Proton-Activated “Off–On” Room-Temperature Phosphorescence from Purely Organic Thioethers

Article abstract of DOI:10.1002/anie.201808861

Room-temperature phosphorescence (RTP)-based sensors have distinctive advantages over the fluorescence counterparts, such as larger Stokes shifts and longer lifetimes. Unfortunately, almost all RTP sensors are operated on quenching-based mechanisms given the sensitive nature of the emissive triplet state. Here we report a type of thioether RTP molecules that shows RTP “turn-on” when volatile acid vapors such as HCl are in contact. To elucidate the underlying mechanism, model thioethers containing different donor/acceptor combinations are investigated via fluorescence spectroscopy and theoretical calculations aided by molecular coordinates obtained from single-crystal X-ray diffraction. It is revealed that a charge-transfer character in the phosphorescence state is crucial. The “turn-on” design concept may significantly broaden the sensing application scope for organic RTP molecules.

Full text of DOI:10.1002/anie.201808861

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Proton-Activated "Off-On" Room-Temperature Phosphorescence
from Purely Organic Thioethers
Authors: Linkun Huang, Biao Chen, Xuepeng Zhang, Carl O. Trindle,
Fan Liao, Yucai Wang, Hui Miao, Yi Luo, and Guoqing Zhang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808861
Angew. Chem. 10.1002/ange.201808861
Link to VoR: http://dx.doi.org/10.1002/anie.201808861
http://dx.doi.org/10.1002/ange.201808861

Angewandte Chemie International Edition
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COMMUNICATION
Proton-Activated “Off-On” Room-Temperature Phosphorescence
from Purely Organic Thioethers
Linkun Huang, Biao Chen, Xuepeng Zhang,* Carl O. Trindle, Fan Liao, Yucai Wang, Hui Miao, Yi Luo,*
and Guoqing Zhang*
Abstract: Room-temperature phosphorescence (RTP)-based
sensors have distinctive advantages over the fluorescence
counterparts, such as larger Stokes shifts and longer lifetimes.
Unfortunately, almost all RTP sensors are operated on quenching-
based mechanisms given the sensitive nature of the emissive triplet
state. Here we report a type of thioether RTP molecules that shows
RTP “turn-on” when volatile acid vapors such as HCl are in contact.
To elucidate the underlying mechanism, model thioethers containing
different donor/acceptor combinations are investigated via
fluorescence spectroscopy and theoretical calculations aided by
molecular coordinates obtained from single-crystal X-ray diffraction. It
is revealed that a charge-transfer character in the phosphorescence
state is crucial. The “turn-on” design concept may significantly
broaden the sensing application scope for organic RTP molecules.
matrices, such as crystals, glasses or other polymeric host
materials, or in the form of crystals.[10]
Recently, we demonstrated that intra-[11] or intermolecular
charge transfer[12] from an electronic donor to an acceptor can
significantly boost the efficiency of intersystem crossing (ISC) and
thus RTP in purely organic systems. A more specific method of
producing solid-state molecular RTP was demonstrated where
the geminal donor and acceptor are attached to a central sp3
carbon atom.[13] In this case, the tetrahedron-like geometry
reduced the proclivity to stacking interactions in the solid state and
generates strong RTP as crystals or powders, irrespective of
molecular oxygen presence. However, many of the carbon-
3
centered sp type of organic phosphors are liquid or easily liquefy
at room temperature also due to their reduced intermolecular
interactions, which poses
a substantial inconvenience for
applications. A plausible design motif is to introduce a larger and
more polar central atom, such as sulfur (Figure 1a), to replace
carbon, which may increase intermolecular interactions through
the bridging atoms while still imposing minimal interactions on
donor/acceptor moieties. Based on the strategy, we first validated
the design strategy and then synthesized two thioethers with
proton-activated acceptor moieties (pyridine and quinoline[14]), the
RTP of which exhibits a “turn-on” in the longer wavelength region
upon contacting protic acids. As a result, the two molecules serve
as rudimentary solid-state “off-on” sensors for volatile acidic
vapors such as HCl with a detection limit of 8.3 mg/m3.
Fluorescence sensing based on organic molecules has distinctive
advantages over other methods including small probe size, non-
invasiveness, high sensitivity, multiplexing capability and low
cost.[1]
temperature phosphorescence (RTP) instead of fluorescence,
A specialized sensing module, which uses room-
[2]
has added benefits of extreme sensitivity, large Stoke shifts and
zero background from gated emission.[2,3] Nonetheless, the
[
4]
majority of RTP sensing is focused on temperature and
oxygen,[ where RTP quenching becomes predominant at
elevated temperatures and/or high oxygen concentrations due to
more competitive thermal decay[5] or increased triplet energy
transfer.[6] For example, Fraser et al.[7] reported an in vivo,
polymeric RTP oxygen sensor with good spatial and temporal
2]
The thioethers (TEs, Figure 1a) are synthesized according to a
published method.[15] We first investigated the optical properties
of these TEs in dilute 2-methyl tetrahydrofuran (mTHF) solutions
resolution, which made real-time monitoring of molecular O
2
(
25 M, Figure 1b and 1c). For all five compounds, the absorption
within tumors[7,8] and wounds[9] possible. Up to date, nearly all
RTP sensing technologies are quenching-based, and cannot be
used in “off-on” modules, which are lower in background.
Alternatively, the RTP sensors may be rendered oxygen-
insensitive by way of embedment in oxygen-impermeable
spectra show relatively strong interactions with light with
a)
[*]
Mr. L Huang, Dr. B. Chen, Dr. X. Zhang, Ms. F. Liao, Prof. H. Maio,
Prof. Y. Luo, Prof. G. Zhang
b) 1.0
30.0k c)
1
8.0k
2.0k
ASA
DSD
DSA
1.2
DSP
DSQ
Hefei National Laboratory for Physical Science at the Microscale,
University of Science and Technology of China, 96 Jinzhai Rd,
Hefei, Anhui 230026 (China)
25.0k
0
.8
.6
20.0k
15.0k
10.0k
5.0k
1
0
0
.8
.4
E-mail: zhangxp@ustc.edu.cn
0.4
0.2
6.0k
0.0
0
yiluo@ustc.edu.cn
gzhang@ustc.edu.cn
0.0
0.0
0.0
2
50 300 350 400 450 500
250 300 350 400 450 500
Prof. C. O. Trindle
Wavelength (nm)
Wavelength (nm)
Department of Chemistry, University of Virginia, VA 22903 (USA)
Prof. Y. Wang
Figure 1. a) Chemical structures of thioethers with a donor moiety and an
acceptor one (abbreviations: S = sulfur, D = donor, A = acceptor, P = pyridine,
Q = quinoline). b) and c) absorption and steady-state fluorescence emission
spectra of ASA (ex = 280 nm), DSD (ex = 280 nm), DSA (ex = 280 nm), DSQ
School of Life Science, University of Science and Technology of
China, 96 Jinzhai Rd, Hefei, Anhui 230026 (China)
Supporting information for this article is given via a link at the end of
the document.
(ex = 325 nm) and DSP (ex = 280 nm, slit: 2/4 nm) in mTHF.
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Angewandte Chemie International Edition
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COMMUNICATION
Table 1. Photoluminescence properties of TEs in the solid state at room temperature.
Lifetime [ms][b]
(A
em [nm][a]
Quantum yield
λ
[
%][d]

PL

1
(A
1
(%))

2
2
(%))
3 3
 (A (%))
2.2010-7 [c]
7.6310-8
40.78)
4.81
7.3810-8
1.1910-6
(17.18)
20.2
0.9[e]
<0.2
ASA
392
(
(42.04)
-
2
550
11.5
5.1410
(36.78)
(14.91)
(48.30)
DSD
DSA
500
480
37.6
20.4
42.53)
0.317
(5.99)
56.2
(51.49)
<0.2
20.0
(
1.3510-2
32.55)
3.53
1.47
82.5
1.28
(49.10)
98.2
4.56
(18.35)
0.138
(4.89)
(
530
(
11.57)
(83.54)
1.0910-6 [c]
1.0610-7
18.25)
1.3110-8
DSQ
DSP
429
480
530
510
_
3.3
<0.2
10.7
5.7
(
(81.75)
4.8110-3
38.46)
8.8110-3
(40.46)
1.69
7.96
(21.08)
(
DSQH+
DSPH+
1.0310-2
5.9110-2
3.0510-3
20.26)
1.7710-5
(23.19)
1.7210-2
(56.56)
(
3.1010-2
24.79)
3.4510-4
(5.12)
7.2510-2
(70.09)
(
[
a] Photoluminescence emission maximum excited at 365 nm; [b] phosphorescence lifetime (ex = 372 nm, SpectraLED); [c] fluorescence lifetime (ex = 370 nm,
NanoLED); [d] absolute quantum yield from 430-600 nm at room temperature; [e] absolute quantum yield from 380-420 nm at room temperature.
wavelengths in between 250 and 300 nm and weak interactions
with light around ~340 nm. The two bands are assigned as the
singlet -* and n-* (orthogonal lone pair on sulfur) transitions of
these TEs, respectively (Figure 1b). It has to be noted that the
stronger absorption at ~340 nm of DSQ arises from the quinoline
substituent.[ All TEs exhibit very weak fluorescence emission in
mTHF solutions with quantum yield values lower than 0.5% at
room temperature.
In the solid state, however, one of the TEs, DSA exhibits
intense blue photoluminescence (PL) under the 365-nm UV
irradiation, while the rest four TEs do not show PL to the naked
eye (Figure 2a). Figure 2b shows the steady-state PL emission
spectra of the solids of ASA, DSD and DSA in the ambient
condition. While the spectrum of DSD is essentially baseline, the
ASA counterpart has a weak near-UV fluorescence peak at 392
nm with a measurable lifetime of ~0.2 ns, indicating dominant
non-radiative decay processes for the ASA solid. For the DSA
solid, in contrast, not only is the PL strong (the absolute quantum
yield, PL = 20.0%) with an emission maximum at 483 nm, it also
exhibits a strong “afterglow” with a yellow color (Figure 2c). When
analyzed with transient emission spectra, it was found that the
wavelength of the delayed emission maximum ( = 483 nm)
started to red-shift conspicuously beyond a delay of 10 ms and
eventually stabilized at 524 nm. The time-correlated decay
profiles were monitored at 483 nm and 524 nm, respectively
organic phosphors. Here we showcase the “turn-on” of RTP
sensing module from solid-state DSP and DSQ (Figure 1a), when
the two TEs are exposed to acid vapors such as HCl or AcOH.
Compared to cyanobenzene, pyridine (P) or quinoline (Q) does
not initially produce RTP. However, the proton-activated acceptor
16]
+
+
moieties PH and QH can then induce a strong RTP state, similar
to the situation found in DSA.[11,17,18] Visually, the DSP and DSQ
solids were originally weakly or not photoluminescent to the naked
eye; after HCl or AcOH vapor treatment, however, they exhibit
bright yellow or green PL under UV irradiation at room
temperature (Figure 2g). The corresponding steady-state PL
emission spectra are presented in Figure 2h and Table 1: for the
DSP solid, the PL spectrum is roughly baseline (PL < 0.2%) but
+
the protonated DSPH PL spectrum has an emission maximum at
507 nm (PL = 5.7%; PL = 58 s), where the sub-ms long lifetime
unambiguously indicates emission involving the triplet excited
state. For the DSQ molecular solid, a fluorescence band (em
=
429 nm; PL = 3.3 %) with a lifetime of 1.1 ns, most likely
originated from the quinoline -*, is only present prior to
exposure to the acid vapor. However, a dramatic PL intensity
increase (PL = 10.7%) and a bathochromic shift (100 nm to em
= 529 nm) with a substantially increased lifetime (PL = 10.2 s)
+
are noted for the DSQH solid after acid exposure. The long
lifetime again indicates triplet-state involvement. The RTP “turn-
on” sensing experiment with non-luminescent DSP solid was
shown in Figure 2i and 2j, where progressively more intense RTP
could be recorded upon exposure to incremental amounts of HCl
gas. The linear fitting of RTP intensity at 507 nm gives a detection
(
Figure 2e and 2f). The PL decay at 483 nm consists of a rapid,
ns-long decay ( = 1.1 ns) and a much slower one ( = 1.5 ms) in
the ms domain, while the decay at 524 nm, attributed to
aggregates RTP, is even slower ( = 82.5 ms). Such long-lived
emission lifetimes are undoubtedly RTP in nature. Given the
successful validation of the general method for producing RTP
3
limit of 8.3 mg/m , a value half of the maximal allowable
3
[22]
concentration (14.9 mg/m ) for human health,
implying the
pratical value of the reported materials.
from organic molecules (i.e., donor-sp3 linker-acceptor),[13]
a
To the best of our knowledge, this is the first example of RTP
“turn-on” type of sensing module. It is thus essential to elucidate
the sensing mechanism: we examined the molecular geometries
rational design of TEs with activatable acceptor is expected to
show “turn-on” RTP, which is essentially unheard of for purely
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Angewandte Chemie International Edition
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COMMUNICATION
a)
c)
h on
h off
DSA
DSD
ASA
a)
b)
b)
d)
ASA
DSA
DSP
DSQ
60
50
40
30
20
10
0
1
0 ms
12 ms
5 ms
20 ms
0 ms
50 ms
0 ms
DSP
1
DSA
3
8
4
50
500
550
600
DSQ
Wavelength (nm)
e)
f)
4
3
2
1
0
4
3
2
1
0
prompt
decay
fit
ASA
4
3
2
1
0
decay
fit
5
5
60 65 70 75 80 85 90
Time (ns)
decay
fit
5
10
Time (ms)
15
20
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Figure 3. a) Stick model showing tetrahedron-like molecular geometries of ASA,
DSA, DSP and DSQ in the solid state (data extracted from single-crystal XRD);
b) packing diagrams of space-fill model showing reduced stacking interactions
among DSA, DSP and DSQ in the solid state.
g)
h)
Time (s)
6
solid state acetic acid 2.0x10
DSQ
+
DSQH
DSP
529nm
6
6
5
1.5x10
1.0x10
5.0x10
+
DSPH
429nm
507nm
acceptor may also play a determining role, given the strong RTP
of DSA, DSPH+ and DSQH . Following the hypothesis, we
launched a set of theoretical calculations for all five TEs and two
protonated species: DSPH+ and DSQH+ (Figure 4). The
calculations were performed using the Head-Gordon Ahlrichs
model.[19,20] While the detailed results are presented in the
Supporting Information (Tables S1-S7), two very important
implications must be noted. First, for the experimentally non-
luminescent TEs, both the lowest singlet and triplet transition
states consist of orbitals from localized regions of the molecules
(SI); for the strongly luminescent DSA and two protonated
+
0.0
4
00
450
500
550
600
650
i)
j)
Wavelength (nm)
5
0 mg/m
3
4.0x10
3.5x10
3.0x10
2.5x10
2.0x10
1.5x10
1.0x10
5.0x10
3
1.0
0.8
0.6
0.4
0.2
5
5
5
5
5
5
4
25 mg/m
3
37.5 mg/m
3
50 mg/m
3
7
5 mg/m
3
1
50 mg/m
y = -0.058+ 0.007*x
0.0
0
.0
4
00
450
500
550
600
650
700
20
40
60
80 100 120 140 160
3
Wavelength (nm)
HCl (mg/m )
Figure 2. a) Image showing DSA, DSD and ASA crystalline solids under 365-
nm UV light irradiation; b) steady-state emission spectra of DSA, DSD and ASA
crystalline solids (ex = 365 nm, slit: 2/4 nm); c) Photo showing DSA crystalline
solid under (left) 365-nm light excitation and after (right) ceased excitation; d)
time-correlated emission spectra of the DSA crystalline solid at different time
points; e) photoluminescence decays of DSA crystalline solid at em = 480 nm
+
+
molecules DSPH and DSQH , the lowest single transition state
has strong or entirely CT character, while the lowest triplet state
depends on the substituent ring (SI). Shown in Figure 4, for
example, a comparison in calculations between ASA and DSA
clearly reveals the marked difference in the fluorescence (F) state,
where the ASA F transition involves the entire molecule for both
origin and destination orbitals, whereas the DSA F transition
consists orbitals residing in either the donor or acceptor region
with a small overlap. Consequently, the ISC rate is expected to
be strongly accelerated for DSA due to the extreme CT character
in the lowest singlet state transition.[18] On the other hand, the
phosphorescence (P) state for DSA is slightly different in that the
destination orbital now extends to the entire molecule while the
origin one is still largely localized on the acceptor moiety. For both
DSP and DSQ, although the calculated F state shows substantial
CT character, the P state is still largely localized -*; after
protonation, however, strong CT transition is noted for the P state.
We thus conclude that strong RTP is only strong when the P state
contains substantial CT character. This may be explained by the
fact that a triplet CT state promotes spin-orbit coupling in this
particular molecular geometry (the origin and destination orbitals
are rotated by 105.3, 104.7 and 105.9 in the P state), which
increases the competitive RTP rate relative to that of the non-
radiative decay process. The explanation is consistent with the
sub-ms PL lifetimes observed while maintaining decent quantum
yields. Finally, in the model used here F energies are
showing
a millisecond component and a nanosecond (inset) one; f)
photoluminescence decays of DSA crystalline solid at em = 530 nm; g) photos
showing visible photoluminescence change of DSQ and DSP crystalline solids
before and after exposure to the acetic acid vapor; h) steady-state emission
spectra showing spectroscopic change before and after HCl vapor treatment for
DSQ and DSP, respectively; i) steady-state emission spectra of DSP solid (15.0
mg) after the treatment of HCl gas at various concentrations; (j) linear fitting of
RTP emission intensity in i) at 507 nm; inset: photo showing (left to right)
progessive RTP “turn-on” with incrementing HCl concentrations.
of these TEs with single-crystal XRD measurements except for
DSD, which always yields polycrystals. Therefore, the rest of the
four TEs were analyzed with structures shown in Figure 3a. As
expected, just like the methylene linker as previously reported,[13]
the sulfur linker also renders the donor and acceptor planes adopt
a highly twisted configuration, close to 109 in the crystalline state,
irrespective of the substituent groups (Figure 3). Furthermore, the
twist angle also makes - stacking interactions among aromatic
rings less accessible, particularly for DSA, DSP and DSQ that do
not have C2 symmetry (in the case of DSD or ASA). It is clear that
the space-fill model shows reduced stacking interactions among
DSA, DSP and DSQ in the solid state. Nonetheless, the
substantial reduction in intermolecular interactions appears only
partially responsible for the strong PL observed for DSA, as DSP
and DSQ show trivial PL in the solid state. Consistent with
spectroscopic results, the CT state between the donor and
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Angewandte Chemie International Edition
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COMMUNICATION
Acknowledgements
We thank the National Key R&D Program of China
2017YFA0303500 to G. Z. and Y. L.), the Fundamental Research
(
Funds for the Central Universities (WK2340000068 to G. Z.), the
General Financial Grant from China Postdoctoral Science
Foundation (2016M602029 to X.Z.) and the fund from Anhui
Provincial Natural Science Foundation (1708085MB38 to X.Z.).
We are especially thankful for Mr. Xinghao Yuan’s generous
donation to the lab and Mr. Jin Li’s contribution to the cover art.
Conflict of interest
The authors declare no conflict of interest.
Keywords: thioethers• room temperature phosphorescence •
acid • donor • acceptor
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“
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8
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3
with a calculated detection limit of 8.3 mg/m . On the other hand,
detailed calculations aided by coordinates obtained from XRD
data reveals that both intermolecular interactions and CT state are
key to the observed RTP. More specifically, weak intermolecular
interactions and more CT character in the phosphorescence state
are likely to be responsible. The report serves as a new design
strategy for purely organic RTP sensors and devices given that
RTP has intrinsically large Stokes shifts and longer lifetimes, two
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module in the presented work implies that instead of focusing on
the matrix effect which inhibits molecular motions, the key lies in
the induction of a new triplet decay pathway upon analyte binding,
3
3
such as from -* to CT. More RTP-based “turn-on” or
ratiometric enhancement will serve as main topics in future
reports.
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Entry for the Table of Contents (Please choose one layout)
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Room-temperature phosphorescence
Linkun Huang, Biao Chen, Xuepeng
Zhang,* Carl O. Trindle, Fan Liao, Yucai
Wang, Hui Miao, Yi Luo* and Guoqing
Zhang*
“
turn on” is realized in a purely organic
thioether system, where protons serve
to activate a charge-transfer
phosphorescence state. The
technology can be very promising for
back-ground free sensing and
imaging.
Page No. – Page No.
Proton-Activated “Off-On” Room-
Temperature Phosphorescence from
Purely Organic Thioethers
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