupled
with a cavitand, they have more steric hindrance and
their structures are twisted on the upper rim of the
cavitand. Thus, one can expect a large change in the
molecular conformation of the protected FurylBz on
the upper rim of the cavitands and its electronic struc-
ture with shortened conjugation length, compared to
those of FurylBz. In addition, the t-BOC group substi-
tuted on FurylBz may affect the conformational and/or
electronic structure of 9 and 10. However, TetraPFDA
11 having the same t-BOC group reveals the Abs and
PL peaks which resemble those of PFDA as seen in
Fig. 3; in the case of 11, the effect of t-BOC group on
the Abs and PL peaks is negligible. Taking this fact
into account, the effect of t-BOC group on the Abs and
PL peaks of 9 and 10 is expected to be negligible.
Consequently, the change of the electronic structure
due to the conformational modification of FurylBz
Figure 3. UV-Visible absorption (Abs) and photolumines-
cence (PL) spectra of PFDA and TetraPFDA 11.

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M. Ree et al. / Tetrahedron Letters 44 (2003) 8211–8215
Table 1. Quantum yields of the flourophores and the
fluorescent cavitands
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Refractive indices of pure 1.0 N H2SO4 and 1,4-dioxane
were used for the standard and the sample solutions in
the estimation of quantum yield. Quantum yields
reported here were averaged over at least three mea-
surements, with a standard deviation below 0.03 (Table
1). PL quantum yields of FurylBz and PFDA are
measured to be 0.52 and 0.92, respectively. In the case
of PFDA, its quantum yield is relatively very high. For
TetraFurylBz 9 and 10, relative quantum yields were
0.09 and 0.06, respectively. They have lower PL
efficiencies than FurylBz. The quantum yield of Tetra-
PFDA 11 was 0.73, which is also lower than that of
PFDA. In comparison, the quantum yield of Tetra-
PFDA 11, however, is much higher than those of
TetraFurylBz 9 and TetraFurylBz 10.
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Overall, cavitands bearing four fluorophores have a
lower PL quantum yield compared to that of the corre-
sponding fluorophore itself. These results might be due
to two major factors as follows. The first contributing
factor may be an increase in the interaction between
four fluorophores attached to the upper rim of cavi-
tands that can affect nonradiative decay processes of
the excited states, in comparison to fluorophore itself
which is not attached onto cavitand. The second factor
may be an unfavorable conformational change of the
fluorophores via their attachment to the cavitands,
which can shorten their p-conjugation length, conse-
quently lowering quantum yield in the photolumines-
cence.
14. Baki, C. N.; Akkaya, E. U. J. Org. Chem. 2001, 66, 1512.
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19. Lee, S. B.; Hwang, S.; Chung, D. S.; Hong, J.-I. Tetra-
hedron Lett. 1997, 38, 8713.
20. Selected data for 9: m.p. >246°C dec.; 1H NMR (l,
DMSO-d6): 9.96 (s, 1H, CH2C(O)NH), 9.57 (s, 1H,
OC(O)NH), 7.61 (d, 1H, Ph-H), 7.54 (s, 2H, fuyl-H),
7.44 (s, 1H, Ph-H), 7.40 (d, 1H, Ph-H), 7.20 (br s, 5H,
Ph-H), 7.10 (d, 1H, Ph-H), 7.01 (d, 1H, Ph-H), 6.22 (d,
2H, furyl-H), 6.05 (d, outer of OCH2O), 5.30 (s, 2H,
furyl-H), 4.69 (br s, 3H, CH2CH2CH, OCH2C(O)), 4.52
(d, 1H, inner of OCH2O), 2.63 (br s, 4H, CHCH2CH2),
1.51 (s, 9H, C(CH3)3); 13C NMR (l, DMSO-d6): 166.92,
152.73, 151.47, 151.21, 146.52, 143.60, 142.14, 142.03,
141.41, 139.37, 138.82, 137.82, 133.05, 131.22, 131.10,
130.76, 129.49, 129.19, 128.36, 128.19, 125.79, 118.53,
117.35, 116.29, 115.70, 114.92, 111.61, 111.47, 108.33,
108.08, 99.50, 79.17, 72.10, 36.88, 33.74, 31.34, 28.07; IR
(KBr, cm−1): 3336, 2976, 2934, 1693, 1609, 1585, 1524,
1494, 1478, 1390, 1368, 1315, 1235, 1157, 1050, 1018, 973,
738, 701; m/z (FAB, NBA) 2842.19 [M+H]+. For 10:
In conclusion, we have demonstrated the synthesis and
photoluminescent properties of fluorophores and cavi-
tands with four fluorophores on their upper rim. These
new fluorescent cavitands 9, 10 and 11 have not only
fluorophores which can emit photoluminescence but
also ionophoric functional groups such as amide link-
ages attached to their phenolic oxygen atoms that can
act as complexing moieties for metal ions and organic
cations. In particular, the cavitand 11 exhibits a high
quantum yield in the photoluminescence. Therefore,
these cavitands have potential applications in optical
and chemical sensors. In addition, these are useful as
intermediate compounds for synthesizing a variety of
new functional cavitands since each fluorophore moiety
owns a reactive amino group at the end after the t-BOC
groups are deprotected.
1
m.p. >267°C dec.; H NMR (l, DMSO-d6): 9.97 (s, 1H,
CH2C(O)NH), 9.61 (s, 1H, OC(O)NH), 8.24 (s, 1H,
Ph-H), 8.14 (s, 1H, Ph-H), 7.65–7.58 (m, 4H, Ph-H,
furyl-H), 7.43 (d, 1H, Ph-H), 7.12 (d, 1H, Ph-H), 7.03 (d,
1H, Ph-H), 6.26 (m, 2H, furyl-H), 6.05 (d, 1H, outer of
OCH2O), 5.32 (d, 2H, furyl-H), 4.88 (d, 1H, CH3CH),
4.70 (s, 2H, OCH2C(O)), 4.50 (d, 1H, inner of OCH2O),
1.88 (d, 3H, CH3CH), 1.55 (s, 9H, C(CH3)3); IR (KBr,
cm−1): 3342, 2976, 2936, 1707, 1609, 1585, 1524, 1494,
1479, 1446, 1391, 1369, 1314, 1238, 1158, 1115, 1052,
1019, 982, 738; m/z (FAB, NBA) 2481.47 [M+H]+. For
11: m.p. >210°C dec.; 1H NMR (l, DMSO-d6): 9.91 (s,
Acknowledgements
This study was supported by the Center for Integrated
Molecular Systems (KOSEF) and by the Ministry of
Education (BK21 Program).

M. Ree et al. / Tetrahedron Letters 44 (2003) 8211–8215
8215
1H, CH2C(O)NH), 9.50 (s, 1H, OC(O)NH), 7.75 (s,
4H, Ph-H), 7.67 (d, 2H, Ph-H), 7.54 (d, 2H, Ph-H),
7.43 (s, 1H, Ph-H), 7.20 (br s, 5H, Ph-H), 6.97 (d, 1H,
furyl-H), 6.93 (d, 1H, furyl-H), 6.84 (d, 2H, furyl-H),
5.95 (d, 1H, outer of OCH2O), 4.67 (br s, 3H,
CH2CH2CH, OCH2C(O)), 4.49 (d, 1H, inner of
OCH2O), 2.62 (br s, 4H, CHCH2CH2), 1.49 (s, 9H,
C(CH3)3); IR (KBr, cm−1): 3338, 2933, 1694, 1596,
1515, 1467, 1413, 1367, 1316, 1233, 1158, 1108, 1052,
1018, 972, 835, 779, 701; m/z (FAB, NBA) 2841.90
[M+H]+.
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