An Unexpected Chromophore–Solvent Reaction Leads to Bicomponent Aggregation-Induced Phosphorescence

Article abstract of DOI:10.1002/anie.202000865

Organic luminogens with persistent room-temperature phosphorescence (RTP) have found a wide range of applications. However, many RTP luminogens are prone to severe quenching in the crystalline state. Herein, we report a strategy to construct a donor-sp

Full text of DOI:10.1002/anie.202000865

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Accepted Article
Title: Unexpected Chromophore-Solvent Reaction Leads to
Bicomponent Aggregation-Induced Phosphorescence
Authors: Guoqing Zhang, Biao Chen, Wenhuan Huang, Hao Su, Hui
Miao, and Xuepeng Zhang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202000865
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Unexpected Chromophore-Solvent Reaction Leads to
Bicomponent Aggregation-Induced Phosphorescence
Biao Chen,[a] Wenhuan Huang,[a] Hao Su,[a] Hui Miao,[a] Xuepeng Zhang*[a] and Guoqing Zhang*[a]
Abstract: Organic luminogens with persistent room-temperature conjugation which renders the system planarity and rigidity, a
phosphorescence (RTP) have found a wide range of applications in good recipe for ACQ. In light of this problem, we recently
sensing, bioimaging, illumination, and information encryption proposed an AIE molecular motif that uses an sp3-type of linker
However, many RTP luminogens are prone to severe quenching in to attach  -conjugated donor and acceptor together, which
the crystalline state. Herein we report a working strategy to construct diminishes the sp3 single-bond rotation but concomitantly
a donor-sp3-acceptor type of luminogen which exhibits aggregation- prevents close molecular packing as aggregates.[14] In the
induced emission (AIE) while the donor-sp2-acceptor counterpart course of working with these molecular systems, we discovered
structure exhibits non-emissive solid state. Unexpectedly, it was
discovered that trace amount (0.01%) of structurally similar
that
5-bromo-2-(3,4-dimethoxybenzyl)isoindoline-1,3-dione
a
(BrBID, Figure 1a) appeared to possess unusual solvent-
derivative, produced by a side reaction with the DMF solvent, could dependent RTP, i.e., BrBID procured from DMF has very strong
induce strong RTP with an absolute RTP yield up to 25.4% and a RTP which is absent when EtOH is used. The observed
lifetime of 48 ms, although the identified substance does not show anomaly intrigued further investigation and was unveiled as a
RTP by itself. Single-crystal XRD-based calculations suggest that n- result of a side reaction from DMF and the AIEgen. More
 * orbital interactions as a result of structural similarity may be
strangely, the side product, identified as an amino derivative of
responsible for the strong RTP in the bicomponent system. The BrBID, lacks or shows very weak phosphorescence even at 77
current study provides a new insight into the design of multi- K in other media including heavily brominated solvents. Herein,
component, solid-state RTP materials from organic molecular
systems.
we present the discovery of the bicomponent AIE-RTP
phenomenon, as well as experimental evidence for a plausible
mechanism involving specific orbital interactions due to
structural similarity, which can be illuminating for the rational
design of solid-state organic RTP systems.
Purely organic RTP materials are important for practical
applications such as biological imaging and sensing,[1]
information encryption[2] and flexible organic light-emitting diode
(OLED) devices.[3] Most of these applications require that these
RTP molecules be utilized in the solid state since mobile phases
usually dissipate the triplet excited state via various quenching
mechanisms such as intramolecular rotations, biomolecular
collisions and electron transfer.[4] Consequently, organic RTP
molecules are commonly used as molecular solids (e.g.,
crystals,[5] films[6]) or dopant[7] in solid-state matrices such as
polymers.[8] In the latter case, deoxygenation can be a nuisance
if the matrix is porous enough to allow gas diffusion;[9] in this
regard, crystalline materials are better suited media for
harvesting RTP[10] since they are usually gas-impermeable.
Unfortunately, many molecules that exhibit long-lived RTP in
polymers are prone to ACQ in aggregated forms such as the
non-doped OLED emitting layer.[11] To solve the conundrum,
AIE[12] materials are frequently designed. Structurally, AIE
molecules often exhibit large rotational and vibrational freedoms
which facilitate the radiationless transitions from the excited
state to the ground state. Upon aggregation, such freedoms are
reduced without inducing additional non-radiative transition (e.g.,
strong  stacking) modes so that the excited state energy is
better preserved for emissive decay.[13] For most organic
luminogens,
a common characteristic is the extended 
Figure 1. a) Stick models showing sp2- and sp3- carbon-linked phthalimide
organic donor-acceptor structures with crystal lattice parameters. b) UV
absorption spectra of BrPID and BrBID in mTHF (3x10-5 M) at 298 K (solid
lines), PL spectra of BrPID and BrBID in mTHF (3x10-5 M) at 298 K (dash
lines, λex = 280 nm), PL spectra of BrPID and BrBID in the solid state in air at
298 K (dot lines, λex = 365 nm), inset: Photographs of BrPID (up row) and
BrBID at daylight and 365 nm irradiation ON. c) SC-XRD showing π-π stacking
distances and d) phthalimide ring overlaps for BrBID (left) and BrPID (right).
[a]
Dr. B. Chen, Mr. W. Huang, Mr. H. Su, Prof. H. Miao, Dr. X. Zhang,
Prof. G. Zhang
Hefei National Laboratory for Physical Science at the Microscale,
University of Science and Technology of China, 96 Jinzhai Rd,
Hefei, Anhui 230026 (China)
E-mail: zhangxp@ustc.edu.cn
gzhang@ustc.edu.cn
Two sp2- and sp3-carbon-linked donor-acceptor phthalimide
molecules BrPID (P for phenyl) and BrBID (B for benzyl), shown
Supporting information for this article is given via a link at the end of
the document.
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in Figure 1a, were facilely synthesized in high yields by a single-
step amine-anhydride condensation reaction in ethanol (Scheme
S1). The target compounds were confirmed by 1H and 13C
nuclear magnetic resonance (NMR) spectra and high resolution
mass spectrometry (HRMS), elemental analysis (EA) and high
performance liquid chromatography (HPLC, Figure S1, Figure
S15-S20). Single-crystal X-ray diffraction (SC-XRD, Figure 1a)
reveals that the dihedral angle between the donor and acceptor
planes is 113.55  imposed by the tetrahedral geometry of the
sp3-CH2- group; the sp2 counterpart has nearly perpendicular
planes. These configuration differences caused by bonding
results clearly demonstrated that an unknown RTP emitter is
involved.
We then employed HPLC to analyze the components of
different samples by monitoring the onset absorptions at both
280 and 365 nm, where several small impurity peaks were
uncovered gradually when the acetonitrile-water ratio was
optimized from 100/0 to 55/45 (v/v, Fig. S5, note that the solvent
ratio optimization is essential because impurity information is
easily buried by the absorption of the dominant BrBID, should
acetonitrile be used as the sole eluent). Although 1 and 2 had
several peaks ranging from ~0.1% to ~1% (relative to BrBID),
the top priority was given to a peak unique to sample 1 (Figure
S6, elution time = 8.3 min). After repeated chromatograph
purifications from a large batch of reaction (~100 g scale),
sufficent impurity X (> 50 mg) was isolated. The structure of X
was determined by 1H NMR, 13C NMR, HPLC and HRMS
(Figure S21-S23), and was also analyzed with SC-XRD (Figure
2c, S24 and table S2) as an derivative of BrBID, which has
previouly been discovered as a by-product from DMF and aryl
halide.[18] The photophysical properties of X are also shown in
Figure 2c. It was found that X is fluorescent in both liquid solvent
and as crystals (Figure S7). However, no emission longer than a
few nanoseconds could be recorded even at 77 K in any form,
indicating rather weak triplet state acitivites of X without BrBID
presence. In order to confirm that the RTP of sample 1 is indeed
from X doping, photoluminescence spectra were compared for
the bicomponent crystal of X/BrBID (0.05% m/m) and sample 1,
respectively. As shown in Figure 2d, the two materials possess
almost identical prompt and delayed spectra. Furthermore, time-
resolved emission decay curves indicate that they have almost
manners have
a profound impact on the photophysical
properties of the two molecules. The absorption and emission
spectra in dilute 2-methyl-tetrahydrofuran (m-THF, 30 μM) are
presented in Figure 1b, along with the solid state emission
spectra (dot lines). The main absorption band lies mainly < 350
nm for both BrPID and BrBID ( 
= 280 and 278 nm,
max
respectively). When excited at the absorption maxima for both
molecules, no discernible fluorescence could be recorded
(Figure 1b, PL < 0.001) at 298 K. In the solid state, under the
irradiation of a hand-held UV lamp, however, bright green-blue
photoluminescence of BrBID (λem = 505 nm,  = 0.49 s) could
be easily observed by the naked eye while BrPID was not
fluorescent at all. Consequently, BrBID exhibits typical AIE
features. There is no obvious difference in the degree of
crystallinity according to the powder XRD data (Figure S3).
Hence, one can speculate that it is likely due to the way in which
molecules stack. From XRD data (Table S1, Figure 1c and 1d),
the phthalimide ring stacking distances are 3.307 Å for BrPID
and 3.591
Å for BrBID, respectively, consistent with the
hypothesis that sp3-linkers tend to hinder tight molecular packing, the same lifetime (48.1 ms for X/BrBID), which confirms the
which explains the solid-state quenching[15] and AIE behaviors[16]
for the two analogs, respectively.
identity of X as the persistent RTP emitter in sample 1.
In the process of optimizing the product yield, two working
solvents, ethanol and DMF, were employed (see Supporting
Information, SI). Surprisingly, BrBID synthesized from DMF (1)
emitted strong yellow photoluminescence (PL, Figure 2a, λem
=
565 nm with a 506-nm shoulder band) with intense delayed
emission under the irradiation of 365-nm UV light, while 2
(synthesized from EtOH) showed an identical emission profile to
that of ultra-purified BrBID (Figure 1b and 2a, λem = 506 nm). At
a closer examination, time-correlated spectroscopy (∆t = 0.05
ms) revealed that for 1 the prompt and delay emission spectra
are nearly identical with a lifetime of 45.3 ms (Figure 2b),
indicating that 1 has nearly pure RTP emission. The RTP
quantum yield is measured to be >0.20 for crystals extracted
from the particular reaction condition. Sample 2, nevertheless,
has a mixed excited state given the Br presence that enhances
the spin-orbit coupling (  @506nm = 0.49 μs) due to the internal
heavy-atom effect. The phenomenon was first proposed as a
result of solvent-dependent polymorph, i.e., crystallizations from
different solvents yield different molecular packings (Figure
S3).[17] However, 1 repeatedly recrystallized with either EtOH or
DMF still showed strong RTP ~560 nm, but with varied
intensities (relative to the 506-nm band) and durations, which
prompted us to consider an impurity giving off RTP. After
purification via silica gel chromatography (2), the strong RTP at
560 nm disappeared (and showed the same luminescence
properties as those for ultrapure BrBID, Figure S4). These
Figure 2. a) Steady-state PL spectra (Ss) and delayed (∆t
= 0.05 ms)
emission spectra (DE) of 1 and 2 in the solid state at 298 K (λex = 365 nm). b)
Emission decay curves of 1 and 2 at 560 nm and 506 nm in solid-state. c)
Steady-state PL spectra of X in mTHF (3x10-5 M) at 298 K and 77 K (λex = 320
nm), steady state PL and delayed (∆t = 0.05 ms) spectra of X in solid state at
298 K (λex = 365 nm). d) SC-XRD of X, steady-state PL spectra and delayed
(∆t = 0.05 ms) spectra of 0.05% X dopant in BrBID and sample 1 in the solid
state at 298 K (λex = 365 nm); inset: emission decay curves of samples 1 and
X monitored at 560 nm.
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To find out how X-doping correlates with RTP in BrBID, we
blended BrBID with different proportions of X ranging from
0.01% to 5% (by mass fraction, Figure 3b and S8) in
dichloromethane. By solution evaporation of the mixture solution
homogenous bicomponent crystals (with no observable phase
separations) could be obtained possibly owing to the very similar
molecular structures of X and BrBID. Surprisingly, with only
0.01% X presence (when X does not even show up on HPLC
spectrum), strong yellow RTP with an afterglow was observed
by the naked eye, which implies that the doping efficacy is
exceedingly high. Under spectroscopic investigation, all samples
exhibit strong yellow RTP bands centered at ~560 nm except for
the sample with 0.01% X doping, which has more intense
shoulder emission at ~506 nm, ascribed to the bulk BrBID
luminescence. The highest PL intensity was recorded for 1% X
doping with an absolute RTP quantum yield >0.25.
Correspondingly, BrBID crystals prapared from DMF terminated
at different reactions times (1h, 4h, 12h and 24h) , and were also
examined by spectroscopy (Figure 3c and Figure S9). While the
1-h sample is barely phosphorescent, the 4h- and 12h-samples
show highly efficient RTP. The X content increases gradually as
the reaction proceeds (Figure S10, HPLC), and the 24h-sample
shows white emission and bears a new band centered at ~490
nm, attributed to the bulk luminescence of phase-separated X
crystals (by comparing with Figure 2c). In this sense, the
reaction time may be used to tune the concentration of X and
thus the luminescence properties of the resulted materials.
and the recorded ML spectrum of 1 (Figure 3d) superimposes
with that of the photoluminescence. To date, the most accepted
explanation for ML is excitation through electrification,[20] in
which case the X/BrBID molecules are driven into charge-
separated excited states via electrical field instead of
electromagnetic field.
As for the RTP-activation mechanism by
X doping, the
external heavy-atom effect (i.e., Br on BrBID in close contact
with X) is highly conducive. However, if such a hypothesis were
true, then doping the sp2 carbon-connected BrPID with X should
result in similar RTP given that the bromine atoms were mainly
responsible. When X was blended with BrPID under the same
condition (as BrBID), no RTP was observed at all (Figure S11);
when X was dissolved in 1,6-dibromohexane, extremely weak
phosphorescence was recorded (Figure S12), excluding the
possibility of external heavy-atom influence alone. Based on
these experimental observations, we proposed
a plausible
model (Figure 4) for the highly efficient RTP, aided by theoretical
calculations, which concerns where the X molecule fits after
doping. For BrBID crystals, X only differs in the 2-substitution on
the phthalimide ring, which implies that as long as the interaction
between C-Br and C-NMe2 is not repulsive, the X molecule has
a large probability to spatially replace a BrBID molecule in the
crystal lattice. Given that the emission spectrum exhibits vibronic
features which indicate localized Frenkel or charge-transfer
excitons, using the dimer coordinates derived from the SC-XRD,
we tried two different methods: 1) arbitrarily replace one C-Br
with C-NMe2 group with fixed BrBID coordinates; 2) optimization
of the dimer energy built upon SC-XRD coordinates for X and
BrBID. Both methods of calculations, employing molecular
orbital (MO, Figure 4 left) and natural bond orbital (NBO, Figure
4 right) models, suggest that there is orbital interaction between
the two groups in that the * (C-Br) and possibly the * spatially
overlap with that of the lone pair electrons on –NMe2.[21] The
findings have two major implications: 1) the X molecule should
be held even stronger when it replaced a BrBID molecule; 2)
with X molecules being the absolute minority species in the
bicomponent crystal, the –NMe2 non-bonding orbital is shared by
two C-Br * orbitals. Since the excited state heavily involves the
-NMe2 participation (Figure S25), the spin-orbit coupling should
be enhanced for X in the bicomponent crystal due to such
external heavy atom effect.
Finally,
it
was
also
discovered
that
persistent
mechanoluminescence[19] (ML) emanating from X/BrBID (e.g.,
sample 1) could be observed at room
Figure 3. a) Steady-state PL spectra of X dopants (0.01%, 0.05%, 0.1%, 0.2%,
0.5%, 1%, 3% and 5%) in BrBID crystals at 298 K (λex = 365 nm); inset: PL
intensity at 560 nm vs. doping content. b) HPLC spectra monitored at the
onset absorption of 365 nm for different dopant ratios. c) PL spectra of crude
BrBID products from different reaction times in DMF; inset: photographs of
crude products in daylight and under 365-nm irradiation. d) ML images and ML
spectrum of sample 1 taken at room temperature.
Figure 4. Molecular orbital (MO, left) and natural bond orbital (NBO, right)
models demonstrate orbital overlaps between an unoccupied  * (C-Br) like
orbital with an occupied lone pair-like (–NMe2) one.
temperature. When the crystals were crushed with various forms
of mechanical force, persistent yellow luminescence with a
duration of ~0.1 s was observed without external light excitation.
The ML was so strong that it was observable even in daylight
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Ling, H. Ma, M. Li, Z. Cheng, C. Ma, S. Cai, Q. Wu, N. Gan, X. Xu, Z.
An, W. Huang, J. Am. Chem. Soc. 2018, 140, 10734-10739.
S. Hirata, Adv. Opt. Mater. 2017, 5, 1700116.
In summary, we have constructed an AIE luminogen, BrBID,
by an sp3 methylene linker, which exhibits short-lived RTP (s-
long) with an emission maximum at 506 nm. An unexpected
side-reaction between the DMF solvent and BrBID results in an
amino derivative X which induces intense (  = 25.4%) and
substantially prolonged (τ = 48 ms) RTP in the X-in-BrBID
bicomponent solids. The impurity X was carefully isolated and
resolved by SC-XRD. It was found that the intense RTP of X
was only activated by the BrBID matrix and was explained in
terms of specific molecular orbital interactions from SC-XRD and
theoretical calculations. The current discovery sets an important
example for designing bicomponent organic RTP systems.
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We thank the National Key R&D Program of China
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Fundamental Research Funds for the Central Universities
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Conflict of interest
The authors declare no conflict of interest.
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Keywords: aggregation-induced emission • room temperature
phosphorescence • chromophore-solvent reaction • sp3 linked
donor–acceptor • persistent mechanoluminescence
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Entry for the Table of Contents
COMMUNICATION
Trace impurity by unexpected
solvent-chromophore
reaction leading to highly
efficient room-temperature
phosphorescence.
Biao Chen, Wenhuan Huang,
Hao Su, Hui Miao, Xuepeng
Zhang* and Guoqing Zhang*
Page No. – Page No.
Unexpected Chromophore-
Solvent Reaction Leads to
Bicomponent Aggregation-
Induced Phosphorescence
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