Two-Photon Ionization Induced Stable White Organic Long Persistent Luminescence

Article abstract of DOI:10.1002/anie.202106472

Organic long persistent luminescence (OLPL) materials with afterglow duration in the scale of minutes or even hours are still rare. Most OLPL systems are based on exciplexes, which require complicated multi-component system in order to realize white after

Full text of DOI:10.1002/anie.202106472

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Accepted Article
Title: Two-Photon Ionization Induced Stable White Organic Long
Persistent Luminescence
Authors: Xiao Liang, You-Xuan Zheng, and Jing-Lin Zuo
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Two-Photon Ionization Induced Stable White Organic Long Persistent
Luminescence
Xiao Liang, You-Xuan Zheng*, Jing-Lin Zuo
Abstract: Organic long persistent luminescence (OLPL) materials with
afterglow duration in the scale of minutes or even hours are still rare. Most
OLPL systems are based on exciplexes, which require complicated multi-
component system in order to realize white afterglow with slightly
compromised duration and color stability. In this work, OLPLs lasting from
20 to 40 minutes are realized with a simple binary system based on two-
photon ionization mechanism, which can harvest simultaneously excitons
from both singlet and triplet excited states, making it potentially the most
promising candidate to achieve stable white OLPL. Through modulation
and optimization of dopant structures in dibenzo[b,d]thiophen-2-yldiphenyl
phosphine oxide host, the emission profiles of afterglow can be readily
tuned from cyan (0.19, 0.22), cold white (0.31, 0.35), standard white (0.33,
0.33) to warm white (0.31, 0.46), with excellent color consistency.
long-lasting white afterglow.
There is another much less reported mechanism that can also trigger
OLPL, two-photon ionization (TPI).[14] It was believed TPI systems
required extremely high excitation power to surpass the ionization
potential as well as low temperature to stabilize the ionized species. In
2020, Samuel and coworkers reported a TPI triggered OLPL system
using a thermal activated delay fluorescence (TADF) molecule as
dopant.[15] Due to the efficient reverse intersystem crossing from T1 to
S1, the afterglow emission appeared to come exclusively from singlet
excited state. In previous reports, additional dopants are required to
achieve white afterglow either with polymer matrix[16] or exciplex-
based OLPL system.[13] However, the complexity of these multi-dopant
systems could potentially accelerate the non-radiative processes,
subsequently leading to compromised afterglow color consistency as
well as duration. In TPI-based OLPL system, the profiles of afterglow
are theoretically solely dependent on the photophysical properties of
the dopant molecules, rendering TPI potentially much superior to
realize stable white OLPL with such a simple binary system.
In this work, a series of triphenylamine (TPA)-based fluorescent
dyes with charge transfer characteristics were chosen as dopants
(Figure 1, namely CNPDTPA, CNPDDTP, KZLTPA and KZLDTPA),
triphenylphosphine (TPP) or dibenzo[b,d]thiophen-2-yldiphenyl
phosphine oxide (DBTSPO) were used as hosts for optimization and
comparison. Obvious RTP can be observed in TPP- based materials,
while TPI can be successfully triggered at room temperature in
DBTSPO-based organic films with duration of afterglow ranging from
20 minutes to around 40 minutes. Meanwhile, the afterglow profiles of
Organic materials capable of light emission even after cessation of
external stimulation are of particular interest to researchers. There are
fundamentally two categories of organic materials that could exhibit
perceivable afterglow.[1] One being organic room temperature
phosphorescence (oRTP) materials with lifetime in the order of micro-
second or second, the other being organic long persistent luminescence
(OLPL) materials with significantly longer duration of afterglow.[2]
The former has been systematically investigated in the past few
decades.[3] With excellent phosphorescence quantum yield and decent
triplet lifetime, these materials have shown huge potential in bio-
imaging,[4] information encryption,[5] anti-counterfeiting,[6] X-ray
scintillator,[7] water-oxygen detection,[8] OLED,[9] white-light
illumination,[10] etc. However, despite tremendous efforts in
eliminating non-radiative decay pathways, promoting the generation of
triplet excitons and stabilizing triplet excited states, the duration of
afterglow still lags far behind the sophisticated inorganic persistent
luminescent materials.
In 2015, Adachi and coworkers reported the first OLPL system
based on trace exciplexes formed between a pair of electron donor and
acceptor molecules.[2] With stabilized radical ions formed in rigid
organic matrix, the duration of afterglow can be extended from seconds
to an hour, which is huge leap forward towards pragmatic applications.
The mechanisms of OLPL based on exciplexes have been
comprehensively investigated in the next few years.[11] The emission
profile of afterglow can be modulated through modification of
donor/acceptor molecules or through secondary or tertiary doping with
additional fluorescent dyes.[12] White OLPL was also achieved in a
quaternary system with two additional fluorescent dyes at the cost of
greatly reducing the duration of afterglow.[13] The introduction of
additional fluorophores could also potentially induce more non-
radiative channels, it is still quite challenging to realize stable and
[*]
Mr. X. Liang, Prof. Y.-X. Zheng, Prof. J.-L. Zuo
State Key Laboratory of Coordination Chemistry
Collaborative Innovation Center of Advanced Microstructures
Jiangsu Key Laboratory of Advanced Organic Materials
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing, 210023 (P. R. China)
E-mail: yxzheng@nju.edu.cn
Figure 1. Structures of host molecule (DBTSPO, TPP) and dopant molecules
(CNPDTPA, CNPDDTPA, KZLTPA, KZLDTPA) (top) and demonstrations of RTP
based on TPP as host and TPI based OLPL (bottom). LPL-S denotes OLPL from
singlet excited states. LPL-T denotes OLPL from triplet excited states.
Prof. Y.-X. Zheng, Prof. J.-L. Zuo
Green Catalysis Center, and College of Chemistry
Zhengzhou University, Zhengzhou 450001 (P. R. China)
1
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Figure 2. (a) Room temperature fluorescence and (b) phosphorescence spectra of films with TPP or DBTSPO as host. (c) Time resolved emission spectra at the initial 6 seconds
and (d) from 1 minute to 21 minutes of DBTSPO + 1 mol% CNPDTPA, CNPDDTPA, KZLTPA and KZLDTPA films. (e) HOMO and LUMO distributions, potential ISC
channels and (f) corresponding SOC coefficients (BDF/B3LYP/6-31g(p)). (g) Photos of four organic films under excitation (left), 1 second after excitation (right top) and 10
minutes after excitation (right bottom). (h) CIE coordinates of afterglow for four films. (i) CIE coordinates of afterglow for DBTSPO + 1 mol% KZLDTPA with variation of time
after cessation of excitation.
these materials exhibit excellent consistency throughout the entire
afterglow process, the color of which can be rationally tuned from cyan
(0.19, 0.22), cold white (0.31, 0.35), standard white (0.33, 0.33) to
warm white (0.31, 0.46) according to intrinsic photophysical properties
of dopant molecules. In this regard, OLPL based on TPI mechanism is
a potential method towards realization of long-lasting and stable white
afterglow. To the best of our knowledge, the simplicity of TPI-based
binary system is superior to other complicated OLPL systems, and the
state-of-art performances could also help promote the application of
these materials.
The dopant molecules were chosen based on two reasons: 1. TPA
group can act as RTP antenna which can promote the generation of
RTP characteristics;[17] 2. TPA units linked to electron deficient units
would introduce charge transfer excited characteristics, which are
beneficial for intersystem crossing process (ISC). However, large
energy gap between singlet and triplet excited states (ΔEST) is
specifically optimized to undermine the RISC process to endow the
afterglow with both singlet and triplet components, consequently
resulting in white OLPL. DBTSPO is designed from a commonly
adopted electron acceptor 2,8-bis(diphenylphosphoryl)dibenzo[b,d]
thiophene (PPT), due to its excellent capability to stabilize the ionic
All films based on DBTSPO display decent transparency and
excellent OLPL properties. Upon excitation with Xe lamp at 380 nm,
all films exhibit intense photoluminescence with emission maxima
located at 490, 493, 462 and 465 nm in DBTSPO doped with 1 mol%
CNPDTPA, CNPDDTP, KZLTPA and KZLDTPA (Figure 2a),
respectively, which evidently can be identified as singlet emission in
accordance to corresponding emission profiles in toluene solutions and
TPP hosts (Figure S2 and S4). Compared with emission profiles in
toluene solutions, the emission maxima of organic films are moderately
red-shifted due to polarized organic matrix and intrinsic charge transfer
characteristics of these dopants (Figure S20). The red-shifts in
emission maxima are much more pronounced with DBTSPO group due
to much larger polarity of the host matrix and stronger induction
between host and dopants (Table 1). The phosphorescence profiles
with TPP and DBTSPO hosts, on the other hand, are much less
affected by organic matrix. From theoretical calculations of neutral
transitional orbitals (NTOs) for these four compounds (Figure S21) it
was found that singlet excited state exhibits both charge transfer (CT)
characteristics and locally excited (LE) characteristics, while the triplet
states exhibit mainly LE characteristics. Since the CT characteristics
are more susceptible to polarity while the LE characteristics are much
less affected by the matrix, the red-shift in fluorescence maxima is thus
quite reasonable. As illustrated in Figure S8, films with KZLTPA and
KZLDTPA as dopants exhibit smaller stokes shift with excitation
maxima located around 380 nm, while films with CNPDTPA and
CNPDDTPA exhibit slightly larger stokes shift.
radicals in exciplex-based OLPL systems with
a much lower
processing temperature of less than 180 ℃. The organic films (0.5 mm
thick) were prepared using melt-cast method, similar to previous
reports.[2] In addition, triphenylphosphine (TPP) is also introduced as
reference host since its high-lying triplet excited state and can serve as
an ideal host for TPA derivatives according to previous report.[18]
2
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Table 1. Emission maxima of four dopant molecules at different states and temperatures.
TPP
(0.1 mol%)
DBTSPO
1 mol%
In toluene
[b]
[c]
[d]
FL
(nm)
452
455
445
433
FL[1]
(nm)
446
411
423
434
Phos[a]
(nm)
525
531
545
ΔEST
(eV)
0.46
0.35
0.65
0.71
FL
(nm)
470
486
457
451
Phos
(nm)
530
540
565
570
ΔEST
FL
(nm)
490
493
462
465
ΔEST
τPhos (ms)
Phos (nm)
(eV)
(eV)
0.32
0.10
0.45
0.43
CNPDTPA
CNPDDTPA
KZLTPA
120.4/129.6
287.7/241.1
301.4
532
538
550
577
0.13
0.20
0.42
0.35
KZLDTPA
565
91.6/112.3
[a] Measured at 78 K. [b] Determined from onset of fluorescence and phosphorescence emission spectra in toluene at 78 K. [c] Determined from onset of fluorescence and
phosphorescence emission spectra at room temperature. [d] Energy level of S1 was calculated from onset of fluorescence at room temperature while energy level of T1 was
approximated from onset of phosphorescence emission in TPP films.
Figure 3. Single point kinetic decay of afterglow for four films at singlet fluorescence emission maxima and phosphorescence maxima with different excitation power: (a) 50 mW,
excited for 5 minutes. (b) 100 mW, excited for 5 minutes. (c) Single point kinetic decay of afterglow for four films at fluorescence emission maxima and phosphorescence
maxima with different excitation time (50 mW, laserland).
With TPP as host, evident afterglow can be observed after cessation
of excitation source with afterglow duration of 1-5 seconds. The
phosphorescence lifetimes of these materials were determined to be
120.42/129.60 ms, 287.7/241.1 ms, 301.4 ms and 91.5/112.3 ms
(Figure S7) for TPP doped with 0.1 mol% CNPDTPA, CNPDDTPA,
KZLTPA and KZLDTPA (Table 1), respectively. The introduction of
rigid TPP host can protect the dopants from oxygen annihilation and
eliminate non-radiative decay processes[19]. With DBTSPO as host,
none of these films exhibited obvious afterglow with either Xe lamp or
LED lamp as excitation source. However, upon excitation with high-
power laser (50 mW), all films exhibited long-lasting afterglow and
fascinating transition of afterglow emission color in the initial stage
(Figure 2c, 2d, 2g). For DBTSPO + 1 mol% CNPDTPA, the film
exhibited blue emission upon excitation with 410 nm laser diode (50
mW, laserland). The emission color dramatically shifted to bright
orange color instatly after cessation of excitation, then gradually
shifted to warm white within 5 seconds as the yellow color slowly
faded. The similar process appears in all four films. With DBTSPO + 1
mol% CNPDDTPA, the emission color shifted from blueish green to
greenish yellow and finally stabilized to cold white color. For
DBTSPO + 1 mol% KZLDTPA, the emission color shifted from
greenish blue to orange yellow and finally to standard white. For
DBTSPO + 1 mol% KZLTPA, however, the emission color remains
almost consistent throughout the entire process, with emission color
varies slightly from cyan to sky blue and finally back to cyan. As
depicted in Figure 2c, at the initial 5 seconds of each film, the emission
3
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profiles display two major components corresponding to singlet and
triplet emissions except for KZLTPA. The afterglow was dominated by
RTP instantly after removal of excitation, then overtaken by singlet
components as relative intensity of triplet emission gradually faded
away with CNPDTPA and KZLDTPA as dopants. While in the case
with CNPDDTPA, the contribution from RTP is still pronounced. With
KZLTPA as dopant, the contribution from triplet excited states can
almost be neglected, resulting in cyan afterglow with only singlet
component.
compromised performance. This could potentially be attributed to
accumulated heat dissipation from long-time laser excitation which
could result in severer quenching by non-radiative factors such as
molecular vibration and oxygen quenching. With 50 mW laser as
excitation source at room temperature, all four films show afterglow
duration of more than 20 minutes. In the case of CNPDDTPA and
KZLTPA, the afterglow can last even longer than 40 minutes
perceivable to bare eyes in the dark. The state-of-art performance of
white OLPL renders TPI mechanism an excellent candidate towards
stable white afterglow and could have great potentials and practical
values.
In summary, through modulation of a series of dopant molecule
structures, efficient RTP was achieved with TPP as host, while stable
warm white, cold white as well as standard white (0.33, 0.33)
afterglows can be realized with DBTSPO as host through TPI
mechanism at room temperature, the afterglow can last from 20
minutes to around 40 minutes. The photophysical characteristics of
these films depend heavily on the SOC and ISC processes of the
dopant molecules. Besides, the kinetic decay characteristics exhibit
minor relevance towards excitation power and time, with simultaneous
singlet and triplet exciton decay processes in sync throughout the entire
decay processes. In this regard, TPI based OLPL systems could
significantly reduce the threshold and complexity of white OLPL and
greatly expand the current design concepts of OLPL systems.
Upon completion of phosphorescence decay process, the emission
profiles stabilized with minor fluorescence/phosphorescence ratio
variations (Figure 2d). The CIE coordinates in this stage are (0.31,
0.35) with DBTSPO + 1 mol% CNPDTPA, (0.31, 0.46) with DBTSPO
+ 1 mol% CNPDDTPA, (0.19, 0.22) with DBTSPO + 1 mol%
KZLTPA and (0.33, 0.33) with DBTSPO + 1 mol% KZLDTPA
(Figure 2h, 2i), respectively. Through time-resolved emission scan it is
evident that the afterglow profiles consist of two stages (Figure 2c, 2d).
The initial photoluminescence decay can be attributed to host-guest
induced RTP process, similar to corresponding films with TPP as host.
The long-lasting afterglow can be attributed to TPI induced OLPL
since the afterglow consists of profiles from both singlet and triplet
excited states and no evidence of exciplex formation was found.
Surprisingly, the OLPL emission maxima with singlet components
exhibited gradual red-shift with DBTSPO + 1 mol% CNPDTPA and
DBTSPO + 1 mol% KZLTPA. The subtle red-shift can be rationalized
with slow molecular relaxation between the excited molecular and
ground state structure despite of the rigid DBTSPO matrix [20]
.
Acknowledgements
In dopants with two TPA units (CNPDDTPA and KZLDTPA), the
afterglow profiles of corresponding films are dominated by triplet
emission, while in dopants with single TPA unit (CNPDTPA and
KZLTPA), the afterglow are dominated by singlet emission. Through
theoretical calculations of the spin-orbit coupling (SOC) coefficients
(Figure 2f) it is evident that the ISC process could be potentially much
faster than the other three samples due to a much small energy gap
between the higher-lying T3 and S1 (0.07 eV). Besides, there are much
more potential intersystem crossing channels for KZLDTPA with
reasonable SOC coefficients, all of which contribute to dramatically
faster ISC processes, resulting in gradual attenuation in relative OLPL
intensity from triplet excited states, as depicted in Figure 2c and Figure
2d. With DBTSPO + 1 mol% KZLTPA, however, the energy gap
between S1 and T2 is merely 0.06 eV, which could drastically facilitate
the RISC process, resulting in OLPL with almost no triplet components.
To further investigate the photodynamic of TPI-based system,
kinetic decay behaviors of these films were conducted. It is of great
significance that the kinetic behaviors of singlet excitons and triplet
excitons remain in sync throughout the entire afterglow process to
ensure the color stability. As illustrated in Figure 3a and Figure 3b, the
kinetic decays of singlet and triplet excitons in all four films remain
almost parallel. Besides, the kinetic behaviors with 50 mW group and
100 mW group exhibit only minor differences with slightly elongated
lifetime in terms of triplet exciton decay with 100 mW group. In the
case of DBTSPO + 1 mol% CNPDDTPA, the kinetic decay curves of
singlet and triplet excitons almost superimpose in both 50 mW and 100
mW group. This clearly demonstrates excellent color consistency of
TPI based OLPL systems. Besides, all kinetic decay curves are
composed of two stages corresponding to RTP decay and OLPL decay.
The RTP decay process follows exponential decay pattern while the
OLPL decay exhibits power-law decay following Debye-Edwards law.
The second decay stage are almost identical with in the tail of kinetic
decay with 1 and 3 minutes of excitation duration (Figure 3c), while
the kinetic decay with 5 minutes excitation duration shows slightly
This work is supported by the National Natural Science Foundation of
China (51773088, 21975119).
Keywords: Organic room temperature phosphorescence • Organic
long persistent luminescence
ionization
• White afterglow • Two-photon
[1]
[2]
[3]
Kenry, C. Chen, B. Liu, Nat. Commun. 2019, 10, 2111.
R. Kabe, C. Adachi, Nature 2017, 550, 384-387.
a) G. C. Smith, Phys. Rev. 1968, 166, 839-847; b) Y. Gong, G. Chen, Q. Peng,
W. Z. Yuan, Y. Xie, S. Li, Y. Zhang, B. Z. Tang, Adv. Mater. 2015, 27, 6195-
6201; c) S. Xu, R. Chen, C. Zheng, W. Huang, Adv. Mater. 2016, 28, 9920-
9940.
[4]
[5]
[6]
[7]
Y. Wang, H. Gao, J. Yang, M. Fang, D. Ding, B. Z. Tang, Z. Li, Adv. Mater.
2021, 33, 2007811.
J. Tan, Q. Li, S. Meng, Y. Li, J. Yang, Y. Ye, Z. Tang, S. Qu, X. Ren, Adv.
Mater. 2021, 33, 2006781.
K. Huang, X. Dou, Y. Zhang, X. Gao, J. Lin, J. Qu, Y. Li, P. Huang, G. Han,
Adv. Funct. Mater. 2021, 2009920. DOI: 10.1002/adfm.202009920.
a) L. Tang, J. Zan, H. Peng, X. Yan, Y. Tao, D. Tian, Q. Yang, H. Li, Q. Chen,
W. Huang, R. Chen, Chem. Commun. 2020, 56, 13559-13562; b) X. Wang, H.
F. Shi, H. L. Ma, W. P. Ye, L. L. Song, J. Zan, X. K. Yao, X. Y. Ou, G. H.
Yang, Z. Zhao, M. Singh, C. Y. Lin, H. Wang, W. Y. Jia, Q. Wang, J. H. Zhi,
C. M. Dong, X. Y. Jiang, Y. G. Tang, X. J. Xie, Y. Yang, J. P. Wang, Q. S.
Chen, Y. Wang, H. H. Yang, G. Q. Zhang, Z. F. An, X. G. Liu, W. Huang, Nat.
Photonics 2021, 15. 187-192.
[8]
[9]
Y. Zhou, W. Qin, C. Du, H. Gao, F. Zhu, G. Liang, Angew. Chem. Int. Ed.
2019, 58, 12102-12106.
a) R. Kabe, N. Notsuka, K. Yoshida, C. Adachi, Adv. Mater. 2016, 28, 655-
660; b) S. Tan, K. Jinnai, R. Kabe, C. Adachi, Adv. Mater. 2021, 2008844.
DOI: 10.1002/adma.202008844.
[10] J. Wang, X. Gu, H. Ma, Q. Peng, X. Huang, X. Zheng, S. H. P. Sung, G. Shan,
J. W. Y. Lam, Z. Shuai, B. Z. Tang, Nat. Commun. 2018, 9, 2963.
[11] Z. Lin, R. Kabe, K. Wang, C. Adachi, Nat. Commun. 2020, 11, 191.
4
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COMMUNICATION
[12] N. Nishimura, Z. Lin, K. Jinnai, R. Kabe, C. Adachi, Adv. Funct. Mater. 2020,
30, 2000795.
[13] K. Jinnai, R. Kabe, C. Adachi, Adv. Mater. 2018, 30, 1800365.
[14] H. S. Pilloff, A. C. Albrecht, Nature 1966, 212, 499-500.
[15] W. Li, Z. Li, C. Si, M. Y. Wong, K. Jinnai, A. K. Gupta, R. Kabe, C. Adachi,
W. Huang, E. Z. Colman, I. D. W. Samuel, Adv. Mater. 2020, 32, 2003911.
[16] S. Kuila, S. Garain, S. Bandi, S. J. George, Adv. Funct. Mater. 2020, 30,
2003693.
[17] I. Bhattacharjee, S. Hirata, Adv. Mater. 2020, 32, 2001348.
[18] Y. Lei, W. Dai, J. Guan, S. Guo, F. Ren, Y. Zhou, J. Shi, B. Tong, Z. Cai, J.
Zheng, Y. Dong, Angew. Chem. Int. Ed. 2020, 59, 16054-16060.
[19] K. Jinnai, N. Nishimura, R. Kabe, C. Adachi, Chem. Lett. 2019, 48, 270-273.
[20] M. Hempe, N. A. Kukhta, A. Danos, M. A. Fox, A. S. Batsanov, A. P.
Monkman, M. R. Bryce, Chem. Mater. 2021, DOI: 10.1021/acs.chemmater.
0c03783.
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Table of Contents
Organic long persistent luminescence with afterglow lasting up to 20-40 minutes was achieved based on two-photon ionization mechanism at room
temperature in a simple binary system. Through modulation of molecular structures and potential ISC channels between excited states, the
afterglow profiles can be rationally tuned from cyan (0.19, 0.22), cold white (0.31, 0.35), standard white (0.33, 0.33) to warm white (0.31, 0.46).
6
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