Synthesis of novel diketopyrrolopyrrole-based luminophores showing crystallization-induced emission enhancement properties

Article abstract of DOI:10.1016/j.dyepig.2011.01.005

A series of diketopyrrolopyrrole-based luminophores were synthesized and characterized. Their photoluminescent properties were affected significantly by the change of π-conjugated units and alkyl groups linked to the diketopyrrolopyrrole-ring. 2,5-Dialkyl

Full text of DOI:10.1016/j.dyepig.2011.01.005

Dyes and Pigments 90 (2011) 311e318
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis of novel diketopyrrolopyrrole-based luminophores showing
crystallization-induced emission enhancement properties
,
Yi Jin a, Yanbin Xu a, Yinling Liu a, Lingyun Wang a, Huanfeng Jiang a, Xianjie Li b, Derong Cao a
*
a School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510641, China
b State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of diketopyrrolopyrrole-based luminophores were synthesized and characterized. Their photo-
luminescent properties were affected significantly by the change of -conjugated units and alkyl groups
Received 16 September 2010
Received in revised form
10 January 2011
Accepted 11 January 2011
Available online 19 January 2011
p
linked to the diketopyrrolopyrrole-ring. 2,5-Dialkyl-3,6-bis(4-((10-oxoanthracen-9(10H)-ylidene)methyl)
phenyl)pyrrolo[3,4-c]pyrrole-1,4-dione showed aggregate-induced emission enhancement and crystal-
lization-induced emission enhancement properties while 3,6-bis(4-[1,3]dioxolan-2-yl-phenyl)-2,5-dia-
lkylpyrrolo[3,4-c]pyrrole-1,4-dione, 2,5-dialkyl-3,6-bis(4-formylphenyl)pyrrolo[3,4-c]pyrrole-1,4-dione,
and 2,5-dioctyl-3,6-bis{4-[2-(4-bromophenyl)-2-cyanovinyl]-phenyl}pyrrolo[3,4-c]pyrrole-1,4-dione did
not exhibit these properties. The results showed that emission enhancements of the luminophores
Keywords:
Diketopyrrolopyrrole
Crystallization-induced emission
enhancement
depended on the competition between the restriction of intramolecular rotation and
pep aggregation
with the former causing emission enhancement and the latter inducing fluorescence quenching.
Ó 2011 Elsevier Ltd. All rights reserved.
Fluorescence
Restriction of intramolecular rotation
Aggregate-induced emission
1. Introduction
emission or crystallization-induced emission enhancement (CIEE)
[9,10]. High-tech applications of the AIE luminogens are explored, for
p
-Conjugated organic materials have recently attracted much
example, in fluorescence sensors, biological probes, immunoassay
markers, poly(acrylamide) gel electrophoresis visualization agents,
monitors for layer-by-layerassembly, reporters for micelle formation
and multi-stimuli-responsive nanomaterials [9]. However, only
a limited number of organic compounds display AIE characteristics
such as siloles, aminobenzoic acids, arylethene and arylbenzene
derivatives [7,9e17].
Some hypotheses have been proposed for the possible mecha-
nism of AIE. Generally, AIE has been attributed to the restriction of
intramolecular rotations (RIR) [9,18e21], formation of J-aggregates
[22,23], intramolecular planarization [5] or restriction of the tran-
sition from the local excited state to the intramolecular charge
transfer state that accompanies twisting [24].
attention because of their potential applications in biological
imaging systems [1,2] and light-emitting diodes [3,4]. It is neces-
sary to understand and to manipulate their solid-state optical
properties for achieving successful functional devices containing
conjugated materials.
p-
In the solid state, luminogenic molecules lie in close proximity
and form aggregates. Chromophore aggregation usually quenches
the emission of organic luminophores which have high lumines-
cence efficiencies in dilute solutions. Such quenching is due to the
formation of detrimental species such as excimers [5e8]. However,
a novel phenomenon, aggregate-induced emission enhancement
(AIEE) or aggregate-induced emission (AIE), has been recently
reported [9]. This emission phenomenon is manifest by compounds
exhibiting significant enhancement of their light-emission in the
amorphous or crystalline states whereas they exhibit weak or almost
no emission in dilute solutions. Furthermore, the crystals of some
compounds emit more efficiently than their solution or amorphous
powders, showing a novel phenomenon of crystallization-induced
The nature of molecular aggregation in bulk is controlled by
a delicate balance of various weak intermolecular forces including
both
not in conjugation [25,26]. Therefore, variations in the structure of
-conjugated units and the remote functional groups could be
p-conjugated units and remote functional groups which are
p
utilized as an effective strategy to fine-tune the solid-state photo-
physical properties of a luminophore [27]. Diketopyrrolopyrrole
(DPP) derivatives are a class of strongly fluorescent heterocyclic
pigments and their structures can be easily optimized through
variations of substituents at the 2,5- and 3,6-positions [28,29].
* Corresponding author. Tel./fax: þ86 20 87110245.
E-mail address: drcao@scut.edu.cn (D. Cao).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.01.005

312
Y. Jin et al. / Dyes and Pigments 90 (2011) 311e318
In this study, in order to enlarge the family of AIE luminophores
added dropwise to the mixture which was stirred at 20 ꢀC for 10 h
under nitrogen. Then, the solvent was evaporated under reduced
pressure and the residue was recrystallized with NMP and methanol
to afford a yellow solid (2c) in 44% yield (1.25 g), m.p. 285e287 ꢀC.
FT-IR (KBr, cmꢁ1): 3060, 2920, 2883, 1687, 1603, 1509, 1451, 1072,
and to gain an insight into luminogenic molecular structur-
eeproperty relationships, a series of new DPP derivatives with
systematically varied substituents at the 2,5- and 3,6-positions
were synthesized. We also studied the influence of p-conjugated
units and alkyl substituents linked to the DPP-ring upon emission
behavior of the DPP-luminophores in both solution and solid state.
840. 1H NMR (400 MHz, CDCl3, ppm):
d
7.75 (d, J ¼ 8.0 Hz, 4H, Ar),
7.53 (d, J ¼ 8.4 Hz, 4H, Ar), 7.31e7.17 (m,10H, Ar), 5.83 (s, 2H, OCHO),
4.95 (s, 4H, NCH2), 4.09e4.02 (m, 8H, OCH2CH2O); 13C NMR
2. Experimental section
(100 MHz, CDCl3, ppm) d 162.7,148.7,141.3,137.4,129.1,128.8,128.4,
127.4, 126.9, 126.6, 109.8, 102.9, 65.3, 45.6; MS (APCI) calcd for
C38H32N2O6 612.2, found 613.1. Anal. Calc. for C38H32N2O6: C, 74.49;
H, 5.26; N, 4.57. Found: C, 74.31; H, 5.34; N, 4.56.
2.1. Instrumentation and materials
NMR (1H and 13C) spectra were collected on a Bruker DRX 400
spectrometer (in CDCl3 or DMSO-d6, TMS as internal standard).
MALDI-TOF mass spectra were recorded on a Bruker Autoflex TOF
mass spectrometer. APCI-MS spectra were recorded with a Bruker
Esquire HCT plus mass spectrometer. Elemental analyses were per-
formed using a Vario EL III instrument. Fourier transform infrared
(FT-IR) spectra were recorded on an RFX-65A (Analect Co.) spec-
trometer. UVevis absorption spectra were obtained on a Shimadzu
UV-2550 spectrometer. PL spectrawere obtained on a Hitachi F-4500
FL Spectrophotometer. PL quantumyields in solutionwere estimated
2.2.3. 2,5-Dibutyl-3,6-bis(40-formylphenyl)pyrrolo[3,4-c]
pyrrole-1,4-dione (3b)
A mixture of 2b (0.916 g, 1.68 mmol), THF (100 mL) and HCl
(2.0 mol/L, 8.5 mL) was stirred at 60 ꢀC for 2 h. The solvent was
removed and the residue was washed with water to afford an
orange solid (3b) in 99% yield (0.737 g), m.p. 184e186 ꢀC. FT-IR (KBr,
cmꢁ1): 2957, 2930, 2869, 2751, 1702, 1667, 1592, 1505, 1457, 1392,
1089, 830, 738. 1H NMR (400 MHz, CDCl3, ppm)
d 10.06 (s, 2 H,
CHO), 8.01e7.94 (m, 8H, Ar), 3.75 (t, J ¼ 8.0 Hz, 4H, NCH2), 1.52 (m,
using Rhodamine 6G
(4F ¼ 95% in ethanol) as the standard,
4H, CH2), 1.25e1.20 (m, 4H, CH2), 0.81 (t, J ¼ 8.0 Hz, 6H, CH3); 13C
lex ¼ 488 nm [30,31]. The absolute PL quantum yields were deter-
mined in an Integrating sphere IS080 (Labsphere) with 325 nm
excitation of HeeCd laser (Mells Griod), as a percent of the total
output photons in all directions vs the total input photons. Single
crystal X-ray diffraction intensity data were collected on a Bruker
Smart 1000 CCD diffractometer. Polarizing optical micrographs were
observed with a polarized light microscope Olympus BX41.
Ethanol was distilled from sodium under nitrogen. N-Methyl-2-
pyrrolidone (NMP) was dried with CaH2 and distilled under
nitrogen atmosphere. Other solvents were obtained from
commercially available resources without further purification.
Compounds 1, 2a, 3a were synthesized according to the published
literature [32].
NMR (100 MHz, CDCl3, ppm) d 191.2, 162.2, 147.6, 137.6, 133.2, 129.9,
129.2, 111.0, 41.7, 31.5, 19.9, 13.5; MS (APCI) calcd for C28H28N2O4
456.2, found 457.1. Anal. Calc. for C28H28N2O4: C, 73.66; H, 6.18; N,
6.14. Found: C, 73.58; H, 6.23; N, 6.14.
2.2.4. 2,5-Dibenzyl-3,6-bis(40-formylphenyl)pyrrolo[3,4-c]
pyrrole-1,4-dione (3c)
It was prepared by using the same synthetic procedure of 3b to
afford an orange solid (3c) in 98% yield, m.p. 258e259 ꢀC. FT-IR (KBr,
cmꢁ1): 2927, 2845, 2742, 1703, 1674, 1594, 1505, 1456, 833. 1H NMR
(400 MHz, DMSO-d6, ppm)
4H, Ar), 7.97 (d, J ¼ 8.0 Hz, 4H, Ar), 7.29e7.09 (m,10H, Ar), 5.00 (s, 4H,
NCH2); 13C NMR (100 MHz, DMSO-d6 ppm)
192.6,161.5,147.5,137.5,
d
10.05 (s, 2H, CHO), 8.01 (d, J ¼ 8.0 Hz,
d
137.1, 132.4, 129.6, 129.4, 128.7, 127.3, 126.5, 110.2, 44.5; MS (APCI)
calcd for C34H24N2O4 524.2, found 525.1. Anal. Calc. for C34H24N2O4:
C, 77.85; H, 4.61; N, 5.34. Found: C, 77.69; H, 4.65; N, 5.32.
2.2. Syntheses of DPP derivatives 2e5
2.2.1. 3,6-Bis(4-[1,3]dioxolan-2-yl-phenyl)-2,5-dibutylpyrrolo
[3,4-c]pyrrole-1,4-dione (2b)
2.2.5. 2,5-Dioctyl-3,6-bis(40-((10-oxoanthracen-9(10H)-ylidene)
methyl)phenyl)pyrrolo[3,4-c]pyrrole-1,4-dione (4a)
A mixture of 3,6-bis(4-[1,3]dioxolan-2-yl-phenyl)pyrrolo[3,4-c]
pyrrole-1,4- dione (1) (2.00 g, 4.63 mmol), potassium tert-butoxide
(2.59 g, 23.1 mmol), and dry NMP (60 mL) was heated to 60 ꢀC and
stirred for 0.5 h. 1-Bromobutane (3.8 g, 27.8 mmol) in dry NMP
(30 mL) was added dropwise to the mixture which was stirred at
60 ꢀC for 10 h under nitrogen atmosphere. The solvent was evap-
orated under reduced pressure and the residue was purified by
silica gel column (eluent: petroleum ether/dichloromethane/ethyl
acetate 5:5:1) to afford a yellow solid (2b) in 45% yield (1.13 g), m.p.
229e230 ꢀC. FT-IR (KBr, cmꢁ1): 3063, 2950, 2928, 2869, 1667, 1603,
Anthrone (404 mg, 2.08 mmol) and 3a (473 mg, 0.832 mmol)
were mixed in absolute ethanol (20 mL). Then the suspension was
saturated with gaseous HCl overnight and then heated under reflux
for 3.5 h. After cooling, the mixture was poured into H2O. Precipi-
tates were washed with methanol and recrystallized with CH2Cl2
and methanol to afford a red solid (4a) in 74.1% yield (568 mg), m.p.
270e271 ꢀC. FT-IR (KBr, cmꢁ1): 3059, 3035, 2951, 2921, 2850, 1659,
1592, 1503, 1462, 1087, 780, 722. 1H NMR (400 MHz, CDCl3, ppm)
d
8.32e8.27 (m, 4H, Ar), 8.05 (d, J ¼ 8.0 Hz, 2H, Ar), 7.82 (d,
1509, 1454, 1384, 1084, 727. 1H NMR (400 MHz, CDCl3, ppm):
d
7.81
J ¼ 8.0 Hz, 4H, Ar), 7.69 (t, J ¼ 8.0 Hz, 2H, Ar), 7.60 (d, J ¼ 8.0 Hz, 2H,
Ar), 7.56e7.47 (m, 10H, Ar and C]CH), 7.26 (t, J ¼ 8.0 Hz, 2H, Ar),
3.78 (t, J ¼ 8.0 Hz, 4H, NCH2), 1.63 (m, 4H, CH2), 1.25e1.21 (m, 20H,
CH2), 0.84 (t, J ¼ 8.0 Hz, 6H, CH3); 13C NMR (100 MHz, CDCl3, ppm)
(d, J ¼ 8.4 Hz, 4H, Ar), 7.62 (d, J ¼ 8.0 Hz, 4H, Ar), 5.87 (s, 2H, OCHO),
4.13e4.05 (m, 8H, OCH2CH2O), 3.74e3.70 (m, 4H, NCH2), 1.58e1.50
(m, 4H, CH2), 1.26e1.20 (m, 4H, CH2), 0.82 (t, J ¼ 7.2 Hz, 6H, CH3);
13C NMR (100 MHz, CDCl3, ppm)
d
162.5, 148.0, 140.9, 128.8, 128.7,
d 184.6, 162.6, 147.7, 140.3, 140.1, 135.7, 133.0, 132.8, 132.4, 131.4,
126.9, 109.9, 102.9, 65.3, 41.6, 31.5, 19.9, 13.5; MS (APCI) calcd for
C32H36N2O6 544.3, found 545.2. Anal. Calc. for C32H36N2O6: C,
70.57; H, 6.66; N, 5.14. Found: C, 70.40; H, 6.69; N, 5.14.
131.0, 130.7, 129.8, 129.2, 128.9, 128.8, 128.1, 127.4, 127.1, 123.0, 110.0,
42.1, 31.6, 29.4, 29.1, 29.0, 26.7, 22.6, 14.0; MALDI/TOF MS calcd for
C64H60N2O4 920.5, found 921.5. Anal. Calc. for C64H60N2O4: C, 83.45;
H, 6.57; N, 3.04; Found: C, 83.40; H, 6.60; N, 3.03.
2.2.2. 3,6-Bis(4-[1,3]dioxolan-2-yl-phenyl)-2,5-dibenzylpyrrolo
[3,4-c]pyrrole-1,4-dione (2c)
A mixture of 1 (2.00 g, 4.63 mmol), potassium tert-butoxide
(2.59 g, 23.1 mmol), and dry NMP (60 mL) was stirred for 0.5 h at
20 ꢀC. Benzyl bromide (4.75 g, 27.8 mmol) in dry NMP (30 mL) was
2.2.6. 2,5-Dibutyl-3,6-bis(40-((10-oxoanthracen-9(10H)-ylidene)
methyl)phenyl)pyrrolo[3,4-c]pyrrole-1,4-dione (4b)
It was prepared by using the same synthetic procedure of 4a to
afford a red solid (4b) in 37% yield, m.p. 246e247 ꢀC. FT-IR (KBr,

Y. Jin et al. / Dyes and Pigments 90 (2011) 311e318
313
cmꢁ1): 3063, 3030, 2954, 2930, 2869, 1661, 1591, 1500, 1460, 1364,
1081, 835, 778, 734. 1H NMR (400 MHz, CDCl3, ppm)
8.29e8.25 (m,
obtain 4 by using a condensation reaction in the presence of pyri-
dine/piperidine failed [33]. Luminophores 2e4 are soluble in
chloroform and tetrahydrofuran (THF), but insoluble in water.
d
4H, Ar), 8.03 (d, J ¼ 8.0 Hz, 2H, Ar), 7.80 (d, J ¼ 8.4 Hz, 4H, Ar), 7.67 (t,
J ¼ 8.0 Hz, 2H, Ar), 7.58e7.26 (m, 12H, Ar and C]CH), 7.25 (d,
J ¼ 6.0 Hz, 2H, Ar), 3.78 (t, J ¼ 7.6 Hz, 4H, NCH2), 1.61e1.58 (m, 4H,
CH2), 1.30e1.25 (m, 4H, CH2), 0.85 (t, J ¼ 7.2 Hz, 6H, CH3); 13C NMR
3.2. Photophysical properties
(100 MHz, CDCl3, ppm)
d
184.6,162.7,147.8,140.4,140.2,135.8,133.1,
Compounds 2e5 had absorption peaks (lab) between 474 and
132.9,132.5,131.4,131.0,130.8,129.8,129.2,129.0,128.8,128.2,127.5,
127.2, 127.2, 123.1, 110.1, 41.8, 31.5, 20.0, 13.6; MALDI/TOF MS calcd
for C56H44N2O4 808.3, found 808.3. Anal. Calc. for C56H44N2O4: C,
83.14; H, 5.48; N, 3.46. Found: C, 83.23; H, 5.50; N, 3.47.
519 nm arising from the pep* transition of the DPP-ring moieties
(Table 1). When changing substituents at the benzene ring with
[1,3]dioxolanyl, aldehyde and anthracenone groups, respectively,
the lab of the DPP derivatives 2e4 was progressively red shifted due
to their extended conjugated structures.
2.2.7. 2,5-Dibenzyl-3,6-bis(4-((10-oxoanthracen-9(10H)-ylidene)
methyl)phenyl)pyrrolo[3,4-c]pyrrole-1,4-dione(4c)
It was prepared by using the same synthetic procedure of 4a to
afford a red solid (4c) in 75.2% yield, m.p. 297e298 ꢀC. FT-IR (KBr,
cmꢁ1): 3059, 3021, 1662, 1660, 1505, 1079, 773. 1H NMR (400 MHz,
The solutions of 2a, 3a and 4a had fluorescence maxima at 533,
570 and 597 nm with the efficiencies of 88%, 31% and 0.09%
(Table 1), respectively. A similar trend was also observed in the
series of 2b, 3b, 4b and 2c, 3c, 4c (Table 1). That is to say, the
emission maximum (lem) values of DPP derivatives were bath-
ochromically shifted and their fluorescence quantum yields
decreased, when changing substituents at the benzene ring from
[1,3]dioxolanyl to aldehyde and anthracenone groups, respectively.
The red-shifted lem should be attributed to the extended conju-
CDCl3, ppm)
d
8.28e8.23 (m, 4H, Ar), 7.98 (d, J ¼ 8.0 Hz, 2H, Ar), 7.73
(d, J ¼ 8.0 Hz, 4H, Ar), 7.64 (t, J ¼ 8.0 Hz, 2H, Ar), 7.53e.37 (m, 12H, Ar
and C]CH), 7.30e7.24 (m, 8H, Ar), 7.18 (d, J ¼ 7.2 Hz, 4H, Ar), 5.02 (s,
4H, NCH2); 13C NMR (100 MHz, CDCl3, ppm)
d 184.6, 162.7, 148.3,
140.6, 140.2, 137.3, 135.7, 133.1, 132.8, 132.4, 131.4, 131.0, 130.7, 129.8,
129.3, 129.2, 128.8, 128.2, 127.5, 127.2, 126.7, 123.1, 110.0, 45.7; MALDI/
TOF MS calcd for C62H40N2O4 876.3, found 876.3. Anal. Calc. for
C62H40N2O4: C, 84.91;H, 4.60; N, 3.19. Found: C, 85.28; H, 4.60; N, 3.21.
gated p systems. The decrease of efficiency might be due to the
carbonyl groups which could increase the probability of inter-
system crossing [34] and intramolecular rotation processes
inducing annihilation of excitations [9,35]. However, the lem of the
DPP molecules was blue-shifted and the fluorescence efficiency
was enhanced when the 2,5-substituents were changed from octyl
to butyl and benzyl. For example, lem of compounds 4aec was 654,
651 and 640 nm, respectively; 4F of 4aec was 0.09%, 0.17% and
0.36%, respectively. The reason for this phenomenon might be RIR
[35]. The intramolecular rotation consumes energy and serves as
a relaxation channel for the excited sate to decay, which decrease
the radiative decay [9]. Moreover, the ability of rotation and
vibrational motion of the substituents on 2e4 decrease following
the sequence of octyl, butyl and benzyl. Therefore, the fluorescence
efficiency increases following this order.
2.2.8. 2,5-Dioctyl-3,6-bis{40-[2-(4-bromophenyl)-2-cyanovinyl]-
phenyl}pyrrolo[3,4-c] pyrrole-1,4-dione (5)
A
mixture of 2-(4-bromophenyl)acetonitrile (122 mg,
0.625 mmol), 3a (142 mg, 0.25 mmol), potassium carbonate
(863 mg, 6.25 mmol) and absolute ethanol (4 mL) was heated
under reflux for 2.5 h. After cooling, the mixture was poured into
H2O (5 mL). Precipitates were washed with water and recrystallized
with CH2Cl2 and methanol to afford a red solid (5) in 43% yield
(100 mg), m.p. 263e264 ꢀC. FT-IR (KBr, cmꢁ1): 3064, 3030, 2952,
2921, 2853, 2220, 1677, 1589, 1505, 1464, 1379, 1096, 828, 726. 1H
NMR (400 MHz, CDCl3, ppm)
d
7.96 (d, J ¼ 8.0 Hz, 4H, Ar), 7.87 (d,
A temperature dependence study of the emission properties of
4a was conducted in THF solution. When a solution of 4a was
cooled from 20 to ꢁ70 ꢀC, its emission peak was slightly red shifted,
and its emission intensity increased (Fig. 1). These experiments
may confirm that the RIR process was a cause for the phenomenon
observed in compounds 4aec since reducing the temperature
would inhibit intramolecular rotation [9].
J ¼ 8.0 Hz, 4H, Ar), 7.52e7.47 (m, 10H, Ar and CH]C), 3.76 (t,
J ¼ 7.2 Hz, 4H, NCH2), 1.52 (m, 4H, CH2), 1.17 (m, 20H, CH2), 0.81 (t,
J ¼ 6.8 Hz, 6H, CH3); 13C NMR (100 MHz, CDCl3, ppm)
d 162.4, 147.5,
140.6,135.5,132.8,132.3,129.8,129.7,129.4,127.4,124.0,117.3,111.8,
110.6, 41.9, 31.7, 29.4, 29.1, 29.0, 26.7, 22.6, 14.1; MS (APCI) calcd for
C52H52N4O2Br2 924.2, found 925.7. Anal. Calc. for C52H52N4O2Br2: C,
67.52; H, 5.67; N, 6.06. Found: C, 67.59; H, 5.70; N, 6.04.
Comparison of the emission spectra of compounds in solution
and in the solid state showed that crystals of all DPP derivatives
exhibited bathochromic shifts. The extent of the spectral shift
varied with molecular structure. Compounds 2ce4c with benzyl
substituents showed relatively small red shifts while 2be4b with
a butyl substituent exhibited relatively larger shifts. This suggests
that the bulky 2,5-substituents could reduce intermolecular inter-
actions [8]. However, the fluorescence quantum yields of DPP
derivatives in the solid state were different from those in solution.
The fluorescence quantum yield (4F) of 2a, 2b and 2c in crystals was
39%, 19% and 8.5%, respectively, which were much lower than that
in solution. Their efficiencies decreased following the sequence of
2a, 2b and 2c, which was contrary to the sequence in solution.
Compounds 4aec were AIEE active because their PL intensities in
the solid state were higher than those in solutions. Their emission
intensities in the solid state were strongly affected by particle size
and degree of crystallization. Therefore, their fluorescence
quantum yields in crystals were not measured. The emission
spectrum of 3a in the solid state could not be obtained on the FL
spectrophotometer. The crystals of 3bec and 5 emitted weakly.
Their fluorescence quantum yields were not reported, considering
the likely measurement error due to the measurement technique.
2.3. Preparation of aggregates
Stock solutions of the DPP derivatives in THF with a concentra-
tion of 1 mM were prepared. Aliquots (100 mL) of the solution were
transferred to volumetric flasks (10 mL). After adding appropriate
amounts of THF, water was added dropwise at a constant rate of
1 mL/min under vigorous stirring to furnish 10 mM solution with
defined fractions of water (fwater ¼ 0e90 vol%). Spectral measure-
ments of the resultant solutions or aggregate suspensions were
performed immediately.
3. Result and discussion
3.1. Synthesis and structural characterization
The synthetic route employed to access the DPP derivatives is
presented in Scheme 1. The key products 4 were obtained by
a straightforward aldol-type condensation reaction of 9(10H)-
anthracenone with appropriately formyl substituted DPP precur-
sors in the presence of dry HCl/ethanol. However, attempts to

314
Y. Jin et al. / Dyes and Pigments 90 (2011) 311e318
O
O
OHC
O
O
R
R
N
HCl
N
NH
R
X
O
O
O
t-C H OK
O
O
4
9
THF/H O
2
O
N
R
N
R
HN
O
CHO
O
O
3
2
O
1
3a: R= C H
2a: R= C H
8
17
9
8
17
9
3b: R= C H
2b: R= C H
4
4
3c: R= benzyl
2c: R= benzyl
O
O
R
N
O
O
N
HCl, EtOH
R
O
4
4a: R= C H
8
17
9
4b: R= C H
4
4c: R= benzyl
Br
C H
O
8
17
N
3a
CN
CN
CN
N
Br
K CO , EtOH
2
3
C H
8
O
17
Br
5
Scheme 1. Syntheses of DPP derivatives 2e5.
3.3. Aggregate-induced emission enhancement
in the H2O/THF mixtures (fwater ꢂ 60%) showed a red-shift and
a long absorption tail, which were characteristic of the nano-
particles’ absorption [36]. Since the light-scattering effect of the
aggregates of 4 in the aqueous mixtures made the quantum yield
Fluorescence properties of 4 were studied in various mixtures of
THF and water (10 M) because compounds 4 were soluble in THF
m
and insoluble in water (Fig. 2). The absorption spectra profiles of 4
Table 1
Absorption and emission characteristics of DPP derivatives 2e5 in solutions and
crystalline states.a
DPP
lab (nm)
3
(Mꢁ1 cmꢁ1
)
lem (nm)
4F (%)
derivatives
Solution
Solution Crystal Solutionb
Crystalc
2a
3a
4a
2b
3b
4b
2c
3c
4c
5
474
498
509
474
498
507
473
496
507
519
17,900
11,300
35,900
16,863
18,300
33,700
23,600
21,500
51,900
37,200
533
570
597
533
570
597
529
565
580
604
568
88 ꢃ 2
31 ꢃ 1
39 ꢃ 2
654
570
610
651
562
604
640
682
0.09 ꢃ 0.01
97 ꢃ 2
19 ꢃ 2
8.5 ꢃ 2
37 ꢃ 1
0.17 ꢃ 0.01
99 ꢃ 2
45 ꢃ 1
0.36 ꢃ 0.01
8.9 ꢃ 0.3
a
Solution ¼ 10
m
M
in CHCl3, lab ¼ absorption maximum, lem ¼ emission
maximum, 4F ¼ fluorescence quantum yield.
b
Estimated using Rhodamine 6G
(4F ¼ 95% in ethanol) as the standard,
lex ¼ 488 nm.
c
Measured using an integrating sphere.
Fig. 1. Effect of temperature on emission of 4a in THF solution (10
mM, lex ¼ 560 nm).

Y. Jin et al. / Dyes and Pigments 90 (2011) 311e318
315
Fig. 2. (A) Emission spectra of 4a in H2O/THF mixtures. (B) Relations between the ratio
of I/I0 and water fraction in H2O/THF mixtures. I0 and I were fluorescence peak
intensity of 4 in pure THF and in H2O/THF mixtures (10 M), respectively; excitation
wavelength: 560 nm for 4a, 520 nm for 4b, 552 nm for 4c.
Fig. 3. Emission spectra of amorphous and crystalline films of luminophor 3a
m
(
lex ¼ 450 nm) and 4a (lex ¼ 560 nm).
treated film displayed birefringent textures. Based on these obser-
vations, 4a was considered to be CIEE active. Furthermore, vapor-
responsiveness of the PL of 4a could enable it to find application in
detecting volatile organic compounds (VOCs) [10].
The fluorescence properties of 4c were compared with those of
2c, 3c and 5 that contained different substituents at the 3- and the
6-positions of the DPP unit (Fig. 2B vs Fig. 4). It was found that only
4c possessing anthracenone groups was AIEE active.
measurements inaccurate [36], the peak intensities of PL spectra
were used for the comparison of their emission properties. The PL
intensity of 4a remained almost constant in the H2O/THF mixtures
with up to 50% (fwater), however, it increased sharply as the water
content increased further (Fig. 2B). The PL intensity of 4a in the 70%
(fwater) H2O/THF mixture was about 55 times higher than that in
THF solution. Moreover, 4a would gradually aggregate in the H2O/
THF mixtures with increase of water content. Therefore, 4a is AIEE
active [9]. lem and full width at half maximum of PL of 4a were
similar in H2O/THF at different water content. Similar trends were
observed for 4b and 4c. However, the increments of PL intensity
depended on the 2,5-substituents of the DPP-ring (Fig. 2). As shown
in Fig. 2B, the maximum of I/I0 of 4aec was 56, 4.8 and 57,
respectively.
The emission intensity of 4 decreased as the fraction of water
further increased in the H2O/THF mixtures. For example, the ratio of
I/I0 of 4a decreased from 56 to 5 as the water fraction increased
from 70% to 90%. This could be attributed to a morphology change
in the aggregates of 4 in the aqueous medium [10]. During the
process of preparation of aggregates, water was added at a constant
rate to the THF solutions of 4. As a result, in the H2O/THF mixtures
of low water content (e.g., fwater ꢄ 70%), the molecules of lumino-
phores may aggregate slowly, which could form more crystalline
particles and less amorphous aggregates. However, in the mixtures
with high water content (e.g., fwater > 80%), the molecules of lumi-
nophores might aggregate abruptly, which might form more
amorphous aggregates and lead to a low ratio of crystalline parti-
cles [10]. The quantum yield of the crystalline particles might be
higher than that of the amorphous particles [10]. Therefore, PL
intensity of 4 would decrease with a higher fraction of water.
In order to confirm this hypothesis, an amorphous thin film of
4a was prepared by melting a sample of 4a on a quartz plate under
N2. Since solvent fuming can induce chromophores to crystallize
[10], the emission intensity transformation of 4a with different
crystallization degree could be tracked by fuming the amorphous
thin film with organic solvent vapor. As shown in Fig. 3, the fluo-
rescence intensity of the thin film of 4a was significantly enhanced
by fuming with CH2Cl2 vapor, while the intensity of 3a was almost
unchanged. The morphological structures of the parent and fumed
films were checked by polarized light microscope (POM), con-
firming the hypothesis that fumigation-induced crystallization was
responsible for the emission enhancement. The untreated film
showed a uniform dark shade under POM observation, while the
3.4. Crystal structures and mechanism
Single crystals of 3c and 4c were grown from CH2Cl2/EtOAc and
CH2Cl2/n-hexane, respectively, and were analyzed by X-ray diffrac-
tion crystallography. Their crystal structures are shown in Fig. 5, and
detailed crystal data are given (see Supporting Information in
Table S1).
In the crystal structure of 4c (Fig. 5), anthracenone groups are
strongly distorted (the dihedral angle between two phenyls of the
anthracenone group is 26.4ꢀ) due to the steric hindrance between H1
ꢀ
and H2 (d ¼ 2.036 A). The dihedral angle between the anthracenone
Fig. 4. Relations between the ratio of I/I0 and water fraction in H2O/THF mixtures. I0
and I were fluorescence peak intensity of 2c, 3c and 5 in pure THF and in H2O/THF
mixtures (10
mM), respectively; excitation wavelength: 485 nm for 2c, 510 nm for 3c,
540 nm for 5.

316
Y. Jin et al. / Dyes and Pigments 90 (2011) 311e318
Fig. 5. Single crystal structures of 3c and 4c.
groups and the middle benzene ring is 41.6ꢀ. The dihedral angle
interactions were not found in the dimer of 3c. These CHep
interactions may lead to relatively tight packing and rigid mole-
cules of 4c in the crystals [37].
Generally, pep aggregation in the solid state could quench the
emission of organic luminophores due to the formation of detri-
mental excimers [5,7,8,38]. That is to say, the degree of fluorescence
(
q
¼ 39.7ꢀ) between the DPP-ring and the benzene ring is nearly
equal to that of 3c (
q
¼ 40.2ꢀ). The angle between NeC7 bond and the
DPP-ring is 72.1ꢀ, while the angle between the C8eC7 bond and the
DPP-ring is 135.4ꢀ. These angles are larger than the corresponding
ones of 3c (28.0ꢀ and 100.5ꢀ, respectively), that could be attributed to
the effect of the anthracenone groups.
quenching would increase with the stronger
luminogenic molecules. As discussed above, it is clear that stronger
interactions exist among the molecules of 4c than among
pep stacking of the
As shown in Fig. 6a, the molecules of 3c pack into “zigzag motif”
in one layer. The stacking arrangement of 3c is shown in Fig. 6b. The
distance between the C]O double bond of aldehyde group and the
pep
those of 3c in crystals. From this point of view, compound 4c should
have a stronger fluorescence quenching behavior than 3c from the
solution to the solid state. However, the experimental results
showed that 4c exhibited significant enhancement of its light-
emission from the solution to the solid state while the emission
intensity of 3c decreased. What should be responsible for this
ꢀ
DPP-ring of neighboring molecule is 3.836 A, and the distance of
ꢀ
two phenyl rings is 3.852 A, indicating that the dimer has weak
pep interactions. In addition, two aromatic CeH/O hydrogen
ꢀ
bonds with distances of 2.443 A are observed in the dimer.
The arrangement of 4c in the crystal is significantly different
from that of 3c as shown in Fig. 7. Two layers of 4c are arranged in
a “V” shaped fashion while layers of 3c adopt a nearly linear
arrangement. The distance between the C]O double bond of
anthracenone groups and the DPP-ring of neighboring molecule is
phenomenon? Besides
exist among the molecules of 3c and 4c in crystals such as hydrogen
bond interactions and CHe interactions, which would inhibit
pep interactions, other interactions also
p
intramolecular rotation. Although two aromatic CeH/O hydrogen
bond interactions exist in the dimer of 3c, they are too weak to hold
the molecules of 3c tightly in crystal lattice [39]. In other words,
intramolecular rotation might still cause the non-radiative decay of
ꢀ
3.426 A (Fig. 7c), while the distance of two anthracenone groups is
ꢀ
3.723 A (Fig. 7d). These
p
ep
interaction distances are shorter than
those in 3c. Therefore, the molecules of 4c should have stronger
interactions than those of 3c. This could be used to explain the
pe
p
excited state of 3c in the solid state [9]. In contrast, many CHep
phenomenon that the angle between NeC7 bond and the DPP-ring,
and the angle between the C8eC7 bond and the DPP-ring of 4c are
interactions exist among the molecules of 4c in crystals, which
could block intramolecular rotation and enhance the radiative
decay [9]. The restriction of intramolecular rotation (RIR) could
larger than the corresponding angles of 3c. Moreover four CHe
p
interactions with interaction distances of 3.166, 3.166, 2.943 and
2.943 A exist in the dimers of 4c (Fig. 7c and d), but such
increase emission intensity while
fluorescence quenching. The results of the competition between
pep aggregation could induce
ꢀ
Fig. 6. (a) Stacking images of 3c, a-axis view. The hydrogen atoms have been omitted for clarity. (b) The intermolecular interactions of 3c in the crystals.

Y. Jin et al. / Dyes and Pigments 90 (2011) 311e318
317
Fig. 7. (a) Stacking images of 4c, b-axis view. (b) A tetramer unit of 4c, a*-axis view. The hydrogen atoms have been omitted for clarity. (ced) The intermolecular interactions of 4c in
the crystals.
the RIR and
p
e
p
aggregation were different for 3c and 4c. The
solution. The fluorescence intensities of 2ae2c, 3ae3c and 5
decreased while fluorescence intensities of 4ae4c increased. The
quantum yields of luminescent molecules in the solid state were
controlled by the competition between RIR causing emission
fluorescence intensity of 3c was remarkably decreased with the
increased accumulation of its molecules, while the emission
intensity of 4c was augmented with the formation of molecular
aggregation. Moreover, the degree of the conformational stiffening
caused by the restricted intramolecular rotation in the crystalline
state of 4c was higher than that in amorphous particles. Therefore
4c exhibited CIEE [10].
enhancement and
p
ep
aggregation inducing fluorescence
quenching. The conformational stiffening caused by the restricted
intramolecular rotation in the crystals of 4 was higher than that in
the amorphous particles, which depicted that 4 possessed CIEE.
These results are important in molecular design to gain expected
fluorescence properties in solution and solid state. Furthermore,
vapor-responsiveness of the PL of 4 could enable it to find appli-
cation in detecting VOCs.
As for 4b,
pep aggregation between anthracenone groups and
the emission group (DPP-ring) should be stronger than that of 4a
and 4c, due to the weak steric hindrance of butyl group. Therefore,
its AIEE effect is weak.
The analysis of the crystal structures of 3c and 4c demonstrates
that supramolecular interactions affect the degree of torsion and
stacking feature of the molecules. The variation of the p-conjugated
Acknowledgment
units and steric hindrance of substituents linked to the DPP-ring
could affect the packing model and photophysical properties in the
solid states.
This work was supported by the National Natural Science
Foundation of China (20872038, 20904010), the Fundamental
Research Funds for the Central Universities (2009ZM0170) and the
Natural Science Foundation of Guangdong Province, China
(10351064101000000).
4. Conclusion
A series of DPP derivatives were synthesized and their fluores-
cence properties in solution and solid state were investigated. PL
spectra and quantum yields of the chromophores were found to be
Appendix. Supporting information
Supplementary data related to this article can be found online at
doi:10.1016/j.dyepig.2011.01.005.
tunable by changing the
p
-conjugated units and alkyl groups linked
system
to the DPP-ring. In solution, the extended conjugated
p
could make a bathochromic shift in the emission spectrum,
meanwhile, the PL efficiency of the chromophore would decrease
due to carbonyl groups and intramolecular rotation processes
inducing annihilation of excitations; the quantum yields of the
luminescent moieties would decrease with the increased rotation
motion of alkyl substituents. However, the fluorescence quantum
yields of DPP derivatives in the crystals were different from those in
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