Solid state emission enhancement of 9,10-distyrylanthracene derivatives and amplified spontaneous emission from a large single crystal

Article abstract of DOI:10.1039/c0nj00300j

Methyl-substituted 9,10-distyrylanthracene was synthesized by a simple Heck reaction and its photophysical properties investigated. Tight intermolecular stacking through supramolecular interactions in the crystal not only induced strong fluorescence emiss

Full text of DOI:10.1039/c0nj00300j

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Solid state emission enhancement of 9,10-distyrylanthracene derivatives
and amplified spontaneous emission from a large single crystalw
Bin Xu,a Honghua Fang,b Yujie Dong,a Feipeng Chen,a Qidai Chen,b Hongbo Sunb
and Wenjing Tian*a
Received (in Montpellier, France) 20th April 2010, Accepted 29th June 2010
DOI: 10.1039/c0nj00300j
Methyl-substituted 9,10-distyrylanthracene was synthesized by
a simple Heck reaction and its photophysical properties investi-
gated. Tight intermolecular stacking through supramolecular
interactions in the crystal not only induced strong fluorescence
emission with a high fluorescence efficiency of 35% but also
conducted the formation of large size and high quality needle-
like single crystals. Amplified spontaneous emission (ASE) with
a low threshold value of 10 lJ pulseÀ1 was observed.
been demonstrated in a broad range of conjugated polymers,
oligomers and dye-doped polymer systems, though the main
factors to affect the applications of those materials are their
low stability and inferior carrier mobility.6 Organic single
crystals may be promising candidates for ASE applications
due to their high thermal stability, good charge transport
properties and remarkable optical anisotropy, induced by
the crystalline assembly of molecules. Nevertheless, only a
few works have reported on ASE in organic single crystals,
because it is difficult to obtain large size and high quality
crystals with a high photoluminescence quantum yield.7
Recent work in our group has demonstrated that
9,10-distyrylanthracene derivatives show high fluorescence
efficiency in their crystals.8b Here, we report on a large size
and high fluorescence efficiency crystal of methyl-substituted
9,10-distyrylanthracene (BMSA). The purpose of the structure
modification is to control the molecular packing. Tight
intermolecular stacking via supramolecular interactions in
the crystal not only induces strong fluorescence emission but
also conducts the formation of large size and high quality
needle-like single crystals. To further demonstrate its potential
applications in an organic crystal-based laser, the amplified
spontaneous emission (ASE) properties of the crystals were
investigated. A low threshold value of 10 mJ pulseÀ1 and a
full-width at half-maximum (FWHM) of 10 nm were obtained
from the large size needle-like crystals.
Fluorescent organic materials have been extensively investi-
gated as active gain media for optically-pumped solid-state
lasers.1 However, most fluorescent organic materials have a
very high fluorescence efficiency in solution, but this generally
decreases in the solid-state. This decreasing fluorescence
efficiency in the solid-state is mainly attributed to high
non-radiative decay rates by excitation energy trapping or
low-radiative rate constants by H-type aggregation, which is a
specific case of excitonic coupling.2 Consequently, increasing
attention has been paid to enhancing the solid-state
fluorescence efficiency of fluorescent materials. Recently,
several intriguing cases have been reported for organic
fluorophores that show unique enhanced emission rather than
fluorescence quenching in the solid-state.3 Although the
origins of these enhanced emissions are still in debate, it is
assumed that this unique fluorescence phenomenon is related
to the effects of intramolecular planarization, the formation
of specific aggregation and the blockage of non-radiative
relaxation pathways of the excited species.4 Recent studies
on the crystals of silole derivatives and other molecules have
revealed that their common structural characteristics are a
twisted conformation in the solid-state, which suggests that
the emission enhancement of the crystals is caused by
restricted intramolecular vibrational and rotational motions
in the solid-state.5 Thus, investigating the single crystal
structure of molecules is an important way to understand
and clarify the relationship between their molecular packing
and emission properties in the solid-state. On the other hand,
the phenomenon of amplified spontaneous emission (ASE) has
BMSA was synthesized by a typical trans-selective Heck
coupling reaction.8 BMSA shows almost no fluorescence
(FF o 1%) in THF solution but its crystals show strong
fluorescence in the green spectral region with a FF efficiency up
to 35% (FF in the solid-state is obtained in a calibrated
integrating sphere), as shown in Fig. 1(b).
A single crystal of BMSA was prepared by vaporizing a
mixture of chloroform and ethanol slowly at room temperature
under the rigorous exclusion of light, and its structure was
characterized by X-ray crystallography.w The crystal data
reveals that the BMSA molecule lies about an inversion centre
and has a trans-conformation in the molecule, with relatively
large torsion angles of 671 between the anthracene center and
the vinylene moieties substituted at the 9- and 10-positions.
On the other hand, the structure of BMSA and the optimized
geometry of isolated BMSA (Fig. 1(a)) demonstrates a twisted
configuration of the molecule with a torsion angle of 541 in the
gas phase or dilute solution due to the large internal steric
hindrance between the hydrogen on the phenyl ring of
the anthracene moieties and the hydrogen on the vinylene
moieties.9 The origin of the non-emission of BMSA in solution
a State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, 2699 Qianjin Avenue, Changchun 130012,
P. R. China. E-mail: wjtian@jlu.edu.cn; Fax: + 86 431 85193421;
Tel: + 86 431 85166368
b State Key Laboratory on Integrated Optoelectronics, College of
Electronic Science and Engineering, Jilin University, 2699 Qianjin
Avenue, Changchun 130012, P. R. China
w CCDC reference number 774731. For crystallographic data in CIF
or other electronic format see DOI: 10.1039/c0nj00300j
c
1838 New J. Chem., 2010, 34, 1838–1842 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Fig. 1 (a) The molecular structure of BMSA (top) and the optimized
geometry of isolated BMSA (bottom). (b) A crystal of BMSA under
UV light irradiation (top) and the BMSA structure fixed by CH/p
interactions in the crystal (bottom).
should be related to the twisted configuration and the corres-
ponding intramolecular torsional motions around the double
bond beside the anthracene moieties. These intramolecular
motions can lead to fast non-radiative relaxation and the
reduction of fluorescence efficiency in solution.5
From an analysis of the structure and packing arrangements
of the crystal, we found that the CH/p interactions play a
crucial role in the molecular packing, and they are also
important in terms of the non-planar conformations of the
BMSA. The unit cell of BMSA is monoclinic with a space
group P21/c. Its packing arrangement is shown in Fig. 2.
A CH/p hydrogen bond (interaction I) is formed between
two molecules with an interaction distance of 2.79 A and an
angle of 147.91, where the vinylene moiety along the long axis
of one molecule acts as a H-donor and the corresponding
phenyl ring of anthrylene moiety of the adjacent molecule acts
as a H-acceptor. At the same time, another CH/p hydrogen
bond (interaction II) is formed between the two molecules with
an interaction distance of 2.91 A and an angle of 166.81, where
the methyl of the phenyl ring acts as a H-donor and the
corresponding anthrylene moiety of the adjacent molecule acts
as a H-acceptor. In crystals of BMSA, the molecules pack
into molecular columns along the a-axis and slide to their
neighbors along the long molecular axis within one column
due to interaction I. On the other hand, the molecules are
packing into a zig-zag motif along a mirror glide plane in one
layer due to interaction II.10 This indicates that the supra-
molecular interactions in the BMSA crystal are relatively
strong.11 It is the CH/p hydrogen bonds that fix the anthrylene
moieties and the peripheral double bonds, preventing free
torsional motions around the double bonds. Assisted by
strong supramolecular interactions, the molecule not only
becomes more stable and distorted in the crystal lattice, but
also tends towards tight intermolecular packing. Therefore,
supramolecular interactions play an important role in the
strong fluorescence of BMSA crystals.12
Fig. 2 (a) Intermolecular interactions I and II in crystals of BMSA.
(b) A molecular packing diagram of BMSA under CH/p interactions
viewed along the a-axis. The hydrogen atoms have been omitted for
clarity.
shows a weak and board emission band. Upon cooling to 77 K,
the non-emitting BMSA becomes strongly fluorescent in the
glassy state, and the intensity of the PL is almost 120-fold
higher than that at room temperature. Meanwhile, the
emission of BMSA at 77 K shows a blue shift compared to
that of a dilute solution at room temperature. This can be
attributed to the enhanced potential barrier for free intra-
molecular torsional motion around the double bonds in such a
rigid medium at low temperature and the formation of
more distorted configurations resulting in a blue-shifted
emission.5,8b Thus, the strong emission of BMSA in a glassy
solvent at low temperature may have the same origin as the
strong emission due to restricted intramolecular motion.
Otherwise, different from the PL spectrum of the single crystal,
the PL spectrum of BMSA in a low temperature solution
clearly shows vibrational structure and
a blue shift.
This feature indicates that the emission properties at low
temperature may arise from single molecules with restricted
intramolecular motion caused by glassy solvent molecules,
which is different from the restriction of the environment in
single crystals due to supramolecular interactions.
Time-resolved measurements of BMSA in dilute THF
solution and in single crystals at room temperature were
performed using time-correlated single-photon counting
(TCSPC) with excitation at lex = 405 nm; typical results
(lfl = 630 nm for solution and lfl = 530 nm for crystal,
respectively) are shown in Fig. 4. The data points are the
experimental data and the solid lines are the fitting curves
based on exponential functions.14 From Fig. 4, one can see
that the fluorescence of crystals of BMSA decays very rapidly
compared to that of its solutions. The BMSA solution
fluorescence shows a single exponential decay with an emission
lifetime of B2.29 ns. However, for BMSA crystals, the
following double-exponential function can simulate its decay
To fully understand the origin of emission enhancement,
Fig. 3 presents the absorption and PL spectra of BMSA at
room temperature and 77 K. Compared to the absorption
spectrum of BMSA at room temperature, the absorption
spectrum at 77 K clearly shows vibrational structure and a
red shift, indicating a reduction in conformational disorder
and a higher polarizability of the low temperature glass.13 The
PL spectrum of BMSA in THF solution at room temperature
c
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Ti:sapphire optical amplifier with a maximum pulse energy
1.5 mJ and a pulse duration of 120 fs. The laser beam was
focused by using a quartz cylindrical lens into a strip whose
spot area was 5 Â 1 mm2 and parallel to the long axis of the
crystal, which was glued to the quartz substrate. The PL
emission was collected from the edge of the crystal and sent
through an optical fiber to the spectrometer. All measurements
were carried out at room temperature. Fig. 5 displays the PL
spectra of this crystal as a function of pump laser energy, and
the insert is the PL emission peak changes as a function
of pump laser energy. The emission spectra (including
conventional PL spectra) for the BMSA single crystal exhibit
a broad feature under weak incident intensities that corres-
ponds to spontaneous emission. As the pump intensity is
increased, a sharp PL spectrum centered at 524 nm and a
gradual blue-shift of the ASE peak maximum are observed,
indicating that the ASE process occurs on a timescale faster
than the spectral diffusion that occurs during the first few ps
after photoexcitation.1c Fig. 6 shows the dependence of the
FWHM and the integrated peak intensity of the PL spectrum
as a function of excitation fluence. At a low pump fluence of
5.1 mJ pulseÀ1, the FWHM of the emission spectrum was
approximately 60 nm. When the pump fluence was gradually
increased, the FWHM decreased to 10 nm. The relationship
between the peak intensity and the pump energy was not
linear, being characterized as ASE caused by stimulated
emission. The threshold of ASE can be extracted from the
change of slope of the pump intensity dependence, which is
located at 10 mJ pulseÀ1. In addition, the FWHM of the
emission spectrum was dramatically reduced from 60 to
10 nm in the region of the threshold. Upon further increasing
the pump energy, gain narrowing was halted and the FWHM
changed no further. The main mechanism for the observed
gain-narrowing is attributed to ASE by the observation of the
waveguiding property and the effect of pump length on light
amplification.15
Fig. 3 Room temperature (TT) and 77 K (Á Á Á) absorption spectra of
BMSA in methyl-THF (1 Â 10À5 mol LÀ1), and the PL spectrum of
BMSA in dilute THF (1 Â 10À5 mol LÀ1). The absorption spectra are
normalized.
behavior better than can the single-exponential one. This
reveals that the excited states of BMSA molecules in single
crystals decay through two pathways: one pathway shows a
rapid decay with a decay coefficient of 0.97 and a lifetime of
B0.85 ns; the other pathway shows a slow decay character
with a coefficient of 0.03 and a lifetime of B7.21 ns. These
observations indicate that the contribution of the slow decay
becomes negligible and that of the rapid decay became
dominant in the emission transient processes of crystals. The
decrease of the lifetime in single crystals against solutions is
again indicative of the role of supramolecular interactions in
the emission process.
Under the strong driving force of CH/p hydrogen bonding
interactions, BMSA molecules can easily form large needle-
like single crystals with a length of around 3 cm and a width of
several hundred microns (a photograph is shown in Fig. 1(b)),
suggesting its potential application in organic solid-state
lasers. To obtain an in-depth characterization of optical gain
properties, ASE of a single crystal was investigated. Optical
pumping was generated by the second harmonic (400 nm) of a
To summarize, we have developed a new molecule, BMSA,
with a solid-state emission enhancement and demonstrated
that tight intermolecular stacking through supramolecular
Fig. 4 Time-resolved fluorescence spectra of BMSA in THF solution
(&) and single crystals (J) at room temperature measured by
TCSPC. The solid lines are the fitting curves based on exponential
functions.
Fig. 5 The PL spectra of a BMSA single crystal as a function of
pump laser energy. Inset: PL emission peak changes as a function of
pump laser energy.
c
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to a board (Becker & HickelSPC-130). A 405 nm picosecond
diode laser (Edinburgh Instruments EPL-405, repetition rate
10 MHz) was used to excite the samples.
Single crystals of BMSA were prepared by vaporizing mixed
solvents of chloroform and ethanol (2 : 1) slowly at room
temperature under the rigorous exclusion of light. Diffraction
experiments were carried out on a Rigaku R-AXIS RAPID
diffractometer equipped with Mo-Ka radiation at 293(Æ2) K.
The structures were solved by direct methods and refined by a
full-matrix least-squares technique using the SHELXS v. 5.1
program.w
Synthesis of BMSA
A round-bottomed flask (25 mL) was oven dried and cooled
under an N2 atmosphere. 1-Methyl-4-vinylbenzene (0.28 g,
2.4 mmol), 9,10-dibromoanthracene (0.34 g, 1 mmol), K3PO4
(0.64 g, 3 mmol) and Pd(OAc)2 were dissolved in dry DMAc
(10 mL). The reaction mixture was heated to 110 1C in an oil
bath and stirred for 24 h at this temperature. After being
cooled to room temperature, the reaction mixture was poured
into water and extracted with CH2Cl2 (3 Â 40 mL). The
combined organic extracts were washed with brine, dried
(Mg2SO4) and concentrated to dryness under vacuum. The
crude product was purified by flash column chromatography
(petroleum ether/CH2Cl2 = 4 : 1) to give BMSA as a yellow
Fig. 6 The dependence of the FWHM and the integrated peak
intensity of the PL spectrum as a function of excitation fluence.
interactions in its single crystals is a mechanism for emission
enhancement. ASE from a large single crystal of BMSA was
observed. The FWHM of the narrowed spectra reached 10 nm
and the threshold of ASE was about 10 mJ pulseÀ1. The
observation of ASE with a low threshold value indicates that
the BMSA crystals are good candidates for organic solid-state
lasers.
We thank the National Basic Research Program of China
(973 program, 2009CB623605) and the Program for New
Century Excellent Talents in Universities of China Ministry
of Education, the 111 Project (Grant no. B06009).
1
solid (0.30 g, 73% yield). H NMR (500 MHz CDCl3) d 2.43
(6H, s, CH3), 6.91 (2H, d, J = 16.5 Hz, CHQCH), 7.27
(2H, d, J = 7.5 Hz, Ar), 7.45–7.47 (4H, m, Ar), 7.59 (2H, d,
J = 8.0 Hz, Ar), 7.88 (2H, d, J = 16.5 Hz, CHQCH),
8.39–8.41 (4H, m, Ar); MALDI/TOF MS: Calc. for C32H36:
410.2. Found: 410.8. Anal. calc. for C32H26: C, 93.62; H, 6.38.
Found: C, 93.56; H, 6.45.
Experimental section
General remarks
Crystal data for BMSA
All reagents and starting materials for the synthesis are
commercially available and were used as received. THF and
methyl-THF were purchased from Aldrich and used without
further purification.
1H NMR spectra were recorded at 298 K on an AVANCZ
500 spectrometer with tetramethylsilame (TMS) as the
standard. Compounds were characterized by a Flash EA
1112, CHNS-O elemental analysis instrument. Time of flight
mass spectra were recorded using a Kratos MALDI-TOF
mass system.
C32H26, M = 410.53, monoclinic, space group P21/c, a =
5.4103(11), b = 9.7287(19), c = 21.814(4) A, a = 90.00, b =
94.84(3), g = 90.001, V = 1144.1(4) A3, T = 293(2) K, Z = 2,
10 691 reflections measured, 2592 independent reflections
(Rint = 0.1441). The final R1 values were 0.0990 (I > 2s(I)).
The final wR(F2) value was 0.1602 (I > 2s(I)). The final R1
value was 0.2362 (all data). The final wR(F2) value was 0.2073
(all data).w
UV-vis absorption spectra were recorded on a UV-3100
spectrophotometer. Fluorescence measurements were carried
out with a RF-5301PC device. The fluorescence quantum yield
(FF) in solution was measured in dilute THF using quinine
sulfate in 0.1 mol LÀ1 sulfuric acid as the standard. Crystalline
state PL efficiencies were measured with an integrating sphere
(C-701, Labsphere Inc.), using a 325 nm Xe light as the
excitation source, and the laser was introduced into the sphere
through the optical fiber. Low temperature measurements for
samples were performed from a liquid nitrogen optical
cryostat (Optistat DN-V). Time-resolved fluorescence
measurements were performed on a time-correlated single
photon counting (TCSPC) system. Fluorescence signals were
collimated and focused onto the entrance slit of a mono-
chromator with the output plane equipped with a photo-
multiplier tube (Hamamat-suH5783p), which was connected
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