Rational Design of Dual-State Emission Luminogens with Solvatochromism by Combining a Partially Shared Donor–Acceptor Pattern and Twisted Structures

Article abstract of DOI:10.1002/chem.201903857

We report a general design strategy for a new class of luminogens with dual-state emission (DSEgens) that are brightly emissive in both the solution and solid state, with solvatochromism properties, by constructing a partially shared donor–acceptor pattern based on a twisted molecule. The DSEgens with bright fluorescence emission in both the solid and solution state demonstrate a unique solvatochromism behaviour depending on solvent polarity and thus may have applications in anti-counterfeiting.

Full text of DOI:10.1002/chem.201903857

DOI: 10.1002/chem.201903857
Communication
&
Fluorescence Spectroscopy |Hot Paper|
Rational Design of Dual-State Emission Luminogens with
Solvatochromism by Combining a Partially Shared
Donor–Acceptor Pattern and Twisted Structures
Qianqian Qiu+, Pengfei Xu+, Yanjun Zhu, Junru Yu, Mengru Wei, Wenbin Xi, Hui Feng,
Jianrong Chen, and Zhaosheng Qian*[a]
haviours. One major strategy to design DSEgens is to add con-
jugation-induced rigidity in twisted molecules, for which conju-
gation-induced rigidity facilitates the emission in solution by
suppressing intramolecular rotation and vibration, and twisted
structures are used to avoid fluorescence quenching in the
solid state caused by p–p stacking.[5–9] Dual-state emission can
also be attained by incorporating bulk substituents into molec-
Abstract: We report a general design strategy for a new
class of luminogens with dual-state emission (DSEgens)
that are brightly emissive in both the solution and solid
state, with solvatochromism properties, by constructing a
partially shared donor–acceptor pattern based on a twist-
ed molecule. The DSEgens with bright fluorescence emis-
sion in both the solid and solution state demonstrate a
unique solvatochromism behaviour depending on solvent
polarity and thus may have applications in anti-counter-
feiting.
ular structures to prohibit p–p stacking in the solid state.[10]
A
zero-twist donor–acceptor molecule was also found as a dual-
state emitter,[11] and a recent study showed that D–p–A struc-
tures can be used to achieve DSE properties.[12] However, there
is no valid design strategy for designing DSEgens with solvato-
chromism because of the lack of an investigation into the rela-
tionship between structure and DSE property.
The design and development of organic fluorescent materials
have fundamental importance because these materials are
widely used in various areas.[1] Traditional dye molecules gener-
ally possess large conjugation, structural planarity, and confor-
mational rigidity, thus exhibiting bright fluorescence in the dis-
solved state with disaggregation-induced emission (DIE) prop-
erties. These diverse dye molecules have long exploited proper
functional design, but their utilization in the solid state has
met great challenges because of their notorious aggregation-
induced quenching (ACQ) behaviours.[2] In contrast to tradi-
tional dyes (DIEgens), a new class of luminogens with aggrega-
tion-induced emission (AIE) has emerged.[3] These AIEgens emit
intense fluorescence in the aggregated or solid state, but
become weakly emissive in the dissolved state. The specific
AIE characteristic of AIEgens greatly promotes the advances of
biosensing and organic optoelectronic systems.[4] However,
both DIEgens and AIEgens can only exhibit bright fluorescence
in a single state (solution or solid state).
Herein, we have designed and synthesized a group of dual-
state emitters by combining the partially shared donor–accept-
or pattern and twisted structures, and have investigated their
dual-state emission behaviours and remarkable solvatochro-
mism properties through experimental data and theoretical
analysis. As shown in Scheme 1, rhodamine 6G as a representa-
tive of DIEgens only exhibit intense fluorescence in solution,
whereas AIEgens such as tetraphenylethylene (TPE) merely
emit intensely in the solid state. The designed products includ-
ing 4-(N,N-diphenylamino)benzaldehyde (DPAB), 1-(4-(N,N-di-
phenylamino)phenyl)ethanone (DPPE), and (4-(N,N-diphenyl-
amino)phenyl)(phenyl)methanone (DPPM) exhibit bright emis-
sion in both the solution and solid state, and this DSE phe-
nomenon is markedly distinct from AIE and DIE. The twisted
structure and a partially shared donor–acceptor pattern greatly
contribute to the generation of dual-state emission. The dis-
tinct emission colours of these DSEgens in different solvents
were explained with the aid of theoretical simulations, and
their anti-counterfeiting applications were further demonstrat-
ed.
There is a growing interest in organic dual-state emission
emitters (DSEgens), intensely emitting in both the solution and
solid states, because these DSEgens fill the gap between ACQ
and AIE. However, only a few examples demonstrated DSE be-
Three electron-withdrawing groups were introduced into a
steric triphenylamine molecule, respectively, by Friedel Crafts
acylation reactions (Scheme S1, Supporting Information). The
obtained products were fully characterized using 1H and
13C NMR and HRMS (Figures S1–S9, Supporting Information).
Their UV/Vis spectra (Figure S10, Supporting Information) show
that they have similar absorption patterns with two apparent
absorption peaks at around 360 and 290 nm. Figure 1 displays
PL spectra of the three compounds in THF solutions and in the
solid state. The fluorescence emission maxima of DPAB, DPPE,
and DPPM in THF are located at 492, 472, and 502 nm, respec-
[a] Q. Qiu,+ P. Xu,+ Y. Zhu, J. Yu, M. Wei, W. Xi, Dr. H. Feng, Prof. J. Chen,
Prof. Z. Qian
Department of Chemistry, College of Chemistry and Life Sciences
Zhejiang Normal University, Yingbin Road 688, Jinhua 321004 (P. R. China)
E-mail: qianzhaosheng@zjnu.cn
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.201903857.
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Scheme 1. Schematic illustration of distinct photophysical behaviours of three classes of luminogens: (a) Disaggregation-induced emission luminogens (DIE-
gens); (b) aggregation-induced emission luminogens (AIEgens); (c) dual-state emission luminogens (DSEgens).
that the solids of all three compounds emit bright blue colours
under the excitation of UV light, and their PL spectra in Fig-
ure 1b show that their emission peaks at 445, 433, and 450 nm
are located at the blue light region, and their lifetimes fall in
the range of 1.2–2.0 ns (Figure S12, Supporting Information),
which are in the same order of magnitude with those in THF.
Table S1, Supporting Information, summarizes photophysical
properties of DPAB, DPPE and DPPM in THF and in the solid
state, from which one can note that both solution and solid
states for each of them possess high fluorescence quantum
yields (ꢀ15%), which definitively demonstrates that all the
compounds have dual-state emission characteristics.
The large difference in emission maximum between solution
and solid state for each of them implies the occurrence of a
significant solvatochromism of fluorescence. As shown in Fig-
ure 2a, DPAB exhibits largely distinct fluorescence colours from
blue to yellow depending on diverse solvents with different
polarities. The emission maximum of DPAB in nonpolar toluene
is located at 449 nm, but its emission peak is largely redshifted
to 554 nm in polar acetonitrile. A gradual redshift of emission
maximum, as the polarity of the solvent increases, is observed.
Their lifetimes in diverse solvents fall in the range of 3.2–
9.0 ns, and their emission efficiencies are all greater than 27%
despite of varied PL intensity as shown in Figure 2b. Similar
solvatochromism phenomena are also observed for DPPE and
Figure 1. (a) PL spectra of DPAB (50.0 mm), DPPE (50.0 mm), and DPPM
DPPM as shown in Figure S13–S14, Supporting Information,
(50.0 mm) in THF. Inset: Fluorescent images. (b) PL spectra of DPAB, DPPE,
and all the photophysical properties of DPAB, DPPE, and DPPM
in diverse solvents are summarized in Tables S2–S4, Supporting
Information. These DSEgens have similar solvatochromism
properties with common push–pull solvatochromic dyes like
dansyl and prodan,[13] but they show bright fluorescence in the
solid state whereas those traditional solvatochromic dyes do
not fluoresce in the solid or aggregated state. Differing from
aggregation-induced emission enhancement of AIEgens, the
PL intensities of DPAB, DPPE, and DPPM first abruptly decline
and DPPM in the solid state. Inset: Fluorescent images.
tively, exhibiting intense green colours under the illumination
of UV light. Their lifetimes are estimated to be 5.8, 5.6 and
5.4 ns from their respective time-resolved PL decay curve in
Figure S11, Supporting Information, and such short emission
durations in nanosecond magnitude clearly suggest their fluo-
rescence nature from singlet states. It is interestingly found
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acid, but then progressively recovers following the introduc-
tion of excess triethylamine into the above system. These ob-
servations suggest that the bright-blue emission of DPAB in
toluene originates from a TICT state because this emission can
be greatly inhibited by the protonation of the triphenylamine
moiety which converts the electron-donating triphenylamine
to an electron-withdrawing triphenylaminonium ion. Similar re-
sults were also observed for DPPE and DPPM (Figures S19–S20,
Supporting Information), which clearly demonstrates that the
three compounds undergo a TICT emission. The solvent relaxa-
tion process of TICT fluorophores can be described by the Lip-
pert–Mataga equation,[16] and thus the dependence of Stokes
shifts on the orientation polarizability (Df) for the three com-
pounds were evaluated as shown in Figure S21, Supporting In-
formation. The increased Stokes shift along with the increase
of Df for each of them clearly suggests that they are all strong
TICT fluorophores with highly localized excited-state charge
separation. DFT calculations[17] and natural bond orbital (NBO)
analysis[18] were performed to identify exact donor parts and
acceptor parts for DPAB, DPPE, and DPPM. As shown in
Figure 3, triphenylamine moieties dominantly contribute to the
HOMOs of all three compounds, whereas the LUMOs mainly
originate from carbonyl-containing phenyl groups, which indi-
cate that the donors and acceptors share a phenyl ring for the
three compounds. The charge-transfer process induced partial-
ly shared donor–acceptor pattern was also confirmed by their
electron density difference maps (Figure S22, Supporting Infor-
mation),[19] which indicate that a major charge-transfer process
occurs among the nitrogen atom, benzene moiety, and car-
bonyl group for each of them. As a result, a twisted intramo-
lecular charge-transfer process induced by a partially over-
lapped donor–acceptor pattern in the molecules is responsible
for intense fluorescence emissions for DPAB, DPPE, and DPPM
in solvents.
Figure 2. (a) Normalized PL spectra and solvatochromism of DPAB (50.0 mm)
in different solvents. (b) PL spectra of DPAB (50.0 mm) in different solvents.
and then gradually increase to the top as shown in Figures
S15–S17, Supporting Information.
A twisted intramolecular charge transfer (TICT) mechanism
may be responsible for solvatochromism behaviours of these
DSEgens, similar to traditional TICT molecules for viscosity
measurement.[14,15] To verify this hypothesis, the PL spectra
changes caused by the protonation and the following depro-
tonation of DPAB were recorded as shown in Figure S18, Sup-
porting Information. The intense blue fluorescence of DPAB in
toluene gradually declines with the addition of trifluoroacetic
A variable-temperature experiment on DPAB in THF was per-
formed to explore the relation between structural conforma-
tion and emission location. Figure 4a shows that the emission
maxima of DPAB are mainly located at around 460 nm when
DPAB molecules are fixed in the solid THF at 77–170 K, but its
Figure 3. HOMOs and LUMOs of DPAB, DPPE, and DPPM calculated at the B3LYP/6–31+G(d,p) level.
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DPAB in toluene (481 nm) and in acetonitrile (571 nm) are very
close to their corresponding experimental data in toluene
(450 nm) and in acetonitrile (552 nm). The accordance between
theoretical data and experimental data further substantiates
the origin of solvatochromism for these D–A luminogens from
subtle conformational changes caused by different solvents.
It is easy to imagine that twisted structures of DPAB, DPPE,
and DPPM could overcome p–p stacking in the crystal or solid
state. To confirm this hypothesis, their crystal structures deter-
mined by single-crystal X-ray diffraction were examined as
shown in Figure S23, Supporting Information.[21] It is readily
found that all the three compounds exist in the twisted struc-
ture due to the distorted conformation of the triphenylamine
moiety. The average intermolecular distances represented by
the bond length of the NÀN bond for the three compounds
are greater than 5.0 ꢁ, and the nearest CÀC bond lengths are
also longer than 3.7 ꢁ, which clearly suggests that no signifi-
cant intermolecular p–p interaction occurs for all the com-
pounds. As a result, a twisted structure of the triphenlamine
moiety mainly contributes to bright emission in solid state,
while partially shared donor-acceptor pattern leads to a TICT
state resulting in bright fluorescence in solution and significant
solvachromism behaviour. The twisted structure and partially
shared donor–acceptor pattern jointly results in dual-state
emission and solvachromic properties of DPAB, DPPE, and
DPPM. Their common DSE properties and solvatochromism be-
haviours suggest that such DSEgens can be designed by com-
bining twisting structures and a partially shared donor–accept-
or model.
Figure 4. (a) PL spectra of DPAB (50.0 mm) in THF at different temperature
from 77 K–270 K. (b) Optimized structures of DPAB at ground state and S1
state at PBE0/6–31+G(d,p) level. (c) Optimized structures of DPAB at S1
state in toluene and in acetonitrile at PBE0/6–31+G(d,p) level.
The unique dual-state emission and solvatochromism prop-
erties of DSEgens in this work provide an opportunity for ap-
plication in data encryption and anti-counterfeiting. Figure 5a
emission peak is largely red-shifted to 520 nm when DPAB
molecules can freely rotate in liquid THF above 190 K, suggest-
ing that the structural conformation of DPAB might dominantly
determine its emission peak because its structure cannot be
changed in solid THF but can be relaxed in liquid THF when it
is excited. Thus, its structure at the ground state in THF was
compared to that at the S1 state from computations as shown
in Figure 4b. It is apparently found that the dihedral angle
qC26-C24N12-C13 between one benzene ring linked to the nitrogen
atom and the benzene ring linked to the carbonyl group at S0
is greatly increased from 100.98 to 154.08 at S1 state. These re-
sults prove that the emission location of DPAB is dominated
by the relative position of the diphenylamino groups and the
benzaldehyde moiety. To further verify this hypothesis, time-
dependent DFT (TDDFT) calculations[20] on excited state struc-
tures and properties of DPAB in nonpolar toluene and in polar
acetonitrile were performed. Figure 4c shows the optimized
structures of DPAB at the S1 state in toluene and acetonitrile.
The formyl group is conjugated to the neighbouring benzene
ring in the same plane for both the S1 structures in toluene
and in acetonitrile, but the relative position of the diphenyl-
amino group to the benzaldehyde moiety in toluene is obvi-
ously different from that in acetonitrile because the dihedral
angle qC26-C24N12-C13 of the S1 structure of DPAB is significantly in-
creased from 96.58 in toluene to 103.98 in acetonitrile. Based
on these S1 structures, the calculated emission maxima of
Figure 5. Data encryption demonstration of DSEgens using a number-based
pattern (a) and anti-counterfeiting applications with DSEgens in a school
badge (b) and a school motto in Chinese (c). The pattern a1 was covered
with DPAB and TPE, and a2—a4 further were treated with toluene, THF and
acetonitrile, respectively. The patterns in b1 and c1 were drawn with DPAB,
and patterns in b2 and c2 were imaged after treating with THF.
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demonstrates a simple example of the use of DSEgens in data
encryption based on a number pattern. The pattern of the
number “8” was first painted with a solution of tetraphenyl-
ethylene, and then part of the pattern was covered with a so-
lution of DPAB. The pattern “8” in bright cyan colour can be
observed under the UV light due to the solid-state emissions
of TPE and DPAB, but the blue “8” can be changed to “6” in
blue, green or yellow colour after spraying the pattern with
toluene, THF or acetonitrile because of the disappearance of
the blue emission from TPE in solution and specific solvato-
chromism behaviour of DPAB. The unique dual-emission prop-
erties of these DSEgens can also be expanded to anti-counter-
feiting applications. Figure 5b and c show a school badge and
a school motto in Chinese drawn with a solution of DPAB on a
paper. The bright cyan colour of the pattern and Chinese char-
acters under the irradiation of UV light can be sharply turned
to yellowish green after a simple treatment of THF, illustrating
a great potential of these DSEgens with solvatochromism
properties in anti-counterfeiting applications.
Keywords: anti-counterfeiting
solvatochromism · twisted intramolecular charge transfer ·
twisted structure
·
dual-state emission
·
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donor–acceptor pattern and by introducing a twisted triphe-
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unique solvatochromism behaviours of these DSEgens. Dual-
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were further used for anti-counterfeiting applications. This
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Experimental Section
Experimental details are included in Supporting Information.
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We 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 Nos. LR18B050001 and LY17B050003).
Conflict of interest
Manuscript received: August 23, 2019
Revised manuscript received: September 16, 2019
The authors declare no conflict of interest.
Accepted manuscript online: September 19, 2019
Version of record online: && &&, 0000
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COMMUNICATION
Twisted chemistry! A general design
strategy for a new class of luminogens
with dual-state emission (DSE) and sol-
vatochromism properties by construct-
ing a partially shared donor–acceptor
pattern based on a twisted molecule
was reported (see figure).
&
Fluorescence Spectroscopy
Q. Qiu, P. Xu, Y. Zhu, J. Yu, M. Wei, W. Xi,
H. Feng, J. Chen, Z. Qian*
&& – &&
Rational Design of Dual-State
Emission Luminogens with
Solvatochromism by Combining a
Partially Shared Donor–Acceptor
Pattern and Twisted Structures
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