9-Anthryl-capped DPP-based dyes: Aryl spacing induced differential optical properties

Article abstract of DOI:10.1039/c6tc02761j

While pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-diones (DPPs) are extensively used as building blocks in field-effect transistors and photovoltaic active layers, less attention has been paid to the construction of novel optical materials. In this work, we have des

Full text of DOI:10.1039/c6tc02761j

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9-Anthryl-capped DPP-based dyes: aryl spacing
induced differential optical properties
Cite this:DOI: 10.1039/c6tc02761j
a
a
a
a
b
Shian Ying, Mingshuai Chen, Zhongwei Liu, Kai Zhang, Yuyu Pan,
a
a
Shanfeng Xue* and Wenjun Yang*
While pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-diones (DPPs) are extensively used as building blocks in field-effect
transistors and photovoltaic active layers, less attention has been paid to the construction of novel optical
materials. In this work, we have designed and synthesized two soluble 9-anthryl-capped DPP derivatives
with benzene and thiophene as spacing units, 1,4-diketo-2,5-dioctyl-3,6-bis(9-anthrylphenyl)pyrrolo-
[3,4-c]pyrrole (ABDPP) and 1,4-diketo-2,5-dioctyl-3,6-bis(9-anthrylthiophen)pyrrolo[3,4-c]pyrrole (ATDPP).
It is found that ABDPP and ATDPP emit strong and weak fluorescence in solution, respectively, but their
crystalline states all exhibit strong fluorescence, implying that only ATDPP exhibits an aggregation-
enhanced emission (AEE) effect although the two dyes all adopt a twisted conjugation backbone.
Interestingly, grinding and pressing can only change the fluorescence color of crystalline ABDPP from
yellow to red, contributing a non-AEE dye exhibiting mechanochromic luminescence. Differential
scanning calorimetry and powder X-ray diffraction analyses confirm that the phase transition between the
crystalline and amorphous states upon application of external stimuli is responsible for the reversible color
change. This work has demonstrated once again that a subtle manipulation of the molecular structure unit
could significantly tune and alter the optical properties of conjugated organic materials.
Received 3rd July 2016,
Accepted 2nd August 2016
DOI: 10.1039/c6tc02761j
www.rsc.org/MaterialsC
small molecules and polymers applicable in field-effect tran-
Introduction
1
7–25
sistors and photovoltaic devices.
In recent years, we and
Conjugated organic molecules have been attracting much others have found that donor-capped DPP derivatives are
2
6–28
attention due to their potential applications in organic optical excellent organic two-photon fluorescent dyes
and optoelectronic fields,
with high
1
–7
and many studies have been light and heat stability because of the large p-system and strong
devoted to adjusting the molecular chemical structures and electron-withdrawing ability (acceptor unit, A) of the DPP core,
the aggregate morphologies for the improvement of material and mono-alkylated DPP derivatives could also be used as one-
2
9–32
performances and the development of material functions. and two-photon fluorescence sensors to fluoride anions.
There are a variety of strategies for the subtle manipulation Recently, to apply DPP derivatives in aqueous media, DPPs have
8
,9
of molecular chemical structures, such as linking bonds,
been modified as water-soluble and water-dispersible fluores-
7
,10,11
5,12,13
14–16
33
linking positions,
linking units,
end groups,
etc. cent dyes. However, the exploitation of fluorescent DPP-based
In this context, the nature of the p-center and peripheral arylene materials with novel solid-state optical and optoelectronic
still plays a crucial role in tuning and altering the molecular properties is still relatively rare at present.
aggregation behavior and the material function. Pyrrolo[3,4-c]-
In the current work, we have designed and synthesized two
pyrrole-1,4(2H,5H)-dione (DPP) derivatives represent a class of 9-anthryl-capped 2,5-dialkylated DPP derivatives (Scheme 1) in
brilliant red and strongly fluorescent high-performance pigments which ABDPP and ATDPP have benzene and thiophene as
having exceptional light, weather and heat stability, and high light spacing units between 9-anthryl and 3,6-positions of pyrrolo-
34–36
fastness.
as building blocks for the construction of promising conjugated goal was to investigate whether different conjugation bridges
benzene and thiophene) and different degrees of steric
In the last ten years, DPP derivatives have been used [3,4-c]pyrrole-1,4(2H,5H)-dione, respectively. Our primary
(
hindrance between the 1,8-positions of the anthracene ring
and the spacing unit could endow the resulting materials with
unique and tunable photophysical properties. We here report
that a subtle manipulation of the aryl spacing unit could endow
9-anthryl-capped DPP dyes with unique and differential solution
and solid-state optical properties.
a
b
Key Laboratory of Rubber-plastics of Ministry of Education/Shandong Province
QUST), School of Polymer Science & Engineering, Qingdao University of Science &
(
Technology, 53-Zhengzhou Road, Qingdao, 266042, China.
E-mail: ywjph2004@qust.edu.cn, sfxue@qust.edu.cn
School of Petrochemical Engineering, Shenyang University of Technology,
3
0 Guanghua Street, Liaoyang, 111003, P. R. China
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Scheme 1 Structure and synthetic route of 9-anthryl-capped DPP dyes ATDPP and ABDPP.
Experimental section
Materials
were recorded, the pressed sample was placed on an oven at
00 1C for 30 minutes, and then cooled to room temperature
before the measurements. The repressing experiment was the
same as the first pressing. The fuming experiment: the pressed
sample was placed into a beaker containing a small amount of
2
3
,6-Bis(4-bromophenyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione was
28
from our previous work. Tetrahydrofuran (THF) over metallic
sodium and N,N-dimethylformamide (DMF) over calcium hydride
were distillated before use. 3,6-Bis(5-bromothiophen-2-yl)pyrrolo-
dichloromethane, and the sample was placed above the CH Cl2
2
level for 5 min at room temperature. Grinding experiment: the
pristine solid was ground on a glass plate using a metal spatula
at room temperature.
[3,4-c]pyrrole-1,4(2H,5H)-dione, Pd(PPh
3 4 8
) , C H17Br, 9-anthracene-
boronic acid, and K CO were obtained from Energy Chemical
2
3
Ltd. Shanghai, China, and used without further purification. The
other solvents were of analytical grade and obtained commercially
from available resources.
Synthesis
1
,4-Diketo-2,5-dioctyl-3,6-dithiophene-pyrrolo[3,4-c]pyrrole (1).
Under nitrogen atmosphere, 1,4-diketo-3,6-dithiophene-
CO (0.82 g,
at 298 K and 5.93 mmol) in 30 mL anhydrous dimethylformamide (DMF) were
Measurements
1
a
13
H (500 MHz) and C NMR (125 MHz) spectra were recorded pyrrolo[3,4-c]pyrrole (0.60 g, 2.00 mmol), and K
using a Bruker-AC500 spectrometer in CDCl
2
3
3
tetramethylsilane (TMS) as the internal standard. The elemental stirred for 1 h at 120 1C. Then 1-bromooctane (1.16 g, 6.00 mmol,
analysis was performed on a Perkin-Elmer 2400 analyzer. in 5 mL DMF) was slowly added to the flask. The reaction was
UV-visible absorption and fluorescence emission spectra were kept for 24 h at 130 1C. After being cooled to room temperature,
respectively recorded on Hitachi U-4100 and Hitachi F-4600 the mixture was extracted with dichloromethane. The organic
spectrophotometers. Powder wide-angle X-ray diffraction (PXWD) phase was dried over anhydrous MgSO
experiments were performed on a Powder X-ray Diffractometer separated by silica column chromatography using dichloro-
INCA Energy, Oxford Instruments). Differential scanning methane as the eluent. A dark purple solid was obtained (0.65 g,
4
, and the product was
(
1
calorimetry (thermal analysis, DSC) curves were determined 62%). H NMR (500 MHz, CDCl
3
, ppm): d 8.90 (d, J = 8.5 Hz, 2H),
on a Netzsch DSC (204F1) instrument at a heating (or cooling) 7.63 (d, J = 7.9 Hz, 2H), 7.29 (t, J = 9.1 Hz, 2H), 4.03 (t, J = 9.6 Hz,
ꢀ1
rate of 10 1C min . The thermogravimetric analysis (TGA) was 4H), 1.82–1.75 (m, 4H), 1.37–1.24 (m, 20H), 0.86 (t, J = 8.1 Hz, 6H).
performed on a Netzsch (209F1) thermogravimetric analyzer Anal. calcd for C H N O S : C, 55.98; H, 2.68; N, 9.33; O, 10.65;
1
4
8 2 2 2
ꢀ
1
under a nitrogen atmosphere (50 mL min ) at a heating rate of S, 21.35. Found: C, 55.91; H, 2.65; N, 9.30.
ꢀ1
2
0 1C min . The solution fluorescence quantum yield (F) was
determined by a dilute solution method using fluorescein in water pyrrole (TDPP). Compound 1 (0.53 g, 1.00 mmol) and NBS
pH = 11) as the reference. The quantum efficiencies of the solid (0.39 g, 2.20 mmol) were dissolved in 30 mL of CHCl . And the
1,4-Diketo-2,5-dioctyl-3,6-di(bromothiophene)pyrrolo[3,4-c]-
(
3
films were measured using a FLS980 spectrometer. The fluores- suspension was stirred for 8 h at room temperature in the dark.
cence decays of the solids were measured on an Edinburgh FLS- The crude product was purified by silica gel column chromato-
9
80 spectrometer equipped with an EPL-375 optical laser.
Pressing and stimulus-recovering experiments: KBr powder obtained (1.15 g, 85%). H NMR (500 MHz, CDCl , ppm): d 8.66
graphy (petroleum ether/CH Cl = 1/5, v/v). A purple solid was
2 2
1
3
(
250 mg) and a mass of solid (B10 mg) were simply mixed in an (d, J = 8.3 Hz, 2H), 7.25 (d, J = 9.2 Hz, 2H), 4.04 (t, J = 9.5 Hz, 4H),
agate mortar, and the mixture was pressed using an IR pellet 1.76–1.69 (m, 4H), 1.45–1.28 (m, 20H), 0.87 (t, J = 8.0 Hz, 6H).
press under the pressure of 1500 psi for 1 min at room Anal. calcd for C30 : C, 52.79; H, 5.61; Br, 23.41;
temperature. After the fluorescence color and emission spectra N, 4.10; O, 4.69; S, 9.40. Found: C, 52.74; H, 5.56; N, 4.14.
2 2 2 2
H38Br N O S
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1,4-Diketo-2,5-dioctyl-3,6-di(bromophenyl)pyrrolo[3,4-c]-pyrrole
Solution photophysical properties
(
BDPP). The compound was synthesized as described for TDPP.
All four compounds are soluble in common organic solvents,
such as tetrahydrofuran, toluene, chloroform, dichloromethane,
etc. Fig. 1 shows the absorption and emission spectra of the four
compounds. It is observed that all the alkylated DPPs with and
without 9-anthryl capping hardly show the solvatochromic effect,
but 9-anthryl capping and spacing units affect significantly the
photophysical properties, and the corresponding spectroscopic
data are summarized in Table 1. BDPP emits yellow fluorescence
with peak wavelengths of B542 nm and quantum yields of
1
The target compound was obtained (0.45 g, 35%). H NMR
500 MHz, CDCl , ppm): d 7.60–7.70 (m, 8H), 3.70 (t, J = 9.3 Hz,
(
3
4
H), 1.52 (m, 4H), 1.21 (m, 20H), 0.86 (t, J = 8.4 Hz, 6H). Anal.
calcd for C34 : C, 60.90; H, 6.31; Br, 23.83; N, 4.18;
O, 4.77. Found: C, 60.87; H, 6.27; N, 4.22.
,4-Diketo-2,5-dioctyl-3,6-bis-(9-anthrylthiophen)pyrrolo[3,4-c]-
pyrrole (ATDPP). Under a nitrogen atmosphere, TDPP (0.30 g,
.46 mmol), Pd(PPh (0.04 g, 0.03 mmol), 9-anthraceneboronic
acid (0.23 g, 1.00 mmol), 2 M K CO (5 mL) and THF (20 mL) were
H
2 2 2
42Br N O
1
0
3 4
)
2
3
0.5–0.9, while TDPP shows emission peak wavelengths and quan-
added to a 50 mL one-neck flask. The mixture was refluxed for
tum yields of B585 nm and 0.38–0.45, respectively (Fig. 1a and b,
Table 1). The remarkably red-shifted emissions and decreased
fluorescence quantum yields observed in TDPP solutions imply
that thiophene is a strong electron-donating unit (D), and TDPP is
obviously characteristic of a D–A–D motif. Also, there is a large
overlap between the absorption and emission spectra in TDPP
solution, which should result in self-absorption that could
generally decrease the fluorescence quantum yields.
4
8 h under nitrogen, and then extracted with CH Cl (3 ꢁ 50 mL).
2
2
4
The organic phase was dried over MgSO and the solvent was
removed via rotary evaporation. The crude product was purified
by silica column chromatography using petroleum ether/CH Cl
3/2, v/v) as the eluent to afford the target compound (0.25 g,
2
2
(
1
62%). H NMR (500 MHz, CDCl
3
, ppm): d 9.27 (d, J = 4.5 Hz, 2H),
8.62 (s, 2H), 8.11 (d, J = 8.0 Hz, 4H), 7.95 (d, J = 9.0 Hz, 4H), 7.53
(t, J = 7.5 Hz, 8H), 7.43 (d, J = 8.2 Hz, 2H), 4.13 (t, J = 8.6 Hz, 4H),
1
3
As shown in Fig. 1, 9-anthryl-capping could red-shift absorp-
tion and emission spectra. However, ABDPP only shows slightly
red-shifted absorption and emission by 4 and 9 nm relative to
those of BDPP, respectively (Table 1). ABDPP is still in the
yellow-emitting region with high fluorescence quantum yields
although the backbone size is extended. In contrast, relatively
large absorption (6 nm) and emission (23 nm) red-shifts are
caused by 9-anthryl capping when thiophene was used as the
conjugation bridge. Quantum chemical calculations indicate
that the twisted angle between anthracene and thiophene is
larger than that between anthracene and benzene in the
optimized geometric structure even if the five-member thio-
phene heterocycle has only one b-hydrogen interacting with the
anthracene ring, but the twisted angle between thiophene and
the DPP core is much smaller than that between benzene and
the DPP core (Fig. 2). The electron distributions of the frontier
molecular orbitals (FMOs) for ABDPP and ATDPP vary with
conjugation spacers. When a benzene ring is used as the aryl
spacing, the HOMO is spread out over the whole p-system, and
the LUMO is located on the 3,6-diphenyl DPP moiety. For
ATDPP, it is not the case, here the FMOs are essentially spatially
super-imposable upon one another, and the electron distribu-
tion in the HOMO is concentrated on the 3,6-dithiophene DPP
moiety, implying that thiophene has a certain electron-donating
2
.05 (m, 4H), 1.35 (m, 20H), 0.89 (t, J = 8.5 Hz, 6H). C NMR
(125 MHz, CDCl
3
, ppm): d 161.87, 145.09, 140.23, 136.11,
1
1
31.51, 131.12, 128.78, 128.44, 126.68, 126.40, 126.09, 125.41,
08.08, 46.03, 39.23, 30.12, 28.33, 25.58, 23.40, 22.31, 14.01.
Anal. calcd for C58
S, 7.31. Found: C, 79.35; H, 6.40; N, 3.17.
,4-Diketo-2,5-dioctyl-3,6-bis(9-anthrylphenyl)pyrrolo[3,4-c]-
56 2 2 2
H N O S : C, 79.41; H, 6.43; N, 3.19; O, 3.65;
1
pyrrole (ABDPP). The compound was synthesized as described
for ATDPP. The target compound was obtained (0.2 g, 58%).
1
3
H NMR (500 MHz, CDCl , ppm): d 8.58 (s, 2H), 8.18 (d, J = 8.0
Hz, 4H), 8.10 (d, J = 8.5 Hz, 4H), 7.78 (d, J = 8.5 Hz, 4H), 7.76
d, J = 8.8 Hz, 4H), 7.54 (t, J = 7.5 Hz, 4H), 7.45 (t, J = 9.0 Hz,
H), 4.00 (d, J = 8.0 Hz, 4H), 1.85 (m, 4H), 1.34 (m, 20H), 0.89
(
4
1
3
(t, J = 6.5 Hz, 6H). C NMR (125 MHz, CDCl
3
, ppm): d 162.89,
1
1
2
48.31, 142.10, 135.55, 131.88, 131.25, 129.90, 128.76, 127.43,
27.14, 126.43, 125.70, 125.20, 109.99, 42.23, 31.69, 29.62,
9.09, 28.90, 26.80, 22.57, 14.05. Anal. calcd for C H N O :
6
2
60 2 2
C, 86.07; H, 6.99; N, 3.24; O, 3.70. Found: C, 86.11; H, 7.02;
N, 3.22.
Results and discussion
Synthesis and characterization
As illustrated in Scheme 1, the target dyes ATDPP and ABDPP ability and is also a better conjugation bridge for effectively
were conveniently synthesized. Firstly, DPPs (BDPP and 1) were extending the molecular conjugation and red-shifting spectra.
treated with K
1
1
2
CO
-bromooctane. And TDPP was obtained from the treatment of convergence, which could explain their lack of solvatochromism.
with NBS/CHCl
. Then the separated alkylated intermediates The calculated band-gaps for ABDPP and ATDPP are respectively
3
in DMF with the dropwise addition of Nevertheless, these FMOs all have symmetrical distribution and
3
(
BDPP and TDPP) in ca. 60% yield were subjected to a Suzuki 2.65 and 2.42 eV, which are qualitatively consistent with their
coupling reaction with 9-anthraceneboronic acid to afford the emission wavelengths.
target dyes in good yields. The intermediates were characterized
One interesting phenomenon is that BDPP, TDPP and
by H NMR spectroscopy and elemental analysis, and the target ABDPP all emit strong solution fluorescence, but ATDPP shows
dyes (ATDPP and ABDPP) were characterized by H and C NMR very low solution fluorescence quantum yields, regardless of
spectroscopy and elemental analysis, and the corresponding solvents (Table 1). As shown in Fig. 3, ATDPP exhibits aqueous
1
1
13
data are shown in the Experimental section.
aggregation-enhanced emission (AEE) activity: weak fluorescence
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Table 1 Solution photophysical data of DPP-based dyes
Fluorescence quantum yield Optical wavelength (nm)
Dyes
PhMe
CHCl
3
THF
l
abs
l
em
BDPP
TDPP
ABDPP 0.54
ATDPP 0.023
0.53
0.45
0.51
0.41
0.62
0.026
0.89
0.38
0.78
0.024
481
565
485
571
542
585
551
608
Fig. 2 The optimized molecular geometry (top) and calculated HOMO and
LUMO density maps (down) of ABDPP and ATDPP at the B3LYP/6-31G(d) level.
quenching (ACQ) dye. This is probably related to their different
steric hindrance effects affecting the intramolecular distortion
motion amplitude and frequency. Fig. 4 shows the internal
rotation barrier curves of the ABDPP and ATDPP molecules,
assuming zero potential energy under the optimized geometry.
The dominant internal rotation barrier is located at the
9-arylanthracene moiety. Since the internal rotation barrier for
ABDPP is higher than that of ATDPP, the steric hindrance and
repulsion between phenyl and the 1,8-positions of anthracene
are stronger. This should limit the 9-anthrylbenzene moiety from
rotating at small amplitude and high frequency and then
decrease the non-irradiation decay channels. Indeed, it is well
known that almost all 9,10-diaryl-anthracene derivatives exhibit
strong fluorescence in solutions. For ATDPP, the relatively weak
steric hindrance and repulsion between the 1,8-hydrogens
of anthracene and one b-hydrogen of thiophene ring render
the 9-anthrylthiophene moiety with a lower internal rotation
barrier and a larger distortion angle, which should make ATDPP
rotate with a larger amplitude and a lower frequency, which
could activate the non-irradiation decay channels and decrease
the fluorescence emission as observed in 9,10-distyrylanthracene
Fig. 1 The absorption and emission spectra (excited at 480 nm) of BDPP
(
a), TDPP (b), ABDPP (c) and ATDPP (d) at 1.0 ꢁ 10ꢀ5 M.
3
7–40
derivatives and other AEE compounds.
,10-di(thiophen-2-yl)anthracenes might be exploited as a new
in THF solution but an enhanced fluorescence in THF/water media class of AEE dyes. This is interesting work and worth further
2/8 in vol.). In contrast, ABDPP is an aqueous aggregation-caused investigation, which is underway in our laboratory. It should be
In this context,
9
(
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Fig. 5 The absorption and emission spectra of ABDPP and ATDPP solids.
respective solutions (Fig. 1 and 5), and they emit orange
(581 nm) and red (649 nm) with the fluorescence efficiency of
70.5 and 19.8%, respectively. ABDPP is a strongly fluorescent
dye in both solution and solid state, but its aqueous dispersion
exhibits an aggregation-caused quenching effect. In contrast,
the fluorescence efficiencies of ATDPP in both aqueous disper-
sion (Fig. 3) and solid state (19.8%) are significantly higher
than its molecularly dissolved solution (2.4% in THF).
Generally, AEE molecules are characterized by a twisted
conjugation skeleton bearing rotatable aryl moieties, which
could provide their solid states with intermolecular loose
stacking and weak p–p interactions to exhibit mechanochromic
luminescence (MFC). Indeed, AEE luminogens have recently
become the mainstay of MFC materials, but non-AEE fluores-
cent solids exhibiting MFC behavior could also provide new
valuable material systems for the potential applications and the
Fig. 3 Emission spectra (excited at 480 nm) of ABDPP and ATDPP in
ꢀ5
water–THF mixtures at 1.0 ꢁ 10 M. The insets are the fluorescence
images under illumination using a 365 nm UV lamp.
4
3–45
investigation of structure–property correlation.
We have
examined the fluorescence color change of ABDPP and ATDPP
solids upon application of external stimuli. When the ATDPP
solid is ground on a glass plate using a metal spatula and then
heat-annealed or solvent-fumed, no change is observed in the
fluorescence color and emission spectra (Fig. 6). In contrast,
the orange-emitting ABDPP solid can be changed into a red-
emitting state upon grinding, and heat-annealing and solvent-
fuming can convert the red fluorescence into a yellow-emitting
one. This process is reproducible between red and yellow states,
indicating a reversible MFC behavior (Fig. 7). At the same time,
a pressing experiment is also conducted at room temperature
using an IR pellet press (1 min at 1500 psi), and the results
Fig. 4 Internal rotation (a or b) barrier curves of ABDPP and ATDPP, show that the effectiveness is the same as the grinding (Fig. 8).
assuming zero potential energy at the optimized geometry.
The spectroscopic measurement reveals that the ground red
state and the annealed yellow state emit fluorescence with peak
wavelengths of 554 and 597 nm, respectively, which affords a
noted that the DPP derivatives capped with typical AIE tetra-
phenylethene do not exhibit the typical AIE activity.
43 nm spectral change. It is found that the fluorescence effi-
4
1,42
ciency for the ground state is only 4.7%, thus ABDPP exhibits
crystallization-enhanced emission and a MFC redshift. ATDPP
does not exhibit MFC behavior, which is possibly due to its
Solid-state optical properties
The as-prepared ABDPP and ATDPP solids show obviously red- larger-amplitude distortion angle with a lower internal rotation
shifted absorption and emission spectra relative to their barrier. In the crystalline state, ATDPP might have adopted a
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Fig. 6 Fluorescence image (under 365 nm UV lamp) and spectra of pure
ATDPP solid under various external stimuli.
Fig. 8 Fluorescence image (a) and spectra (b, excited at 480 nm) of
ABDPP–KBr mixture under various external stimuli.
higher melt point (250 1C). As a result, the molecular confor-
mation and packing structure of ATDPP solid could not be
changed further by an external mechanical force. Fig. 9 shows
the fluorescence decay kinetics of ABDPP solid under different
external stimuli. Overall, the mean fluorescence lifetimes hti of
the unground states (including pristine, annealed, and fumed
yellow-emitting crystalline solids) with a higher fluorescence
efficiency are slightly longer than that of the ground amorphous
state with a lower fluorescence efficiency. Nevertheless, these
fluorescence decay lifetimes are all within a few nanoseconds
4
6
scale, which could be assigned to exciton fluorescence. That is,
Fig. 7 Fluorescence image (under 365 nm UV lamp) and spectra (excited
at 480 nm) of pure ABDPP solid under external stimuli.
more planar conformation and formed stronger intermolecular
Fig. 9 The fluorescence decay profile of ABDPP solid under different
interactions, which could be evidenced by its red emission and external stimuli at room temperature.
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no excimer emission is observed even in the red-emitting
ground state.
Thermal and X-ray diffraction analysis
As DPP-based molecules with fully aromatic structures, ABDPP
and ATDPP all have excellent heat stability and no weight loss
before 380 1C (Fig. 10). DSC measurements (Fig. 11) reveal that
the pristine ABDPP solid obtained by evaporating solution
shows one endothermic peak (229 1C) following an exothermic
peak (232 1C) before the isotropic melt transition (245 1C) upon
the first heating process, and the resulting melt can crystallize
at 174 1C to form a yellow solid upon a cooling process.
However, only the isotropic melt transition is detected when
heating the yellow solids, implying that the pristine orange
solid is in a thermodynamically metastable crystalline state. For
the ground solid, there is one new exothermic peak appearing
at 96 1C besides the normal melting point, and this new
exothermic peak could be ascribed to the cold-crystallization
Fig. 12 Powder wide-angle X-ray diffraction patterns of different solid
ABDPP at room temperature.
transition of the amorphized ABDPP caused by grinding.
Powder wide-angle X-ray diffraction (Fig. 12) reveals that the
pristine orange and annealed yellow ABDPP solids are in
different crystalline states characterized by multiple sharp
but differential reflection peaks. In contrast, the ground solid
displays a broad, weak and featureless diffractogram, reflecting
the appearance of notably amorphous feature. The existence of
the part crystalline feature in the PWXD pattern of the ground
state could probably be regarded as the result of incomplete
amorphization (Fig. 12), and the residual crystal seeds can
facilitate the re-crystallization of the mechanically induced
amorphous state upon heat-annealing and solvent-fuming. Overall,
the PWXD and DSC experiments confirm that the phase transfor-
mation between the crystalline and amorphous states should be
responsible for the MFC behavior.
Fig. 10 The thermogravimetric (TGA) curves of ABDPP and ATDPP solids.
Conclusions
We have capped 1,4-diketo-2,5-dioctyl-3,6-bis(phenyl/thiophen)-
pyrrolo[3,4-c]pyrrole (DPP) with 9-anthracene to design two new
DPP derivatives (ABDPP and ATDPP) with different twisted moieties
for the development of new DPP-based optical materials.
Like other 9,10-diphenyl-anthracene derivatives, ABDPP with
benzene as the spacing unit is highly fluorescent in both
solution and solid state. In contrast, ATDPP with thiophene
as the spacing unit is an aggregation-enhanced emission dye.
The influence of the conjugation spacing unit on optical
properties is also observed in the stimuli responsive behavior.
It is the ABDPP solid rather than the ATDPP solid that exhibits
remarkable mechanochromic luminescence, contributing a
non-AEE dye to the MFC family. PWXD and DSC measurements
confirm that the phase transformation between the crystalline
and amorphous states upon application of external stimuli is
responsible for the reversible MFC behavior. It should be noted
that while DPP derivatives are extensively used as excellent
building blocks in field-effect transistors and photovoltaic
materials, their optical properties are still rarely developed
Fig. 11 The differential scanning calorimetry (DSC) curves of ABDPP solids
under pristine (as-prepared), ground solid-state and annealed solid-state.
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Journal of Materials Chemistry C
at present. This work has demonstrated that manipulating the 18 W. Hong, B. Sun, H. Aziz, W. T. Park, Y. Y. Noh and Y. N. Li,
molecular structure of DPP-based derivatives could signifi-
Chem. Commun., 2012, 48, 8413.
cantly tune and alter their aggregation behaviors and optoelec- 19 J. Lee, A. R. Han, J. Hong, J. H. Seo, J. H. Oh and C. Yang,
tronic properties.
Adv. Funct. Mater., 2012, 22, 4128.
2
2
0 J. Yue, J. Liang, S. Sun, W. Zhong, L. Lan, L. Ying, W. Yang
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1 W. Li, K. H. Hendriks, A. Furlan, M. M. Wienk and
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Acknowledgements
We thank the support through the funds from the NSF of China 22 A. P. Zoombelt, S. G. J. Mathijssen, M. G. R. Turbiez,
(
51303091, 51473084 and 51573081), the NSF of Shandong
M. M. Wienka and R. A. J. Janssen, J. Mater. Chem., 2010,
20, 2240.
Province (ZR2014EMQ013), and the Specialized Research Fund
for the Doctoral Program of Higher Education (201333719120005). 23 P. Sonar, G. M. Ng, T. Lin, A. Dodabalapur and Z. Chen,
We are also very grateful for China Postdoctoral Science Fund
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(
2015M582061 and 2014M560537), and the Open Project of State 24 W. W. H. Wong, J. Subbiah, S. R. Puniredd, B. Purushothaman,
Key Laboratory of Supramolecular Structure and Materials of Jilin
University (SKLSSM-201627).
W. Pisula, N. Kirby, K. Mullen, D. J. Jones and A. B. Holmes,
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