Reversible Switching between Phosphorescence and Fluorescence in a Unimolecular System Controlled by External Stimuli

Article abstract of DOI:10.1002/chem.201801739

Manipulation of the emission properties of pure organic molecules through external stimuli is attractive but challenging. Herein, a dual-emissive hexathiobenzene-based molecule is reported with significant aggregation-induced phosphorescence characteristics, and demonstrates reversible switching among blue, green, and yellow phosphorescence by controlling molecular aggregation state or protonation state. Variation of solvent or pH value manipulates the interconversion between fluorescence and phosphorescence, while the change in protonation state in organic solvent switches two short-lived emissions in a controllable manner. Such a controlled manipulation is achieved by the rational design of combining a twisted structure and the proper arrangement of energy gaps among different excited states. This work provides a new design principle for organic molecules with efficient room-temperature phosphorescence and tunable singlet–triplet emissive properties, and contributes to the design and development of smart materials and intelligent optoelectronic devices.

Full text of DOI:10.1002/chem.201801739

A Journal of
Accepted Article
Title: Reversible Switching between Phosphorescence and
Fluorescence in a Unimolecular System Controlled by External
Stimuli
Authors: Jing Xu, Hui Feng, Hao Teng, Guilin Chen, Saifei Pan,
Jianrong Chen, and Zhaosheng Qian
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
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To be cited as: Chem. Eur. J. 10.1002/chem.201801739
Link to VoR: http://dx.doi.org/10.1002/chem.201801739
Supported by

10.1002/chem.201801739
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COMMUNICATION
Reversible Switching between Phosphorescence and
Fluorescence in a Unimolecular System Controlled by External
Stimuli
Jing Xu,† Hui Feng,† Hao Teng, Guilin Chen, Saifei Pan, Jianrong Chen and Zhaosheng Qian*
Abstract: Manipulation of the emission properties of pure organic
molecules through external stimuli is attractive but challenging.
Herein, we report a dual-emissive hexathiobenzene-based molecule
fluorescence dual emission was reported by Fraser and
coworkers,[8] which demonstrated smart modulation of
fluorescence and phosphorescence for a single component
with significant aggregation-induced phosphorescence characteristic, through systematic variation of poly(lactic acid) chain length
and demonstrate reversible switching among blue fluorescence,
green fluorescence and yellow phosphorescence by controlling
molecular aggregation state or protonation state. Variation of solvent
or pH value manipulates the interconversion between fluorescence
and phosphorescence, while the change in protonation state in
organic solvent switches two short-lived emissions in a controllable
manner. Such a controlled manipulation is achieved by rational
design of combining twisted structure and proper arrangement of
energy gaps among different excited states. This work provides a
new design principle for organic molecules with efficient room-
temperature phosphorescence and tunable singlet-triplet emissive
properties, and contributes to design and development of smart
materials and intelligent optoelectronic devices.
combined with heavy atom substitution. Singlet-triplet emissive
switching in single molecular system was accomplished by
helical self-assembly and dissociation of a hexathiobenzene-
based single luminophore in a recent study,[9] and helical self-
assembly driven by intermolecular hydrogen bonding and π-π
stacking interactions are responsive for the reversible switching
between fluorescence and phosphorescence,[10] which further
inspired the design of a single luminophore with dual emissions
controlled by molecular stacking.[11] For such a unimolecular
system, crystallization-induced self-assembly is required for
simultaneous emission of phosphorescence and fluorescence,
which makes the shift between phosphorescence and
fluorescence in a non-controllable way. Precise manipulation of
the switch between phosphorescence and fluorescence in a
single molecule controlled by external stimuli still remains a
great challenge.
Persulfurated arenes have long been exploited for designing
redox sensors, coordination polymers and organic conductors
due to their conformational patterns of the sulfur ligands,[12] but
their unique solid-state phosphorescence was reported by
Bergamini et al. till 2013,[13] uncovering unique room-
temperature phosphorescence[14] and extremely high quantum
yields of persulfurated benzene molecules.[15] This specific
property was further employed to develop a phosphorescence
turn-on sensor for magnesium ion.[16] These distinctive
Precious photoluminescence of organic materials has shown
great power in a variety of areas including organic light-emitting
diodes, sensors and life sciences.[1] Fluorescence from emissive
singlet of a molecule has long dominated in diverse applications
by the way of various traditional organic dyes with rigid
conjugated structures,[2] and recent discovery and flourishing of
aggregation-induced emission (AIE) have further boosted the
development of fluorescence.[3] However, room-temperature
phosphorescence (RTP) originated from molecular triplet state
has only observed from a small portion of particular organic
molecules.[4]
phosphorescence (CIP) provided
Recently
proposed
crystallization-induced
promising strategy to
molecules
with
significant
aggregation-induced
a
phosphorescence (AIP) characteristic provide an ideal platform
to manipulate phosphorescence and fluorescence in the
unimolecular system in a controlled manner. In this work, a
novel dual-emissive molecule with significant aggregation-
induced phosphorescence characteristic was designed and
construct organic materials with efficient room-temperature
phosphorescence by enhancing intersystem crossing process
and suppressing nonradiative deactivation pathways of triplet
excitons.[5] Under such a background, it is still extremely difficult
to achieve organic materials with dual emissions from two
singlets[6] or two triplets[7] via rational structural design and
molecular packing. More challenging work is to achieve
controllable manipulation of phosphorescence and fluorescence
at single molecule level.
synthesized,
and
reversible
switching
between
phosphorescence and fluorescence can be achieved by external
stimuli such as aggregation state and pH value. As shown in
Scheme 1, the twisted structure of hexakis(3-hydroxy-1-
phenylthio)-benzene (compound 1, HHPB) contributes to its
efficient room-temperature phosphorescence in aggregated
state, and this molecule also shows appreciable blue
fluorescence or green emission under different conditions. The
shift between phosphorescence and blue or green fluorescence
can be realized by controlling aggregation state of HHPB via
variation in solvent or pH value, while the switching between
blue fluorescence and green emission is conducted by
modulating the protonation state of isolated molecules. Except
for the contribution from twisted structure of HHPB, a proper
arrangement of energy gaps of S1 and T1 or the neighboring T
First example of organic molecules with phosphorescence-
[a]
J. Xu, H. Feng, H. Teng, G. L. Chen, S. F. Pan, J. R. Chen, Z. S.
Qian*
Department of Chemistry, College of Chemistry and Life Sciences
Zhejiang Normal University
Yingbin Road 688, Jinhua 321004, People’s Republic of China
E-mail: qianzhaosheng@zjnu.cn
[†] 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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Scheme 1. Schematic Illustration of Reversible Switching between
Phosphorescence and Fluorescence Controlled by External Stimuli and
Underlying Contributions from Twisted Structure and Proper Energy
Arrangement.
Figure 2. (A) HOMO and LUMO of HHPB molecule with charge-transfer
feature. (B) A Jablonski diagram of HHPB molecule with labelled excitation
energies.
most polar organic solvent like THF and ethanol, but has poor
solubility in water. As seen from Figure 1B, HHPB shows an
intense blue emission around 400 nm with a small emission
peak at 530 nm, which largely differs with that of its solid. The
quantum yield for the emission at 530 nm is lower than 0.001,
implying its long-lived nature. After carefully measuring and
analyzing their time-resolved decay curves in Figure 1C and 1D,
we found that the intense blue emission and the weak green-
yellowish luminescence belong to fluorescence and
phosphorescence, respectively, in terms of their extremely
different lifetimes by three orders of magnitude. These results
clearly suggest HHPB in THF possesses a dual fluorescence-
phosphorescence emission, but its solid only shows an intense
phosphorescence band, which provides the opportunity to
modulate their emission intensity in a controlled way.
Figure 1. (A) Excitation and emission spectra of HHPB powder. Inset:
Photograph for HHPB powder excited by UV light. (B) Excitation and emission
spectra of HHPB in THF. Inset: image for HHPB solution in THF irradiated by
UV light. (C) Time-resolved luminescence decay curve of HHPB in THF at 400
nm. (D) Time-resolved luminescence decay curve of HHPB in THF at 530 nm.
state of S1 is required to attain a large population of triplet
excitons, promoting the generation of intense phosphorescence.
The revealing of underlying nature in photophysical process is
helpful and valuable to design such an efficient S-T
interconversion approach in a controllable manner.
The synthetic route in Scheme S1 and procedures for target
compound HHPB were shown in the Supporting Information.
Among a diversity of organic materials with room-temperature
phosphorescence, hexathiobenzene-based organic molecules
showed the most precious features because their RTP does not
Hexakis(3-methoxy-1-phenlthio)benzene
(HMPB)
as
the
intermediate product illustrates an emission band around 528
nm and a long lifetime of 13.6 µs as shown in Figure S1. Similar
to its precursor HMPB, the solid powder of HHPB exhibits
extremely bright luminescence under the irradiation of UV light,
suggesting its significant aggregation-induced room-temperature
phosphorescence (RTP) nature. Figure 1A shows PL spectra of
HHPB powder, and its bright yellow luminescence is readily
observed by naked eyes. Its emission maximum is located
around 558 nm with a substantial red-shift relative to that of
HMPB powder, and can be efficiently excited by a wide light
range of 300 − 450 nm. The long lifetime of 8.8 µs from time-
resolved luminescence decay curve in Figure S2 and a high
emission efficiency of 0.13 demonstrated its excellent
phosphorescence characteristic. HHPB can be well dissolved in
require the introduction of heavy atoms and the occurring of
[15,17]
strictly crystalline state.
efficient
To gain a deep insight into the
room-temperature
aggregation-induced
phosphorescence of HHPB, we performed first- principle density
functional theory (DFT) and time-dependent DFT (TD-DFT)
calculations on HHPB molecule. The optimized structure in
Figure S3 demonstrates that six 3-hydroxy-benzene subunits
are perpendicular to central benzene showing as a twisted
structure in whole. Such a propeller-like conformation, similar to
that of the famous AIEgen tetraphenylethylene, is responsible
for its apparent AIE behavior because nonradiative deactivation
pathways can be efficiently blocked under a constrained
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Figure 3. (A) The change of PL spectra of HHPB versus the ratio of
ethanol/water. (B) Time-resolved luminescence decay curve of HHPB in water
at 570 nm.
environment.[18] NBO analysis provides visualized HOMO and
LUMO of HHPB shown in Figure 2A, and it is easily noted that
HOMO is mainly comprised of atomic orbitals of six sulfur atoms
whereas LUMO consists of atomic orbitals from central benzene.
Such a separated location of LUMO and HOMO clearly indicates
a significant charge-transfer character, and also implies a
relatively small S-T slitting energy resembling organic thermally
Figure 4. (A) The change of PL spectra of HHPB in varied pHs. Inset:
Photoimages of HHPB solution at different pHs when excited by UV light. (B)
Time-resolved luminescence decay curve of HHPB in pH 12 solution at 500
nm. (C) Schematic illustration of phosphorescence-fluorescence switching
controlled by pH and optimized structures of HHPB and its deprotonated form.
(D) Optimized structure of deprotonated form of HHPB.
activated delayed fluorescence materials.[19] However,
a
Joblonski diagram with labeled excitation energies for HHPB in
Figure 2B shows that the energy gap between S1 and T1 (ΔEST)
is estimated to be 0.48 eV, which is much larger than those of
typical organic molecules with thermally activated decayed
fluorescence (around 0.1 eV).[20] The large energy gap between
S1 and T1 blocks the reverse intersystem crossing for T1 to S1. A
close examination on these excitation energies of singlets and
triplets from calculated data in Table S1 leads to the conclusion
that the small energy gap between S1 and T3 states (0.13 eV)
promotes intersystem crossing process between them,
appreciably increasing the population of triplet excitons.
Effective suppression of nonradiative deexcitaiton pathways
under constrained environments allows T3 triplet to reach T1
triplet via internal conversion, finally releasing photons in the
form of phosphorescence. Thus, charge-transfer character at the
lowest singlet and triplet states and small energy difference
between the lowest singlet and its neighboring triplet dominantly
introduced by naked eyes. The lifetime of the generated yellow
emission was determined to be 10.7 µs from Figure 3B, clearly
revealing its phosphorescence nature. Time-resolved PL decay
curves in Figure S4 shows the huge change in lifetimes of these
solutions from 1.1 ns to 10.7 µs, which confirms the gradual
conversion from fluorescence to phosphorescence. To further
verify water-caused aggregation of HHPB, dynamic light
scattering was used to characterize the size change during this
process. When water content in THF solution of HHPB is lower
than 67%, no substantial nanoparticles in hundreds nanometer
are detected, but appreciable amount of aggregates with the
size at 300±100 nm appears when the water content reaches
83%. A gradual rise in average size of generated aggregates as
water ratio is also observed from Figure S5, which verifies the
progressive aggregation of HHPB molecules as the addition of
water into THF. These results suggest aggregation or dissolution
induced by variation in solvent is capable of switching
fluorescence and phosphorescence in a single HHPB molecule.
Six hydroxyl groups of HHPB allow this hydrophobic molecule
to dissolve in alkaline aqueous solution by adjusting pH value,
and thus the occurring state of HHPB molecules can be
controlled between aggregation and dissolution by external pH
value. The hypothesis of pH-mediated aggregation-induced
phosphorescence was tested by changing pH value of HHPB
suspension from pH 7.0 to 12.0 in Figure 4A. However, it is
surprisingly found that yellow phosphorescence of HHPB
suspension is progressively turned to an intense green emission
as pH value varies from pH 7.0 to 12.0, and the suspension
finally becomes a homogenous solution. The aggregation state
of HHPB molecules at pH 7 and 8 was also confirmed by
dynamic light scattering as shown in Figure S6, where average
diameters fall in the range of 100 – 400 nm. The generated
green emission is quite bright since its absolute quantum yield is
determined to be 0.08. A short lifetime on the order of
nanoseconds (2.5 ns) determined from time-resolved
contribute
to
the
occurring
of
aggregation-induced
phosphorescence and its dual emission behavior in solution, and
these findings are promising to develop as a design guideline for
efficient RTP materials without the presence of heavy atoms.
Twisted molecular structure and specific electronic
configuration of HHPB result in its efficient RTP in solid state,
and thus the change in physical state would be used to
modulate its fluorescence and phosphorescence. This
hypothesis was verified by the observation in Figure 3A, which
shows the variation of fluorescence and phosphorescence of
HHPB by gradually aggregating HHPB molecules in ethanol
solution using a poor solvent water. A moderately intense blue
emission of HHPB in ethanol can be observed without the
addition of water, but the blue fluorescence progressively
decreases as the rise in ratio between water and ethanol. A new
intense yellow emission centered at 570 nm is generated when
the ratio is 83%, and its intensity reaches the top when pure
water is used as the solvent. It is readily noticed that
aggregation of HHPB is enhanced as more water content is
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design smart materials and intelligent devises.[21] We attempt to
uncover underlying nature for such properties by analyzing and
comparing optical properties of compound 1 and its isomer
hexakis(4-hydroxy-1-phenylthio)benzene (compound 2). The
schematic structure of compound 2 is shown in Figure S8, and
Figure S9 demonstrates compound 2 has bright RTP in solid
state and a long lifetime of 3.2 µs. The Joblonski diagram in
Figure S10 shows that intersystem crossing process can be
greatly promoted due to a very small energy gap between S1
and T9 (0.03 eV), but the relative energy difference between S1
and T1 effectively blocks the reverse process of intersystem
crossing, facilitating the generation of phosphorescence. Both
compound 1 and compound 2 meet such two requirements. In
Figure 5. (A) The change of PL spectra of HHPB in THF versus the content of
triethylamine (Et3N). (B) The change of PL spectra of HHPB in THF in the
presence of 200.0 µL triethylamine versus the content of trifluoroacetic acid
(TFA).
contract with compound 1, compound
2
has
a
weak
fluorescence peak around 330 nm except for a long-lived
emission at 580 nm as shown in Figure S11. The lacking of
emissive singlet in the visible range excludes the possibility of
manipulating phosphorescence and fluorescence. Such a small
change in substitution position between compound 1 and
luminescence decay curve in Figure 4B clearly implicates its
fluorescence nature. These results lead to a hypothesis that the
green emission probably originates from singlet of deprotonated
form of HHPB due to such an alkaline environment. Figure 4C
depicts the optimized structures of HHPB and its deprotonated
form, and an appreciable difference in conformation can be
concluded. HHPB as a neutral molecule exists a twist but
compact configuration, whereas its deprotonated form shows a
more spread conformation because of electrostatic repulsion
among six phenoate subunits with a negative charge. This
relatively rigidified structure facilitates the generation of
fluorescence to some extent in comparison with HHPB. Table
S2 lists excitation energies, oscillator strengths and contributions
of molecular orbitals for the deprotonated form of HHPB. The
accordance between theoretical and experimental values further
confirms the origin of green emission from singlet of
deprotonated form of HHPB. This unique pH-triggered
phosphorescence-fluorescence switch is also reflected from a
sharp decline from 10.7 µs to 2.5 ns as the rise of pH value from
7 to 12 as shown in Figure S7.
compound
2
leads to extremely different behavior in
photophysical properties, which suggests the rational design for
organic luminogens with controllable emissions can be achieved
by more efficient design of molecular structure with aid of
theoretical analysis.
In summary, a twisted hexathiobenzene-based molecule with
dual emissions was designed and synthesized, and it shows a
significant aggregation-induced phosphorescence behavior and
a regulation of singlet and triplet emissions by external stimuli.
Switching between phosphorescence and fluorescence at the
unimolecular level was realized by regulating the physical states
of aggregation and dissolution. Twisted structure and the proper
arrangement in excited energies mainly contribute to precise
modulation of singlet and triplet emissions of the designed
molecule. This work demonstrates an example of reversible
switching among three colored emissions from a single molecule
in a controlled way, which is extremely valuable to design smart
materials and intelligent optoelectronic devices.
Two emissions from singlets were identified from the
preceding results, and they originate from singlets of HHPB and
its deprotonated form in the isolated state. As
a result,
triethylamine and trifluoroacetic acid were chosen as the organic
base and acid, respectively, to achieve switching between blue
and green fluorescence in organic solvents. As shown in Figure
5A, HHPB in THF only shows a blue emission around 400 nm
initially, but the blue emission is gradually quenched as the
addition of triethylamine in volume while a new emission at 500
nm is progressively enhanced. When 200.0 µL of trimethylamine
is introduced, the blue fluorescence completely disappears while
the green emission reaches the top. During this process, all the
neutral molecules of HHPB have transformed to deprotonated
forms, thus exhibiting intense green fluorescence. To further
evaluate its reversibility of this emissive switch, different
amounts of trifluoroacetic acid is continuously added into the
above solution. The reverse process is demonstrated in Figure
5B, which shows the gradual enhancement of blue emission and
progressive quenching of green fluorescence. These results
prove fluorescence switching between two colors can be
achieved by protonation and deprotonation in a reversible and
controllable way.
Experimental Section
Experimental details are included in Supporting Information.
Acknowledgements
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Grant Nos. 21675143,
21775139 and 21775138) and Natural Science Foundation of
Zhejiang Province (Grant No. LR18B050001 and LY17B050003).
Keywords: Room-Temperature Phosphorescence •
Aggregation-Induced Phosphorescence• Molecular Switch •
Triplet-Singlet Switching• External Stimuli
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Entry for the Table of Contents
COMMUNICATION
External Stimuli-Controlled Switching
among Blue Fluorescence, Green
Fluorescence and Yellow
Jing Xu, Hui Feng, Hao Teng, Guilin
Chen, Saifei Pan, Jianrong Chen,
Zhaosheng Qian*
Phosphorescence in a Unimolecular
System is Achieved.
Page No. 1 – Page No.5
Reversible Switching between
Phosphorescence and Fluorescence
in a Unimolecular System Controlled
by External Stimuli
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