Long-Lived Room-Temperature Phosphorescence for Visual and Quantitative Detection of Oxygen

Article abstract of DOI:10.1002/anie.201906312

An unconventional organic molecule (TBBU) showing obvious long-lived room temperature phosphorescence (RTP) is reported. X-ray single crystal analysis demonstrates that TBBU molecules are packed in a unique fashion with side-by-side arranged intermolecular aromatic rings, which is entirely different from the RTP molecules reported to date. Theoretical calculations verify that the extraordinary intermolecular interaction between neighboring molecules plays an important role in RTP of TBBU crystals. More importantly, the polymer film doped with TBBU inherits its distinctive RTP property, which is highly sensitive to oxygen. The color of the doped film changes and its RTP lifetime drops abruptly through a dynamic collisional quenching mechanism with increasing oxygen fraction, enabling visual and quantitative detection of oxygen. Through analyzing the grayscale of the phosphorescence images, a facile method is developed for rapid, visual, and quantitative detection of oxygen in the air.

Full text of DOI:10.1002/anie.201906312

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Accepted Article
Title: Long-Lived Room-Temperature Phosphorescence for Visual and
Quantitative Detection of Oxygen
Authors: Yusheng Zhou, Wei Qin, Cheng Du, Haiyang Gao, Fangming
Zhu, and Guodong Liang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201906312
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Angewandte Chemie International Edition
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Long-Lived Room-Temperature Phosphorescence for Visual and
Quantitative Detection of Oxygen
Yusheng Zhou,[a]† Wei Qin,[a]† Cheng Du, Haiyang Gao, Fangming Zhu, and Guodong Liang*
[a]
[a]
[b]
[a]
Abstract: An unconventional organic molecule (TBBU) showing
obvious long-lived room temperature phosphorescence (RTP) is
reported. X-ray single crystal analysis demonstrates that TBBU
molecules are packed in a unique fashion with side-by-side arranged
intermolecular aromatic rings, which is entirely different from the
RTP molecules reported so far. Theoretical calculation verifies that
the extraordinary intermolecular interaction between neighboring
molecules plays an important role in RTP of TBBU crystals. More
importantly, the polymer film doped with TBBU inherits its distinctive
RTP property, which is highly sensitive to oxygen. The color of the
doped film changes and its RTP lifetime drops abruptly through a
dynamic collisional quenching mechanism with increasing oxygen
fraction, enabling visual and quantitative detection of oxygen. Last
but not least, through analyzing grayscale of the phosphorescence
carbonyl, are introduced to promote spin-orbit coupling (SOC)
and subsequent ISC. In the latter, H-aggregation,[6] host-guest
doping,[ and crystallization are used to suppress non-radiative
decay of excited triplet state.[8] Therefore, most of the organic
RTP compounds reported are constructed from aromatic
[5]
7]
[
9]
moieties bearing halogen atoms and/or carbonyl groups. In
crystals, aromatic rings are packed in a face-to-face manner
(Figure 1a), so that intermolecular π-π interaction suppresses
non-radiative decay of triplet excitons and further promotes
RTP.[10]
RTP of organic compounds is susceptible to oxygen due to
the strong interaction of triplet excitons with oxygen molecules,[
which offers a unique opportunity for oxygen sensing. However,
most organic RTP materials are crystals of aromatic compounds,
in which planar aromatic rings are tightly-packed in a face-to-
face fashion through strong intermolecular π-π interaction.[
Although such π-π stacking is favorable for RTP, the compact
molecules prevent oxygen from diffusing through, which is
detrimental to oxygen sensing. Indeed, although several types of
long-lived RTP organic molecules are reported (Figure S1,
Supporting Information, SI), few of them are explored for oxygen
detection. Herein, we report an unconventional RTP molecule,
namely TBBU, which does not contain any heavy atoms such as
halogen atoms, carbonyl group, or face-to-face π-π stacking in
crystals. TBBU has asymmetric non-coplanar D-π-A skeleton,
which avoids π-π stacking of the intermolecular aromatic rings.
The presence of electron donor (D) and electron acceptor (A)
units enhances intermolecular interaction through multiple D-A,
D-π, and A-π interactions. TBBU molecules are aligned in a
head-to-tail manner with intermolecular aromatic rings side-by-
side arranged (Figure 1a), so that lots of large pores retain in
crystals, which allows oxygen molecules defusing through.
TBBU crystals show obvious RTP with high efficiency (0.13) and
long lifetime (350 ms) in vacuum. Moreover, the polymer film
doped with TBBU inherits its RTP, which is sensitive to oxygen.
The color of the film changes from pink-blue to deep blue with
increasing oxygen fraction. Meanwhile, the reciprocal of the RTP
lifetime of the doped film increases linearly with increasing
oxygen fraction, which allows quantitative detection of oxygen.
Finally, through analyzing grayscale of the phosphorescence
images of the TBBU doped film, a facile and reliable method is
developed for oxygen detection. Only tens of milligrams of TBBU
doped polymers, coupled with a UV lamp and a cell phone allow
quick, visual, and quantitative determination of oxygen fraction.
Taking advantages of oxygen-sensitive and long-lived RTP,
easy synthesis, and cost-efficiency, TBBU opens up a new
avenue to facile detection of oxygen.
11]
images,
a facile method is developed for rapid, visual, and
8]
quantitative detection of oxygen in the air. With the merits of oxygen-
sensitive RTP, cost-efficiency, and simple devices, TBBU opens up
a new avenue to facile detection of oxygen.
Oxygen detection is very important in various fields such as
biomedicine, food storage, vacuum technique, and so on.[1] The
common methods for oxygen detection are based on
electrochemical technique using metal oxides (zirconia or titania)
as sensing units. The presence of oxygen changes the potential
of the metal oxide film, which allows oxygen detection.
Unfortunately, electrochemical methods suffer from some
inherent limits such as slow response, susceptibility to moisture,
and poor sensitivity due to the nonlinear relationship of output
signals against oxygen concentration. Therefore, the
development of a rapid and sensitive method for oxygen
detection is highly desirable.[2]
Organic room-temperature phosphorescence (RTP) materials
have attracted increasing attention due to their low toxicity and
cost, long lifetime, large Stokes shift, and good processability,
with potential applications in optoelectronic devices, high-
[
3]
resolution bioimaging, molecular sensing. However, organic
RTP materials generally show a shorter lifetime (<100 ms) at
room temperature, because of their weak intersystem crossing
(
ISC) between excited singlet and triplet states, and rapid non-
radiative decay of triplet excitons through thermal perturbation
and molecular motion. Correspondingly, two strategies including
enhancement of ISC and stabilization of triplet excitons are
generally employed to achieve RTP in organic materials.[4] In the
former, heavy atoms with a lone pair such as Br and I, and
[
[
a]
b]
Y. Zhou, Dr. W. Qin, C. Du, Prof. H. Gao, Prof. G. Liang
PCFM Lab, School of Materials Science and Engineering, Sun
Yat-sen University, Guangzhou, 510275, China
E-mail: lgdong@mail.sysu.edu.cn.
TBBU and its counterpart (TBU) were facilely synthesized
from cheap starting compounds with satisfactory yields through
two- and one-step simple reactions, respectively (Scheme S1-
S3, SI). They are soluble in common organic solvents, such as
tetrahydrofuran (THF), dichloromethane, and chloroform, but
insoluble in methanol or water. The UV spectrum of TBBU
shows an absorbance band at 310 nm with a shoulder peak at
Prof. F. Zhu
PCFM Lab, School of Chemistry, Sun Yat-sen University,
Guangzhou, 510275, China
†
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end
of the document.
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Angewandte Chemie International Edition
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3
25 nm (Figure S2, SI), red-shifted compared to that of TBU due
two phosphorescence peaks at 440 and 460 nm. Compared to
the diluted solutions, TBBU crystals show much red-shifted
phosphorescence emissions (525 and 565 nm), implying that the
interaction among TBBU molecules plays an important role in
phosphorescence of TBBU crystals. TBBU possesses a D-π-A
system, where bis(4-(tert-butyl)phenyl)amine acts as electron
donor and tetramethyldioxaborolane as electron acceptor. The
interactions among D, π, and A moieties promote the RTP of
TBBU crystals. To specify which moiety necessary for RTP,
to the presence of D-π-A system.
a
b
face-to-face
π-π stacking
D
A
D
In vacuum
A
D
photoluminescence
property of TB and TBU was also
A
investigated. No RTP is observed for either TB or TBU crystals
even in vacuum, which demonstrates that both D and A units are
crucial to RTP of TBBU.
TBBU
In the air
side-by-side
c 4
00
50
00
50
8
6
4
00
00
00
To understand the mechanism of RTP of TBBU, single
crystals of TBBU, TB, and TBU were analyzed. TBBU is packed
in triclinic space group. The dihedral angle between phenyl and
borate rings is 172.5 °. Adjacent TBBU molecules are aligned in
a head-to-tail manner (Figure 2). Two adjacent TBBU molecules
were extracted as a dimer from crystals for investigating
molecular packing. Three dimers are recognized in TBBU
crystals. Side-by-side arrangement of intermolecular phenyl and
borate rings nearly in a plane with abundant intermolecular
interactions including C-H---O, π-H---O, and C-H---π prevails.
The multiple interactions between neighboring TBBU molecules
prefer to block non-radiation decay through stabilizing the
excited state, and lead to the enhanced RTP. Such unique side-
by-side packing of intermolecular phenyl rings of TBBU is
entirely different from that of the RTP molecules reported, where
intermolecular phenyl rings are stacked in a face-to-face manner
through strong π-π interaction.[8] In TBBU, the asymmetric non-
coplanar skeleton favors the head-to-tail alignment of the
adjacent molecules, which maximizes their geometric contact.
The abundant intermolecular interactions due to the introduction
of D-π-A system further stabilize the side-by-side arrangement
of intermolecular phenyl rings. In contrast, TB is in orthorhombic
space group (Table S1 and Figure S7, SI). The dihedral angle
between phenyl and borate rings is 176.2 °, larger than that of
TBBU (172.5 °). 6 dimers are discerned in TB crystals. Adjacent
TB molecules pack loosely in a head-to-head manner. Limited
intermolecular interactions including C-H---O, π-H---O, C-H---π,
C-H---B, C-H---O, π-H---π and π-H---O are available between
adjacent TB molecules. The weak interactions between the
neighboring TB molecules disfavor stabilization of its excited
state, leading to the faint phosphorescence of TB. TBU is in
monoclinic space group. It is noted that the tert-butyl moiety still
moves even in crystals (Figure S8, SI). Movements of the tert-
butyl moiety consume the energy of the excited state, leading to
the weak phosphorescence of TBU. Moreover, 10 dimers are
found in TBU crystals. Most TBU molecules pack loosely, which
is adverse to RTP.
3
3
2
7
7 K
200
0
7
7 K
5
00
600
700
Solution
Emission Wavelength (nm)
4
00
500
600
700
Wavelength (nm)
d 2.0
e1
800
phosphorescence
00
80
60
fluorescence
600
400
200
76
1.5
1.0
0.5
6
4
4
0
0
0
2
13
0
1.4
In the air
500
550
600
650
700
Wavelength (nm)
In vac
Figure 1. (a) Arrangement of aromatic rings of RTP molecules. (b)
Fluorescence (solid) and phosphorescence (dot) spectra of TBBU crystals.
Inset: digital photos of TBBU crystals under UV lamp on (left) and off (right).
Excitation-phosphorescence
photoluminescence decay mapping (d) of TBBU crystals. (e) Fluorescence
and phosphorescence quantum yields of TBBU crystals.
emission
mapping
(c)
and
transient
Crystals of TBBU show obvious RTP in vacuum. TBBU
crystals reveal pink-blue emissions in the range of 380-650 nm
at room temperature in vacuum upon UV excitation at 365 nm
(
Figure 1b). After switching off UV lamp, the crystal emissions
turn to green with a wavelength from 500 nm to 650 nm, and
fade gradually. Two emission peaks at 525 and 565 nm and a
shoulder peak at 620 nm are observed in the phosphorescence
spectrum of TBBU crystals, showing long lifetimes of 348, 354,
and 369 ms (Figures S3 and S4, SI), respectively. The
phosphorescence with a wavelength from 500 to 650 nm is
sensitive to oxygen, which weakens sharply in the air (Figure 1b
and Figure S5). Moreover, lifetimes of RTP at 525 nm, 565 nm,
and 620 nm drop drastically by an order of magnitude in the air
compared to those in vacuum (12.8, 13.3, and 14.9 ms,
respectively). Furthermore, the phosphorescence emission is
also sensitive to temperature. Lowering temperature to 77 K
results in a significant enhancement of the phosphorescence
emission even in the air (Figure 1b and Figure S6).
Photoluminescence of TBBU in diluted solutions was further
investigated. The diluted THF solution of TBBU at 77 K shows
To gain deep insight into RTP of TBBU, theoretical calculation
was carried out in crystalline phase. The energy gap between
1
the lowest excited singlet state (S ) and the lowest excited triplet
state (T ), ∆EST, is 0.84 eV for the TBBU monomer with 2 inter-
1
system crossing (ISC) channels, which decreases to 0.74 eV for
TBBU dimers with 4 ISC channels (Figures S9-S11 and Table
S2 and S3, SI). The lower ∆EST and more ISC channels of TBBU
dimers than the monomer show that interaction between
adjacent TBBU molecules is favorable for its RTP. TB dimers
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show a higher ∆EST of 0.84 eV and less ISC channels of 2
compared to TBBU dimers (Figures S12-S16 and Table S4-S7,
SI), which is unfavorable for RTP. Possible thermal transitions of
TBBU crystals were further monitored. TBBU is thermostable up
to 200 °C. TBBU crystals do not undergo any detectable thermal
transitions in the temperature range from -50 to 70 °C, as shown
in the differential scanning calorimetry (DSC) curve (Figure S17). the
The glass transition and melting temperatures of TBBU crystals
oxygen fractions are summarized in Figure 3a. CIE chromaticity
coordinates shift linearly with increasing oxygen fraction. Such
color change of the film upon exposure to oxygen allows visual
detection of oxygen. The values of I405/I525 (I405 and I525 are
emission intensity at 405 and 525 nm, respectively) increase
with increasing oxygen fraction. This further demonstrates that
presence
of
oxygen
efficiently
quenches
the
phosphorescence of the film. On the other hand, the lifetime of
the emission at 525 nm decreases sharply with increasing
oxygen fraction (Figure 3b and Figure S23, SI). The values of
(
89.9 and 128.2 °C, respectively) are well above room
temperature. Thus, TBBU molecules are solidified at room
temperature. Such vitrified TBBU molecules are beneficial to
their strong intermolecular interactions and subsequent RTP.
0 0
τ /τ (τ and τ are the lifetimes of the excited state in the absence
and presence of oxygen, respectively), increase linearly with
increasing oxygen fraction (Figure 3c). This demonstrates that
phosphorescence emission is quenched through a dynamic
collisional mechanism in the presence of oxygen. The dynamic
collision between excited TBBU and oxygen molecules offers an
extra pathway of the excited triplet state, leading to the
decreased lifetime. In TBBU crystals, lots of pores with size
larger than oxygen molecules are available (Figure 3d), where
oxygen molecules can diffuse through easily. The large contact
area of TBBU and oxygen molecules facilitates quenching of
RTP of TBBU.
a
c
Head-to-tail
Head-to-head
C—H---O
π—H---π
b
C—H---π
π—H---O
3
10
10
10
a0.8
0%
(0.21, 0.13)
0.2%(0.20, 0.11)
1% (0.19, 0.09)
b
O
2
fraction (%)
0
2
.92
3
.42
2
.82
3
.36
4
2
%
(0.18, 0.08)
1% (0.17, 0.06)
3
3
3
3
.03
0.6
2
1
2
3
3
2
.87
.60
.60
.87
2
.38
.38
.03
y 0.4
0
0
0
.2
.0
21 %
3
.36
2
.82
3
.42
1
2
.92
Dimer 1
Dimer 2
Dimer 3
0.0
0.2
0.4
x
0.6
0.8
0.0
0.5
Time (s)
1.0
1.5
c
d c
Figure 2. Crystal structures of TBBU (a and b) and TB (c).
PL
video
a
b
30
TBBU crystals are difficult to form films for sensing
applications. Thus, TBBU was used to dope polymers. A
commercial poly(ethylene oxide)/poly(propylene oxide) triblock
copolymer was used as polymer matrices due to its superior film
processability and environmental friendship. The TBBU doping
fraction and film thickness were systematically optimized
20
10
0
y=1.259x+1
R=0.9989
3
.4 Å
0
5
10
15
20
O
2
Oxygen fraction (%)
(
Figures S18 and S19, Table S8). The polymer film doped with
0 wt% TBBU and a thickness of approximately 464 μm shows
1
Figure 3. (a) Commission Internationale de l’Eclairage (CIE) coordinates of
obvious RTP in vacuum, inherited from TBBU crystals. The
green phosphorescence emission fades gradually in vacuum,
which can be readily observed by the naked eye, and recorded
using a commercial cell phone camera. X-ray diffraction (XRD)
spectra verify that TBBU forms crystals in the doped polymer
film (Figure S20, SI). Fluorescence microscopy images (Figure
S21) show that the TBBU microcrystals with a diameter of 100
μm are uniformly dispersed in the polymer matrix. RTP of the
doped polymer film is also sensitive to oxygen. With increasing
photoluminescence spectra of TBBU doped film at various oxygen fractions.
(b) Time-resolved photoluminescence spectra of TBBU doped film. (c) Plots
of τ /τ against oxygen fraction for TBBU doped film. (d) Illustration of diffusion
0
of oxygen in TBBU crystals.
Encouraged by the linear relationship of the reciprocal of the
long RTP lifetime against oxygen fraction, we further developed
oxygen sensors based on the TBBU doped polymer film. The
doped film was subjected to oxygen/nitrogen mixed gas and
excited using a UV flashlight at 365 nm (Figure 4). Videos were
2
O fraction from 0 to 21 vol%, the color of the film changes from
pink-blue to deep blue. And the phosphorescence emission
weakens gradually (Figure S22, SI). The corresponding
Commission Internationale de l’Eclairage (CIE) coordinates of
photoluminescence spectra of the TBBU doped film at various
recorded using
a commercial cell phone camera. Video
screenshots at various times were analyzed. The red, green,
and blue (RGB) values of images were calculated using a
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Matlab program (Figure S24, SI). The brightness of each image
was quantized to grayscale (G) according to the following
equation:
is entirely different from the RTP molecules reported so far.
Theoretical calculation further verifies that the extraordinary
intermolecular interaction of neighboring molecules plays an
important role in RTP of TBBU crystals. More importantly, the
polymer film doped with TBBU inherits its RTP property, which is
highly susceptible to oxygen. The color of the doped film
changes from pink-blue to deep blue with increasing oxygen
fraction. While the reciprocal of the RTP lifetime of the doped
film increases linearly with increasing oxygen fraction, following
a dynamic collisional quenching mechanism. Finally, through
analyzing grayscale of the phosphorescence images, a simple
and reliable method is developed for rapid detection of oxygen.
Only tens of milligrams of TBBU doped polymers, coupled with a
Grayscale (G) = 0.2989×Red + 0.5870×Greed + 0.1140×Blue (1)
The plot of G values versus time for each oxygen fraction was
obtained. The G values decreased exponentially against time.
The phosphorescence lifetime was obtained by data fitting. To
check reproducibility, at least six measurements were performed
for each oxygen fraction (Figure S25-S28 and Table S9, SI). The
phosphorescence lifetimes of the doped polymer film exposed to
oxygen at various fractions are close to those determined using
time-resolved photoluminescence spectra (Figure 3c). For
instance, the phosphorescence lifetime of the doped polymer
film exposed to 2% oxygen is determined to be 103±9.2 ms.
Based on the working curve (Figure 3c), the oxygen fraction is
calculated to be 2.08%, close to the designed oxygen fraction
UV flashlight and
a cell phone allow quick, visual, and
quantitative determination of oxygen fraction in the air. Taking
advantages of oxygen-sensitive long-lived RTP, easy synthesis,
cost-efficiency, and simple devices, TBBU offers a new avenue
to facile detection of oxygen.
(
2%).
Acknowledgements
a
UV flashlight
Y.Z. and W.Q. contributed equally to this work. The financial
support is partially from NSFC (21374136), the Fundamental
Research Funds for the Central Universities (17lgjc03 and
cell phone
18lgpy04).
Keywords: room temperature phosphorescence (RTP) • oxygen detection •
quantitative • visual
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Angewandte Chemie International Edition
10.1002/anie.201906312
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
An unconventional organic molecule
showing obvious long-lived room
temperature phosphorescence (RTP)
Yusheng Zhou, Wei Qin, Cheng Du,
Haiyang Gao, Fangming Zhu, and
Guodong Liang*
O2
is
syntheiszed
for
viual
and
quantitative detetion of oxygen
Page No. – Page No.
Long-Lived
Room-Temperature
Phosphorescence for Visual and
Quantitative Detection of Oxygen
Long-lived RTP
Quenching
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