Aggregated Excimer Formation through Hydrogen Bonding in a Pyrene-Urea Derivative

Article abstract of DOI:10.1246/bcsj.20150060

We examined intermolecular hydrogen-bonding interactions of pyrene derivatives bearing a urea unit, referred to as1PUP (one pyrenyl group) and 1DPU (two pyrenyl groups). 1DPU showed a new emission band around 530 nm along with a locally excited emission (LE) attributed to the pyrene moiety, even at 10^-7M, in contrast to 1PUP, which only had an LE, indicating that the symmetrical configuration allowed stable stacking of 1DPU. Time-resolved emission spectroscopy revealed that the intensity of the new band increased relative to that of the LE with the concentration of 1DPU. Furthermore, addition of tetrabutylammonium acetate (TBAAc) resulted in the disappearance of the new band, due to the formation of hydrogen bonds with 1DPU. These results indicate that the new band may originate from intermolecular hydrogen-bonding interactions between 1DPU molecules through the urea unit, even at very low concentrations, and can be attributed to an excimer emission. This implies that hydrogen bond formation by the urea units of 1DPU favors a conformation that results in the formation of the excimer in the absence of anions, and the strength of the hydrogen-bonding interaction between 1DPU molecules is weaker than the interaction between a 1DPU molecule and an anion.

Full text of DOI:10.1246/bcsj.20150060

Aggregated Excimer Formation through Hydrogen Bonding
in a Pyrene-Urea Derivative
Hisato Matsumoto, Yoshinobu Nishimura, and Tatsuo Arai*
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571
E-mail: arai@chem.tsukuba.jp
Received: February 24, 2015; Accepted: April 19, 2015; Web Released: April 30, 2015
We examined intermolecular hydrogen-bonding interactions of pyrene derivatives bearing a urea unit, referred to as
1PUP (one pyrenyl group) and 1DPU (two pyrenyl groups). 1DPU showed a new emission band around 530 nm along with
a locally excited emission (LE) attributed to the pyrene moiety, even at 10¹7 M, in contrast to 1PUP, which only had an LE,
indicating that the symmetrical configuration allowed stable stacking of 1DPU. Time-resolved emission spectroscopy
revealed that the intensity of the new band increased relative to that of the LE with the concentration of 1DPU. Furthermore,
addition of tetrabutylammonium acetate (TBAAc) resulted in the disappearance of the new band, due to the formation of
hydrogen bonds with 1DPU. These results indicate that the new band may originate from intermolecular hydrogen-bonding
interactions between 1DPU molecules through the urea unit, even at very low concentrations, and can be attributed to an
excimer emission. This implies that hydrogen bond formation by the urea units of 1DPU favors a conformation that results
in the formation of the excimer in the absence of anions, and the strength of the hydrogen-bonding interaction between
1DPU molecules is weaker than the interaction between a 1DPU molecule and an anion.
The detection of anions is important in biology, agriculture,
and medicine due to their effect on living cells.1-3 Some of the
most important substances in this regard are pyrene derivatives,
since their fluorescence lifetimes are very long and they may
interact with each other to form excimers.4,5 Excimers have
been utilized to investigate interactions between proteins, detect
transition metals and aromatic nitro compounds, and con-
firm the triple-strand helix of polypeptides.6-10 The longer-
wavelength emission of pyrene is suitable for various applica-
tions.11,12 However, a relatively concentrated pyrene solution
is required for excimer formation, unless cyclodextrin is present
or there are intramolecular interactions of pyrene moieties in
one molecule, since the pyrene molecules must be in close
proximity to generate excimers.13-17 One method of achieving a
suitable geometry for excimer formation is to utilize hydrogen-
bonding interactions.18,19
Urea derivatives are also used for anion recognition, which is
accomplished by hydrogen bond formation followed by
changes in the electronic structure.20 We have been studying
the photochemistry of 1-anthracen-n-yl-3-phenylurea (nPUA;
n = 1, 2, and 9) in the presence of TBAAc, which contains
an acetate anion in DMSO. nPUA gives a longer-wavelength
emission at around 600 nm, along with an LE at around 450 nm,
in the presence of TBAAc, whereas in the absence of TBAAc it
shows an anthracene-like emission spectrum localized around
450 nm. Irrespective of whether TBAAc was present, TBABr
caused no changes in the emission spectra, with no emission at
600 nm. It was thought that hydrogen-bonding interaction might
take place between nPUA and TBAAc, with significant changes
in the electronic structure in the excited state, followed by the
formation of a tautomeric form, which gives the 600-nm
species.21-23 Mosca et al. reported that a pyrenyl urea compound
also showed dual emissions in the presence of TBAAc.24
The aim of this study was to elucidate the photochemistry
of a urea compound, N,N¤-1-dipyrenylurea, 1DPU (Chart 1),
bearing a pyrenyl group at both sides of the urea moiety. 1DPU
allows us to investigate the hydrogen-bonding ability of a urea
unit based on its charge-transfer properties by comparing its
behavior with that of 1PUP, which has more charge-transfer
character than 1DPU due to the asymmetric structure of the
pyrenyl moiety, and 1-aminopyrene, 1AP having an amino unit
instead of a urea. In addition, 1DPU can engage in three
different interactions: intramolecular hydrogen bonding of urea
moieties, intramolecular repulsion between pyrene rings in the
ground state, and intramolecular stacking of pyrene rings in the
excited state followed by the excimer formation. Spectroscopic
techniques including steady state and transient absorption
measurement play an important role in examining fluorescence
quenching of the locally excited pyrenyl moiety of 1DPU as
well as subsequent excimer formation. Furthermore, the addi-
Chart 1. Pyrene derivatives used in the study.
1100 | Bull. Chem. Soc. Jpn. 2015, 88, 1100–1104 | doi:10.1246/bcsj.20150060
© 2015 The Chemical Society of Japan

tion of TBAAc or TBAF to 1DPU may affect this interaction
due to hydrogen bond formation with the urea moiety, leading
to the development of a new type of anion sensor.
Hz), 8.33 (2H, d, J = 8.6 Hz), 8.54 (2H, d, J = 8.47 Hz), 8.75
(2H, d, J = 8.36 Hz), 9.75 (2H, s). 13C NMR (DMSO-d6, 400
MHz): ¤ 154.1, 133.7, 133.6, 131.8, 131.1, 127.7, 127.5, 127.1,
126.7, 126.2, 125.9, 125.5, 125.1, 125.0, 124.7, 121.7, 120.7.
Anal. calcd. for C33H20N2O: C, 86.07; H, 4.38; N, 6.08; O,
3.47%. Found: C, 85.86; H, 4.63; N, 6.06; O, 3.45%.
Experimental Methods
Methods. Absorption and fluorescence spectra were mea-
sured on a Shimadzu UV-1600 and a Hitachi F-4500 fluores-
cence spectrometer, respectively. Fluorescence decay measure-
ments 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 a power
control unit (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 maxi-
mum (FWHM) value of the instrument response function was
51 ps.25 The values of »2 and the Durbin-Watson parameters
were used to determine the quality of the fit obtained by
nonlinear regression.26 DMSO (spectroscopic grade, Wako
Pure Chemical Industries, Japan) was used as a solvent without
further purification. Anions were in the form of tetrabutylam-
monium acetate and fluoride referred to as TBAAc and TBAF,
respectively, which contains a tetrabutylammonium cation
(Sigma-Aldrich, Japan). 1-Aminopyrene is commercially avail-
able (Tokyo Chemical Industry Co., Ltd.). All measurements
were carried out at room temperature under an Ar atmosphere.
The concentrations were adjusted so that the absorption maxi-
mum of the excitation wavelength was about 0.1 for each
sample. DFT calculations were performed using the Spartan ’04
program (Wavefunction, Inc., Irvine, CA).
Synthesis. 1PUP:24 To a two-neck round-bottom flask
(100 mL) were added 1-aminopyrene (300 mg, 1.39 mmol), dry
THF (40 mL), and phenyl isocyanate (0.15 ¯L, 1.39 mmol). The
mixture was refluxed for 23 h, after which the solvent was
filtered by suction filtration. The residue was washed by MeOH
to give a white solid (320 mg, 70%). 1H NMR (DMSO-d6, 400
MHz): ¤ 7.01 (1H, t, J = 7.6 Hz), 7.34 (2H, dd, J = 7.6 Hz,
J = 7.6 Hz), 7.57 (2H, d, J = 7.6 Hz), 8.07 (1H, t, J = 8.8 Hz),
8.08 (1H, d, J = 8.93 Hz), 8.14 (1H, d, J = 8.93 Hz), 8.26 (3H,
d, J = 8.4 Hz), 8.28 (1H, d, J = 8.4 Hz), 8.37 (1H, d, J =
8.47 Hz), 8.63 (1H, d, J = 8.36 Hz), 9.15 (1H, s), 9.19 (1H, s).
13C NMR (DMSO-d6, 400 MHz): ¤ 153.5, 140.2, 133.5, 131.6,
131.1, 129.4, 127.8, 127.4, 127.1, 126.8, 126.1, 125.8, 125.5,
125.0, 124.9, 124.6, 122.5, 121.6, 121.5, 120.5, 118.7. Anal.
calcd. for C23H16N2O: C, 82.12; H, 4.79; N, 8.33; O, 4.76%.
Found: C, 81.99; H, 4.98; N, 8.15; O, 4.88%.
Results and Discussion
Absorption Spectra. The absorption spectrum of 1DPU
was measured to investigate the electronic properties in the
ground state along with 1PUP and 1AP as a reference com-
pound, as shown in Figure 1. All three compounds showed
sharp peaks around 286 nm, which were ascribed to a higher
electronic transition of a pyrenyl moiety. The peak and shoul-
ders of the absorption spectrum of 1DPU were located at 362,
374, and 390 nm, which were similar to those of 1PUP. The
electronic band ranging from 310 to 420 nm seemed to be due
to the lowest transition of the pyrenyl moiety. In contrast, the
absorption spectrum of 1AP was broadened and red-shifted
relative to both 1DPU and 1PUP. These features may be due
to intramolecular charge-transfer interactions originating from
a nonbonding electron pair of the amino group of 1AP.28
The absorption spectra of 1DPU did not show concentration
dependence under the same conditions (see Supporting Infor-
mation, Figure S1), indicating that direct intramolecular inter-
action of the pyrene ring of 1DPU was not likely to occur in the
ground state.
Fluorescence Spectra. Figure 2 shows the fluorescence
spectra of 1PUP, 1DPU, and 1AP normalized at the maximum
intensity, excited at 355 nm, at 1.4 © 10¹6, 1.5 © 10¹6, and
2.5 © 10¹6 M, respectively. 1PUP and 1AP showed narrowed
emission bands, in contrast to 1DPU. The red-shifted emission
maximum of 1AP relative to that of 1PUP was consistent with
the absorption spectra behaviors of both. In contrast to 1PUP
and 1AP, 1DPU gave dual emission spectra consisting of LE
and a longer-wavelength emission even at 10¹6 M. The spectral
shape of the longer-wavelength emission was very close to that
of a pyrene excimer.4 Since excimer formation takes place
between the excited and ground states of pyrene molecules, the
efficiency of excimer formation largely depends on the distance
between them.4,29 In homogeneous solution, pyrene excimers
may be generated at concentrations greater than 10¹4 M due
to the diffusion-limited collision process.4 Neither 1AP nor
1DPU:27 To a two-neck round-bottom flask (100 mL) were
added 1-pyrenecarboxylic acid (105 mg, 0.426 mmol), toluene
(20 mL), triethylamine (100 ¯L, 0.72 mmol), and diphenylphos-
phoryl azide (250 ¯L, 1.0 mmol). After the mixture had been
stirred for 30 min at 80 °C, 1-aminopyrene (40 mg, 0.184 mmol)
in THF (10 mL) was added, and the mixture was refluxed for
15 h. The solvent was filtered by suction filtration and the
residue was washed with methanol a number of times. The
product was recrystallized from DMF-H2O and washed with
1
MeOH to give a light yellow solid (84 mg, 80%). H NMR
(DMSO-d6, 400 MHz): ¤ 8.08 (2H, t, J = 8.8 Hz), 8.10 (2H, d,
J = 8.93 Hz), 8.16 (2H, d, J = 8.93 Hz), 8.26 (6H, d, J = 8.4
Figure 1. Absorption spectra of 1PUP, 1DPU, and 1AP in
DMSO.
Bull. Chem. Soc. Jpn. 2015, 88, 1100–1104 | doi:10.1246/bcsj.20150060
© 2015 The Chemical Society of Japan | 1101

Figure 4. Excitation spectra of 1DPU monitored at 420,
450, and 600 nm in DMSO.
Figure 2. Fluorescence spectra of 1PUP, 1DPU, and 1AP at
an excitation wavelength of 375 nm in DMSO.
interaction. Since the spectral shape of I540 is similar to that of a
pyrene excimer, I540 may be ascribed to intermolecular exci-
mer formation. Hereafter, we focus on the longer-wavelength
emission of 1DPU.
[1DPU] / 10-7
M
31
6.1
2.4
Excitation Spectra.
Figure 4 shows excitation spectra
of 1DPU monitored at various wavelengths from 420 to 600
nm and normalized at peaks around 370 nm. The spectrum
consists of double peaks at around 285 and 370 nm, which are
consistent with those of the absorption spectra shown in
Figure 1, except for the wavelength dependence of the longer-
wavelength peak. The peak wavelength was red-shifted from
362 to 377 nm for monitoring at 420 and 600 nm, respectively.
In addition, the spectral shape ranging from 310 to 440 nm
showed wavelength dependence. Since the excitation spectrum
monitored at 420 nm was similar to the absorption spectrum
shown in Figure 1, it was thought that the spectrum monitored
at 600 nm might be another electronic state in the ground state.
These results indicate that intermolecular association in 1DPU
results in intermolecular interactions of the pyrene moiety of
1DPU, leading to an excimer-like emission, as well as wave-
length dependence in excitation spectra monitored at 600 nm.
Some types of precursor responsible for excimer formation
may be formed in various configurations between 1DPU mole-
cules through hydrogen-bonding interactions of the urea moiety
in the ground state. It is noteworthy that pyrene has a repulsive
potential surface, leading to a lack of stable dimer formation
in the ground state, which suggests that the binding energy, BE
(1DPU-1DPU), of the hydrogen-bonding interaction between
1DPU molecules is stronger than the repulsive energy, RE
(pyrene-pyrene), of pyrene molecules in the ground state.
Lifetimes and Time-Resolved Spectra of 1DPU. Table 1
lists the fluorescence lifetimes of 1DPU: 4.0 © 10¹7 and 4.0 ©
10¹6 M in DMSO (Figures S2, S3, and S4). The lifetimes
observed at 440 nm showed triexponential decay, with 15 ps as a
main component along with 1.22 and 6.96 ns. Those monitored
at 540 nm gave rise (236 ps) and decay (1.48 ns) components,
which are attributed to the formation and deactivation of a
longer emission band. This is a similar situation to the lifetimes
monitored at 640 nm. To confirm the emissive species, time-
resolved emission spectra (TRES) of 1DPU were measured in
400
450
500
550
600
650
700
Wavelength / nm
¹7
Figure 3. Fluorescence spectra of 1DPU (2.4 © 10 to
3.1 © 10¹6 M) at an excitation wavelength of 375 nm in
DMSO.
1PUP showed any pyrene-like aspect in its emission spectrum.
However, 1AP may have charge-transfer character in the
ground state from the amino group to the pyrene moiety as
shown in Figure 1, since the spectrum of 1AP was more red-
shifted than that of 1PUP. However, the emission spectrum of
1AP showed no dual-emission bands containing a significant
red-shifted charge transfer band, since the spectral profile was
similar to that of 1PUP. This suggests that the longer emission
band of 1DPU was not due to charge transfer; a charge transfer
band was not observed in the absorption spectrum.
Figure 3 demonstrates the concentration dependence of the
emission spectra of 1DPU normalized at 440 nm. The ratio of
the intensity at 540 nm (I540) to that at 440 nm (I440) increased
with the concentration of 1DPU. This implies that I540
originates from intermolecular interactions rather than intra-
molecular interactions, even at extremely low concentrations
(10¹7 M). Since the profiles of emission spectra longer than
500 nm have no concentration dependence, the electronic struc-
ture responsible for I540 must be only one emissive mechanism,
which plays an important role in the origin of I540. However,
this is inconsistent with the lack of concentration dependence
in the absorption spectra, as described above. This indicates
that the electronic state responsible for the I540 band is not the
ground state but the excited state resulting from intramolecular
¹6
two different concentrations (4 © 10 and 4 © 10¹7 M), as
shown in Figure 5. The longer emission band gradually disap-
1102 | Bull. Chem. Soc. Jpn. 2015, 88, 1100–1104 | doi:10.1246/bcsj.20150060
© 2015 The Chemical Society of Japan

Table 1. Fluorescence Lifetimes of 1DPU in DMSO at an
Excitation Wavelength of 375 nm
we assume that the spectra obtained at 0.92-1.05 and 11.0-
12.3 ns correspond, respectively, to the sum of the LE and
the new emission, and the LE emission only, subtraction may
give the new emission spectrum. In fact, these calculations
successfully yielded broad spectra at 4 © 10¹6 and 4 © 10¹7 M
(Figure S5). Interestingly, these spectra were in good agree-
ment with each other. Considering the concentration depend-
ence of the efficiency of formation of the new emission,
intramolecular excimers of the intermolecular pyrene moiety of
1DPU are the most plausible explanation for the spectroscopic
results obtained. However, a certain distance between pyrene
moieties is an essential requirement for excimer formation.
Therefore, DFT calculations were performed to determine the
structure of 1DPU hydrogen bonded through urea moieties
(Figure S6). The two pyrene rings retained almost coplanar
configuration, with a center-to-center distance of ca. 4 ¡, which
is a distance comparable with those found in pyrene crystals
and pyrenophanes.4,5
-/nm ¸1/ps
¸2/ns
¸3/ns
[1DPU] = 4 © 10¹7 M
440
540
640
15
236
231
(0.832) 1.22 (0.154) 6.96 (0.014)
(¹0.031) 1.48 (1.000)
(¹0.085) 1.45 (1.000)
[1DPU] = 4 © 10¹6 M
440
540
640
12
250
270
(0.945) 1.42 (0.046) 4.20 (0.009)
(¹0.023) 1.47 (1.000)
(¹0.089) 1.44 (1.000)
Association of 1DPU with TBAAc.
The addition of
TBAAc to 1DPU resulted in a downfield shift for the NH
1
proton upon H NMR measurement, as shown in Figure S7,
indicating a hydrogen bonding interaction between them.
Besides, Spectral red-shift of the absorption band was observed
upon the addition of TBAAc, as shown in Figure S8. This is
quite similar to the phenomenon found in nPUA.21,22 Increases
in the TBAAc concentration yielded remarkable changes in the
fluorescence spectrum, as shown in Figure S9. In the presence
of 15.6 mM TBAAc, the longer wavelength emission band
almost disappeared following an increase in LE band intensity.
This may be related to the manner of association of 1DPU with
TBAAc. We obtained similar results as for the addition of
TBAF, as shown in Figures S10-S12 except for the absorption
spectra in the addition of more than 5.5 mM TBAF. This is
consistent with the result reported by Amendola et al.24
The association constants of 1DPU with TBAAc and TBAF
¹1
were determined, according to eq 1, as 920 and 830 M
ꢀ
ꢁ
I0
I0
1
¼
þ 1
ð1Þ
I ꢀ I0
Imax ꢀ I0 Ka½Aꢁ
where I0 and I are the fluorescence intensities of the host
monitored at 530 nm in the absence and presence of anions,
respectively, [A] is the total anion concentration, and Imax is
the fluorescence intensity of 1DPU fully hydrogen bonded with
the anion. The disappearance of the excimer emission in the
presence of TBAAc implies that there is no intermolecular
interaction between 1DPU units due to the hydrogen-bonding
interaction with TBAAc. Since the intensity of emission in the
presence of TBAAc exists up to 700 nm, an electronic change
in the excited state takes place, as found in nPUA, leading to
tautomer formation.
Characteristics of 1DPU. An extremely dilute solution of
1DPU (ca. 10¹7 M) showed excimer emissions caused by inter-
molecular interactions of 1DPU, indicating close proximity
between the pyrenyl moieties of different 1DPU molecules.
It was revealed that 1DPU formed stable aggregates through
intermolecular hydrogen bonding, with time-resolved spectra
showing a concentration dependence for excimer formation.
Although the reason for the disagreement in the rise and decay
time constants is still unknown, it is worthwhile to note that
Figure 5. Time-resolved fluorescence spectra of 1DPU in
DMSO, excited at 375 nm. [1DPU]: (a) 4 © 10¹6 M, (b)
4 © 10¹7 M. Spectra were normalized at LE peak intensity.
peared relative to the LE band from 0.92 to 12.3 ns. Interest-
ingly, the TRES of 1DPU at 4 © 10¹7 M showed less intense
emission of the longer species than that at 4 © 10¹6 M, as
shown in Figure 5, implying that the formation of the longer
emissive species is concentration-dependent. This indicates that
intermolecular association of 1DPU cannot be ignored even at
10¹7 M. Although the reason for the disagreement in the rise
and decay time constants is still unknown, it seems that the exci-
mer formation process may be complicated due to the hetero-
geneous character of the excimer potential surface in association
with wavelength dependence of the excitation spectra.5
TRES gives important information about the spectral profile
of LE and new emissive species, since these have different
lifetimes, leading to the possibility of separation of TRES. If
Bull. Chem. Soc. Jpn. 2015, 88, 1100–1104 | doi:10.1246/bcsj.20150060
© 2015 The Chemical Society of Japan | 1103

multiple types of the 1DPU complex may be generated in the
ground state and give a nonexponential decay curve due to
inhomogeneous distribution, which requires a different method
to analyze the decay curve.30,31 In contrast, 1PUP gave no exci-
mer emission. Although 1PUP can also form a complex through
the urea moiety, the stacking style may be different from that
of 1DPU due to its asymmetric structure, which may favor
phenyl-pyrenyl rather than pyrenyl-pyrenyl interaction to avoid
pyrenyl-pyrenyl repulsion in the ground state. However, the
formation of a complex has not yet been confirmed.
Since the addition of TBAAc to 1DPU decreased excimer
fluorescence, hydrogen bond formation by the urea units of
1DPU favors a conformation that promotes excimer forma-
tion in the absence of acetate anions, and the strength of the
hydrogen bond interaction between 1DPU molecules is weaker
than the BE (1DPU-TBAAc) between a 1DPU molecule and
an anion. Consequently, we can assume the order of interac-
tion energy: BE (1DPU-TBAAc) > BE (1DPU-1DPU) > RE
(pyrene-pyrene) qualitatively, which represents a significant
indicator of individual interaction. This may be useful from the
point of view of the development of anion sensors, because
excimer emissions can be used to identify the existence of an
anion or a hydrogen-bonded unit. Furthermore, dual emissions
enable us to use the ratiometric method to determine asso-
ciation parameters.32 The phenomenon of excimer formation
even at extremely dilute concentrations (ca. 10¹7 M) may be
advantageous in terms of lowering the risk of toxicity in
biological applications.
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using spectroscopic measurement and DFT calculations, while
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This work was partially supported by a Grant-in-Aid for
Scientific Research (B) (No. 23350075) from the Ministry of
Education, Culture, Sports, Science and Technology (MEXT),
Japan.
Supporting Information
The concentration dependence of absorption spectra, fluo-
rescence decay analyses, TRES, DFT calculations, 1H NMR
spectra, and the fluorescence spectra upon addition of TBAAc.
This material is available free of charge on J-STAGE.
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1104 | Bull. Chem. Soc. Jpn. 2015, 88, 1100–1104 | doi:10.1246/bcsj.20150060
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