Tropolone-terminated oligomeric fluorophores with responsive properties to external environment

Article abstract of DOI:10.1246/bcsj.82.236

Tropolone-terminated oligomeric fluorophores (Tp20P, Tp20PV, and Tp20PE) were prepared by palladium-catalyzed coupling reactions. A phenylene-type Tp20P showed an emission maximum in the blue fluorescent region and the fluorescence quantum yield was relat

Full text of DOI:10.1246/bcsj.82.236

236
Bull. Chem. Soc. Jpn. Vol. 82, No. 2, 236–241 (2009)
© 2009 The Chemical Society of Japan
Tropolone-Terminated Oligomeric Fluorophores with
Responsive Properties to External Environment
Koji Takagi,* Kazuhiro Saiki, Hirokazu Hayashi, Hiroyasu Ohsawa,
Shin-ichi Matsuoka, and Masato Suzuki
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso, Showa-ku, Nagoya 466-8555
Received July 15, 2008; E-mail: takagi.koji@nitech.ac.jp
Tropolone-terminated oligomeric fluorophores (Tp2OP, Tp2OPV, and Tp2OPE) were prepared by palladium-
catalyzed coupling reactions. A phenylene-type Tp2OP showed an emission maximum in the blue fluorescent region and
the fluorescence quantum yield was relatively low. Tp2OPV and Tp2OPE, which included vinylene and ethynylene units
as connecting groups, respectively, had emission maxima at longer wavelength regions than Tp2OP. The UV spectra of
Tp2OP gradually red shifted as the dielectric constant of solvent increased. Tp2OP exhibited positive solvatofluoro-
chromic behavior, which related to an increase of the dipole moment due to the charge-transfer characteristics of the
emitting state. The fluorescence quantum yields (QYs) of fluorophores exponentially fell with increasing MeOH content
in THF solution. Upon addition of CuCl2 to Tp2OP until the ratio of [Cu2+]/[Tp2OP] reached 1, the UV spectra exhibited
a red-shift. The emission maximum wavelength of Tp2OP blue shifted and a remarkable decrease of the PL intensity was
observed. Tp2OP showed metal ion-response, especially in PL spectra.
The development of novel luminescent molecules1 is an
to have fascinating optical properties.9 On the other hand, the
emission properties of fluorophores having a metal ion-
chelating group are influenced by the coordinated metal ions
(and of course their counter ions). It has been reported that
tropolone complexes with transition-metal ions are more
stable than those of ¢-diketones of comparable acidity10 and
tropolones are suitable ligands for lanthanide metal ions with
high coordination number.11 Accordingly, conjugated oligo-
meric fluorophores containing the tropolone structure also
belong to a rather attractive family due to the responsive optical
properties to metal ions.
essential topic in chemical applications including optoelec-
tronic devices,2 biological chemosensors,3 and metal ion
analyses.4 Environment-sensitive fluorophores are a special
class of chromophores showing responsive spectroscopic
behaviors, namely shifts of emission maximum wavelength
and changes of fluorescence intensity that are dependent upon
the physicochemical properties of the surrounding environ-
ment. The emission properties of these kinds of organic
materials are tunable via chemical modifications of a base
structure that has been extensively explored. Among many
organic materials, oligomeric fluorophores bearing ³-conju-
gated systems would be better candidates compared to
luminescent conjugated polymers, because they have precisely
defined molecular length and can be purified to obtain defect-
free compounds. The choice of aromatic segments and their
connecting groups can provide access to diverse types of
functional fluorophores. We have previously reported the
synthesis of a series of conjugated polymers possessing
seven-membered non-benzenoid aromatic troponoid deriva-
tives and found interesting optical properties.5 Tropolone, 2-
hydroxy-2,4,6-cycloheptatrien-1-one, has an equal double-
minimum multidimensional potential energy surface allowing
its two equivalent tautomers to interconvert and has been
extensively studied from experimental and theoretical points of
view.6 There are a few reports utilizing tropolone as the key
element of functional molecules such as liquid crystalline
materials7 and antibiotic polymers.8 However, to the best of our
knowledge, conjugated oligomeric fluorophores containing the
tropolone moiety have been hitherto unknown. Since tropolone
has a relatively large dipole moment (3.71 debye), a completely
planar structure, and an amphiphilic character (pKa = 6.7 and
pKBH = Õ0.86), the corresponding fluorophore can be expected
In the present paper, we would like to describe the
preparation and optical properties of tropolone-terminated
oligomeric fluorophore in the first section and the optical
response to metal ions in the second section.
Experimental
General. All reactions were performed under nitrogen unless
otherwise noted. Dry diethyl ether (ether) and tetrahydrofuran
(THF) were purchased from Aldrich and Kanto Chemical Co.,
respectively. Dichloromethane (CH2Cl2), toluene, hexane, and N-
methyl-2-pyrrolidone (NMP) were dried over CaH2 and distilled
under nitrogen prior to use. Tetrakis(triphenylphosphine)palla-
dium(0) was purchased from Tokyo Chemical Industry (TCI) Co.
Palladium(II) acetate was purchased from Wako Pure Chemical
Industry Co. Dichlorobis(triphenylphosphine)palladium(II) was
purchased from Aldrich Chemical Co. Other reagents were used
as received. Silica gel 60N obtained from Kanto Chemical Co. was
used for column chromatography.
Instrumentation. 1H and 13C nuclear magnetic resonance
(NMR) spectra were recorded on a Bruker Avance 200 FT-NMR
spectrometer using tetramethylsilane (1H NMR, ¤ 0.00) and CDCl3
(13C NMR, ¤ 77.0) as internal reference peaks. Infrared (IR)
spectra were recorded on a Jasco FT-IR 460Plus spectrophotometer
Published on the web February 13, 2009; doi:10.1246/bcsj.82.236

K. Takagi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 2 (2009)
237
in ATR method. Melting points (Mp) were determined on a
Yanagimoto micro melting point apparatus MP-J3. Elemental
analyses (EA) were performed on a Perkin-Elmer PE2400II in
CHN mode. Ultraviolet (UV) and photoluminescence (PL) spectra
were recorded on a Shimadzu UV-1650PC spectrophotometer and
a Shimadzu RF-5300PC spectrofluorometer, respectively, using a
1-cm quartz cell. Quantum yields in solution were determined
relative to quinine sulfate in 0.05 M H2SO4 with a quantum yield
of 0.55.
(0.04 g, 0.33 mmol) were added an aqueous solution (0.6 mL) of
K2CO3 (0.09 g, 0.66 mmol) and Pd(PPh3)4 (0.02 g, 0.02 mmol).
The mixture was heated to reflux for 2 days and extracted with
CH2Cl2. The organic phase was washed with diluted aq. HCl,
distilled H2O, and brine. After drying over MgSO4, purification by
SiO2 chromatography (CHCl3/MeOH = 19/1 in volume, Rf =
0.4) gave 0.05 g of green solid (79% yield). 1H NMR (200
MHz, CDCl3): ¤ 7.63 (d, J = 12 Hz, 2H), 7.48Í7.36 (br, 7H).
13C NMR (50 MHz, CDCl3): ¤ 170.8, 142.4, 142.4, 137.7, 132.2,
132.0, 129.0, 128.1, 127.5, 124.1. IR (ATR) (cmÕ1) 3189, 2923,
2852, 1617, 1552, 1486, 1458, 1423, 1360, 1257, 1213, 1118,
1072, 853, 753, 719, 696. Found: C, 71.89; H, 5.53%. Calcd for
C13H10O2¢H2O: C, 72.21; H, 5.59%.
Metal Ion Coordination (Typical Procedure): To a THF
solution (10Õ5 M (1 M = 1 mol dmÕ3)) of Tp2OP was added a
MeOH solution of NaOH (10Õ3 M) where the molar ratio of
base relative to tropolone was kept at unity. A varying amount
of CuCl2 (MeOH solution, 10Õ3 M) was subsequently added
while maintaining the volume ratio of THF/MeOH at 20/1. After
shaking the solution in a vial several times, UV and PL spectra
were collected.
Synthesis of Tropolone-Containing Fluorophores. 1,4-Di-
decyloxy-2,5-bis(4-hydroxy-5-oxocyclohepta-1,3,6-trienyl)ben-
zene (Tp2OP): To a THF (1.7 mL) solution of 5-bromotropolone
(0.05 g, 0.25 mmol) and 2,5-didecyloxy-1,4-phenylenediboronic
acid12 (0.05 g, 0.11 mmol) were added an aqueous solution
(0.4 mL) of K2CO3 (0.06 g, 0.45 mmol) and Pd(PPh3)4 (0.01g,
0.01 mmol). The mixture was heated to reflux for 2 days and
extracted with CHCl3. The organic phase was washed with diluted
aq. HCl, distilled H2O, and brine. After drying over MgSO4,
precipitation into cold hexane gave 0.06 g yellowish-green solid
(79% yield). Mp 127Í129 °C. 1H NMR (200 MHz, CDCl3): ¤ 7.57
(d, J = 12 Hz, 4H), 7.37 (d, J = 12 Hz, 4H), 6.87 (s, 2H), 3.91 (t,
J = 6.3 Hz, 4H), 1.66 (m, 4H), 1.42Í1.01 (br, 28H), 0.86 (t, J =
3.4 Hz, 6H). 13C NMR (50 MHz, CDCl3): ¤ 170.9, 149.8, 139.5,
139.2, 123.3, 115.4, 69.8, 31.8, 29.5, 29.5, 29.3, 29.2, 26.1, 22.6,
14.1. IR (ATR) (cmÕ1) 3136, 2915, 2850, 1619, 1549, 1509, 1458,
1399, 1360, 1293, 1262, 1214, 1037, 848, 817, 678. Found: C,
74.39; H, 8.73%. Calcd for C40H54O6¢H2O: C, 74.04; H, 8.70%.
1,4-Didecyloxy-2,5-bis[(E)-(4-hydroxy-5-oxocyclohepta-1,3,6-
trienyl)ethenyl]benzene (Tp2OPV): To a NMP (5 mL) solution
of 5-bromotropolone (0.05 g, 0.25 mmol), 1,4-didecyloxy-2,5-
diethenylbenzene13 (0.05 g, 0.11 mmol), and NaOAc (0.03 g, 0.34
mmol) was added Pd(OAc)2 (2.5 mg, 0.01 mmol). The mixture was
heated at 100 °C for 2 days and poured into a large amount of H2O.
CHCl3 was added and the separated organic phase was washed
with diluted aq. HCl, distilled H2O, and brine. After drying over
MgSO4, the crude product was dissolved again in THF and
purified by precipitating into cold hexane to give 0.03 g red solid in
Results and Discussion
Preparation.
5-Bromotropolone was synthesized by
modifying methods15 reported earlier starting from 1,4-
cyclohexadiene in four steps (See, Supporting Information,
Figures S1ÍS4). Three different tropolone-terminated oligo-
meric fluorophore (Tp2OP, Tp2OPV, and Tp2OPE) bearing
connecting groups were prepared by using palladium-catalyzed
coupling reactions, as shown in Scheme 1. Although the
tropolone unit was likely to deactivate the palladium catalyst
because of the coordination ability to the metal center, the cross
coupling reactions successfully proceeded to give desired
1
products. Those H NMR spectra are shown in Figures S5ÍS7.
The configuration of C=C double bond in Tp2OPV was
proven to be trans judging from the coupling constant of
vinylene protons (J = 16 Hz). The characteristics of the C=O
double bonds were dependent upon the chemical structure of
the connecting group. Namely, the carbonyl stretching vibra-
tion of Tp2OP, Tp2OPV, and Tp2OPE were observed at 1619,
1611, and 1614 cmÕ1, respectively, in the IR spectra. This trend
agrees with our previous report about conjugated polymers
with a tropone ring in the main chain, where the carbonyl
stretching vibration of the tropolone ring embedded in the
phenylenevinylene structure was observed in the lowest wave-
number region.5b The results of elemental analyses indicated
the inclusion of a water molecule, which may be due to the
intermolecular hydrogen-bonding interaction with the tropo-
lone unit. All materials, especially Tp2OP, demonstrated good
solubility in common organic solvents. Furthermore, Tp1OP
bearing one tropolone ring (Figure S8) and OP without any
tropolone ring (Figure S9) were synthesized as reference
compounds by Suzuki coupling reaction. The carbonyl stretch-
1
39% yield. Mp 129Í130 °C. H NMR (200 MHz, CDCl3): ¤ 7.58
(d, J = 10 Hz, 4H), 7.44Í7.31 (br, 6H), 7.11 (d, J = 16 Hz, 2H),
7.07 (s, 2H), 4.06 (t, J = 6.3 Hz, 4H), 1.87 (m, 4H), 1.65Í0.98 (br,
28H), 0.88 (t, J = 6.0 Hz 6H). IR (KBr) (cmÕ1) 3221, 2922, 2852,
1611, 1555, 1495, 1455, 1418, 1332, 1261, 1202, 1024, 952, 859,
744. Found: C, 75.09; H, 8.68%. Calcd for C44H58O6¢H2O: C,
75.39; H, 8.63%.
1,4-Didecyloxy-2,5-bis[(4-hydroxy-5-oxocyclohepta-1,3,6-tri-
enyl)ethynyl]benzene (Tp2OPE): To a solution of 5-bromo-
tropolone (0.05 g, 0.25 mmol) and 1,4-didecyloxy-2,5-diethynyl-
benzene14 (0.05 g, 0.11 mmol) in toluene/NEt3 (2.5 mL/1 mL)
were added CuI (2.2 mg, 0.01 mmol) and PdCl2(PPh3)2 (7.9 mg,
1.1 µmol). The mixture was heated at 70 °C for 2 days and poured
into a large amount of H2O. CHCl3 was added and the separated
organic phase was washed with diluted aq. HCl, distilled H2O, and
brine. After drying over MgSO4, the crude product was dissolved
again in THF and purified by precipitating into cold Et2O to give
1
0.04 g of an orange solid in 54% yield. Mp 124Í125 °C. H NMR
ing vibration of Tp1OP was observed at 1617 cmÕ1
.
(200 MHz, CDCl3): ¤ 7.60 (d, J = 12 Hz, 4H), 7.29 (d, J = 12 Hz,
4H), 6.97 (s, 2H), 4.03 (t, J = 6.2 Hz, 4H), 1.82 (m, 4H), 1.63Í0.97
(br, 28H), 0.88 (br, 6H). IR (ATR) (cmÕ1) 3207, 2923, 2852, 1614,
1556, 1420, 1260, 1207, 1018, 720. Found: C, 75.35; H, 8.08%.
Calcd for C44H54O6¢H2O: C, 75.83; H, 8.10%.
Optical Properties. The color of powdery Tp2OP was
yellowish-green, which was much different from that of a
terphenyl analog OP (colorless). The colors of Tp2OPV and
Tp2OPE were red and orange, respectively. Thus the tropolone
unit seems to have a significant influence upon the electronic
state of the material. The UV and PL spectra of tropolone-
5-Phenyltropolone (Tp1OP): To a THF (3 mL) solution of
5-bromotropolone (0.07 g, 0.33 mmol) and phenylboronic acid

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Bull. Chem. Soc. Jpn. Vol. 82, No. 2 (2009)
Tropolone-Terminated Oligomeric Fluorophores
OC
H
10 21
OC
H
10 21
HO
O
O
Pd(PPh )
3 4
(HO) B
B(OH)
2
2
OH
C
H O
10 21
C
H O
10 21
Tp2OP
HO
O
OC
H
OC H
10 21
10 21
O
Pd(OAc)
2
OH
O
+
C
H
O
C H O
10 21
Br
OH
10 21
Tp2OPV
OC
H
10 21
OC
H
10 21
HO
O
PdCl (PPh )
2
3 2
O
OH
C
H O
C
H O
10 21
10 21
Tp2OPE
Scheme 1. Synthetic route of tropolone-terminated oligomeric fluorophores.
responsible for the low quantum yield. Tp2OP, bearing two
tropolone units in the opposite direction at both termini, is
nevertheless considered to be relatively less polar because the
local dipole moments arising from tropolone cancel each other.
It is believed however, that Tp1OP is highly polar although we
could not experimentally evaluate the molecular dipole mo-
ment. Accordingly, Tp1OP did not emit light at all. Tp2OPV
and Tp2OPE had emission maxima at 539 and 489 nm,
respectively (Figure S10). The fluorescence quantum yield
was 8% for Tp2OPE and was lower (1%) for Tp2OPV. The
high or low fluorescence quantum yield of fluorophore was not
affected by the Stokes shift but correlated with the connecting
group between aromatic rings. The shorter triple bond in
Tp2OPE produces stronger bond alternation, which increases
the intrinsic band gap and is unfavorable for conjugation.16
Ring rotation around carbonÍcarbon triple bonds is rather more
feasible than Tp2OP and Tp2OPV to release the excited energy
by non-radiative process.
300
400
500
Wavelength/nm
600
700
Figure 1. UV (open mark) and PL (closed mark) spectra of
tropolone-terminated oligomeric fluorophores and refer-
ence compounds in THF solution (10Õ5 M) (circle: Tp2OP,
triangle: Tp2OPV, square: Tp2OPE, diamond: OP, and star:
Tp1OP).
Subsequently, the UV and PL spectra of Tp2OP were
collected in various solvents. Tp2OP could dissolve not only in
apolar solvents such as THF and CHCl3 but also in highly polar
solvents such as DMF and MeOH. The large dipole moment of
the tropolone ring may be a consequence of good solubility
even in MeOH, which is often a poor solvent for rigid ³-
conjugated molecules. The UV spectra gradually red shifted as
the dielectric constant of solvent increased (Figure S11). The
absorption maxima in toluene and DMF were observed at 362
and 377 nm, respectively. In DMF and MeOH, broad peaks
were detected at ca. 400 nm. Meanwhile, Tp2OP exhibited
positive solvatofluorochromic behavior. The PL spectra dem-
onstrated a bathochromic shift and a good linear relationship
was found between the emission maximum wavelength and the
dielectric constant of solvent (Figure 2). The shift of emission
maximum with increasing solvent polarity corresponds to an
increase in the dipole moment, indicating the charge-transfer
character of the emitting state.17 The fluorescence quantum
yields in THF and DMF were 0.11 and below 0.01, respec-
tively. It is conceivable that the central dialkoxybenzene unit
and terminal tropolone unit act as electron-donor and electron-
acceptor, respectively. In highly polar MeOH, the fluorescence
terminated oligomeric fluorophores together with reference
compounds were measured in a THF solution (10Õ5 M). Tp2OP
showed an absorption maximum at 364 nm, which obviously
red shifted as compared with that of OP (320 nm) possessing a
similar three aromatic ring sequence (Figure 1). The tropolone
ring has therefore a rather delocalized ³-electron system. Even
Tp1OP with one tropolone ring demonstrated an absorption
maximum at 340 nm. Tp2OPV and Tp2OPE bearing vinylene
and ethynylene connecting groups, respectively, showed
absorption maxima at lower energy region (446 and 398 nm)
as previously reported for other conjugated molecules. They
also had smooth shoulders at 375 and 358 nm. Meanwhile,
Tp2OP showed an emission maximum at 479 nm in the
blue fluorescent region. The fluorescence quantum yield was
determined to be 11% using quinine sulfate as a standard. In
contrast, OP without any tropolone unit showed an emission
maximum at 383 nm (deep blue) and the fluorescence quantum
yield was 35%. Tp2OP has a polarized carbonyl group and part
of the excited energy might be released through a non-radiative
process by the C=O stretching vibration. The intersystem
crossing of excited singlet state to triplet state is also

K. Takagi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 2 (2009)
239
0.42
0.38
0.34
0.3
540
520
500
480
460
440
Toluene
Chloroform
THF
0
10
20
30
40
Dielectric Constant
QY
0.12
Acetone
DMF
0.08
0.04
0
350
400
450
500
550
600
650
0
0.1
0.2
0.3
0.4
Wavelength/nm
MeOH/(MeOH+THF)
Figure 2. PL spectra of Tp2OP in various solvents
(10Õ5 M) (circle: toluene, triangle: CHCl3, square: THF,
diamond: acetone, and cross: DMF). A spectrum in NMP
is omitted for clarity. The inset shows the emission
maximum wavelength as a function of dielectric constant
of solvent.
Figure 4. Fluorescence quantum yields (QYs) of fluoro-
phores as a function of volume ratio of MeOH in THF
(circle: Tp2OP, triangle: Tp2OPE, diamond: Tp2OPV, star:
OP).
tration of MeOH (Figure 3). Since no detectable shifts were
observed in the spectra of a terphenyl analog OP within the
same range, the solvatochromic and solvatofluorochromic
characteristics are ascribed to the introduction of tropolone.
The change of PL intensity of tropolone-terminated fluoro-
phores was much different from that of OP without a tropolone
ring (Figure 4). The fluorescence quantum yields (QYs) of
Tp2OP, Tp2OPV, and Tp2OPE exponentially dropped and
those were below 0.01 as the content of MeOH reached
30 vol %, that can be attributed to the interaction with a protic
solvent (vide supra). In sharp contrast, QY of OP gradually
increased. Many molecules tend to quench fluorescence in the
condensed state such as film, which can be understood by the
intermolecular vibronic interactions to induce nonradiative
deactivation. However, unique enhanced emission rather than a
fluorescence quenching in the solid state was recently reported
for molecules with internal rotation around carbonÍcarbon
bonds.18 It is assumed that this unique fluorescence change is
more or less related to the effects of intramolecular planariza-
tion or a specific aggregation (H- or J-aggregation) in the solid
state. OP might form nanoparticles by adding a poor solvent
MeOH to induce an aggregation-induced enhanced emission
(AIEE).
515
505
495
485
475
0
0.2
0.4
0.6
0.8
1
MeOH/(MeOH+THF)
400
500
600
700
Wavelength/nm
Figure 3. Change of PL spectra of Tp2OP by increasing
concentration of MeOH from 0 to 50 vol % in THF. The
inset shows the emission maximum wavelength as a
function of MeOH content.
quantum yield of Tp2OP was almost zero, which originates
from intermolecular hydrogen bonding with MeOH. When a
13C NMR spectrum was measured in CDCl3 containing ca.
30 vol % MeOH-d4, the carbon signal assignable to C=O in
Tp2OP shifted from 170.9 to 172 ppm. This finding actually
supports the protonation of C=O by MeOH in the ground state.
The PL emission behavior can be visualized in Figure S12.
To evaluate the influence of MeOH upon the optical
properties of fluorophores in detail, the volume percentage
of MeOH against THF was consecutively changed in the
solution of Tp2OP, Tp2OPV, and Tp2OPE together with OP.
Figure S13a illustrates the UV spectra of Tp2OP under various
concentrations of MeOH, in which the absorption maximum
wavelength shifts from 364 nm in THF/MeOH = 100/0
to 373 nm in THF/MeOH = 0/100. Tp2OPV and Tp2OPE
also demonstrated red-shifts of 16 and 25 nm, respectively
(Figures S13b and S13c, respectively). Although an accurate
emission maximum of Tp2OPV and Tp2OPE could not be
determined due to the weak fluorescence, that of Tp2OP
obviously shifted from 479 to 514 nm with increasing concen-
Metal Ion Coordination. In order to gain insights into the
potential ability of tropolone-terminated fluorophores as a
sensory material for metal ions, a UV titration experiment
was carried out. After the deprotonation of Tp2OP by NaOH,
the absorption maximum red shifted from 36819 to 402 nm.
Subsequent addition of CuCl2 until the ratio of [Cu2+]/
[Tp2OP] reached unity, the spectra revealed a further red shift
of absorption maximum peak from 402 to 412 nm as shown in
Figure 5. As discussed later, it is conceivable that the ion
exchange occurs from Na+ ion to Cu2+ ion by adding CuCl2.
These red shifts in the presence of metal ions may have arisen
from a charge-transfer interaction between the electron-rich
dialkoxybenzene unit and the electron-deficient metal-coordi-
nated tropolone moiety. The coordination of Cu2+ ion was also
followed by means of the IR spectrum of a material obtained
under more concentrated conditions (10Õ4 M). The carbonyl
stretching vibration in Tp2OP was detected at 1619 cmÕ1
,

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Tropolone-Terminated Oligomeric Fluorophores
400
450
500
550
600
650
300
350
400
450
500
550
Wavelength/nm
Wavelength/nm
Figure 6. Change of PL spectra upon addition of CuCl2 to
Tp2OP in the presence of NaOH (2 equiv relative to
Tp2OP) in THF/MeOH (20/1 by volume) (10Õ5 M). The
spectra are shown at selected [Cu2+]/[Tp2OP] ratios
ranging from 0 to 1. The excitation wavelengths were
those of the absorption maximum.
Figure 5. Change of UV spectra upon addition of CuCl2 to
Tp2OP in the presence of NaOH (2 equiv relative to
Tp2OP) in THF/MeOH (20/1 by volume) (10Õ5 M). The
spectra are shown at selected [Cu2+]/[Tp2OP] ratios
ranging from 0 to 1.
Table 1. UV and PL Responses of Tp2OP upon Addition of Various Metal Ionsa)
Metal-free
Na+
Ca2+
Zn2+
Cu2+
Al3+
Fe3+
-
-
abs/nmb)
em/nmc)
368
489
402
485 (m)
395
518 (m)
399
502 (m)
408
470 (w)
401
479 (m)
394
484 (w)
a) Conditions: THF/MeOH = 20/1 by volume. The concentrations of metal ion [metal ion]/[Tp2OP] were 2 for
monovalent ion, 1 for divalent ions, and 2/3 for trivalent metal ions, respectively. b) The absorption maximum.
c) The emission maximum. The characters in parenthesis indicate relative PL intensity compared to metal-free
Tp2OP (m: middle and w: weak).
which shifted to 1601 cmÕ1 by forming the complex with Cu2+
ion (Figure S14).
Metal-free
Na(I)
Ca(II)
Zn(II)
Al(III)
Metal ion coordination was also confirmed by the change of
emission behavior as was evident from the PL spectra observed
upon the addition of CuCl2 into a solution of Tp2OP. The
emission maximum wavelength blue shifted from 485 nm20 to
approximately 470 nm and spontaneously a remarkable de-
crease of the PL intensity was observed (Figure 6). The
fluorescence emission gradually decreased until the molar ratio
of [Cu2+]/[Tp2OP] reached unity, and beyond this point, a
fluorescence signal almost vanished. The intrachain energy
transfer to the electron-deficient metal-coordinated tropolone
moiety came about to effectively quench the fluorescence
emission.21
Finally, the metal ion coordination to Tp2OP was likewise
carried out for other divalent ions (Ca2+ and Zn2+) and trivalent
ions (Al3+ and Fe3+). The ionochromic responses of Tp2OP
with these metal ions are summarized in Table 1 in conjunction
with the results of Na+ and Cu2+ ions. The absorption
maximum always red shifted as compared with that of metal-
free Tp2OP. The magnitude of shift was independent of the
valence number of ions as there was no great difference
between Ca2+ and Fe3+ (ca. 5 nm) or Zn2+ and Al3+ (ca.
10 nm). Chelate complex coordinated with Fe3+ and Cu2+ ions
showed the shortest and longest absorption maximum wave-
length, respectively. The coordination of Fe3+ ion also
demonstrated a shoulder at ca. 473 nm. The ion responsive
property of Tp2OP was confirmed more remarkably from
the PL spectra as illustrated in Figure 7. According to the
Fe(III)
Cu(II)
400
450
500
550
600
650
Wavelength/nm
Figure 7. Change of PL spectra upon addition of various
metal ions to Tp2OP in THF/MeOH (20/1 by volume)
(10Õ5 M) (closed circle: metal-free, open circle: Na+,
triangle: Ca2+, square: Zn2+, diamond: Al3+, star: Fe3+
,
and cross: Cu2+). The concentrations of metal ion [metal
ion]/[Tp2OP] were 2 for monovalent ion, 1 for divalent
ions, and 2/3 for trivalent metal ions, respectively.
spectroscopic influence of metal ion upon the emission
property of Tp2OP, the metal ions can be categorized into
two groups. The first group causes red-shift of the fluorescence
spectra, which includes typical element such as Ca2+ and Zn2+
ions. The PL intensity of these materials was moderate. The
second group conversely induces blue-shift of the fluorescence
spectra, which includes Cu2+, Al3+, and Fe3+ ions. The
coordination with transition-metal ions (Cu2+ and Fe3+
)

K. Takagi et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 2 (2009)
241
Suzuki, Polym. J. 2007, 39, 813. b) K. Takagi, K. Mori, H.
Kunisada, Y. Yuki, Polym. Bull. 2004, 52, 125.
substantially quenched fluorescence signals that could not be
seen. The difference in the PL spectra may be a consequence of
the following two factors. One is the ability of metal ions to
substitute pre-coordinated Na+ ion generated by the addition
of NaOH to Tp2OP. The complete substitution is believed
to cause the drastic change of PL spectra. Another can be
attributed to electron density variations on the fluorophore,
caused by introducing positively charged metal ions. It is noted
that Tp2OP has a potential ability to bind not only transition-
metal ions but also alkali and alkaline earth metal ions.
6
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rophore has a widespread conjugated system and demonstrated
interesting optical response to solvent polarity. The addition of
metal ion to the tropolone unit brought about the formation of
chelate complexes and their optical spectra were dependent
upon the nature of the metal ions.
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This work was financially supported by Iketani Science and
Technology Foundation (ISTF). We also thank Ms. Yachiyo
Taniyama for elemental analyses.
Supporting Information
Synthetic procedures and 1H NMR spectra of 5-bromotropolone
1
and reference compounds. H NMR spectra of fluorophores. UV
spectra and light-emitting behavior of fluorophores. IR spectra of
metal-free and metal-coordinated fluorophores. This material is
available free of charge on the web at http://www.csj.jp/journals/
bcsj/.
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