Observation of anthracene excimer fluorescence at very low concentrations utilizing dendritic structures

Article abstract of DOI:10.1016/j.jphotochem.2011.07.008

We prepared water-soluble anthracene-cored poly(aryl ether) dendrimers wG1 and wG2, together with lipophilic dendrimers G1 and G2, and examined their photochemical properties. Water-soluble dendrimers wG1 and wG2 produced excimer emissions peaking at 463

Full text of DOI:10.1016/j.jphotochem.2011.07.008

Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 1–5
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Observation of anthracene excimer fluorescence at very low concentrations
utilizing dendritic structures
Ryosuke Akatsukaa, Atsuya Momotakea, Yoshihiro Shinoharab, Yoko Kannac, Tomoo Satoa,
Masaya Moriyamad, Kayori Takahashie, Yoshinobu Nishimuraa, Tatsuo Araia,∗
a Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
b Research Facility Center for Science and Technology, University of Tsukuba, Japan
c Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan
d Department of Applied Chemistry, Faculty of Engineering, Oita University, Oita 870-1192, Japan
e National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba City, Ibaraki 305-8565, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We prepared water-soluble anthracene-cored poly(aryl ether) dendrimers wG1 and wG2, together with
lipophilic dendrimers G1 and G2, and examined their photochemical properties. Water-soluble den-
drimers wG1 and wG2 produced excimer emissions peaking at 463 nm and 447 nm, respectively, in
water at a concentration of 10−5 M, indicating that these dendrimers could form aggregates under highly
diluted conditions. The results presented here are the first clear report of the observations of the excimer
emission of anthracene in very low fluid solutions in which appropriate substituents were introduced.
© 2011 Published by Elsevier B.V.
Received 18 April 2011
Received in revised form 13 June 2011
Accepted 15 July 2011
Available online 30 July 2011
Keywords:
Anthracene
Dendrimers
Self-aggregations
Excimer emission
1. Introduction
While pyrene and naphthalene have singlet lifetimes as long
as 400 ns and 100 ns, respectively, in non-polar solvents and can
Dendrimers are supramolecules possessing well-defined,
repeating units and a central core [1–4]. When the cone-shaped
dendrimers [5–7], having hydrophilic surfaces with hydrophobic
cores and branching units, are dissolved in water, self-aggregation
to form a micelle-like structures occurs. In this case, the generation
effect of the dendrimers on the aggregate formation is of interest.
We have already prepared cone-shaped, water-soluble dendrimers
of polyaromatic hydrocarbons such as pyrene that showed excimer
emission due to the formation of aggregates of the core even at low
concentrations of approximately 10−5 M [8]. Thus, water-soluble
fluorescent molecules may give aggregates with characteristic
fluorescences at longer wavelengths in contrast to the monomer
emissions. In the case of naphthalene, researchers have observed
excimer fluorescence in the solid state [9], polymers [10,11],
photodimers [12], and dendrimers having several naphthalene
chromophores at the periphery [13]; however the excimer fluo-
rescence by intermolecular interaction in a very dilute solution
could be observed only by use of cone-shaped dendrimer [14].
easily form dimers or intermolecular aggregated forms within the
lifetime of their singlet excited states, anthracene does not easily
form intermolecular excimers because of the relatively short life-
time (4 ns) of its excited singlet state. Therefore, we attempted to
observe the anthracene excimer at very low concentrations using
cone-shaped dendrimers (Fig. 1).
Although anthracene-cored dendrimers have been prepared
[15–17] where photochemical intermolecular dimerization reac-
tion was observed [18,19], the anthracene excimer fluorescence
in dilute aqueous solutions has not been reported thus far. We
report a photochemical study on the generation and concentration-
dependent aggregation of anthracene dendrimers in aqueous
solution. The excimer fluorescence of anthracene was observed at
concentrations as low as 2.0 × 10−5 M.
2. Experimental
2.1. Materials
2.1.1. 9-(3ꢀ,5ꢀ-Dihydroxyphenoxymethyl)anthracene (1)
A solution of 9-(bromomethyl)anthracene (1.00 g, 3.72 mmol)
in 50 mL of DMF was added dropwise to a mixture of phlorogluci-
nol (900 mg, 7.13 mmol) and K2CO3 (1.49 g, 10.7 mmol) in 10 mL
∗
Corresponding author. Tel.: +81 298 853 4315; fax: +81 298 853 6503.
E-mail address: arai@chem.tsukuba.ac.jp (T. Arai).
1010-6030/$ – see front matter © 2011 Published by Elsevier B.V.
doi:10.1016/j.jphotochem.2011.07.008

2
R. Akatsuka et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 1–5
Fig. 1. Chemical structures of anthracene dendrimers.
of DMF, and the mixture was stirred for 6 h at room tempera-
ture under N2. After evaporation to remove the solvent, water
was added, and the resulting precipitate, including the desired
product, was filtered and purified via silica gel column chromatog-
raphy (dichloromethane/ether = 9/1) to give 1 as a pale yellow solid
(177 mg) in a 15% yield.
The solvent was evaporated after the resulting salts were filtered.
The residue was purified via silica gel column chromatography
(dichloromethane/ether = 9/1) followed by GPC (chloroform) to
give G1 as a white solid (118 mg) in a 39% yield.
1H NMR (270 MHz, DMSO-d6) ı 8.49 (1H, s), 8.23 (2H, d,
J = 8.4 Hz), 7.99–8.04 (6H, m), 7.44–7.54 (8H, m), 6.41 (3H, d,
J = 2.0 Hz), 6.28 (1H, t, J = 2.0 Hz), 5.88 (2H, s), 5.04 (4H, s), 3.90
(6H, s); 13C NMR (67.5 MHz, DMSO-d6) ı 166.8, 161.2, 160.5, 142.0,
131.5, 131.1, 130.1, 130.0, 129.2, 129.2, 127.0, 126.7, 126.6, 125.1,
124.0, 95.2, 95.1, 69.6, 63.0, 52.3;MALDI–TOF–MS (m/z) calcd. for
1H NMR (270 MHz, acetone-d6) ı 8.65 (1H, s), 8.43 (2H, d,
J = 8.6 Hz), 8.13 (2H, d, J = 7.6 Hz), 7.50–7.63 (4H, m), 6.20 (2H, d,
J = 2.1 Hz), 6.10 (1H, t, J = 2.1 Hz), 5.96 (2H, s).
C
39H32O7 [M + Na]+ = 635.65; found, 634.92.
2.1.2. G1 (Typical procedure)
G2 was obtained by the same procedure in a 43% yield (250 mg).
A solution of 1 (0.150 g, 0.473 mmol) in 10 mL of THF was
added to a mixture of 18-crown-6-ether (66 mg, 0.25 mmol),
K2CO3 (287 mg, 2.1 mmol), methyl p-hydroxybenzoate (254 mg,
1.10 mmol) in 10 mL of acetone, and was stirred at 60 ◦C for 18 h.
1H NMR (270 MHz, DMSO-d6) ı 8.52 (1H, s), 8.27 (2H, d,
J = 8.6 Hz), 7.98–8.04 (10H, m), 7.44–7.53 (12H, m), 6.66 (4H, d,
J = 2.16 Hz), 6.53 (2H, t, J = 2.16 Hz), 6.41 (2H, d, J = 1.9 Hz), 6.28
Fig. 2. (a) Steady state UV absorption spectra of G1 (thin line) and G2 (solid line) in THF. (b) Those of wG1 (thin line) and wG2 (solid line) in 0.1 M KOH aq. (c) Steady state
fluorescence spectra of G1 and G2 in THF and (d) those of wG1 and wG2 in 0.1 M KOH aqueous solution. The excitation wavelength was 350 nm for all the experiments.

R. Akatsuka et al. / Journal of Photochemistry and Photobiology A: Chemistry 223 (2011) 1–5
3
Fig. 3. Concentration dependence of the absorption spectra for (a) wG1 and (b) wG2, and that of the fluorescence spectra for (c) wG1 and (d) wG2 in 0.1 M KOH aq.
(1H, m), 5.92 (2H, m), 5.09 (8H, s), 4.95 (4H, s), 3.90 (12H, s);
MALDI–TOF–MS (m/z) calcd. for C71H60O15 [M + Na]+ = 1175.89;
found, 1175.87.
3. Results and discussion
3.1. Steady state absorption and fluorescence spectra
3.1.1. Generation dependence
Table 1 summarizes the photochemical properties of anthracene
dendrimers. All measurements were performed under argon at
room temperature. The spectral shapes and the values of the molar
extinction coefficients of G1 and G2 in THF were very similar to each
other (Fig. 2a), suggesting that the substituted dendron did not pro-
vide the “generation effect” either electronically or sterically. The
absorption spectra showed vibrational structures, and the peak of
the lowest absorption band appeared at 386 nm for all lipophilic
dendrimers. Fig. 2b shows the absorption spectra of the water-
soluble analogs wG1 and wG2 in a 0.1 M KOH aqueous solution.
In this case, the absorption band broadened and red shifted more
significantly in higher generation. In addition, the molar extinction
coefficients of the anthracene sites in wG1 and wG2 were less than
that of G1 and G2, probably due to the formation of aggregate even
at a concentration lower than 10−5 M.
The fluorescence spectra of lipophilic dendrimers G1 and G2 in
THF exhibited similar profiles and intensities, peaking around 396,
417, and 440 nm with the edges at 525 nm, which was assigned
to the anthracene monomer emission (Fig. 2c). The water-soluble
dendrimer wG1 exhibited a slightly higher intensity at longer
wavelengths, indicating the contribution of excimer-like emission
at longer wavelengths. This observation was more pronounced in
wG2 because we observed the broad fluorescence up to 600 nm,
accompanied by vibrational structures peaking at 402, 426, and
2.1.3. wG1 (Typical procedure)
A solution of G1 (135 mg, 0.223 mmol) in 14 mL of mixed sol-
vent (1 M KOH/THF/MeOH = 1/12/2) was stirred at 60 ◦C for 8 h. A
mixture was then acidified by the addition of 1 M HCl. The result-
ing precipitate was filtered and washed with distilled water to give
wG1 as a pale yellow solid (122 mg) in a 93% yield.
1H NMR (270 MHz, DMSO-d6) ı 8.71 (1H, s), 8.33 (2H, d,
J = 8.2 Hz), 8.15 (2H, d, J = 8.2 Hz), 7.95 (4H, d, J = 8.2 Hz), 7.63–7.53
(9H, m), 6.50 (2H, s), 6.38 (1H, s), 6.02 (2H, s), 5.17 (4H, s). 13C
NMR (67.5 MHz, DMSO-d6) ı 166.8, 160.5, 159.8, 141.8, 130.8,
130.3, 130.0, 129.3, 128.8, 128.4, 127.2, 127.0, 126.5, 125.1, 124.0,
94.9, 94.8, 68.8, 62.3; MALDI–TOF–MS (m/z) calcd. for C37H28O7
[M + Na]+ = 607.16; found, 607.33.
wG2 (89 mg) was obtained by the same procedure in a 78% yield.
1H NMR (270 MHz, DMSO-d6) ı 8.80 (2H, d, J = 8.6 Hz), 8.58 (1H,
s), 8.27–7.86 (12H, m), 7.49–7.44 (12H, m), 6.70 (4H, s), 6.61 (2H,
s), 6.42 (2H, s), 6.29 (1H, s), 5.98 (2H, s), 5.11 (8H, s), 4.97 (4H,
s); MALDI–TOF–MS (m/z) calcd. for C67H52O15 [M + Na]+ = 1096.33;
found, 1096.72.
2.2. Measurements
We measured the 1H NMR spectra with a JEOL EX-270 (270 MHz
for 1H NMR) or a Bruker ARX-400 (400 MHz for 1H NMR) spec-
trometer in a solution of DMSO-d6 with tetramethylsilane as an
internal standard. The UV absorption and fluorescence spectra
were recorded on a Shimadzu UV-1600 spectrophotometer and
a Hitachi F-4500 fluorescence spectrometer, respectively. Fluo-
rescence decay measurement was performed by time-correlated
single photon counting method reported previously [20]. The time
resolution is 20 ps.
Table 1
Absorption and fluorescence maxima and fluorescence quantum yield for
anthracene dendrimers.
ꢀabs/nm
ε/M−1 cm−1
ꢀFL/nm
˚
f
G1
G2
wG1
wG2
348, 366, 386
347, 365, 386
348, 366, 386
352, 373, 391
9600
10,000
5500
4100
396, 416, 439
395, 416, 439
398, 418, 440
402, 426, 446
0.16
0.11
0.067
0.097

4
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Table 2
Fluorescence lifetimes of wG1 and wG2 at various concentrations in 0.1 M KOH aq
with excitation at 375 nm.
Concentration/M
ꢁs (at 430 nm)/ns (population (%))
wG1
wG2
3.1 × 10−6
6.2 × 10−5
3.2 × 10−6
5.3 × 10−5
0.02 (84), 0.62 (14), 5.9 (2)
0.34 (20), 9.4 (80)
0.27 (70), 3.0 (26), 16.1 (4)
0.31 (64), 3.9 (28), 19.4 (8)
446 nm (Fig. 2d). We ascribed the broadened fluorescence band to
the anthracene excimer.
3.1.2. Concentration dependence
Fig. 3 shows the superposition of normalized spectra at varying
concentrations. Both the absorption and the fluorescence spectra of
wG1 in aqueous solution (Fig. 3a and c) dramatically changed with
increasing concentration. At low concentrations (3.1 × 10−6 M),
wG1 exhibited absorption and fluorescence bands similar to those
of the lipophilic dendrimer whereas the absorption spectra at
higher concentrations (10−4 M) was totally different (Fig. 3a). We
attributed these results to the anthracene aggregation in the ground
state. The emission band also broadened and red shifted at 10−4 M
(Fig. 3c), assigned to the anthracene excimer. Interestingly, the
concentration dependence of the higher generation wG2 was dif-
ferent than that of wG1. The normalized absorption spectrum at
10−4 M was not very different from that at 5.5 × 10−6 M (Fig. 3b)
because wG2 formed aggregates even at a lower concentration
(5.5 × 10−6 M) in which the anthracene unit partially existed as a
monomer within the aggregates and the phenomenon did not sig-
nificantly change by increasing the concentration. The fluorescence
spectrum of wG2 (Fig. 3d) at a low concentration (5.5 × 10−6 M)
was probably the sum of the emission from the excited monomer
and the excimer. At a high concentration (10−4 M), the fluorescence
spectrum slightly broadened and red shifted, but we still observed
weak vibrational structures at 405 and 427 nm, supporting our
deduction that the excited monomer and excimer existed within
the aggregates and that the population of the excimer increased
at higher concentrations. The peak wavelengths and the spectral
shapes of the anthracene excimer emission strongly depend on the
manner of overlapping of the anthracene ring as described below.
Table 2 lists the fluorescence lifetimes of wG1 and wG2 in
aqueous solution at different concentrations. We analyzed the life-
times at low concentrations via multi-component analysis, which
suggested a complicated aggregation structure and/or a partially
monodispersed wG1 and wG2. At higher concentrations, the longer
lifetime component with ꢁs = 9.4 ns, probably due to the excimer
emission produced by excitation of aggregated form, was domi-
nant for wG1 whereas the still fitted value of the multi-component
of wG2 supported a complicated formation of aggregate. The
shorter lifetime components of which the population decreased
with increasing dendrimer concentration can be assigned to the
monomer fluorescence. The lifetime of excimer emission varies in
different concentration or dendrimer generation because it may be
dependent of the excimer structure.
Fig. 4. Change in the fluorescence spectra upon the addition of KCl for (a) wG1
(2.0 × 10−5 M) and (b) wG2 (2.0 × 10−5 M) in 0.001 M KOH aqueous solution. (c)
Comparison of the spectra of wG1 and wG2 in the presence of 800 mM KCl. The
excitation wavelength was 350 nm for all the experiments.
3.1.3. Effective formation of aggregate via KCl addition
The addition of KCl could also induce aggregation of wG1 and
wG2 in water, because the masking a carboxylate anion at the
dendrimer periphery by added counterions (K+) would reduce
charge repulsion between dendrimers. In this case, we used 0.001 M
KOH solutions instead of the 0.1 M KOH solution to observe the
salt effect more clearly. Fig. 4a shows the change in the fluo-
rescence spectra of wG1 upon the addition of KCl wherein the
excimer fluorescence dramatically increased with increasing KCl
concentration. The fluorescence spectra of wG2 also changed in
a similar manner: a decrease of the monomer fluorescence at
402 nm and a simultaneous increase of the excimer fluorescence
at 448 nm were observed (see Fig. 4b). Interestingly, the excimer
fluorescence of wG1 (ꢀmax = 463 nm) and wG2 (ꢀmax = 447 nm) at
high KCl concentrations did not overlap (see Fig. 4c), indicating
that the excimer structure of wG2 was different from that of
wG1. Researchers have reported the structural dependence of the
anthracene excimer for a variety of anthracenophane and bisan-
thrylethane [21] and 1-(1-anthryl)-3-(9-anthryl)propane (1,9-DAP,
(ꢀmax = 520 nm)) and 1-(1-anthryl)-3-(2-anthryl)propane (1,2-
DAP, (ꢀmax = 470 nm)) [22]. According to the results, the anthracene
excimer can exist in a variety of geometries, and each one can
have a different stability, emitting fluorescence over a range of
460–620 nm. Waldeck et al. deduced that the overlap of one of the
phenyl rings in anthracene resulted in the excimer emission peak-
ing around 460 nm, and the overlap of two or three of the phenyl
Table 3
Fluorescence lifetimes of wG1 (2.0 × 10−5 M) and wG2 (2.0 × 10−5 M) at various
concentrations in 0.001 M KOH aq with excitation at 375 nm.
Concentration of KCl/mM
ꢁs (at 430 nm)/ns (population (%))
wG1
wG2
0
800
0
0.08 (58), 0.87 (40), 5.3 (2)
9.3 (100)
0.93 (43), 3.0 (51), 7.1 (6)
1.3 (34), 3.8 (63), 10.1 (3)
800

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5
intensity of this emission band increased progressively in the time
period of 1–10.3 ns. However, the lack of observation of the rise
component indicated that the formation of the excimer within the
dendrimer aggregates should be faster than the time resolved limit
of the instrument. Fig. 5b depicts the time-resolved spectra of wG2
(3.1 × 10−6 M) in the same measurement conditions. The time evo-
lution of wG2 was similar to that of wG1, but the excimer emission
was more intense, agreeing with the steady state fluorescence spec-
tra (see Fig. 2d, solid line).
4. Conclusions
In conclusion, the anthracene-cored poly(aryl ether) den-
drimers formed aggregates at low concentrations in aqueous
solution (2.0 × 10−5 M) to form micelles via the addition of
KCl, results that were very different from the CMC of usual
detergents (∼10−2 M); the aggregate formation was highly depen-
dent upon the generation. Furthermore, the results presented
here are the first clear report of the observations of the
excimer emission of anthracene in very low fluid solutions in
which appropriate substituents were introduced. A compari-
son of the excimer fluorescence of wG1 and wG2 indicated
that the different overlapping structures depended on the den-
drimer generations. These findings of dendrimer aggregation
revealed by photochemical techniques may be useful for devel-
oping new drug delivery systems using amphiphilic cone-shaped
dendrimers.
Acknowledgement
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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Fig. 5. Normalized time-resolved fluorescence spectra with peak intensities of (a)
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