Ultralong Phosphorescence from Organic Ionic Crystals under Ambient Conditions

Article abstract of DOI:10.1002/anie.201710017

A new type of materials, organic salts in the crystal state, have ultralong organic phosphorescence (UOP) under ambient conditions. The change of cations (NH₄⁺, Na⁺, or K⁺) in these phosphors gives access to tun

Full text of DOI:10.1002/anie.201710017

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Accepted Article
Title: Ultralong Phosphorescence from Organic Ionic Crystals under
Ambient Conditions
Authors: Zhichao Cheng, Huifang Shi, Huili Ma, Lifang Bian, Qi Wu,
Long Gu, Shuzhi Cai, Xuan Wang, Wei-wei Xiong, Zhongfu
An, and Wei Huang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201710017
Angew. Chem. 10.1002/ange.201710017
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10.1002/anie.201710017
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Ultralong Phosphorescence from Organic Ionic Crystals under
Ambient Conditions
Zhichao Cheng, Huifang Shi, Huili Ma, Lifang Bian, Qi Wu, Long Gu, Suzhi Cai, Xuan Wang, Wei-wei
Xiong, Zhongfu An* and Wei Huang*
Abstract: We report on a new type of material system of organic salts
in crystal state with ultralong organic phosphorescence (UOP) under
+
ambient conditions. The change of cations (NH4 , Na+ or K+) in these
phosphors gives access to tunable UOP colors ranging from sky blue
to yellow green, along with ultralong emission lifetimes of over 504 ms.
Single-crystal analysis reveals that unique ionic bonding can promote
an ordered arrangement of organic salts in crystal state, which then
can facilitate molecular aggregation for UOP generation. Additionally,
Figure 1. Schematic representation of rigid material architectures for ultralong
reversible ultralong phosphorescence can be realized through the
organic phosphorescence. a) Molecular crystal, b) Metal-organic framework
alternative employment of fuming gases (ammonia and hydrogen
(MOF), and c) Organic ionic crystal as presented in this work.
chloride), demonstrating its potential as a candidate for visual
ammonic or hydrogen chloride gas sensing. The results provide an
excitons, from thermal, oxygen and moisture perturbation, as well
environmental responsible and practicable synthetic approach to
as molecular motions[9]. To date, it remains a formidable
expanding the scope of ultralong organic phosphorescent materials
challenge to develop a new material system for ultralong organic
as well as their applications.
phosphorescence under ambient conditions.
Here we report on rigid ionic crystals with UOP via unique ionic
bonding based on a new class of ionic compounds: organic salts.
The type of ionic crystals can be easily prepared with low cost and
under moderate and environmental-friendly conditions. With
cations changing from NH4+ to K+, the emission color of UOP was
expressly tuned from yellow green to sky blue. Also the lifetime of
ultralong phosphorescence simultaneously changed from 504 to
586 ms. Importantly, we found that reversible and repeatable
ultralong phosphorescence could be obtained with ammonia and
hydrogen chloride gas alternative fuming, making them ideal
candidates as visible gas sensors.
Ultralong phosphorescence, i.e., persistent luminescence or
afterglow emission, has drawn forth extensive attention due to its
extraordinarily long emission lifetime within a wide range of
potential applications, such as display, document security, and
bioimaging, as well as emergency signage[1]. Most known
luminogens with ultralong phosphorescence are limited to
inorganic materials with noble-metal or rare-earth elements[2].
However, their intrinsic drawbacks, such as high bio-toxicity,
harsh preparation conditions, scarcity of rare-metal resources,
and insulation nature set limits on further development of
inorganic phosphors, thus giving birth to a new class of analogues
based on organics, i.e., ultralong organic phosphorescence
(UOP)[3]. Very recently, significant progress in organic luminogens
with UOP has been achieved in which UOP materials were
designed and prepared through crystallization[4], host-guest
doping[5], the construction of metal-organic frameworks (MOFs)[6],
H-aggregation[7], and other methods[8]. Notably, among these
phosphors, the material architectures are mainly restricted on
metal-free molecular crystals with rich intermolecular interactions
and MOFs with coordinate bonds, as shown in Figure 1a and 1b.
Such architectures can provide rigid environments for enhanced
phosphorescence by suppressing non-radiative decay of triplet
The rigid crystals based on ionic compounds were incubated
gradually in aqueous solution. Terephthalic acid (TPA) is a weak
acid, whose solubility in water is only 1.5×10-5 g/mL at 20°C[10]
.
But, as an organic weak acid, TPA can easily dissolve in ammonia
solution due to its ionization process. First, TPA powder was
added to ammonia water to form a transparent solution. Then, the
water was evaporated slowly at 50°C to render ammonium
hydrogen terephthalate (AHT) as block transparent crystals with
a yield of over 50%. The molecular structure of AHT was verified
by single crystal X-ray diffraction. The phase purity of the
synthesized crystals was further validated by powder X-ray
diffraction (PXRD) (Figure S1a).
Subsequently, the photophysical properties of AHT in crystal
under ambient conditions were investigated. Under irradiation by
a hand-held 365 nm lamp blue-violet emission can be observed.
Unexpectedly, upon removal of the excitation light source, the
emission changed to green and faded gradually with a remarkably
persistent luminescence of several seconds (Figure 2a). As
shown in Figure 2b, the steady-state photoluminescence (PL) of
AHT in crystal showed a red-edge effect[11]. With excitation
wavelength increased from 300 to 400 nm, the emission peak
[*]
Z. Cheng[+], Dr. H. Shi[+], Dr. H. Ma, L. Bian, W. Qi, L. Gu, S. Cai, X,
Wang, Prof. W.-w. Xiong, Prof. Z. An, Prof. W. Huang
Key Laboratory of Flexible Electronics (KLOFE) & Institute of
Advanced Materials (IAM)
Nanjing Tech University (NanjingTech), 30 South Puzhu Road,
Nanjing 211800, China
E-mail: iamzfan@njtech.edu.cn; iamwhuang@njtech.edu.cn
Prof. W. Huang
Shaanxi Institute of Flexible Electronics (SIFE)
Northwestern Polytechnical University (NPU), 127 West Youyi
Road, Xi'an 710072, China
[+]
These authors contributed equally to this work.
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Figure 3. Intermolecular interaction analysis of AHT in crystal. a) Illustration of
torsion angle between carboxyl group and benzene ring. b) Molecular packing
in dimer. The distance between intermolecular planes or centers is highlighted
in blue. c) Intermolecular interactions viewed along b axis. d) Intermolecular
Figure 2. Photophysical properties of AHT in crystal under ambient conditions.
a) Chemical structure and photographs of AHT crystal before and after
irradiation with a hand-held 365 nm UV lamp. b) Normalized photoluminescence
(PL) spectra of AHT under different excitation wavelengths. c) Excitation-
phosphorescence mapping of AHT crystal. d) Time-resolved phosphorescence
decay curve of the emission band at 502 nm excited at 328 nm.
stacking viewed along
c axis. With the ammonium cation inducement,
terephthalate anions show well-aligned intermolecular stacking through face–
to-face interaction.
which may facilitate intersystem crossing (ISC) for triplet exciton
generation[14]. With amnonia ion inducement the benzene
chromorphores show a face-to-face stacking with a distance of
3.510 Å. Figure 3b demonstrates that the angle (θ=80.02o)
between the interconnected axis and the long axis in the
terephthalic acid group is larger than the critical value of 54.7o for
distinction of H- and J-aggregation[15], proving the presence of H-
aggregation in crystal[16]. Apart from intermolecular interactions,
including C=O···H-O, C=O···H-N, C-H···π, etc. in crystal (Figure
3c), ionic bonding between ammonium cations and terephthalate
anions also play an important role in locking the terephthalate
anion conformation and restricting the molecular motions and
vibrations, thus reducing the non-radiative transition of triplet
excitons to boost ultralong phosphorescence at room temperature.
In view of the above-mentioned results and H-aggregation‘s
increasing luminescence lifetimes[17], we speculate that H-
aggregation with strong coupling may stabilize the triplet excitons
for ultralong phosphorescence[7].
showed a continuous red-shift from 370 to 460 nm, which were
confirmed to be fluorescence accroding to the lifetime
measurement (Figure S2a). In light of these photophysical
properties, combined with the excitation spectra shown in Figure
S3, it is speculated that multiple excited states may exist for this
variable feature in PL spectra[12]. According to the literatures[13]
,
there may exist proton transfer between terephthalate anions and
ammonium cations, resulting in more than two molecular
configurations in crystal state due to protonation. Combining with
theoretical calculation shown in Figure S4, we speculated that
multiple excited states might result from the different degrees of
luminophore protonation in crystal upon photo-excitation.
Whereas, the profiles of phosphorescence spectra were fixed with
a dominant peak at 502 nm as excitation wavelengths changed
from 250 to 450 nm (Figure 2c). The time-resolved
phosphorescence decay curve of AHT crystal revealed a
luminescent lifetime of 586 ms under amibient conditions,
demonstrating the UOP feature (Figure. 2d). The time-resolved
emission spectra (TRES) were also conducted for AHT. As shown
in Figure S5a, the emission of AHT crystal at short wavelengths
first disappeared with time delays. The profiles of the emission
spectra at long wavelengths are similar at different delay times.
Addtionally, it is worthy to note that this unique UOP was stable
even under oxygen atmosphere (Figure S6a).
Notably, as shown in Figure 3d, an ordered face-to-face
arrangement between terephthalic acid chromorphores was
induced by ammonium cations, which may promote UOP
generation. Inspired by this concept, we further synthesized two
organic salts, namely, dipotassium terephthalate (DPT) and
disodium terephthalate (DST), by replacing ammonium cations
with potassium and sodium cations, respectively. Both DPT and
DST crystals showed UOP at 456 and 508 nm, respectively,as
shown in Figure 4a and 4c, with the UOP color changed from sky
blue to yellow green (Figure 4b and 4d). With excitation
wavelength variation, no UOP change was observed. The UOP
lifetimes of DPT and DST crystals reach 504 and 585 ms under
ambient conditions, respectively. Like AHT crystal, UOP of both
DPT and DST crystals were also little affected by oxygen (Figure
S6b and S6c). In order to investigate the sourse of their UOP
In order to gain insights into the molecular mechanism of
ultralong phosphorescence in AHT crystal, the intermolecular
interactions and stackings were investigated through single-
crystal annalysis, as shown in Figure 3. In AHT crystal there exists
a torsion angle of 9.56o between carbonyl group and benzene ring,
which is larger than that in terephthalic acid crystal (5.37o) and
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Figure 4. Phosphorescence properties and crystal structures of DPT and DST crystals under ambient conditions. a) and c) Excitation-phosphorescence mapping
of DPT and DST, respectively. b) and d) Lifetime decay profiles of the emission bands at 456 and 508 nm for DPT and DST crystals, respectively. e), f), g), h), i)
and j) Illustration of torsion angle between carboxyl group and benzene ring; molecular stacking diagrams, and crystal structures viewed along the b axis in DPT
and DST crystals, respectively.
coupled with emission color change, their single-crystal structures
were analyzed, along with theoretical simulation. As anticipated,
both DPT and DST exhibited well-organized arrangements in their
crystal state, as shown in Figure 4e-4j, owing to the ionic
inducement (K+ and Na+). In addition to the face-to-face molecular
packing in these organic ionic crystals, the intermolecular π-π
stacking distance decreases from 3.571 Å (DPT) to 3.268 Å
(DST), which is consistent with the increasing tendency of their
UOP lifetimes. The shorter π-π stacking distance may lead to
more stable H-aggregation for longer UOP emission. To gain
insight into the change of UOP color, the lowest triplet states of
these chromophores were evaluated through time-dependent
density functional theory (TD-DFT) method at M062X/cc-pVDZ
level. Because of the ion inducement, the geometric structures of
terephthalic chromophore units in crystal show significant
distortion, with torsion angles changing from 13.20o (DST) to 6.62o
(DPT) (Figure 4e and 4h). As shown in Table S4 and S5, the
variation of triplet excitation energies from DST to DPT model
Figure 5. Structural characterization and phosphorescence spectra of TPA powders before and after exposure to fumigants under ambient conditions. a) PXRD
patterns of a series of TPA phosphors fumed with ammonia or hydrogen chloride gas and with different exposure times. Inset: Corresponding photographs of the
samples taken after the irradiation source is removed. The X-ray diffraction peaks with stars are the characteristic peaks of ammonium chloride. b) The corresponding
phosphorescence spectra of the same products used in PXRD measurement.
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agree well with the blue-shift tendency of UOP spectra in crystal.
Therefore, it is speculated that the inducement of cations play a
critical role for colorful UOP in organic ionic crystals.
Keywords: crystal engineering • ultralong phosphorescence •
organic ionic crystal • gas sensing
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In summary,
a
new material sourse of ultralong
phosphorescence, organic ionic crystals, was formulated and
prepared through ionic bonding in water or through gas fuming.
With cationic variations from Na+, NH4+ to K+, the UOP color could
be intentionally tuned from sky blue to yellow green. The emission
lifetimes of these phosphors exceed 504 ms. In light of both
experimental and simulated results, it is proposed that this unique
ionic bonding can promote an ordered molecular arrangement to
influence molecular aggregation, which in turn can enhance
ultralong phosphorescence. More importantly, reversible and
repeatable ultralong phosphorescence was observed through the
alternating application of fuming gases (ammonia and hydrogen
chloride), demonstrating the potential for visual ammonic or
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Natural Science Fund for Colleges and Universities
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Entry for the Table of Contents
COMMUNICATION
Zhichao Cheng, Huifang Shi, Huili
Ma, Lifang Bian, Qi Wu, Long Gu,
Suzhi Cai, Xuan Wang, Wei-wei
Xiong, Zhongfu An*, Wei Huang*
A new type of material system—organic
ionic crystals with ultralong organic
phosphorescence (UOP) was reported.
The change of cations (NH4+, Na+ or K+)
in these phosphors gives access to
tunable UOP colors ranging from sky
blue to yellow green. Additionally,
reversible ultralong phosphorescence
can be realized through the alternative
employment of fuming gases (ammonia
and hydrogen chloride), demonstrating
its potential as a candidate for visual
gas sensing.
Page No. – Page No.
Ultralong Phosphorescence
from Organic Ionic Crystals
under Ambient Conditions
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