Force-Induced Turn-On Persistent Room-Temperature Phosphorescence in Purely Organic Luminogen

Article abstract of DOI:10.1002/anie.202101994

Research of purely organic room-temperature phosphorescence (RTP) materials has been a hot topic, especially for those with stimulus response character. Herein, an abnormal stimulus-responsive RTP effect is reported, in which, purely organic luminogen of

Full text of DOI:10.1002/anie.202101994

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How to cite:
International Edition: doi.org/10.1002/anie.202101994
German Edition: doi.org/10.1002/ange.202101994
Phosphorescence
Force-Induced Turn-On Persistent Room-Temperature
Phosphorescence in Purely Organic Luminogen
Jia Ren, Yunsheng Wang, Yu Tian, Zhenjiang Liu, Xiangheng Xiao, Jie Yang, Manman Fang,*
and Zhen Li*
Abstract: Research of purely organic room-temperature
phosphorescence (RTP) materials has been a hot topic,
especially for those with stimulus response character. Herein,
an abnormal stimulus-responsive RTP effect is reported, in
which, purely organic luminogen of Czs-ph-3F shows turn-on
persistent phosphorescence under grinding. Careful analyses of
experimental results, coupled with the theoretical calculations,
show that the transition of molecular conformation from quasi-
axial to quasi-equatorial of the phenothiazine group should be
mainly responsible for this exciting result. Furthermore, the
applications of stylus printing and thermal printing are both
successfully realized, based on the unique RTP effect of Czs-
ph-3F.
Phenothiazine is a well-known building block for lumi-
nescent materials, and its derivatives usually have quasi-axial
(ax) and quasi-equatorial (eq) conformers that can convert to
each other under the external stimulus (Figure 1A). Accord-
ingly, a series of stimulus-responsive emissive effects have
been carried out based on the changed molecular conforma-
tion of phenothiazine derivatives. For example, Minakata and
co-workers successfully realized the tricolor-changing mecha-
nochromic luminescence through conformational change of
phenothiazine upon grinding.[3] Li and co-workers utilized the
unique conformation-dependent emission property of phe-
nothiazine derivative in the development of dynamic mecha-
noluminescence, another kind of emission process excited by
mechanical stimulus without UV irradiation.[4] Xu and co-
workers reported the force-induced delayed fluorescence in
two phenothiazine derivatives based on the transition of their
conformations from quasi-axial to quasi-equatorial ones.[5]
Thus, could the adjustable RTP emission be realized through
changing the molecular conformation of phenothiazine
derivatives with external stimulus? If it could, the stimulus-
responsive RTP emission and the clarification of relationship
between RTP effect and molecular conformation should be
both achieved, and also the related information could be
obtained for the further rational molecular design and deep
understanding of the molecular behaviors in aggregated
states.
R
ecently, purely organic RTP luminogens have been attract-
ing increasing attention due to their extensive applications in
various fields, such as anti-counterfeiting, sensor, bioimaging
and OLEDs.[1] For the further development of RTP materials,
a deep understanding of the internal mechanism is of great
importance. Accordingly, thanks to the great efforts of
scientists, a series of corresponding mechanisms have been
proposed, such as H-aggregation, strong intermolecular
hydrogen bonds, p–p stacking, intermolecular n–p electron
coupling, and so on.[2] Most of them are focusing on molecular
stacking, while there are fewer researches on molecular
conformation, regardless of its important role for the
intermolecular interactions.
[*] J. Ren, Y. Wang, Y. Tian, Z. Liu, Dr. J. Yang, Dr. M. Fang, Prof. Z. Li
Institute of Molecular Aggregation Science, Tianjin University
Tianjin 300072 (China)
E-mail: manmanfang@tju.edu.cn
lizhentju@tju.edu.cn
Prof. Z. Li
Department of Chemistry, Wuhan University
Wuhan, Hubei 430072 (China)
and
Joint School of National University of Singapore and Tianjin
University, International Campus of Tianjin University
Binhai New City, Fuzhou, Fujian 350207 (China)
and
Tianjin Key Laboratory of Molecular Optoelectronic Sciences,
Department of Chemistry, Tianjin University
Tianjin 300072 (China)
E-mail: lizhen@whu.edu.cn
Prof. X. Xiao
School of Physics and Technology, Wuhan University
Wuhan, Hubei 430072 (China)
Figure 1. A) The molecular structure and two possible conformations
of CzS-ph-3F. B) A proposed diagram for the stimulus-responsive RTP
effect based on conformational change and photographs of Czs-ph-3F
taken before and after turning off the 365 nm UV irradiation under
ambient conditions.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202101994.
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Herein, a new organic luminogen of Czs-ph-3F was
designed and synthesized, in which electron donor of
phenothiazine and acceptor of trifluorobenzene were con-
À
nected by C N bond. For comparison, another analogue
molecule of Czs-ph-F was also prepared, with only one Fatom
instead of three ones in Czs-ph-3F. With this molecular
structure of Czs-ph-3F, three issues could be expected: 1) the
existence of N/S heteroatoms could facilitate n–p* transition,
to promote the RTP emission; 2) the multiple fluorine atoms
could lead to the formation of effective hydrogen-bonding
interaction, then contributing to the restricted non-radiative
transition and the resultant emission; 3) the conformational
change potential of phenothiazine group might give rise to the
stimulus-responsive emission effect.[6] Upon excitation, the
crystal of Czs-ph-3F just gave blue fluorescence emission,
while a turn-on phosphorescence with obvious green after-
glow appeared after grinding, showing the unique force-
induced RTP effect (Supporting Information, Movie S1). To
the best of our knowledge, this should be the first example of
persistent RTP luminogen with mechano-responsive turn-on
RTP effect, as the mechanical stimulus usually led to the
quenched RTP emission for the damaged lattice.[7] Further-
more, the grounded powder would return to the non-RTP
phase after heating or fuming stimulus, and the cycle process
could be repeated for heaps of times, which were much
beneficial for the practical applications.[8] Careful analyses of
experimental results, coupled with the theoretical calcula-
tions, demonstrated that the transition of molecular confor-
mation from quasi-axial to quasi-equatorial of partial mole-
cules should be mainly responsible for these exciting results.
Herein, we present the synthesis, photophysical property,
crystal structure, and theoretical calculations of Czs-ph-3F in
detail, to fully understand its abnormal force-induced RTP
effect and summarize the molecular conformation–property
relationship.
Czs-ph-3F was conveniently synthesized in just one step
(Supporting Information, Scheme S1). In the photolumines-
cence (PL) process, Czs-ph-3F showed different emission
behaviors in different phases. As shown in Figure 2A, its
crystal just gave a blue fluorescence peaked at 437 nm with
the PL quantum yield (FPL) of 2.30%, while no RTP emission
could be detected. Interestingly, when the crystal was
grounded, a new peak at about 505 nm appeared and
gradually increased (Supporting Information, Figures S1,S2,
Table S1). At last, the PL emission was changed to green with
the FPL of 4.27%. At this time, the emission lifetimes for the
peaks of 437 and 505 nm were measured to be 11.56 ns and
20.44 ms, respectively (Supporting Information, Figure S3).
After turning-off the UV irradiation, the obvious green
afterglow could last more than 0.5 s. Furthermore, the
temperature-dependent PL behaviors were studied for
grounded sample of Czs-ph-3F, it was found that the emission
intensity and lifetime at 505 nm both increased with the
decreased temperature, indicating that the new peak should
be assigned to phosphorescence (Figure 2B,C; Supporting
Information, Figure S4). Thus, the first example of persistent
RTP luminogen with mechano-responsive turn-on RTP effect
was demonstrated. Further on, the grounded sample of Czs-
ph-3F could return to the non-RTP phase after heating at
Figure 2. A) The steady-state PL and phosphorescence (P) spectra for
the crystal and grounded samples of Czs-ph-3F. B) Temperature-
dependent phosphorescence decays for the grounded sample of Czs-
ph-3F from 100 K to 300 K. C) Temperature-dependent PL spectra for
the grounded sample of Czs-ph-3F from 100 K to 300 K. D) The solid
UV/Vis absorption spectra of Czs-ph-3F in different solid phases.
E) The PXRD patterns of Czs-ph-3F in different solid phases. F) Stim-
ulus-responsive RTP behaviors of Czs-ph-3F and the corresponding
reversible cycle diagram.
758C for about 0.5 h or fuming with dichloromethane for
about 0.5 h, showing the reversible stimulus-responsive RTP
effect. Especially, after more than 10 times of cycles for both
grinding-heating or grinding-fuming, the turn-on RTP effect
could still be readily realized under mechanical stimulus,
showing excellent stability and recyclability.
To investigate the origin for this peculiar stimulus-
responsive RTP effect, the powder X-ray diffraction
(PXRD) measurements were carried out for Czs-ph-3F
samples in different phases (Figure 2E). Interestingly, similar
PXRD patterns with sharp peaks could be observed for the
grounded sample with RTP and grounded-heated/grounded-
fumed ones without RTP. This disclosed their similar molec-
ular packing mode and crystalline degree. As the PL behavior
for the same luminogen was mainly determined by the
corresponding molecular conformation and packing, it could
be expected that the changed molecular conformation, rather
than packing, should be mainly responsible for the stimulus-
responsive RTP effect. To further explore the changes that
occurred during the grinding process, their solid UV/Vis
absorption spectra were measured (Figure 2D). In compar-
ison with Czs-ph-3F crystal, the UV/Vis absorption at about
350 nm to 400 nm was much weakened after grinding, then re-
enhanced to some extent with heating or fuming stimulus.
Thus, the changed molecular conformation as the main origin
for stimulus-responsive RTP effect could be certified.
Then why didnꢀt the conformational change influence the
molecular stacking? It was deduced that the conformational
change only happened in a small fraction of Czs-ph-3F
molecules. As shown in the Supporting Information, Fig-
ure S5, little change was found for the solid 13C NMR spectra
of Czs-ph-3F crystal and grounded sample, indicating the
negligible change of chemical environment for C atoms in
Czs-ph-3F during the grinding process. On the other hand, the
DSC measurements also showed the similar results that the
melting points of the crystal and grounded samples were
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almost the same (Supporting Information, Figure S6). There-
Then the relationship between molecular conformation
fore, during the grinding process, the conformational change
only happened for a small fraction of molecules, which was
not strong enough to influence the resultant packing mode.
Accordingly, a self-doping system should be formed for the
molecules with initial conformation as host and those with
new conformation as guest.
The single crystal of Czs-ph-3F was cultured and the
corresponding crystal structure was analyzed. The crystal of
Czs-ph-3F gave a space group of P21/C1 (Supporting
Information, Table S2), in which efficient intermolecular
hydrogen bonds and p–p interactions were observed. To
simplify the crystal analyses, eight dimers with close inter-
molecular interactions in the crystal were selected (Fig-
ure 3B). p–p interactions existed in dimer 1 and 2 with the
central-central distances of 3.954 and 3.883 ꢁ respectively,
and RTP effect was further investigated. Although Czs-ph-3F
presented a typical quasi-axial (ax) conformation in crystal
phase, the optimized molecular structure in gas state by TD-
DFT calculation gave a quasi-equatorial (eq) one (Supporting
Information, Figure S7), indicating the existence of two
possible conformations. Furthermore, these two conforma-
tions of Czs-ph-3F presented much different HOMO/LUMO
orbital distributions (Supporting Information, Figure S8).
Among them, Czs-ph-3F(ax) showed large overlap for their
HOMO and LUMO, while obvious charge transfer could be
found for Czs-ph-3F(eq). Accordingly, the much stronger spin
orbit coupling between singlet and triplet states could be
achieved in Czs-ph-3F(eq) (Figure 3C). Particularly, the spin
orbit coupling constant (SOC) between S0 and T1 of Czs-ph-
3F(eq) was calculated to be as large as 33.34 cmÀ1, much
larger than that (5.64 cmÀ1) of Czs-ph-3F(ax), indicating the
better phosphorescence emission ability of quasi-equatorial
conformation. Based on it, the ground-induced RTP effect
could be well understood: in crystal, Czs-ph-3F showed no
RTP emission with quasi-axial (ax) conformation; then some
of Czs-ph-3F molecules were converted into quasi-equatorial
(eq) one after grinding, thus resulting in the appearance RTP
emission.
À
while C H···S bonds were found in dimer 1 and 3 with
distances of 3.128/3.950 and 3.193 ꢁ respectively. Particularly,
thanks for the introduction of the three fluorine atoms, there
À
were many strong C H···F bonds in crystal, especially in
dimer 1 (distances of 2.968 ꢁ), dimer 5 (distances of 2.732–
À
2.993 ꢁ), dimer 6 (distances of C H···F 2.645 ꢁ), dimer 7
À
(distances of 2.576–2.882 ꢁ) and dimer 8 (distances of C
H···F 2.967 ꢁ). With these strong intermolecular interactions,
a rigid environment could be formed for Czs-ph-3F crystal,
which could endow it good lattice stability.[9] Therefore, once
stimulated by mechanical force, the crystal was just broken
into micro-crystals. At this time, the resultant molecular
stacking mode showed no difference, while a small number of
molecules underwent conformational change, then leading to
the resultant RTP emission.
Further on, the theoretical absorption spectra of Czs-ph-
3F(ax) and Czs-ph-3F(eq) were calculated. As shown in
Figure S9A, another shoulder peak could be observed in the
long wavelength region for Czs-ph-3F(ax), while not for Czs-
ph-3F(eq). This could be well consistent with the experimen-
tal results: a shoulder peak could be observed in the long
wavelength region for Czs-ph-3F crystal with quasi-ax con-
Figure 3. A) Molecular packing of Czs-ph-3F crystal observed from the a, b, and c directions, respectively. B) Dimers in Czs-ph-3F crystal and their
intermolecular interactions. C) Calculated spin–orbit coupling constants (x) between the S0/S1 and Tn (DES1Tn <0.3 eV) for Czs-ph-3F(ax) and Czs-
ph-3F(eq).
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former, while the grounded Czs-ph-3F sample was not
(Supporting Information, Figure S9B). Thus, the existence
of Czs-ph-3F(eq) in grounded phase could be further
certified.
identical to Czs-ph-3F crystal, regardless of the trace amounts
of Czs-ph-F (Supporting Information, Table S2). Just as
expected, the cocrystal showed green RTP with the lifetime
of 36.90 ms (Supporting Information, Figure S20), which was
longer than those of the Czs-ph-F crystal and the grounded
On the other hand, the measurement of Raman spectra
was carried out for Czs-ph-3F crystal and grounded phases. As
shown in the Supporting Information, Figure S10A, the
Raman peaks centered at 162 and 191 cmÀ1 (central ring
boat deformation) decreases obviously after grinding, which
is consistent well with the decreased distortion of phenothia-
zine group from Czs-ph-3F(ax) to Czs-ph-3F(eq) (Supporting
Information, Figure S10B).[10] Based on the Raman spectra,
the conformation change from Czs-ph-3F(ax) to Czs-ph-
3F(eq) under grinding could be further certified.
Then the photophysical properties of Czs-ph-3F in
solution phase were studied carefully (Supporting Informa-
tion, Figures S11–S13). As shown in Figure S11, Czs-ph-3F
solution showed similar UV/Vis absorption spectrum to
grounded phase, while that of crystal phase was much
different with an additional shoulder peak ranging from 350
to 400 nm. This indicated that Czs-ph-3F molecules mainly
showed the quasi-equatorial conformation (Czs-ph-3F(eq)) in
solution phase, just like that after grinding. Also, the PL
spectrum of Czs-ph-3F solution at 77 K is similar to the
grounded sample at low temperature (Supporting Informa-
tion, Figure S13), further certifying their similar molecular
conformation. Besides, dominant phosphorescence emission
could be achieved for Czs-ph-3F solution at 77 K, indicating
the great phosphorescence emission ability for Czs-ph-3F(eq)
when its non-radiative transition was restrained. Based on
these experimental results, the existence of Czs-ph-3F(eq)
with great phosphorescence emission ability could be well
certified. Then, the transition of Czs-ph-3F(ax) to Czs-ph-
3F(eq) could lead to the force-induced RTP effect.
To further prove that quasi-equatorial conformation
favored RTP emission, one analogue, Czs-ph-F, consisting of
phenothiazine and fluorobenzene units, was synthesized and
studied (Supporting Information, Figures S14–S19). As
shown in the Supporting Information, Figure S17, Czs-ph-F
crystal gave fluorescence-phosphorescence dual emissions
with lifetimes of 1.36 ns and 0.70 ms, respectively, just like that
of Czs-ph-3F in grounded phase. The analysis of the single-
crystal structure found that it presented typical quasi-
equatorial conformation with fluorobenzene perpendicular
to phenothiazine group in radial direction (Supporting
Information, Figure S18). As just one fluorine atom existed
in compound Czs-ph-F, the corresponding intermolecular
interactions were much weaker than those of Czs-ph-3F,
which might be the main reason for the much shorter RTP
lifetime in Czs-ph-F crystal. With the experimental results
from Czs-ph-F crystal, the significant role of quasi-equatorial
conformation in RTP emission of phenothiazine derivatives
could be further verified.
À
phase of Czs-ph-3F, because of more C H···F intermolecular
interactions and better crystalline in cocrystal. The RTP
spectrum of co-crystal could be well consistent with the that in
doped PMMA film for Czs-ph-F, indicating the RTP emission
in cocrystal was indeed from Czs-ph-F (Supporting Informa-
tion, Figure S21). On the other hand, obvious overlap could
be found between the PL spectrum of Czs-ph-3F crystal and
UV/Vis absorption spectra of Czs-ph-F crystal/PMMA film,
then the energy transfer could happen in the cocrystal
(Supporting Information, Figure S22). Thus, Czs-ph-3F not
only acted as a rigid host, but also an energy donor in the
cocrystal, which might work together to promote the RTP
emission. And this effect might also exist in Czs-ph-3F at
grounded phase. Besides, when Czs-ph-3F was doped in the
PMMA with mass ratio of 1:100, the resultant film also
showed the green RTP with lifetime of 14.01 ms (Supporting
Information, Figure S23), since Czs-ph-3F tended to show
quasi-equatorial conformation in non-crystal phase, just like
that after grinding. These experimental results further con-
firmed that the quasi-equatorial conformation of phenothia-
zine derivatives could promote RTP emission.
Inspired by the unique force-responsive RTP effect of
Czs-ph-3F, the corresponding stylus printing was carried out,
in which the stylus printer created a typeface by hitting carbon
paper with a needle in the print head. Firstly, the typing paper
was constructed with two layers, namely transparent encap-
sulation film and Czs-ph-3F layer, in which Czs-ph-3F layer
has been fumed with dichloromethane to reach the non-RTP
phase. When the typing paper was passed over the print head,
the preset pattern was printed by hitting paper with a needle.
As shown in Figure 4A, the green pandas were successfully
printed on the paper for the turn-on RTP effect of Czs-ph-3F
after force stimulus. It was a pity that the afterglow of pandas
was not so clear for the inferior force-stimulus sensitivity.
Further on, the cocrystal of Czs-ph-F/Czs-ph-3F with mass
ratio of 1:100 was cultured, in which the small amount of Czs-
ph-F with quasi-equatorial conformation acted as RTP
emitter and the large one of Czs-ph-3F with quasi-axial
conformation as rigid host. The measurement of single crystal
structure found that the packing of cocrystal was almost
Figure 4. A) Application of stylus printing: First, the typing paper was
made of Czs-ph-3F and transparent encapsulation film; then, it was
passed over the print head, and the preset pattern was printed by
hitting paper with a needle in the print head. B) Application of thermal
printing: the preset pattern was printed under the combined actions of
heating and pressure.
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H. Shi, Z. An, W. Huang, Chem. Asian J. 2020, 15, 947 – 957;
To further promote the RTP emission of the printed
pattern, the thermal printer was utilized. Similar to a stylus
printer, the thermal printer produces image by selectively
heating the thermochromic paper, coupled with a certain
force. Because of the advantages of fast speed, low noise, clear
printing and easy to use, thermal printer has been widely used
in receipt printers, mobile label printers, and fax machines.[11]
In previous works, we have successfully realized the thermal
printing based on the host-guest doping materials with
stimulus-responsive RTP emission.[12] As multi-components
were needed for the doping systems, the structure of thermal
paper was a little bit complicated. In this work, the simplified
structure of thermal paper with just two layers could be
obtained. When it passed through the print heat, a green
pattern of a tree with clear afterglow appeared due to the
combined actions of heating and pressure.
In summary, we have demonstrated the first example of
force-induced turn-on persistent RTP emission in a purely
organic luminogen of Czs-ph-3F. Through measuring the
changed solid UV/Vis spectra under external stimulus,
coupled with theoretical calculations, the relationship
between molecular conformation and RTP effect was clearly
clarified. It was found the quasi-equatorial conformation of
the phenothiazine derivatives was more likely to give RTP
emission than the quasi-axial one, and the transition between
them led to interesting stimulus-responsive RTP effect of Czs-
ph-3F. This demonstrates that the tuning of molecular
conformation is a good approach to modify the material
properties, once again showing the molecular uniting set
identified characteristic (MUSIC) proposed in 2018.[2b] Fur-
thermore, with the unique force-induced RTP effect, this
material was successfully utilized in thermal printing and
stylus printing for the first time, which further extended the
application of RTP materials. Thus, the control of molecular
conformation should be considered during the rational
molecular design for achieving excellent material perfor-
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21905197), the starting grants of
Tianjin University and Tianjin Government.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: conformers · phenothiazine · phosphorescence ·
stimulus response
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Manuscript received: February 8, 2021
Revised manuscript received: March 9, 2021
Accepted manuscript online: March 14, 2021
Version of record online: && &&, &&&&
6
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Communications
Phosphorescence
The first example of force-induced turn-on
persistent RTP luminogen named Czs-ph-
3F is reported. The abnormal stimulus-
responsive RTP effect is attributed to the
transition of molecular conformation
from quasi-axial to quasi-equatorial of the
phenothiazine group. This material was
successfully applied in thermal printing
and stylus printing.
J. Ren, Y. Wang, Y. Tian, Z. Liu, X. Xiao,
J. Yang, M. Fang,* Z. Li*
&&&& — &&&&
Force-Induced Turn-On Persistent Room-
Temperature Phosphorescence in Purely
Organic Luminogen
Angew. Chem. Int. Ed. 2021, 60, 1 – 7
ꢀ 2021 Wiley-VCH GmbH
www.angewandte.org
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These are not the final page numbers!