Amplified spontaneous emission, optical waveguide and polarized emission based on 2,5-diaminoterephthalates

Article abstract of DOI:10.1016/j.cclet.2017.08.045

A series of 2,5-diaminoterephthalates with a simple structure were synthesized through one-step reaction, and their bar-shaped single crystals with a large size and a smooth surface have been obtained via the solvent-evaporation method. These crystals exh

Full text of DOI:10.1016/j.cclet.2017.08.045

Accepted Manuscript
Title: Amplified spontaneous emission, optical waveguide and
polarized emission based on 2,5-diaminoterephthalates
Authors: Baolei Tang, Zuolun Zhang, Huapeng Liu, Hongyu
Zhang
PII:
S1001-8417(17)30331-5
DOI:
Reference:
http://dx.doi.org/10.1016/j.cclet.2017.08.045
CCLET 4194
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Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
26-6-2017
23-8-2017
23-8-2017
Please cite this article as: Baolei Tang, Zuolun Zhang, Huapeng Liu,
Hongyu Zhang, Amplified spontaneous emission, optical waveguide and
polarized emission based on 2,5-diaminoterephthalates, Chinese Chemical
Lettershttp://dx.doi.org/10.1016/j.cclet.2017.08.045
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Communication
Amplified spontaneous emission, optical waveguide and polarized
emission based on 2,5-diaminoterephthalates
Baolei Tang, Zuolun Zhang*, Huapeng Liu, Hongyu Zhang*
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012,
China.
 Corresponding authors.
E-mail addresses: zuolunzhang@jlu.edu.cn; hongyuzhang@jlu.edu.cn
ARTICLE INFO
ABSTRACT
Article history:
A series of 2,5-diaminoterephthalates with a simple structure were synthesized through one-ste
reaction, and their bar-shaped single crystals with a large size and a smooth surface have been
obtained via the solvent-evaporation method. These crystals exhibit bright emission with
fluorescence quantum yields higher than 0.2. They display the waveguide property, and low
optical loss coefficients for waveguide have been determined for the crystal of one compound.
addition, the crystal can cause linear polarization of the light emitted from it, with a high
polarization contrast of 0.70. Most importantly, these crystals can realize amplified spontaneou
emission (ASE), including the red ASE, with appreciable energy thresholds of 72–198 kW/cm
and high gain coefficients, which suggests the potential of these crystals for the application in
organic solid-state lasers.
Received 27 June 2017
Received in revised form 21 July 2017
Accepted 6 August 2017
Available online
Keywords:
2,5-Diaminoterephthalates
Waveguide
Polarization
Amplified spontaneous emission
Laser
In recent years, organic optoelectronic materials have attracted considerable attention due to their various
applications, such as organic light-emitting diodes, field-effect transistors, photovoltaics, sensors and
solid-state lasers [1‒8]. As a kind of morphological forms studied for optoelectronic materials, single
crystals are featured by the ordered molecular arrangement and well-defined surface, which endow the
crystals with diverse and outstanding properties, such as good thermal stability, high charge-carrier
mobilities, low light propagation loss and reflecting mirror structures [9‒14]. As such, they have attracted
increasing research interest in optoelectronics.
Amplified spontaneous emission (ASE) is a phenomenon that has long been studied, which is closely
related to pumping laser [15‒21]. In general, the ASE is accompanied by the waveguide and emission
polarization properties of the active materials. Some organic crystals have been found to possess good
waveguide and ASE performance because of their low light scattering, smooth surfaces and high
fluorescence quantum yields (F) [22,23]. To date, the ASE of crystals with blue, green and red colors has
all been reported. However, the crystals that can realize red ASE with low energy thresholds and high gain
coefficients are still lacking [24‒27].
In this work, we have employed a series of 2,5-diaminoterephthalates, 1–4, for ASE study (Fig. 1). The
existence of electron-donating amino and electron-withdrawing ester groups in the molecule is beneficial to
the realization of desirable low-energy emission. The substituents on the peripheral phenyl rings are adopted
to tune the material properties. The intramolecular hydrogen bonds between the amino and the carbonyl
groups are expected, which could lead to a relatively planar and rigid conformation for the central part of the
molecules. Such a conformation may enhance the luminescence efficiency in the dispersed state and thus
provide a good opportunity for realizing high solid-state F that benefits the ASE [24]. Although compounds
1, 3 and 4 are known [28,29], their optoelectronic properties have not been disclosed. In addition to the ASE,
the waveguide and polarized emission properties have also been explored.

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The syntheses of compounds 1, 3 and 4 have been reported previously, and compound 2 is newly
synthesized according to the literature procedure (Scheme S1 in Supporting information) [29]. These
compounds can be easily crystalized by slow evaporation of their solutions in the CHCl3/CH3OH mixed
solvent (V:V = 1:2), and bar-shaped single crystals with a large size and a smooth surface can be obtained.
The thermal properties of 1–4 were characterized by thermogravimetric analysis (TGA) and differential
scanning calorimetry (DSC) under a nitrogen atmosphere at a heating rate of 10°C/min (Figs. S1 and S2 in
Supporting information). 1–4 exhibit high thermal decomposition temperatures (Td, corresponding to 5%
weight loss) of 233, 250, 251 and 275 °C, respectively, as can be seen from the TGA curves. The melting
points (Tm) of 1–4 determined by DSC measurements are moderate to high, being 167, 230, 227 and 232 °C,
respectively. Considering the close molecular weights and structures of 1–4, the much lower Tm of 1 may
imply relatively weaker intermolecular interactions. The good thermal properties of 1–4 benefit their
optoelectronic application.
To get insight into molecular conformations and packing features, the crystal structure of 1 was resolved
by X-ray diffraction. The crystal belongs to the triclinic system, space group P-1. There is one molecule in the
asymmetric unit (Fig. 2a), and no solvation is observed. Within the molecule, there are two kinds of
intramolecular hydrogen bonds: The N–HO hydrogen bond (HO distance: 1.964 Å, ∠N–HO: 136.4°)
between the N–H group and the carbonyl oxygen; and the CHO hydrogen bond (HO distance: 2.298 Å,
∠C–HO: 103.48°) between the central benzene ring and the ester group (Fig. 2a). As expected, these
interactions lead to a relatively planar conformation for the central part of the molecule. The two peripheral
phenyl rings twist out of the plane defined by the central benzene ring by an angle less than 34°. In the
packing structure, there are ππ interactions between the peripheral phenyl rings of adjacent molecules (Fig.
2b). Notably, all the molecules in crystal 1 are arranged in the same conformation and orientation (Fig. 2c and
2d), resulting in the formation of so-called uniaxially oriented crystal, which is beneficial to the waveguide,
polarized emission and ASE of organic crystals [30,31].
Fig. 3a shows the absorption and emission spectra of 1–4 measured in CH2Cl2 (10‒5 mol/L), and the
photophysical data are summarized in Table S1 in Supporting information. All these compounds possess a
broad absorption band in the visible-light region. The absorption maxima of 1 and 2 are both located at 469
nm, while those of 3 (479 nm) and 4 (484 nm) are gradually red shifted. 1–4 show a broad and structureless
emission spectrum in CH2Cl2, with the emission maxima located at 564, 564, 578 and 594 nm, respectively.
Correspondingly, the emission colors range from yellow to orange-red. The bar-shaped single crystals of
these compounds display bright orange-red emission. The emission maxima of crystals 1–4 are 597, 604,
597 and 608 nm, respectively (Fig. 3b), which are significantly red shifted compared with those of
corresponding emission spectra measured with CH2Cl2 solutions. The F values of crystals 1–4 are 0.22,
0.38, 0.48 and 0.24, respectively. Notably, these values are at a high level as for organic solids with the
emission maxima around 600 nm, as the non-radiative decay of the excited state becomes more serious for
low-energy emission according to the energy-gap law. The good F values are favorable for the
optoelectronic application of the compounds.
Interestingly, when the bar-shaped crystals of 1–4 are irradiated with UV light, the two ends of crystal
emit brighter light than the body, suggesting that these crystals are optical waveguides and may be useful for
ASE or lasing [32‒35]. To evaluate the waveguide performance, optical loss coefficients were studied with
crystal 1. The method adopted is to irradiate different positions of a single crystal using the same excitation
light and collect, at one end of the crystal, the emission spectrum corresponding to each irradiation position
(Fig. 4a) [36,37]. As can be seen in Fig. 4b, the emission intensity at the end decreases gradually when the
distance between the end and the irradiated position is increased, due to a longer propagation distance of
light. In addition, a spectral redshift can be observed with increased propagation distance. The optical loss
coefficients are estimated to be 0.361 and 0.058 dB/mm at 617 and 638 nm, respectively, by fitting the data
of Fig. 4b according to literature procedures [38,39]. Notably, these values are significantly smaller than
those of many organic crystals ever reported [32], suggesting the good optical waveguide performance of
crystal 1. The relatively larger loss coefficient for the light of 617 nm should be related to the reabsorption,
as deduced from the overlap between the emission and absorption spectra of crystal 1 in the range of ca. 525–
620 nm (Fig. S4 in Supporting information). Upon increasing the propagation distance, more light within
525–620 nm is reabsorbed, which is responsible for the observed spectral redshift of long-distance
propagation.
a
b

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In addition to the waveguide property, the polarization property of the light emitted from the bar-shaped
crystal of 1 was also studied. A polarization analyzer (polarizer) was set between the end of a crystal and a
CCD spectrometer [40,41]. Upon irradiating the crystal with a laser (λex = 355 nm) with the energy of 510
kW/cm2 or 70 kW/cm2, the light emitted from the crystal transmitted through the analyzer and was detected
by the CCD spectrometer. As can be seen in Fig. 5, the detected light intensity shows dependence on the
rotation angle of the analyzer. The minimum intensities of emission appear at ca. 30 and 210°, and the
maximum intensities occur at ca. 120 and 300°, which are approximately perpendicular to the direction in
which the maximum emission intensity is detected. Moreover, for both powers of the pumping laser, the
emission intensity can be well fitted by a cosine quadratic (cos2 θ) function, where θ is the angle between the
transmission axes of the analyzer and the crystal. In addition, the polarization contrast calculated with
literature procedures [42] is as high as 0.70. These results suggest that the light emitted from crystal 1 is well
linearly polarized [22].
Finally, the ASE properties of these crystals were explored. A single bar-shaped crystal of each compound
was selected and excited with pulsed laser beams (λex = 355 nm) with different energy, and the emission
spectrum for each energy was collected at one end of the crystal using a CCD spectrometer. For crystal 1,
the emission spectrum displays a broad band with the full width at half maximum (FWHM) of ca. 58 nm
when the laser power is low (57 kW/cm2), as can be seen in Fig. 6a and b. The rapid narrowing of emission
spectra and the nonlinear increase of the emission intensity, caused by the increase of laser power, can be
observed when the laser power is over ca. 143 kW/cm2, suggesting the ASE behavior of crystal 1 with the
energy threshold of ca. 143 kW/cm2. The narrowest spectrum with the FWHM of ca. 5.9 nm is achieved at
792 kW/cm2. Crystals 2–4 also display the ASE property, with the energy thresholds of ca. 198, 72 and 98
kW/cm2 and the smallest FWHM of ca. 6.1, 8.6 and 6.1 nm, respectively (Figs. S5–S7 in Supporting
information). Notably, crystals 1 (λem: ca. 615 nm) and 4 (λem: ca. 613 nm) display the red ASE. Moreover,
they possess the energy thresholds comparable to those of the reported organic crystals [43]. The gain
coefficient of crystal 1 was measured using a successively varied pump stripe method. Fig. 6c shows the curve
of emission-spectra intensities (at 615 nm) versus pump stripe lengths at three different intensities of the
pulsed laser. Upon enlarging the pump strip, there is a nonlinear increase in the emission intensity, consistent
with a higher gain at a higher laser intensity for ASE [44‒47]. As shown in Fig. 6d, the net gain coefficients
at 615 nm are 48 cm−1 at a pump intensity of 548 kW/cm2 and up to 62 cm−1 at a pump intensity of 668
kW/cm2.
In summary, a series of structurally simple 2,5-diaminoterephthalates were readily synthesized, and
high-quality single crystals of these compounds were prepared through simple solvent-evaporation method.
These compounds possess good thermal properties, and their crystals display bright orange-red emission.
The crystals can realize ASE with appreciable energy thresholds. More importantly, red ASE with good
performance is achieved by the crystals of 1 and 4. Crystal 1 was also selected to explore the waveguide and
polarized emission properties, and the waveguide featured by low optical loss coefficients, as well as the
linear polarization of the light emitted from crystal 1, have been demonstrated. The uniaxially oriented
packing of 1, as observed in the single-crystal structure, could be a reason for the good performance of
crystal 1. The success in adopting structurally simple and easily synthesized organic compounds to realize
low-energy emissive organic crystals with high material performance is of guiding significance for the
further development and practical application of organic optoelectronic materials.
Acknowledgment
This work was supported by the National Natural Science Foundation of China (Nos. 51622304, 51603082)
and Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences (No. 201631).

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Fig. 1. Molecular structures of 1–4 (a) and the photographs of crystal 1 under daylight (b, d) and UV irradiation (c, e).
Fig. 2. Single-crystal structures of compound 1: (a) molecular structure of 1 in the asymmetric unit; (b)
intermolecular ππ interactions; and (c) and (d) crystal packing mode.
1.0
1.0
0.8
0.6
0.4
0.2
0.0
a
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
4
1
2
3
4
0.8
0.6
0.4
0.2
0.0
500
550
600
650
700
750
400 450 500 550 600 650 700 750
Wavelength / nm
Wavelength / nm
Fig. 3. (a) Absorption and emission spectra of 1–4 in CH2Cl2 (10‒5 mol/L); (b) Emission spectra of crystals 1–4.
4k
8 mm
7 mm
3k
2k
1k
0
6 mm
5 mm
4 mm
3 mm
2 mm
1 mm
550
600
650
700
750
Wavelength / nm
Fig. 4. (a) Fluorescence microscopy images collected upon excitation of the identical crystal of 1 at five different
positions; (b) Emission spectra measured at the end of a single crystal of 1 with the distance between the end and
the excitation point in the range of 1–8 mm.

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00
510 kW/cm2
300
3300
70 kW/cm2
16.0k
12.0k
8.0k
3000
600
4.0k
0.0 2700
900
4.0k
8.0k
2400
1200
12.0k
16.0k
2100
1500
1800
Fig. 5. Dependence of the intensity of polarized light from crystal 1 on the rotation angle of the polarization analyzer situated
between the end of crystal 1 and the CCD spectrometer. 510 kW/cm2 and 70 kW/cm2 is the energy of the pumping laser
irradiated on the crystal.
70k
c
a
1.0
0.8
0.6
0.4
0.2
0.0
668 kW/cm2
574 kW/cm2
548 kW/cm2
60k
50k
40k
30k
20k
10k
0
60
50
40
30
20
10
0
b
104
103
102
0
1
2
3
4
5
6
7
8
9 10 11
0
100 200 300 400 500 600 700 800
Pump intensity (kW/cm2)
500
550
600
650
700
Pump strip length (mm)
Wavelength (nm)
d
60
50
40
30
20
10
0
668 kW/cm2
574 kW/cm2
548 kW/cm2
600 605 610 615 620 625 630
Wavelength (nm)
Fig. 6. ASE property of crystal 1: (a) emission spectra as a function of the energy of pump laser; (b) dependences of peak
intensity and FWHM of the emission spectra on the energy of pump laser; (c) emission peak intensity as a function of the pump
stripe length at different pump energy; (d) the net gain coefficient as a function of wavelength at different pump energy.