Anion-regulated transient and persistent phosphorescence and size-dependent ultralong afterglow of organic ionic crystals

Article abstract of DOI:10.1039/c9tc04951g

It is a formidable challenge to achieve highly efficient organic afterglow and readily tunable transient and persistent phosphorescence of organic materials because of the lack of valid design principles. Herein, we report the regulation of the transient and persistent room-temperature phosphorescence of organic ionic crystals by the alteration of anions, and the unique size-dependent ultralong afterglow of tetraphenylphosphonium perchlorate (TPP ClO₄) crystals under ambient conditions. Three organic ionic crystals show sharply distinct triplet emissions based on three heavy-atom free anions by proper construction of donor-acceptor patterns in the crystals. TPP ClO₄ crystals exhibit ultrabright transient phosphorescence and ultralong afterglow, and the gradual enhancement of the afterglow with the crystal size directly proves the generation of ultralong afterglow from aggregates with strong intermolecular interactions in the crystals. These results greatly contribute to the general design principles of highly efficient transient and persistent phosphorescence and their regulations by rational design. Highly sensitive detection and imaging of perchlorate ions are achieved in a phosphorescence turn-on manner based on the bright room-temperature phosphorescence of TPP ClO₄ crystals, and valuable anti-counterfeiting applications of the ultralong afterglow of TPP ClO₄ crystals are also demonstrated in practical examples.

Full text of DOI:10.1039/c9tc04951g

Showcasing research from Key Laboratory of the
Ministry of Education for Advanced Catalysis Materials,
College of Chemistry and Life Sciences,
As featured in:
Volume
7 Number 46 14 December 2019 Pages 14399–14754
Zhejiang Normal University, Jinhua 321004, China
Journal of
Materials Chemistry C
Materials for optical, magnetic and electronic devices
rsc.li/materials-c
Anion-regulated transient and persistent phosphorescence and
size-dependent ultralong afterglow of organic ionic crystals
Anion-regulating transient and persistent phosphorescence and
size-dependent ultralong afterglow of organic ionic crystals are
demonstrated by alteration of anion and crystal size.
ISSN 2050-7526
PAPER
Runfeng Chen, Guohua Xie, Wei Huang et al.
Thermally activated delayed fluorescence enantiomers
for solution-processed circularly polarized
electroluminescence
See Hui Feng, Zhaosheng Qian et al.,
J. Mater. Chem. C, 2019, 7, 14535.
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PAPER
Anion-regulated transient and persistent
phosphorescence and size-dependent ultralong
afterglow of organic ionic crystals†
Cite this: J. Mater. Chem. C, 2019,
, 14535
7
a
a
a
b
Guilin Chen, Sidan Guo, Hui Feng * and Zhaosheng Qian
*
It is a formidable challenge to achieve highly efficient organic afterglow and readily tunable transient
and persistent phosphorescence of organic materials because of the lack of valid design principles.
Herein, we report the regulation of the transient and persistent room-temperature phosphorescence of
organic ionic crystals by the alteration of anions, and the unique size-dependent ultralong afterglow of
tetraphenylphosphonium perchlorate (TPP ClO
crystals show sharply distinct triplet emissions based on three heavy-atom free anions by proper
construction of donor–acceptor patterns in the crystals. TPP ClO crystals exhibit ultrabright transient
4
) crystals under ambient conditions. Three organic ionic
4
phosphorescence and ultralong afterglow, and the gradual enhancement of the afterglow with the crystal
size directly proves the generation of ultralong afterglow from aggregates with strong intermolecular inter-
actions in the crystals. These results greatly contribute to the general design principles of highly efficient
transient and persistent phosphorescence and their regulations by rational design. Highly sensitive detection
and imaging of perchlorate ions are achieved in a phosphorescence turn-on manner based on the bright
Received 7th September 2019,
Accepted 27th September 2019
DOI: 10.1039/c9tc04951g
4
room-temperature phosphorescence of TPP ClO crystals, and valuable anti-counterfeiting applications of
rsc.li/materials-c
the ultralong afterglow of TPP ClO
crystals are also demonstrated in practical examples.
4
6
intramolecular vibration and rotation. However, to achieve
highly efficient room-temperature phosphorescence, another
Introduction
Organic materials that exhibit room-temperature phosphorescence obstacle regulated by the general principle that electronic
have been emerging as a promising alternative to long-lived transitions from excited singlet states to triplet states are
phosphorescent inorganic and organometallic counterparts widely forbidden should be properly handled. There are two dominant
1
applied in energy and life sciences. Although great advances ways to promote intersystem crossing processes between singlet
in discovering and designing organic molecules with room- and triplet states currently. One of them is to introduce heavy
2
temperature phosphorescence have been made recently, it is atoms like bromine into luminogens to enhance the promotion
still a formidable challenge to achieve efficient organic phos- from singlet to triplet states due to the spin–orbit coupling effect
7
phorescence because of the following two major obstacles. of the heavy atoms, while the other means is to boost the
The inherent long-lived emission of phosphorescence makes population of triplet excitons by tuning the energy level via the
the generated triplet excitons to be readily consumed by the introduction of particular functional groups like carbonyl and
8
surroundings like oxygen. To overcome this problem, diverse nitro groups or mixing appropriate electron donor and electron
3
4
9
approaches, such as polymer embedding, crystallization and acceptor pairs. In comparison with the former approach relying
5
host–guest capsulation, have been proposed to effectively on the heavy atom effect, rational design of luminogens of donor–
protect triplet excitons by constraining molecular motion and acceptor type is more promising to achieve diverse and highly
phosphorescent organic materials from the few examples
a
with extraordinary emission efficiency (40.4) reported in the
literature.
College of Chemistry and Life Sciences, Zhejiang Normal University, Yingbin Road
88, Jinhua 321004, People’s Republic of China. E-mail: fenghui@zjnu.cn
4
a,8a,10
6
b
Key Laboratory of the Ministry of Education for Advanced Catalysis Materials,
Zhejiang Normal University, Yingbin Road 688, Jinhua 321004,
Persistent phosphorescence referring to the emission that
can last for more than several seconds after ceasing the excitation
frequently takes place along with transient room-temperature phos-
People’s Republic of China. E-mail: qianzhaosheng@zjnu.cn
†
Electronic supplementary information (ESI) available: Computational methods,
11
phorescence on the microsecond timescale. Such a long-lived
afterglow has indispensible advantages in time-gated luminescence
imaging and anti-counterfeiting areas relative to transient
NMR spectra, NBO analysis, time-resolved and time-delayed PL spectra, and
Cartesian coordinates. CCDC 1880252 and 1880253. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9tc04951g
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phosphorescence due to its time-dimensional resolution by important to discover the unique size-dependent behaviour of the
1
2
4
the naked eye. A variety of organic crystals with ultralong ultralong afterglow of TPP ClO crystals, and such direct and solid
afterglow have been discovered and designed, but its exact proof clearly substantiates the aggregate-based photophysical
generation mechanism is still an open question because of properties for ultralong afterglow. The ultrahigh quantum yield
the lack of direct and solid evidence. Recent studies have of the transient blue phosphorescence of TPP ClO microcrystals
4
revealed that the generation of persistent phosphorescence is allows us to establish an ultrasensitive detection method for
1
3
probably associated with the formation of H-aggregates and perchlorate ions in aqueous solution and on solid-substrates,
1
4
strong intermolecular interaction or electronic coupling in and a further in vivo imaging study of seedlings of Arabidopsis
the crystals. However, there are only very few examples that thaliana validates its practicability in complex matrices. The
4
exhibit both ultra-bright and ultralong afterglow because of the distinguishable afterglow of TPP ClO by the naked eye is demon-
unclear generation mechanism and design principles of strated in the time-dimensional anti-counterfeiting of a school
1
5
afterglow. Our recent study showed that significant inter- badge and a banknote.
molecular electronic coupling among molecules in the crystal
causes the occurrence of persistent phosphorescence, and the
introduction of heavy bromine atoms is capable of appreciably Results and discussion
enhancing the emission efficiency, but its lifetime is largely
Synthesis and characterization of tetraphenylphosphonium
1
6
shortened to a transient phosphorescence.
Therefore, a
tetrafluoroborate (TPP BF ) and tetraphenylphosphonium
4
further exploration of the influence of crystal engineering and
structural design on organic afterglow will greatly contribute to
the general principles of achieving highly efficient ultralong
4
perchlorate (TPP ClO )
Tetraphenylphosphonium tetrafluoroborate (TPP BF ) and tetra-
4
1
7
phenylphosphonium perchlorate (TPP ClO ) were synthesized
4
phosphorescence in organic materials.
by precipitating reactions of tetraphenylphosphonium chloride
In contrast to ordinary organic crystals consisting of neutral
molecules, organic ionic crystals always retain their ordered
crystal packing and can exhibit bright room-temperature phos-
(
TPP Cl) with NaBF
4 4 4
and NaClO , respectively. TPP Cl, TPP BF
and TPP ClO were purified by multiple recrystallization, and fully
4
1
13
31
19
1
8
characterized using H NMR, C NMR, P NMR, F NMR and
high-resolution mass spectrometry. The purities of these crystals
were further verified using high-performance liquid chromato-
graphy. The HPLC results in Fig. S1 (ESI†) show that the
purities of TPP Cl, TPP BF4 and TPP ClO4 were satisfactory
phorescence through rational design, and thus are a class of
desirable platforms to investigate the generation mechanism of
persistent phosphorescence. In this work, we report anion-
regulated transient and persistent phosphorescence based on
tetraphenylphosphonium (TPP) cations and unique size-dependent
(499.0%), excluding the possibility of intense luminescence
4
afterglow of tetraphenylphosphonium perchlorate (TPP ClO )
from impurities included in the crystals. Their structures were
determined using single-crystal X-ray diffraction and are dis-
played in Fig. S2 (ESI†), which gave their absolute structural
configurations and ionic arrangements for theoretical analysis.
crystals. As shown in Scheme 1, three TPP crystals show sharply
distinct luminescence behaviours relying on the diversity of anions,
and only TPP ClO crystals exhibit highly efficient transient blue
4
phosphorescence and bright green afterglow with a long emission
time of 4 s. The underlying mechanisms for the enhancement of
both transient and persistent phosphorescence are investigated by
crystal structure analysis and theoretical computations. It is more
Distinct photophysical behaviours of TPP Cl, TPP BF
TPP ClO crystals
4
and
4
The three tetraphenylphosphonium compounds showed com-
pletely different luminescent behaviours depending on diverse
anions. TPP Cl crystals exhibited a very weak green emission
located at 500 nm with an ultralong lifetime of 253.2 ms, and
their observable afterglow can last for a short period of 1.5 s
1
8b
(Fig. S3, ESI†).
The TPP BF crystals emit intense blue light
4
under UV light irradiation, and the emission maximum is
located at 430 nm (Fig. 1a). After 5.0 ms decay time is applied,
a new weak green emission is detected at 490 nm (Fig. 1b),
4
indicating that TPP BF exhibits an ultralong luminescence like
TPP Cl. As shown in Fig. 1c, the bright blue emission quickly
disappears after ceasing the excitation, and a faint green after-
glow of up to 1.5 s can be observed by the naked eye. The time-
resolved PL decay curves in Fig. S4 (ESI†) show that the lifetime
at 420 nm is 4.5 ms, but the lifetime of one component is as long
as 272.6 ms, indicating its ultralong afterglow of 1.5 s. In
contrast to TPP Cl, TPP BF
emissions including a strong ordinary room-temperature phosphor-
escence and a weak ultralong afterglow lasting for a few seconds.
4
simultaneously exhibits two
Scheme 1 Schematic illustration of anion-regulated transient and persis-
tent phosphorescence (a) and size-dependent afterglow of tetraphenyl-
phosphonium perchlorate crystals (b).
1
4536 | J. Mater. Chem. C, 2019, 7, 14535--14542
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emission (Fig. 2c). It is noted that the emission at 475 nm originated
from two major components, and the lifetime of the longer-emitting
component is nearly 1 s (952.6 ms) from its decay curve (Fig. 2d).
The ultra-high brightness of the blue phosphorescence can be
readily observed by the naked eye when TPP ClO4 crystals are
irradiated with UV light, and the green afterglow also was almost
as intense as the blue emission and persisted for more than 4 s after
ceasing the excitation (Fig. 2e). In comparison with TPP Cl and TPP
BF
afterglow of TPP ClO
efficiencies, and their total quantum yield is as high as
.56 determined by an absolute method (Table S1, ESI†). Their
4
, both the room-temperature phosphorescence and ultralong
4
crystals have appreciably high emission
0
quantum yield for transient phosphorescence is just only
Fig. 1 (a) PL spectra of TPP BF
4
crystals, (b) time-gated PL spectra of TPP
crystals.
4a
inferior to those of 1,4-dibenzoyl-2,5-bis(siloxy)-benzenes (0.64)
4
BF crystals and (c) ultralong afterglow of TPP BF
4
10a
and 1,2,3,4,5,6-hexakis(arylthio)-benzenes (1.0)
without after-
glow, and their efficiency for persistent phosphorescence is the
highest compared to those for previously reported ultralong
4
The phosphorescence quantum yield of TPP BF crystals was
8a
organic phosphorescence emissions (o0.345). It is also the first
example with equivalently bright transient and persistent phos-
phorescence to the best of our knowledge. The aggregation-
induced emission behaviors of TPP BF4 and TPP ClO4 were
further demonstrated using water as a poor solvent as shown in
Fig. S5 (ESI†), indicating that the aggregated or solid state
guarantees the generation of transient phosphorescence for
determined to be as high as 0.48, which is much higher than those
of most reported organic phosphors. It is surprisingly found that
an intense blue room-temperature phosphorescence and a strong
green ultralong afterglow occurred simultaneously for TPP ClO4
crystals. Two separate emissions with equivalent PL intensity located
2
4
at 420 nm and 475 nm were recorded for TPP ClO crystals (Fig. 2a),
and the PL spectra at different decay times from 1.0 to 4.5 s clearly
suggest that an intense and broad emission centered at 500 nm
can last for 4 s after stopping the excitation light (Fig. 2b). The time-
resolved PL decay curve at 420 nm shows that this emission
has a relatively short lifetime of 2.8 ms, and such a lifetime on
the microsecond timescale illustrates that it comes from triplet
4 4
TPP BF and TPP ClO .
Mechanistic investigation on anion-regulated transient and
persistent phosphorescence
Low-temperature experiments were performed with dilute
solutions of TPP Cl, TPP BF and TPP ClO to explore the
4 4
origins of their transient emissions at 420 nm. Fig. S6 (ESI†)
shows that an apparent emission peak is gradually intensified
with the decrease of the temperature from 300 K to 77 K for
each solution, indicating that these emissions around 420 nm
originate from dissolved ion pairs. The consistency in emission
maxima and lifetimes (Fig. S7, ESI†) between the low-
temperature glassy state and the solid state for each of them
suggests that separated monomers mainly contribute to the
transient emissions at around 420 nm in the crystals. The
emission enhancement trend with the decrease of the tempera-
ture excludes the possibility of the emission origin from
thermally activated delayed fluorescence. It is generally known
that the heavy atom effect due to heavy atoms like bromine and
iodine apparently facilitates intersystem crossing processes,
1
9
thus greatly boosting the triplet emission. However, these
three crystals do not contain heavy atoms. As a result, it is
rational to deduce that intermolecular charge transfer during
the excitation process appreciably promotes the intersystem
9
1
crossing rate from the S state to its neighbouring triplet state.
To verify this hypothesis, density functional theory (DFT) and
time-dependent density functional theory (TDDFT) were employed
to analyze their molecular orbitals and electron density change
between ground states and excited states. All the HOMOs and
LUMOs of TPP Cl, TPP BF and TPP ClO can be visualized in
4 4
Fig. 3. It is surprisingly noted that all the HOMOs are mainly
composed of atomic orbitals of the anions, while the p orbitals of
4 4
Fig. 2 (a) PL spectra of TPP ClO crystals, (b) PL spectra of TPP ClO
crystals at different decay times from 1.0 to 4.5 s, (c) time-resolved PL
decay curve at 420 nm, (d) time-resolved PL decay curve at 475 nm, and
4
(e) ultralong afterglow of TPP ClO crystals.
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Fig. 4 Ionic arrangements with labeled distances of TPP BF
the a-axis, b-axis and c-axis (a–c), and ionic arrangements with labeled
distances of TPP ClO viewed along the a-axis, b-axis and c-axis (d–f).
4
viewed along
Fig. 3 HOMOs and LUMOs of TPP Cl (a), TPP BF
monomers, and Jablonski diagrams of TPP Cl (d), TPP BF
(
4
(b) and TPP ClO
(e) and TPP ClO
f) monomers with labeled energy gaps from TDDFT calculations. The
energy gaps between T and S for TPP Cl, TPP BF and TPP ClO
4
4
(c)
4
4
4
1
0
4
determined experimentally were 2.500, 2.884 and 2.938 eV, respectively,
according to their respective PL spectra.
1
4
of pure organic materials, we postulate that appreciable
interionic electronic coupling occurs for TPP ClO , whereas
interionic interaction in TPP BF is relatively weak. To validate
4
4
aromatic carbon atoms in the TPP moiety dominantly contribute this hypothesis, ionic arrangement in a single crystal is
to the LUMOs of TPP Cl, TPP BF and TPP ClO . Such separated analyzed for TPP BF and TPP ClO . It is readily concluded
locations of the HOMOs and LUMOs are an indication of the that the ionic packing diagrams of TPP BF4 and TPP ClO are
4
4
4
4
4
significant charge transfer character for these ionic compounds. very similar from their crystallographic data listed in Table S3
The electron density difference maps in Fig. S8 (ESI†) clearly (ESI†) because both the crystals belong to the same space group
demonstrate appreciable charge transfer trends from anions to I4 and possess very similar lattice parameters. Fig. 4 shows
the TPP moiety for all of them. It is reported that the occurrence of ionic packing diagrams of TPP BF and TPP ClO along the
4 4
charge transfer during excitation is favoured to lower the energy a-axis, b-axis and c-axis, which clearly demonstrates that they
gap between S and its neighbouring triplet state, and thus results have similar ionic packing patterns in the different directions.
1
in the enhancement of the intersystem crossing rate and phos- By viewing the ionic arrangements along the c-axis for TPP BF
4
20
phorescence proportion. Consequently, the energy differences and TPP ClO
4
4ꢁ
as shown in Fig. 4c and f, it is noted that the
ꢁ
between S
1
and its neighbouring triplet state (DEST) for TPP Cl, BF
4
or ClO anions are homogeneously filled in the cavities
TPP BF and TPP ClO
4
4
are calculated using TDDFT and labelled in surrounded by TPP cations, and the distances between the
ꢁ
their Jablonski diagrams in Fig. 3. According to the Fermi golden central atoms of BF
4
and its two neighboring TPP cations for
21
rule, the intersystem crossing rate can be described as
4
TPP BF (6.28 and 8.01 Å) are almost identical to those for TPP
ClO (6.31 and 8.03 Å), suggesting that the sizes of the cavities
surrounded by TPP cations are almost the same for TPP BF4
4
2
p
1
3 2
kISC ¼ hc jHSOjc i ꢀ FCWD
ꢀh
ꢁ
and TPP ClO
4
. However, the thermochemical radius of ClO
4
ꢁ
where the former term in this equation is related to the spin–orbit (2.25 Å) is much larger than that of BF
4
(2.05 Å) according to
ꢁ
coupling effect caused by heavy atoms, and the latter term FCWD ref. 22, which leads to a greater volume of ClO
4
by 33% than
ꢁ
4ꢁ
ꢁ
is inversely proportional to the value of DEST. The much smaller BF
4
. Such a greater space occupation of ClO than BF
4
in
values calculated for TPP BF
that of TPP Cl (0.185 eV) provide direct proof that TPP BF
4
(0.008 eV) and TPP ClO
4
(0.001) than the identical cavity would cause a stronger interaction between
ꢁ
4
and ClO
4
and TPP cations in the crystal, and such a strong ionic
ꢁ
TPP ClO have much greater intersystem crossing rates (kIST) than interaction between ClO4 and TPP cations might greatly
4
that of TPP Cl, which indicates the highly bright room-temperature contribute to the generation of the afterglow of TPP ClO4
phosphorescence of TPP BF and TPP ClO crystals. Lowering crystals. As a result, it is rational to ascribe the ultrabright
4
4
the energy gap between S
1
4
and its neighboring triplet state afterglow of TPP ClO crystals to the combination of the
through construction of donor–acceptor patterns in molecular enhancing intersystem crossing rate and the intensifying inter-
structures can act as a general design strategy to realise heavy- ionic interaction in the crystals by lowering the energy gap DEST
atom free organic phosphors with highly efficient room-temperature and unique crystal arrangement. To further substantiate the
phosphorescence.
ultralong green emission due to the aggregation behaviour of
Differing from TPP BF4 crystals, TPP ClO4 crystals not TPP ClO crystals, the photophysical properties of three TPP
4
only exhibit ultrabright transient phosphorescence under the ClO crystals with different sizes are compared. Large crystals
4
irradiation but also exhibit highly efficient green afterglow after are several millimeters in length, but small crystals obtained by
ceasing the excitation. Since it has been revealed that strong grinding the large crystals are not uniform with diameters
intermolecular electronic coupling with specific molecular greater than 25 ꢂ 10 mm, while microcrystals obtained by fast
arrangement in crystals is responsible for persistent emission precipitation have the smallest size around 1 ꢂ 0.5 mm with uniform
1
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precipitated from water. The high quantum yield of TPP ClO
0.56) enables TPP Cl to act as a phosphorescence turn-on
4
(
molecular probe for perchlorate ions in aqueous solution.
Fig. 6a shows the gradual increase in phosphorescence intensity
ꢁ
when different amounts of ClO4 are added into a TTP Cl solution
4
(100.0 mM), where an increasing amount of TPP ClO micro-
crystals is formed with continuous introduction of perchlorate
ions. A good linear relationship between the PL intensity and
ꢁ
ClO
4
concentration in the range of 3.3–60.0 mM is found
(Fig. S17, ESI†), and the detection limit from three time standard
deviations is estimated to be 0.08 mM, which is comparable to that
23
based on an ionic iridium complex (0.05 ppm). No response is
observed when the other 14 common anions are introduced into
the same TPP Cl solution because these anions do not create
luminescent precipitates under such conditions (Fig. 6b). The
4
long lifetime of TPP ClO microcrystals (3.5 ms) allows us to
Fig. 5 XRD patterns (a), PL spectra (b) and afterglows (c) of large crystals, establish the first time-gated luminescent detection method for
4
small crystals and microcrystals of TPP ClO .
perchlorate ions. Human serum as a typical example with strong
blue autofluorescence is used to evaluate the time-gated lumines-
ꢁ
cent assay for ClO4 . PL spectra of the serum sample are recorded
shape (Fig. S9, ESI†). The almost identical XRD patterns among
these crystals in Fig. 5a indicate that their crystal structures are the
same, but their PL emissions completely change as shown in
Fig. 5b. In comparison with large crystals, the afterglow intensity
at 470 nm of small crystals apparently decreases, and the afterglow
of microcrystals completely disappears. The time-resolved PL decay
curve of small crystals demonstrates that their afterglow lifetime is
also largely shortened to 417.9 ms (Fig. S10, ESI†), and the afterglow
emission almost completely disappears for microcrystals according
to its lifetime change (Fig. S11, ESI†). Fig. 5c also illustrates both the
emission duration and brightness sharply decrease in the order of
large crystals, small crystals and microcrystals. To further acquire
quantitative relation between crystal size and the relative intensity
of the afterglow, additional three crystals with different average
diameters were prepared and characterized using XRD and SEM
ꢁ
upon progressively increasing the concentration of ClO4 after a
delay time of 5.0 ms is applied because under such conditions
the interference from autofluorescence is almost completely
ꢁ
eliminated. The PL intensity gradually increases with ClO
4
concentration, and a broad linear scope in the range of
.1–120.0 mM is established (Fig. S18, ESI†). Because the bright
0
blue phosphorescence originates from the solid form of TPP
ClO , a facile test strip containing a certain amount of TPP Cl
4
(Fig. S12 and S13, ESI†). Their crystal packing arrangements are the
same, but their average diameters fall in a large size range from 0.5
to 5000 mM. The PL spectra of these crystals in Fig. S14 (ESI†) show a
clear decreasing trend for the afterglow emission with the decrease
in the size of the crystals. A quantitative relation can be obtained
between the relative intensity of the afterglow and the average size of
the crystals (Fig. S15, ESI†), and a similar decreasing trend in the
lifetime of the afterglow with the decrease in crystal size is also
found (Fig. S16, ESI†). These observations evidently support that the
4
afterglow of TPP ClO crystals is a result of their aggregation
behaviour and their ultrabright RTP is not size-dependent due to
its origin from a couple of ionic pairs. This is the first time that
direct evidence has been provided to show that the ultralong
afterglow of organic crystals is size-dependent and originates from Fig. 6 (a) PL spectra of TPP Cl solution (100.0 mM) with the addition of
different amounts of perchlorate ions from 0.0 to 100.0 mM. (b) Selectivity
aggregates in the crystals, and it offers a convincing design rule to
achieving efficient ultralong afterglow of organic materials.
test of TPP Cl (100.0 mM) towards perchlorate ions (50.0 mM). Letters a–o
ꢁ
ꢁ
3ꢁ
2ꢁ
ꢁ
2ꢁ
2ꢁ
2ꢁ
2ꢁ
represent NO
3
, PO
4
, CO
3
, CH
3
CO
2
, SO
4
, SO
3
, S
2
O
3
, S ,
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ClO
3
, IO
3
, F , Cl , Br , I and ClO
4
, respectively. (c) Laser scanning
Phosphorescence turn-on detection of perchlorate ions and
in vivo imaging
confocal microscopy image of a seedling root of Arabidopsis thaliana with
TPP Cl (100.0 mM). (d) Laser scanning confocal microscopy image of a
ꢁ
seedling root of Arabidopsis thaliana with TPP Cl (100.0 mM) and ClO
4
TPP Cl is soluble in water, but TPP ClO
4
is highly insoluble in
(
100.0 mM). Scale bar: 50 mm; excitation wavelength: 355 nm; collection
aqueous solution and exists as microcrystals when it is wavelength: 410–450 nm.
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4ꢁ
can be fabricated for the determination of ClO . The PL
intensity of the test strip progressively increases upon continu-
ꢁ
ously adding ClO4 solution, and a very low detection limit
(
5.0 ng) can be achieved in terms of its calibration curve
ꢁ
between the PL intensity and the ClO4 amount in Fig. S19
ꢁ
(ESI†). Even by the naked eye a very small amount of ClO
4
as
low as 25.0 ng can be readily identified from its blue phospho-
rescence on the test strip, and the test strip also shows excellent
ꢁ
selectivity for ClO
4
.
The high brightness and outstanding specificity of the
ꢁ
established assay for ClO4 encourage us to validate it in more
complex biological matrices. Perchlorate ions as a hazardous Fig. 7 (a) A demonstration of anti-counterfeiting function in a school
pollutant frequently can accumulate in plants, and cause slow badge of Zhejiang Normal University. (b) A demonstration of time-
2
4
dimensional anti-counterfeiting of a 100 banknote. The images are taken
under a 365 nm UV lamp and after ceasing the excitation.
growth and death of the plants. We adopt a seedling root of
Arabidopsis thaliana as a model to evaluate the feasibility of the
ꢁ
established method for ClO
4
because its seedling root is able
to allow blue light to penetrate through it and absorb a small
anti-counterfeiting function is introduced besides the addition
of two different colours. In this case, anti-counterfeiting of
banknotes can be greatly enhanced with several dimensions.
These demonstrations offer attractive and huge application
prospects in anti-counterfeiting, and will greatly promote the
fast development of organic materials with ultrabright room-
temperature phosphorescence and ultralong afterglow.
ꢁ
amount of ClO
4
. Fig. 6c shows a low-intensity blue back-
ground from the autofluorescence of Arabidopsis thaliana after
the seedlings are incubated in a TPP Cl solution (100.0 mM) for
3
0 min, but bright blue images can be recorded after introdu-
ꢁ
cing a certain amount of ClO
4
. A gradual enhancement of the
blue emission from the root tip to the body in Fig. 6d dictates
ꢁ
that ClO
4
penetrates into the root of Arabidopsis thaliana
through root tips. Differing from the homogeneous emission
of traditional dyes, the signals from TPP ClO microcrystals
4
Conclusions
scattered inside the root emerge in dot-like shape without any
ꢁ
diffusion. A progressive absorption of ClO4 into the root of In summary, the transient and persistent room-temperature
Arabidopsis thaliana is clearly reflected through the gradual phosphorescence of tetraphenylphosphonium-based organic
extension of the blue signal from the root tip to the body ionic crystals can be regulated by anions, and appropriate
(
Fig. S20, ESI†). The photostability of TPP ClO
also assessed by continuous irradiation with UV light, and Fig. S21 lowering the energy gap between the first singlet state and its
ESI†) illustrates that the blue phosphorescence intensity shows neighboring triplet state of the formed ionic crystals. Achieve-
no appreciable decrease after 15 min, indicating the excellent ment of extraordinarily efficient transient phosphorescence by
4
microcrystals is anions greatly promote the occurrence of triplet emission by
(
resistance of precipitating TPP ClO to photobleaching.
rational construction of such a donor–acceptor pattern in the
crystals contributes to general design principles. The afterglow
brightness of tetraphenylphosphonium perchlorate largely
relies on the size of the crystals, and such a size-dependent
4
Time-dimensional anti-counterfeiting of school badges and
banknotes
The short-emitting blue phosphorescence of microcrystals and behavior provides direct and solid proof that the afterglow is
the ultralong green afterglow of small and large crystals of TPP generated from aggregates with strong interionic interactions
ClO provide an opportunity to demonstrate their specific in the crystals. In combination with structural design boosting
4
function in anti-counterfeiting. As shown in Fig. 7a, the outer the intersystem crossing processes, large aggregates consisting
circle of our school badge is made of TPP ClO microcrystals, of ordered units with significant intermolecular electronic
4
while the inner pattern is composed of small crystals and large coupling greatly favor the occurrence of organic afterglow.
crystals. A bright blue school badge appears under a UV lamp, This design rule will be enormously valuable to design and
but a sharp contrast in emission can be achieved after turning accomplishment of diverse organic materials with highly
off the lamp. The outer circle is quickly darkened in one bright organic afterglow in the future. The establishment of
second, whereas the inner pattern still shines in green color a highly specific detection method for perchlorate ions and the
for more than 4 s. Another important application of emissive demonstration of time-dimensional anti-counterfeiting based
4
anti-counterfeiting is on banknotes, which generally contain on the unique phosphorescence properties of TPP ClO crystals
short-lived fluorescent dyes in specific areas. For example, the illustrate their bright prospects and potential use in various
denomination 100 and the serial number for a 100 banknote fields. Organic phosphonium crystals are expected to exhibit
are separately marked with bright yellow and green fluores- bright room-temperature phosphorescence and ultralong
cence as part of anti-fake function. When a similar pattern afterglow by proper molecular design and anion regulation,
4
made of small TPP ClO crystals to the yellow-emitting area and these organic crystals will show huge potential in diverse
on the banknote is utilized, an additional time-dimensional applications.
1
4540 | J. Mater. Chem. C, 2019, 7, 14535--14542
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Conflicts of interest
9 (a) R. Kabe and C. Adachi, Nature, 2017, 550, 384–387;
(b) I. Bhattacharjee, N. Acharya, S. Karmakar and D. Ray,
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J. Phys. Chem. C, 2018, 122, 21589–21597; (c) Z. Lin, R. Kabe,
N. Nishimura, K. Jinnai and C. Adachi, Adv. Mater., 2018,
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803713; (d) H. Bhatia, I. Bhattacharjee and D. Ray, J. Phys.
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1
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4542 | J. Mater. Chem. C, 2019, 7, 14535--14542
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