Functionalized Salen ligands linking with non-conjugated bridges: unique and colorful aggregation-induced emission, mechanism, and applications

Article abstract of DOI:10.1039/c5tc02555a

A series of novel, simple, and colorful Salen ligands (56 samples), salicylaldehyde-based bis-Schiff bases, linking with different non-conjugated alkyl bridges ((CH₂)_n, n = 2-9, 12; cyclohexyl) and containing different electron-accepting (-NO₂, -F, and -Cl), electron-donating (-OMe, -OH, and -NEt₂), or sterically hindering (-t-butyl) substituents or a π-extended system (naphthalene ring) have been designed and synthesized. The photophysical properties of these Salen ligands can be well-tuned by the introduction of side functional substituents, π-extended systems, and central N-alkyl chain bridges. It is unusual that they contain a small π-conjugated system but display strong blue, green, and red aggregation-induced emission (AIE) with large Stokes shifts (up to 162 nm) and high fluorescence quantum yields (up to 0.44 and 0.75 in water and in solid, respectively). Combining with their advantages of AIE and good stability and biocompatibility, the Salen ligands can be potentially used in mechanofluorochromism (crystal-defect-induced emission) and living cell imaging. Moreover, the inherent relationships between their chemical structures and AIE properties are studied, which provide unequivocal insights for the design of AIE-active dyes.

Full text of DOI:10.1039/c5tc02555a

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Functionalized Salen ligands linking with non-
conjugated bridges: unique and colorful aggregation-
induced emission, mechanism, and applications†
Cite this: DOI: 10.1039/x0xx00000x
Jinghui Cheng,‡a Yuanxi Li,‡b Rui Sun,a Jiaoyan Liu,a Fei Gou,a Xiangge Zhou,a
Haifeng Xiang,*a and Jin Liu*b
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
A series of novel, simple, and colorful Salen ligands (56 samples), salicylaldehydeꢀbased bis
ꢀ
Schiff bases, linking with different nonꢀconjugated alkyl bridges ((CH2)n, n = 2
cyclohexyl) and containing different electronꢀaccepting ( NO2, F, and Cl), electronꢀ
donating ( OMe, OH, and NEt2), or steric hindrance ( ꢀbutyl) substituents or ꢀextended
system (naphthalene ring) have been designed and synthesized. The photophysical properties
−9, 12;
www.rsc.org/
−
−
−
−
−
−
−
t
π
of these Salen ligands can be wellꢀtuned by the introduction of side functional substituents,
extended system, and central alkyl chain bridges. It is unusual that they contain a small
π
π
ꢀ
ꢀ
N
ꢀ
conjugated system but display strong blue, green, and red aggregationꢀinduced emission (AIE)
with large Stokes shifts (up to 162 nm) and high fluorescence quantum yields (up to 0.44 and
0.75 in water and solid, respectively). Combining with their advantages of AIE and good
stability and biocompatibility, the Salen ligands can be potentially used in
mechanofluorochromism (crystalꢀdefectꢀinduced emission) and living cell imaging. Moreover,
the inherent relationships between their chemical structures and AIE properties are studied,
which provide unequivocal insights for the design of AIEꢀactive dyes.
luminophores are in solid state and might have a more severe ACQ
effect. Tang et. al. recently discovered an exactly opposite
Introduction
The design and synthesis of organic luminescent (fluorescent or
phosphorescent) materials1 have attracted great interests in a variety
of scientific communities due to their wide and useful applications,
such as organic lightꢀemitting diodes (OLEDs),2 lightꢀemitting
electrochemical cells (LECs),3 triplet–triplet annihilation based
upconversion,4 fluorescence probes,5 therapy,6 and bioꢀimaging.7 In
general, a typical organic luminescent material is a polycyclic
aromatic molecule with one plane of big πꢀconjugated system, which
would be favorable to the larger wavelength of absorption (λabs) and
emission (λem) bands with the stronger absorption intensity and
higher luminescence quantum yield (Ф). However, it is notorious
that luminescence is often weakened or quenched at high
concentrations or in solid state, due to the aggregationꢀcaused
quenching (ACQ) effect8 that is usually arisen from the
intermolecular π−π stacking interactions of πꢀconjugated plane
molecules. In fact, the ACQ effect is usually harmful for some
practical applications. In the case of sensing applications, the
preferred work conditions are aqueous media, but it is incongruous
that most of organic luminescent probes containing hydrophobic
polycyclic aromatics have a poor solubility in water, which might
cause aggregation in water and subsequently lead to luminescence
quenching by the ACQ effect. Moreover, the ACQ effect is more
harmful and challenging for OLED and LEC applications, where the
phenomenon of aggregationꢀinduced emission (AIE).9 Since then
AIEꢀactive materials have been becoming one of the most active and
hottest research areas in material field.10
N,N′ꢀBis(salicylidene)ethylenediamine (Salen), a particular class
of tetradentate chelating bisꢀSchiff base (Fig. 1), can be synthesized
by the condensation of diamine with
2
equivalents of
salicylaldehyde. Because of their facile preparation, good stabilities,
biological activities, and rich photophysical properties, Salen ligands
and their associated metal complexes have attracted much attention
in many fields, including catalysts,11 DNA cleavage,12 optical and
magnetic materials,13 supramolecular materials,14 cell imaging,15 and
OLEDs.16 Our research work is focusing on the synthesis,
photophysical properties, and sensing applications of Salen
ligands.17 Like a typical organic luminescent material, most of these
Salen ligands containing one big plane of πꢀconjugated system
usually exhibit much stronger fluorescence in dilute organic solution
than in solid state or water, indicating the possible existence of
ACQ. For example, one maleonitrileꢀbased Salen ligand (CN) (Fig.
1) has one big plane of πꢀconjugated system, resulting in the strong
fluorescence with an unexpectedly high quantum yield of 0.80 in
dilute MeCN but no fluorescence in water or solid state.17c
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are rotatable N
because the existence of intramolecular hydrogen bonds
restricts the rotations of C C single bonds (Fig. 1). Plane SA
molecules are cross stacking with a short interplanar distance
) of 3.45 Å and weak intermolecular faceꢀtoꢀface π−π
interactions, which ensures to eliminate the rotations of N
−N single bonds rather than C−C single bonds,
−
(
d
~
−
N
single bonds and results in AIE thereby.20d Moreover, the recent
works also demonstrated that the presence of side alkyl chains at the
AIEꢀactive core would affect the mechanochromic properties.21 Is it
possible to achieve and improve AIE properties of Salen
ligands if more rotations of C−C single bonds (alkyl chains) are
introduced to act as the central linker (Fig. 1)? To the best of our
knowledge, however, in the literature there is still a scarcity of
reports on such AIEꢀactive materials, because they linking with
nonꢀconjugated alkyl bridges have a very small πꢀconjugated system
and are taken for granted as nonꢀemissive materials hereby.
Especially, the effects of the length of the central alkyl chain on AIE
properties have been still unclear yet. The detailed study on the
central alkyl bridges in cooperation with other side functional
substituents should be required to clarify the intrinsic AIE
properties. Herein, we report the unique AIE properties,
mechanofluorochromism, and cell imaging of Salen ligands (56
samples), which contain different nonꢀconjugated alkyl bridges
((CH2)n, n = 2−9, 12; cyclohexyl) and different electronꢀaccepting
(−NO2, −F, and −Cl), electronꢀdonating (−OMe, −OH, and −NEt2),
or steric hindrance (−tꢀbutyl) substituents or πꢀextended system
(naphthalene ring). Although these Salen ligands have a small πꢀ
conjugated system, they display strong blue, green, and red AIE with
large Stokes shifts (up to 162 nm) and high fluorescence quantum
yields up to 0.44 and 0.75 in water and solid, respectively.
Fig. 1 Chemical structures of AIEꢀactive silole, TPE, and SA and ACQꢀactive
Salen ligand of CN.
Up to now, most of AIEꢀactive materials have nonꢀplane AIE
cores, such as silole10,18 or tetraphenylethene (TPE) (Fig. 1) 9,19
In
,
dilute organic solution, their phenyl groups rotor undergo dynamic
intramolecular rotations (IRs), which provide a possible way to nonꢀ
radiatively annihilate its excited states and result in the absence of
luminescence. In the aggregates, however, these IRs are greatly
restricted owing to the physical constraint. In addition, these nonꢀ
plane molecules with propeller shape cannot be packed through
intermolecular faceꢀtoꢀface π−π interactions (Fig. 1). This
restriction of both IRs and π−π interactions blocks the nonꢀradiative
pathway and opens up the radiative channel. On the other hand, the
previous works of our group and other groups revealed that
salicylaldehyde azine (SA) (Fig. 1) molecules exhibit the nature of
AIE,20 even though they have a plane structure and similar synthetic
process with Salen ligands. Unlike TPE and silole, the IRs of SA
.
Fig. 2 Chemical structure of organic dyes studied in this work.
2 | J. Name., 2012, 00, 1ꢀ3
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effect on their absorption. The lower energy absorption band of
C4 λabs = 316 nm) is reproduced well using gasꢀphase timeꢀ
dependentꢀDFT (TDꢀDFT) calculations, which predict one
(
Results and discussion
Synthesis and characterization
absorption maximum at 311 nm (Fig. S1 in Supporting
information). The lower energy absorption (S0
corresponds to the highest occupied molecular orbital (HOMO)
lowest unoccupied molecular orbital (LUMO) and
HOMO LUMO+1. The energy level diagram of C4 with
orbital isosurfaces is also given in Fig. S2. Based on these
frontier molecular orbitals and absorption spectra, the lower
energy absorption can mainly be assigned to the π → π*
transition involving molecular orbitals essentially localized on
the C=N group and the phenol ring17a,c but little contribution
from nonꢀconjugated alkyl bridges. Moreover, the presence of
→ S1) mainly
Compared with other wellꢀknown robust and expensive
tetradentate porphyrin ligands,22 inexpensive Salen ligands are
facile to prepare and tune fluorescence bands.17c The general
method of preparation of Schiff bases is quite straightforward
and consists of the condensation reaction of primary diamines
with an aldehyde precursor usually in alcoholic solution or
toluene and sometimes under reflux conditions. The reaction is
acid catalyzed but catalysts are not generally required when
aliphatic amines are involved. In order to achieve high yield,
the water produced in the reversible reaction can be removed by
azeotropic distillation, when conducting the synthesis in toluene
solution. Finally, the condensation of primary diamine with 2
equivalents of salicylaldehyde precursor in ethanol under
refluxing condition was adopted for the preparation of Salen
ligands in this work. Most of Salen ligands have a good
solubility in organic solvent, such as CH2Cl2, CH3CN, EtOH,
and DMF, and bad solubility in hexane and petroleum ether,
especially bad solubility in water. Usually, in order to avoid the
degradation of Schiff bases during the purification step through
hydrolysis, it is better to purify Schiff bases by reꢀ
crystallization (e.g. CH2Cl2/hexane) rather than running
chromatography even though most of Salen ligands are stable
enough for running chromatography. Most of Salen ligands can
grow goodꢀquality single crystals by the method of slow
solvent diffusion/evaporation (CH2Cl2/hexane). Unlike many
free Schiff bases that are not always stable in solution and need
to prepare by template synthesis in the presence of a metal ion,
all Salen ligands either in solution or in solid state are stable
within several months under air. For the purpose of the
→
−1 →
steric hindrance
electronꢀaccepting
λabs = 322 nm) substituents at C2
effect on absorption spectra. However, if
−
−
t
ꢀbutyl (3-
F (3-F-C2
t
,
-Bu-C2
abs = 316 nm), or
λabs = 318 nm) have little
ꢀextended system
λabs = 420 nm), strong electronꢀdonating OMe (3-
abs = 332 nm; 4-OMe-C2 abs = 392 nm; 5-OMe-C2
NEt2 (4-NEt2-C2 λabs = 336 nm), or strong
electronꢀaccepting −ΝO2 (3-NO2-C2 λabs = 442 nm; 3,5-NO2-
C2 λabs = 378 nm) substituents are introduced, obvious red
,
λabs = 323 nm), weak
λ
−
Cl (3-Cl-C2,
(
π
(
Naph-C2
,
−
OMe-C2
,
λ
,
λ
,
λabs = 346 nm),
−
,
,
,
shifts in absorption spectra would be observed (Fig. 3b and
Table 1).
(a)
3x104
2x104
1x104
C
C
C
C
C
C
C
C
C
2
5
8
3
6
9
4
7
12
Cy
mechanism evaluation, the reference substances, N,N
′
ꢀ
ꢀ
0
dibenzylideneethaneꢀ1,2ꢀdiamine
dibenzylideneethaneꢀ1,8ꢀdiamine
((ethylimino)methyl)phenol
(
Ref1),
Ref2),
and
N,N′
2ꢀ
(E)ꢀ2ꢀ
(b)
3x104
(
3-t-Bu-C2
3-F-C2
3-Cl-C2
3-NO2-C2
Naph-C2
(
Ref3),
((octylimino)methyl)phenol (Ref4) (Fig. 2) were prepared as
well.
2x104
3-OMe-C2
1x104
Photophysical and AIE properties
The roomꢀtemperature optical properties of the synthesized
Salen ligands were investigated by UV/visible absorption and
fluorescence emission spectra and are listed in Table 1. As
0
250
300
350
400
450
500
examples, the absorption spectra of C2−C9, C12 and Cy in pure
Wavelength / nm
3
organic solvent of CH2Cl2 (1 × 10−5 mol dm− ) are given in Fig.
3a. They containing the same phenol and different alkyl bridge
have similar absorption intensity and bands (λabs: 314−318 nm),
Fig. 3 Absorption spectra of some selected Salen ligands in CH2Cl2.
indicating that the nonꢀconjugated alkyl bridges have little
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As expected, most of the Salen ligands have weak This aggregation is further confirmed by the fact of its redꢀ
fluorescence in pure organic solvent of CH2Cl2 or MeCN shifted absorption spectra (Fig. S3). When the volume fraction
(Table 1). For example, the emission spectra of C2 are depicted
in Fig. 4. In the dilute organic solution of MeCN, the IRs of maximum (
central ethane bridge in C2 provide a possible way to nonꢀ Moreover, C2 in solid state emits strong blueꢀgreen emission (
radiatively annihilate its excited states, resulting in the absence 0.030) under 360 nm UV lamp (Fig. 5). All the
(
f) of water is increased to 94 %, its emission attains the
Φ
= 0.025) with a large Stokes shift of 119 nm.
Φ
=
of luminescence. Nonetheless, when water is added into MeCN, aforementioned findings confirm the fact of its AIE nature.
its emission is enhanced due to the formation of aggregation.
Table 1 Photophysical data of the Salen ligands and complexes at room temperature. Sample without solidꢀstate emission data is nonemissive in solid state.
Salen ligand
medium
CH2Cl2
Water
Solid
λ
abs /nm (ε /dm3 mol−1 cm−1)
λ
em /nm
369
497
500
500
577
588
441
440
504
507
370
500
555
432
466
473
369
368
369
369
408
539
406
552
369
445
504
513
458
478
369
499
500
499
558
490
556
560
364
499
508
434
501
Stokes shift /nm
119
Φ
f /%
C2
258(3.1×104); 318(1.23×104)
0.0072
0.025
0.030
0.071
0.036
0.037
0.026
0.003
0.24
314; 378
94
3-NO2-C2
CH2Cl2
Water
Solid
344(1.2×104); 442(4.7×103)
430
137
98
3,5-NO2-C2
3-F-C2
CH2Cl2
CH2Cl2
Water
Solid
378(6.3×103)
256(2.93×104); 316(7.0×103); 404(5.0×102)
318; 380
124
112
80
96
0.38
3-Cl-C2
CH2Cl2
Water
Solid
260(3.22×104); 322(1.05×104); 422(2.0×102)
318; 388
0.0092
0.11
0.12
4-NEt2-C2
Naph-C2
CH2Cl2
CH2Cl2
Solid
342(7.88×104)
0.070
0.092
0.086
0.065
0.063
0.044
0.0076
0.0027
0.09
306(1.62×104); 364(8.3×103); 420(8.9×103); 424(8.8×103)
3-OMe-C2
4-OMe-C2
5-OMe-C2
3-tBu-C2
4-OH-C2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Solid
264(3.04×104); 332(7.2×103)
276(3.73×104); 308(2.63×104); 392(2.3×103)
260(1.35×104); 346(8.6×103)
262(3.29×104); 324(1.24×104)
275(6.1×104); 310(4.55×104); 385(2.1×103)
3-OH-C2
CH2Cl2
Solid
310(3.3×104); 350(2.0×103); 430(3.3×104)
0.0013
0.011
0.0066
0.0030
0.12
C3
CH2Cl2
CH2Cl2
Water
Solid
256(3.32×104); 316(1.31×104)
258(3.75×104); 320(8.3×103); 408(1.0×103)
268; 380
3-F-C3
124
117
80
92
0.13
Naph-C3
C4
CH2Cl2
Solid
308(1.29×104); 360(5.6×103); 402((7.6×103); 4204(7.7×103)
0.077
0.080
0.011
0.13
CH2Cl2
Water
Solid
254(2.61×104); 316(9.5×103)
314; 382
0.19
3-NO2-C4
5-NO2-C4
CH2Cl2
Solid
346(7.3×103); 438(9.6×103)
348(3.3×104); 404(3.46×104)
0.0065
0.082
0.0055
0.052
0.020
0.0039
0.11
CH2Cl2
Solid
3,5-NO2-C4
3-F-C4
CH2Cl2
CH2Cl2
Water
Solid
396(1.8×103)
256(2.95×104); 312(8.6×103); 410(2.6×103)
314; 380
119
109
98
70
0.19
3-Cl-C4
CH2Cl2
Water
254(1.39×104); 320(3.7×103); 410(8.0×102)
276; 392
0.0094
0.17
4 | J. Name., 2012, 00, 1ꢀ3
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Solid
CH2Cl2
Water
Solid
524
356
536
542
425
470
471
410
514
521
488
457
489
410
409
491
404
553
369
496
498
445
500
504
457
489
369
496
495
445
498
506
446
503
522
498
542
457
476
409
357
443
501
518
457
483
364
497
498
498
550
563
445
498
509
484
0.11
3-OMe-C4
4-OMe-C4
5-OMe-C4
264(2.77×104); 326(6.2×103) ; 420(6.0×102)
0.0038
288; 408
128
100
88
0.091
0.11
90
92
94
CH2Cl2
Water
Solid
276(8.79×104); 302(6.06×104) ; 388(8.1×103)
292; 370
0.0035
0.11
0.12
CH2Cl2
Water
Solid
230(1.06×105); 258(3.91×104) ; 342(2.45×104)
334; 426
0.0034
0.14
0.17
4-Net2-C4
Naph-C4
CH2Cl2
CH2Cl2
Solid
336(5.91×104)
0.097
0.089
0.091
0.0066
0.0024
0.097
0.0017
0.025
0.0086
0.076
0.073
0.0027
0.14
308(3.22×104); 362(1.18×103); 402(2.13×103); 422(2.18×103)
3-tBu-C4
4-OH-C4
CH2Cl2
CH2Cl2
Solid
262(6.55×104); 324(2.44×104)
275(7.22×104); 305(5.15×104); 385(4.1×103)
3-OH-C4
C5
CH2Cl2
Solid
315(1.7×104); 345(1.7×104); 430(2.2×104)
CH2Cl2
Water
Solid
254(3.14×104); 314(1.2×104)
314; 386
110
114
3-F-C5
CH2Cl2
Water
Solid
254(3.38×104); 316(7.7×103); 412(1.9×103)
270; 386
50
0.18
Naph-C5
C6
CH2Cl2
Solid
306(2.49×104); 362(8.6×103); 402(1.78×103); 422(1.82×103)
0.038
0.051
0.0069
0.095
0.11
CH2Cl2
Water
Solid
256(3.39×104); 316(1.33×104)
314; 384
112
110
109
80
70
80
3-F-C6
CH2Cl2
Water
Solid
254(3.37×104); 320(7.1×103); 410(1.8×103)
272; 388
0.0053
0.12
0.41
3-Cl-C6
CH2Cl2
Water
Solid
258(2.97×104); 320(9.6×103); 416(2.7×103)
274; 394
0.0083
0.34
0.36
3-NO2-C6
Naph-C6
CH2Cl2
Solid
350(7.6×103); 442(1.87×104)
0.0047
0.034
0.078
0.086
0.0023
0.006
0.0012
0.11
CH2Cl2
Solid
308(2.58×104); 360(8.6×103); 402(1.87×103); 422(1.9×103)
4-OH-C6
C7
CH2Cl2
CH2Cl2
CH2Cl2
Water
Solid
275(5.68×104); 305(4.13×104); 385(4.5×103)
256(3.21×104); 314(1.13×104)
254(3.48×104); 316(7.2×103); 410(1.8×103)
268; 380
3-F-C7
121
70
0.11
Naph-C7
C8
CH2Cl2
Solid
306(2.42×104); 358(7.9×103); 402(1.82×103); 422(1.84×103)
0.069
0.067
0.0092
0.093
0.16
CH2Cl2
Water
Solid
254(3.46×104); 316(1.35×104)
314; 378
118
122
114
98
98
70
3-NO2-C8
3-F-C8
CH2Cl2
Water
Solid
344(8.5×103); 440(2.03×104)
428
0.0039
0.18
0.17
CH2Cl2
Water
Solid
252(2.98×104); 316(6.4×103); 410(1.6×103)
268; 384
0.005
0.12
0.12
3-Cl-C8
CH2Cl2
256(3.14×104); 320(9.6×103); 418(3.4×103)
0.0026
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Water
Solid
314; 388
500
523
457
483
403
365
498
500
445
499
508
457
486
390
498
500
445
501
514
434
502
519
457
484
409
458
112
0.12
0.11
98
Naph-C8
CH2Cl2
Solid
306(2.45×104); 362(7.9×103); 402(1.87×103); 422(1.91×103)
0.045
0.049
0.0021
0.0066
0.12
4-OH-C8
C9
CH2Cl2
CH2Cl2
Water
Solid
275(4.71×104); 305(3.58×104); 385(4.9×103)
256(3.31×104); 316(1.26×104)
312; 398
100
121
98
80
0.17
3-F-C9
CH2Cl2
Water
Solid
254(2.45×104); 320(5.4×103); 414(1.5×103)
268; 378
0.0011
0.13
0.11
Naph-C9
C12
CH2Cl2
Solid
306(2.59×104); 362(8.6×103); 402(1.97×103); 422(1.99×103)
0.037
0.035
0.0064
0.014
0.028
0.004
0.11
CH2Cl2
Water
Solid
258(1.85×104); 314(9.0×103)
268; 336
162
121
108
98
70
98
3-F-C12
3-Cl-C12
Naph-C12
CH2Cl2
Water
Solid
254(2.1×104); 314(4.6×103); 412(1.2×103)
268; 380
0.091
0.0045
0.44
CH2Cl2
Water
Solid
258(3.99×104); 322(1.26×104); 420(2.0×103)
276; 394
0.75
CH2Cl2
Solid
306(2.41×104); 360(7.6×103); 402(1.85×103); 422(1.87×103)
0.037
0.044
0.0022
0.067
4-OH-C12
Cy
CH2Cl2
CH2Cl2
275(3.77×104); 305(3.66×104); 385(7.6×103)
254(2.19×104); 316(8.3×103)
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
92%
94%
96%
98%
Fig. 5 Photographs (a, c: under room light; b, d: under 360 nm UV light) of some
selected Salen ligands in solid state (a, b; C6 and C9 are in crystal) and casting
films (c, d; CH2Cl2 solvent).
400
450
500
550
600
650
Wavelength / nm
Fig. 4 Emission spectra and photographs (under 360 nm UV light) of C2 in
The aggregated Cn (n = 2, 4−6, 8, 9, 12), 3-F-Cn (n = 2−9, 12), 3-
Cl-Cn (n = 2, 4, 6, 8, 12), 3-NO2-Cn (n = 2, 4, 6, 8), 3-OH-Cn (n = 2,
4), 4-OH-Cn (n = 2, 4, 6, 8, 12), 5-NO2-C4, 3-OMe-C4, 4-OMe-C4,
5-OMe-C4, and Naph-Cn (n = 2−9, 12) have strong emission in
solid state or water, but other aggregated Salen ligands are nonꢀ
emissive (Table 1). Most of AIEꢀactive Salen ligands have similar
emission spectra in solid state and water, but solid samples exhibit
much stronger emission (Fig. 5 and Table 1), and thus the discussion
will be focused on the AIE characteristics of solid samples. Since
MeCN H2O with different
−
f
values (1 × 10−5 dm3 mol−1).
t
he molecule arrangements play a key role in AIE, it’s not a
strange phenomenon that different solid forms of AIEꢀactive
materials have different fluorescence properties. For most of AIEꢀ
6 | J. Name., 2012, 00, 1ꢀ3
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active Salen ligands, however, the different solid forms, such as quantum yield up to 0.44 and 0.75 in water and solid,
crystal, powder, and casting film, have similar emission, except that respectively. Such chlorination was also found to be an efficient
C6 and C9 in powder and casting film exhibit much stronger way to improve fluorescence quantum yield in our previous
fluorescence than in crystal (see the later discussion).
The length effect of the central alkyl chain bridges in AIEꢀ
work.17a,20d
Nꢀ
active Salen ligands are explored firstly. As depicted in Fig. 6,
like their absorption spectra, C2, C4, C5, C6, C8, C9, and C12 in
solid state have similar emission spectra (λem: 495−500 nm),
indicating that the nonꢀconjugated alkyl bridges have little effect on
the emission bands of this serial of Salen ligands. However, if
different side functional substituents are introduced to C2, different
substituents have different effects on the emission bands. Compared
with C2 (λem = 500 nm), Naph-C2 (λem = 473 nm) with a πꢀextended
system shows blueꢀshifted emission, in contrast, 3-F-C2 (λem = 513
nm), 3-Cl-C2, (λem = 555 nm), and 3-NO2-C2, (λem = 588 nm)
containing electronꢀaccepting substituents display redꢀshifted
emission (Fig. 8). This substituent effect is a simple and useful tool
to achieve red, green, and blue (RGB) emission (Fig. 5 and 7), which
was observed in SAꢀbased AIE materials as well.20
C2
Naph-C2
3-F-C2
3-Cl-C2
3-NO2-C2
450
500
550
600
650
700
Wavelength / nm
Fig.
7 Normalized emission spectra of C2 with different side functional
substituents in solid state.
MeCNꢀwater
3-NO -C
2
2
C2
C4
C5
3-NO -C
2
4
6
8
3-NO -C
2
3-NO -C
2
C6
Solid
C8
C9
C12
400
450
500
550
600
650
500
550
600
650
700
Wavelength / nm
Wavelength / nm
Fig. 8 Normalized emission spectra of 3-NO2-Cn in solid state.
4
1
Fig. 6 Normalized emission spectra of Cn in MeCN
−
H2O (1 × 10− dm3 mol−
,
f
values is listed in Table 1) and solid ( 6 and C9 in powder).
C
Mechanism of AIE
All fluorescence quantum yields (Table 1) of the aggregated
Salen ligands in water and solid state were measured by the
optical dilute method of Demas and Crosby23 with a standard of
It is interesting that, with similar chemical structures, some of
the aggregated Salen ligands are highly emissive, but others are
nonꢀemissive. The molecule arrangements play a key role in
AIE. For AIEꢀactive materials, slight modification of the
chemical structures could alter the molecule arrangements and
AIE properties significantly. Nonꢀcovalent molecular
interactions are frequently involved in the crystallization
process to assist the molecules to orient and assemble. In order
to achieve a high fluorescence quantum yield, AIEꢀactive dyes
should stack closely with a short interplanar distance (for plane
molecules, such as SA20d) or intermolecular aromatic HAr⋅⋅⋅π
hydrogen bonds (for nonꢀplane molecules, such as TPE or
silole10) and weak intermolecular faceꢀtoꢀface π−π interactions
The former can ensure to eliminate the molecular rotation, the
later would prevent the formation of excimer.10 For our case,
however, the Salen ligands have plane
iminomethylphenol and nonꢀplane alkyl chain bridges. The
3
quinine sulfate (Фr = 0.55, quinine in 0.05 mol dm− sulfuric
acid) and an integrating sphere, respectively. Although the
Salen ligands linking with nonꢀconjugated alkyl bridges have a
small
strong fluorescence, as shown in Table 1. In general, the
introduction of F and Cl would help to achieve AIE with a
high fluorescence quantum yield, but the introduction of bulky
πꢀconjugated system, most of them have unexpected
−
−
20d
hindrance substituents of
adversely affects. Since the central
have an obvious effect on fluorescence quantum yield,
OMe, and naphthalene substituents might influence
fluorescence quantum yield positively or negatively. Except C5
−
t
ꢀbutyl and
ꢀalkyl chain bridges also
NO2,
−
NEt2
would bring
N
−
−
.
,
Salen ligands Cn linking with oddꢀcarbon chain bridges have
much weaker AIE than these linking with evenꢀcarbon chain
bridges. Especially, 3-Cl-C12 has a highest fluorescence
πꢀconjugated units of
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intramolecular and intermolecular interactions ensure to
eliminate the rotation of N C and C C single bonds and result
in the presence of AIE thereby.
restriction of C−C single bond rotations in the central alkyl chain
−
−
bridges is another key to achieve AIE, and thus other nonꢀcovalent
intermolecular interactions, such as N⋅⋅⋅H, O⋅⋅⋅H, and H⋅⋅⋅H,
should be considered.
Firstly, in order to investigate the length effect of the central
Nꢀalkyl chain bridges on molecule arrangements, the xꢀray
single crystals of C2 C3 C4 C5 C6, and C9 are depicted in Fig.
,
,
,
,
9. Unlike planeꢀlike SA molecules whose atoms are located in
one plane of πꢀconjugated system,20d stepꢀlike Cn molecules
have two
separated by the alkyl chain bridges (Fig. 9). It is an interesting
phenomenon that the two ꢀconjugated planes are almost
parallel for oddꢀcarbon chain bridges (C2 C4, and C6),
nonetheless they are almost vertical for evenꢀcarbon chain
bridges (C3 C5, and C9).
πꢀconjugated planes of iminomethylphenol which are
π
,
,
Fig. 10 Xꢀray single crystal structures and packing of C2 molecules (a: side view;
b: top view; d: intermolecular interactions of the two closest molecules).
Unlike C2 molecules, C3 molecules have a very complicated
and magic packing (Fig. 11a), even though they have similar
chemical structures. There are many strong intermolecular
interactions including HBz⋅⋅⋅π (Bz = benzyl) (2.878 Å) and
HAk⋅⋅⋅ (2.548 and 2.828 Å) interactions in the two closest C3
molecules (Fig. 11b) as well. On the other hand, C3 molecules
have strong faceꢀtoꢀface π−π interactions (Fig. 11d) with a
d
value of 3.409 Å (Fig. 11c), resulting in the absence of
fluorescence.
Fig. 9 Xꢀray single crystal structures of Cn
.
As expected, the intramolecular N⋅⋅⋅H hydrogen bonds (1.673
and 1.787 Å) between H atom in –OH and N atom are found in
C2. Crossꢀstacking C2 molecules exhibit an edgeꢀtoꢀface
herringbone packing motif (Fig. 10) without any efficient
intermolecular faceꢀtoꢀface π−π interactions (overlaps in
πꢀ
conjugated units, Fig. 10a), which is beneficial to prevent the
formation of excimer. On the other hand, many other strong
intermolecular interactions including HAr⋅⋅⋅π hydrogen bonds
(2.847 and 3.022 Å), and HOH⋅⋅⋅HAk (2.690 Å) (Ak = alkyl),
HAk⋅⋅⋅HAk (2.516 Å), and N⋅⋅⋅HAk (2.784 Å) interactions are
found in the two closest molecules (Fig. 10c). These above
8 | J. Name., 2012, 00, 1ꢀ3
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Fig. 12 Xꢀray single crystal structures and packing of
C4 molecules (a: top view; b:
side view; c: intermolecular interactions of the two closest molecules).
Unlike C4 molecules, C6 molecules have a cross lamellar
packing (Fig. S8). C4 molecules exhibit an edgeꢀtoꢀface
herringbone packing motif without efficient intermolecular
faceꢀtoꢀface π−π interactions, but C6 molecules show a slipped
faceꢀtoꢀface lamellar packing with moderate intermolecular
faceꢀtoꢀface π−π interactions (
Moreover, the intermolecular interactions
HAk⋅⋅⋅HAk and HAr⋅⋅⋅ Ar interactions, respectively, Fig. S8c) of two
d
=
3.413 Å, Fig. S8d and S8e).
(2.738 and 2.794 Å for
H
neighboring C6 molecules is a bit weaker than those of C4
molecules. And thus the single crystals of C6 are nonemissive.
Since the florescence properties of AIEꢀactive materials are
dependent on their molecule arrangements, they have been
found to show mechanochromism upon the transition of
crystalline and amorphous states under external stimuli.10 In
general, upon applying mechanical force, most of AIEꢀactive
materials that own mechanochromic properties exhibit a red
shift in emission bands with a decrease in fluorescence intensity.
Mechanochromic AIEꢀactive materials showing turnꢀon
fluorescence are still rare.10,24 Moreover, as the above
discussion, the previous works21 reveal that the presence of side
alkyl chains at the AIEꢀactive core would affect the
mechanochromic AIE properties, and thus we tried to examine
the mechanochromic AIE properties of these Salen ligands.
Most of the Salen ligands have no mechanochromic AIE
properties. However, it is fortunate to find that C6 is a
mechanochromic material showing turnꢀon fluorescence. As
shown in Fig. 13 and S9, when the pristine crystal solids of C6
are ground with a spatula, the resulted powders of C6 have
Fig. 11 Xꢀray single crystal structures and packing of C3 molecules (a: side view;
b: intermolecular interactions of the two closest molecules; c: side view of faceꢀ
toꢀface π−π interactions; d: top view of faceꢀtoꢀface π−π interactions).
With a longer length of alkyl chain, C4 molecules have a
uniform packing (Fig. 12a and 12b). Like C2 molecules, C4
molecules exhibit a similar edgeꢀtoꢀface herringbone packing
with little intermolecular faceꢀtoꢀface π−π interactions (Fig.
12b). Moreover, it should be noted that the intermolecular
HAk⋅⋅⋅HAk interaction (2.443 Å) of two neighboring C4
molecules is stronger than that of C2 molecules, which might be
one possible reason that the aggregated C4 molecules have a
higher fluorescence quantum yield (0.19). C5 molecules have
similar molecular packing and intermolecular interactions (Fig.
S7), resulting in the strong AIE like C4 molecules.
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strong green emission under the irradiation of UV light. The
same emission enhancement is also observed if CH2Cl2 solution
is added to the pristine crystal solids of C6 molecules and then
the added CH2Cl2 solution evaporates fast by reduced pressure
distillation. The powders of C9 show a similar fluorescence
enhancement like C6, but both the powders and crystals of C4
emit strong green fluorescence (Fig. 14).
of C6 molecules may be damaged by molecule movement, and
thus the fluorescence can be turned on. For the case of fast
evaporated solids, the solvent molecules of CH2Cl2 may invade
into the C6 crystal lattice and partially dissolve the crystal.
Simultaneously, there is not enough time for the dissolved
molecules to stack closely and regularly by π−π interactions
during fast CH2Cl2 evaporation, leading to the formation of
crystal defects at the etched area. Such crystalꢀdefectꢀinduced
emission was also observed in a diaminomaleonitrileꢀbased
Schiff base.24
simulated pattern
evaporated solids
Fig. 13 Photographs of C6 solids (a, b, e, f: crystals; c, d: ground solids; g, h: fast
evaporated solids) under sunlight or 360 nm UV light.
ground solids
pristine crystal solids
10
20
30
40
50
2θ / degree
Fig. 15 Xꢀray powder diffraction of
C6 solids.
Owing to the similar lamellar packing (Fig. S10), the single
crystals of C9 also show a very lower fluorescence quantum yield
like the single crystals of C6 (Fig. S8). It is worth noting that C9
molecules containing the second longest chain can selfꢀ
assemble to build very beautiful and wonderful molecular
constructions, such as batman (Fig. S10b) and luxuriant palace
lantern (Fig. S10c), and big single crystals (Fig. 14), which
might have some potential applications in molecular selfꢀ
assembly.
Next, the xꢀray single crystal structures of 3-OMe-C2 and 3-
F-C2 are discussed to investigate the substituent effect on
molecule arrangements. Our previous work20d demonstrated
that the presence of different electronꢀaccepting and electronꢀ
donating substituents at SA molecule is an efficient way to tune
molecular packing and AIE properties thereby. As shown in Fig.
S11a, 3-OMe-C2 molecules exhibit a brickꢀwall packing that
provides twoꢀdimensional percolation paths for strong faceꢀtoꢀ
Fig. 14 Photographs of C4 and C9 solids (the solids at the middle of the tubes are
powders, but they are crystals below the middle of the tubes) under sunlight or
360 nm UV light (the diameter of dime is about 2.0 cm).
To gain an insight into the mechanochromic behavior of C6
molecules, powder wideꢀangle Xꢀray diffraction experiments
were conducted on the pristine crystal, ground and evaporated
solids. As shown in Fig. 15, all the solid samples show intense
and sharp reflection peaks, which are in good agreement with
the simulated pattern of single crystal of C6. This indicates that
all the solid samples contain microcrystallites and accords with
the fact that C6 molecules containing a long chain bridge are
easy to crystallize (Fig. S9). Most of mechanochromic AIEꢀactive
materials process a phase transition from a crystalline to an
face π−π interactions
to the absence of AIE consequently.
However, if OMe substituents are taken placed by
ubstituents, 3-F-C2 molecules have a totally different packing
(d = 3.310 Å, Fig. S11b and S11c) and lead
−
−F
s
from 3-OMe-C2 molecules in spite of the fact that there is no
any strong halogen bond (Fig. 16). As shown in Fig. 16a and
16b, 3-F-C2 molecules exhibit a very uniform edgeꢀtoꢀface
herringbone packing like C2 molecules, resulting in the
formation of big single crystals (~2.0 cm, Fig. 16c)
conveniently. There are no faceꢀtoꢀface π−π interactions but many
other strong intermolecular interactions in two neighboring 3-F-
amorphous phase 10,21
however, for C6 solids, no such phase
,
transition is found. Therefore, the mechanochromic behavior of
C6 might be responsible for its lamellar packing (Fig. S8a) that is
in favor of molecule movement under the external mechanical
pressure. If an external mechanical pressure is applied on the
crystal, the regular intermolecular faceꢀtoꢀface π−π interactions
10 | J. Name., 2012, 00, 1ꢀ3
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DOI: 10.1039/C5TC02555A
Salen ligands are of greatest importance for AIE. BisꢀSchiff base
structure presumably enables strong intermolecular interactions
C2 molecules. The strongest HOH⋅⋅⋅HAk interaction of 3-F-C2
molecules is 2.465 Å, which is close to the HAk⋅⋅⋅HAk interaction
between πꢀconjugated iminomethylphenol groups in the two closest
(2.443
Å) of C4 molecules. And thus, the aggregated 3-F-C2
molecules (Fig. 10c, 12c, S7c, S8c, and 16d) to achieve AIE.
Through the intramolecular hydrogen bonds, the –OH groups can
efficiently prevent two phenolic rings from IRs and nonꢀradiative
annihilations. Additionally, these intramolecular hydrogen bonds
would lead to keto and enol tautomers through an ultrafast excitedꢀ
state intramolecularꢀproton transfer (ESIPT) process.20d The AIEꢀ
active dyes with ESIPT offer some additional advantages, such as a
large Stokes shift with a low background signal from the excitation
light source. These large Stokes shifts combining with the facts of
molecules have strong AIE like C4 molecules.
small πꢀconjugated systems but highly emissive solids enabled us to
wonder whether the emission of these Salen ligands originated from
fluorescence, because recently some reports25 demonstrated that
some pure organic compounds containing small
πꢀconjugated
systems show strong solidꢀstate phosphorescence at room
temperature. However, timeꢀresolved emission spectra (Fig. S13)
reveal that the emission decay lifetime of C4, 3-F-C2, and 3-Cl-C12
(powder) is 1.7, 3.3, and 5.5 ns, respectively. This confirms the fact
that their emission originates from fluorescence rather than
phosphorescence. The tendency in emission decay lifetime (C4
<
3-F-C2 3-Cl-C12) is consistent with that in fluorescence quantum
<
yield (Ф = 0.19, 0.38, and 0.75 for C4, 3-F-C2, and 3-Cl-C12,
respectively).
Living cell imaging
Fluorescent dyes are promising tools for turnꢀon, realꢀtime, onꢀ
site and nonꢀinvasive visualization of biological molecules and
processes in live cells and organisms. Furthermore, AIEꢀactive
dyes offer higher resistance to photobleaching and thus superior
photostability and higher signal reliability relative to
conventional dyes.26 For a useful fluorescent biological tracer,
it should neither inhibit nor promote the growth of living cells.
The HeLa cells were imaged by the Salen ligands using a
standard cellꢀstaining protocol. The living cells were incubated
with the Salen ligands for 2 h and then washed three times with
phosphate buffered saline (PBS) solution. As can be seen from
Fig. 16 Xꢀray single crystal structures and packing of 3-F-C2 molecules (a: side
view; b: side view; c: photographs of the single crystal under sunlight and under
360 nm UV light; d: intermolecular interactions of the two closest molecules; the
diameter of dime is about 2.0 cm).
Then, the xꢀray single crystal structures of Cy are studied to
investigate the different effects on molecule arrangements
Fig. 17, incubated with Naph-C2, C4, 3-F-C2, 3-F-C8, 3-Cl-C12,
and 3-NO2-C8, the HeLa cells grow similar as in the control
experiments in the absence of the dyes, indicating that the Salen
ligands exert less toxicity on the living cells. HeLa cells show
negligible background fluorescence (Fig. 17b). However, after
incubation with the Salen ligands, intense intracellular blue,
green, yellow, or red fluorescence with high signalꢀtoꢀnoise
between cycloalkane and
S12, unlike Cn molecules with
with a cyclohexane bridge are not stepꢀlike but Vꢀshaped. The
two ꢀconjugated planes of iminomethylphenol in one Cy
molecule ace to each other obliquely, indicting the possible
N
ꢀalkane bridges. As shown in Fig.
Nꢀalkane bridges, Cy molecules
π
f
existence of intramolecular π−π interactions. Moreover, the
cyclohexane bridges in the neighboring Vꢀshaped Cy molecules
ratio is observed (Fig. 17c−d). Some commercially available
fluorescence dyes, such as CellTracker Green, stain the entire
cells, and only the cytoplasmic regions of the cells are stained
by TPEꢀ or siloleꢀbased dyes.27 It is interesting that the Salen
ligands, especially for 3-Cl-C12 (Fig. 17f) and 3-NO2-C8 (Fig.
17g), mainly stain the nucleus of living HeLa cells. The
colorful AIEꢀactive Salen ligands may thus be used to probe or
monitor biologically important processes in vitro as well as in
vivo.
are separated without any strong intermolecular interactions (> 4
Therefore, the aggregated Cy molecules have no AIE.
Å).
Finally, for the purpose of the mechanism evaluation, the
photophysical properties of reference substances, Ref1 Ref2 Ref3,
,
,
and Ref4, were tested. Like the Salen ligands, Ref1 and Ref2 are
bisꢀSchiff bases, and thus they are solid at room temperature.
Conversely, monoꢀSchiff bases Ref3 and Ref4 are oil. All of them
(solid or oil samples) are nonemissive at room temperature,
indicating that both bisꢀSchiff base structure and –OH groups of
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and IR spectra. CCDC 1417706 (C4), 1417707 (3-F-C2), 1417708 (C6),
1417709 (C5), and 1417710 (C9). See DOI: 10.1039/b000000x/
‡ These authors contributed equally to this work.
1
2
B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley,
2002.
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L.S. Hung and C. H. Chen, Mater. Sci. Eng. R, 2002, 39, 143; (c) K.
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Xiang, J. H. Cheng, X. F. Ma , X. G. Zhou and J. Chruma, J. Chem. Soc.
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Fig. 17 Brightfield (a, incubated without dye) and fluorescence images (b,
incubated without dye; c, incubated with Naph-C2; d, incubated with
C4; e,
incubated with 3-F-C2; e, incubated with 3-F-C8; f, incubated with 3-Cl-C12; g,
incubated with 3-NO2-C8; h, incubated with 3-NO2-C2) of HeLa cells.
3
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Conclusion
We have systematically synthesized and studied the unique AIE
properties, mechanofluorochromism, and cell imaging
applications of a series of Salen Schiff Bases. The chemical
structures of these Salen ligands have a significant influence on
their molecule arrangements and consequently AIE properties.
Both weak intermolecular faceꢀtoꢀface π−π interactions and
strong nonꢀcovalent intermolecular interactions such as N⋅⋅⋅H,
5
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DOI: 10.1039/C5AN01398D.
O
⋅⋅⋅H, H⋅⋅⋅H, and H⋅⋅⋅π are the key factors to achieve AIE.
Although these Salen ligands linking with nonꢀconjugated alkyl
bridges have a small ꢀconjugated system, they show strong
red, green, and blue AIE. The fluorescence colors can be wellꢀ
tuned by the introduction of side functional substituents,
extended system, or central ꢀalkyl chain bridges. The
introduction of electronꢀaccepting substituents of NO2, F,
and Cl and ꢀextended system of naphthalene ring would help
π
π
ꢀ
N
−
−
−
π
to induce redꢀ and blueꢀshifted emission respectively.
Especially, combining with the help of side functional
substituents, the central nonꢀconjugated alkyl bridges have a
much obvious effect on tuning fluorescence bands. Moreover, it
has been revealed that chlorination and fluorination are an
efficient way to achieve high fluorescence quantum yield.
Therefore, we believe that these simple Salen ligands provide a
new paradigm in the design of AIEꢀactive dyes for developing
advanced organic optoelectronic devices, florescent probes, and
cell imaging, and so on.
6
7
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21172160, 21272161, and 21372169).
Notes and references
a College of Chemistry, Sichuan University, Chengdu, 610041, China. Fax:
(+86) 28-8541-2291. E-mail: xianghaifeng@scu.edu.cn.
b Laboratory of Stem Cell Biology, State Key Laboratory of Biotherapy,
West China Hospital, Sichuan University, Chengdu, 610064, China. E-
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Electronic Supplementary Information (ESI) available: General,
materials, computational details, crystallographic information files (CIF),
crystallographic data, absorption, emission data, SEM, DLS, TGA, DSC,
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J. Name., 2012, 00, 1ꢀ3 | 13

Journal of Materials Chemistry C
Page 14 of 14
View Article Online
DOI: 10.1039/C5TC02555A
Functionalized Salen ligands linking with non-conjugated
bridges: unique and colorful aggregation-induced emission,
mechanism, and applications
Jinghui Cheng, Yuanxi Li, Rui Sun, Jiaoyan Liu, Fei Gou, Xiangge Zhou, Haifeng
Xiang, and Jin Liu
Graphical abstract
A series of novel, simple, and useful Salen ligands (56 samples) linking with different
nonꢀconjugated alkyl bridges have a small πꢀconjugated system but exhibit strong red,
green, and blue AIE.