Manipulating the Stacking of Triplet Chromophores in the Crystal Form for Ultralong Organic Phosphorescence

Article abstract of DOI:10.1002/anie.201907572

Provided here is evidence showing that the stacking between triplet chromophores plays a critical role in ultralong organic phosphorescence (UOP) generation within a crystal. By varying the structure of a functional unit, and different on-off UOP behavior

Full text of DOI:10.1002/anie.201907572

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Accepted Article
Title: Manipulating the Triplet Chromophore Stacking for Ultralong
Organic Phosphorescence in Crystal
Authors: Nan Gan, Xuan Wang, Huili Ma, Anqi Lv, He Wang, Qian
Wang, Mingxing Gu, Suzhi Cai, Yanyun Zhang, Lishun Fu,
Meng Zhang, Chaomin Dong, Wei Yao, Huifang Shi, Zhongfu
An, and Wei Huang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201907572
Angew. Chem. 10.1002/ange.201907572
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10.1002/anie.201907572
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Manipulating the Triplet Chromophore Stacking for Ultralong
Organic Phosphorescence in Crystal
Nan Gan+,[a] Xuan Wang+,[a] Huili Ma,[a] Anqi Lv,[a] He Wang,[a] Qian Wang,[a] Mingxing Gu,[a] Suzhi Cai,[a]
Yanyun Zhang,[a] Lishun Fu,[a] Meng Zhang,[a] Chaomin Dong,[a] Wei Yao,[a] Huifang Shi,*[a] Zhongfu An,*[a]
and Wei Huang*[a,b]
Abstract: Molecular packing has been of great importance for
tailoring the ultralong organic phosphorescence (UOP) properties in
crystal, whereas the underlying mechanism for the UOP properties is
still unclear because of the complex influence factors on
phosphorescence. Herein, we provide some deeper understanding,
from the perspective of structural group, that the stacking between
triplet chromophores really plays a critical role in UOP generation in
crystal. Through adjusting the substitution positions of chlorine atoms,
we found there was a UOP on and off variation from 24CPhCz to
35CPhCz. Remarkably, 24CPhCz with the strongest intermolecular
coupling between carbazole units exhibited the most impressive UOP
with a long lifetime of 1.06 s and a phosphorescence quantum yield
of 2.5%. 34CPhCz showed UOP and TADF dual-emission with a
moderately decreased phosphorescence lifetime of 770 ms, while
35CPhCz only displayed TADF owing to the absence of strong
electronic coupling between triplet chromophores. This study will
provide a clear understanding for explaining UOP generation in crystal
and new guideline for obtaining UOP materials.
been reported.[7] It was revealed that the molecular packing style
in the solid state usually plays a vital role in tailoring the UOP
lifetime,[8] emission efficiency,[9] luminescent color,[10] and even
realizing the unique dynamic UOP and mechano-induced
persistent phosphorescence emission.[11] Despite great success
in studying the relationship between the phosphorescence
properties and molecular packing styles, the underlying
mechanism for the UOP properties is still unclear owing to the
complex influence factors on phosphorescence. Therefore, a
deeper understanding of the effective molecular packing is of
great importance. Generally, each organic phosphor consists of
several groups. In fact, not all the organic group coupling is
effective for the UOP generation. The broad defined molecular
packing involves the stacking between various groups. Some
groups, such as carbazole,[1a,5d,8a] phenothiazine,[8b-c] etc.,
favoring for the triplet excited states could be regarded as triplet
chromophores to manipulate phosphorescence generation, while
the others can be seen as functional groups that may be inclined
to manipulating molecular packing by tuning intermolecular
interactions (Figure 1a). It is noteworthy to think that the stacking
of which group is really essential for UOP generation in crystal.
Ultralong organic phosphorescence (UOP), as a kind of novel
long persistent luminescent phenomenon, is especially attractive
in recent years,[1] because materials with such a feature show
great potential in sensing, displays, bioimaging and anti-
counterfeiting.[2] To obtain UOP materials, tremendous efforts are
mainly devoted to two prerequisites: one is the introduction of
heavy halogen atoms, aromatic carbonyl groups or other
substituents to accelerate intersystem crossing (ISC) between
singlet and triplet excited states; the other is to provide a rigid
molecular environment to suppress the non-radiation transitions
for favoring UOP emission.[3] Among the reported strategies, such
as the polymerization, host-guest doping methods, etc.,[4] crystal
engineering is indispensable to obtain UOP, because molecular
motions can be efficiently restricted in the rigid crystal
environment.[5] In this context, the analysis of molecular packing
in crystal is crucial for understanding the UOP generation.[6]
Up to now, a plenty of investigations on the relationship
between the UOP behaviors and the molecular packing have
[a]
N. Gan+, X. Wang+, Dr. H. Ma, A. Lv, H. Wang, Q. Wang, M. Gu, S.
Cai, Y. Zhang, L. Fu, M. Zhang, C. Dong, Dr. W. Yao, Prof. H. Shi*,
Prof. Z. An*, Prof. W. Huang*
Key Laboratory of Flexible Electronics (KLOFE) & Institute of
Advanced Materials (IAM)
Nanjing Tech University (NanjingTech), 30 South Puzhu Road,
Nanjing 211816, China
E-mail: iamhfshi@njtech.edu.cn; iamzfan@njtech.edu.cn;
iamwhuang@njtech.edu.cn
Figure 1. a) Schematic illustration of different triplet chromophore stacking
styles for tailoring UOP on and off behaviors. b) Chemical structures of three
compounds and their luminescent properties.
[b]
[+]
Prof. W. Huang
Institute of Flexible Electronics (IFE)
Northwestern Polytechnical University (NPU), 127 West Youyi Road,
Xi'an 710072, China
These authors contributed equally.
Herein, we proposed a specific explanation that the stacking
between triplet chromophores really plays a critical role in UOP
generation in crystal. We designed three isomers to manipulate
the triplet chromophore stacking by tiny modulation on the
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201YXXXXX.
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Figure 2. Photophysical properties of aromatic amide derivatives in the solid state. a) Steady-state photoluminescence (blue line) and phosphorescence
(red line) spectra of 24CPhCz, 34CPhCz and 35CPhCz. Insets show photographs of 24CPhCz, 34CPhCz and 35CPhCz crystals under 365 nm UV light
on and off. b) Excitation-phosphorescence mapping of 24CPhCz crystal under ambient conditions. c) Lifetime decay profiles of the ultralong
phosphorescence bands of 24CPhCz and 34CPhCz at 532 and 537 nm, respectively. d) Lifetime decay profile of fluorescence band of 35CPhCz at 488
nm. e) Steady-state fluorescence spectra of 35CPhCz at different temperatures. The inset shows the luminescent intensity changes.
functional group (Figure 1b). With isomerization of chlorine atoms
at phenyl unit, we found that there was an interesting UOP
switching on−off behavior in these isomers. Remarkably,
24CPhCz exhibited an ultralong phosphorescence lifetime of up
to 1.06 s, and 34CPhCz showed UOP (770 ms) and thermally
activated delayed fluorescence (TADF, 1.9 μs) dual-emission. For
35CPhCz, however, there displayed only TADF with a short
lifetime of 2.7 μs. Taken X-ray single crystal analysis and
theoretical calculations together, we proposed that the
intermolecular coupling between carbazole chromophores played
a critical role in tailoring the two competing processes of UOP and
TADF emission in crystal at room temperature.
spectra (Figure S1-6).
Firstly, the photophysical properties of these isomers were
systematically investigated in dilute solution. As depicted in
Figure S7, the absorption spectra in dichloromethane solution
(DCM, 50 μM) of these molecules showed similar peaks at 300
and 315 nm. Accordingly, the steady-state photoluminescence
(PL) spectra in DCM revealed splitting peaks at 340 and 353 nm,
which might be attributed to locally excited states (Figure S8).[12]
Whereas the wide bands at around 533, 550 and 590 nm were
ascribed to intramolecular charge transfer (ICT) states owing to
their twisted D–A architectures, which was proved by the red-shift
of the wide emission bands with the solvent polarity increased.[13]
Besides, these isomers all present similar phosphorescence
profiles with peaks at 419 and 448 nm in dilute dimethyl
tetrahydrofuran (50 μM) at 77 K, indicating the phosphorescence
in single molecule state originated from the same chromophore
(Figure S9). Interestingly, the PL and phosphorescence spectra
were well-overlapped at 77 K, indicating a very high ISC efficiency
of singlet excitons. Obviously, these compounds show the similar
photophysical properties in dilute solution.
In this work, the aromatic amide derivatives, namely, (9H-
carbazol-9-yl) (2,4dichlorophenyl) methanone (24CPhCz), (9H-
carbazol-9-yl) (3,4-dichlorophenyl) methanone (34CPhCz) and
(9H-carbazol-9-yl) (3,5-dichlorophenyl) methanone (35CPhCz)
were easily synthesized by one-step reaction with high yields of
over 50% (Scheme S1). Their chemical structures were fully
1
characterized by H NMR, 13C NMR, elemental analysis, X-ray
single-crystal diffraction analysis and high-resolution mass
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In solid state, 24CPhCz, 34CPhCz, and 35CPhCz performed
distinct emission features. As shown in Figure 2a, their emission
colors in crystals are obviously tailored through altering the
substitution positions of two chlorine atoms, displaying blue,
green-white and green emission, respectively. Correspondingly,
34CPhCz and 35CPhCz presented a broad, structureless
24CPhCz
and
34CPhCz
demonstrated
unchanged
phosphorescence emission position with the excitation
wavelength changed from 260 to 460 nm (Figure 2b and Figure
S10). The UOP properties were further demonstrated by the
lifetime decay measurements and oxygen sensitivity experiments
(Figure S11). As depicted in Figure 2c and Figure S12, 24CPhCz
emission band at 454 and 488 nm in their PL spectra, respectively, and 34CPhCz showed an ultralong lifetime of 1.06 s and 0.77 s
which was different from the fine structural peaks with blue shift
of 24CPhCz. Impressively, after removal of UV light, ultralong
yellow phosphorescence for 24CPhCz and 34CPhCz was
observed over 5 seconds with the naked eye, but it could not be
observed for 35CPhCz crystal (SV1-SV3). With a delay time of 5
ms, it was found that 24CPhCz displayed two phosphorescent
emission peaks at 532, 574 nm and a shoulder at around 620 nm,
while 34CPhCz showed a major emission peaks at 537 nm and
two shoulders at 582 and 636 nm. However, there was no
emission signal detected for 35CPhCz at room temperature. The
3D-scanning of excitation-phosphorescence emission for
with a phosphorescence efficiency (Phos) of 2.5% and 3.4%,
respectively (Table S1). Different from 24CPhCz (Figure S13), the
transient PL decay of 34CPhCz and 35CPhCz displayed a TADF
feature with nanosecond-order (4.3 and 3.1 ns) prompt
components and microsecond-order (1.9 and 2.7 μs) delayed
components (Figure 2d, S14a and Table S2).[14] The TADF
properties of 34CPhCz and 35CPhCz were further confirmed by
the temperature dependent PL spectra changes in PL intensity
(Figure 2e and S14b). With the temperature increased, the
emission intensity of the structureless bands exhibited decrease
first and then increased, demonstrating a typical TADF feature.
Figure 3. A probable explanation of ultralong organic phosphorescence in crystal state. Intermolecular interactions of a) 24CPhCz, b) 34CPhCz and c) 35CPhCz
single molecules. The packing modes of d) 24CPhCz, e) 34CPhCz and f) 35CPhCz in crystals, respectively (The depth cue is to tell the spatial distance between
two molecular groups not on the same plane).
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The initial decrease of the total emission intensity was attributed
to the competing process between phosphorescence (Figure
S15) and TADF emission with the temperature increase.[15] In the
amorphous films, however, characteristic phosphorescence
emissions in crystal disappeared when dispersed in the PMMA
matrix (Figure S16, S17 and Table S3). In a more rigidified
Zeonex matrix, blue-green phosphorescence can only be
detected in 24CPhCz/Zeonex film with main peak at 495 nm and
In order to reveal the underlying reason of why strong C-H···π
stacking between carbazoles could play such a critical role in
UOP generation in these isomers, the density-functional theory
(DFT) calculations were carried out (Figure S27). It was found that
the natural transition orbitals (NTOs) of the lowest triplet state (T1)
were mainly distributed on carbazole units (Figure 4a), indicating
carbazole acts as triplet chromophore for phosphorescence
occurrence in this system. Therefore, the strong C-H···π stacking
between carbazoles could stabilize the triplet excitons for
ultralong phosphorescence emission (Figure 4b). To further verify
our speculation, we made the independent gradient model (IGM)
analysis to directly show the intermolecular weak interactions.
From Figure 4c and S28, it is obvious that 24CPhCz displayed the
largest isosurface between two carbazoles, which indicated the
strongest weak interactions as well as the efficient electronic
communications among triplet species. Meanwhile, the isosurface
between benzene units was much more obvious in 35CPhCz.
Besides, the isosurface displayed a gradual decreasing trend
from 24CPhCz, 34CPhCz to 35CPhCz, which was in accordance
with the UOP lifetime in the three compounds. Combining the
experimental data and theoretical calculations, we provide a
deeper understanding that the stacking between triplet
chromophores really plays a critical role in UOP generation in
crystal.
a lifetime of 30.55 ms under N2 atmosphere Figure S18). For
(
34CPhCz and 35CPhCz/Zeonex films, structureless TADF
emission bands were observed (Figure S19-S21).[16] From these
experimental results, we speculated that the distinct UOP
behaviors in the three isomers were mainly attributed to their
packing modes in crystal state rather than single molecules. To
further ascertain the intermolecular interaction effect in crystal, we
compared the photophysical properties of compounds in
crystalline and amorphous states under ambient conditions
(Figure S22 and S23). Note that the amorphous material is
prepared by the treatment of fusing by heating and quick freezing
at 77 K. It was found that the UOP of 34CPhCz disappeared in
amorphous state, indicating the UOP generation was closely
related to the special molecular packing in crystal state.
To gain a deep insight into the various UOP properties of these
isomers, we carefully analyzed the intermolecular interactions
and packing modes in their single crystal (Table S4). All of the
three compounds showed a twisted molecular configuration with
dihedral angle more than 60o between carbazole and benzene
units (Figure S24). As shown in Figure 3a-3c, there exists plenty
of intermolecular interactions in 24CPhCz crystal. The molecules
were rigidly restricted by abundant interactions, including C-
Cl···H-C (2465~2.886 Å), C=O···Cl (3.248 Å), Cl···Cl (3.180 Å), C-
Cl···π (3.128~3.414 Å) and C-H···π (2.828, 2.887 Å), which
contributes to restricting nonradiative transitions through
molecular motions to prolong the UOP lifetime. In comparison,
there are much fewer short contacts around 34CPhCz and
35CPhCz, meanwhile the individual interactions of the two
compounds are almost equal. However, their luminescent
properties are very different in crystal state, thereinto 34CPhCz
displayed obvious UOP while 35CPhCz exhibited no
phosphorescence emission. This finding indicates the molecular
confinement effect is an important but not key factor to the UOP
generation. From the perspective of the entire packing styles of
these crystals, we found that 24CPhCz and 34CPhCz showed a
similar stacking mode to carbazole in crystal (Figure S25),[17]
which can’t be observed in 35CPhCz crystal (Figure 3d-3f). More
carefully analyzing the localized stacking modes between two
adjacent carbazoles, it can be found that there existed very strong
coupling between two adjacent carbazoles with C-H···π
interactions (2.828 Å) in 24CPhCz crystal, which was slightly
weaker in 34CPhCz with a longer distance between carbazoles
(C-H···π=2.917) (Figure S26a and S26b). In contrast, for the
TADF emitter 35CPhCz, there were only C-H···π (3.307 Å), C-
Cl···π (2.873 Å) and C-H···Cl (2.892 Å) between benzene units or
between benzene and carbazole (Figure S26c). Nevertheless, the
coupling between carbazoles was very week, which may be
mainly responsible for the exhaustive shut of UOP emission in
35CPhCz crystal. Based on these analyses, we speculated that
the UOP generation is closely related to the stacking style
between carbazole units in the three isomers.
Figure 4. a) Natural transition orbitals (NTOs) for the lowest triplet state of
24CPhCz. b) The mechanism of the different emission behaviors in the three
compounds. c) The calculated intermolecular weak interactions (green
isosurface) in dimers of 24CPhCz, 34CPhCz and 35CPhCz (the isovalue is
0.007).
In summary,
a
new insight into ultralong organic
phosphorescence in crystal was proposed in several aromatic
amide derivatives. Through adjusting chlorine substitution
positions, there was an interesting UOP switching on−off behavior
in these isomers. Experimental and calculated results revealed
that 24CPhCz with the strongest intermolecular coupling between
carbazole chromophores exhibited the most impressive UOP
emission with a long lifetime of 1.06 s and a Phos of 2.5%.
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X. Zhou, D. Jana, G. Liu, W. Q. Lim, W. K. Ong, C. Yang, Y. Zhao, Sci.
Adv. 2018, 4, eaas9732.
34CPhCz was UOP and TADF dual-emissive with a moderately
decreased phosphorescence lifetime of 770 ms and a Phos of
3.4%. However, the too weak coupling between triplet
chromophores but strong coupling between benzene units
resulted in the disappearance of UOP and only TADF emission
with a luminescent lifetime of 2.7 μs for 35CPhCz. These results
demonstrate it is the triplet chromophores stacking that really
plays a critical role in UOP generation. Our viewpoint can provide
some deeper understanding for the inherent mechanism of
ultralong phosphorescence emission in purely organic
compounds and new guideline for obtaining UOP materials.
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Acknowledgements
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This work is supported by the National Natural Science
Foundation of China (21875104, 51673095, 91833304 and
91833302), National Basic Research Program of China (973
Program, No. 2015CB932200), Natural Science Fund for
Distinguished Young Scholars of Jiangsu Province (BK20180037).
We are grateful to the High Performance Computing Center of
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resources.
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Keywords: crystal engineering • triplet chromophore stacking •
ultralong organic phosphorescence • intermolecular Interactions
• thermally activated delayed fluorescence
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10.1002/anie.201907572
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Ultralong organic phosphorescence
can be controlled by manipulating the
triplet chromophore stacking in crystal.
The stronger coupling between
carbazole chromophores favours the
UOP generation of 24CPhCz and
34CPhCz, while the too weak coupling
between triplet chromophores but
strong coupling between benzene units
resulted the disappearance of UOP
and only TADF emission in 35CPhCz.
Nan Gan+, Xuan Wang+, Huili Ma, Anqi
Lv, He Wang, Qian Wang, Mingxing
Gu, Suzhi Cai, Yanyun Zhang, Lishun
Fu, Meng Zhang, Chaomin Dong, Wei
Yao, Huifang Shi,* Zhongfu An,* and
Wei Huang*
Page No. 1– Page No. 6
Manipulating the Triplet
Chromophore Stacking for
Ultralong Organic
Phosphorescence in Crystal
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