Excited-state intermolecular proton transfer dependent on the substitution pattern of anthracene-diurea compounds involved in fluorescent ON1-OFF-ON2 response by the addition of acetate ions

Article abstract of DOI:10.1039/c7ob01376k

We report anthracene-diurea compounds which behave as anion sensors based on the fluorescence emission regulated by the substitution position on the anthracene ring. Anthracene-diurea compounds exhibit different excited-state intermolecular proton transfer (ESIPT) reactions depending on the pattern of the substituents. Three new anthracene-diurea compounds that have two phenylurea groups substituted at different positions on anthracene were synthesized. These compounds formed complexes with acetate ions through intermolecular hydrogen bonding between N-H and CO moieties in the ground state. The positions of the substituents greatly affected the excited-state intermolecular proton transfer. 1,5BPUA with urea groups at the 1 and 5 positions exhibited ESIPT reaction, which is energetically favorable for tautomer formation, in the presence of TBAAc. In contrast, 2,6BPUA with urea groups at low-electron-density positions (2 and 6 positions) showed no ESIPT reaction due to the inversion of the lowest unoccupied molecular orbital (LUMO) energy levels of the normal and tautomer states. Detailed spectroscopic measurements showed that the LUMO energy level of the normal form was lowered because the urea group acted as an electron-withdrawing group. In addition, 9,10BPUA exhibited strong electronic interactions between the two phenylurea moieties at the 9 and 10 positions, resulting in an ON¹-OFF-ON² response for acetate ions. Our findings offer guidelines for the molecular design of materials with anthracene moieties based on the substitution patterns of anthracene derivatives.

Full text of DOI:10.1039/c7ob01376k

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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DOI: 10.1039/C7OB01376K
Organic & Biomolecular Chemistry
PAPER
Excited State Intermolecular Proton Transfer Dependent on
Substitution Pattern of Anthracene–Diurea Compounds
involved in Fluorescent ON1–OFF–ON2 response by the
Addition of Acetate Ions
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Hisato Matsumoto, Yoshinobu Nishimura, and Arai Tatsuo*
www.rsc.org/
We report anthracene–diurea compounds allows us an anion sensor by using the fluorescence emission control
depending on the substitution position on the anthracene ring. Anthracene–diurea compounds exhibit different
excited-state intermolecular proton transfer (ESIPT) reactions depending on the pattern of the substituents.
Three new anthracene–diurea compounds, which have two phenylurea groups substituted at different positions
on anthracene, were synthesized. These compounds formed a complex with acetate ions through
intermolecular hydrogen bonding between N–H and C=O moieties in the ground state. It was revealed that the
difference in the position of the substituents greatly impacted the excited state intermolecular proton transfer.
Whereas 1,5BPUA having the urea group at 1 and 5 positions exhibited ESIPT reaction, which is energetically
favourable to form its tautomer, in the presence of TBAAc, 2,6BPUA having the urea group at low electron
density positions, 2 and 6 positions, showed no ESIPT reaction due to the inversion of the LUMO energy
levels of normal and tautomer states. As a result of detailed spectroscopic measurements, we found that the
LUMO energy level of the normal form was lowered due to the urea group acting as an electron-withdrawing
group. In addition, we revealed that 9,10BPUA had strong electronic interactions between the two phenylurea
moieties at 9 and 10 positions, and caused an ON1–OFF–ON2 response for acetate ions. Our findings offer
molecular design guidelines for the material having an anthracene moiety dependent on the substitution
pattern of anthracene derivatives.
A typical proton donor group is urea. Its N–H group has a δ+
character and can form a hydrogen bond with a chemical
species having a δ– character. Thus, much research has been
Introduction
Proton transfer reactions are one of the most basic reactions
and are functionally important in numerous biological,
chemical and physical processes.1-6 Excited state intermolecular
proton transfer (ESIPT) reactions are used in various
applications in pH,7 photolithography,8 proton transfer
lasers,9,10 organic electro-luminescence materials,11 and probes
of the environment around proteins,12-14 micelles15-17 and
cyclodextrin.18-21 An ESIPT reaction requires a molecule with a
proton donor group and one with a proton accepting group that
form intermolecular hydrogen bonds in the ground state. Upon
exciting the hydrogen bonds, the acid−base properties of the
proton donor and the proton accepting groups reverse, causing
the excited normal form (N*) to be at a higher energy than the
conjugate excited tautomer (T*).
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1
Tennoudai, Tsukuba, Ibaraki 305-8571, Japan. E-mail: arai@chem.tsukuba.ac.jp;
Fax: +81-29-853-6503; Tel: +81-29-853-4315
Electronic supplementary information (ESI) available. See DOI:
10.1039/x0xx00000x
†
Scheme 1. Synthetic routes of 1,5BPUA, 9,10BPUA and 2,6BPUA.
J. Name., 2013, 00, 1-3 | 1
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devoted to anion sensors utilizing intermolecular hydrogen calculation of free energy in the excited state. As a result of
View Article Online
bonding interactions between urea and anion species.22,23
these studies, we found that it is possibleDtoOIr: e10g.u10la3t9e/Ct7hOeBE0S13I7P6TK
In our previous research, we studied the photochemistry of reaction and the ON1–OFF–ON2 response by changing the
urea derivatives containing aromatic fluorophore molecules and substitution pattern of the two phenylurea groups.
their hydrogen-bonding interactions with anion species.24-29 In
particular, 1-anthracen-n-yl-3-phenylurea (nPUA; n = 1, 2 and
9) in the presence of tetrabutylammonium acetate (TBAAc)
Experimental
formed a complex through intermolecular hydrogen bonding
between N–H and C=O moieties in the ground state, followed
Methods
by an ESIPT reaction. These studies revealed the influence of
1H NMR spectra were recorded on a Bruker ARX-400 (400
the substitution pattern of anthracene for ESIPT by introducing
1
MHz for H) spectrometer using DMSO-d6 as a solvent and
phenylurea groups at positions 1, 2 and 9 of anthracene. In fact,
tetramethylsilane as an internal standard. Absorption and
fluorescence spectra were measured on Shimadzu UV-1600 and
Hitachi F–4500 fluorescence spectrometers, respectively.
Fluorescence decay measurements were performed using a
time-correlated single-photon counting method. Laser
excitation at 375 nm was performed using a diode laser
(PicoQuant, LDH-P-C-375) with
(PicoQuant, PDL 800-B), with a repetition rate of 2.5 MHz.
The temporal profiles of the fluorescence decays were detected
by a microchannel plate photomultiplier (Hamamatsu, R3809U)
equipped with a TCSPC computer board module (Becker and
Hickl, SPC630). The full width at half maximum (fwhm) of the
instrument response function was 51 ps.39 The values of χ2 and
the Durbin–Watson parameters were used to determine the
quality of the fit obtained by nonlinear regression.40 DMSO
(spectroscopic grade, Wako Pure Chemical Industries, Japan)
was used as a solvent without further purification. Acetate ions
we found that the rate constant of ESIPT (kPT) depended
significantly on the position of the phenylurea substituents. The
kPT of 1PUA, with a phenylurea group introduced at the 1-
position of anthracene, is 2.4×109 s–1, while the kPT of 2PUA is
5×107 s–1, i.e., the kPT of 1PUA is 50 times larger than that of
2PUA. Other experiments show that the photodimerization,
a power control unit
luminescence and electrochemical properties depend on the
position of the anthracene substituents.25,28,30-35 However,
research on the ESIPT of anthracene–urea compounds has been
limited.
Many compounds having two or more anion recognition
sites have been reported as anion sensors. The fluoride ion
sensor, in which two phenylurea groups are immobilized on the
1,8-position of anthracene, recognizes one anion by using two
urea groups.36 Chemosensors, designed as “receptor-spacer-
fluorophore-spacer-receptor”
conjugates,
recognize
dicarboxylates and pyrophosphate by photoinduced electron
transfer quenching of the anthracene moiety.37 Recently, we
reported an anthracene–diurea compound having two
phenylurea groups on the 9,10-position of anthracene. This
compound exhibited unique emission. While the fluorescence
of the urea compound (ON1) was quenched in the presence of a
low concentration of acetate ions (OFF), fluorescence
enhancement occurred upon addition of a high concentration of
acetate ions (ON2). Thus, we succeeded in developing an ON1–
OFF–ON2 sensor for acetate ions, depending on their
concentration.29 As described above, different anion recognition
mechanisms can be realized by the manipulation of the
substitution.38
were in the form of TBAAc, which contains
a
tetrabutylammonium cation (Sigma-Aldrich, Japan). All
measurements were carried out at room temperature under an
Ar atmosphere. The concentrations were adjusted so that the
absorption maximum of the excitation wavelength was about
0.1 for each sample. Density functional theory (DFT)
calculations were performed using the Spartan ’04 program
(Wavefunction, Inc., Irvine, CA, USA).
Synthesis
All solvents and 1,5-diaminoanthraquinone, 2,6-
diaminoanthraquinone,
diphenylphosphoryl
phenyl
azide, triethylamine
isocyanate,
and
aniline,
9,10-
In this study, we synthesized anthracene–diurea compounds,
1,5BPUA, 9,10BPUA and 2,6BPUA, in which two phenylurea
anthracenedicarboxylic acid were purchased from Wako Pure
Chemical Industries, Ltd., Japan, Tokyo Chemical Industry Co.,
Ltd., Japan, or Aldrich, USA and were used without further
purification.
groups are immobilized on the 1,5-, 9,10- and 2,6-positions of
anthracene, respectively (Scheme 1). These compounds were
examined spectroscopically for changes in fluorescence
properties upon addition of TBAAc. Among these compounds,
only 1,5BPUA caused ESIPT in the presence of TBAAc,
1,5-Diaminoanthracene: To a 100-mL two-necked round
bottom flask were added 1,5-diaminoanthraquinone (940 mg,
3.94 mmol), NaBH4 (4.4 g, 0.12 mol) and 2-propanol (20 mL).
After stirring for 14 h at 80 °C, the solvent was removed by
suction filtration. The residue was washed by MeOH to give a
whereas all anthracene–monophenylurea compounds (nPUA
)
caused ESIPT. In 9,10BPUA, it was revealed that the two
phenylurea moieties showed electronic interactions with each
other, and this interaction was closely related to for ON1–OFF–
ON2 response. Furthermore, to explore the ESIPT reaction and
ON1–OFF–ON2 response by changing the substitution pattern
of the two phenylurea groups, we used kinetic analysis and
1
black solid (713 mg, 87%). H NMR (DMSO-d6, 270 MHz): δ
5.28 (s, 4H), 5.38 (d, J = 4.0 Hz, 2H), 6.62 (d, J = 5.0 Hz, 2H),
6.88 (d, J = 5.0 Hz, 2H), 7.05 (t, J = 5.0 Hz, 2H)
2 | J. Name., 2012, 00, 1-3
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were added 1,5-diaminoanthracene (150 mg, 0.72 mmol),
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phenyl isocyanate (40 µL, 0.33 mmol) aDnOdI:d1r0y.10T3H9/FC7(O2B001m37L6)K.
After stirring for 70 h at 70 °C, the solvent was removed by
suction filtration. The residue was washed by MeOH to give a
1
black solid (89 mg, 28%). H NMR (DMSO-d6, 270 MHz): δ
7.02 (t, J = 7.8 Hz, 2H), 7.34 (dd, J = 8.3 Hz, J = 6.9 Hz, 4H),
7.41-7.50 (m, 6H), 7.85 (dd, J = 8.4 Hz, J = 6.7 Hz, 2H), 8.07
(d, J = 6.9 Hz, 2H), 8.78 (s, 2H), 9.02 (s, 2H), 9.15 (s, 2H).
+
HRMS m/z (ESI-MS) calcd for C28H22N4NaO2 ([M + Na]+),
469.1635; found, 469.1630.
9,10BPUA: To a two-necked round bottom flask (100 mL)
were added 9,10-anthracenedicarboxylic acid (206 mg, 0.773
mmol), toluene (80 mL), triethylamine (210 μL, 1.51 mmol),
and diphenylphosphoryl azide (150 μL, 1.85 mmol). After
stirring for 30 min at 80 °C, aniline (150 mg, 1.64 mmol) in
toluene (5 mL) was added, and the mixture was refluxed for
91 h. The solvent was filtered by suction filtration and the
residue was washed with methanol a number of times to give a
1
light yellow solid (43.5 mg, 12%). H NMR (DMSO-d6, 400
MHz): δ 6.97 (t, J = 7.4 Hz, 2H), 7.29 (t, J = 7.4 Hz, 4H), 7.51
(t, J = 7.4 Hz, 4H), 7.58-7.61 (m, 4H), 8.23-8.26 (m, 4H), 8.79
(s, 2H), 9.08 (s, 2H). HRMS m/z (ESI-MS) calcd for
C28H22N4NaO2+ ([M + Na]+), 469.1635; found, 469.1645.
2,6-Diaminoanthracene: To a 100-mL two-necked round
bottom flask were added 2,6-diaminoanthraquinone (1100 mg,
4.61 mmol), NaBH4 (4.2 g, 0.11 mol) and 2-propanol (40 mL).
After stirring for 12 h at 80 °C, the solvent was removed by
suction filtration. The residue was washed with MeOH to give a
1
light yellow solid (866 mg, 90%). H NMR (DMSO-d6, 270
MHz): δ 5.15 (s, 4H), 6.71 (s, 2H), 6.85 (d, J = 7.0 Hz, 2H),
7.55 (d, J = 7.0 Hz, 2H), 7.74 (s, 2H).
2,6BPUA: To a 100-mL two-necked round bottom flask
were added 2,6-diaminoanthracene (153 mg, 0.73 mmol),
phenyl isocyanate (190 µL, 1.59 mmol) and dry THF (55 mL).
After stirring for 24 h at 70 °C, the solvent was removed by
suction filtration. The residue was washed by MeOH to give a
Fig 1. Changes in the absorption spectra of (a) 1,5BPUA (9.8 × 10–6
M), (b) 9,10BPUA (8.6 × 10–6 M), (c) 2,6BPUA (1.1 × 10–5 M) in
the presence of TBAAc.
1
light yellow solid (40 mg, 12%). H NMR (DMSO-d6, 270
MHz): δ 6.99 (t, J = 6.8 Hz, 2H), 7.31 (t, J = 6.8 Hz, 4H), 7.43-
7.52 (m, 6H), 7.97 (d, J = 9.7, 2H), 8.23 (s, 2H), 8.32 (s, 2H),
8.86 (s, 2H), 8.99 (s, 2H). HRMS m/z (ESI-MS) calcd for
C28H22N4NaO2+ ([M + Na]+), 469.1635; found, 469.1638.
Measurement of 13C NMR spectra was unsuccessful due to
the low solubilities of 1,5BPUA, 9,10BPUA and 2,6BPUA.
Results
Absorption spectra
Figure 1 shows the absorption spectra of 1,5BPUA
,
9,10BPUA and 2,6BPUA in DMSO with various
concentrations of TBAAc. In the absence of TBAAc, the
spectral shape for 1,5BPUA was broad compared to that of the
parent molecule, anthracene, which had vibrational structure
(Fig. 1a). The wavelength maxima were 400 and 421 nm, with
shoulders at 381 and 368 nm. There was a slight increase in the
absorption band in the presence of TBAAc. The spectral shape
Fig 2. 1H NMR spectra of (a) 1,5BPUA, (b) 9,10BPUA, (c)
2,6BPUA in the presence (top) and absence (bottom) of TBAAc in
DMSO-d6.
1,5BPUA: To a 100-mL two-necked round bottom flask
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exhibited downfield shifts from 9.08 and 8.79 ppm to 12.26 and
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11.96 ppm, respectively (Fig. 2b). SimDilOarI:ly10, .1t0h3e9/sCi7gOnBa0ls137f6oKr
2,6BPUA exhibited downfield shifts from 8.99 and 8.86 ppm to
12.24 and 12.14 ppm, respectively (Fig. 2c). These shifts
indicate that BPUAs interact with TBAAc through hydrogen
bonding.
Association constants
The association constants for 1,5BPUA, 9,10BPUA and
2,6BPUA with TBAAc were determined according to eq. 1 to
be 1.9×103, 5.3×103 and 5.2×103 M–1, respectively,41
1
1
1
=
( [ꢅ] + 1)
(1)
ꢀ–ꢀ0
ꢀꢁꢂꢃ–ꢀ0 ꢄꢂ
where I0 and I are the fluorescence intensities for the host monitored
at the wavelength of the fluorescence maximum in the absence and
presence of anions, respectively, [A] is the total anion concentration,
and Imax is the fluorescence intensity for BPUAs fully hydrogen
bonded with the anion (Fig. S6 in the ESI). However, since
1,5BPUA and 2,6BPUA showed only a one-step change in the
absorption and fluorescence spectra, we calculated only Ka1
(association constant of 1:1 complex). 9,10BPUA showed a two-step
change in the fluorescence spectra (ON1–OFF–ON2), therefore Ka1
and Ka2 (association constants for 1:1 complex and 1:2 complex,
respectively) could be calculated by the fluorescence change (Ka2
=
270 M–1).29 The difference in the association constants for BPUAs
may arise from the acidity of the urea protons. The association
constants for 1,5BPUA and 2,6BPUA were similar to those for
1PUA and 2PUA, respectively, while that for 9,10BPUA was 3.3
times larger than that for 9PUA.25,26
Fluorescence spectra
Fig 3. Changes in the fluorescence spectra of (a) 1,5BPUA
(9.8 × 10–6 M), (b) 9,10BPUA (8.6 × 10–6 M), (c) 2,6BPUA
(1.1 × 10–5 M) in the presence of TBAAc. The insets show
normalized spectra at maximum intensity.
Figure 3a shows changes in the fluorescence spectra of 1,5BPUA in
the presence of various concentrations of TBAAc in DMSO. While
the spectrum in the absence of TBAAc had a maximum at 468 nm
with a shoulder at 447 nm, the fluorescence intensity decreased as
the TBAAc concentration increased from 0 to 3.0 mM followed by
the appearance of a new band with a maximum at 665 nm as shown
in the inset of Fig. 3a. The peak wavelength for 1,5BPUA remained
unchanged even in the presence of 3.0 mM TBAAc. The
fluorescence changes in 9,10BPUA were very characteristic (Fig.
3b). While the spectrum in the absence of TBAAc had a maximum
at 480 nm, the fluorescence intensity decreased with increasing
concentration of TBAAc from 0 to 0.8 mM followed by the
appearance of a weak new band in the long-wavelength region. As
shown in Fig. S7 in the ESI, when the TBAAc concentration was
increased further to 91.8 mM, the intensity of the band at 483 nm
increased. This indicated that while the fluorescence of 9,10BPUA
in DMSO (ON1) was quenched in the presence of low concentration
of acetate anion due to suppressing ESIPT reaction (OFF),
fluorescence enhancement occurred by the addition of high
concentration of acetate anion due to suppressing ESIPT (ON2)29.
The fluorescence intensity for 2,6BPUA showed a smaller decrease
than that for 1,5BPUA in the presence of 0.6 mM TBAAc (Fig. 3b).
For 2,6BPUA, an increase in the TBAAc concentration resulted in a
decrease in fluorescence intensity and a red-shift of the spectrum
for 9,10BPUA was broader than that for 1,5BPUA (Fig. 1b).
The wavelength maxima for 9,10BPUA were 381 and 396 nm
with shoulders at 359 and 342 nm. The red-edge absorption
band longer than 383 nm was red-shifted in the presence of
TBAAc. In contrast, the spectral shape for 2,6BPUA was
completely different from those for 1,5BPUA and 9,10BPUA
,
with strong vibrational bands from 320 to 420 nm, as shown in
Fig. 1c. The red-edge absorption band (greater than 404 nm)
was red-shifted in the presence of TBAAc, while the
absorbance below 300 nm increased significantly.
1H NMR spectra
To confirm the complexation of 1,5BPUA, 9,10BPUA and
1
2,6BPUA with TBAAc, H NMR spectra were measured in the
absence and presence of TBAAc (Fig. 2). This revealed
hydrogen-bonding interactions between 1,5BPUA and TBAAc,
since downfield shifts of the N–H protons from 9.15 and 9.01
ppm to 11.94 and 11.32 ppm, respectively, were observed in the
presence of TBAAc (Fig. 2a). The signals for 9,10BPUA also
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Table 1. Lifetimes of 1,5BPUA, 9,10BPUA and 2,6BPUA in the absence and presence of TBAAc
ARTICLE
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DOI: 10.1039/C7OB01376K
2,6BPUA
1,5BPUA
9,10BPUA
τ / ns
630 nm
τ / ns
600 nm
τ / ns
600 nm
[TBAAc]/mM 450 nm
[TBAAc]/mM
0
450 nm
[TBAAc]/mM
0
450 nm
0
6.89 (1.00)
6.78 (0.44)
0.83 (0.56)
–
3.39 (1.00)
3.65 (0.77)
1.40 (0.23)
–
16.5 (1.00)
15.2 (0.97)
3.03 (0.03)
–
5.40 (1.00)
0.54 (–1.00)
1.67 (0.41)
3.49 (0.59)
2.49 (–1.00)
21.0 (0.33)
11.3 (0.58)
1.21 (0.09)
0.3
0.8
1.0
The values in parentheses are the normalized amplitudes for the respective lifetimes.
without a new emission band around 600-700 nm. The new band of
1,5BPUA had a maximum at 665 nm and may be the tautomer
emission, which is generated by the ESIPT reaction.24 In contrast,
2,6BPUA showed no tautomer emission due to the lack of an ESIPT
Discussion
reaction and 9,10BPUA emitted only
a slight tautomeric
Difference in the fluorescence properties of BPUAs
fluorescence in the presence of 0.8 mM TBAAc. This indicates that
the complexes of these two compounds with TBAAc caused almost
no ESIPT reaction, and/or the fluorescence quantum yield of the
tautomer was very low. The fact that only 1,5BPUA caused an
ESIPT reaction in BPUAs by the addition of TBAAc is in contrast to
all anthracene–monourea compounds (nPUA) that cause an ESIPT
reaction.24,25
To investigate the substitution effects of urea moieties on
the photochemical behavior, we focused on the changes in the
fluorescence properties and the reactivity of intermolecular
proton transfer. Differences in the interactions between urea
groups were observed in the fluorescence spectra in the
presence of TBAAc. 1,5BPUA showed no spectral shift and a
monotonic decrease in emission with increasing TBAAc
concentration. To examine the emissive properties of the
Lifetimes of BPUAs in the excited state
complex of BPUA
s with TBAAc in more detail, we
investigated the relationship between the proportion of free
BPUA, which is uncomplexed BPUA with TBAAc, and the
fluorescence intensity. The proportion of free BPUA can be
The fluorescence decay of 1,5BPUA in the absence of
TBAAc under Ar gave a monoexponential curve with a lifetime
of 6.89 ns (Table 1, see Figs. S8–S13 in the ESI). The
fluorescence curve observed at 440 nm in the presence of 3.0
mM TBAAc exhibited a biexponential decay with lifetimes of
6.78 and 0.83 ns, while the curve at 630 nm showed rise and
decay times of 0.54 and 5.40 ns, respectively. The fluorescence
decay for 9,10BPUA in the absence of TBAAc under Ar
exhibited a monoexponential curve with a lifetime of 3.39 ns.
The fluorescence curve observed at 450 nm in the presence of
1.0 mM TBAAc exhibited a biexponential decay with lifetimes
of 3.65 and 1.40 ns, and the curve at 650 nm showed a
biexponential decay with lifetimes of 3.49 and 1.67 ns and rise
of 2.49 ns. The fluorescence decay for 2,6BPUA in the absence
of TBAAc under Ar gave a single exponential curve with a
lifetime of 16.5 ns. However, the fluorescence curve observed
at 440 nm in the presence of 10 mM TBAAc showed
biexponential decay with lifetimes of 15.2 and 3.03 ns, while
the curve at 630 nm showed a tri-exponential decay with
lifetimes of 21.0, 11.3 and 1.21 ns. The time development of
the new emission was confirmed by time-resolved spectra of
1,5BPUA (see Fig. S14 in the ESI). The build-up of new
emission spectra maximized around 650 and 520 nm was
calculated by equations (2)-(4),
ꢊ–�ꢊ2–4[TBAAc][ꢆꢇꢈꢉ]0
[ꢆꢇꢈꢉ ⋯ AcO–] =
(2)
(3)
2
1
ꢋ = [TBAAc] + [ꢆꢇꢈꢉ]0 +
�
ꢌꢍ
[free ꢆꢇꢈꢉ]
[ꢆꢇꢈꢉ]0
[ꢆꢇꢈꢉ]0–[ꢆꢇꢈꢉ⋯AcO–]
=
(4)
[ꢆꢇꢈꢉ]0
where [BPUA···AcO−], [TBAAc], [BPUA]0 and [free BPUA
are the concentrations of the complex of BPUA with TBAAc,
the total TBAAc, the initial BPUA and the BPUA
]
,
uncomplexed with TBAAc, respectively. In the presence of 3.0
mM TBAAc, the proportion of free 1,5BPUA is about 15%,
which is consistent with the relative fluorescence intensity for
free 1,5BPUA (see Fig. S17 in the ESI). This indicates that the
complex of 1,5BPUA with TBAAc may have almost no
emissive properties. In contrast, the fluorescence spectra of the
complex of 2,6BPUA with TBAAc exhibited different
behavior. Although the proportion of free 2,6BPUA was less
than 1% in the presence of 30.5 mM TBAAc, the relative
fluorescence intensity for 2,6BPUA was 0.57 times that in the
absence of TBAAc (see Fig. S18 in the ESI). Furthermore, the
fluorescence spectrum of 2,6BPUA under the addition of
TBAAc showed an emissive red-shifted band relative to that for
free 2,6BPUA. This indicates that unlike 1,5BPUA, the
complex of 2,6BPUA with TBAAc is luminescent. We can
confirmed at 5.3 and 7.2 ns for 1,5BPUA and 9,10BPUA
,
whereas no new emission spectra at longer wavelengths in the
time-resolved spectrum of 2,6BPUA appeared (see Figs. S14–
S16 in the ESI).
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surmise an associative form by the fluorescence lifetimes.27,28
Journal Name
A
View Article Online
DOI: 10.1039/C7OB01376K
complicated associative state may exist due to two phenylurea
moieties of 2,6BPUA. We inferred that the associative form
between 2,6BPUA and TBAAc consists of multiple
luminescent lifetimes, which suggests that there are multiple
complexes such as 2,6BPUA···AcO– and 2,6BPUA···2AcO–.
By contrast, the sum of the proportion of free 9,10BPUA and
9,10BPUA···2AcO– is roughly agreement with the relative
fluorescence intensity. This indicates that free 9,10BPUA and
9,10BPUA···2AcO– are luminescent while 9,10BPUA···AcO–
is non-luminescent (see Fig. S19 in the ESI).
Scheme 2. Reaction scheme for ESIPT.
∗
∗
ꢓ1 + ꢓ2 = ꢒ + ꢙ = ꢐN + ꢐT + ꢐPT + ꢐ–PT
(12)
Kinetic analysis of excited state intermolecular proton transfer
reactions
An iterative fitting method was applied to the deconvoluted
decay curve to determine the four unknown rate constants,
which are correlated with eq. (12), in the presence of TBAAc,
according to a previously reported calculation method.50
The kN*, kT*, kPT and k–PT values were calculated to be
1.6×108, 3.5×108, 5.3×108, and 2.5×108 s–1, respectively, for
1,5BPUA and 2.2×108, 3.9×108, 1.6×108, and 2.3×108 s–1 for
9,10BPUA. The Gibbs free energy change, ΔG*, as shown in
Scheme 2, and equilibrium constant (Keq) for the complex and
the tautomer in the excited state at 298 K were estimated as
follows: 26,48
As described above, the fluorescence properties of BPUA
s
in the presence of TBAAc depend on the position of urea
substituents on the anthracene ring. In particular, it is
noteworthy that only 1,5BPUA gave an ESIPT reaction in the
presence of TBAAc, whereas 2,6BPUA and 9,10BPUA
showed no remarkable tautomer emission. These differences in
the fluorescence properties of BPUAs may arise from the
electronic interactions between urea groups depending on the
substitution pattern of the anthracene ring. To examine the
interaction between urea groups, we compared the relevant
spectra and rate constants for BPUAs with those for nPUA
,
which has only one phenylurea moiety.
ꢘꢢꢣ
ꢚꢛ∗ =– ꢜꢝꢞꢟꢌeq, ꢌꢠꢡ
=
(13)
According to the reaction scheme shown in Scheme 2, the
differential rate equations for the change in concentration of
BPUA···AcO– (normal, N*) and the tautomer (T*) in the
excited state can be expressed as follows:25,42-49
ꢘ–ꢢꢣ
The Keq and ΔG* values were calculated to be 2.1 and –1.9
kJ/mol, respectively, for 1,5BPUA and 0.70 and 0.90 kJ/mol
for 9,10BPUA. The ΔG* value for 2,6BPUA might be slightly
greater than that for 9,10BPUA since no tautomer of 2,6BPUA
was observed in the presence of TBAAc. An exergonic ESIPT
reaction involving 1,5BPUA···AcO– is responsible for the dual
fluorescence consisting of free 1,5BPUA and the tautomer. In
contrast, the endergonic ESIPT reaction involving
9,10BPUA···AcO– may explain the much weaker tautomer
emission due to the small amount of T* generation.48,51,52
The differences between the energy levels of the normal and
tautomer forms play an important role in the relevant rate
constants for an ESIPT reaction. To calculate the 0–0 transition
energy for the normal form (E0–0), the intersection of absorption
and fluorescence spectra was used. The E0–0 energies for
ꢎ[N∗]
=– (ꢐN ∗ + ꢐPT) × [N∗] + ꢐ–PT × [T∗]
(5)
∗
ꢎꢏ
ꢎ[T∗]
=– (ꢐT + ꢐ–PT) × [T∗] + ꢐPT × [N∗]
(6)
∗
ꢎꢏ
where kPT and k–PT denote the forward and reverse ESIPT rate
constants, respectively, and kN* and kT* are the decay rate
constants for the excited normal and tautomer forms,
respectively. The integration of equations (5) and (6) with the
given initial conditions at t = 0, [N*] = [N*]0 and [T*] = [T*]0 =
0 gives equations (7) and (8) for [N*] and [T*]:
[N∗]0
[N∗] =
× [(ꢒ– ꢓ2)ꢔꢕꢖ(– ꢓ1ꢗ)– (ꢒ– ꢓ1)ꢔꢕꢖ(– ꢓ2ꢗ)] (7)
ꢑ1–ꢑ2
ꢘPT[N∗]0
ꢑ1–ꢑ2
[T∗] =
× [ꢔꢕꢖ(– ꢓ2ꢗ)– ꢔꢕꢖ(– ꢓ1ꢗ)]
(8)
1,5BPUA
are 275.0, 273.1 and 270.0 kJ/mol, respectively, while those for
1PUA 9PUA and 2PUA are 281.0, 289.7 and 279.5 kJ/mol,
, 9,10BPUA and 2,6BPUA in the presence of TBAAc
,
where
respectively. The introduction of two urea groups results in a
decrease in the E0–0 energy, which is associated with the
LUMO energy level.53,54 Since the phenylurea moiety acts as an
electron-withdrawing group, the E0–0 energies of BPUAs might
be affected by the stabilization of the LUMO energy resulting
from the introduction of two phenylurea groups.
To examine E0–0 in a different way, it is worthwhile to
determine the spectral shape for the tautomer to estimate its
energy levels. The emission spectral shape for the tautomer
(9)
(10)
(11)
∗
∗
ꢒ = ꢐN + ꢐPT
,
ꢙ = ꢐT + ꢐ–PT
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form except for 2,6BPUA was derived by simply subtracting sequentially in the order of 2 < 1 < 9 positions.55 TVhieewreAfrtoicrlee,Ointlinies
DOI: 10.1039/C7OB01376K
the emission spectrum of 1,5BPUA and 9,10BPUA in the expected that the electronic interaction of the anthracene ring
absence of TBAAc from that in the presence of TBAAc. The with the urea group also increase in the same order. The
resulting peak wavelengths for 1,5BPUA and 9,10BPUA were vibrational structures in the absorption spectra of 1,5BPUA and
estimated to be 660 and 615 nm, respectively (see Fig. S20, S21 2,6BPUA were quite similar to those for 1PUA and 2PUA
in the ESI), which correspond to the energy gap between the respectively, whereas that for 9,10BPUA is broader than that
ground and excited states for the tautomer. As described above, for 9PUA 24
Differences in the vibrational structure of the
,
.
the substitution effect on the energy level may appear to be absorption spectra indicate that the electronic interaction
more significant stabilization of the excited state than the between the anthracene and urea moieties of 9,10BPUA is
ground state.53,54 By calculating the energy for the tautomer greater than that for 2,6BPUA and 1,5BPUA. The maximum
based on its fluorescence wavelength, the emissive state of the wavelengths in the fluorescence spectra of 1,5BPUA and
tautomer of 1,5BPUA was found to be stabilized by 13.28 2,6BPUA were similar to those for 1PUA and 2PUA (see Fig.
kJ/mol relative to that for 9,10BPUA, which is consistent with S22, S23 in the ESI), while those for 9,10BPUA and 9PUA
the contribution of more extended π-conjugation in 1,5BPUA were distinctly different from each other (see Fig. S24 in the
than in the tautomer of 9,10BPUA. The reason why the ESI). The introduction of a phenylurea moiety resulted in a red
tautomer of 1,5BPUA is stabilized compared with those of shift of the fluorescence peak of 9,10BPUA up to 480 nm,
2,6BPUA and 9,10BPUA is unknown due to the yet-to-be- compared with that of 9PUA (450 nm). This indicates that the
defined structure of the tautomer. However, we revealed that phenylurea moiety acts as an electron-withdrawing group,
stabilization of the tautomer of 1,5BPUA might cause a thereby significantly reducing the LUMO energy level.
significant excited-state energy gap between the normal and
tautomer forms leading to the negative Gibbs free energy anthracene and urea moieties affect the fluorescence properties
change involved in the tautomer generation.
and association constants in the presence of TBAAc.25 This
These differences in fluorescence properties of BPUAs may electron–withdrawing effect can also be considered to affect the
be related to differences in the electronic state of BPUA difference in the association constants for TBAAc. The
Differences in the electronic interaction between the
s
originated by the substitution position of an anthracene ring, association constant for 9,10BPUA was 5300 M−1, which is 3.3
which may affects to the electronic interaction between the two times larger than that for 9PUA (1600 M−1). In general, the
phenylurea moieties.29 The vibrational structure of the strength of hydrogen bonding depends on the acidity of the
absorption spectrum of 2,6BPUA was more remarkable than hydrogen donor. This implies that one phenylurea moiety may
those of 1,5BPUA and 9,10BPUA. Differences in vibration act as an electron-withdrawing group against the other
structure of the absorption spectra imply that the electronic phenylurea group, such that the electron density in the nitrogen
interaction between the anthracene and urea moieties of atom of the urea moiety bound with an acetate ion is lower in
2,6BPUA is weaker than those of 9,10BPUA and 1,5BPUA 9,10BPUA than in 9PUA. However, the association constants
due to low electron density positions, 2 and 6 positions25. for 1,5BPUA and 2,6BPUA were similar to those for 1PUA
Therefore, since the electronic coupling of 2,6BPUA between and 2PUA, respectively, indicating that the phenylurea moieties
the anthracene and urea moieties is weaker than those of of 1,5BPUA and 2,6BPUA have less electron-withdrawing
1,5BPUA and 9,10BPUA
, the charge delocalization of ability than that for 9,10BPUA. These results suggest that
2,6BPUA followed by ESIPT reaction reduces relative those of 9,10BPUA has strong interactions between the two phenylurea
1,5BPUA and 9,10BPUA. Consequently, the substitution moieties, whereas 1,5BPUA and 2,6BPUA have negligibly
position dependence of the electronic coupling for charge weak interactions. The difference in this interaction is closely
delocalization might affect ΔG* although the details remain related to the ON1–OFF–ON2 response observed in 9,10BPUA
unknown.
for acetate ions. The urea moiety hydrogen-bonded with
Since the decay rate constants for the tautomer (kT*) are the TBAAc may behave as an electron-donating group while the
sum of the radiative and nonradiative rate constants, they relate phenylurea group without TBAAc acts as an electron-
to the amount of T* produced. The fluorescence intensities for withdrawing group. The resonance effect of the two electron-
T* depend on the product of the amount of T* generated donating urea groups that interact with each other may cause
multiplied by the fluorescence quantum yield of T*. Although the ON1–OFF–ON2 response.29 Thus, 9,10BPUA, which has a
the tautomer of 1,5BPUA was clearly seen, that of 9,10BPUA strong interaction between phenylurea moieties, induces an
could barely be observed by fluorescence. This may be ON1–OFF–ON2 response, while 1,5BPUA and 2,6BPUA
,
explained by the small amount of T* generated and the which have weak interactions between phenylurea moieties,
relatively low fluorescence quantum yield for T*.
cause only an ON–OFF response.
The origin of the ON1–OFF–ON2 reaction in 9,10BPUA
Conclusions
To investigate the mechanism of the ON1–OFF–ON2
reaction in 9,10BPUA, differences in the electronic interactions
depending on the substitution position were examined. It is
well-known that the electron density for anthracene increases
We found that anthracene–diurea compounds (1,5BPUA
,
9,10BPUA and 2,6BPUA) exhibited a significant difference in
the ESIPT reaction due to their ΔG*, which depends on the
positions of the two urea moieties substituted on the anthracene
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Journal Name
V. Amendola, L. Fabbrizzi and L. Mosca, Chem. Soc. Rev.
22
23
24
25
26
27
28
29
30
31
32
33
ring, i.e., only 1,5BPUA underwent the ESIPT reaction in the
presence of TBAAc through the form of 1,5BPUA···AcO–.
Furthermore, from a kinetic analysis of the ESIPT reaction, we
revealed that the substitution position affects both the energy
levels of the LUMO and the rate constants associated with
ESIPT. In addition, we found that the ON1−OFF−ON2 response
of 9,10BPUA for acetate ions arises from a specific charge
density of the 9,10-positions in the anthracene ring, which is
involved in π-conjugation with the anthracene ring. This study
might provide new knowledge for the molecular design of
fluorescent materials using a substitution position effect of
anthracene rings.
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Conflicts of interest
There are no conflicts of interest to declare.
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DOI: 10.1039/C7OB01376K
Anthracene–diurea compounds exhibit different excited-state intermolecular proton transfer (ESIPT) reactions
depending on the pattern of the substituents.