Synthesis of 9,10-distyrylanthracene derivative and its one- and two-photon induced emission in solid state

Article abstract of DOI:10.1016/j.jphotochem.2018.05.013

Solid fluorescence plays an important role in optoelectronic devices. In this paper, 9,10-bis(6-dimethylamino benzoxazole styryl)-anthracene (1) was synthesized. 1 displayed a strong emission in solid state (microcrystalline state, amorphous solid state and polymeric thin film) resulted from aggregation-induced emission mechanism, and a large absolute fluorescence quantum yield (?_f = 0.65) was obtained. 1 also exhibited up-conversion emission in solid state and a strong yellow emission was observed upon excitation with 800–1064 nm, the quadratic dependence of the fluorescence on the excitation laser intensity confirmed that the up-conversion emission resulted from two-photon process.

Full text of DOI:10.1016/j.jphotochem.2018.05.013

JournalofPhotochemistry&PhotobiologyA:Chemistry361(2018)62–66
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis of 9,10-distyrylanthracene derivative and its one- and two-photon
induced emission in solid state
⁎
Cong Qua,b, Zheng Gaoa,b, Yi Chena,b,
a
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing,
100190, China
b
University of Chinese Academy of Sciences, Beijing, 100190, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Solid fluorescence plays an important role in optoelectronic devices. In this paper, 9,10-bis(6-dimethylamino
benzoxazole styryl)-anthracene (1) was synthesized. 1 displayed a strong emission in solid state (micro-
crystalline state, amorphous solid state and polymeric thin film) resulted from aggregation-induced emission
mechanism, and a large absolute fluorescence quantum yield (ϕf = 0.65) was obtained. 1 also exhibited up-
conversion emission in solid state and a strong yellow emission was observed upon excitation with 800–1064
nm, the quadratic dependence of the fluorescence on the excitation laser intensity confirmed that the up-con-
version emission resulted from two-photon process.
9,10-Distyrylanthracene derivative
Solid fluorescence
Up-conversion emission
Two-photon process
Synthesis
1. Introduction
absorption process and tunability within a quite broad spectral range.
Moreover, organic materials offer easier processing, faster response
Solid fluorescence has played an important role in optoelectronic
devices such as display, laser, and biomedicine [1–6]. The molecular
systems which exhibit emission in solid are usually from inorganic
materials or hybrid materials, and a few examples are based on organic
small molecules due to aggregation-induced emission quenching. For-
tunately, some molecular systems [7–9] show unique enhanced emis-
sion rather than fluorescence quenching upon aggregation in their solid
states based on aggregation induced emission or aggregation induced
enhanced emission, and the discoveries have promoted research on
newly fluorescent materials for their potential applications [10–12].
Up-conversion emission, generating a higher energy light from a
lower energy light [13], has attracted a great deal of attention for its
potential wide range of applications [14–18]. Recently, up-converted
lasing based on two-photon pumped mechanism is of increasing interest
since it is a new approach to accomplish frequency up-conversion of
coherent light without phase-matching requirements [19,20]. The early
reported materials which exhibit up-conversion emission are usually
from inorganic rare earth [21–23] or metal oxides semiconductors
[24–26], and few are based on organic material due to the quench of
emission of organic dye in solid state. Now, more and more organic
materials which exhibit up-conversion emission have been reported
[27–33]. As compared to inorganic materials, organic materials exhibit
more efficient nonlinear optical properties through two or more photon
time, and lower lasing threshold. The challenge is to develop ideal or-
ganic materials with strong two-photon pumped emission in solid
states.
9,10-Distyrylanthracene derivatives are a class of promising and
attractive aggregation induced emission molecules [34–36], they
showed many potential applications including fluorescent probe, sen-
sors, bioimaging, optical waveguide and others [37–41]. Herein, we
synthesized
a new 9,10-distyrylanthracene-based fluorophore 1
(Scheme 1) and studied its one- and two-photon pumped emission in
solid state. It was found that 1 exhibited weak emission in solvents but
strong emission in microcrystalline or amorphous solid state, with one-
or two-photon excitation, a strong emission was observed.
2. Experimental
2.1. Materials and equipment
1H and 13C NMR spectra were recorded at 400 and 100 MHz, re-
spectively, with TMS as an internal reference. MS spectra were recorded
with TOC-MS spectrometer, respectively. UV absorption spectra and
fluorescence spectra in solution were measured with an absorption
spectrophotometer (Hitachi U-3010) and
a fluorescence spectro-
photometer (F-2500), respectively. Fluorescence quantum yields in
⁎
Corresponding author at: Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences,
Beijing, 100190, China.
E-mail address: yichen@mail.ipc.ac.cn (Y. Chen).
https://doi.org/10.1016/j.jphotochem.2018.05.013
Received 7 March 2018; Received in revised form 7 May 2018; Accepted 11 May 2018
1010-6030/©2018PublishedbyElsevierB.V.

C. Qu et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry361(2018)62–66
Scheme 1. Chemical structure of 1 and its synthesis.
solid state were measured with a fluorescence spectrophotometer
(Edinburgh Instruments FLS-920). Solid absorption spectra were mea-
sured with UV–vis-NIR spectrophotometer (Cray 50000, Varian). All
chemicals for synthesis were purchased from commercial suppliers, and
solvents were purified according to standard procedures. Reaction was
monitored by TLC silica gel plates (60 F-254). Column chromatography
was performed on silica gel (70–230 mesh).
the solvent, the crude 2-methylbenzoxazol-6-amine (2.77 g, 85% yield)
was obtained for next step reaction without purification. (c) To a so-
lution of 2-methylbenzoxazol-6-amine (2.66 g, 18.0 mmol) in methanol
(30.0 mL) was added formaldehyde (37%, 12 mL, 144 mmol) and
NaBH3CN (2.27 g, 36.0 mmol). The solution was stirred at ambient
temperature for 36 h till the starting material disappeared (TLC detec-
tion). The solution was poured into H2O (30.0 mL), and the mixture
solution was extracted with ethyl acetate (30.0 mL × 3). The combined
organic solution was dried over Na2SO4, the solution is concentrated
and 2-methyl-6-(N, N-dimethylamino) benzoxazole (2.50 g, 79% yield)
is obtained by flash column chromatography (elute: petroleum ether /
ethyl acetate = 10 / 1, v/v, Rf =0.11). 1H NMR (400 MHz, CDCl3):
δ = 7.46 (d, J = 8.8 Hz, 1 H), 6.78 (d, J = 2.0 Hz, 1 H), 6.74 (dd,
J = 8.8 Hz, J = 2.4 Hz, 1 H), 2.97 (s, 6 H), 2.46 (s, 3 H). (d) To a so-
lution of 2-methyl-6-(N, N-dimethylamino) benzoxazole (1.76 g,
10.0 mmol) and anthracene-9,10-dicarbaldehyde (1.18 g, 5.00 mmol)
in dry DMF (20 mL) was added KOH (0.56 g, 10.0 mmol). The solution
was stirred at 110 °C for 8 h till the starting material disappeared (TLC
detection). The solid was filtered and washed with MeOH (30.0 ml),
and target compound 1 (1.37 g) is obtained by flash column chroma-
tography (elute: CH2Cl2). Yield: 48%. M.p. ≥ 300 °C. 1H NMR
2.2. Experiment for two-photon pumped emission
In two-photon pumped emission experiment, a Ti-sapphire laser
system (Mai Tai HP, Spectra-Physics) with a pulse width of 120 fs and a
repetition rate of 82 MHz was employed as an excitation light source.
The energy of excited pulse can be controlled by neutral density filters,
and the beam of exciting laser was focused into the central of the
cuvette with a plan-convex lens (focal length = 100 mm) under ex-
periment. The fluorescence emitted from the sample was collected by a
fiber spectrometer (SD 2000, Ocean Optics) and the direction of the
detection is perpendicular to the laser transmission direction.
2.3. Molecular orbital calculations
(400 MHz, CDCl3):
δ (ppm) 7.91 (d, J = 16 Hz, 2 H), 7.52 (d,
J = 8.8 Hz, 2 H), 7.23 (d, J = 16 Hz, 1 H), 6.73 (s, 1 H), 6.80 (d,
J = 5.2 Hz, 1 H), 6.87 – 6.83 (m, 2 H), 3.05 (s, 6 H). 13C NMR
(100 MHz, CDCl3): δ = 151.1, 150.4, 142.2, 133.8, 133.1, 129.5, 129.4,
126.2, 125.6, 124.8, 120.1, 100.3, 95.8, 41.9. TOF MS (EI) calcd for
The optimized structure of the 9,10-bis(6-dimethylamino benzox-
azole styryl)-anthracene molecule was calculated at the density func-
tional theory (DFT) level with the hybrid B3LYP functional and the
6–31 G basis set. The electron densities of the HOMO and LUMO were
calculated with the Gaussian 03 package.
C
36H30N4O2 (M+): 550.2369, found: 550.2372.
3. Results and discussion
2.4. Synthesis of 1
3.1. Synthesis of 1
The synthetic route for 1 was outlined in Scheme 1, and the detailed
procedures were presented as follows: (a) To a solution of 2-amino-5-
nitrophenol (7.70 g, 50.0 mmol) and pyridine (3.96 g, 50.0 mmol) in
dry xylene (150 mL) at 0 °C was added dropwise acetyl chloride (4.32 g,
55.0 mmol). The solution was stirred at ambient temperature for 2 h. To
above solution was added p-toluenesulfonic acid (1.72 g, 10.0 mmol),
the solution was refluxed till no water was discharged. After cooling to
room temperature, the solution was washed with water (100 mL × 3)
and a saturated solution of NaCl (50.0 mL), respectively. The organic
solution was collected and dried over Na2SO4, after evaporation of the
solvent, the crude 2-methyl-6-nitrobenzoxazole (pale solid, 8.80 g, 95%
yield) was obtained for next step without purification. (b) To a solution
of 2-methyl-6-nitrobenzoxazole (3.92 g, 22.0 mmol) in methanol
(60.0 mL) at 70 °C was added NH4Cl (11.77 g, 220 mmol) in H2O
(40.0 mL) and Fe (4.48 g, 80.0 mmol). The mixture solution was stirred
at 70 °C for 4 h till the starting material disappeared (TLC detection).
The mixture solution was cooled to ambient temperature and filtered,
the solution was extracted with ethyl acetate (30.0 mL × 3). The
combined organic solution was dried over Na2SO4, after evaporation of
The synthetic details for 1 are described in the Experimental. In
brief, as depicted in Scheme 1, 1 was obtained in 30% total yield by a 4-
step reaction. The key intermediate N, N, 2-trimethylbenzoxazol-6-
amine was obtained starting from 2-amino-5-nitrophenol, which con-
densed with acetyl chloride in xylene (98% yield), followed by reduc-
tion with NH4Cl and Fe powder in MeOH-H2O mixture solution (85%
yield). Amino methylation was performed by the reaction of 2-me-
thylbenzoxazol-6-amine with formaldehyde using NaBH3CN as reduc-
tion reagent to afford N, N, 2-trimethylbenzoxazol-6-amine in 78%
yield. Treatment of N, N, 2-trimethylbenzoxazol-6-amine with anthra-
cene-9,10-dicarbaldehyde in dry DMF afforded 1 in 48% yield.
3.2. Optical properties of 1 in solution
It has demonstrated [39,42] that centrosymmetric 9,10-dis-
tyrylanthracene derivatives usually possess two main absorption bands:
one located at ∼310 nm corresponded to the absorption of the styryl
63

C. Qu et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry361(2018)62–66
Fig. 1. Absorption and fluorescence of
1 in MeCN solution (10 μM,
λ
ex = 420 nm).
Fig. 3. Fluorescence of 1 in microcrystalline state with one-photon excitation
(inset: the fluorescent imaging of 1 in microcrystalline state, λex = 420 nm).
segment, and the other at ∼410 nm attributed to the π-π* transition.
The optical properties of 1 in solution were conducted by measuring the
absorption and the emission of 1 in dilute solutions (10 μM) in different
organic solvents. In acetonitrile (MeCN), 1 showed two main absorption
bands at ∼360 nm and ∼410 nm, respectively (Fig. 1), ∼360 nm
should be deduced to the absorption of benzoxazole styryl segment, a
red-shift was probably due to the donor part of N,N-dimethylamino in
benzoxazole ring, which strengthened the electron-donating ability,
and ∼410 nm was attributed to the π→π* transition. No significantly
change in absorption spectral was obtained when 1 dissolved in other
solvents such as ethanol (EtOH, λmax = 410 nm) and dimethyl sulfoxide
(DMSO, λmax = 414 nm). A little solvatochromic effect suggested that
the charge transfer transition in 1 is very small in ground state due to a
centrosymmetric molecule with D-π-D-π-D structure.
Upon excitation the solution of 1 in MeCN solution, a orange
fluorescence with the maximum emission wavelength at 585 nm was
detected (Fig. 1), by using fluorescein (ϕf = 0.95, NaOH) as reference, a
very small fluorescence quantum yield (ϕf = 0.07) was obtained. Si-
milar results were obtained when MeCN was replaced by EtOH
(λem = 587 nm, ϕf = 0.07). Further investigation found that 1 ex-
hibited a positive solvatochromism: the emission wavelength was
shifted to long wavelength with the increase of the polarity of solvents.
In DMSO, the emission wavelength of 1 was red-shifted to 610 nm
(ϕf = 0.01), the large solvatochromism suggested that a strong in-
tramolecular charge transfer (ICT) occurred in the excited state, which
was confirmed by both HOMO density and LUMO density of target
compound 1 calcuated at the density functional theory (DFT) level with
B3LYP/6-31G*. As shown in Fig. 2, the HOMO density of 1 is almost
spread out over the whole π-system, while the LUMO density almost
concentrates on the anthracene ring, which implies a strong ICT effect
occurred when 1 was in excited state and resulted in a large solvato-
chromism.
Fig. 4. Fluorescence of 1 in microcrystalline state with two-photon excitation.
(inset: the fluorescent imaging of 1 in microcrystalline state, λex = 800 nm).
yield ϕf = 65%), narrow (full with half maximum fwhm = 40 nm) and
blue-shifted (λem = 580 nm) emission was observed (Fig. 3). As com-
pared to solution, the emission of 1 in microcrystalline state exhibited
much stronger, the enhanced fluorescence was resulted from the ag-
gregation-enhanced emission. It has demonstrated [42] that 9,10-dis-
tyrylanthracene derivatives display a nonplanar conformation in their
crystalline states due to the CH/π hydrogen bonds and exhibit strong
emission as a result of the restricted torsional motion by supramolecular
interaction in the crystal. As shown in Fig. 3, the intense, narrow, and
blue-shifted emission of the crystal compared to that of the solutions
implies that the torsional motion of 1 is unambiguously restricted by
steric interaction, which leads to the closure of the nonradiative decay
channel [43,44]. The distorted conformation of 1 in crystalline state
limits effective conjugation and resulted in the blue-shifted emission. It
is worth mentioning that a strong yellow emission was also obtained
when 1 was in amorphous solid state or doped in polymeric thin films
such as PMMA (polymethyl methacrylate) thin film and PS
3.3. One-photon induced emission of 1 in solid state
Fig. 3 represents the fluorescence spectra of 1 in microcrystalline.
Upon excitation with 420 nm, a strong (absolute fluorescence quantum
Fig. 2. Optimized molecular structure and the HOMO and LUMO electron densities of 1.
64

C. Qu et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry361(2018)62–66
microcrystalline state. Using fluorescein solution (pH = 11) as re-
ference, the TPA cross sections of 1 in dichloromethane with different
excitation wavelength were obtained. Fig. 6 displayed the TPA cross
section of 1 in wavelegnth range of 800–1000 nm. As shown in Fig. 6,
two main TPA cross sections appeared at 810 nm and 920 nm, respec-
tively, and the largest TPA cross section (151 GM, 1 GM = 10−50 cm4
photon-1 s-1) was obtained with 810 nm excitation wavelength, which is
consistent with the results of similar organic compounds [46,47]. The
result confirmed that 1 exhibited two-photon absorption properties
which resulted in two-photon induced up-conversion fluorescence.
4. Conclusions
In summary, a new fluorophore based on 9,10-distyrylanthracene
derivative with a centrosymmetric D-π-D-π-D structure has been syn-
thesized. The fluorophore shows very weak fluorescence in solution but
strong emission in microcrystalline state, the enhanced fluorescence
results from aggregation-induced emission as a result of the restricted
torsional motion by supramolecular interaction in crystalline state. It
has demonstrated that the fluorophore exhibits nonlinear absorption
property in microcrystalline state, with near-infrared excitation
(800–1064 nm), a strong up-conversion emission is obtained. The
quadratic dependence of the fluorescence on the excitation laser in-
tensity confirms the two-photon process.
Fig. 5. Two-photon excited fluorescence changes of 1 (1.0 × 10−4 M, di-
chloromethane) with different excitation power (periods: 80, 100, 120, 140,
160 mW, λex = 800 nm, inset: logarithmic plots of the dependence of two-
photon induced fluorescence on pulse intensity).
Acknowledgment
This work was supported by the National Natural Science
Foundation of China (No 21572241).
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