Butterfly-shaped π-extended benzothiadiazoles as promising emitting materials for white OLEDs

Article abstract of DOI:10.1039/c9tc01059a

A series of butterfly-shaped D-A-D and D-A-A compounds composed of a benzothiadiazole core and corresponding terminal alkynes were synthesized. Their photophysical properties in different states (solution, film, crystal, and powder) were investigated. Meanwhile, the linear-shaped compounds were prepared accordingly for the comparative investigation of their photophysical properties. Three phenomena (aggregation caused quenching, crystal induced emission, and aggregation induced emission enhancement) were all observed for the butterfly-shaped compounds. Although the linear-shaped compounds emitted brighter light in pristine powders with higher quantum yields, the butterfly-shaped compounds formed crystals and emitted colorful light from 473 nm to 622 nm in the crystalline state. Moreover, most of them exhibited a mechanochromic character. The color of 2POZ-56 could be reversibly tuned from yellow to red by grinding and fuming. Furthermore, polymorphism-dependent emission was observed for 2POZ-56, allowing for its use as an emissive layer in white organic light emitting diodes.

Full text of DOI:10.1039/c9tc01059a

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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DOI: 10.1039/C9TC01059A
Journal Name
ARTICLE
Butterfly-shaped -extended benzothiadiazoles as promising emitting materials for white
OLEDs
Zaibin Wang, Zhixing Peng, Kai Huang, Ping Lu*, Yanguang Wang*
Received 00th January 20xx,
Accepted 00th January 20xx
A series of butterfly-shaped D-A-D and D-A-A compounds, composed of benzothiadiazole core and corresponding terminal
alkynes were synthesized. Their photophysical properties in different states (solution, film, crystal, and powder) are
investigated. Meanwhile, the linear-shaped compounds were prepared accordingly for comparative investigation on their
photophysical properties. Three phenomena (aggregation causing quenching, crystal induced emission, aggregation
induced emission enhancement) are all observed for the butterfly-shaped compounds. Although the linear-shaped
compounds emit brighter light in pristine powders with higher quantum yields, the butterfly-shaped compounds are likely
to form crystals and emit colorful light from 473 nm to 622 nm in crystalline state. Moreover, most of them exhibit the
mechanochromic character, the color of 2POZ-56 could be reversibly tuned from yellow to red by grinding and fuming.
Furthermore, polymorphism-dependent emission was observed for 2POZ-56, and it could also be used as emissive layer in
white organic light emitting diodes.
DOI: 10.1039/x0xx00000x
www.rsc.org/
near-infrared (NIR) fluorescence and is fabricated for NIR-
8
Introduction
OLED . A nearly 800 nm electroluminescent emission could be
realized
by
using
9
naphthothiadiazole
instead
of
Fluorescent compounds containing benzothiadiazole (BTD)
core have been widely investigated and applied in many
disciplines. They are interested as a new class of bioprobes for
its absorption in the visible-light region with large molar
absorptivity, bright emission with large Stokes shift and high
quantum yield, no blinking with good signal-to-noise ratio,
high optical and thermal stability in solutions as well as in
solids. With these significant characters, the π-extended BTD-
derivatives easily break down some limitations normally
appeared in the classical scaffolds, such as coumarins,
benzothiadiazole . In above-mentioned examples of the

-
extended BTD derivatives, the conjugation is normally
extended through 4,7-positions of the BTD core. In another
word, the intramolecular charge transfer (ICT) in the excited
state occurs from donor to acceptor through 4,7-positions of
BTD core in linear shape. But, change the substituent position
as well as the molecular geometry would largely effect on the
ICT process as well as their luminescent properties . For
instance, experiments show that it is effective to improve the
device performance through structural modification. For
instance, higher power conversion efficiency could be
approached when 5,6-positions of BTD were fluorinated . On
this basis, we hereby synthesized a series of conjugated
compounds through 5,6-positions of BTD core in butterfly
shape. Thus, four different models of the triads with the
connection of D-A-D or D-A-A in butterfly or linear shaped
were obtained for the systematically investigation of the
fluorescent structure-property relationship of the BTD-based
conjugated compounds (Scheme 1).
10
BODIPYs,
fluoresceins,
rhodamines,
cyanines,
and
phenoxazines, and become star probes for a number of
biological analyses in recent years . On the other hand, the
1
1
1
electron-deficient BTD core has been extensively used as a key
building unit in the construction of donor-acceptor conjugated
compounds for optoelectronic applications, such as organic
2
3
photovoltaics , solar-hydrogen conversion , organic field effect
4
5
transistor , organic light emitting diode and so on. BTD could
also be added between donor and acceptor to enlarge the
conjugation and enhance the D-A effect to produce AIEgen
with deep red emission for large third-order optical
6
nonlinearity and large reversible mechanochromic behavior
Results and discussion
Synthesis and characterization
7
for mechano-sensoring . When the phenothiazine has two
wings of benzothiadiazoles, the resulting A-D-A displays strong
Five butterfly-shaped D-A-D compounds were synthesized in a
single step through Sonogashira coupling reactions between
5,6-dibromobenzothiadiazole and corresponding terminal
alkynes in moderate to excellent yields. Two butterfly-shaped
D-A-A compounds were prepared by firstly coupling of 5,6-
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. China.
*
E-mail: pinglu@zju.edu.cn, orgwyg@zju.edu.cn
†
Electronic supplementary information (ESI) available: Structural characterization
data, TGA, crystallographic data, photophysical spectra, DFT, and CV. CCDC
1
otherelectronic format See DOI: 10.1039/x0xx00000x
887112-1887113 and 1850544–1850549. For ESI and crystallographic data in CIF or dibromobenzothiadiazole with electron-rich alkyne and then
This journal is © The Royal Society of Chemistry 20xx
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the electron-deficient alkyne. Linear-shaped D-A-D and D-A-A The synthesized compounds are soluble in normal organic
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compounds were prepared through similar manner in order to
DOI: 10.1039/C9TC01059A
solvents, such as cyclohexane (CH), toluene (Tol), dioxane
(
DIO), tetrahydrofuran (THF), and dichloromethane (DCM).
-5
Absorption spectra were recorded in a concentration of 2*10
mol L in tetrahydrofuran (Figure 1, Table 1). 2Ph-56 absorbs
-1
light at two bands with the maximum absorption wavelength
4
−1
−1
of 373 nm and the molar absorptivity of 1.87*10 L mol cm .
It emits light at 424 nm with a Stokes shift of 51 nm and a
quantum yield of 19%. By tethering two methxoy at both ends,
2
PMP-56 presents red-shifted absorption as well as red-shifted
emission with higher molar absorptivity and higher quantum
yield as we expected. Moreover, a larger Stokes shift (85 nm) is
observed for 2PMP-56 in comparison with that of 2Ph-56
.
Replacement of methoxy with dimethylamino, absorption and
emission spectra of 2DMAP-56 are further bathochromically
shifted. Thus, 2DMAP-56 absorbs light at 439 nm with molar
4
−1
−1
absorptivity of 2.58*10 L mol cm and emits light at 600 nm
with quantum yield of 13%. The large Stokes shift (161 nm)
indicates the excellent electronic communication between
donor and acceptor which directly results in the decreased
quantum yield when two compounds with similar molar
1
4
absorptivities are compared . With diphenylamino substituted
2TPA-56), the electron donating ability from nitrogen to BTD
core is partially inhibited because of the propeller structure of
triphenylamine. Therefore, larger molar absorptivity (2.76*10
Scheme 1 -extended BTD derivatives through 5,6-positions and 4,7-positions
(
comparative investigation of the fluorescent structure-
property relationship of these BTD-containing compounds .
12
4
Unfortunately, we failed to obtain 2POZ-47 although many L mol− cm ) and smaller Stokes shift are recorded (140 nm)
1
−1
efforts had been paid. Among these butterfly-shaped for the dilute THF solution of 2TPA-56. It shows the largest
compounds, 2Ph-56 has been previously reported as an
quantum yield (35%) among these butterfly-shaped D-A-D as
well as D-A-A compounds. With phenoxazinyl substituted,
1
3
intermediate in the preparation of quinoxalenediynes .
1
Others were firstly synthesized and fully characterized by H
2
1
POZ-56 absorbs light at 325 nm with the molar absorptivity of
.84*10 L mol cm , but without fluorescence.
and 1 C NMR (Figures S1-S22), IR, HRMS and further confirmed
by their single crystal analysis. The crystallographic data are
summarized in Table S1. Compounds in butterfly-shaped are
thermally stable on the basis of TGA analysis (Figure S23,
Table1). The decomposition temperature, obtained at 5%
3
4
−1
−1
To linear-shaped D-A-D compounds, absorption and emission
spectra are all red-shifted with higher molar absorptivities and
higher quantum yields in comparison with those of the
corresponding butterfly-shaped D-A-D compounds. The
o
weight loss, is 291 C for 2Ph-56. Substitution on 2Ph-56 with
methoxy, dimethylamino, and diphenylamino groups highest quantum yield (95%) is obtained for 2PMP-47, while
significantly increase the thermal stability. With phenoxazine the largest maximum emission wavelength (607nm) is
substituted, decomposition temperature of 2POZ-56 is recorded for 2DMAP-47
.
o
determined to be 436 C. In comparison to these butterfly-
shaped compounds, the linear-shaped compounds presented
higher decomposition temperatures, accordingly (Figure S24).
Absorption and emission of these compounds in solutions
2
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9TC01059A
Figure 1 (a) Absorptions and emissions of butterfly-shaped compounds in THF; (b) Absorptions and emissions of linear-shaped compounds in THF.
Table 1 Photophysical properties of these compounds in different states as well as their thermal stability
abs
4
em
[c]
em
max
em
max
λ
max (ε×10 /L
λ
max
λ
λ
Φ (%)
Φ(%)
em [a, b]
max
o
[f]
Compounds
Ph-56
λ
Stokes
51
d
T ( C)
mol−1 cm )
−1
[a]
[d]
[e]
[e]
powder
crystal
film
THF
film
2
3
81(5.20)
73(1.87)
2
424 (386)
478 (398)
600 (498)
572 (433)
------
484
------
------
468
19
31
13
35
0
2.2
1.8
291
395
385
410
436
524
457
389
508
418
2
3
83(6.20)
93(2.47)
2
PMP-56
85
161
140
------
72
498
577
550
563
478
562
630
630
511
473
592
3
4
09(5.93)
39(2.58)
2
DMAP-56
550
1.5
3
4
29(5.17)
32(2.76)
2
TPA-56
557
------
------
469
7.1
2
3
87(5.13)
25(1.84)
573g
2
POZ-56
2.9
h
627
3
4
00(4.03)
11(2.64)
2
Ph-47
483(416)
519 (449)
607 (510)
577 (492)
494 (380)
------
------
------
------
492
88
95
38
70
22
20
3
4
12(4.35)
33(2.82)
2
PMP-47
86
530
24.4
13.4
25.2
1.2
3
4
24(5.38)
92(3.52)
2
DMAP-47
115
99
------
576
3
4
39(5.5)
78(4.0)
2
TPA-47
3
3
00(6.05)
68(1.83)
PMP-5-BN-6
126
------
2
98(5.75)
DMAP-5-BN-6
350(3.28)
626 (352)
184
585
601
------
2
3.7
389
4
42(1.25)
3
4
13(4.38)
23(3.22)
PMP-4-BN-7
517 (436)
------
94
526
637
------
------
533
92
0
14.6
11.8
466
436
3
4
25(3.93)
80(2.60)
DMAP-4-BN-7
------
------
[
a] Measured in THF solution, c=2*10-5 M; [b] excitation wavelength indicated in parenthesis; [c] pristine powder; [d] Films are prepared by spin-coating of dilute
chlorobenzene solution on quartz plate; [e] Quantum yields are obtained from integrating sphere; [f] Decomposition temperature is determined by TGA when the
weight loss is 5%; [g] emission from crystal I; [h] emission from crystal II
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To butterfly-shaped D-A-A compounds, PMP-5-BN-6 absorbs wavelength of 2DMAP-56 changed from 463 nm,V5ie3w1Arnticmle,O5nl6in1e
light at two bands (300 nm and 368 nm) and emits light at 494 nm, 600 nm, to 602 nm as the solvent cDhOaIn: 1g0e.1s03fr9o/Cm9TCCH01,0T5o9Al,
nm with Stokes shift of 126 nm and quantum yield of 22%.
DIO, THF, to DCM, while the solution color under UV lamp
DMAP-5-BN-6 absorbs light at three bands (298 nm, 350 nm, changes from blue, green, yellow, red, to purple, accordingly.
and 442 nm) and emits light at 626 nm with the largest Stokes Similar solvatofluorochromic effect is observed for 2TPA-56
shift of 184 nm and relative low quantum yield of 2%. A better (Figure 2, up left). 2POZ-56 is fluorescent in cyclohexane,
conjugation is seen in the case of the linear-shaped D-A-A. toluene, and dioxane. The maximum emission wavelength is
Thus, absorption and emission spectra of PMP-4-BN-7 are all 481 nm, 569 nm, and 595 nm, accordingly. However, it is non-
red-shifted with Stokes shift of 94 nm and quantum yield of fluorescent in tetrahydrofuran and dichloromethane.
9
2%. DMAP-4-BN-7 absorbs light at 325 nm and 480 nm, but When linear-shaped D-A-D compounds are tested,
without fluorescence. solvatofluorochromic effect still exists but with less influenced
Based on these findings, some preliminary conclusions could in comparison with those corresponding butterfly-shaped D-A-
be made. The maximum absorption and emission wavelengths D compounds. In contrast to 2TPA-56 2TPA-47 presents
,
largely depend on the type of the substituents and the smaller solvatofluorochromic effect (Figure 2, down left). The
resultant electronic communication between the donor and largest solvatofluorochromic effect is observed for the
acceptor. Among these donors, dimethylamino is the most butterfly-shaped DMAP-5-BN-6, which emits light at 480 nm in
powerful electron donor followed by diphenylamino and cyclohexane and 626 nm in tetrahydrofuran, respectively.
methoxy. Substitution on 4,7-positions of BTD provides better When the molecule is excited, electron transition occurs from
conjugation and higher quantum yields in comparison with
substitution on 5,6-positions of BTD.
Solvent effect on absorption and emission of these compounds
S
0
to S
the molecule surround by solvents because of the solvation.
The decreased degree depends on the dipole moments of S
and S as well as the solvent polarity. Generally speaking, the
dipole moment of S is larger than that of S that directly
1 0 1
. Both energy levels of S and S would be decreased as
0
1
It is common that polar molecule shows the
solvatofluorochromic effect and the extent depends on the
dipole moment change of the molecule in excited state and
ground state. In general speaking, the larger the dipole
moment changes, the bigger the solvatofluorochromic effect
1
0
results in the Stokes shift in non-polar solvent. As the D-A
compound is tested, not only locally excited (LE) emission
could be observed but the intramolecular charge transfer (ICT)
emission also. In these cases, the polar solvent is more likely to
solvate the excited state of the ICT which further reduces the
energy gap and results in the red-shifted emission. In
comparison with linear-shaped D-A-D compounds, the
butterfly-shaped D-A-D compounds obviously possess larger
dipole moments in ground states because of the molecular
geometry. Theoretical calculation confirmed this order (Table
15
is . To these butterfly-shaped and linear-shaped compounds,
we record their absorption and emission in various solvents
(Figure 2, Figures S25-S26) and the corresponding data are
summarized in Table S2. Absorption spectra of 2Ph-56 in
various solvents are almost identical, but with red-shifted
emission from 400 nm, 412 nm, 412 nm, 424 nm, to 430 nm as
the solvent changes from CH, Tol, DIO, THF, to DCM. When
S3). To the series of 2Ph-56
, the dipole moments of these compounds in ground states
are calculated to be 5.0, 5.6, 12.2, and 8.1 D, respectively. By
, 2PMP-56, 2DMAP-56, and 2TPA-
5
6
16
applying the modified Lippert-Mataga equation , the dipole
moment changes for these compounds are found to be 10.1,
1
6.5, 18.3, and 22.4 D, respectively. The bigger the dipole
moment change, the more red-shifted of emission. To the
linear-shaped D-A-D compounds of 2Ph-47 2PMP-47 2DMAP-
, and 2TPA-47, the dipole moment changes are obtained in
,
,
4
7
smaller values (8.3, 11.0, 10.4, and 12.6 D, respectively) in
comparison with those of the butterfly-shaped compounds.
Therefore, the linear-shaped D-A-D compounds present
smaller solvatofluorochromism, accordingly. To D-A-A
compounds, the linear-shaped compounds presented larger
dipole moment in the ground state than the one in butterfly-
shaped because of longer efficient conjugation length17 from
the donor to acceptor. Thus, to the pair of PMP-5-BN -6 and
PMP-4-BN-7, although PMP-4-BN-7 has larger dipole moment
in ground state, PMP-5-BN-6 presents larger dipole moment
change based on the modified Lippert-Mataga equation (Table
S3). Thus, as the solvent changes from cyclohexane to
dichloromethane, the emission of PMP-5-BN-6 shifts from 422
nm to 500 nm in DCM, while PMP-4-BN-7 shifts from 475 nm
to 525 nm in DCM accordingly. Above all, the larger
Figure 2 Solvatofluorochromism of 2TPA-56 (up left) and 2TPA-47 (down left); solutions
of butterfly-shaped (middle) and linear-shaped compounds (right) under UV lamp
methoxy substituted on both ends, absorption spectra of
2
PMP-56 keep the same, while their emission spectra change a
lot. The maximum emission wavelength ranges from 416 nm to
88 nm as the solvent changes from CH to DCM. With
dimethylamino substituted, the maximum emission
4
4
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solvatofluorochromism is observed for those compounds in emission turns off. The precipitation could even beViesweAernticbleyOntlhinee
butterfly-shaped because these compounds possess larger naked eyes, probably due to the limitedDOsoI:l1u0b.1il0it3y9./CIn9TcCre0m10e59nAt
diploe moment changes.
Emissions of these compounds in nanoparticles or tiny crystals in
aqueous THF solutions
of water fraction to 95%, both TEM and DLS indicate the
formation of nanoparticles with the diameter about 100 nm
(Figure S27). At this point, the fluorescence is completely
turned off. As a comparison, PMP-5-BN-6 presents almost
identical phenomenon (Figure 3, right; Figure S28). On the
contrary, with the dimethylamino substituted, emission of
These compounds are easily to form nanoparticles or tiny
crystals as we quickly pour water into their dilute THF
solutions. 2Ph-56 presents a gradually red-shifted emission
with a gradually decreased quantum yield as water fraction
increases. When the water fraction increased to 95%, TEM
shows the formation of nanoparticle with the diameter about
2
DMAP-56 in THF solution is turned off as the water fraction
ranges from 20% to 60% (Figure S29). The emission comes
back when the water fraction reaches 70% but with
significantly blue-shifted. A much broad emissive peak is
recorded as the water fraction reaches 95%. In this case,
nanoparticles are formed that are confirmed by TEM and DLS.
When the unsymmtrical D-A-A is tested, a relatively broad and
weak emission from DMAP-5-BN-6 is quenched as the water
1
00 nm which was further confirmed by DLS (Figure 3, left
column). Thus, a typical aggregation causing quenching (ACQ)
occurs in this case. With methoxy substituted, 2PMP-56 shows
the red-shifted emission along with the decreased quantum
yield as water fraction reaches 70%. As water fraction
increased to 80% and beyond, precipitation occurs with the
Figure 3 Emissions and shapes of 2Ph-56 (left column) and 2TPA-56 (middle column), as well as aqueous THF solutions of these butterfly-shaped compounds under UV lamp (right
column)
fraction ranges from 20% to 60% (Figure S30). When the water three normal phenomena (ACQ, CIE, and AIEE) are all observed
fraction reaches 70%, tiny crystal forms and emits bright light relying on the different substituted groups and different
at 566 nm. The bright tiny crystals could be seen by naked eyes fraction of water.
and confirmed by TEM. In this case, it is a typical crystalline Emission of these compounds in solid states
induced emission (CIE). Beyond 70%, DMAP-5-BN-6 forms
We then move onto the investigation of the emissions of these
nanoparticles with the aggregation caused quenching emission
compounds in solid states which is important because the
(ACQ). To compound 2TPA-56, emission at 572 nm in pure THF
practical usage of these compounds in optoelectronic devices
is mostly in solid state. Solid states are classified into four
categories, including crystalline, pristine powder, ground
powder and pure film.
Six single crystals for six butterfly-shaped compounds are
obtained by slowly evaporation of the appropriate solvents
solution is gradually quenched as water fraction ranges from
0% to 60% (Figure 3, middle column). As water fraction
2
reaches 70%, tiny crystals form and emit light at 567 nm.
When the water fraction reaches 80% and beyond,
nanopartiles form and emit light at 563 nm, indicating the
aggregation induced emission enhancement (AIEE). After all,
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except for the 2Ph-56. Single crystal of 2PMP-56 is S31b). Crystal I emits light at 573 nm, while crystalVIieIweAmrtiictlesOlinglihnet
transparency and emits light at 473 nm which is slightly blue- at 627 nm because of the different molecDuOlaI:r1s0t.1a0c3k9i/nCg9mTCo0d10e5s9.A
shifted in comparison with its emission in dilute THF solution To the emissions of these compounds in pristine powders
(478 nm) (Figures 4 and S31 a&b, Table 1). With (Figures 4, S31c and S32a, Table 1), it could be clearly seen
dimethylamino substituted, single crystal of 2DMAP-56 is from photos that the linear-shaped compounds, no matter it is
bright red under UV lamp. Similar to the observations in symmetrical or unsymmetrical, are all brighter than the
solution, crystal emission of 2TPA-56 is blue-shifted in corresponding compounds in butterfly-shaped under UV lamp.
comparison with the crystal emission of 2DMAP-56 because of Undoubtedly, the linear-shaped compounds present higher
the propeller structure of triphenylamine and the decreased quantum yields in pristine powders. Moreover, most of the
conjugation. With cynao substituted, the crystal emission of linear-shaped compounds provide red-shifted emissions in
PMP-5-BN-6 is bright green, while the crystal emission of comparison with those corresponding compounds in butterfly-
DMAP-5-BN-6 is orange red. Polymorphism-dependent shaped except for the pair of the unsubstituted 2Ph-56 and
emission (PDE) was observed for 2POZ-56. By slowly 2Ph-47. The biggest change could be seen for the pair of 2TPA-
evaporating the dilute solution of 2POZ-56 in 56 and 2TPA-47. 2TPA-56 emits light at 550 nm in pristine
DCM/MeOH/Acetone/n-Hexane, two different crystals (I and powder, while 2TPA-47 emits light at 630 nm. It might be the
II) were separated and picked up by tweezers (Figures 5 and
Figure 4 Photos of these compounds under UV lamp, butterfly-shaped compounds in crystalline (above), butterfly-shaped compounds in pristine (middle), linear-shaped
compounds in pristine (down)
steric hindrance existing in 2TPA-56 which inhibits the another word, the mechanofluorochromic effect is not
effective conjugation between TPA and BTD in a certain reversible for 2Ph-47. XRD tells us that the molecular stacking
degree.
mode in pristine, fir-grinding, and sec-grinding are not the
Some of them are fabricated in films by spin coating the dilute same.
chlorobenzene solutions on quartz plates (Figures S31d and With methoxy substituted, 2PMP-56, 2PMP-47, PMP-5-BN-6
S32b) and the fluorescent results are summarized in Table 1. and PMP-4-BN-7 show slightly change in emission spectra
To these films, blue-shifted emissions are observed in before and after grinding (Figure S35). With dimethylamino
comparison with those compounds in pristine powders except
the linear-shaped unsymmetrical PMP-4-BN-7
.
Mechanochromic characters of these compounds
Some compounds show mechanochromic characters. By
grinding the pristine powder of 2Ph-56, the emission spectra
kept same. Both of the two states (in pristine and in ground)
emit light at 484 nm (Figure S33). In comparison with this,
linear-shaped 2Ph-47 presents mechanochromic property. By
grinding the pristine powder of 2Ph-47, the maximum
emission wavelength changes from 478 nm to 504 nm (Figure
S34). By fuming the sample with DCM and grinding the sample,
the maximum emission wavelength changes into 534 nm.
Fuming the ground sample with DCM again, the emission
spectrum keeps the same and could not be tuned back. In
Figure 5 Crystals (I and II) of 2POZ-56 and their molecular stacking modes
substituted, 2DMAP-56 shows the reversible emissive spectra
from 607 nm to 581 nm by grinding and fuming. The color
change could be clearly seen by putting the sample under UV
lamp (Figure S36). Similar alternation is observed for 2DMAP-
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, but with smaller difference (10 nm) in maximum emission and the power efficiency (0.27 lm W ) were obtained. The
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wavelengths (Figure S37). To the butterfly-shaped DMAP-5- origin of the blue band emission couldDObIe: 10a.1s0c3ri9b/Ce9dTCto010P5V9KA
BN-6 (Figure S38) and linear-shaped DMAP-4-BN-7 (Figure emission on the basis of the controlled experiment.
S39), reversible emissive spectra are also recorded. Both of
them show 15 nm differences in maximum emission
wavelength by grinding and fuming. Grinding 2TPA-56 brings a
slightly red-shifted emission from 550 nm to 556 nm, while
grinding 2TPA-47 brings a slightly blue-shifted emission from
6
30 nm to 626 nm (Figure S40). The biggest and reversible
change is seen for 2POZ-56. By grinding the pristine powder of
2
POZ-56, the maximum emission wavelength changes from
5
63 nm to 621 nm. Fuming the ground powder with DCM for 2
minutes, the emission wavelength turns back to 569 nm and
the color changes accordingly from red to yellow under UV
lamp (Figure 6).
Figure 7 EL a) Spectrum of 2POZ-56 based device operating at a voltage of 7 v. Inset:
Image of device; b) Current-density-voltage (L-J-V) characteristics
Conclusions
In this work, we designed and synthesized a series of butterfly-
shaped D-A-D and D-A-A compounds by using benzothiadiazole
core as electron acceptor. Meanwhile, the linear-shaped
compounds were prepared accordingly for comparative
investigation on their photophysical properties in different
states. Although the linear-shaped compounds present higher
molar absorptivities and higher solution quantum yields
because of the better conjugation, larger solvatofluorochromic
effects are observed for the butterfly-shaped compounds. By
pouring water into THF solutions, three phenomena
Figure 6 Emission spectra (a) and XRD diffractions (b) of 2POZ-56 before and after
grinding/fuming; (c) Reversible changes of emission intensities of 2POZ-56 by grinding
and fuming; (d) Photos under UV lamp
(aggregation causing quenching, crystal induced emission,
aggregation induced emission enhancement) are all observed
for the butterfly-shaped compounds and the variation largely
depends on the substituted group and the water fraction.
Although the linear-shaped compounds emit bright light in
pristine powders with higher quantum yields, the butterfly-
shaped compounds are likely to form crystals and emit colorful
light from 473 nm to 622 nm in crystalline state. Furthermore,
Electrochemical properties of the butterfly-shaped compounds
Cyclic voltammetric studies are carried out in solutions
(
MeCN:DCM = 3:1) in order to obtain the oxidation and
reduction potentials and to estimate the frontier orbital levels
HOMO and LUMO) of the butterfly-shaped compounds.
(
Results are summarized in Figure S41 Table S4. The energy
gaps obtained from the electrochemical studies are in good
accordance with the energy gaps obtained from theoretical
calculation as well as from UV measurements in dilute THF
solutions except for an incomprehensible drop of energy gap
for 2POZ-56 from CV measurement. Compounds substituted
with dimethylamino group (2DMAP-56 and DMAP-5-BN-6) or
phenoxazinyl group (2POZ-56) present smaller energy gaps
with longer emission wavelengths.
polymorphism-dependent emission was observed for 2POZ-56
.
It is also noticeable that most of them exhibit the
mechanochromic character. By grinding and fuming, the color
of 2POZ-56 could be reversibly tuned from red to yellow.
Finally, 2POZ-56 could be used in the fabrication of white OLED
device.
Conflicts of interest
There are no conflicts to declare.
Electroluminescent properties
Several butterfly-shaped compounds were further applied to
estimate their utilities for OLEDs. Thus, three devices with a
sandwiched structure of ITO / PEDOT : PSS (40 nm) / mCompd
:
Acknowledgements
The authors thank Prof. Yizheng Jin for providing help in the
fabrication of OLED devices and thank the National Natural
Science Foundation of China (21871229).
mPVK (1 : 16) / TPBI (30nm) / LiF (1 nm) / Al (60 nm) were
designed and fabricated by respectively using DMAP-5-BN-6
,
2
TPA-56, and 2POZ-56 (Figures S42, S43 and 7). Results are
summarized in Table S5. A white OLED device was achieved by
doping 2POZ-56 in PVK. The device emits light at a broad range
with two peaks of 403 nm and 555 nm (CIE = 0.32, 0.35) at a Notes and references
-
2
voltage of 7v. At a brightness of 100 cd m , the external
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DOI: 10.1039/C9TC01059A