Control of the intermolecular photodimerization of anthracene derivatives by hydrogen bonding of urea groups in dilute solution

Article abstract of DOI:10.1039/c6pp00125d

The photodimerization reaction of anthracene derivatives was performed by capitalizing on intermolecular hydrogen bonds. Anthracene derivatives that can control the dimerization reaction depending on the substitution site were designed by using two anthryl moieties and one urea group, referred to as N,N′-dianthracen-n-ylurea, nDAU (n = 1, 2 and 9), which are symmetrically substituted by 1-anthryl, 2-anthryl and 9-anthryl groups, respectively. We investigated the excimer emission and photodimerization reaction of these anthracene-urea derivatives using absorption, emission, and ¹H NMR spectroscopy along with fluorescence decay measurements. All derivatives showed a concentration dependence of their fluorescence spectra and multiple fluorescence lifetime components even at 10^-6 M. Significantly, 9DAU resulted in an intermolecular photodimerization reaction. These differences in photoreactivity of nDAU may depend on variations in the overlap of the intermolecularly associated anthracene rings of nDAU by hydrogen bonding between intermolecular urea moieties. Furthermore, the dimerization quantum yield of 9DAU was reduced by the addition of tetrabutylammonium acetate (TBAAc). Consequently, we revealed that the substitution site and the addition of TBAAc affected the dimerization reaction of anthracene-urea derivatives.

Full text of DOI:10.1039/c6pp00125d

Photochemical &
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Control of the intermolecular photodimerization
of anthracene derivatives by hydrogen bonding of
urea groups in dilute solution†
Cite this: Photochem. Photobiol. Sci.,
2016, 15, 1071
Hisato Matsumoto, Yoshinobu Nishimura and Tatsuo Arai*
The photodimerization reaction of anthracene derivatives was performed by capitalizing on inter-
molecular hydrogen bonds. Anthracene derivatives that can control the dimerization reaction depending
on the substitution site were designed by using two anthryl moieties and one urea group, referred to as
N,N’-dianthracen-n-ylurea, nDAU (n = 1, 2 and 9), which are symmetrically substituted by 1-anthryl,
2-anthryl and 9-anthryl groups, respectively. We investigated the excimer emission and photodimeriza-
tion reaction of these anthracene–urea derivatives using absorption, emission, and 1H NMR spectroscopy
along with fluorescence decay measurements. All derivatives showed a concentration dependence of
their fluorescence spectra and multiple fluorescence lifetime components even at 10−6 M. Significantly,
9DAU resulted in an intermolecular photodimerization reaction. These differences in photoreactivity of
nDAU may depend on variations in the overlap of the intermolecularly associated anthracene rings of
nDAU by hydrogen bonding between intermolecular urea moieties. Furthermore, the dimerization
quantum yield of 9DAU was reduced by the addition of tetrabutylammonium acetate (TBAAc). Con-
sequently, we revealed that the substitution site and the addition of TBAAc affected the dimerization
reaction of anthracene–urea derivatives.
Received 27th April 2016,
Accepted 12th July 2016
DOI: 10.1039/c6pp00125d
www.rsc.org/pps
restricted circumstances. An excimer is defined as a dimer that
is stabilized in the excited state and dissociated in the ground
Introduction
Control of photochemical reactions by reactive conditions state. Since an excimer is more stable than the monomer in
would be an important advancement with respect to various the excited state and deactivates to the dissociative potential
photoactive organic materials.1–3 For example, anthracene is surface in the ground state, excimer emission appears in a
well known to undergo an intermolecular [4π + 4π] photo- longer wavelength region than the monomer emission.29
cycloaddition reaction upon irradiation at around 350 nm,4–10 At the low temperature of 77 K, a concentrated solution of
and the generated photoproduct is reversibly photodissociated anthracene exhibits excimer fluorescence.4,7 It is worthwhile to
by irradiation at less than 300 nm.11 This reversible reaction achieve photodimerization and excimer emission under
has been widely studied for photochemical cross-linking of ambient conditions, especially at low concentrations. With
DNA,12 in polymers containing anthracene moieties,13 and as regard to the photochemical and photophysical properties of
photoresistive materials14–16 and recording materials.17
excimers, several studies have been reported in restricted
Furthermore, anthracene derivatives play an important role environments such as cyclodextrin,30 phospholipids,26 and
in the design of a wide range of applications as fluorescent duplexes of DNA.31 We have also investigated photodimeriza-
substances, e.g., laser dyes,18 organic light emitting tion with molecular assemblies.32,33
displays,19–22 fluorescence sensors,23–26 and solid-state emis-
Much attention has been focused on the hydrogen bonding
sion.27,28 One of the characteristic emissions of anthracene is interactions in urea derivatives.25,34–40 We previously studied
its excimer fluorescence emission under certain specific con- the photochemistry of urea derivatives bearing pyrene fluoro-
ditions, e.g., at extremely low temperature or in spatially phores.41 While the urea derivative 1-pyren-1-yl-3-phenylurea,
containing one pyrenyl group, shows only monomer emission,
the urea derivative N,N′-1-dipyrenylurea, containing two
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1
pyrenyl groups, forms an excimer due to the intermolecular
Tennoudai, Tsukuba, Ibaraki 305-8571, Japan. E-mail: arai@chem.tsukuba.ac.jp;
interaction of the pyrenyl groups. We revealed that N,N′-1-
Fax: +81-29-853-6503; Tel: +81-29-853-4315
dipyrenylurea forms an excimer via an intermolecular hydro-
†Electronic supplementary information (ESI) available. See DOI:
gen bonding interaction through the urea moiety at extremely
10.1039/c6pp00125d
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2DAU. Into a 100 mL two-necked round-bottom flask were
added 2-anthracene carboxylic acid (185 mg, 0.832 mmol),
toluene (20 mL), triethylamine (250 µL, 1.80 mmol), and
diphenylphosphoryl azide (450 µL, 1.8 mmol). After the
mixture had been stirred for 30 min at 80 °C, 2-aminoanthra-
cene (145 mg, 0.751 mmol) in THF (10 mL) was added, and
the mixture was refluxed for 17 hours. The solvent was
removed by suction filtration and the residue was washed with
methanol several times. The product was a light yellow solid
(285 mg, 92%). 1H NMR (DMSO-d6, 400 MHz): δ 7.45–7.56 (6H,
m), 8.03–8.08 (6H, m), 8.36 (2H, s), 8.44 (2H, s), 8.51 (2H, s),
9.11 (2H, s). Anal. calcd for C29H20N2O: C, 84.44; H, 4.89;
N, 6.79; O, 3.88; found: C, 84.62; H, 5.01; N 6.76; O, 3.61.
9DAU. Into a 100 mL two-necked round-bottom flask were
added 9-anthracene carboxylic acid (555 mg, 2.50 mmol), THF
(25 mL), triethylamine (250 µL, 1.80 mmol), and diphenylphos-
phoryl azide (300 µL, 1.2 mmol). The mixture was refluxed for
15 hours. The solvent was removed by suction filtration and
the residue was washed with methanol several times. The
product was a light yellow solid (206 mg, 40%). 1H NMR
(DMSO-d6, 400 MHz): δ 7.56 (4H, t, J = 6.96 Hz), 7.62–7.70 4H,
m), 8.13 (4H, d, J = 7.4 Hz), 8.40–8.44 (4H, m), 8.58 (2H, s).
Anal. calcd for C29H20N2O: C, 84.44; H, 4.89; N, 6.79; O, 3.88;
found: C, 84.32; H, 5.07; N 6.45; O, 4.16.
Scheme 1 Structures of symmetrical anthracene–urea derivatives.
low concentrations. Consequently, we found that this inter-
molecular interaction of the pyrenyl moiety is promoted by the
introduction of double pyrene rings to a urea group. In this
sense, the photodimerization reaction of anthracene through
intermolecular interactions between aromatic rings even at
quite low concentrations could be expected.
We synthesized three types of anthracene–urea derivatives,
N,N′-dianthracen-n-ylurea, nDAU (n = 1, 2 and 9), bearing the
n-anthryl group at both sides of the urea (Scheme 1), and
investigated excimer formation and photodimerization result-
ing from intermolecular hydrogen bonding between urea moi-
eties. In addition, we examined the differences in
photoreactivity such as photodimerization and excimer for-
mation relative to the substitution position on the anthracene
ring. By the irradiation at 365 nm, 9DAU showed significant
excimer emission and photodimerization compared with
1DAU and 2DAU, indicating remarkable dependence on the
substitution site of the anthracene moiety. Furthermore, we
succeeded in controlling the dimerization reaction by the
addition of tetrabutylammonium acetate (TBAAc) due to
the dissociation of 9DAU aggregates, indicating that inter-
molecular hydrogen bonding interactions are necessary for the
intermolecular dimerization reaction.
Method
1H NMR spectra were recorded on a JEOL EX-270 (270 MHz for
1
1H NMR) or a Bruker ARX-400 (400 MHz for H NMR) spectro-
meter using DMSO-d6 as a solvent and tetramethylsilane as an
internal standard. Absorption and fluorescence spectra were
recorded on a Shimadzu UV-1600 and a Hitachi F-4500 fluo-
rescence spectrometer, respectively. JASCO F-777 was used as
an irradiation source. 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 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
using a microchannel plate photomultiplier (Hamamatsu,
R3809U) equipped with a TCSPC computer board module
Experimental
Synthesis
1DAU. Into a 100 mL two-necked round-bottom flask were (Becker and Hickl, SPC630). The full width at half maximum
added 1-anthracene carboxylic acid (161 mg, 0.724 mmol), (FWHM) value of the instrument response function was
toluene (20 mL), triethylamine (200 µL, 1.44 mmol), and 51 ps.42 The values of χ2 and the Durbin–Watson parameters
diphenylphosphoryl azide (300 µL, 1.2 mmol). After the were used to determine the quality of the fit obtained by non-
mixture had been stirred for 30 min at 80 °C, 1-aminoanthra- linear regression.43 DMSO (spectroscopic grade, Wako Pure
cene (160 mg, 0.745 mmol) in THF (10 mL) was added, and Chemical Industries, Japan) was used as a solvent without
the mixture was refluxed for 15 hours. The solvent was further purification. Acetate anions were in the form of tetra-
removed by suction filtration and the residue was washed with butylammonium acetate (TBAAc), which contains a tetrabutyl-
methanol several times. The product was a light yellow solid ammonium cation (Sigma-Aldrich, Japan). All measurements
(260 mg, 87%). 1H NMR (DMSO-d6, 400 MHz): δ 7.51–7.60 (6H, were carried out at room temperature under an Ar atmosphere.
m), 7.86 (2H, d, J = 8.08 Hz), 8.11 (2H, d, J = 8.08 Hz), 8.14 (6H, The concentrations were adjusted so that the absorption
m), 8.22 (2H, d, J = 9.36 Hz), 8.65 (2H, s), 8.92 (2H, s), 9.47 (2H, maximum of the excitation wavelength was about 0.1 for each
s). Anal. calcd for C29H20N2O: C, 84.44; H, 4.89; N, 6.79; sample. DFT calculations were performed using the Spartan
O, 3.88; found: C, 84.15; H, 5.05; N 6.64; O, 4.16.
’04 program (Wavefunction, Inc., Irvine, CA).
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Determination of the dimerization quantum yield
Fig. S2† illustrates the concentration dependence of the
absorption spectra of 1DAU, 2DAU and 9DAU. Although the
absorption spectra of 1DAU and 2DAU did not change with the
concentration, that of 9DAU increased in absorbance ranging
from 290 nm to 380 nm with increasing the concentration.
This indicates that 9DAU molecules interact with each other in
the ground state depending on the concentration, in contrast
to 1DAU and 2DAU.
The light quantity, I, was determined to be I = 4.70 × 10−7
mol min−1 according to the literature.44 Using the light quan-
tity, the photodimerization quantum yield was calculated
using eqn (1):
AV
Φdim
¼
ð1Þ
Ið1 ꢀ 10ꢀA Þε
365
where A is the slope of the change in absorbance against time,
V is the volume of the photoirradiated sample (3.0 × 10−3 L),
ε is the molar extinction coefficient of the derivative and A365
is the absorbance at 365 nm.
Fluorescence spectra
Fluorescence spectra of nDAU and nPUA were recorded at
various concentrations ranging from 1.0 × 10−7 to 5.5 ×
10−6 M. The fluorescence spectrum of 1DAU at 3.3 × 10−7
M
showed a broad profile having a maximum at 445 nm along
with a shoulder at 415 nm, in contrast to its absorption spec-
trum with vibronic bands (Fig. 2), indicating that the emissive
Results and discussion
Absorption spectra
To investigate the electronic interaction of anthryl moieties of
nDAU via the urea group in the ground state, the absorption
spectra were recorded (Fig. 1). The spectral profiles of nDAU
were very similar to those of previously reported urea–anthryl
derivatives, nPUA, having a single anthryl moiety. The nDAU
derivatives showed about 2-fold higher molar extinction coeffi-
cients compared with the corresponding nPUA as shown in
Scheme S1 and Fig. S1† and listed in Table 1.25,38 These
results indicate that the electronic interaction between anthryl
moieties through the urea unit is relatively weak in the ground
state.
Fig. 1 Absorption spectra of 1DAU, 2DAU and 9DAU in DMSO under Ar.
Table 1 Photochemical properties of nDAU and nPUA
λAbs/nm
ε/104 M−1 cm−1
λFl/nm
1DAU
2DAU
9DAU
1PUA
2PUA
9PUA
366, 388, 405
340, 358, 384, 403
353, 368, 392
384
340, 359, 384, 403
353, 370, 389
1.0, 1.5, 1.3
450
450
426
462
447
435
1.4, 1.0, 0.99, 0.76
0.88, 1.4, 1.5
0.79
0.46, 0.43, 0.43, 0.35
0.46, 0.69, 0.65
Fig. 2 Concentration dependence of fluorescence spectra: (a) 1DAU,
(b) 2DAU and (c) 9DAU in DMSO under Ar.
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state of 1DAU is electronically delocalized, while the con-
centration dependence was not observed in the spectra of
nPUA (Fig. S3†). The fluorescence spectrum was red-shifted to
455 nm at 3.4 × 10−6 M and showed an increase in fluo-
rescence intensity around 500 nm relative to the peak intensity
following the disappearance of the shoulder. A similar concen-
tration dependence was observed in the spectra of 9DAU
accompanying a negligible peak shift and a relatively slight
increase of fluorescence intensity around 550 nm. In contrast
to 1DAU and 9DAU, the fluorescence spectrum of 2DAU
showed no increase in the 550 nm region with a change in the
concentration. Intermolecular interactions in the excited state
are the main cause of the concentration dependence of the
fluorescence spectra. As previously reported, 1DAU molecules
bearing a pyrenyl group at both sides of the urea moiety
are allowed to interact with each other by hydrogen bonding
interactions with a urea unit.41 The same situation may apply
to nDAU.
Differences in the photoreaction depending on the
substitution site of the anthracene ring
Photoirradiation experiments of nDAU were performed to
investigate their photoreactive properties. Fig. 3 shows the
changes in the absorption spectra of nDAU upon photoirradia-
tion. The irradiation wavelength was set at 365 nm for all
derivatives. The spectra of 1DAU and 9DAU showed a decrease
in the absorbance under irradiation. The absorbance of 1DAU
decreased from 350 nm to 425 nm, showing isosbestic points
at 350 nm and 425 nm, while an increment of absorbance was
observed at shorter and longer wavelength regions, indicating
that the photoproduct has a different electronic structure than
the parent 1DAU. In contrast to 1DAU, the absorption spec-
trum of 9DAU almost disappeared after 250 min of irradiation.
Although isosbestic points were confirmed at 290 nm and
425 nm, photoproducts seem to be produced through a
different pathway from 1DAU. On the other hand, 2DAU gave
no remarkable spectral changes in the course of photoirradia-
tion. It is noteworthy that nPUA, which have only a single
anthracene moiety, exhibited fewer changes upon photo-
irradiation than nDAU (Fig. S4†). This suggests that inter-
Fig. 3 Changes in absorption spectra of (a) 1DAU, (b) 2DAU and
(c) 9DAU with irradiation at 365 nm in DMSO under Ar.
molecular interactions may be involved in the photoreaction. intermolecular dimer C58H40NaO2 was m/z 847.30 for
With respect to 1DAU and 9DAU, we investigated the changes [M + Na]+, the calculated and the measured values were con-
in fluorescence spectra by photoirradiation. The fluorescence sistent with each other. To elucidate the photodimerization
spectrum of 1DAU having a maximum at 445 nm gradually mechanism of 9DAU, the absorption spectra of 9DAU were
red-shifted and this was accompanied by a new fluorescence recorded, as shown in Fig. S5b.† The absorbance of 9DAU
band with a 498 nm maximum along with a shoulder at gradually decreased with irradiation time, with isosbestic
520 nm (Fig. S5a†). Although the isoemissive point was less points at 290 nm and 425 nm. Similar phenomena were
remarkable than the isosbestic point, the fluorescence obtained for 1DAU. In order to identify the structure of 9DAU
changes of 1DAU were consistent with the absorption spec- after photoirradiation, we recorded the 1H NMR spectrum.
trum. The photoirradiation of 9DAU yielded an increase in the After 150 minutes of photoirradiation, the anthracene proton
intensity of the fluorescence spectrum along with the disap- signals vanished completely, and new signals appeared
pearance of the long tail, which may be ascribed to an inter- (Fig. S6†). These peaks were assigned as follows. Signals at
molecular excimer emission analogous to 1DAU.41 ESI-MS 7.65 ppm, 7.14–6.85 ppm and 4.72 ppm were attributed to the
measurements were performed to examine the photoproduct N–H of urea groups, C–H of the benzene ring and C–H of the
after photoirradiation and a new peak was observed at m/z cross-linking site of the methyl protons, respectively. These
845.25 for 9DAU. Since the calculated ESI-MS value of the symmetric peaks and the observation of the intermolecular
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dimer in the ESI-MS revealed the structure of the photoreac- where A0 and A are the absorbances of the host monitored at
tion product as shown in Fig. S6.† The photodimerization 370 nm in the absence and presence of TBAAc, respectively,
quantum yield of 9DAU was determined to be 0.0093. Except [A] is the total TBAAc concentration, and Amax is the absor-
for absorption and fluorescence spectra, there is little infor- bance of 9DAU under full association with TBAAc. Using the
mation on photoproducts of 1DAU due to a lack of successful association constant of 9DAU, we examined the dimerization
1
analyses of H NMR and ESI-MS spectra. However, it is note- reaction in the presence of 0, 1.37 × 10−3 and 1.37 × 10−2
M
worthy that the photoreaction of 1DAU yields unknown pro- TBAAc. The ratio of free 9DAU was derived from the following
ducts (Fig. S7†) in contrast to the remarkable photostability of equations:
2DAU (Fig. 3b). There have been previously reported photo-
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
a ꢀ a2 ꢀ 4b
½9DAU–AcOꢀꢁ ¼
dimerization reactions that depend on hydrogen bonding, e.g.,
thymine–adenine interaction,46 dimerization of trans-cinnama-
ð3Þ
2
mide,47 transformation of one coordination polymer to
another,48 alkene photodimerization in the solid state,49 and
[2 + 2] photodimerization in the solid state.50 However, to the
best of our knowledge, this is the first example of an anthra-
cene photodimerization reaction under low concentration that
depends on urea hydrogen bonding.
a ¼ ½TBAAcꢁ þ ½9DAUꢁ0 þ 1=Ka; b ¼ ½TBAAcꢁ½9DAUꢁ0
½free 9DAUꢁ=½9DAUꢁ0 ¼ ꢀ½9DAUꢁ ꢀ ½9DAU–AcOꢀꢁ=½9DAUꢁ ð4Þ
where [9DAU–AcO−], [TBAAc], [9DAU]0 and [free 9DAU] are the
concentrations of the complex of 9DAU with TBAAc, total
TBAAc, initial 9DAU, and 9DAU without interaction with
TBAAc, respectively. From eqn (4), the ratios of free 9DAU were
calculated to be 1, 0.51 and 0.09, and dimerization yields were
determined to be 9.32 × 10−3, 5.04 × 10−3 and 1.52 × 10−3 for 0,
1.37 × 10−3 and 1.37 × 10−2 M TBAAc, respectively (Fig. 3c, 4a
and b). Plotting the dimerization yield against the concen-
The origin of photodimerization in the ground state examined
by the addition of acetate
Among the three urea derivatives, 9DAU showed the most sig-
nificant photodimerization reaction. nDAU derivatives form
intermolecular aggregates by hydrogen bonding between inter-
molecular N–H and CvO in the ground state,51 and these
aggregates may promote photodimerization due to the close
proximity of the anthracene rings. Thus, it is anticipated that
dissociation of the aggregates by hindering intermolecular
hydrogen bonding inhibits the photodimerization reaction.
We attempted to inhibit intermolecular hydrogen bonding
between intermolecular anthryl moieties of 9DAUs by the
addition of tetrabutylammonium acetate (TBAAc) to cleave
intermolecular hydrogen bonds between urea groups. In fact,
inhibition of intermolecular hydrogen bonding in the presence
of TBAAc was revealed by research in the disappearance of
excimer emission in the case of N,N′-1-dipyrenylurea.41
tration of free 9DAU gave
a good linear relationship
(Fig. S10†), indicating that the dimerization yield was directly
related to the concentration of 9DAU associated with no
acetate anion. This implies that the dimerization reaction of
To examine the interaction between 9DAU and TBAAc,
1
absorption and H NMR spectra were recorded. Fig. S8† shows
the absorption spectra of 9DAU in DMSO with various concen-
trations of TBAAc. In the presence of TBAAc, the spectral
shape at the red-edge region (wavelengths longer than 350 nm)
was broad compared to that in the absence of TBAAc due to
the interaction with 9DAU.
To confirm the complex formation of 9DAU with TBAAc,
1H NMR spectra were recorded in the absence and presence of
TBAAc. This revealed the existence of hydrogen bonding
between 9DAU and TBAAc through the urea moiety, since
downfield shifts of the urea N–H protons in 9DAU from
9.01 ppm to 12.39 ppm were observed in the presence of
5 equiv. TBAAc (Fig. S9†). The association constant of 9DAU
with TBAAc was determined according to eqn (2) to be
7.7 × 102 M−1
:
ꢀ
ꢁ
Fig. 4 Changes in absorption spectra of 9DAU in the presence of (a)
1.37 × 10−2 M TBAAc and (b) 1.37 × 10−3 M TBAAc with irradiation at
365 nm in DMSO under Ar.
1
1
1
¼
þ 1
ð2Þ
A ꢀ A0 Amax ꢀ A0 Ka½Aꢁ
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9DAU proceeds not from the 9DAU⋯TBAAc complex in the 9DAU had a triexponential decay (Fig. S11†). The observed life-
presence of TBAAc, but from the aggregates between 9DAUs times of 14.3, 16.0 and 2.12 ns at 560 nm for 1DAU, 2DAU and
without interaction with TBAAc (Scheme 2). Aggregate for- 9DAU, respectively, seemed to agree with those of nPUA, imply-
mation in the ground state by intermolecular hydrogen ing that these are attributed to monomer emission.25,38 In
bonding interactions promotes the photodimerization reaction general, two types of anthracene excimers are known: a 55°
even in dilute solution.
dimer and a sandwich dimer.5,52,53 The 55° dimer is defined
as having the long molecular axes of two anthracene mono-
mers nearly parallel, while the short molecular axes are at an
angle of about 55°. Since this dimer is relatively unstable, the
lifetime of the excimer emission becomes shorter than that of
the monomer. On the other hand, the sandwich dimer has two
anthracene moieties arranged by parallel stacking. This
excimer emits a red-shifted wavelength region and has a
longer lifetime than the stable excimer.52–54 Thus, the life-
times of 0.65 and 2.24 ns observed at 560 nm for 1DAU and
2DAU, respectively, are attributed to the 55° dimer. With
respect to 9DAU, the fluorescence lifetime of 0.13 ns is
ascribed to the 55° dimer and 10.8 ns is ascribed to the sand-
wich dimer. All DAU derivatives emit both monomer and
excimer emissions. Since a significant concentration depen-
dence of the fluorescence spectrum was observed, this excimer
originates from intermolecular stacking of anthryl moieties.
The above attribution is also supported by a pyrenyl–urea
derivative that emits intermolecular excimer emission.41
Accordingly, anthracene–urea derivatives promote inter-
molecular interactions followed by excimer emission in the
excited state in dilute solution. Since the anthracene molecules
must be in close proximity to generate excimers, a relatively
concentrated anthracene solution is required for inter-
molecular excimer formation, unless restricted environments
such as cyclodextrin and micelles are used.45,54 Similar to our
previous work, we found that nDAU can generate hydrogen
bonding assemblies even at extremely low concentrations, by
introducing anthracene instead of pyrene.41
Excimer formation examined by fluorescence lifetime analysis
Only 9DAU underwent the photodimerization reaction in con-
trast to 1DAU and 2DAU. These differences in photoreactivity
of nDAU may result from the excimer formation since the
photodimer is related to the excimer intermediate in anthra-
cene derivatives according to the fluorescence measurements.
Usually, the attribution of excimer fluorescence should be per-
formed on the basis of the fluorescence spectra. However,
since fluorescence spectral separation was difficult in the
present case due to a lack of characteristic spectra of various
excimers, we measured fluorescence decay curves to examine
fluorescence lifetimes of nDAU, as listed in Table 2. Those of
1DAU and 2DAU consisted of a biexponential decay and that of
DFT calculations were performed to investigate the stacking
conformation of two nDAU molecules by intermolecular hydro-
gen bonding through the urea groups. Since intermolecular
anthracene rings of 9DAU are overlapped in a face-to-face
fashion, 9DAU is allowed to form the sandwich dimer, which
may undergo photodimerization (Fig. S12e and f†).29 On the
other hand, although 1DAU and 2DAU may form the 55°
dimer, it seems that no dimerization reaction proceeds due to
the dihedral angle of 55° between anthracene rings
(Fig. S12a–d†).
Consequently, we revealed that the intermolecular hydrogen
bonding interactions with a urea unit in the ground state play
a significant role in the dimerization reaction of nDAU in the
excited state resulting from the dependence of the substitution
site of an anthracene ring as well as the inhibition of the
photodimerization by the addition of TBAAc.
Scheme 2 Photodimerization reaction of 9DAU.
Table 2 Fluorescence lifetimes of nDAU
Sample
λ/nm
τs/ns
1DAU
450
560
450
560
450
560
13.7 (0.07), 0.62 (0.93)
14.3 (0.07), 0.65 (0.93)
14.2 (0.01), 2.19 (0.99)
16.0 (0.01), 2.24 (0.99)
9.62 (0.01), 1.78 (0.05), 0.34 (0.94)
10.8 (0.04), 2.12 (0.16), 0.13 (0.80)
2DAU
9DAU
Conclusions
We synthesized three types of symmetric anthracene–urea
derivatives, 1DAU, 2DAU and 9DAU, to investigate the differ-
The values in parentheses are the normalized amplitudes for the
respective lifetimes.
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ences in photoreactivity. In dilute solution, all derivatives
formed aggregates by hydrogen bonding between inter-
molecular urea moieties in the ground state. These aggregates
were confirmed by the addition of TBAAc. The photodimeriza-
tion reaction occurred in the case of 9DAU photoirradiation at
Emission from Anthracene Included in Nax Zeolite, Chem.
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365 nm, but no photoreaction of 2DAU was observed. This 10 R. O. Al-Kaysi, A. M. Mueller and C. J. Bardeen, Photo-
observation indicates that the substitution position plays an
important role in the photodimerization reaction of anthra-
chemically Driven Shape Changes of Crystalline Organic
Nanorods, J. Am. Chem. Soc., 2006, 128, 15938.
cene. nDAU exhibited multi-component fluorescence lifetimes. 11 H. D. Becker, Unimolecular Photochemistry of Anthra-
The excimer fluorescence was assigned to the intermolecular cenes, Chem. Rev., 1993, 93, 145.
excimer due to intermolecular stacking of anthracene rings 12 M. Mukae, T. Ihara, M. Tabara and A. Jyo, Anthracene-DNA
through hydrogen bonding. Furthermore, under photoirradia-
tion with 365 nm light, photodimerization was confirmed only
in the case of 9DAU. DFT calculations revealed that the most
Conjugates as Building Blocks of Designed DNA Structures
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important factor is the orientation of the anthracene rings 13 J. T. Goldbach, T. P. Russell and J. Penelle, Synthesis and
induced by the intermolecular hydrogen bonding between
urea derivatives. Consequently, we successfully controlled the
photodimerization of anthracene rings by the substitution
position of the urea moiety as well as by intermolecular hydro-
Thin Film Characterization of Poly(Styrene-Block-
Methyl Methacrylate) Containing an Anthracene Dimer
Photocleavable Junction Point, Macromolecules, 2002, 35,
4271.
gen bonding interactions through the CvO and N–H of the 14 S. Paul, O. Halle, H. Einsiedel, B. Menges, K. Mullen,
urea moiety, even though anthracene rings have a repulsive
potential surface in the ground state. This may provide new
insight into the molecular design of desirable photoreactive
W. Knoll and S. MittlerNeher, An Anthracene-Containing
PMMA Derivative for Photoresist and Channel Waveguide
Applications, Thin Solid Films, 1996, 288, 150.
molecules. While 1DAU and 9DAU are photoreactive, 15 K. H. Chae, Y. W. Kim and T. H. Kim, Application of an
2DAU has excellent photostability. Therefore, changing the
Anthracene Containing Polymer to a Negative Type Photo-
substitution site of the anthracene derivatives can remarkably
resist, Bull. Korean Chem. Soc., 2002, 23, 1351.
modify the properties of anthracene. 9DAU undergoes the 16 H. J. Park, H. Jung, S. H. Kim, M. Park, J. H. Kim, J. Son,
intermolecular photodimerization reaction even in dilute solu-
tion. Furthermore, its applications can be extended by using
intermolecular cross-linking based on the inhibition of the
dimerization reaction of 9DAU by the addition of TBAAc.
B. J. Jung, D. H. Hwang, C. Lee, J. K. Lee, J. G. Yoon and
S. Y. Yoon, Fluorous Solvent-Soluble Imaging Materials
Containing Anthracene Moieties, J. Polym. Sci., Part A:
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17 H. Kihara and M. Yoshida, Reversible Bulk-Phase Change
of Anthroyl Compounds for Photopatterning Based on
Photodimerization in the Molten State and Thermal Back
Reaction, ACS Appl. Mater. Interfaces, 2013, 5, 2650.
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