Wide-Range Color-Tunable Organic Phosphorescence Materials for Printable and Writable Security Inks

Article abstract of DOI:10.1002/anie.202003585

Organic materials with long-lived, color-tunable phosphorescence are potentially useful for optical recording, anti-counterfeiting, and bioimaging. Herein, we develop a series of novel host–guest organic phosphors allowing dynamic color tuning from the cy

Full text of DOI:10.1002/anie.202003585

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Wide-Range Color-Tunable Ultralong Organic Phosphorescence
Materials for Printable and Writable Security Inks
Authors: Yunxiang Lei, Wenbo Dai, Jianxin Guan, Shuai Guo, Fei Ren,
Yudai Zhou, Jianbing Shi, Bin Tong, Zhengxu Cai, Junrong
Zheng, and Yuping Dong
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10.1002/anie.202003585
Angewandte Chemie International Edition
RESEARCH ARTICLE
Wide-Range Color-Tunable Ultralong Organic Phosphorescence
Materials for Printable and Writable Security Inks
Yunxiang Lei,† Wenbo Dai,† Jianxin Guan,† Shuai Guo, Fei Ren, Yudai Zhou, Jianbing Shi, Bin Tong,
Zhengxu Cai,* Junrong Zheng, and Yuping Dong*
[a]
Y. Lei, W. Dai, S. Guo, F. Ren, Y. Zhou, Prof. J. Shi, Prof. B. Tong, Prof. Z. Cai, Prof. Y. Dong
Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science & Engineering
Beijing Institute of Technology
Beijing 100081, China
E-mail: caizx@bit.edu.cn (Z. C.); chdongyp@bit.edu.cn (Y. D.).
[b]
J. Guan, Prof. J. Zheng
Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering
Peking University
Beijing 100871, China
[†]
These authors contributed equally.
Supporting information for this article is given via a link at the end of the document.
Abstract: Organic materials with long-lived, color-tunable
phosphorescence are potentially used for optical recording, anti-
counterfeiting, and bioimaging. Herein, we develop a series of novel
host-guest organic phosphors allowing dynamic color tuning from the
cyan (502 nm) to orange red (608 nm). Guest materials are employed
to tune the phosphorescent color, while the host materials show a
synergistic effect to the guest to activate the phosphorescence
emission. These organic phosphors have an ultra-long lifetime of 0.7
s and a maximum phosphorescence efficiency of 18.2%. Although
color tunable inks have already been developed using visible dyes,
solution-processed security inks that are temperature dependent and
display time-resolved printed images are unprecedented. This
strategy can provide a crucial step towards the next-generation of
security technologies for information handling.
such as steroid analogue, the cavity of cyclodextrin, poly(vinyl
alcohol), and poly(methyl methacrylate).[6] The rigidity of the
matrix avoids quenching of the triplet excitions by interactions with
the humidity and oxygen.[6a,6c,6e] In addition, the rigid host also
restricts the molecular motion of the guest molecules, thereby
promoting their phosphorescence.[6a,6b,6e] However, the major role
of the matrix molecules in the multi-component RTP system
remains elusive. Researchers still need to carefully choose the
matrix for a guest phosphor. A rigid or crystalline matrix is
unsuitable for all guest phosphors, resulting in a lack of wide
range color tunable RTP materials.[6a,6c-e] Tunable emission color
endows RTP materials with superior performance in
optoelectronic applications. For instance, multicolor-encoded
materials can serve as information carriers for encrypted data
storage, high-density, and anti-counterfeiting.[7] Furthermore, long
wavelength RTP materials are ideal biomarkers for bioimaging.[8]
To date, the development of wide-range color-tunable emission
RTP materials remains a formidable challenge, which greatly
limits application of multi-component phosphorescence
materials.[1b,1h,6f,6g]
In this work, we demonstrate a host-guest doping approach
that can achieve wide-range color tuning of RTP emission. Both
the host and guest molecules have no phosphorescence at the
room temperature. When the guest molecules were doped into
the host molecules, maximum phosphorescent quantum yield
reaches 18.2%. The experimental results also reveal that the host
molecules not only restrict the motion of the guest molecules, but
also show a synergistic effect to the guest at excited state. The
host materials of both triphenylphosphine (TPP) and
triphenylarsenic (TPAs) exhibit low melting points and highly
stable subcooling states. Therefore, the host-guest RTP system
can be solution-processed in both molten state and solution state.
The hand-writing characters of RTP inks and printing figure by the
ink solution display similar color under sunlight and UV light, but
show a dynamic color change after removal of UV light. In addition,
RTP performance of the host-guest materials at high temperature
is highly dependent on guest molecules. The three-component
Introduction
Purely organic room temperature phosphorescence (RTP)
materials with long-lived, persistent luminescence in the visible
spectrum are applicable in optical recording, anti-counterfeiting,
and high contrast background independent bioimaging.[1]
Fundamental understanding of luminescence process through
molecular design is the central theme of organic RTP. A
seemingly general strategy is to enhance the intersystem
crossing (ISC) from the singlet state to triplet state.[2] Therefore,
most traditional RTP materials are carbazole or phenothiazine
based donor (D)-acceptor (A) small molecules with a distorted
configuration.[2-3] The intramolecular D-A interaction can narrow
the energy gap between the lowest singlet and triplet state
(ΔEST).[4] Aldehyde group and halogens can also be introduced
into small molecules to increase the spin-orbit coupling (SOC)
constant.[4a,5] Small ΔEST and large SOC constant are conducive
to enhance the ISC ability of excition.[2-4] However, this molecular
design strategy faces several urgent issues, such as short
phosphorescence wavelengths, low brightness, and lack of new
systems.[1d-g,2-3]
RTP system possesses
a phosphorescent thermochromic
Recently, a novel method to generate RTP was reported by
embedding guest materials into a rigid or crystalline host matrix,
property. Therefore, multiple dimensional, including color tunable,
temperature dependent, and time-resolved, anti- counterfeiting
1
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RESEARCH ARTICLE
Figure 1. (a) The molecular structures of guest and host molecules. (b) Schematic diagram of host-guest phosphorescence materials. (c) Fluorescence (dash line)
and phosphorescence (solid line) spectra of host-guest crystalline materials. Insets show photographs of guest/TPAs materials with and without UV irradiation. (d)
Phosphorescence decay curves of host-guest materials with TPAs as the host, excitation wavelength: 370 nm. (e) The fluorescence (top) and phosphorescence
(down) images of the DQD/TPAs crystalline powders with different amount of DQD (Molar ratio).
techniques are achieved in the printable and writable host-guest
inks.
afterglow. To the best of our knowledge, this value is the lowest
guest concentration found in organic host-guest materials that
can achieve RTP.
The melt-casting host-guest materials with 0.1 mol% guests
dispersed in either TPP or TPAs exhibiting the best fluorescence
and phosphorescence performance were selected for the
photophysical investigation. The host-guest materials produce
blue fluorescence emission with the wavelength ranging from 433
to 462 nm (Figure 1c, Table 1). The phosphorescence
wavelengths of the host-guest materials gradually increase from
502 to 608 nm, achieving full emission coverage from cyan, to
green, to yellow, and to orange-red (Figure 1c, Table 1). In
addition, eight host-guest crystalline materials all exhibit high
luminescent efficiency with phosphorescence quantum yields (Q.
Y.) between 8% and 18%, and fluorescence Q. Y. higher than
70% (Table 1). The strong luminous ability of host-guest materials
indicates that the host materials restrict the non-radiation
transition, thus increasing the light conversion efficiency. On the
basis of the RTP-decay curves, the phosphorescence lifetimes of
doped crystalline powders ranging from 0.22-0.70 s (Figures 1c
and S3c, Table 1), result in multicolor afterglow that last as long
as 2-4 s under ambient conditions (Figure S5). In addition, the
phosphorescence intensities of TPAs based materials are
stronger than those of the TPP based materials (Table 1), likely
due to the heavy atomic effect of arsenic. To calculate the triplet
gap between T1 to ground state of the guest molecules,
phosphorescence spectra of the guests were measured in
toluene solution at 77 K (Figure S1c). The results indicate that
ΔEST of the guest in host-guest systems are smaller than the
guests in toluene (Table S1). The smaller energy gap obviously
facilitates the intersystem crossing of excitons, resulting in strong
Results and Discussion
Figure 1a shows the molecular structures of the hosts and
guests. TPP and TPAs were selected as the host molecules
according to our previous results because triphenylamine (TPA)
displayed good host capacity for TPA-based RTP guest materials.
Since nitrogen, phosphorus and arsenic are in the same main
element group, and heavy atom facilitates the intersystem
crossing, TPP and TPAs as the host materials were expected to
process enhanced RTP performance than TPA. Four TPA-based
molecules with electron withdrawing groups (Figure 1a) were
designed and synthesized as the guests (Schemes S1 and S2).
Both TPP and TPAs have low melting points (Figure S2, 81.7 oC
for TPP and 63.1 oC for TPAs) and stable subcooling states.
Therefore, the guests dispersed in the host can be fabricated by
the melt-casting method, in which blends of the materials are
simply heated by hot water. All host-guest materials show visible
afterglow after removal of irradiation, indicating the present of
RTP properties. The concentration of the guest molecules play an
important role in the performance of RTP.[9] Therefore, a series of
host-guest materials with molar ratio of DQD to TPAs ranging
from 1:50 to 1:20000 were prepared (Figure 1e). As ratio of DQD
increases, the emission wavelength of the host-guest materials
slightly red shift (Figure S4), likely due to the molecular
aggregation of the guest molecules at high concentration.[9a]
Notably, even the concentration of DQD is as low as 0.005 mol%
(1:20000), the host-guest materials still produce obvious visible
2
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Table 1. Photophysical properties of host-guest materials.[a]
Fluo.
Phos.
Sample
λem
ФF
τ
λem
ФP
τ
(nm)
(%)
(ns)
(nm)
(%)
(s)
FDA/TPP
DTA/TPP
DBC/TPP
DQD/TPP
433
438
448
459
82.5
77.3
81.2
82.8
2.8
2.8
2.5
2.2
502
531
556
608
8.1
16.2
12.8
9.3
0.22
0.58
0.70
0.46
FDA/TPAs
DTA/TPAs
DBC/TPAs
DQD/TPAs
434
442
448
462
80.9
78.6
77.6
78.4
3.2
3.0
2.1
2.0
504
536
559
605
9.2
0.25
0.61
0.68
0.47
18.2
15.5
10.9
[a] Note: Guest: Host = 1:1000 (Molar ratio). Excitation wavelength: 370 nm. All samples are in the crystalline states.
Figure 2. (a) Fluorescence spectra of DQD/TPP. (b) Phosphorescence spectra of DQD/TPP. (c) Photographs of DQD/TPP in different state.
phosphorescence emissions of the host-guest systems at room
temperature.[2-3] The color tunable property of host-guest
materials provides an opportunity for constructing the white
phosphorescence materials with two complementary emission of
blue and yellow. FDA, DBC and DQD with TPAs (molar ratio:
1:1:1:1000) show a broad phosphorescence peak from 490 to 580
nm (Figure S6a). The Commission Internationale de L’Eclairage
(CIE) chromaticity coordinate of FDA-DBC-DQD/TPAs powder
was calculated as (0.31, 0.38) (Figure S6c), which is a bright cold
white phosphorescence emission.
Generally, host molecules provide a rigid environment to
avoid the quenching of the triplet excitons by interaction with
oxygen.[6a-e] However, the degassed DQD/TPP (0.1 mol%) molten
o
solution at 85 C and room temperature (subcooling state) only
show fluorescence but no phosphorescence (Figure 2). When the
host-guest system begins to solidify, the fluorescence of
DQD/TPP was enhanced and bright phosphorescence was
generated (Figure 2). Therefore, the rigid host molecules not only
play a role to avoid the quenching of the triplet excitons by oxygen,
but also can restrict the motion of the guest molecules and
decrease the non-radiation transitions.
3
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RESEARCH ARTICLE
were performed in different solvents. Compared to the
fluorescence spectra of DQD in common solvents, DQD shows a
shorter emission wavelength in the molten host (Figure S9a),
indicating that the molten host with a low polarity showed a weak
solvent effect. However, the maximum excitation and absorption
wavelengths in the molten host are longer than those in common
solvents (Figures S9b, S9c). Hence, the molten host not only acts
as the solvent, but also displays a strong interaction with the guest.
To further investigate the host-guest interaction, waiting time-
dependent spectra of TPAs host, five guests and corresponding
host-guest materials were measured (guest and host molar ratio
= 1:10000; excitation wavelength: 370 nm). Transient infrared
(IR)-absorption spectra and vibrational relaxation kinetics of five
guests are presented in Figure S11 and S12. The absorption
peaks of solid samples (1375~1625 cm-1) correspond to benzene
skeleton vibration modes. Upon excitation, the molecules reach
high vibrational level of their excited states without conformation
changes, according to the Frank-Condon principle.[13] Then the
excited molecules relax back to the ground state via vibration
relaxation from excited vibrational level to the ground vibrational
level. The fast relaxation lifetime, within 2 ps (Figure S12), are
heavily distorted by nonlinear effects arising from the temporal
overlap of the pump and probe pulse, at the first several
picoseconds of our decay signal.[14] All the guest molecules
exhibit the similar vibrational lifetimes ranging from 120 to 170 ps,
whereas the vibrational lifetime of the TPAs is only 62 ps, which
is significantly shorter than those of the guest molecules (Figure
S12). Since the maximum absorption wavelength red shifts from
321 nm to 407 nm from FDA to NDBA (Figure S8a), and DQD
shows the maximum molar absorption coefficient at 370 nm, DQD
possesses the highest excitation efficiency. The broad absorption
in the transient IR-absorption spectra illustrates that the free
electron moves among solid molecules.[15] In addition, all guest
molecules show a long-lived process, where the persistent mid-
IR absorption may arise from a population of singlet excited state
that cannot undergo singlet decay, probably due to singlet
trapping or disorder in the films.
Transient IR-absorption spectra of five host-guest materials
are presented in Figure 4g-k. TPAs show a strong absorption
peak with an excitation wavelength at 280 nm, but almost no
absorption peak under 370 nm excitation (Figures 4d-f). However,
when the four host-guest materials are excited by an excitation
source at 370 nm, TPAs molecules display relatively strong
absorption peaks (Figures 4g-j). Once the guest molecules are
excited, the excitation energy can transfer from the guest to the
host, indicating the existence of intermolecular interaction
between these molecules. The vibrational lifetimes of the host in
host-guest materials are three-fold longer than that of pure host
molecule excited under 280 nm (Figures 4c, S13). In addition, the
interaction between DBC to TPAs is strongest among the four
host-guest materials, which is consistent with the fact that
DBC/TPAs has the longest phosphorescence lifetime.
Furthermore, the NDBA/TPAs with non-RTP phenomenon shows
an extremely weak absorption signal in transient IR-absorption
measurement (Figure 4k). Therefore, the rigid host is not only
used to minimize oxygen effects and restrict the motion of the
Figure 3. (a) Proposed energy transfer path between guest and host. (b) The
energy levels of the TPAs and the guests.
Tunable emission color produces RTP materials that exhibit
superior performance in bioimaging. Especially the materials with
long emission wavelength display low phototoxicity and high
penetrability for cells and tissues.[8,10] Therefore, RTP materials
with long emission wavelength were achieving by our host-guest
doping strategy. DQD guest molecules show
a
broad
phosphorescence peak with a wavelength around at 600 nm at
77 K (Figure S1c), affording phosphorescence emission of 605
nm in DQD/TPAs powder. However, the material with a
phosphorescence longer than 605 nm were failed to achieve in
our host-guest system. For example, TPA based molecule with a
nitro group which has stronger electron-withdrawing ability as the
acceptor, namely, 4'-nitro-N,N-diphenyl-[1,1'-biphenyl]-4-amine
(NDBA, Figure 1a), display a phosphorescence emission at 625
nm at 77 K (Figure S7b). Unexpectedly, TPAs materials with any
amount of NDBA produce only fluorescence, but non-
phosphorescence (Figure S7d). Hence, TPA-based guest
materials with longer emission wavelength will not be suitable for
TPP and TPAs based host-guest systems. Density functional
theory (DFT) calculations were carried out to offer further insight
into host-guest systems. Large bandgap between the S1 and T1 in
guest molecules do not facilitate to the intersystem crossing
(Figure 3a).[2,3,11] Therefore, non-phosphorescence was observed
in guest powders. T1 of the host can be the bridge between the S1
and T1 of the guest molecules (Figure 3a). However, the large
energy gap between T1 of TPAs and T1 of NDBA were not
suitable for intermolecular energy transfer (Figure 3b).[12] The host
can only restrict the molecular motion of NDBA and results in an
enhanced fluorescence. The host with low T1 energy level is
needed for pursuing long wavelength emission RTP materials.
The solid-state UV-absorption experiments of DQD/TPAs and
NDBA/TPAs were carried out to confirm the intermolecular
interaction between the host and guest. The maximum absorption
wavelength of DQD/TPAs shows a significant red shift compared
to DQD powder, indicating the absence of the intermolecular
charge transfer between the DQD and TPAs (Figure 4a).
However, the maximum absorption wavelength of NDBA/TPAs is
almost identical to NDBA powder due to lack of intermolecular
interaction (Figure 4b). The solvatochromic experiments of DQD
guest molecules on the phosphorescence processes.
A
synergistic interaction between the host and guest is an
indispensable factor for the realization of RTP in host-guest
system.
4
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Figure 4. (a) UV absorption spectra of TPAs, DQD and DQD/TPAs. (b) UV absorption spectra of TPAs, NDBA and NDBA/TPAs. (c) The vibrational relaxation
kinetics of TPAs, DQD/TPAs, NDBA/TPAs (TPAs, excitation wavelength: 280 nm. DQD/TPAs and NDBA/TPAs, excitation wavelength: 370 nm). (d) IR-absorption
of TPAs. (e) Transient IR-absorption spectra of TPAs, excitation wavelength: 280 nm. (f) Transient IR-absorption spectra of TPAs, excitation wavelength: 370 nm.
(g)-(k) Transient IR-absorption spectra of five host-guest materials, excitation wavelength: 370 nm.
The vibrational freedoms of host-guest systems are largely
affected by noncryogenic temperature changes below their
melting points. This phenomenon offers the possibility of
enhancing the effect of temperature on phosphorescence
intensity. The phosphorescence intensity of FDA/TPP, DTA/TPP,
DBC/TPP, DQD/TPP quench by 96%, 85%, 36%, 21%,
respectively, when the temperature increases from 15 to 65 C
(Figure 5a and S14). The difference in intensity quenching can be
attributed to the difference in vibrational freedoms between the
host and guest. Strong electron withdrawing substitute promotes
strong host-guest interaction, thus decreasing the molecular
vibration. As shown in Figure 5b, the pattern displays different
information at different temperatures, which can be exploited for
thermochromic anti-counterfeiting. Three-component host-guest
material of DTA-DQD/TPP (molar ratio: 1:1:1000) was also
prepared for the investigation of phosphorescent thermochromic
property. DTA-DQD/TPP shows green phosphorescence at 15 oC
due to high phosphorescence Q. Y. of DTA/TPP. As the
o
5
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Figure 5. (a) In situ phosphorescence intensity of host-guest materials in the temperature range of 15-65 oC. (b) Photographs of the flower before (in the middle)
and after removing the excitation source at different temperatures. (c) Phosphorescence wavelength and intensity of DTA-DQD/TPP materials at different
temperatures. (d) In situ phosphorescence wavelength of DTA-DQD/TPP materials during four cycles. (e) Phosphorescence color of DTA-DQD/TPP host-guest
material at different temperatures. (f) Application of DTA-DQD/TPP in anti-counterfeiting of calligraphy and artworks.
temperature increases, a dramatic decrease in phosphorescence
intensity of DTA/TPP and slight decrease in phosphorescence
intensity of DQD/TPP are observed. Therefore, the
phosphorescence wavelength of DTA-DQD/TPP gradually red-
shift to orange with increasing temperature (Figure 5c and 5e).
The in situ phosphorescence spectra of the heating and cooling
importance. In addition to be used as the security inks at
subcooling state, our host-guest materials are also considered to
be conjoined with a facile process, such as writing and printing in
solution state. The filter paper can be first soaked with the molten
host molecules. The guest molecules can be dissolved in their
good solvents, such as DCM or tetrahydrofuran at a concentration
of 1×10-4 mol/L. Furthermore, various complicated patterns have
been written or printed with good resolution by using guest
solution (Figure S16). As shown in Figure S16a, a butterfly was
drawn with four types of guest inks. After thorough drying under
ambient conditions, it is invisible under ambient light. Upon
irradiation with 365 nm UV lamp, a bright blue pattern emerges
due to similar fluorescence of the four guest-host system. In
addition, the butterfly changes to a brilliant colorful self-protective
RTP after removal of the UV lamp. The security badge was also
printed on A4 paper using an ijd300 printer where a vacant black
cartridge is injected with guest solutions (1×10-4 mol/L in DCM).
As shown in Figure S16b, the information on the badge was
invisible both under ambient light and under UV light due to the
strong fluorescence background of the A4 paper (Figure S15).
The colorful information, including the portrait, the name, the
organization, and the badge number is readable after removal the
irradiation. Therefore, the information on the badge can only be
read and identified, but cannot be photocopied and duplicated
even when was stolen. Notably, the long-lived afterglow
o
in the temperature range of 15-65 C was recorded in order to
monitor the thermal response reversibility. Reversible
phosphorescence is repeated in four consecutive cycles,
indicating that thermochromic anti-counterfeiting that utilize this
system can be reusable (Figure 5d). The host-guest materials can
be easily heated to the molten state (85 oC) as the ink for
calligraphy with the writing brushes. As shown in Figure 5f, the
characters show a bright cyan fluorescence under UV-radiation.
After removing the excitation source, the characters display green
phosphorescence at room temperature. Slight heating results in a
bright orange afterglow. Therefore, the time-resolved
thermochromic property of DTA-DQD/TPP can be explored as
sensitive, inert, and cheap materials for anti-counterfeiting.
The counterfeiting of currency, valuable documents, and
branded commodities has become a challenging issue that has
serious economic, security and health ramifications for
governments and businesses.[16] Therefore, the development of
security inks that are not able to be photocopied, but are readable
only under special conditions has become of increasing
6
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RESEARCH ARTICLE
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Conclusion
In conclusion, we developed a series of host-guest materials
with RTP properties. The phosphorescence color can be tuned
from 502 to 608 nm under ambient conditions by selecting
different guests. This opens up a broad class of pure organic
compounds to new applications in phosphor design. In light of
both the experimental results and simulation, we believe that the
host molecules can not only restrict the molecular motion and
avoid the triplet quenching from the oxygen, but also exhibit a
synergistic effect for the realization of RTP. These materials with
color tunable, temperature dependent, and time-resolved
emissive properties can be used as writable and printable inks for
multiple-dimensional anti-counterfeiting.
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Acknowledgements
This work was financially supported by the National Natural
Scientific Foundation of China (Grant Number: 51803009,
21975021, 51673024, 51328302, 21404010). Beijing Institute of
Technology Research Fund Program for Young Scholars.
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Keywords: color-tunable • host-guest synergistic effect • room
temperature phosphorescence • printable and writable security
inks • multi-dimension anti-counterfeiting
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7
This article is protected by copyright. All rights reserved.

10.1002/anie.202003585
Angewandte Chemie International Edition
RESEARCH ARTICLE
A facile doping approach was developed for color-tunable organic phosphorescence materials. The hosts not only restrict the
molecular motion and avoid the triplet quenching from the oxygen, but also exhibit a synergistic effect to the guests for the realization
of phosphorescence. Color tunable, temperature dependent, and time-resolved anti-counterfeiting techniques are achieved by the
printable and writable materials.
Yunxiang Lei,† Wenbo Dai,† Jianxin Guan,† Shuai Guo, Fei Ren, Yudai Zhou, Jianbing Shi, Bin Tong, Zhengxu Cai,* Junrong Zheng,
and Yuping Dong*
8
This article is protected by copyright. All rights reserved.