A Dual Emissive Coumarin–urea Derivative with an Electron-withdrawing Substituent in the Presence of Acetate Anion

Article abstract of DOI:10.1111/php.13165

We investigated the fluorescent properties, including the excited-state intermolecular proton transfer, of urea derivatives comprising a coumarin ring, which is a widely used fluorophore. We prepared two different coumarin–urea derivatives, 6CU and 7CU, which bear a urea-based substituent at the 6 and 7 positions of a coumarin ring, respectively. In the presence of the acetate ion, 7CU showed additional tautomer fluorescence emission with respect to 6CU, indicating that tautomer formation depends on the positions of the urea-based substituent on the coumarin ring. Thus, the resonance structures of urea derivatives might play an important role in the behavior of dual fluorescence, which is an important phenomenon applicable to photochemical anion sensing. Moreover, in order to further improve the fluorescence properties of the mentioned derivatives, a CF₃ group was introduced in a phenyl ring opposite to a coumarin ring. The fluorescence quantum yield of 7CUCF₃ thus synthesized was 65 times as large as that of 7CU, an observation that renders 7CUCF₃ a suitable anion sensor candidate. The results of this study will contribute to the development of new molecular designs for highly fluorescent sensing.

Full text of DOI:10.1111/php.13165

Photochemistry and Photobiology, 20**, **: *–*
A Dual Emissive Coumarin–urea Derivative with an Electron-withdrawing
Substituent in the Presence of Acetate Anion
Tomoyuki Shinoda, Yoshinobu Nishimura*
and Tatsuo Arai
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Japan
Received 28 July 2019, accepted 18 September 2019, DOI: 10.1111/php.13165
ABSTRACT
intramolecular charge transfer (ICT) (15,16). Furthermore, the
modification of a coumarin ring consisting of the introduction of
substituents into the said ring causes the compound to display
spectral changes (17). In this sense, coumarin derivatives are
suitable to be used as laser dyes and fluorogenic probes (18–20).
In addition, various fluorescence cation sensors (21,22) and anion
sensors (23,24) consisting of coumarin derivatives have been
synthesized and reported.
Urea derivatives have been used as anion receptors based on
the fact that they engage in hydrogen bonding interactions with
the “target anionic” species followed by fluorescence changes
due to the reformation of the emissive state (8,9,25,26). Notably,
few urea derivatives that comprise a coumarin ring as fluo-
rophore have been reported, although some coumarin derivatives
have been frequently used as fluorophores for various applica-
tions. Since coumarin–urea derivatives are expected to be poten-
tial high-sensitivity anion sensors, we found it worthwhile to
investigate their photochemical behavior in the absence and pres-
ence of target anions.
We have investigated the photochemistry of aromatic urea
derivative in the presence of tetrabutylammonium acetate
(TBAAc), an acetate anion-releasing species, in DMSO (27–33).
Most urea derivatives form complexes with TBAAc, a type of
reactivity made evident by the observation of a new emission
spectrum at longer wavelength. This longer-wavelength emission
band has been attributed to the formation of a tautomer resulting
from an excited-state intermolecular proton transfer reaction
between the complexes and the acetate anion through intermolec-
ular hydrogen bonds. A dual fluorescence urea derivative is
expected to work as a practical emitter (34).
We have revealed that the wavelength and intensity of tau-
tomer fluorescence depend on the position of the urea-based sub-
stituent in the fluorescent moiety. For example, in the presence
of TBAAc, a tautomer of 4PUP, a pyrene–urea derivative,
showed more intense fluorescence with larger stokes shift than
its isomer 1PUP; in other words, 4PUP proved more suitable as
a fluorescent anion sensor of the acetate anion than 1PUP (35).
In light of the results described above, coumarin–urea derivatives
may display a photochemical behavior that would render them
suitable anion sensors as a result of the modification of the posi-
tion of the urea moiety in the coumarin ring. In fact, the fluores-
cence properties of a coumarin derivative depend on the position
of urea unit in the coumarin ring more remarkably than it is the
case for a pyrene derivative, as a consequence of the difference
in emission mechanisms between these two fluorophores (36).
Furthermore, we prepared coumarin–urea derivatives having a
We investigated the fluorescent properties, including the
excited-state intermolecular proton transfer, of urea deriva-
tives comprising a coumarin ring, which is a widely used flu-
orophore. We prepared two different coumarin–urea
derivatives, 6CU and 7CU, which bear a urea-based sub-
stituent at the 6 and 7 positions of a coumarin ring, respec-
tively. In the presence of the acetate ion, 7CU showed
additional tautomer fluorescence emission with respect to
6CU, indicating that tautomer formation depends on the
positions of the urea-based substituent on the coumarin ring.
Thus, the resonance structures of urea derivatives might play
an important role in the behavior of dual fluorescence, which
is an important phenomenon applicable to photochemical
anion sensing. Moreover, in order to further improve the flu-
orescence properties of the mentioned derivatives, a CF3
group was introduced in a phenyl ring opposite to a cou-
marin ring. The fluorescence quantum yield of 7CUCF3 thus
synthesized was 65 times as large as that of 7CU, an observa-
tion that renders 7CUCF3 a suitable anion sensor candidate.
The results of this study will contribute to the development
of new molecular designs for highly fluorescent sensing.
INTRODUCTION
Anion sensors have been the focus of much research in the fields
of biological, supramolecular and industrial chemistry, since
anionic species play important roles in each of these fields (1–4).
In particular, fluorescent anion sensors have been extensively
studied because of their high detection sensitivity (5–7). In gen-
eral, a large number of fluorescent anion sensors consist of an
anion recognition unit and a fluorophore (8,9). In order for them
to be useful, these sensors need to be characterized by a high
enough fluorescence quantum yield (ΦFl) and a favorable interac-
tion with the target anionic species (10).
One of the most important classes of fluorophores comprises
coumarin derivatives. Some coumarin derivatives occur naturally
in Apiaceae and citrus fruits and have pharmacological properties
with respect to some diseases (11–14). Coumarin derivatives like
Coumarin120 and Coumarin102, which have an electron-donat-
ing substituent in their 7-position, are well known to be charac-
terized by relatively high ΦFl values resulting from an
*Corresponding author's email: nishimura@chem.tsukuba.ac.jp (Yoshinobu Nishi-
mura)
© 2019 American Society for Photobiology
1

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Tomoyuki Shinoda et al.
yellow solid (158 mg, 45% yield). 1H NMR (DMSO-d6 270 MHz)Á7.87
(d, 1H, J = 10.8 Hz) 7.09 (d, 1H, J = 8.1 Hz) 6.83 (dd, 1H) 6.73 (s,
1H) 6.34 (d, 1H, J = 10.8 Hz) 5.23 (s, 2H).
CF3 group on the phenyl ring bound to the other side of a urea
unit to a coumarin ring, since it was reported that the introduc-
tion of a CF3 group caused the value of the association constant
(Ka) of the complexation reaction between a coumarin-derived
compound and an anion to increase (37,38).
In this work, we synthesized coumarin–urea derivatives nCU
(n = 6 and 7) and nCUCF3 (n = 6 and 7) (Scheme 1) to investi-
gate the changes in fluorescence properties that result from the
addition of TBAAc to solutions of these species. Moreover, we
investigated how the differences in tautomer emission depended
on the substituent position in the coumarin moiety and how the
ΦFl parameter of the anion sensor was affected by the introduc-
tion of a CF3 group into a phenyl ring.
6CU. To a 50-mL three-neck flask were added 6-aminocoumarin
(61 mg, 0.38 mmol), dry THF (9 mL) and phenyl isocyanate (100 lL,
0.92 mmol). The mixture was then refluxed for 20 h; subsequently, the
supernatant was removed by suction filtration. The residue, the crude
desired product, was recrystallized from methanol to produce a light-
yellow solid (52.1 mg, 49%). 1H NMR (DMSO-d6 400 MHz)Á9.2 (s, 1H)
9.0 (s, 1H) 8.07 (d, 1H, J = 9.4 Hz) 7.59 (dd, 1H) 7.47 (d, 2H,
J = 8.8 Hz) 7.34 (d, 1H, J = 8.8Hz) 7.27 (t, 2H, J = 8.3 Hz) 6.96 (t,
1H, J = 7, 6 Hz). Anal. Calcd for C16H12N2O3: C, 68.56; H, 4.32; N,
9.99; Found: C, 68.58; H, 4.30; N, 9.96.
6CUCF. To a 50-mL three-neck flask were added 6-aminocoumarin
(79 mg, 0.49 mmol), dry THF (4 mL), and 4-trifluoromethylphenyl
isocyanate (80 lL, 0.57 mmol). The mixture was heated to a temperature
of 50°C and incubated at this temperature for 1 h. Subsequently, the
supernatant was removed by suction filtration. The residue was washed
with dichloromethane, and the pure desired product was obtained by
recrystallization of the residue from ethanol to produce a light-yellow
solid (46 mg, 27%). 1H NMR (DMSO-d6 400 MHz) 9.24 (s, 1H) 9.09
(s, 1H) 8.08 (d, 1H, J = 9.2 Hz) 7.91 (s, 1H) 7.69–7.62 (m, 4H) 7.59
(dd, 1H, J = 8.8 Hz) 7.36 (d, 1H, J = 9.2 Hz) 6.49 (d, 1H, J = 9.6 Hz).
Anal. Calcd for C17H11N2O3F3: C, 58.63; H, 3.18; N, 8.04; Found: C,
58.86; H, 3.15; N, 8.11.
MATERIALS AND METHODS
Methods. 1H NMR spectra were recorded on
(400 MHz for 1H) spectrometer and on
a
Bruker ARX-400
a
JOEL JNM-EX27000
(270 MHz for 1H) spectrometer using DMSO-d6 as solvent and
tetramethylsilan as internal standard. Absorption and fluorescence spectra
were measured on a Shimadzu UV-1600 spectrometer and a Hitachi F-
4500 and F-7000 fluorescence spectrometer, respectively. Fluorescence
decay measurements were carried out employing the time-correlated
single-photon counting method. Laser excitation at 375 nm was achieved
using a diode laser (PicoQuant, LDH-P-C-375) with a power control unit
7CU. To
a 50-mL three-neck flask were added 7-amino-4-
methylcoumarin (97 mg, 0.55 mmol), dry THF (12 mL) and phenyl
isocyanate (65 lL, 0.59 mmol). The mixture was refluxed for 18 h.
Subsequently, the supernatant was removed by suction filtration. The
residue was decanted with THF and methanol. The pure desired product
was obtained by recrystallization of the residue from dimethyl formamide
to produce a light-yellow solid (14 mg, 9%). 1H NMR (DMSO-d6
400 MHz) 9.38 (s, 1H) 9.02 (s, 1H) 7.68 (d, 1H, J = 8.0 Hz) 7.64 (s,
1H) 7.48 (d, 2H, J = 8.0 Hz) 7.37–7.28 (m, 3H) 7.00(t, 1H, J = 8.0 Hz)
6.22 (s, 1H) 2.40 (s, 3H). Anal. Calcd for C17H14N2O3: C, 69.38; H,
4.79; N, 9.52; Found: C, 69.23; H, 4.88; N, 9.45.
(PicoQuant, PDL 800-B), with
a repetition rate of 2.5 MHz. The
temporal profiles of the fluorescence decay were recorded using a
microchannel plate photomultiplier (Hamamatsu, R3809U) equipped with
a TCSPC computer board module (Becker and Hickl, SPC630). DMSO
(spectroscopic grade, Wako Pure Chemical Industries, Japan) was used as
solvent without further purification. The acetate ion was provided in the
form of TBAAc, which contains the tetrabutylammonium cation (Sigma-
Aldrich, Japan). All measurements were carried out at room temperature
under Ar atmosphere. The concentrations were adjusted so that the
absorption maximum of the excitation wavelength was about 0.1 for each
sample.
Syntheses. 6-Aminocoumarin. Coumarin was nitrated by an acid mix
in an ice bath. In particular, coumarin (590 mg, 4.03 mmol) was
dissolved in concentrated H2SO4 (3 mL), and the temperature of the
mixture thus obtained was maintained below 0°C. HNO3 (1 mL) was
then added to it, and the reaction mixture was stirred while it was being
kept at room temperature (rt) for 1 h. The reaction mixture was then
7CUCF. To
a 50-mL three-neck flask were added 7-amino-4-
methylcoumarin (108 mg, 0.618 mmol) dry THF (4 mL), dry dimethyl
formamide (1 mL) and 4-trifluoromethylphenyl isocyanate (100 lL,
0.716 mmol). The mixture was heated to 50°C and kept at that
temperature for 1 h. Subsequently, the supernatant was removed by
suction filtration. The residue was washed with THF and
dichloromethane. The pure desired product was obtained by
recrystallization of the residue from dimethyl formamide to give a white
solid (77 mg, 34%). 1H NMR (DMSO-d6 400 MHz) 9.56 (ss, 2H) 7.71–
7.64 (m, 6H) 7.39 (dd, 1H, J = 8.84 MHz) 6.24 (s, 1H) 2.49 (s, 3H).
Anal. Calcd for C18H13N2O3F3: C, 59.67; H, 3.62; N, 7.73; Found: C,
59.73; H, 3.66; N, 7.80.
poured into an ice–water mixture, and the formation of
a white
precipitate was observed. This precipitate (415 mg 4.03 mmol) was
dissolved in ethanol (15 mL) under N2 atmosphere at rt. To the solution
thus obtained was added SnCl2Á2H2O (2.44 g 10.8 mmol), the
temperature of the reaction mixture was raised to 90°C and the reaction
mixture was incubated at this temperature for 2 h. The reaction mixture
was cooled to rt and was poured onto an ice–water mixture rendered
basic (pH 8–9) with NaHCO3. The aqueous layer was extracted with
ethyl acetate (50 mL 9 3), and the combined organic layers were
washed first with water (50 mL) then with brine (50 mL). The solvent of
organic layer was then evaporated under reduced pressure to give the
crude desired product. This product was purified by column
RESULTS AND DISCUSSION
To investigate the interaction between nCU and TBAAc in the
ground state, absorption and 1H NMR spectroscopies were uti-
lized. The maxima in the absorption spectra of 6CU and 7CU in
the absence of TBAAc were observed at 353 and 336 nm,
respectively (Table 1). A redshift in the spectra of nCU was
observed in the presence of TBAAc, with little change evident in
the spectral shapes (Fig. 1). In 1H NMR spectrometry experi-
ments, the addition of TBAAc to nCU led to downfield shifts of
the resonance signals due to the N-H protons (Fig. 2). These
shifts are similar to those observed previously for urea com-
pounds (28). Therefore, the results described above imply that
nCU compounds form complexes in the ground state with an
acetate ion via intermolecular hydrogen bonding interactions
chromatography
(the
eluent
utilized
was
diethyl
ether:
dichloromethane = 2:1) and recrystallized from ethyl acetate to yield a
1
through the urea moieties. Notably, the absorption and H NMR
spectra of nCUCF3 in DMSO in the absence and presence of
TBAAc were also measured (Figures S1 and S2). In the absence
of TBAAc, the absorption maxima of the spectra of 6CUCF3
and 7CUCF3 were observed at 350 and 334 nm, respectively.
6CU
R = H
7CU
R = H
6CUCF3 R = CF3
Scheme 1. Chemical structures of coumarin–urea derivatives.
7CUCF3 R = CF3

Photochemistry and Photobiology
3
The effects that the addition of TBAAc had on the absorption
and 1H NMR spectra of nCUCF3 compounds were similar to
those observed in the case of nCU compounds.
The association constants of 6CU, 7CU, 6CUCF3 and
7CUCF3 with the acetate ion were 5700, 12 000, 11 000 and
28 000 MÀ1, respectively. The position on the ring of the urea
moiety may affect the acidity of the urea proton, as observed for
urea derivatives (28,35,37,39). These observations may also
explain the present results. By contrast, since CF3 is an electron-
withdrawing group, the strength of the hydrogen bonding interac-
tion of nCUCF3 with TBAAc may be increased with respect to
the nCU case. In fact, the association constants of nCUCF3 with
the acetate ion were about twice as large as those of the corre-
sponding nCU species.
In Fig. 3 are reported data that reflect the changes in the fluo-
rescence spectra of nCU in the presence of various concentra-
tions of TBAAc in DMSO. In the absence of TBAAc, the ΦFl
values for 6CU and 7CU were estimated to be 0.05 and 0.01,
respectively (Table 1). The fluorescence spectrum of 6CU in the
absence of TBAAc was characterized by a maximum at 473 nm.
As the TBAAc concentrations increased from 0 to 0.8 mM, how-
ever, the intensity of the mentioned maximum decreased progres-
sively, while almost no change was observed in the shape of the
fluorescence spectrum. On the other hand, the fluorescence spec-
trum of 7CU was characterized by a maximum at 402 nm. The
presence of TBAAc caused the intensity of the mentioned maxi-
mum to decrease, although in this case the increase in TBAAc
concentration was also associated with the appearance of a new
band with a maximum at 485 nm.
The time-resolved fluorescence spectra of nCU in the presence
of TBAAc were also recorded (Figure S3). Although the spec-
trum of 6CU showed almost no time-dependent change in shape,
the spectrum of 7CU was characterized by the appearance of a
longer-wavelength fluorescence band in the later time region.
The time development of a rise-up component of 7CU was
extracted from the spectra according to a previous study (40),
since no corresponding rise-up component was observed due to
canceling the rise-up curve with the same decay curve. The
longer-wavelength component was estimated by subtracting the
complex emission observed just after laser excitation from each
spectrum. The resulting intensities of the longer-wavelength and
complex components were plotted versus time (Figure S4).
Although the fluorescence at 430 nm consists of 68 ps and
2.2 ns, the calculated fluorescence decay consists of a rise-up
component with a lifetime of 55 ps, whose value is in good
agreement with the lifetime of the complex decay observed at
430 nm, along with a decay component of 3.3 ns. These results
imply that the longer-wavelength band observed in the time-re-
solved fluorescence spectrum of 7CU is due to tautomer emis-
sion generated by excited-state proton transfer to the acetate ion,
as described in previous studies (27,28). Evidence thus indicates
that whether tautomer formation is observed in nCU-type
Figure 1. Changes in the absorption spectra of (a) 6CU and (b) 7CU
upon the addition of TBAAc in DMSO under Ar.
compounds depends on the location on the coumarin ring of the
urea unit.
The fluorescence spectra of nCUCF3 in the presence of vari-
ous concentrations of TBAAc in DMSO were also collected
(Figure S5). In the absence of TBAAc, the fluorescence spectra
of 6CUCF3 and 7CUCF3 were characterized by maxima at 470
and 395 nm, respectively. In the presence of TBAAc, only the
fluorescence spectrum of 7CUCF3 displayed a new longer-wave-
length band, which had a maximum at 472 nm, similarly to what
had been observed for 7CU.
Insertion of a CF3 group in the nCU compounds resulted in a
noticeable change in the value of ΦFl. In the absence of TBAAc,
the ΦFl values for 6CUCF3 and 7CUCF3 were estimated to be
0.04 and 0.65, respectively. The ΦFl value for 7CUCF3 was 65
times as large as that for 7CU (see Fig. 4), whereas the ΦFl value
for 6CUCF3 was almost the same as that for 6CU. The large
increase in the ΦFl value for 7CUCF3 with respect to 7CU
implies that 7CUCF3 represents a great improvement over 7CU
as a fluorescence sensor for the acetate ion. The fluorescent col-
orimetric response of 7CU and 7CUCF3 to TBAAc is reflected
by the data reported in Fig. 5. As was expected, given the large
difference in ΦFl value between 7CUCF3 and 7CU, the fluores-
cence emission of 7CUCF3 was distinctively observed even after
addition of TBAAc, in contrast to the case of 7CU. The change
in fluorescent color from bright blue to pale blue observed in the
case of 7CUCF3 is ascribed to the appearance of a new fluores-
cence band resulting from the addition of TBAAc.
Table 1. Photochemical properties of nCU and nCUCF3 in DMSO.
kabs/nm
e/M À1cmÀ1
kFI/nm
ΦFl
6CU
7CU
6CUCF3
7CUCF3
353
336
350
334
3500
29 000
3300
473
402
470
395
0.05
0.01
0.04
0.65
26 000

4
Tomoyuki Shinoda et al.
Figure 2. 1H NMR spectra of (a) 6CU and (b) 7CU in the presence (top) and absence (bottom) of TBAAc in DMSO-d6.
We also measured the time-resolved fluorescence spectra of
respect to that of the tautomer of 6CU by electronic delocaliza-
tion. The above discussion may also apply to nCUCF3 tautomer
formation, except for the effect of the CF3 group. These results
imply that the available resonance structure in the tautomer of
the urea derivative form plays an important role in tautomer for-
mation.
Some important evidence was collected with respect to the
ΦFl values of coumarin–urea derivatives. The ΦFl of 7CU
(ΦFl = 0.01) was found to be much smaller than that of 7-
amino-4-methylcoumarin (ΦFl = 0.78), which is a synthetic pre-
cursor of 7CU (17). The ΦFl value of 7CUCF3 (ΦFl = 0.65) was
about 65 times higher than that of 7CU, whereas the ΦFl values
for 6CU and 6CUCF3 were similar in magnitude to each other
(both were of the order of 10À2). Furthermore, such values were
also similar to the ΦFl value measured for 6-aminocoumarin,
which is a synthetic precursor of both 6CU and 6CUCF3. To
explain these trends in the values of ΦFl, the possibility of fluo-
rescence enhancements induced by an ICT reaction and nonra-
diative deactivation processes was considered.
In general, coumarin derivatives with an electron-donating
group in the 7-position of the coumarin ring are known to dis-
play strong fluorescence. On the other hand, the presence of an
electron-donating group in the of 6-position the coumarin ring
has little effect on ΦFl, because of the unfavorable resonance
contribution (15,16,41). These properties are interpreted in terms
of an ICT character, which is illustrated by the resonance struc-
tures of 7-aminocoumarin derivatives in the excited state. In
nCUCF3 to estimate the time-dependent formation of the tau-
tomer by the same procedure applied to nCU (Figures S6 and
S7). The results thus obtained indicated that nCUCF3 was similar
time-course to that of nCU. The fluorescence decay of 7CUCF3
at 430 nm gave 92 ps and 1.7 ns, and the tautomer consisted of
a rise-up component of 96 ps and a decay component of 3.2 ns.
The value of the lifetime of the rise-up component of 7CUCF3
was in good agreement with that of the complex decay observed
for 7CU. By contrast, no rise component was observed for either
6CUCF3 or 6CU. Consequently, evidence indicates that insertion
of a CF3 substituent in a phenyl ring of the present sample has a
negligibly small influence on tautomer formation. The lifetimes
of corresponding compounds in the absence of TBAAc are listed
in the supporting information (Table S1).
Evidence indicated that nCU tautomer formation in the pres-
ence of TBAAc depended on the position of the urea-based sub-
stituent on the coumarin ring. We speculated that this
dependence might result from the available resonance structures
of the relevant tautomer. The resonance structure of a tautomeric
form of 7CU may be characterized by intramolecular charge
transfer (ICT) from the urea group to the coumarin ring as
shown in Scheme 2 (also see Figures S8–S10 for ICT). By con-
trast, no resonance structure of this type would appear to be
available to the tautomer of 6CU, because of the disadvantageous
position of the urea-based substituent (Scheme S1). Therefore,
the electronic state of the tautomer of 7CU is stabilized with

Photochemistry and Photobiology
5
(a) 7CU
TBAAc
(b) 7CUCF3
TBAAc
Figure 5. Pictures of (a) 7CU and (b) 7CUCF3 taken in the dark under
UV illumination before and after addition of TBAAc.
Scheme 2. Resonance structures of tautomer of 7CU.
reduction in ΦFl of 7CU. By contrast, since the presence of a
urea-based substituent in 6-position of the coumarin ring has lit-
tle effect on ICT, the fact that ΦFl of 6CU is comparable to that
of its parent molecule seems reasonable. On the basis of ICT
contribution to ΦFl, however, the fact that ΦFl of 7CUCF3 recov-
ers as a consequence of the introduction of the electron-with-
drawing CF3 group cannot be reasonably explained.
Furthermore, the fluorescence spectrum of 7CUCF3 in DMSO
displayed similar features to its counterpart recorded in ethanol
(Figure S11). This evidence suggests that ICT has a small influ-
ence on the fluorescence properties of 7CUCF3 given that the
fluorescence spectra of molecules with ICT properties are red-
shifted in highly polar solvents.
Figure 3. Changes in the fluorescence spectra of (a) 6CU and (b) 7CU
in DMSO by the addition of TBAAc. The insets show normalized fluo-
rescence spectra.
We next evaluated the effect that nonradiative deactivation
has on the value of ΦFl. Evidence from previously published
studies indicates that the value of ΦFl decreases as the contribu-
tion of nonradiative deactivation processes (including vibrational
relaxation), intersystem crossing and electron transfer quenching
processes, increases (42). The radiative rate constants of 7CU
and 7CUCF3 were estimated based on the obtained ΦFl values
and the fluorescence lifetimes (Table 1 and Table S1). The radia-
tive and nonradiative deactivation rate constants for 7CU were
calculated to be kr = 1.7 9 108 sÀ1 and knr = 1.7 9 1010 sÀ1
,
Figure 4. Comparison of fluorescence spectra of 7CU and 7CUCF3 in
respectively. Those for 7CUCF3 were calculated to be
kr = 3.4 9 108 sÀ1 and knr = 1.8 9 108 sÀ1, respectively. Based
on these results, the fluorescence quantum yields of 7CU and
7CUCF3 were concluded to depend on the nonradiative deactiva-
tion of these compounds. However, the origin of the nonradiative
deactivation process for these molecules is still unknown. There-
fore, we propose two hypotheses to explain the described results.
According to the first hypothesis, nonradiative deactivation is
the result of electron transfer quenching. Amendola et al. have
DMSO.
contrast to an amino group, a urea group is usually electron-
withdrawing, since the C=O moiety of urea may withdraw a lone
pair from the N-H group of the urea moiety. Therefore, the elec-
tron-donating properties of the N–H group of a urea moiety
located at the 7-position of the coumarin ring are reduced, and
the ICT is suppressed. The decrease in ICT may result in a

6
Tomoyuki Shinoda et al.
reported that an electron transfer from the carbonyl unit of a urea
group to a pyrene chromophore in the presence of the acetate
ion might result in the quenching of the fluorescence of a
pyrene–urea derivative (39). Based on these results, we assume
that an electron transfer process from the carbonyl unit of a urea
group to a coumarin ring may cause fluorescence quenching in
7CU. In this case, the electron transfer-based quenching of
7CUCF3 in the excited state may be suppressed as a conse-
quence of the low electron density in the carbonyl of the urea
unit induced by the electron-withdrawing effect of the CF3
group. Thus, the remarkable increase in ΦFl value for 7CUCF3
with respect to 7CU may be the result of the suppression of a
quenching process whereby an electron transfer occurs between
the urea unit and the coumarin ring. However, it has not been
experimentally confirmed that 7CU and 7CUCF3 causes electron
transfer in the absence of acetate ion so far. Furthermore, the fact
that no increase in fluorescence intensity was observed between
6CUCF3 and 6CU is difficult to explain on the basis of the men-
tioned hypothesis.
In the second hypothesis, the energy level of the p–p* transi-
tion derived from the coumarin is relatively stabilized. In the
case of the parent coumarin, the ΦFl value is quite low as a
result of the presence of the n–p* transition adjacent to the p–p*
transition of the lowest excited state (43). The introduction of an
electron-donating group in the 7-position of a coumarin ring
results in the formation of an ICT state that enhances the fluores-
cence quantum yield due to a reduction in the n–p* character of
the emissive state as a consequence of the fact that the energy
difference between the n–p* and p–p* transitions increases (44).
However, as discussed above, 7CUCF3 displays a negligible ICT
character in the coumarin ring. Density functional theory calcula-
tions (Figure S12) reveal that, with respect to 7CU, 7CUCF3 is
characterized by a large delocalization of LUMO electron density
on the phenyl ring, indicating that the energy level of the p–p*
transition might be relatively stabilized. Thus, this delocalization
implies that in 7CUCF3 the n–p* transition character of the
emissive state may be reduced without causing ICT on the cou-
marin ring. Indeed, this hypothesis will require further quantita-
tive discussion and the calculation of the energies of the relevant
excited states. Nevertheless, this hypothesis might account for
the negligible difference in ΦFl value between 6CU and
6CUCF3; in fact, contrary to the evidence with respect to 7CU
and 7CUCF3, the LUMOs of 6CU and 6CUCF3 are localized in
the coumarin ring, irrespective of the presence of a CF3 sub-
stituent.
SUPPORTING INFORMATION
Additional supporting information may be found online in the
Supporting Information section at the end of the article:
Table S1. Fluorescence lifetimes of nCU and nCUCF3 in the
absence of TBAAc
Scheme S1. Resonance structures of a tautomer of 6CU.
Figure S1. Changes in the absorption spectra of (a) 6CUCF3
and (b) 7CUCF3 upon the addition of TBAAc in DMSO under
Ar.
1
Figure S2. H NMR spectra of (a) 6CUCF3 and (b) 7CUCF3
in the presence (top) and absence (bottom) of TBAAc in
DMSO-d6.
Figure S3. Time-resolved fluorescence spectra of (a) 6CU
(3.44 9 10À5 M) in the presence of TBAAc (4 mM) (b) 7CU
(3.39 9 10À5 M) in the presence of TBAAc (5 mM) excited at
375 nm under Ar.
Figure S4. Extraction of (a) the complex component at
430 nm and (b) the rise-up component (observed from 484 to
522 nm) from the fluorescence spectra of 7CU. The lines were
fitted by using a nonlinear least-squares fitting method.
Figure S5. Changes in the fluorescence spectra of (a)
6CUCF3 and (b) 7CUCF3 in DMSO upon the addition of
TBAAc. The insets show normalized fluorescence spectra.
Figure S6. Time-resolved fluorescence spectra of (a) 6CUCF3
(4.7 9 10À5 M) in the presence of TBAAc (2.4 mM) and (b)
7CUCF3 (9.2 9 10À5 M) in the presence of TBAAc (2.1 mM)
excited at 375 nm under Ar.
Figure S7. Extraction of (a) the complex component at
430 nm and (b) the rise-up components (observed from 484 to
522 nm) from the fluorescence spectra of 7CUCF3. The lines
were fitted by using a nonlinear least-squares fitting method.
Figure S8. Normalized absorption spectra of 7CUCF3 in the
presence of 1 mM TBAAc in DMSO, DMF and MeCN.
Figure S9. Normalized fluorescence spectra of 7CUCF3 in the
presence of 1 mM TBAAc. Complex (blue line), tautomer (green
line), and complex and tautomer (red line) in (a) DMSO, (b)
DMF, and (c) MeCN separated by subtraction.
Figure S10. Normalized fluorescence spectra of tautomer of
7CUCF3 in DMSO, DMF and MeCN separated by subtraction.
Figure S11. Comparison of fluorescence spectra of 7CUCF3
in DMSO and EtOH.
Figure S12. Molecular orbitals of (a) 6CU, (b) 7CU, (c)
6CUCF3 and (d) 7CUCF3 calculated by TD-DFT B3LYP6-
31G*.
We found that different coumarin–urea derivatives exhibited
significant differences in tautomer formation, which depended on
the position of the urea-based substituent in the coumarin ring.
Our results indicate that the resonance structures available to the
urea derivatives play an important role in tautomer formation. In
addition, the evidence we gathered indicated that the introduction
of a CF3 group in the coumarin rings of the coumarin–urea
derivatives to form in particular 7CUCF3 remarkably increased
the ΦFl of this species with respect to the corresponding species
lacking the CF3 group (7CU). This increase in ΦFl value might
arise from the suppression of nonradiative deactivation by way
of the two possible routes hypothesized above. Consequently,
this study provides new insights that can be applied to the
molecular design of fluorescent urea derivatives based on the
effect of the urea substituent position in the coumarin ring and
the contribution of a CF3 group.
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