Multistage Stimulus-Responsive Room Temperature Phosphorescence Based on Host–Guest Doping Systems

Article abstract of DOI:10.1002/anie.202107639

Compared with inorganic long-lasting luminescent materials, organic room temperature phosphorescent (RTP) ones show several advantages, such as flexibility, transparency, solubility and color adjustability. However, organic RTP materials close to commerci

Full text of DOI:10.1002/anie.202107639

Angewandte
Communications
Chemie
How to cite:
International Edition: doi.org/10.1002/anie.202107639
German Edition: doi.org/10.1002/ange.202107639
Luminescence
Hot Paper
Multistage Stimulus-Responsive Room Temperature Phosphorescence
Based on Host–Guest Doping Systems
Yu Tian, Jie Yang, Zhenjiang Liu, Mingxue Gao, Xiaoning Li, Weilong Che, Manman Fang,* and
Zhen Li*
Abstract: Compared with inorganic long-lasting luminescent
materials, organic room temperature phosphorescent (RTP)
ones show several advantages, such as flexibility, transparency,
solubility and color adjustability. However, organic RTP
materials close to commercialization are still to be developed.
In this work, we developed a new host–guest doping system
with stimulus-responsive RTP characteristics, in which triphe-
nylphosphine oxide (OPph3) acted host and benzo-
(dibenzo)phenothiazine dioxide derivatives as guests. Turn-
on RTP effect was realized by mixing them together through
co-crystallization or grinding, in which the efficient energy
transfer from host to guest and the strong intersystem crossing
(ISC) ability of the guest have played significant role. Further
on, multistage stimulus-responsive RTP characteristics from
grinding to chemical stimulus were achieved via introducing
pyridine group into the guest molecule. In addition, the anti-
counterfeiting printings were realized for these materials
through various methods, including stylus printing, thermal
printing and inkjet printing, which brings RTP materials closer
to commercialization.
and once thought to be too inefficient to be realized at room
temperature, recent advances have vastly increased ISC
efficiency by enhancing spin-orbit coupling (SOC) with the
use of heteroatoms,[6] heavy atoms,[7] and multimers.[8] How-
ever, most of them rely on special crystal structures[9] with
certain maintaining difficulties in cultivation and practical
applications, which greatly limits actual application scenarios.
In addition, some multi-component phosphorescent systems
have been developed, mainly through co-crystallization, rigid
matrix encapsulation, hardening in the polymer matrix, or
interacting with other molecules of the same or different
types.[10] However, in actual operation, the preparation
process of these multi-component materials is slightly com-
plicated, and the application conditions are also subject to
certain restrictions, especially in terms of flexibility. Based on
this, RTP materials based on host–guest doping systems that
do not rely on crystalline are most likely to solve the above
problems. Although this kind of materials has been reported
sporadically, the systematic studies are still scarce.[11] There-
fore, the mechanism and influencing factors of their RTP
generation need to be further studied in detail, because this is
important for the design of efficient RTP materials.
Compared with commonly phosphorescent materials,
RTP materials with stimulus response effect, especially
those that donꢀt rely on special crystal forms, have advantages
and commercial value in application fields such as anti-
counterfeiting, sensing, and detection.[9b,11b,12] Herein, we
construct a series of guest molecules (energy acceptors),
namely Pph, BPph and DBPph (Figure 1b), which can be
mixed with the host OPph3 (energy donor) to realize turn-on
RTP effect by co-crystallization or grinding for the promotion
of energy transfer between them (Figure 1a). Among them,
BPph doped system exhibits the best RTP performance, in
which efficient energy transfer from host to guest and the
strong ISC ability of the guest molecules are found to play the
significant role (Figures 1c and d). In addition, the introduc-
tion of pyridine group into guest molecules endows the
resultant doping system with acid-base reversible and acid-
heat reversible stimulus-responsive RTP effects. With this, the
multistage stimulus-responsive RTP characteristic from
grinding to chemical stimulus is achieved for the first time.
The photophysical properties related to Pph, BPph,
DBPph and OPph3 in tetrahydrofuran solution (10À5 M) at
room temperature were measured, as well as their solid
powders (Figure S2 and Table S2). The UV-vis absorption
spectra show that the maximum absorption wavelengths of
them are 370 nm, 361 nm, 330 nm and 221 nm (Figure S2a),
respectively, which accord with their conjugation degrees.
Single components of BPph, DBPph and OPph3 show no
O
rganic RTP materials, also known as organic afterglow
materials, are organic systems with triplet emission at room
temperature developed in recent years.[1] Because phosphor-
escence has the characteristics of large stokes shift, long
lifetime and full utilization of excited state energy, it has
received extensive attention from scientists and industry in
the fields of organic optoelectronic materials,[2] biological
imaging,[3] and anti-counterfeiting labels.[4]
In recent years, the preparation and application of pure
organic RTP materials have made great progress, especially
the single-component system.[5] Though accessing to and from
the triplet state is a forbidden process for organic luminogens
[*] Y. Tian, J. Yang, Z. Liu, M. Gao, X. Li, W. Che, 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 430072 (China)
and
Joint School of National University of Singapore and Tianjin
University, International Campus of Tianjin University
Binhai New City, Fuzhou 350207 (China)
E-mail: lizhen@whu.edu.cn
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.202107639.
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OPph3 crystal is at 295 nm,
which shows large overlap
with the absorption of
guest molecules. So, it is
considered
energy levels are matched
to certain extent. In
that
their
a
addition, the emission life-
time of OPph3 crystal at
295 nm is 8.64 ns, while in
co-crystal systems of Pph-
C, BPph-C, and DBPph-C,
the emission lifetimes at
295 nm are shortened to
6.63 ns, 4.95 ns and 6.01 ns
(Figure 2b), respectively.
This means the energy
transfer from host to
guest, and the correspond-
ing efficiencies are calcu-
lated to be 23%, 43% and
30% (Figure 2c), respec-
tively. It can be seen from
Figure 2a, the absorption
capacity of Pph under
same concentration is the
Figure 1. a) Energy transfer between donor (host) and acceptor (guest). b) Molecular structures of acceptors
in this work. c) Photographs of co-crystals under and after excitation, lex =254 nm, mguest/mhost =1%. “Pph-C”
represents the co-crystal formed by mixing Pph and OPph3. d) Phosphorescence quantum yields and lifetimes
of Pph-C, BPph-C and DBPph-C.
RTP phenomena (Figure S3 and Table S2), while a strong
afterglow with a duration of ꢀ 9 s is observed from a co-
crystal sample of BPph-C (FP = 3.33%, tP = 980 ms) obtained
by removing the solvent from a solution of BPph and OPph3
(1:100, mass ratio) in ethanol. On the other hand, only weak
RTP can be observed from co-crystals of Pph-C (FP = 0.37%,
tP = 98 ms) and DBPph-C (FP = 1.13%, tP = 334 ms) (Fig-
ure 1c,d and S4c). The PL spectrum of BPph-C shows
emission bands at 295 nm and 399 nm, which are attributed
to the characteristic emission peaks of OPph3 and BPph,
respectively, and the new emission band ranging from 460–
620 nm is considered to be the spectrum of the green
afterglow (Figure S4a and S4b).
The phosphorescence spectrum (Figure S8b) of BPph in
tetrahydrofuran solution (10À5 M) at 77 K is consistent with
the RTP spectrum (Figure S4b) of BPph-C, which indicates
the green afterglow of BPph-C is derived from the triplet
emission of BPph. DBPph-C exhibits similar properties to
BPph-C with its phosphorescence emission at 519 nm, while
Pph-C gives two distinct emission bands, at 390–440 nm and
460–620 nm, respectively (Figure S4b). Compared to the low
temperature phosphorescence spectrum (Figure S8a) of Pph
solution, the RTP emission band at 460–620 nm is considered
to be caused by the dimer in Pph aggregates, which may be
caused by the non-uniform dispersion of guest molecules in
the host.
worst, which is considered to be one of the reasons for its
lowest energy transfer efficiency. On the other hand, DBPph
and BPph give similar absorption ability to the emission from
host. Further on, the energy levels of singlet and triplet
excited states were calculated by TD-DFT based on their
single crystal structures (Table S4 to S7). The energy of the S1
state for OPph3 is 5.60 eV (Table S4), and the Sn states of
guests with energy within ES1Æ0.3 eV of OPph3 were enum-
erated (Figure S9 to S11), in which the energy transfer from
OPph3 to guest could occur easily. As shown in Figure S10,
BPph-C has four energy levels (5.31 eV, 5.70 eV, 5.77 eV and
5.85 eV, respectively) that can be matched with the S1 state of
OPph3 to realize energy transfer, while Pph-C and DBPph-C
have only two (5.55 eV and 5.79 eV, Figure S9) and three
(5.35 eV, 5.50 eV and 5.65 eV, Figure S11) matched energy
levels, respectively. These results may further explain why the
most effective energy transfer occurs in co-crystal system of
BPph-C.
After energy transfer, the triplet emission ability of guest
should be another important factor to affect the resultant
RTP emission of co-crystal. In order to explain these
phenomena in more depth, spin-orbit coupling constants (x)
between S1 and Tn for OPph3, Pph-C, BPph-C, and DBPph-C
were analyzed in detail and the energy levels considered
suitable for ISC were listed (Figures 2d, e, f and S12). As
shown in Figure S12, although in OPph3, there are larger
coupling constants between S1 and T11/T13, which are
1.28 cmÀ1 and 3.50 cmÀ1 respectively, the coupling constant
between T1 and S0 is only 0.24 cmÀ1, indicating the weak
phosphorescence emission ability from T1 to S0. This is
considered to be the reason why OPph3 itself does not
produce RTP. Pph, BPph, and DBPph all have good ISC
ability from T1 to S0 (Figures 2d, e and f), and their coupling
Now two problems must be solved urgently. One is why
RTP can be achieved from co-crystals, and the other is why
the phosphorescence efficiencies and lifetimes of these three
co-crystals are so different.
To answer these two questions, the emission spectrum of
the host and the absorption spectra of the guests were
compared. As shown in Figure 2a, the emission band of
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Figure 2. a) The PL spectrum of OPph3 crystal and the UV-vis absorption spectra of Pph, BPph and DBPph in tetrahydrofuran solution.
b) Emission decay characteristics (lem =295 nm) of OPph3 crystal and three co-crystals. c) Energy transfer efficiencies of OPph3 in three co-
crystals. Theoretically-calculated energy levels and spin-orbit coupling constants (x) between S1 and Tn based on the corresponding molecular
geometries of Ppy (d), BPpy (e) and DBPpy (f). Solid blue arrows represent major ISC channels with x over 1 cmÀ1, and dashed blue arrows
represent minor ISC channels with x less than 1 cmÀ1. The black solid triplet states are those levels with energy within ES1Æ0.3 eV. g) Molecular
packing (left) and intermolecular interactions (right) of OPph3.
constants are 2.05 cmÀ1, 1.78 cmÀ1 and 2.96 cmÀ1, respectively.
However, compared with Pph (xS1T7 = 0.81 cmÀ1, xS1T6
grinding (Figure S13–S21 and Table S8), as long as the host
and the guest are in contact with each other at an appropriate
distance to occur energy transfer. As illustrated in Figure S19,
a gradually enhanced and prolonged RTP emission could be
realized with the increase of grinding time for a mixture of
BPph and OPph3 (1:100 for mass ratio, namely BPph-G). At
about 20 min, a greenish afterglow lasting ꢀ 3 seconds was
observed under ambient conditions. During the grinding
process, the energy transfer efficiency from OPph3 to BPph
increased from nearly zero to 54%. Thus, a unique force-
responsive RTP effect is achieved based on the enhanced
energy transfer under grinding.
To further increase the performance of the phosphores-
cent material applied, multistage stimulus response effect is
considered a good choice. As we all know, the pyridine group
shows a stimulus response behavior to proton acid, so the
pyridine substituted guest molecules of Ppy, BPpy and
DBPpy were designed and synthesized (Figure 3a and
=
0.20 cmÀ1, xS1T5 = 0.05 cmÀ1) and DBPph (xS1T4 = 0.20 cmÀ1,
xS1T3 = 0.24 cmÀ1), there are more Tn states that match S1 with
larger coupling constant in BPph, which are 0.33 cmÀ1 (xS1T6),
1.10 cmÀ1 (xS1T5), 0.52 cmÀ1 (xS1T4) and 0.47 cmÀ1 (xS1T3),
respectively. So, the ISC ability of guest itself is also believed
to have a certain impact on the phosphorescence efficiency
and lifetime. Lastly, the good crystallinity of the host provides
a suitable rigid environment for the guest, which can inhibit
the non-radiative transition of the guest. Figure 2g shows
À
OPph3 can be tightly packed by a large amount of C H···O
À
(2.62–2.84 ꢁ) and C H···p (2.89–3.39 ꢁ) intermolecular
interactions, which is beneficial to restrict thermal motion of
molecule and oxygen diffusion, then contributing much to the
resultant RTP emission in co-crystals.
Not only in co-crystal, obvious RTP emission could also
be observed for the mixtures of host and guest samples after
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Figure 3. a) BPpy’s design concept and stimulus response principle. b) Stimulus-responsive PL behaviors of BPpy-G and the corresponding
reversible cycle diagram. c) The normalized UV-vis and PL spectra for BPpy and BPpy+HCl. Calculated HOMO, LUMO orbits and energy levels,
band gaps of d) BPpy and e) BPpy +HCl.
Scheme S1). Ppy, BPpy and DBPpy exhibit similar photo-
physical properties to their analogues of Pph, BPph and
DBPph (Figure S22 to S39). Thus, when they are doped into
host of OPph3 through co-crystal or grinding, efficient RTP
emission could be also observed. Further on, the physically
ground powders of BPpy and OPph3 mixture (1:100 for mass
ratio, namely BPph-G) are found to show reversible stimulus-
responsive RTP effect upon acid-base or acid-heat stimula-
tion. As shown in Figure 3b, the reversible PL changing
behavior can be cycled many times for the interaction
between nitrogen atom in the pyridine group and proton in
acid. Under the irradiation of UV lamp, BPph-G emits blue
light, then a green afterglow appears after the lamp is turned
off. However, when BPph-G interacts with protonic acid,
namely BPph-G-HCl, it would turn to be RTP inactive due to
the changed electronic structure of guest, which could be well
proved by the newly formed absorption band around 400 nm
(Figure 3c). Then, the smaller energy gap leads to the sky-
blue fluorescence (lem = 483 nm, t = 9.61 ns, FF = 6.31%) of
BPph + HCl (Figure 3d–e, Figure S40,S41 and Table S13).
Since the BPpy guest can be dissolved into the OPPh3
host at molecular level via facile evaporation and grinding
methods, the system is of great potential to serve as security
ink, stylus/thermal printing paper (Figure 4). First, the stylus/
thermal printing paper was prepared based on BPpy and
OPPh3, just as shown in Figure 4a. After stylus printing, the
papers look like as a normal one under natural and UV lights
(Figure 4b). However, after stopping 254 nm UV excitation,
the patterns of “Plum”, “Orchid”, “Bamboo” and “Chrys-
anthemum” with green afterglow could be observed clearly.
In addition, the afterglows of the “dragon” and “phoenix”
patterns printed by thermal printer are brighter (Figure 4c).
This is mainly due to thermal stimulation accelerates the
movement of molecules and makes the host and guest better
contact.
Not only that, environmentally friendly and economical
safety inks were prepared based on this doping system, and
used for inkjet printing and anti-counterfeiting (Figure S42
and Figure 4d–k). Safety ink based on BPpy/OPPh3 mixture
can be used not only for printing, but also for writing. As
shown in Figure 4l, the words “Good luck!” written in three
kinds of handwritings can be displayed after the excitation is
stopped. After fumigation with acid, the afterglow disappears,
and the green afterglow could resume when it is fumigated
with NH3. Thus, the multiple anti-counterfeiting is realized
based on the chemical-responsive RTP effect of this doping
system. These cases fully illustrate the feasibility of success-
fully achieving multiple printing and writing methods. The
preparation of security ink is closer to commercialization, and
it is expected to be used in anti-counterfeiting encryption in
the fields of certificates, trademarks, calligraphy and painting.
In summary, a new host–guest doping system with
stimulus-responsive RTP characteristics was developed, in
which OPph3 acted as host and benzo(dibenzo)phenothiazine
dioxide derivatives as guest to turn-on RTP by co-crystal-
lization or grinding. Through in detail research, it is found that
the energy transfer efficiency from host to guest and the
effective ISC ability of the guest are two essential factors for
obtaining high phosphorescence efficiency, which maybe
provide ideas for the design of organic RTP materials based
on host–guest doping. Further on, the introduction of pyridine
group into guest achieves reversible fluorescence-phosphor-
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Keywords: energy transfer · host–guest doping systems ·
printing · room temperature phosphorescent ·
stimulus-responsive
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21905197) and the starting grants of
Tianjin University and Tianjin Government.
Conflict of Interest
Manuscript received: June 8, 2021
Accepted manuscript online: July 8, 2021
Version of record online: && &&, &&&&
The authors declare no conflict of interest.
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Communications
Luminescence
The influence of different conjugation
degrees on the quantum yield and life-
time of room temperature phosphor-
escence in a host–guest system is dis-
cussed. Multistage stimulus-responsive
RTP characteristics from grinding to
chemical stimulus were achieved by
introducing a pyridine group into the
guest molecule. Anti-counterfeiting
printings were realized through various
methods, including stylus printing, ther-
mal printing and inkjet printing.
Y. Tian, J. Yang, Z. Liu, M. Gao, X. Li,
W. Che, M. Fang,* Z. Li*
&&&& — &&&&
Multistage Stimulus-Responsive Room
Temperature Phosphorescence Based on
Host–Guest Doping Systems
6
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