Self-Stabilized Amorphous Organic Materials with Room-Temperature Phosphorescence

Article abstract of DOI:10.1002/anie.201906881

The stability of pure organic room-temperature phosphorescent (RTP) materials in air has been a research hotspot in recent years. Without crystallization or encapsulation, a new strategy was proposed to obtain self-stabilized organic RTP materials, based

Full text of DOI:10.1002/anie.201906881

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Accepted Article
Title: Self-stabilized Amorphous Organic Room Temperature
Phosphorescence
Authors: Wei Xu, Yaguo Yu, Xiaonan Ji, Huarui Zhao, Jinming Chen,
Yanyan Fu, Huimin Cao, Qingguo He, and Jiangong Cheng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906881
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10.1002/anie.201906881
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Self-stabilized Amorphous Organic Room Temperature
Phosphorescence
Wei Xu*[a] [b], Yaguo Yu[a] [b], Xiaonan Ji[a] [b], Huarui Zhao[a], Jinming Chen[a] [b], Yanyan Fu[a] [b], Huimin
Cao[a], Qingguo He*[a] [b] and Jiangong Cheng*[a] [b]
influence of O2[20]. Whether in crystalline and tight packaging
ABSTRACT: The stability of pure organic room-temperature
phosphorescent (RTP) materials in air is one of the research hotspot
in recent years. Without crystallization or encapsulation, a new
strategy was proposed to obtain self-stabilized organic RTP
materials, based on a complete ionization of photo-induced charge
separation system. The ionization of aromatic phenol 4-carbazolyl
salicylaldehyde (CSA), formed a stable H-bonding anion-cation
radical structure, and led to the completely amorphous CSA-I film.
Long phosphorescent lifetime was gained as long as 0.14 s at room
temperature and a direct exposure in air. The emission intensity was
also increased by 21.5 times. Such amorphous RTP material
reconciled the contradiction between phosphorescence stability and
vapor permeability, and has been successfully utilized for peroxide
vapor detection.
state, non-radiative vibration deactivation or contact with the
triplet quenching agents could be suppressed, leading to a higher
phosphorescence efficiency. However, the permeability of its film
would be sacrificed together with them, which limited its potential
applications in the fields of catalysis and sensing.
In 2017, Adachi group reported an ultra-long persistent
luminescent material based on organic photo-induced charge
[21]
separation system.
Via low energy density laser excitation,
the neuter host-guest molecule pair of strong electron-accepting
molecule 2,8-bis(diphenylphosphoryl)dibenzo[b,d] thiophene
(PPT) and strong electron-donating molecule N,N,N,N-
tetramethylbenzidine (TMB) can be effectively converted into
radical ion pairs, lengthening the phosphorescence lifetime to
Recently, pure organic room-temperature phosphorescence
(RTP) with long afterglow has become a research hotspot for its
[22]
minute level.
However, although the structure was in
amorphous state, the luminescent components still need the
protection of tight polymer encapsulation against air.
potential applications in wide areas such as biological imaging,
[1,
digital encryption, opto-electronic devices.
2]Meanwhile,
because of the long excited-state relaxation lifetime,
phosphorescent materials also show great value in catalysis,
sensing, and photovoltaic fields. [3]
Constructing ionic structure with multiple hydrogen bonds is
another effective method for lengthening RTP lifetime, reported
as RTP organic ionic crystals. [23-25] Inspired by the photo-induced
charge separation and ionic crystals, we considered about
constructing a completely ionized system to gain ultra-stable
amorphous phosphorescent system. As Figure 1a and 1b shows,
instead of crystalline protection or polymer encapsulation, the
electrostatic interaction among ion pairs, together with other
Much progress has been achieved in long lifetime RTP
materials, based on different kinds of molecule structures, from
[4, 5]
cabazole derivatives
, ,
diphenyl ketone derivatives[6-8]
phenothiazine derivatives,[9,
imide compounds[11], aromatic
10]
carboxylic acid derivatives[12] to AIE luminogens[13]. Among these
materials, the RTP efficiency can be as high as 34.5% via 1-
(dibenzo[b,d]furan-2-yl)phenylmethanone structure, while the
RTP lifetime can be as long as 1360 ms via isophthalic acid
structure. However, phosphorescent materials still suffered a
vital problem-- stability. The triplet excited states of RTP
materials are so unstable that they can be quenched under many
circumstances, from triplet state molecules like oxygen, to non-
radiative molecular vibration at room temperature.
inter-molecular interaction, for example, hydrogen bonding
[26]
interaction,
can probably enhance the occurrence of triplet
excited state and gain more stable long lifetime RTP in air.
To obtain such a completely ionized system, aromatic
aldehyde molecules were chosen as the basic structure for its
potential phosphorescent capacity. Phenol group was introduced
into aromatic aldehyde molecules to form a salicylaldehyde
structure for the ionization upon a simple reaction between amine
and the phenol group. So an anion-cation pair was obtained with
the electron donating phenol anion and the electron accepting
amine cation. Due to the existence of both oxygen and nitrogen
and hydrogen atoms, hydrogen bonding interaction may also be
formed between the anion-cation pairs. With this design, it is
expected to realize an air stable room phosphorescence
emission despite its amorphous morphology.
To avoid these adverse factors, most RTP molecules have
been prepared as crystalline structure [14-16], with restricted
molecule motion and low air permeability.[17] Other strategies
were also adopted such as encapsulation in polymer package [18,
19]
or formation of host-guest complexes to eliminate the
Based on the discussion above, CSA-I with a carbazole
backbone was designed as the target compound. The
salicylaldehyde CSA and carbazolyl p-benzaldehyde CBA were
used for comparison to understand the effect of the ionization
and the phenol unit on the phosphorescent property. The
synthetic routes of the above-mentioned compounds were given
in Scheme 1. CBA and CSA were synthesized under microwave
condition. CSA-I was synthesized by a reaction of CSA and
diethylamine (DEA) at room temperature.
[a]
Dr. Wei Xu, Yaguo Yu, Xiaonan Ji, Huarui Zhao, Jinming Chen, Dr. Yanyan
Fu, Huimin Cao, Prof. Qingguo He, Prof. Jiangong Cheng
State Key Lab of Transducer Technology, Shanghai Institute of Microsystem and
Information Technology, Chinese Academy of Sciences, Changning Road 865,
Shanghai 200050, China.
E-mail:windxu@mail.sim.ac.cn;hqg@mail.sim.ac.cn;jgcheng@mail.sim.ac.cn;
Fax:+86-21-62511070-8934; Tel: +86-21-62511070-8967.
[b]
Dr. Wei Xu, Yaguo Yu, Xiaonan Ji, Jinming Chen, Dr. Yanyan Fu, Prof.
Qingguo He, Prof. Jiangong Cheng
Center of Materials Science and Optoelectronics Engineering, University of the
Chinese Academy of Sciences, Yuquan Road 19, Beijing, 100039, China
Supporting information for this article is given via a link at the end of the
document.
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Figure 1 (a),(b)The sketch diagram of the transformation from crystalline CSA structure to the ionized CSA-I structure. (c),(d) XRD
spectra of CBA (black),CSA (blue) and CSA-I (red) films.
To evaluate the effect of the ionization of CSA by DEA, the
morphology of the films was characterized by X-ray diffraction
(XRD) and scanning electron microscopy (SEM). Both CBA and
CSA film showed a diffraction peak near 9° diffraction angle and
a broad diffuse scattering peak from 15º to 22º, while CSA-I film
has no such a sharp peak but with only the diffuse scattering
peak corresponding to an amorphous structure as indicted in
Figure 1c and 1d. SEM images in Figure S1a~e indicate that
CBA tends to form crystalline micro-rod structure (~1 μm), CSA
is incline to show a crystalline nanoparticles (~100nm) together
with micro-rod structures (~1 μm) , while CSA-I film is composed
of amorphous nanoparticles (~100 nm), suggesting that the
introduction of phenol and/or DEA will have a great influence on
the morphology of their films.
As mentioned above, amorphous film will be much better in
vapor permeability than that of crystal or encapsulated film.
Associated with our previous work about peroxide probe[27], we
demonstrated the phosphorescent sensing properties of CSA-I
to H2O2 vapor. As shown in Figure 2c, after reacting with
saturated H2O2 vapor for 30 s, the lifetime of CSA-I film rapidly
declined from 0.14 s to 27 ms. Besides lifetime change, the
relationship curves of both fluorescent and phosphorescent peak
intensity with time of CSA-I film were also measured as shown in
Figure 2d. The phosphorescence of CSA-I film was so stable that
the emission intensity even increased by 2.8% in air condition at
room temperature. When it is exposed to H2O2 vapor, the
phosphorescence was quenched by 30% in 300 s. As
comparison, the fluorescence of CSA-I film was bleached by 24.3%
in air and quenched by 41.1% in H2O2 vapor.
Figure 2 showed the fluorescent and phosphorescent spectra
of the three film samples. Under the UV excitation at 340 nm, the
fluorescent peaks of CBA sample centred at 393, 416 and 443
nm, while the peaks of CSA and CSA-I samples were located
around 430 nm and 450 nm, as shown in Figure 2a. The
fluorescent lifetimes of CBA, CSA and CSA-I were 4.11, 2.56 and
1.99 ns, respectively. Different from their fluorescent spectra,
Figure 2b indicates that the phosphorescent peaks centred at
~540 nm for CSA and CSA-I. The phosphorescent intensity of
CSA-I film is 21.5 times stronger than that of CSA film, even 3
times stronger than that of CBA film (centred at 590 nm) under
the same condition. Meanwhile, the phosphorescence quantum
yield of CSA-I increased by 19.3 times relative to CSA (from 0.22%
to 4.16%). The phosphorescent lifetime curves of CSA and CSA-
I were measured with emission at 540 nm and excitation at 340
nm. According to the curve fitting data, the lifetime of CSA-I film
was 0.14 s, much longer than that of CSA film (30 ms). Evidently,
the ionization of CSA could greatly increase both the
phosphorescent intensity and the phosphorescent lifetime.
Both the FL and PL quenching processes were caused by the
oxidation of aldehyde group to carboxylic acid unit (reaction
schematic shown in Scheme 1), which is reactive in its ionization
state.[27] The active carboxylic acid made the phenol anion
protonated to form neutral phenol, therefore, the charge
separation system was destroyed and the RTP emission was
quenched efficiently. The RTP effect of CSA-I film and its
phosphorescence sensing process in H2O2 vapor were directly
captured by camera as shown in Figure 2e. From 0 ms to 40 ms,
the emission intensity gradually decreased, which shows the
intensity of oxidized CSA-I (right) remains the weaker emission
level compared with the sample without reaction (left). The photo
induced charge separation process continuously enhances the
emission intensity at phosphorescent mode, meanwhile,
decrease the emission intensity at fluorescent mode. Therefore,
the phosphorescent mode of CSA-I amorphous film can better
avoid photo-bleaching effect and reduce noise signal, leading to
a better sensing performance than fluorescent mode.
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Figure 2 The film state (a) fluorescent spectra and (b) phosphorescent spectra of CBA (black), CSA (red) and CSA-I (blue). (c) Lifetime
curves of CSA (black), CSA-I (red) and CSA-I reacted with saturated H2O2 vapor, excitation at 340nm, emission at 540 nm. (d)
Phosphorescence time curves in air (black) and in H2O2 vapor (red) and fluorescent time curves in air (blue) and in saturated H2O2
vapor (green). (e) Photographs of film state CSA-I before (left) and after (right) reacting with saturated H2O2 vapor, at 0ms, 10ms, 20ms,
30 ms,40ms,50ms and 60 ms after switching off 254 nm UV lamp, captured by camera SONY α6300 at 1/100 s shutter speed.
Different with the crystal and encapsulated phosphorescence
material, the reason for the such a stable amorphous RTP
material CSA-I should be uncovered. The mechanism was
systematically analysed by both theoretical simulations and
transient absorption spectra characterizations.
Figure 3b indicated that both the ground state (blue) and triplet
state T1 orbital (red) of CSA-I dimer delocalized on the two
different aromatic units as π orbitals, hence the transition was
long-lifetime 3(π,π*) type. In contrast, as Figure 3c shows, the T1
orbital of CSA-I monomer mainly localized on the Et2NH2+ cation,
while its S1 orbital localized near the formaldehyde group,
indicating a short-lifetime 3(n,π*) transition for T1→S0 and a 1(n,π*)
type transition for S1→S0. Since inter-system transition between
the same orbital type is inhibited, and the existence time of (π,π*)
type T1 state in dimer structure is longer, thus the formation of
ionized dimer structure is advantageous for increasing both the
phosphorescence efficiency and prolonging the lifetime of the
RTP material.
The self-assembling behaviours of CSA-I, CBA, and CSA
molecules were simulated by putting their four respective
molecules into an isothermal-isobaric system (Figure 3a). After a
50 ps dynamic process, each two CSA-I ion pairs tend to form a
stable dimer structure, connected by quadruple hydrogen bonds.
With the formation of dimer structure, the potential energy
decreased by 79.2% (from 87.23 to 18.10 kcal/mol). Meanwhile,
the density of CSA-I system also greatly increased by 61.8%
(from 0.089 to 0.144 g/cm3). After the same dynamic process,
CBA and CSA molecules demonstrated ordered arrangement
(Figure S2), but the potential energy change of CSA was just
8.6%, far lower than that of CSA-I. Movies of the dynamic
simulation processes are available in SI (avi format movies).
Therefore, in spite of lack crystal lattice or package polymer for
a stable structure, the formation of stable dimers in the CSA-I film
could act the same role. As a result, the non-radiative relaxation
was suppressed by electrostatic and hydrogen bonding
interactions without sacrificing its permeability in the amorphous
film of CSA-I.
Furthermore, Ma et al. reported that the strong electrostatic
interaction can further enhance the radiative decay and hinder
the non-radiative decay of phosphorescent materials [28]. For the
dimer structure in CSA-I system, both the strong electrostatic
interaction of ionic bonds and the strong interaction of the
quadruple hydrogen bonds, mainly distributed between the two
phenol anions. Such highly centralized strong interactions can
obviously enhance the triplet radiative transition process. Here
the anion-π interactions of the ionized system also enhance the
inter-system crossing process, act like external heavy atom
effect. [29]
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Figure 3 (a) Space distribution of CSA-I before and after 50 ps isothermal-isobaric dynamics simulation, simulated with the Forcite plus
module in Materials Studio 8. (H-bonds highlighted as shining dot-lines) (b) Ground state S0 (blue) and triplet excited state T1 (red)
orbital distributions of CSA-I H-bonding dimer structure. (c) Ground state S0, S1 and triplet excited state T1 orbital distributions of CSA-
I monomer. (d) Electron paramagnetic resonance spectra of CSA (black), CSA-I (green), CSA under UV light (red) and CSA-I under
UV light (blue). (e) 3D broadband transient absorption spectra of CSA-I film. (f) Transient absorption spectra kinetics curves of CSA
(red) and CSA-I (blue) film.
Electron para-magnetic resonance spectrum was used for
CSA-I film characterization whatever before or after UV irritation
at a wavelength of 365 nm (Figure 3d). Compared with CSA film
before UV excitation, an extra peak at 3508 Gs was observed,
the intensity doubled under UV excitation. These phenomena
suggest the film has a radical character before the UV excitation
and enhanced radicals after UV irritation. The comparison of the
UV-vis absorption spectra of CSA and CSA-I (Figure S10)
suggests that, the π-π* related absorption peak in CSA at 250
nm split into two peaks in CSA-I at 240 nm and 260 nm, probably
caused by the formation of the radical specie. The increased
radical character means that more ion pairs were transferred to
the radiation cycle, which is supported by the ground state
bleaching signal (a continuous concentration loss of ground state
exciton) via its kinetics curves of transient absorption spectrum.
Figure 3f demonstrated, during a 7 ps kinetic process, the
intensity of CSA-I rapidly decreased to negative value in 300 fs,
while the intensity of CSA just gradually approached the zero line.
process as CSA-I, the lifetime of the amorphous TFP-I and DFN-
I were 0.33 ms and 55 μs, respectively, far longer than their
neuter state lifetimes of 0.4 ns and 2 ns.
In conclusion, we have developed a new strategy to obtain
ultra-stable organic RTP materials without crystallization or
encapsulation, based on completely ionized photo-induced
charge separation system. Two RTP materials, CSA-I and TFP-
I has been obtained, with lifetimes as long as 0.16 s and 0.33 ms,
respectively. Both the stability and permeability of these
amorphous RTP materials are so good that they have even been
successfully applied in detection of peroxide vapor. This work
may bring new designing strategy of pure organic RTP materials,
and expand their application area to many new fields such as
catalysis and vapor detection.
Acknowledgment
As reported, the formation of extra radical pair is beneficial for
enhancing the spin-orbit coupling and the intersystem crossing
process from singlet state to triplet state [30], which also contribute
to the increased phosphorescent performance.
This work is supported by the research Programs from Ministry
of Science and Technology (Grant No.: 2016YFA0200800), the
National Natural Science Foundation of China (Grant Nos.
61831021, 61771460 and 51641307), and Shanghai Sailing
Program (No. 19YF1455700). We would also like to express
thanks to Prof. Jinquan Chen and Dr. Xiaoxiao He (East China
Normal University) for their assistance about transient absorption
spectra.
Such designing strategy of ultra-stable amorphous RTP
system is also applicable to other aromatic molecules. For
example, through the introduction of phenol group on 2,4,6-
triformyl benzene and naphthaldehyde, two materials without
phosphorescent property were gained：2,4,6-triformyl-phenol
(TFP) and formyl-naphthol (DFN). After the same ionization
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COMMUNICATION
Ultra-stable organic RTP
without crystallization or
encapsulation, based on new
designing strategy of
completely ionized photo-
induced charge separation.
The RTP lifetime was
Wei Xu*, Yaguo Yu, Xiaonan
Ji, Huarui Zhao, Jinming Chen,
Yanyan Fu, Huimin Cao,
Qingguo He* and Jiangong
Cheng*
Page No. – Page No.
lengthened to 0.16 s while the
intensity was increased by
21.5 times. The excellent
permeability of amorphous
RTP system expand their
application to vapor detection.
Self-stabilized Amorphous
Organic Room Temperature
Phosphorescence
6
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