Amorphous Pure Organic Polymers for Heavy-Atom-Free Efficient Room-Temperature Phosphorescence Emission

Article abstract of DOI:10.1002/anie.201803947

Pure organic, heavy-atom-free room-temperature phosphorescence (RTP) materials have attracted much attention and have potential applications in photoelectric and biochemical material fields owing to their rich excited state properties. They offer long luminescent lifetime, diversified design, and facile preparation. However, recent achievements of efficient phosphorescence under ambient conditions mainly focus on ordered crystal lattices or embedding into rigid matrices, which require strict growth conditions and have poor reproducibility. Herein, we developed a concise approach to give RTP with a decent quantum yield and ultralong phosphorescence lifetime in the amorphous state by radical binary copolymerization of acrylamide and different phosphors with oxygen-containing functional groups. The cross-linked hydrogen-bonding networks between the polymeric chains immobilize phosphors to suppress non-radiative transitions and provide a microenvironment to shield quenchers.

Full text of DOI:10.1002/anie.201803947

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Accepted Article
Title: Heavy-Atom-Free Amorphous Pure Organic Polymers with
Efficient Room-Temperature Phosphorescence Emission
Authors: He Tian, Xiang Ma, Chao Xu, and Jie Wang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201803947
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10.1002/anie.201803947
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Heavy-Atom-Free Amorphous Pure Organic Polymers with
Efficient Room-Temperature Phosphorescence Emission
Xiang Ma,* Chao Xu, Jie Wang and He Tian*
Abstract: Pure organic, especially heavy-atom-free room-
temperature phosphorescence (RTP) materials have attracted much
attention and presented potential applications in photoelectric and
biochemical material fields due to rich excited state properties, with
advantages of long luminescent lifetime, diversified design and facile
single crystals. [18] Kim et al. developed a bright mixed crystal
material with high RTP quantum yield through heavy-atom effect
directed by halogen bonding. [19] However, it is still a challenge for
crystallization-induced phosphorescent systems to prepare thin
solid film light-emitting devices with facile processability and good
repeatability, which further hampers the commercialized
applications of RTP materials.
Thus, it is essential to develop pure organic amorphous RTP
materials. The common method to obtain such materials is to
embed the phosphors into rigid polymer matrixes, [20, 21] steroidal
compound, [22, 23] the cavity of macrocycle hosts, [24-27] and so forth.
In addition, our group designed a series of phosphors to
copolymerize with acrylamide that could engender effective
phosphorescence emission of different colors under ambient
conditions, [27-29] in which the hydrogen bonding network restricted
the molecular motions and the halogen atom (e.g. bromine)
enhanced the spin-orbit coupling. In these cases, the halogen
atoms are essential units in order to facilitate the intersystem
crossing. [30-32] While, it is well-known that the use of heavy atoms
will cause the RTP emission lifetime shortly, which is a typical
blinking time for human eyes and hard to observe. [33] Although
great efforts have been devoted, RTP from pure organic
preparation.
However,
recent
achievements
of
efficient
phosphorescence under ambient condition mainly focused on ordered
crystal lattice or embedding into rigid matrices, which means that strict
growth conditions and difficulty repeatability are inevitable. Herein, we
developed a concise approach to realize RTP exhibiting considerably
decent quantum yield and ultralong phosphorescence lifetime in
amorphous state by radical binary copolymerization of acrylamide and
different phosphors with oxygen-containing functional groups. The
cross-linked hydrogen bonding networks among polymeric chains
immobilize phosphors to suppress non-radiative transitions and
provide microenvironment to shield quenchers in pursuit of ultralong
phosphorescence. This facile and universal strategy may pave the
way toward expanding amorphous heavy-atom-free organic RTP
materials and their applications.
Room-temperature phosphorescence (RTP) materials with
persistent luminescence have attracted considerable attention
because of unique generation process and long-lived excited
state leading to visual observation of ultralong lifetimes emission
from seconds to several hours.[1, 2] Thus, these RTP materials can
be applied to the fields of biological imaging, [3, 4] photodynamic
therapy, [5] molecular sensing, [6] information storage, [7] and so
amorphous materials without halogen atoms remains
formidable challenge.
a
In this work, a concise approach was provided to realize RTP
by radical binary copolymerization of acrylamide and different
phosphors containing oxygen atom and benzene moiety without
heavy atom substituents (Scheme 1, P1-P10). Polyacrylamide is
an ideal polymer matrix to immobilize phosphors, which can also
be strengthened by the hydrogen bond cross-linking between
polymeric chains. Meanwhile, oxygen atom favors n–π* transition
and hence facilitates the spin-forbidden transfer of singlet-to-
triplet excited states through intersystem crossing to populate
triplet excitons. As a result, the materials prepared herein can
engender intense RTP signals and show an ultralong
phosphorescent lifetime. To our knowledge, this would be the first
report about heavy-atom-free pure organic amorphous RTP
materials.
A series of pure organic molecules with oxygen-containing
functional groups (compound 1-10) were employed to
copolymerize with acrylamide and prepare amorphous polymers
P1-P10. The general synthetic route of the polymer and the
structures of monomers were shown in Scheme 1. All the
compounds were purchased from commercial sources directly or
synthesized via a few simple steps (Figure S1-S3, SI). And
compound 1-10 were copolymerized with acrylamide at the molar
ratio of 1:50 unless otherwise stated. Among all the polymers, P3
possesses the longest phosphorescent lifetime and a decent
quantum yield in the solid state. Under 254 nm irradiation, the
solid powder of P3 exhibited intense RTP emission in air. And the
powder showed blue-colored persistent luminescence after
removing the ultraviolet source, which could last for several
seconds (Figure 1b). Thus P3 was investigated as a model in the
following discussions.
forth. Until now, phosphors with RTP are mainly coordination
[8-11]
complexes that contain noble metals.
Since these
organometallic compounds are inevitably expensive and relatively
toxic, novel pure organic phosphorescent materials is necessary
to be developed. For pure organic systems with RTP, two vital
factors should be considered carefully when designing molecules:
(1) facilitating the spin-forbidden transfer of singlet-to-triplet
excited states through intersystem crossing (ISC) to populate
triplet state, (2) inhibiting the non-radiative relaxation pathways as
much as possible. Based on fundamental molecular design
principles, crystallization-induced phosphorescent systems have
been reported as an effective approach to construct RTP
materials not only from the rigidity of system to suppress non-
radiative transitions, but also at the isolation of oxygen molecules,
[12-17]
which could easily quench the triplet state.
Huang et al.
designed a series of pure organic molecules with ultralong RTP
lifetimes ranging from 0.21 to 1.35 s through H-aggregation in
Prof. X. Ma, C. Xu, J. Wang and Prof. H. Tian
Key Laboratory for Advanced Materials and Joint International Research
Laboratory of Precision Chemistry and Molecular Engineering, School of
Chemistry and Molecular Engineering, East China University of Science &
Technology, Meilong Road 130, Shanghai 200237, China.
E-mail: maxiang@ecust.edu.cn & tianhe@ecust.edu.cn
Supporting information for this article is given via a link at the end of the
document. ((Please delete this text if not appropriate))
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H2N
m
O
a)
n
O
O
O
O
O
O
O
O
COOH
NH2
EDCI/DMAP
CH2Cl2
O
+
P3
AIBN, DMF, 65 o
C
COOH
OH
n
m
b)
O
NH2
O
O
O
O
O
O
O
O
O
O
O
COOH
COOH
O
O
1
2
4
3
5
O
O
O
O
CHO
COOH
OH
COOH
O
6
7
8
9
10
Scheme 1. a) The general synthetic route of the polymers; b) Molecular
structures of compounds 1 to 10.
As shown in Figure 1a, P3 solid powder exhibited a delayed
luminescence at a wide wavelength region (λmax = 427 nm) with
an ultralong lifetime (τ up to 537 ms, quantum yield Φ = 15.39%),
along with fluorescence emission at λmax = 322 nm (λex = 289 nm).
The lifetime of the persistent luminescence is exceptionally long
for pure organic compounds. And the blue-colored afterglow
could be traced in a time range of 0-4 s by the naked eye (Figure
1b). The large Stokes shift of RTP emission of P3 is around 140
nm, corresponding to the RTP characteristics. Meanwhile, the
maximum emission in phosphorescence mode (λmax = 427 nm)
Figure 1. a) UV-Vis absorption (blue), steady-state fluorescence (black) and
RTP (red) spectra of P3. Inset: luminescent decay lifetime of P3 at 427 nm. b)
Luminescence photographs of P1-P10 powder under 254 nm UV irradiation and
at different time intervals after removal of ultraviolet lamp.
redshifts 105 nm compared with fluorescence emission (λmax
=
322 nm), which further validates the afterglow belongs to
phosphorescence instead of possible delayed fluorescence.
Moreover, as we expected, the RTP intensity of polymers varied
with the molar ratios of two monomers. Generally, an intense RTP
emission could be observed at lower amount of phosphor
benefiting from more efficient embeddedness and wider
distribution. In contrast, a higher amount of phosphor leads to
weaker rigidity and shielding effect from oxygen. Even so, an
ultralow ratio will also lead to weak RTP emission. Among
different ratios of copolymerization, the RTP intensity of 1/50 was
determined to be the highest (Figure 2a). And the photophysical
properties of P3 with different ratios was shown in Table S1. X-
ray powder diffraction (XRD) analysis was carried out to explore
the microstructure of these polymers (Figure 2b). P1, P3, P5, P9
and polyacrylamide (PAM) as reference polymers were analyzed.
And the results verify the amorphous state of these polymers.
Figure 2. a) RTP intensity of P3 with different molar ratios of m/n at 427 nm.
(Phosphorescence mode; excitation slim = emission slim =10 nm; excitation
voltage = 600 V in solid state. b) XRD patterns of PAM, P1, P3, P5 and P9.
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To probe the mechanism of the observed ultralong
luminescence from pure organic compounds, photophysical
properties of compounds with different oxygen-containing
functional groups were discussed. All these compounds exhibited
RTP emission and triplet lifetimes ranging from 16 ms to 537 ms
at the molar ratio of 1:50 with acrylamide (Table 1, Figure 1b),
which validates the type and position of functional groups, length
of branch chain and the cross-linked structure are not necessary
key factors to RTP. The oxygen atoms with lone pair electrons are
favorable for the n–π* transition, which enhances the spin–orbit
coupling. This in turn populates the triplet states through singlet–
triplet intersystem crossing. [18] Meanwhile, the hydrogen bonding
among polymeric chains can immobilize the RTP phosphor to
reduce energy dissipation from non-radiative relaxation pathways.
In addition, the micro-environment with rich hydrogen bonds can
shield phosphorescent moieties from oxygen quenching effect
and weaken the collisional deactivation as well. As a result, pure
organic ultralong phosphorescence without heavy atom under
ambient conditions could be realized.
structure of P3’ were shown in Figure S11. With the low degree
of hydrogen bond cross-linked network formed by N,N-dimethyl
acrylamide, no signal of RTP at 427 nm was detected (Fig. S12).
Table 1. Photophysical data of P1 to P10 in the solid at 300 K[a]
.
λFluo
λPhos
[nm]
λex[nm]
τp[ms]
Φp
[%]
[nm]
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
243
289
289
277
321
259
260
280
280
278
368
434
123
522
537
81
6.40
17.87
15.39
4.06
5.45
0.01
5.15
4.04
30.55
6.38
n.d.[b]
322
425
427
n.d.[b]
413
426[c]
497[c]
430
218
28
n.d.[b]
313
418
198
106
195
16
314,383
n.d.[b]
n.d.[b]
424
433
Figure 3. Jablonski diagram showing radiative and non-radiative processes
from the lowest excited triplet state in aromatic compounds (kr is the radiative
rate constant of phosphorescence, knr(T) is the non-radiative rate constant from
the T1 state at T [K], kq(T) is the tripet exciton quenching rate constant derived
from interactions with oxygen at T [K]). Material design of significant decrease
of the non-radiative decay rate provides P3 with efficient ultralong RTP in air.
423[c]
[a] The spectral data are shown in Figures S13-S21. [b] Not detected. [c]
The intensity of the phosphorescence emission is weak.
In order to confirm the necessity of copolymerization between
the phosphorescent monomer and acrylamide for the
achievement of RTP emission, compound 2 was doping into
polyacrylamide (PAM) as control experiments because PAM was
only water-soluble and compound 2 exhibited better water-
solubility than other monomers. Comparing the emission
intensities of phosphorescence (Fig. S10), P2 manifested better
RTP-emissive property superior to the mixture (dP2), which
indicated that the covalent bonding from copolymerization
effectively restricted the translational motion of the phosphor. As
mentioned above, the hydrogen bond cross-linked networks could
restrict the molecule motion of phosphor monomer, which greatly
enhanced the intensity of RTP emission. To evaluate this
influence, copolymerization of compound 3 and N,N-dimethyl
acrylamide was conducted (P3’). The experiment detail and
From Fig. 3 Equations (1) and (2), knr(RT) + kq(RT)<kr and a high
Φisc(RT) value are required to realize bright ultralong RTP
emission under ambient conditions. For terephthalic acid (TPA)
with similar structure to compound 3, intrinsic kr is 1.75×100 s−1 in
crystal.[33] Therefore, the key is to minimize the non-radiative
decay rate (knr) and the quenching rate constant (kq) of the lowest
triplet excited state (T1 state) at R.T. The cross-linked hydrogen
bonding networks among polymeric chains hinder molecular
motions (rotations, vibrations and collisions) significantly, thus
suppressing non-radiative deactivations. Meanwhile, such
microenvironment with rich hydrogen bonds can efficiently shield
quenchers like oxygen. As a result, the value of knr(RT) +
kq(RT)<<<kr<1.75×100 s−1. Although the use of heavy atoms is
useful in attaining a high Φisc(RT) and thus contributes to a high
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Φp(RT), it also accelerates kr and knr(RT), which typically results
in a shorter τp(RT) lifetime based on Equation (2). Therefore,
oxygen-containing functional groups with lone pair electrons in
favour of n–π* transition were introduced to enhance the spin–
orbit coupling,[34] realizing heavy-atom-free amorphous RTP
materials with larger ΦP(RT). Compared with other molecules, P3
is ideal in this regard with a quantum yield =15.39 % and a lifetime
=0.537 s because of the promotion of the n-π* transition from
constituting oxygen atoms and the intrinsic cross-linked structure
with more rigidity as well. The existence of heavy atom in similar
polymers just resulted in a shorter lifetime (τ=1.15 ms). [28]
emission without the rigorous condition including deoxygenization
and low temperature. The understanding gained from the
experimental results will allow for the construction of next-
generation amorphous RTP materials that may inspire the
development of organic devices, molecular imaging, water
sensing and data security.
Acknowledgements
We gratefully acknowledge the financial support from NSFC
(21788102, 21722603, 21421004 and 21476075), Programme of
Introducing Talents of Discipline to Universities (B1607), the
Innovation Program of Shanghai Municipal Education
Commission and the Fundamental Research Funds for the
Central Universities (222201717003).
Keywords: room-temperature phosphorescence • heavy-atom-
free • amorphous polymer
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
We developed a concise approach
to realize efficient RTP with ultralong
lifetime in amorphous state by
radical binary copolymerization of
acrylamide and different phosphors
with oxygen-containing functional
groups. This facile and universal
strategy may pave the way toward
expanding amorphous heavy-atom-
free organic RTP materials and their
applications.
Xiang Ma,* Chao Xu, Jie Wang and He
Tian*
Page No. – Page No.
Heavy-Atom-Free Amorphous Pure
Organic Polymers with Efficient
Room-Temperature
Phosphorescence Emission
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