Kinetics of hydrogen bonding between anthracene urea derivatives and anions in the excited state

Article abstract of DOI:10.1021/jp2022693

Urea compounds are useful anion sensors due to their hydrogen-bonding capabilities. We investigated the emissive properties of complexes consisting of urea-anthracene (nPUA, n = 1, 2) host compounds and acetate anions held as guests through intermolecular hydrogen bonding. The kinetics of a new emission band formed by conformational changes in the excited singlet state were revealed. The new band is thought to arise from a charge-transfer interaction between the anthracene and urea moieties after intermolecular hydrogen-bond reconfiguration in the excited state, with rate constants of 2.4 × 10 ⁹ and 4.0 × 10⁷ s^-1 for 1PUA and 2PUA, respectively.

Full text of DOI:10.1021/jp2022693

ARTICLE
pubs.acs.org/JPCA
Kinetics of Hydrogen Bonding between Anthracene Urea Derivatives
and Anions in the Excited State
Satomi Ikedu, Yoshinobu Nishimura, and Tatsuo Arai*
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan
S Supporting Information
b
ABSTRACT: Urea compounds are useful anion sensors due to
their hydrogen-bonding capabilities. We investigated the emis-
sive properties of complexes consisting of ureaꢀanthracene
(nPUA, n = 1, 2) host compounds and acetate anions held as
guests through intermolecular hydrogen bonding. The kinetics
of a new emission band formed by conformational changes in
the excited singlet state were revealed. The new band is thought
to arise from a charge-transfer interaction between the anthra-
cene and urea moieties after intermolecular hydrogen-bond
reconfiguration in the excited state, with rate constants of 2.4 ꢁ 109 and 4.0 ꢁ 107 sꢀ1 for 1PUA and 2PUA, respectively.
’ INTRODUCTION
Since the interaction of the urea moiety at the 1- or 2-position
of anthracene was expected to be different from that at the
9-position, we synthesized the ureaꢀanthracene compounds
nPUA (n = 1, 2, where n is the substituted position of the parent
anthracene; see Scheme 1) to investigate the emissive state.
1PUA and 2PUA exhibited absorption spectra similar to those of
1-aminoanthracene and 2-aminoanthracene, respectively.14
nPUA also showed dual emission spectra in the presence of
acetate anions in DMSO, indicating the formation of emissive
species comparable to those of 9PUA. Interestingly, the rising
component, which was attributed to a new emissive species in the
presence of acetate anions, was observed at a longer wavelength,
the time constant of which agreed well with the decay constant of
the LE state. This is a specific phenomenon of 1PUA and 2PUA,
since 9PUA has no rising component.
Much effort has been devoted to the development of anion
sensors in various fields.1 Urea has been used as an anion receptor
since the discovery of remarkable properties that contribute to its
usefulness in this area.2,3 The hydrogen-bonding capability of
urea compounds containing chromophore groups allows for
spectroscopic detection of anions including phosphate, acetate,
and sulfate ions.4ꢀ12 However, there have been few studies on
the kinetics of chromophores interacting with anions in the
excited singlet state, or on the origin of a new emission band
additional to that observed in the absence of anions. The aim of
this study was to elucidate the fluorescence behavior of urea
compounds containing an anthracene moiety as a fluorophore by
using time-resolved fluorescence spectroscopy, which enabled us
to estimate the rate constants of elemental processes involved in
hydrogen-bonding interactions in the excited singlet state.
Previous investigations have dealt with the photochemistry of
urea derivatives containing an anthracene fluorophore and a urea
moiety which acts as a probe for anions such as bromide or
acetate.13 In a previous paper, such a compound, 1-anthracen-9-
yl-3-phenylurea (9PUA), was studied in the presence of a variety
of anions in dimethyl sulfoxide (DMSO) solution. 9PUA was
found to show dual fluorescence spectra, maximized at 450 and
580 nm, in the presence of acetate anions, while fluorescence
enhancement was observed only in the presence of bromide
anions. Thus, acetate anions were found to interact with 9PUA
followed by the formation of a new emissive species with a large
Stokes shift of 8300 cmꢀ1. Since the acetate anions appeared not
to form a charge-transfer state with the urea moiety of 9PUA, the
newly formed emissive state (1X*) was suggested to be intrinsi-
cally different from the locally excited (LE) state of 9PUA,
because of the large Stokes shift. A plausible emissive structure,
based on proton transfer from the urea moiety in the LE state to
the acetate anion, was proposed.
In this study, by taking advantage of spectroscopic techniques,
we demonstrate that the interaction of nPUA with acetate anions
and the formation of emissive species upon irradiation are
dependent upon the substitution position. We also elucidate
the rate constants in the excited singlet state on the basis of a two-
state model and consistent with the steady-state emission
1
spectra. It was found that the rate constant of X* formation
was completely dependent on the substituted position of the
anthracene moiety, with 1PUA forming 1X* 2 orders of magni-
tude faster than 2PUA.
’ EXPERIMENTAL METHODS
Methods. Absorption and fluorescence spectra were mea-
sured on Shimadzu UV-1600 and Hitachi F-4500 fluorescence
spectrometers, respectively. Fluorescence decay measurements
Received: March 10, 2011
Revised:
June 8, 2011
Published: June 20, 2011
r
2011 American Chemical Society
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Scheme 1. Structures of 1PUA and 2PUA
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-maximum (fwhm) value of
the instrument response function was 51 ps.15 The values of χ2
and the DurbinꢀWatson parameters were used to determine the
quality of the fit obtained by nonlinear regression.16 DMSO
(spectroscopic grade, Wako Pure Chemical Industries, Japan)
was used as a solvent without further purification. Acetate anions
were in the form of tetrabutylammonium acetate (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).
Figure 1. Changes in the absorption spectra of (a) 1PUA and (b) 2PUA
in the presence of TBAAc. [TBAAc] = 0 (—), 1 ( ), and 10 mM (---).
3 3 3
(d, 1H, J = 8.1 Hz, anthryl), 6.76 (d, 1H, J = 7.0 Hz, anthryl), 4.31
(s, 2H, ꢀNH2).
1-Anthracen-1-yl-3-phenylurea (1PUA)9. 1-Aminoanthra-
cene (161.6 mg) and phenylisocyanate (90 μL, 0.831 mmol)
were stirred in dry tetrahydrofuran (THF; 8 mL) at room
temperature under N2 for 17 h. After the addition of another
portion of phenylisocyanate (90 μL, 0.831 mmol) dissolved in
dry THF (4 mL) to the mixture, and 17 h stirring, the solvent was
evaporated under reduced pressure, and the residue was dried
and purified by recrystallization in ethanol to yield 1PUA (45.2
1
mg, 0.145 mmol) as a yellow solid. H NMR (DMSO-d6, 270
MHz): δ 9.12 (s, 1H, ꢀNH), 8.97 (s, 1H, ꢀNH), 8.76 (s, 1H,
anthryl), 8.60 (s, 1H, anthryl), 8.12ꢀ8.03 (m, 2H, anthryl), 8.02
(d, 1H, J = 9.1 Hz, anthryl), 7.81 (d, 1H, J = 8.4 Hz, anthryl),
7.57ꢀ7.46 (m, 5H, anthryl and phenyl), 7.33 (t, 2H, J =
6.9 Hz, phenyl), 7.00 (t, 1H, J = 7.3 Hz, phenyl); 13C NMR
(DMSO-d6, 67.5 MHz): δ 152.8, 139.6, 133.9, 131.7, 130.9,
130.7, 128.8, 128.1, 127.7, 126.6, 125.8, 125.4, 125.1, 122.9,
121.8, 119.7, 118.0, 115.7; Anal. Calcd for C16H21N2O: C, 80.75;
H, 5.16; N, 8.97; found: C, 80.67; H, 5.40; N, 8.83.
Synthesis. 1-Aminoanthracene17,18. A mixture of 1-aminoan-
thraquinone (1.66 g, 7.44 mmol) and NaOH (106.4 mg, 2.66
mmol) in 2-propanol (60 mL) was bubbled with N2 for 15 min
before the addition of NaBH4 (2.86 g, 75.6 mmol), and the
mixture was refluxed for 100 h at 80ꢀ90 °C under N2. The
addition of another portion of NaBH4 (0.56 g, 14.8 mmol) to the
mixture followed by 15 h of stirring gave a precipitate, which was
filtered off, washed with water, and dried in vacuo. The residue
was dissolved in chloroform and dried over anhydrous Na2SO4
before being concentrated in a rotary evaporator under reduced
pressure. This yielded 532 mg of 1-aminoanthracene as a pale
yellow solid, which was used for the subsequent step without
2-Aminoanthracene17,18. A mixture of 2-aminoanthraqui-
none (2.99 g, 13.4 mmol) in 2-propanol (200 mL) was bubbled
with N2 for 15 min before addition of NaBH4 (6.31 g, 168
mmol), and the mixture was refluxed for 66 h at 80 °C under N2.
The solution was then poured into water, and the resulting
precipitate was filtered off, washed with water and 2-propanol,
and dissolved in ethanol. The solution was then filtered again to
exclude impurities and concentrated in a rotary evaporator under
reduced pressure. The residue was recrystallized from ethanol
1
further purification due to difficulties with recrystallization. H
NMR (CDCl3, 270 MHz): δ 8.39 (d, 2H, J = 3.5 Hz, anthryl),
7.99 (t, 2H, J = 8.5 Hz, anthryl), 7.52ꢀ7.47 (m, 3H, anthryl), 7.31
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Figure 2. Changes in the fluorescence spectra of (a) 1PUA and (b)
Figure 3. Fluorescence emission (—) and excitation spectra (
and ---)
3 3 3
of (a) 1PUA and (b) 2PUA in the presence of 10 mM TBAAc.
2PUA in the presence of TBAAc. [TBAAc] = 0 (—), 1 ( ), and
3 3 3
10 mM (---). The insets show normalized spectra at maximum intensity.
with shoulders at 344, 363, and 405 nm. The characteristics of the
absorption spectrum seemed to be similar to that of 1-amino-
anthracene rather than unsubstituted anthracene.14,19 The red-edge
region (wavelengths longer than 380 nm) shifted to longer
wavelengths (up to 440 nm) in the presence of 10 mM TBAAc,
while the absorption band below 380 nm showed no shift,
although there was a slight increase in absorbance.
and dried in vacuo to yield 2-aminoanthracene (122 mg, 0.631
mmol, 4.7% yield) as a pale yellow solid. 1H NMR (acetone-d6,
270 MHz): δ 8.28 (s, 1H, anthryl), 8.05 (s, 1H, anthryl),
7.92ꢀ7.82 (m, 3H, anthryl), 7.38ꢀ7.25 (m, 2H, anthryl), 7.11
(d, 1H, J = 9.2 Hz, anthryl), 7.02 (s, 1H, anthryl), 5.05 (s, 2H,
anthryl).
1-Anthracen-2-yl-3-phenylurea (2PUA). 2-Aminoanthracene
(105 mg, 0.543 mmol) and phenylisocyanate (70 μL, 0.646
mmol) were stirred in dry THF (5 mL) at room temperature
under N2 for 22 h. The resulting precipitate was filtered off and
washed with THF and dichloromethane to yield 2PUA (104 mg,
0.333 mmol, 61.1% yield) as a yellow solid. 1H NMR (DMSO-d6,
270 MHz): δ 8.96 (s, 1H, ꢀNH), 8.79 (s, 1H, ꢀNH), 8.48
(s, 1H, anthryl), 8.40 (s, 1H, anthryl), 8.27 (s, 1H, anthryl),
8.05ꢀ8.02 (m, 3H, anthryl), 7.52ꢀ7.40 (m, 5H, anthryl and
phenyl), 7.31 (t, 2H, J = 7.7 Hz, phenyl), 7.00 (t, 1H, J = 7.4 Hz,
phenyl); 13C NMR (DMSO-d6, 67.5 MHz): δ 152.4, 139.4,
136.5, 131.8, 131.6, 129.9, 128.8, 128.7, 128.0, 127.9, 127.4,
125.7, 125.4, 124.5, 124.2, 121.8, 120.8, 118.2, 111.6. Anal. Calcd
for C16H21N2O: C, 80.75; H, 5.16; N, 8.97; found: C, 80.93; H,
5.35; N, 8.96.
As expected, the spectral shape of 2PUA was totally different
from that of 1PUA, with strong vibrational bands from 320 to
380 nm, as shown in Figure 1b. In contrast to 1PUA, the red-edge
absorption band (greater than 390 nm) was red-shifted in the
presence of TBAAc, while the absorbance below 340 nm in-
creased significantly. This indicates that the electronic state of
2PUA has a more complicated nature than that of 1PUA.
Fluorescence Spectra. Figure 2a shows changes in the
fluorescence spectrum of 1PUA in the presence of TBAAc in
DMSO. While the spectrum in the absence of TBAAc had a
maximum at 460 nm, the fluorescence intensity decreased as the
TBAAc concentration increased from 0 to 10 mM, followed by
the appearance of a new band maximized at 620 nm, as shown in
the inset of Figure 2a. The peak wavelength of 1PUA remained
unchanged even in the presence of 10 mM TBAAc. The changes
in 2PUA in the presence of TBAAc were different from those of
1PUA. An increase in the TBAAc concentration resulted in a
decrease in fluorescence intensity and red shifting of the spec-
trum; in addition, a new emission band appeared, with a peak at
600 nm. The relative intensity of the new band compared to the
LE emission of 2PUA was greater than for 1PUA. Since the peak
wavelength of 2PUA was red-shifted as the concentration of
’ RESULTS
Absorption Spectra. Figure 1a shows the absorption spectra
of 1PUA in DMSO with various concentrations of TBAAc (0, 1,
and 10 mM). In the absence of TBAAc, the spectral shape was
broad compared to that of the parent molecule, anthracene, with
vibrational structure.14 The maximum wavelength was 384 nm,
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Table 1. Fluorescence Lifetimes of 1PUA in the Presence and
Absence of TBAAc
Table 2. Fluorescence Lifetimes of 2PUA in the Presence and
Absence of TBAAc
τ (ns)
τ (ns)
[TBAAc] (mM)
450 nm
650 nm
freea (%)
[TBAAc] (mM)
460 nm
640 nm
freea (%)
0
5
14.6 (1.00)
13.8 (0.38)
0.3 (0.62)
10.6 (0.17)
0.3 (0.83)
100
0
5
18.9 (1.00)
14.7 (0.08)
5.0 (0.92)
13.1 (0.06)
5.1 (0.94)
100
3.4
4.7 (1.00)
12.5
13.4 (1.00)
4.0 (ꢀ1.00)
13.4 (1.00)
4.0 (ꢀ1.00)
0.1 (ꢀ1.00)
4.4 (1.00)
10
6.7
10
1.7
0.1 (ꢀ1.00)
a Ratio of uncomplexed 1PUA to TBAAc. The values in parentheses are
the normalized amplitudes for the respective lifetimes.
a Ratio of uncomplexed 2PUA to TBAAc. The values in parentheses are
the normalized amplitudes for the respective lifetimes.
TBAAc increased, other emissive species were also expected to
be present. Thus, it was concluded that there were three different
emissive species for nPUA: one with no association between
nPUA and TBAAc, the complex between nPUA and TBAAc, and
a long-wave emission. Hereafter, the latter is referred to as 1X*, by
analogy with 9PUA.13
TBAAc showed biexponential decay, with lifetimes of 13.8 and
0.3 ns, while the lifetimes at 650 nm showed rise and decay
behaviors of 0.1 and 4.7 ns, respectively. In the presence of
10 mM TBAAc, the longer-lifetime component at 450 nm
reached 10.6 ns, although the shorter-lifetime component re-
mained constant at 0.3 ns. The decrease in lifetime from 14.6 to
10.6 ns may be ascribed to dynamic quenching by free TBAAc.
Since the amplitude of the 0.3 ns component observed at 450 nm
increased from 0.62 to 0.83 in the presence of 5 and 10 mM
TBAAc, it may be attributed to a hydrogen-bonded complex of
1PUA with TBAAc. The lifetimes of 2PUA, observed at 460 nm,
were 5.0 and 14.7 ns in the presence of 5 mM TBAAc, with quite
a high amplitude (0.92) for the shorter-lifetime component due
to the relatively high association constant, which is 4 times larger
than that for 1PUA (Table 2). In contrast to this, the shorter
lifetime was 16 times longer than the equivalent in 1PUA,
reflecting the relatively slow quenching process of the 2PUAꢀT-
BAAc complex in the excited singlet state. This may reflect the
intramolecular interaction between anthracene and the hydro-
gen-bonded urea moieties in the excited state. Since the observed
rate constant was less than 30 ps for 9PUA, as estimated from the
rising component of fluorescence at 580 nm, the rate constant
seems to be related to the electronic interaction between the urea
and anthracene moieties; the smaller the interaction, the lower
the rate constant of proton transfer in the excited singlet state.
The time development of the new emission was confirmed by
time-resolved spectra of nPUA (see Supporting Information,
Figures S2 and S3). The buildup of new emission spectra,
maximized around 650 and 640 nm, was confirmed at 0.1 and
4.0 ns for 1PUA and 2PUA, respectively.
Excitation Spectra. Figure 3a shows the excitation spectrum
of 1PUA in the presence of 10 mM TBAAc. The excitation
spectrum observed at 640 nm was red-shifted relative to the
spectrum observed at 460 nm, which indicates that the longer-
wavelength emissive species (1X*) has a electronic state different
1
from that of the shorter-wavelength species. X* may be the
species in which 1PUA is associated with TBAAc, as it is
consistent with the absorption changes in the presence of
TBAAc, as shown in Figure 1a. This is also applicable to
2PUA, as shown in Figure 3b. Thus, the inclusion complex with
TBAAc was concluded to be responsible for the different
excitation behaviors.
1H NMR Spectra. To confirm the complexation of 1PUA or
1
2PUA with TBAAc, H NMR spectra were obtained in the
absence and presence of TBAAc. This revealed the existence of
hydrogen bonds between 1PUA and TBAAc, since downfield
shifts of the NH protons from 9.13 and 8.97 ppm to 11.50 and
11.03 ppm were observed in the presence of TBAAc. These were
attributed to the NH proton signals of one urea group and
another urea group adjacent to the anthracene moiety, respec-
tively (see Supporting Information, Figure S1). The equivalent
signals in 2PUA also exhibited downfield shifts, from 8.95 and
8.79 ppm to 12.36 and 12.17 ppm, respectively. These shifts
imply that nPUA interacts with TBAAc by hydrogen bonding
through the urea moiety of nPUA.
Association Constants. The association constants of 1PUA
and 2PUA with TBAAc were determined, according to eq 1, as
’ DISCUSSION
1.4 ꢁ 103 and 5.7 ꢁ 103 Mꢀ1 20
:
Formation of Complex with TBAAc. As shown by the
spectroscopic measurements, nPUA may form a complex with
TBAAc through hydrogen-bonding interactions between the
urea moiety of nPUA and the carboxylate group of TBAAc,
which causes a red shift in the absorption spectrum, the ap-
pearance of a new emission band, and a downfield shift in the 1H
NMR spectrum. Comparison of the association constants in-
dicates that 2PUA interacts with TBAAc 4 times as strongly
as 1PUA.
The absorption spectra indicate that the electronic interaction
between the anthracene and urea moieties of 1PUA is greater
than that of 2PUA. DFT calculations for 1PUA showed a
delocalized highest occupied molecular orbital (HOMO) com-
pared to 2PUA (see Supporting Information, Figure S4). Since
the HOMO orbital of 2PUA is similar to that of unsubstituted
ꢀ
ꢁ
I0
I0
1
¼
þ 1
ð1Þ
I ꢀ I0
Imax ꢀ I0 Ka½Aꢂ
where I0 and I are the fluorescence intensities of the host
monitored at 450 nm in the absence and presence of anions,
respectively, [A] is the total anion concentration, and Imax is the
fluorescence intensity of nPUA fully hydrogen bonded with the
anion. The difference in the association constants of 1PUA and
2PUA may arise from the acidity of the urea protons.21,22
Lifetimes of nPUA in the Excited State. The fluorescence
decay of 1PUA in the absence of TBAAc under Ar gave a single-
exponential curve with a lifetime of 14.6 ns (Table 1). The
fluorescence curve observed at 450 nm in the presence of 5 mM
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Scheme 2. Resonant Structures of nPUA Depending on the Electronic Nature of the Substituent Ra
a R = 1- or 2-substituted anthracene.
anthracene, this is consistent with the vibrational structure of the
absorption spectrum of 2PUA, which is ascribed to localized
excitation in the anthracene moiety, in contrast to 1PUA. In
general, the strength of hydrogen bonding depends on the acidity
of the hydrogen donor.21ꢀ23 The electron density of the nitrogen
atom of the urea group adjacent to the anthracene moiety is
lower in 2PUA than in 1PUA, since the anthracene moiety may
work as an electron-donating group to the urea moiety. This
indicates that the acidity may be higher in 2PUA than in 1PUA,
which may be responsible for the larger association constant of
2PUA.21,22
Orbital Nature. The absorption spectra of 1PUA and 2PUA
showed profiles similar to those of 1-aminoanthracene and
2-aminoanthracene, respectively. The localized n-orbital of the
urea nitrogen can interact with the π-orbital of the anthracene
moiety, leading to a πl, π* orbital nature for the first excited
singlet state, if the geometry of nPUA is planar.24ꢀ26 Since the
stable structure of 1PUA shows a small difference compared to
that of 2PUA, based on DFT calculations, the electronic inter-
action between the urea and anthracene moieties may depend on
the substituent position of the urea group relative to the
anthracene group rather than on planarity between the urea
and anthracene moieties.27
that of 1PUA in the presence of 10 mM TBAAc. Figure 2a shows
that the complex of 1PUA with TBAAc may have almost no
emissive properties, in comparison with the fluorescence beha-
vior of 2PUA (Figure 2b), as there was no spectral shift and a
monotonic decrease in emission with increasing TBAAc con-
centration. In contrast, determination of the spectral shape of
1X2* required shifting the emission spectrum of 2PUA in the
absence of TBAAc to the longer-wavelength side by 760 cmꢀ1
;
the spectral shape of the complex was derived by subtracting the
spectrum of 1X2* from that of 2PUA in the presence of 10 mM
TBAAc. The resulting peak wavelengths of 1PUA and 2PUA
were estimated as 640 and 600 nm (see Supporting Information,
Figure S5). Although the method for determining the spectral
shapes of 1Xn* is based on an assumption, as mentioned above,
the wavelengths of the emission maxima of nPUA may be used to
compare the emissive states of nPUA. The emissive state of 1X1*
is stabilized by 1040 cmꢀ1 relative to that of X2*, which is
1
consistent with the existence of more extended π-conjugation in
1PUA than in 2PUA.
The extended π-conjugation of anthracene with urea moieties
may mean that anthracene functions as an electron-donating,
rather than an electron-accepting, moiety. This may lead to the
faster electron-transfer reaction in 1PUA compared with 2PUA.
However, no explanation has yet been found for the difference
between their time constants.
Origin of New Emissive Species (1X*). The resonant struc-
ture of nPUA in the presence of acetate was inferred from a
phenylurea compound, as shown in Scheme 2.28 In general, the
substituent R determines the charge density of each atom in the
urea moiety. If R has electron-accepting ability, the resonance
equilibrium may move to the right, as shown in Scheme 2; if R has
electron-donating ability, it may move to the left. Since these
resonances are allowed even in the ground state, as seen from
steady-state spectroscopic measurement, a charge-transfer reac-
tion may occur more efficiently in the excited singlet state than in
the ground state.29 Again, DFT calculations show that the urea
moiety of 1PUA has an electron density that is consistent with a
lack of LE absorption in the broad absorption spectrum, in
contrast to 2PUA. This is supported by the observation that the
molar extinction coefficient of 1PUA is twice as large as that
of 2PUA.
A plausible reaction scheme for 2PUA is shown in Figure 4.
The electronic structure of 1Xn* is still under consideration. As
described above, the complex of 1PUA with TBAAc may be less
fluorescent, with a lifetime of 100 ps, or have a spectrum similar
to that of uncomplexed 1PUA. A major difference between 1PUA
and 2PUA may arise from the variation in the substituted
position of the anthracene ring. As seen from the DFT calcula-
tions, the electron density of the substituted 2-position of
anthracene is less than that of the substituted 1-position, leading
to weak interaction between the anthracene and urea groups.
This may explain the slower formation of 1X2* compared to 1X1*,
since the rate constant of the charge-transfer reaction depends on
electronic coupling between the donor and acceptor moieties.
Furthermore, in general, proton transfer takes place on an
ultrafast time scale of less than 100 fs or even less than 1 ps. As
far as we are aware, this is the first example of proton transfer with
a rate constant as slow as 4 ns. One possible explanation for this
slow reaction is delocalization of the excitation accompanying the
For the purpose of discussing the emissive electronic state of
nPUA, it is worthwhile to determine the spectral shapes of the
1Xn* (n = 1, 2) emissions. An assumption was required to
estimate these shapes from the fluorescence spectra; the spectral
shape of the complex form was identical to that of free nPUA,
except for the energy level of the corresponding emissive state.
1
formation of Xn* triggered by proton transfer in the excited
singlet state. If this were the case, it would demonstrate that the
proton-transfer rate constant may be controlled by appropriate
design of proton-transfer systems.
1
The emission shape of X1* was derived by simply subtracting
the emission spectrum of 1PUA in the absence of TBAAc from
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Figure 4. Reaction process of 2PUA in the presence of TBAAc.
Kinetic Analysis of the Formation and Relaxation of 1Xn*.
We may assume that there is reverse electron transfer from 1X2*
to the excited singlet state of the complex referred to as
Table 3. Determined Rate Constants of 1PUA and 2PUA in
the Presence of TBAAc
rate constants (107 sꢀ1
)
1(2PUA Ac)*, with a rate constant k
(as shown in
ꢀET
3 3 3
Figure 4), since the longer-lifetime component observed at
460 nm is in good agreement with that observed at 640 nm in
the case of 2PUA. If kꢀET has a significant value relative to kET, a
two-state model may be used to analyze the kinetic rate constants
involved in the excited state.30,31 If we suppose
[1(2PUA Ac)*] = [1(2PUA Ac)*]0 = 0 at t = 0, the
C
X
[TBAAc] (mM) λobs (nm)
kET
k
ꢀET
kD
kD
1PUA
2PUA
2PUA
2PUA
2PUA
10
5
450
460
640
460
640
240
47
14
15
17
15
18
7.2
4.8
4.6
6.3
4.0
4.1
3.3
4.5
2.3
5.3
5
5.5
4.0
5.7
3 3 3
3 3 3
10
10
following equations allow us to perform calculations to deter-
mine the rate constants involved.32ꢀ36
1
ꢃ
½ ð2PUA AcÞ ꢂ
3 3 3
1
Supporting Information, Figures S6 and S7). This is why more
than 96% of 2PUA can associate with TBAAc (Table 2). The
formation rate constant of 1X2*, kET, was determined to be about
4 ꢁ 107 sꢀ1, which was comparable to the reverse electron
transfer rate, kꢀET, indicating nonnegligible regeneration of
¼
¼
fðX ꢀ γ2Þ expðꢀγ1tÞ
1
ꢃ
½ ð2PUA AcÞ ꢂ0
3 3 3
γ1 ꢀ γ2
ꢀ ðX ꢀ γ1Þ expðꢀγ2tÞg
ð2Þ
ð3Þ
1
ꢃ
½ X2 ꢂ
kET
1(2PUA Ac)* from 1X2*. This is consistent with the appear-
fexpðꢀγ2tÞ ꢀ expðꢀγ1tÞg
3 3 3
1
ꢃ
γ1 ꢀ γ2
½ ð2PUA AcÞ ꢂ0
ance of fluorescence spectra emitted from the complex, as shown
in the insets of Figure 2. Although the profiles of fluorescence
decay for 1PUA monitored at 450 and 650 nm did not give
lifetimes similar to those of 2PUA, the same procedure was
applied to determine rate constants. In the presence of 5 mM
TBAAc, no appropriate fitting was obtained for either the 450-
nm or the 650-nm decay curve due to the considerable amount of
free 1PUA (12.5%). The decay curve observed at 650 nm in the
presence of 10 mM TBAAc also resulted in no fitted curve, which
implied extreme difficulty in solving a time development profile
comprising fast rise and slow decay. Accordingly, only the decay
3 3 3
where
1
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
γ1 ¼ fðX þ YÞ þ ðX ꢀ YÞ þ 4kꢀETkET
g
ð4Þ
ð5Þ
2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
2
γ2 ¼ fðX þ YÞ ꢀ ðX ꢀ YÞ þ 4kꢀETkET
g
2
C
X
profile monitored at 450 nm was solved, giving values of 2.4 ꢁ
and kDX, respectively (see also Supporting Information, Figure
X ¼ kD þ kET
,
Y ¼ kD þ k
ð6Þ
ð7Þ
ꢀET
109, 4.7 ꢁ 108, 1.4 ꢁ 108, and 7.2 ꢁ 107 sꢀ1 for kET, kꢀET, kD
,
C
C
X
γ þ γ ¼ X þ Y ¼ kD þ kET þ kD þ k
ꢀET
1
2
S8). It is noteworthy that kꢀET was significantly smaller than kET
.
The rate constants kDC and kDX are the deactivation constants,
The difference between kET and kꢀET is allowed to explain the
fact that 1PUA gives no detectable emission for a complex in the
steady state fluorescence spectrum, as shown in the inset of
Figure 2a. Consequently, the kET value of 1PUA is 2 orders of
magnitude greater than that of 2PUA. A plausible explanation is
that this reflects faster charge delocalization in 1PUA, arising
from significant electronic coupling between the anthracene and
urea moieties, which is consistent with the absorption spectrum
and DFT calculations, as discussed in the previous section.
1
including radiative and nonradiative decay constants, for
-
(2PUA Ac)* and 1X2*, respectively. An iterative fitting method
3 3 3
was applied to a deconvoluted decay curve to determine four
unknown rate constants, which are correlated with eq 7, in the
presence of 5 and 10 mM TBAAc, according to a previously
reported calculation method using IGOR Pro 4.08J.37 The
resulting rate constants were independent of both the concen-
tration of TBAAc and the monitor wavelength (Table 3; see also
8232
dx.doi.org/10.1021/jp2022693 |J. Phys. Chem. A 2011, 115, 8227–8233

The Journal of Physical Chemistry A
ARTICLE
However, the origin of the 2-order-of-magnitude difference in
the kET values of 1PUA and 2PUA is still unknown.
In contrast, since kD and kD are independent of the sub-
stituted position of the anthracene moiety, it was concluded that
the relaxation kinetics (except for electron transfer) are deter-
mined predominantly by the electronic properties inherent in
anthracene in the excited singlet state.
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Bull. Chem. Soc. Jpn. 2007, 80, 747–751.
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C
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’ CONCLUSIONS
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The effect of substitution position on excited-state intermo-
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considerable effect on the proton-transfer rate constant. It was
revealed that the charge density of the substituted position of the
anthracene moiety determines the rate constant of the formation
of new emissive species. However, the rate constant for the
generation of new emissive species was found to be 2 orders of
magnitude greater for 1PUA than for 2PUA, which was not
expected based on the difference in charge density at the
respective substituted positions.
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Supporting Information. 1H and 13C NMR spectra,
S
b
time-resolved spectra, DFT calculation results, and estimation
of new emissive spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
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London, 1970; p 301.
Corresponding Author
*Tel.: +81-29-853-4315. Fax: +81-29-853-6503. E-mail: arai@
chem.tsukuba.ac.jp.
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This work was supported by a Grant-in-Aid for Scientific
Research in a Priority Area “New Frontiers in Photochromism”
(No. 471) from the Ministry of Education, Culture, Sports,
Science, and Technology (MEXT), Japan.
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