Solid-State Fluorophore Based on π-Extended Heteroaromatic Acceptor: Polymorphism, Mechanochromic Luminescence, and Electroluminescence

Article abstract of DOI:10.1021/acs.cgd.9b01630

A novel light-emitting material 4-(4-diphenylaminophenyl)-benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (4-TPA-BBI) is synthesized and characterized. 4-TPA-BBI exhibits excellent polymorphism, mechanochromic luminescence (MCL), and electroluminescence properties. It can form three different aggregate species: yellow rod-like crystal (Y-crystal), orange needle-like crystal (O-crystal), and red solid (R-solid). It is observed that the Y-crystal can be reversibly changed to R-solid upon external stimuli, while O-crystal can be irreversibly transformed to R-solid via sublimation under a vacuum. Crystallographic and photophysical studies indicate that the Y-crystal featuring loose molecular packing demonstrates a high photoluminescence quantum yield (φ_PL ≈ 1) and obvious MCL property, while the compactly packed O-crystal shows a lower φ_PL of 0.646 with negligible MCL. A nondoped light-emitting device based on 4-TPA-BBI as an emitter exhibits bright orange emission with a peak at 588 nm, a high external quantum efficiency of 4.4%, and a maximum luminance of 22470 cd m^-2

Full text of DOI:10.1021/acs.cgd.9b01630

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Article
Solid-State Fluorophore Based on #-Extended Heteroaromatic Acceptor:
Polymorphism, Mechanochromic Luminescence and Electroluminescence
Bin Huang, Xiaomin Gu, Yan Feng, Ying Zhang, Dawei Jiang, Wen-
Cheng Chen, Gu Dai, Yigang Ji, Qiang Zhao, and Chun-Sing Lee
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b01630 • Publication Date (Web): 05 Mar 2020
Downloaded from pubs.acs.org on March 5, 2020
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Solid-State Fluorophore Based on π-Extended
Heteroaromatic Acceptor: Polymorphism,
Mechanochromic Luminescence and
Electroluminescence
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Bin Huang a,b,*, Xiaomin Gu , Yan Feng , Ying Zhang , Dawei Jiang , Wen-Cheng
a
a
a
a
Chen b, c, *, Gu Dai , Yigang Ji , Qiang Zhao , and Chun-Sing Lee *
a
a
a
b,
a
College of Life Sciences and Chemistry, Jiangsu Key Laboratory of Biofunctional
Molecule, Institute of New Materials for Vehicles, Jiangsu Second Normal University,
Nanjing, 210013, P. R. China
b
Center of Super-Diamond and Advanced Films (COSDAF) and Department of
Chemistry, City University of Hong Kong, Hong Kong SAR, P. R. China
c
School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou, 510006, P. R. China
Correspondent Author: Bin Huang; Wen-Cheng Chen; Chun-Sing Lee
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ABSTRACT:
A
novel
light-emitting
material,
4-(4-diphenylaminophenyl)-
benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (4-TPA-BBI) is synthesized and
characterized. 4-TPA-BBI exhibits excellent polymorphism, mechanochromic
luminescence (MCL) and electroluminescence properties. It can form three different
aggregate species: yellow rod-like crystal (Y-crystal), orange needle-like crystal (O-
crystal) and red solid (R-solid). It is observed that Y-crystal can be reversibly changed to
R-solid upon external stimuli, while O-crystal can be irreversibly transformed to R-solid
via sublimation under vacuum. Crystallographic and photophysical studies indicate that
Y-crystal featuring loose molecular packing demonstrates a high photoluminescence
quantum yield (Φ ~1) and obvious MCL property, while the compactly packed O-crystal
PL
shows a lower ΦPL of 0.646 with negligible MCL. A non-doped light-emitting device
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based on 4-TPA-BBI as an emitter exhibits bright orange emission with a peak at 588
nm, a high external quantum efficiency of 4.4% and a maximum luminance of 22470 cd
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m .
KEYWORDS: polymorphism, mechanochromic luminescence, electroluminescence,
triphenylamine, benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one, emitter
INTRODUCTION
In recent years, highly emissive solid-state organic materials have received increasing attention
due to their potential applications in sensors, optoelectronic devices etc.1-5 Tuning the
luminescence properties of organic materials is also very important for practical applications.6-8
One feasible strategy for achieving efficient and tunable luminescence is to develop new light-
emitting molecular structures or to decorate the known luminescent skeletons through
chemical modifications.9-10 Alternatively, manipulation of molecular aggregation states is
believed to be efficient for fine-tuning the emission properties without any chemical
reactions, especially for the solid-state luminophores. In comparison with the former, the
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latter has the advantage of facile preparation and low pollution. Moreover, the study of
the influence of aggregation state on luminescence properties helps us to further understand
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the structure-property relationship.
Mechanochromic luminescent (MCL) emitters which show reversible color switching
upon external stimuli have aroused increasing interest in recent years.12 They often
display polymorphism and multiple molecular packing arrangements, which are
considered to be the main reason for the MCL behavior, are potential materials to be
used in solid-state sensors, optoelectronic devices, and security inks.13-15 Up till now,
although some MCL-active luminophores have been reported, the underlying MCL mechanism
at the molecular level remains unclear.16-29 Generally, the solid-state emission of a
luminophore depends on many factors, such as molecular conformation, intermolecular
interaction and packing or their combination. Therefore, the development of polymorph-
dependent MCL materials with tunable and predicable crystal structures facilitates the
elaboration of the MCL mechanism. In 2015 and 2016, Chi’s group and Li’s group reported
two tetraphenylethene derivatives P4TA and TMPE with two polymorphs, respectively.30-31
Different conformations and intermolecular interactions in the crystals account for their similar
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MCL behavior. Very recently, Tian’s group reported another MCL material APMOB and
revealed the influence of the molecular conformation and π-π stacking interactions on the
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photophysical properties. However, so far MCL materials showing more than one single crystal
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structures remain limited. It is thus important to enrich the library of polymorph-dependent
MCL materials and unveil how aggregates influence MCL. On the other hand, for EL application
an efficient OLED often need a highly emissive solid thin film as the emissive layer. However,
most MCL materials show strong emission in crystal but low efficiency in film; therefore,
efficient non-doped OLEDs based on polymorph-dependent MCL materials remain limited.14
Benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (BBI) is a novel heteroaromatic
building block for organic optoelectronics application.33 BBI has a π-extended conjugated
skeleton ensuring high photoluminescence quantum yield (Φ ) and can be used as an electron
PL
acceptor to construct bipolar donor-acceptor (D-A) emitters because of its electron-withdrawing
3
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carbonyl group. More importantly, the heteroaromatic framework with nitrogen and oxygen
atom can provide abundant intermolecularly interacting sites (such as forming intermolecular
N···H or O···H bonds) to manipulate aggregation states for realizing polymorphism and MCL.34
Recently, we have exploited a D-A type MCL molecule 3-TPA-BBI by employing
34
triphenylamine (TPA) and BBI as the D and the A moieties, respectively. 3-TPA-BBI can form
two aggregates and its photoluminescence (PL) can be reversibly switched between orange and
yellow upon external stimuli. Unfortunately, we failed to obtained more than one type of single
crystal of 3-TPA-BBI for illustrating the underlying MCL mechanism. Herein, an isomer of 3-
TPA-BBI, 4-(4-diphenylaminophenyl)-benzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-
one (4-TPA-BBI) is synthesized and characterized. 4-TPA-BBI is a versatile fluorophore
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that exhibits polymorphism, high-contrast MCL and efficient electroluminescence (EL)
properties. Two types of single crystals (Y- and O-crystal) of 4-TPA-BBI have been
obtained and the crystallographic study indicates that Y-crystal with weak π-π interaction
and low intermolecular overlap exhibits a respectably high ΦPL approaching unity and
high-contrast MCL property, while O-crystal with strong π-π interaction and high
intermolecular overlap displays a lower ΦPL of 0.646 and negligible MCL. Importantly,
the 4-TPA-BBI’s thin film displays a high ΦPL of 0.723. A non-doped organic light-
emitting device (OLED) with 4-TPA-BBI as an emissive layer exhibits orange EL with a
decent external quantum efficiency (EQE) of 4.4% and a high maximum luminance of
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22470 cd m .
EXPERIMENTAL SECTION
General information. All reagents and solvents were purchased from commercial
sources and used as received without further purification. A mixture of 3-
bromobenzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one
(3-Br-BBI)
and
4-
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bromobenzo[d,e]benzo[4,5]imidazo[2,1-a]isoquinolin-7-one (4-Br-BBI) was obtained
according to the reported method and used for the next step without separation.33 1
H
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and C NMR spectra were recorded on a Bruker AV 300 NMR spectrometer. Si(CH )
3
4
was used as the internal standard. Mass spectra were measured with a Thermo
Electron Corporation Finnigan LTQ mass spectrometer. The UV-vis absorption spectra
were measured on a UV-vis spectrophotometer (Agilent 8453). The PL properties were
measured with an Edinburgh FLS980 spectrophotometer. The X-ray diffraction (XRD)
measurements were obtained through a diffractormeter (Ultima IV) with an X-ray source
of Cu Kα at 40 kV and 40 mA, at a scan rate of 2° (2θ) per min. Single crystal XRD
measurements were carried out on a single crystal X-ray diffractometer (Oxford Gemeni
S Ultra) with Cu Kα radiation. CCDC 1963811 (Y-crystal) and 11963810 (O-crystal)
contain the supplementary crystallographic data for this paper.35
All DFT calculations were performed using Gaussian 09 program package.36 The
molecular models were selected from the single crystal structures. The molecular
structures of 4-TPA-BBI monomers at ground state were optimized at the B3LYP/6-
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1G(d) level.37 In view of the intermolecular interactions, the molecular structures at
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ground state of 4-TPA-BBI dimers were further calculated with M062X/6-31G(d). The
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excitation energies in the singlet and triplet states were obtained using TD-DFT method
based on the optimized molecular structures at ground state.37
Synthesis of 4-TPA-BBI. A mixture of 3-Br-BBI/4-Br-BBI (10 mmol), 4-
(diphenylamino)phenyl boronic acid (10 mmol) in 100 mL of toluene and 40 mL of
ethanol, K CO aqueous solution (2.0 M, 10 mL) was added in a three-necked flask.
2
3
After purging with N for 10 min, Pd(PPh ) (1 mmol) was added. Then the reaction
2
3 4
mixture was refluxed under N for 24 h. After cooling to room temperature, the mixture
2
was poured into 100 mL water, and extracted with dichloromethane (DCM) (2 × 100
mL). The combined organic layer was washed with saturated NaCl aqueous solution (2
×
40 mL), dried over anhydrous Na SO , and concentrated under vacuum. The residue
2
4
was purified by silica gel column chromatography using petroleum ether/ethyl acetate
(v:v = 10:1) as eluent to afford 3-TPA-BBI (R = 0.21) and 4-TPA-BBI (R = 0.16) with
f
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yields of 21.5% and 30%, respectively. The structure analysis of 4-TPA-BBI is shown in
Table S1.
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Different aggregates of 4-TPA-BBI. The block-shaped yellow crystals (Y-crystal) of 4-
TPA-BBI were grown from DCM/ethanol (v:v = 1:1) solution by slow evaporation at room
temperature. The needle-like orange crystals (O-crystal) were grown from DCM/hexane
(v:v = 1:1) solution by slow evaporation at room temperature. The red solid (R-soid) is
obtained by sublimating the as-prepared powder or crystalline samples at 170 °C under
-6
high vacuum (10 Torr).
Device fabrication and performance measurement. Before device fabrication, indium-
tin oxide (ITO)-coated glass substrates were cleaned with detergent and dried carefully
and treated by UV ozone for 20 min. Then the samples were transferred into an
-7
evaporation chamber with a base vacuum of 5 × 10 Torr. Subsequently, the hole-
injection material 4,4'-cyclohexylidenebis[N,N-bis(p-tolyl)aniline (TAPC), hole-transporting
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material 4,4’,4’’-tris(carbazol-9-yl)-triphenylamine (TCTA), the exciton blocking material
1,3-di(9H-carbazol-9-yl)benzene (mCP), the emitter 4-TPA-BBI, and electron-
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transporting material 2,2',2''-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBI)
-1
were deposited in sequence via thermal evaporation at a rate of 1.0 Å s . Finally, LiF
-1
and Al were deposited at rates of 0.1 and 5 Å s , respectively. The current-voltage
characteristics of the device are characterized with a Keithley 2400 Sourcemeter. The
electroluminescent spectra, luminance, and Commission Internationale de l’Éclairage
(CIE) color coordinates were obtained with a Spectrascan PR650 photometer.
RESULTS AND DISCUSSION
Photophysical Properties of 4-TPA-BBI in Solvents. 4-TPA-BBI shows distinctive
photophysical properties in solvents with different polarities. Figure 1a shows the photographs of
4
-TPA-BBI in various solvents under daylight (upper panel) and daylight plus 365-nm UV
excitation (bottom panel). UV-vis absorption spectra of 4-TPA-BBI in solvents with varying
polarity are shown in Figure 1b. In DCM, 4-TPA-BBI shows two absorption bands at 298 and
4
39 nm (Figure 2b), which can be ascribed to π-π* transition and intramolecular charge transfer
ICT), respectively. The absorption spectra of 4-TPA-BBI exhibit insignificant change upon
increasing solvent polarity.
(
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In sharp contrast, the PL spectra show obvious variation upon increasing solvent polarity. As
shown in Figure 1c and Table 1, 4-TPA-BBI in non-polar hexane shows cyan emission peaking
-1
at 487 nm with a small Stokes shift (Δν) of 2885 cm , while the emission peak of 4-TPA-BBI
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markedly red-shifted to 623 nm in polar DCM with a dramatically increased Δν of 6728 cm .
These results indicate that the excited state of 4-TPA-BBI can be stabilized in polar solvents,
implying typical ICT characteristics. The change in dipole moment between S and S upon
1
0
excitation (Δμ) is analyzed using the following Lippert-Mataga Eq. (1)39-40
:
2
3

   

2 / hca

f

，n

 A
a
f
(1);
where, Δf(ε, n) is calculated from Eq. (2):
2
2
f

，n

 

 1

/

2 1

  
 

n 1

/

2n 1




(2).
Here, h is the Plank’s constant (6.6 × 10-34 J s ), c is the velocity of light in the
-1
8
-1
vacuum (3.0 × 10 m s ), α is the Onsagar cavity radius, n and ε are the reflective index
and the static dielectric constant of the solvents, respectively. The slope value is
-1
calculated to be 11714 cm . Because 3-TPA-BBI and 4-TPA-BBI have similar
structures, the calculated α of 4-TPA-BBI is estimated to be 0.9575 nm, which is
34
comparable to that of 3-TPA-BBI. Accordingly, the Δμ value of 4-TPA-BBI is calculated
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to be 30.4 D (Figure 1d), which is similar to the Δμ value 3-TPA-BBI (33.8 D). The large
Δμ value suggests an efficient ICT between the donor TPA and the acceptor BBI within the
molecular framework.41
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Polymorphism and MCL. Through controlling the precipitation conditions, 4-TPA-BBI
can form three different aggregated solid states: Y-crystal, O-crystal and R-solid (Figure
2a). Among them, Y-crystal can be changed to R-solid via sublimation under high
o
vacuum, and R-solid can be transformed back to Y-crystal form upon heating (150 C, 5
s) or fuming with hexane vapor. The reversible transformation between Y-crystal and R-
solid in response to external stimuli indicates the obvious MCL property of 4-TPA-BBI. In
comparison, O-crystal can be changed into R-solid via sublimation under high vacuum,
but R-solid cannot be directly recovered to O-crystal upon external stimuli. It seems like
that the O-crystal can be regarded as a metastable state between Y-crystal and R-solid.
The different MCL features of the two crystals might be attributed to different molecular
conformations and intermolecular interactions (vide infra).
As shown in Figure 2b and Table 2, the three aggregates of 4-TPA-BBI exhibit broad
fluorescence with PL peaks at 553 nm (Y-crystal), 576 nm (O-crystal), and 595 nm (R-solid),
respectively. Absolute ΦPL values of the three aggregates were also measured (Table 2).
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Compared with O-crystal and R-solid, Y-crystal exhibits a high Φ ~1, which may be attributed
PL
to the suppressed non-radiative transition without any strong intermolecular interactions. To
better understand the PL characteristics of 4-TPA-BBI, transient PL decays were investigated
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(Figure 2c and Table 2). All the three aggregates of 4-TPA-BBI exhibit short-lived nanosecond-
scaled lifetimes. Among them, Y-crystal exhibits the largest radiation rate constant of 1.3 × 108 s-
1.
XRD patterns of 4-TPA-BBI in different solid states are shown in Figure 2d. The
different XRD patterns confirm the three different aggregate states. Because of their
well-ordered crystalline structures, the Y-crystal and O-crystal show strong and evident
diffraction peaks. In contrast, the R-solid show weak diffraction patterns, suggesting its
disordered amorphous nature. After heating at 150 °C for 5 s or fuming with hexane
vapor, R-solid was transformed to Y-crystal via an amorphous-crystalline phase
transformation process, which is confirmed by the similar XRD patterns of the heated R-
solid and the Y-crystal. Note that Y-crystal has two sharp peaks at 2θ 4.7° and 13.8° while the
heated R-solid has only one sharp peak at 2θ 4.7°, indicating that the Y-crystal grown from
organic solution by slow evaporation has more ordered structure than the heated R-
solid by quickly thermal treatment. The subtle difference in ordered structure between
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Y-crystal and the heated R-solid may cause different intermolecular interactions, which
should be responsible for the red-shifted PL emission (562 nm, Figure 2b) of the heated
R-solid compared with that of Y-crystal (553 nm, Figure 2b). We consider that the heat
energy forces the molecules in the Y-crystal lattice to experience a re-arrangement
process. In this case, the BBI π-extended moieties are likely to pack more compactly,
like the case of O-crystal, to reach a more stabilized state, leading to the red-shifted
emission. In addition, upon repeated treatment by heating or fuming with hexane and scratching
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(Figure S1), we found that the PL emission of R-solid can be changed between orange and
yellow emission with good reversibility and repeatability. The versatility of the MCL of 4-TPA-
BBI is further demonstrated. A 20-nm neat thin-film was prepared by thermal evaporation at a
-1
rate of 1.0 Å s . As shown in Figure 2e, the film emits orange fluorescence with λ of 583
PL
nm and Φ of 0.723. After heating at 150 °C for 5 s, the emission color of the film is switched to
PL
green (Φ = 0.802, λ = 541 nm). Next, a capital letter “M” was written on the film by a slight
PL
PL
touch with a cotton swab. The scratched area is changed to give orange emission, showing high-
contrast MCL property. After fuming with hexane vapor, the emission switch back to green
again (Φ = 0.748, λ = 536 nm). The results indicate that the emission color of the film can
PL
PL
be reversibly changed in response to external stimulus.
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Crystallographic study. To investigate the mechanism of the MCL behaviors of 4-TPA-
BBI, the single-crystal structures of Y-crystal and O-crystal were measured. Interestingly,
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both Y-crystal and O-crystal are monoclinic with space groups P2 /c (Table 3), but
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exhibit different molecular configuration with the D-A dihedral angles between the
adjacent phenyl planes of 54.23° and 42.35° for Y-crystal and O-crystal, respectively
(
Figure 3a-b).
In the Y-crystal, the two neighboring molecules adopt a face-to-tail stacking
arrangement along b-axis. The adjacent BBI rings exhibit a small interlayer overlap of
approximately 20% with a centroid-centroid (C -C ) distance of 3.649 Å. These weak π-
g
g
π interactions in the Y-crystal are beneficial to inhibiting non-radiative transition, which can
explain the observed high Φ . Meanwhile, the low overlap and long C -C distance induce a
PL
g
g
3
loose packing (the Vcell of Y-crystal is 2604.3(9) Å (Z = 4) as shown in Table 3), and the
molecular packing is easy to be disturbed, resulting in evident MCL upon stimulus.
In the case of O-crystal, the neighboring molecules show similar stacking mode along c-
axis. However, the C -C distance between two adjacent BBI rings is shortened to 3.584
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Å. Notably, the interlayer overlaps between two parallel BBI planes increases to ~70%.
Compared with Y-crystal, these stronger π-π interactions in O-crystal induce red-shift PL
emission and decreased ΦPL value. In addition, the shorter C -C distance and the
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enhanced intermolecular overlap in O-crystal lead to a more compact packing. Therefore, the
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Vcell of O-crystal decreases to 2509.0(5) Å (Z = 4), comparing to that of 2604.3(9) Å (Z = 4)
in Y-crystal, as shown in Table 3. It is believed that compact packing along with stronger π-π
interactions can lead to a less stable aggregation state.32 By comparing the single crystal
structures of Y-crystal and O-crystal, the molecular packing may account for the
significantly different MCL behavior of the two crystals.
Theoretical Calculations. To further clarify the influence of packing modes on the emissions
of the different polymorphs, density functional theory (DFT) and time-dependent density
functional theory (TD-DFT) calculations based on the single crystals were carried out. As
shown in Figure 4, the highest occupied molecular orbitals (HOMOs) and lowest unoccupied
molecular orbitals (LUMOs) of the 4-TPA-BBI monomers in different states are almost the same,
which agree well with the theoretically optimized geometry. All the calculated S values (2.47
1
eV) of the monomers are comparable, which cannot explain the emission variation of Y-crystal
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and O-crystal. The results indicate that the conformation change between Y-crystal and O-crystal
has negligible influence on the emission properties. As mentioned above, intermolecular
interactions in the crystals may have prominent influence on the emissions. Therefore, dimer
models of Y-crystal and O-crystal are selected. As shown in Figure 4 and Table 4, the dimers
exhibit lower excited state energies in comparison with the monomers. Meanwhile, due to the
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enhanced intermolecular interactions, the dimer of O-crystal (2.20 eV) have smaller calculated S1
values than that of the dimer of Y-crystal (2.40 eV), which agree well with the experimental
results that O-crystal has a red-shifted PL emission.
Electroluminescence Performance. To investigate the EL performance of 4-TPA-BBI, we
fabricated an OLED with a configuration of ITO/TAPC (50 nm)/TCTA (20 nm)/mCP (10 nm)/4-
TPA-BBI (20 nm)/TPBI (50 nm)/LiF (1 nm)/Al (100 nm). The EL characteristics of the device
are shown in Figure 5. The non-doped OLED exhibits orange emission with a peak at 588
nm and the CIE coordinate of (0.55, 0.45), which is slightly red-shifted compared to the
corresponding PL spectrum of the neat film. The OLED shows a low turn-on voltage of 3.0 V,
-1
a maximum current efficiency of 10.4 cd A , a peak external quantum efficiency (EQE)
-2
of 4.4%, and a maximum luminance of 22470 cd m . The performance of this OLED is
comparable with the non-doped OLEDs based on other MCL emitters.3, 42 It is worth
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noting that the EQE roll-off of the device is small. The EQE remains 4.2% at 5000 cd m-
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CONCLUSIONS
In conclusion, we have developed a solid-state emitter 4-TPA-BBI which exhibits
polymorphism, high-contrast MCL and decent EL properties. 4-TPA-BBI can form two
types of single crystals with different photophysical properties. Change of molecular
packing is responsible for the significantly different MCL behaviors: Y-crystal with a loose
packing exhibits a high Φ ~1 and evident MCL, while O-crystal with a compact packing
PL
shows a moderate ΦPL and insignificant MCL. This work may help us to understand the
underlying MCL and provide a guideline for further design of MCL emitters.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
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Maximum PL emission of 4-TPA-BBI (R-solid) upon repeated treatment by heating and
scratching; maximum PL emission of 4-TPA-BBI (R-solid) upon repeated treatment by
fuming with hexane and scratching; normalized EL spectra of the device; the structure
analysis of 4-TPA-BBI (PDF)
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Accession Codes
CCDC 1963811 (Y-crystal) and 11963810 (O-crystal) contain the supplementary
crystallographic data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif, or by e-mailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033.
AUTHOR INFORMATION
Correspondent Authors
*
E-mail: huangbinhb31@sina.com
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*
E-mail: wencchen@gdut.edu.cn
E-mail: apcslee@cityu.edu.hk
*
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ACKNOWLEDGMENTS
This work was supported by the National Key R&D Program of China (No.
2
016YFB0401002), Qing Lan Project of Jiangsu Province, and the Natural Science
Foundation of the Jiangsu Higher Education Institutions of China (No. 19KJB150006).
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21) Jadhav, T.; Choi, J. M.; Shinde, J.; Lee, J. Y.; Misra, R. Mechanochromism and
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Excited State of Mechanoluminescence in Organic Luminogens with Room-Temperature
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25) Yang, J.; Qin, J.; Geng, P.; Wang, J.; Fang M.; Li, Z. Molecular Conformation-Dependent
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Reversible Stepwise Tricolor Switching and Thermochromism of Anthracene-o-Carborane Dyad.
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in a Single Molecule. Angew. Chem. Int. Ed., 2018, 57, 12473-12477.
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(35) CCDC 1963811 (Y-crystal) and 1963810 (O-crystal) contain the supplementary
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Scheme 1. Synthesis route of 4-TPA-BBI.
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Figure 1. (a) The photographs of 4-TPA-BBI in various solvents under daylight (upper panel) and daylight
plus 365 nm UV excition (bottom panel); (b) UV-vis absorption spectra of 4-TPA-BBI in various solvents; (c)
PL spectra of 4-TPA-BBI in various solvents; (d) Lippert-Mataga plots of 4-TPA-BBI.
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Figure 2. (a) Photographs of the three solid states of 4-TPA-BBI under the 365 nm UV excitation; (b) PL
spectra of 4-TPA-BBI in different aggregation states; (c) PL decay curves of 4-TPA-BBI in different
aggregation states; (d) XRD spectra of 4-TPA-BBI in different solid states; (e) the fluorescence image of a 4-
TPA-BBI film on quartz under the 365 nm UV lamp.
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Figure 3. (a) The single crystal structure of the Y-crystal; (b) the single crystal structure of the O-crystal; (c)
molecular interactions (upper panel) and overlap (along the b axis, bottom panel) of the Y-crystal; (d)
molecular interactions (upper panel) and overlap (along the c axis, bottom panel) of the O-crystal. (Hydrogen
atoms have been omitted for clarity)
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Figure 4. Calculated energy levels for 4-TPA-BBI monomers and dimers selected from single crystals.
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