Four organic crystals displaying distinctively different emission colors based on an ESIPT-active organic molecule

Article abstract of DOI:10.1016/j.cclet.2018.08.003

Four crystals A?D based on a structurally simple ESIPT-active molecule 4MPP were obtained by subtly controlling the crystallization conditions. Notably, crystals A and C display single emission bands, which correspond to the keto* (K*) and enol* (E*) emission, respectively. B and D exhibit dual emission with different proportion of E*/K* emissions while D sucessfully achieves white emssion. The distinctively different emission properties of A?D is mainly because of the change in crystal structures. In addition, A displays amplified spontaneous emission, which indicates its potential as single crystal lasers.

Full text of DOI:10.1016/j.cclet.2018.08.003

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CCLET 4624 No. of Pages 4
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Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Four organic crystals displaying distinctively different emission colors
based on an ESIPT-active organic molecule
a,
Hao Liua, Xiao Chenga,b, Zhengyi Biana, Kaiqi Yea, , Hongyu Zhang
*
*
a
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China
MoE Key Laboratory of Macromolecule Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou
b
310027, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Four crystals A–D based on a structurally simple ESIPT-active molecule 4MPP were obtained by subtly
controlling the crystallization conditions. Notably, crystals A and C display single emission bands, which
correspond to the keto* (K*) and enol* (E*) emission, respectively. B and D exhibit dual emission with
different proportion of E*/K* emissions while D sucessfully achieves white emssion. The distinctively
different emission properties of A–D is mainly because of the change in crystal structures. In addition, A
displays amplified spontaneous emission, which indicates its potential as single crystal lasers.
© 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Received 11 July 2018
Received in revised form 2 August 2018
Accepted 6 August 2018
Available online xxx
Keywords:
Polymorph
ESIPT
Luminescent materials
Fluorescent single crystal
Amplified spontaneous emission
Organic fluorescent materials in the solid state have been
attracting increasing attention because of their widely applications
in many fields such as organic light-emitting diodes, bio-imaging,
optical communications, laser devices and so on [1]. As key factors
to evaluate the designed materials, the fluorescence color and
efficiency are mainly focused on. Generally, the ideal materials
could be achieved by molecular structure modification [2,3]. For
emission maxima difference (Dl) less than 100 nm by modulating
the molecular packing structure in crystalline state. That is to say,
the different packing structures merely fine-tune the fluorescence
color and usually is not an efficient regulatory factor to obtain
multi-colored fluorescence in obvious contrast. The fundamental
cause of the small Dl color change lies in that the emission region
of most molecules mainly depends on the intrinsic molecular
structures instead of the packing modes.
example, lowering the
p
-conjugation benefits the blue emission
skeleton
and the quantum yield. In the contrary, enlarging the
p
To solve this problem and achieve multi-colored fluorescence in
sharp contrast based on a single molecule in solid state, excited-
state intramolecular proton transfer (ESIPT)-active molecules
which likely show dual emission may be chosen. As shown in
Fig. 1a, an intrinsic four-level photocycle (E→E*→K*→K→E) is
involved in the ESIPT process. On absorption of light, a fast
tautomerization process occurs, leading to either the sole emission
of the tautomer or a dual emission if there exists a balance between
the E* and K* states. For instance, chemical modification of
structurally simple 7-hydroxy-1-indanone successfully balances
the E* and K*emission and achieves pure white light emission, as
reported by Chou and his colleagues [7]. Different with general
emitters, the ESIPT-active molecules are likely more sensitive to
the molecular conformation or packing modes [8]. This is probably
because the proton transfer rate or the K* energy level is easily
affected by small changes in molecular conformation or intermo-
lecular interactions. As a consequence, the balance between the
local emission band and the long-wavelength K* emission band of
ESIPT molecules in solid state may be easily tuned by preparation
has adverse effect on the efficiency but is good for the long-
wavelength fluorescence. Consequently, highly efficient red
emissive organic material is relatively rare because of the
contradiction between the long-wavelength and the high efficien-
cy. Much effort have been made to resolve this problem [4]. For
example, designing three-dimensional molecular structures or
modulating the molecules with non-planar arms can effectively
eliminate the
p-p interactions and then facilitate the target
molecule highly efficient in solid state [5]. On the other hand, in
recent years, simply changing the molecular conformation or
molecular packing structures has been attracting considerable
interest in the field of modulating the solid state fluorescence
which is demonstrated by a lot of work successively [6]. However,
the fluorescence color is usually tuned on a small scale with
* Corresponding authors.
E-mail addresses: yekq@jlu.edu.cn (K. Ye), hongyuzhang@jlu.edu.cn (H. Zhang).
https://doi.org/10.1016/j.cclet.2018.08.003
1001-8417/© 2018 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: H. Liu, et al., Four organic crystals displaying distinctively different emission colors based on an ESIPT-active
organic molecule, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.08.003

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relationship between molecular conformation/packing mode and
the emission behavior of ESIPT molecule.
Compound 4MPP was synthesized according to our previous
method [9]. A solution of 1,3-diaryl β-diketones (10 mmol) in 20 mL
tetrahydrofuran (THF) was heated to 60 ℃. After addition of
hydrazine hydrate (100 mmol), the mixture was heated at 60 ℃ for
another 12 h. The solution was evaporated and dried in vacuum to
get the crude product as canary yellow solid. Vacuum sublimation
in 210 ℃ of the crude product gave rise to pure green emissive
solid. Different crystals are prepared as following:
Crystal A: In a 20 mL test tube, 5 mg 4MPP was dissolved in 5 mL
CHCl3 or THF, then 10 mL methanol was added without destroying
the surface of the CHCl3/THF solution. A few days later, upon
evaporation at room temperature, sheet like crystals which were
marked as crystal A precipitated out from the concentrated
solution.
Crystal B: In a round-bottom flask, a concentrated solution with
4MPP in 20 mL CHCl3 was added 80 mL petroleum ether without
destroying the surface of the CHCl3 solution. After 24 h at 0 ℃, large
amounts of small crystals (crystal B) precipitated at the CHCl3-
petroleum ether interface.
Crystal C: In a 20 mL test tube, 5 mg 4MPP was dissolved in 4 mL
CHCl3, and then 0.5 mL CH3COOH was added dropwise. 6 mL
petroleum ether was added slowly along the side wall of the test
tube. After about 72 h, colorless crystals (crystal C) were found in
the side wall of the test tube.
Fig. 1. (a) Four level ESIPT process and the keto–enol tautomerism, (b) the excited
state of 4MPP and (c) polymorphs of 4MPP displaying E*, K* and E*& K* emissions.
condition such as solvent polarity, temperature, pH level.
Generally, K* emission of the ESIPT molecules displays large
Stokes’ shifts, many of which are even over 10,000 cmÀ1. Thus the
Dl between E* and K* emission bands is usually very large.
Modulating the ratio of the two emission bands in ESIPT-active
molecules, single-molecule based multi-colored fluorescence with
large Dl in solid state may be available. Moreover, the relationship
between the emission behavior and molecular conformation/
packing structures of ESIPT-active molecules is of great importance
on disclosing the reasons that may affect the proton transfer rate.
Indeed, polymorph-dependent multi-colored fluorescence based
on a single ESIPT-active molecule has been achieved by Araki and
co-workers [[8]8a]. However, all the polymorphs display pure K*
emission without a very large Dl. In our previous work recently,
the E*& K* white emissive crystal and the pure E* blue emissive
crystal based on a single ESIPT-active molecule have been obtained
[9]. Nevertheless, multi-colored fluorescence covering pure K*
emission, E*& K* emission and pure E* emission, which inevitably
is more colorful, significant and has a larger Dl, is still an
unopened field.
Crystal D: In the preparation process of crystal B, very small
amounts of D was obtained.
4MPP is well soluble in organic solvent such as CH2Cl2, CHCl3,
THF or dimethylsulfoxide (DMSO). As shown in Fig. 2a, 4MPP
displays an intense absorption band peaking at 313 nm in CHCl3.
4MPP is weekly emissive in CHCl3 and its emission spectra is
featured as a dual emission band which peaking at 355 nm and
452 nm respectively. The weaker emission band corresponds to the
E* emission, and the bathochromic-shifted emission peak of
452 nm can be assigned to the K* emission due to the large stokes
shifts. The spin-coating film of 4MPP in DMSO or CHCl3 is non-
luminous, indicating that the fluorescence quenches in disordered
aggregated state. Crystals A–D of the compound show different
emission colors (Fig. 2b), the main reason is the difference of K*
state. As shown in Fig. S1 (Supporting information), the maximum
absorption bands of crystals A–D are in the range of 350–370 nm.
Compare with stokes shift of solution, the emission behaviors of
crystal A–D could be assigned to the various K* state. Crystal A
displays a single green color emission band peaking at 530 nm,
which can be ascribed to an absolute K* level emission. This also
demonstrates that the inverse intramolecular proton transfer rate
k(K*→E*) is far behind the positive proton transfer rate k(E*→K*)
when molecules in crystal A excited by UV light. The fluorescence
quantum yield of crystal A is 0.26. Crystal B shows typical dual
emission peaking at 432 nm and 593 nm which corresponds to
On this consideration, a structurally simple molecule named 4-
methyl-2-(5-[4-dimethylaminophenyl]-lH-pyrazole-3-yl)phenol
(4MPP, Fig. 1b) is employed as
a prototype, showing the
polymorph-dependent multi-colored fluorescence (Fig. 1c) based
on a single ESIPT-active organic small molecule. By different
preparation processes, we prepared four crystals based on this
molecule and three of them have been successfully analyzed by
single-crystal X-ray diffraction. Because of the different K* state,
the emission color of these crystals displays huge differences.
Furthermore, the crystal structures function well in disclosing the
Fig. 2. (a) UV–vis absorption and emission spectra of 4MPP in CHCl3, (b) emission spectra of crystals A–D. Inset: photographs of crystal A–D.
Please cite this article in press as: H. Liu, et al., Four organic crystals displaying distinctively different emission colors based on an ESIPT-active
organic molecule, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.08.003

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3
local emission and K* emission, respectively. The quantum yield of
crystal B is 0.16. Crystal C shows deep blue emission with a sharp
peak at 439 nm and a quantum yield of 0.17. Unlike the two-
dimensional extended sheet-like or slab-like crystals of A or B,
crystals of C display one-dimensional needle-like structure and
some of them can reach a length of 1 cm. Crystal D displays pure
white emission with CIE coordinate of (0.323, 0.293). Its local
emission band is centered at 462 nm with a small shoulder at
435 nm. Its K* emission band is centered at 588 nm. Appropriate
proportion of the two emission bands endows the crystal D with
white light emission.
Crystals A–C can be assigned to single-crystal X-ray diffrac-
tion. Fig. 3 shows the molecular configuration and packing
structures of crystals A–C. The unit cell of crystal A is monoclinic,
space group P21/c, containing one molecule. In the individual
molecules, the OÁ Á ÁN distance between oxygen atom of the
hydroxyl group and its neighboring nitrogen atom of the pyrazole
ring is 2.579 Å, appropriate for forming a hydrogen bond. Strong
intramolecular hydrogen bond OÀÀHÁ Á ÁN bridges the hydroxyl
group and its neighboring nitrogen atom. The molecules in crystal
A display planar molecular conformation with the dihedral angles
of 5.51ꢀ and 8.09ꢀ between the pyrazole ring and adjacent two
phenyl rings. Every two molecules connect with each other by
intermolecular hydrogen bonds (NÀÀHÁ Á ÁO) between the hydroxyl
group and pyrazole ring (Fig. S2 in Supporting information). The
molecules aggregate into infinite molecular chains in an end-to-
end packing mode along crystallography a direction, adverse to
and 2.603 Å, respectively. Every molecule connects with its
neighboring two molecules by NÀÀHÁ Á Á
p
interaction or non-
direction,
classical hydrogen bond. Along crystallography
b
molecules pack into infinite chains in an edge-to-face mode,
with the dihedral angle of about 76.50ꢀ. Every two molecules
assemble into
a slip-packing unit along crystallography a
direction. And molecules display slipped packing structure along
b direction. These packing structures in crystal B effectively
prevent the molecules from
p-p stacking. In crystal C, the unit
cell is orthorhombic, space group P212121, containing one
molecule which connects with an acetic acid molecule. The
nitrogen atom (N1) close to the p-(dimethylamino) phenyl group
share one hydrogen atom with the acetic acid molecule by an
extremely strong intermolecular hydrogen bond. The previous
hydrogen atom on this nitrogen atom resonates to the other
nitrogen atom (N2) which is close to the hydroxyl group.
Consequently, intramolecular hydrogen bond between the
hydroxyl group and N2 cannot form any more. That is to say,
ESIPT process would not happen in crystal C. Molecules in crystal
C display planar conformation with the dihedral angles between
pyrazole ring and two phenyl rings of 6.86ꢀ and 3.67ꢀ. Taking the
pyrazole molecule and the connected acetic acid molecule as a
whole, it is easy to found that, each pair of molecules connect
with its neighboring three molecular pairs by CÀÀHÁ Á Á
p inter-
actions and intermolecular hydrogen bond. Along crystallography
a and b direction, molecules in crystal C display slip-packing
modes with different slope and spacing (Fig. S3 in Supporting
information). Along crystallography c direction, molecules display
the
yield. In the three-dimensional packing structure of crystal A, no
interaction exists. The planar molecular conformation and
the tightly packing structure without -overlap, together with
the strong intramolecular hydrogen bond, endow the crystals
with bright green emission. The unit cell of crystal is
p-p stacking and thus beneficial to the fluorescence quantum
an interesting helical packing structure. No
p-p interaction is
p
-
p
observed in the crystal packing structure of C. Crystal D is
obtained in very small amount and the crystal quality is not high
enough to be subjected to the single-crystal X-ray diffraction.
Instead, single crystal powder X-ray diffraction was measured for
D. As shown in Fig. 4, crystal D displays different diffraction
signals which also differ from those simulated signals of single
crystals A–C, demonstrating that crystal D must have different
new packing structures. This also explains why they show
different emission behaviors compared with crystals A–C.
p
B
monoclinic, space group P21/n, containing two molecules. The
dihedral angles between the pyrazole ring and the two phenyl
ring of the two molecules are (22.01ꢀ, 7.11ꢀ) and (7.03ꢀ, 8.97ꢀ),
respectively. The NÁ Á ÁO distances, which relate to how easy (or
difficult) it is to form intramolecular hydrogen bond, are 2.663 Å
Fig. 3. Molecular structures and conformations of (a) crystal A, (d) crystal B and (g) crystal C with thermal ellipsoids at 50% probability, and the packing structures of (b) (c)
crystal A, (e) (f) crystal B and (h) (i) crystal C (H-atom are omitted for clarity).
Please cite this article in press as: H. Liu, et al., Four organic crystals displaying distinctively different emission colors based on an ESIPT-active
organic molecule, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.08.003

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gain coefficient is measured by using a slit in front of the laser
beam. As shown in Fig. S4 (Supporting information), there is an
exponential growth in the emission intensity on increasing the
width of the slit. And the maximum gain coefficient value is
46.57 cmÀ1
In summary,
.
a
structurally simple 4MPP molecule is
synthesized as a prototype, showing the multi-color fluores-
cence over large-area tunable emission region. Changing the
preparation condition, crystals A–D which show different-
proportion characteristic emission bands are obtained. These
crystals display distinctively different emission which covers deep
blue, bright green, white and pink white region. The crystals
structures clearly give evidence that the fluorescence of ESIPT-
active molecules is very sensitive to the changes in crystal
structure and thus are changeful and easily tunable. Furthermore,
the crystal structures of the multiple emissive crystals would play
important roles in disclosing the relationship between molecular
conformation/packing mode and the proton transfer rate in ESIPT
molecules
Fig. 4. Powder X-ray diffraction patterns of crystals A–D.
Acknowledgment
This work was supported by the National Natural Science
Foundation of China (Nos. 51622304 and 51773077).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.cclet.2018.08.003.
Fig. 5. PL spectra as a function of the pump laser energy (inset: dependences of
emission peak intensity and FWHM of emission spectra).
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Please cite this article in press as: H. Liu, et al., Four organic crystals displaying distinctively different emission colors based on an ESIPT-active
organic molecule, Chin. Chem. Lett. (2018), https://doi.org/10.1016/j.cclet.2018.08.003