Chiral and non-conjugated fluorescent salen ligands: AIE, anion probes, chiral recognition of unprotected amino acids, and cell imaging applications

Article abstract of DOI:10.1039/c7ra08267c

Natural products are usually non-conjugated and chiral, but organic luminescent materials are commonly polycyclic aromatic molecules with extended π-conjugation. In the present work, we combine with the advantages of non-conjugation and chirality to prepare a series of novel and simple salen ligands (41 samples), which have a non-conjugated and chiral (S,S) and (R,R) cyclohexane or 1,2-diphenylethane bridge but display strong blue, green, and red aggregation-induced emission (AIE) with large Stokes shifts (up to 186 nm) and high fluorescence quantum yields (up to 0.35). Through hydrogen and halogen bonds, these flexible salen ligands can be used as universal anion probes and chiral receptors of unprotected amino acids (enantiomeric selectivity up to 0.11) with fluorescence quantum yields up to 0.29 and 0.27, respectively. Moreover, the effects of different chiral bridges on the molecule arrangement, AIE, and anion and chiral recognition properties are also explored, which provide unequivocal insights for the design of non-conjugated chiral and soft fluorescent materials.

Full text of DOI:10.1039/c7ra08267c

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PAPER
Chiral and non-conjugated fluorescent salen
ligands: AIE, anion probes, chiral recognition of
unprotected amino acids, and cell imaging
applications†
Cite this: RSC Adv., 2017, 7, 40640
Guangyu Shen, Fei Gou, Jinghui Cheng, Xiaohong Zhang, Xiangge Zhou
*
and Haifeng Xiang
Natural products are usually non-conjugated and chiral, but organic luminescent materials are commonly
polycyclic aromatic molecules with extended p-conjugation. In the present work, we combine with the
advantages of non-conjugation and chirality to prepare a series of novel and simple salen ligands
(41 samples), which have a non-conjugated and chiral (S,S) and (R,R) cyclohexane or 1,2-diphenylethane
bridge but display strong blue, green, and red aggregation-induced emission (AIE) with large Stokes shifts
(up to 186 nm) and high fluorescence quantum yields (up to 0.35). Through hydrogen and halogen
bonds, these flexible salen ligands can be used as universal anion probes and chiral receptors of
unprotected amino acids (enantiomeric selectivity up to 0.11) with fluorescence quantum yields up to
0.29 and 0.27, respectively. Moreover, the effects of different chiral bridges on the molecule
arrangement, AIE, and anion and chiral recognition properties are also explored, which provide
unequivocal insights for the design of non-conjugated chiral and soft fluorescent materials.
Received 27th July 2017
Accepted 12th August 2017
DOI: 10.1039/c7ra08267c
rsc.li/rsc-advances
Organic luminescent (uorescent or phosphorescent) mate-
rials6 are commonly polycyclic aromatic molecules with
Introduction
Chirality is a universal phenomenon throughout nature.1 Not
only many biologically active molecules, such as the naturally
occurring amino acids and sugars, are chiral, but also many key
chemical and biological processes are profoundly inuenced by
chiral structures. Therefore, chiral recognition2 through uo-
rescence method have attracted increasing attention due to its
unique advantages, including high sensitivity, good selectivity,
short response time, real-time and parallel monitoring, cell
imaging, and in situ and non-invasive measurement. Up to date,
a lot of different uorescent chiral receptors have been designed
and prepared for enantioselective recognition of chiral amines,
acids, or modied amino acids. In the literature, however, there
are only a few 1,10-bi-2-naphthol3 (BINOL)- and tetraphenyl-
ethylene4 (TPE)-based uorescent receptors for chiral recogni-
tion of unprotected amino acids, because zwitterionic amino
acids have not only a poor solubility in organic solvents but also
little interaction with organic receptors in water.5
extended p-electron conjugation, but these rigid materials
might suffer some disadvantages, such as the synthesis diffi-
culty, poor solubility, and uorescence aggregation-caused
quenching7 (ACQ). On the other hand, non-conjugated so
materials that might have a better solubility, lower cost, higher
exibility, lower cytotoxicity, better biocompatibility, and
naturally occurring,8 are usually taken for granted as non-
emissive materials. Until recently, some non-conjugated mate-
rials, such as 1,1,2,2-tetraphenylethane,9 poly[(maleic anhy-
dride)-alt-(vinyl acetate)],10 polyisobutene succinic anhydrides
and imides,11 and steroidal saponin digitonin,12 were reported
to show amazing ultraviolet (UV) or blue aggregation-induced
emission (AIE).9,13 Our research work continuously focus on
the synthesis, optical properties, and sensing applications of
N,N0-bis(salicylidene)ethylenediamine (salen), a particular class
of salicylaldehyde-based bis-Schiff base (Fig. 1), owing to its
facile preparation, good stability, and rich optical proper-
ties.7b,14 Our recently work demonstrated that step-like salen
ligands14g (R-Cn) and tripod-like tri-Schiff bases15 (TSBs, Fig. 1a)
linking with non-conjugated long alkyl bridges display strong
red-green-blue (RGB) AIE. If chiral (S,S) and (R,R) cyclohexane or
1,2-diphenylethane bridges are employed, it is easy to prepare
chiral salen ligands (Fig. 1b). These non-conjugated chiral
ligands are widely used as asymmetric catalysts,16 and they are
deemed to be non-emissive and used as turn-on uorescence
College of Chemistry, Sichuan University, Chengdu, 610041, China. E-mail:
xianghaifeng@scu.edu.cn; Fax: +86-28-8541-2291
† Electronic supplementary information (ESI) available: General, materials,
computational details, crystallographic information les (CIF), crystallographic
data, absorption, and uorescence emission data. CCDC 3-F-Cy (1551151),
3-F-(S,S)Cy (1496280), 3-F-(R,R)Cy (1496279), 3-F-diPh (1551150), and 3-Cl-diPh
(1551147). For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c7ra08267c
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Photophysical and AIE properties
The UV/visible absorption and uorescence data of all synthe-
sized Cys and diPhs at room-temperature are listed Tables S1,
S2, Fig. S1, and S2 (in ESI†). Since the photophysical and AIE
properties of (S,S), (R,R), and racemic salen ligands are similar,
only racemic salen ligands are discussed in this section. As we
expected, all salen ligands exhibit weak uorescence in pure
organic solvent of MeCN or DMSO (Tables S1–S3†). For
example, racemic 3,5-Cl-Cy shows very weak blue uorescence
(V ¼ 0.010) in the dilute MeCN, because the dynamic intra-
molecular rotations (IRs) of central cyclohexane bridge in 3,5-
Cl-Cy molecules provide a possible way to non-radiatively
annihilate its excited states and result in the absence of uo-
rescence consequently. However, if water is added into MeCN,
the green uorescence of 3,5-Cl-Cy is strengthened (Fig. 2),
because 3,5-Cl-Cy molecules cannot be dissolved in water,
which causes 3,5-Cl-Cy molecules to precipitate and aggregate.
At the same time, adding water would red shi its absorption
spectrum (Fig. S3†), which further conrms this aggregation. Its
green uorescence reaches the maximum (lem ¼ 511 nm, V ¼
0.064) with a large Stokes shi of 117 nm, when the volume
fraction (f) of water is increased to 90%. Furthermore, solid 3,5-
Cl-Cy displays strong green-yellow uorescence (lem ¼ 532 nm,
V ¼ 0.32) under 360 nm UV lamp (Fig. 2 and 3). All these
ndings reveal the fact of its AIE nature.
Fig.
1 Non-conjugated fluorescent salicylaldehyde-based Schiff
bases: (a) step-like R-Cn and tripod-like TSBs in our previous works; (b)
chiral V-type Cys and propeller-type diPhs in this work.
All the salen ligands have stronger uorescence in solid state
or water than in organic solvents (Tables S1 and S2†). Most solid
samples, such as crystals, powders, and casting lms (Fig. S4†),
probes for detecting metal ions.17 Moreover, non-conjugated exhibit not only red-shied but also much stronger uorescence
so materials might be used as anion probes, because they compared with water samples (Fig. 3, Tables S1, and S2†), and
are exible and might have multiple and strong intermolecular thus we will focus on their solid AIE characteristics. As shown in
interactions with anions through hydrogen bonds.15 Particu- Fig. 4, Tables S1, and S2,† for solid salen ligands, different
larly, the effects of different chiral bridges on the molecule substituents have different effects on the emission peaks
arrangement, AIE, and chiral recognition properties are still (lem: 474–565 nm) and colors (blue, green, and red). The pres-
unexplored. Herein, we demonstrate a novel and simple class of ence of a p-extended system (Naph-Cy, lem ¼ 486 nm) to the
colorful chiral salen ligands (41 samples, Fig. 1b) that are linked simplest Cy (lem ¼ 502 nm) leads to a blue shi in uorescence
by a non-conjugated cyclohexane (Cy) or 1,2-diphenylethane spectra. On the other hand, electron-donating –OMe (3-OMe-Cy,
(diPh) bridge but have strong AIE and potential applications in lem ¼ 523 nm; 5-OMe-Cy, lem ¼ 548 nm) or electron-accepting
cell imaging and recognition of anion and chiral unprotected –F (3-F-Cy, lem ¼ 509), –Cl (3-Cl-Cy, lem ¼ 520 nm; 3,5-Cl-Cy,
amino acids.
lem ¼ 532 nm), or –NO2 (3-NO2-Cy, lem ¼ 565 nm; 3,5-NO2,
lem ¼ 550 nm) substituents result in red-shied emission (Fig. 4
Results and discussion
Synthesis and characterization
All salen ligands were reasonably easy to synthesize by the
condensation of primary diamine with 2 equivalents of salicy-
laldehyde precursor in ethanol under reuxing condition.14
Most salen ligands have a bad solubility in petroleum ether,
hexane, and water but a good solubility in CH3CN, CH2Cl2,
DMF, and DMSO. All salen ligands either in solution or in solid
state are stable within several months under air. For most salen
ligands, good-quality single crystals can be obtained by the
method of slow solvent diffusion/evaporation (CH2Cl2/hexane).
For the purpose of comparison, the reference substance C2
(ref. 14g) (Fig. 1a) was prepared as well.
Fig. 2 Emission spectra and photographs (under 360 nm UV light) of
3,5-Cl-Cy in MeCN–H2O with different f values (1.0 ꢀ 10ꢁ5 mol dmꢁ3
)
and solid.
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measured by the optical dilute method of Demas and Crosby18
with a standard of quinine sulfate (Vr ¼ 0.55, quinine in
0.05 mol dmꢁ3 sulfuric acid) and an integrating sphere,
respectively. Although the salen ligands with a non-conjugated
linker have a small p-conjugated system, the uorescence
quantum yields of some salen ligands are unexpected high (up
to 0.35 and 0.22 for Cys and diPhs, respectively, Tables S1 and
S2†). In general, the introduction of –F (3-F-Cy, V ¼ 0.060; 3-F-
diPh, V ¼ 0.12), –Cl (3-Cl-Cy, V ¼ 0.14; 3,5-Cl-Cy, V ¼ 0.32; 3-Cl-
diPh, V ¼ 0.16; 3,5-Cl-diPh, V ¼ 0.20) substituents to Cy (V ¼
0.018) or diPh (V ¼ 0.10) would improve its uorescence
quantum yield, but the presence of other substituents would
bring positive or negative effects on uorescence quantum
yield. Combining with the previous results,14g,15 we can draw
a conclusion that chlorination or uorination is an much more
efficient way to improve the uorescence quantum yields of
non-conjugated materials than those of p-conjugated materials.
Mechanism of AIE
The molecular structures and arrangements play a key role in
AIE. For most AIE-active materials, they are p-conjugated
molecules and stack closely with not only a short interplanar
distance (d, for plane molecules14f) or intermolecular aromatic
HAr/p hydrogen bonds (for non-plane molecules, such as
silole19 or TPE20) but also weak intermolecular face-to-face p–p
interactions. The former can ensure to eliminate the molecular
rotation, the later would prevent the formation of excimer. For
non-conjugated AIE-active R-Cn (ref. 14g) and TSBs,15 however,
the restriction of C–C single bond rotations in the central alkyl
chain bridges through some other non-covalent intermolecular
interactions, such as N/H, O/H, C/H, H/H, F/H, Cl/H,
and Cl/Cl must be considered as well.
Fig. 3 Photographs (top: under room light, bottom: under 360 nm UV
light) of Cys (a) and diPhs (b) 1 ¼ diPh, 2 ¼ 3-F-(R,R)-diPh, 3 ¼ 3-F-
(S,S)-diPh, 4 ¼ 3-Cl-diPh, 5 ¼ 3-Cl-(R,R)-diPh, 6 ¼ 3-Cl-(S,S)-diPh,
7 ¼ 3,5-Cl-diPh, 8 ¼ 3,5-Cl-(R,R)-diPh, 9 ¼ 3,5-Cl-(S,S)-diPh, 10 ¼ 4-
NEt2-diPh; 11 ¼ Naph-diPh, 12 ¼ 3-NO2-diPh, 13 ¼ 5-NO2-diPh, 14 ¼
3,5-NO2-diPh) powders.
and Table S1†). A similar substituent effect was observed for
solid diPhs (Fig. 4 and Table S2†), R-Cn,14g and TSBs15 (Fig. 1a) as
well, indicating that a substituent has a very obvious effect on
lem of these non-conjugated AIE-active materials.
All uorescence quantum yields (V) (Tables S1 and S2†) of
the aggregated salen ligands in water and solid state were
In order to investigate the effect of molecule arrangements,
the X-ray single crystal structures of Cy,21 3-F-Cy (CCDC
1551151), 3-F-(S,S)Cy (CCDC 1496280), 3-F-(R,R)Cy (CCDC
1496279), 3,5-Cl-Cy,22 3-NO2-(R,R)Cy,23 3-F-diPh (CCDC 1551150),
and 3-Cl-diPh (CCDC 1551147) are depicted in Fig. 5–7 and
Fig. 5 X-ray single crystal structures and packing of Cy and 3,5-Cl-Cy
molecules: (a) (S,S) and (R,R) enantiomers of Cy; (b) and (c) packing in
a unit cell (H atoms are omitted).
Fig. 4 Normalized emission spectra of some Cys and diPhs solids.
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radiatively annihilate its excited states and quench the uo-
rescence consequently. In the solid state, however, Cy molecules
exhibit a head (cyclohexane)-to-tail (phenol ring) orientation
(Fig. 5b). No intermolecular face-to-face p–p interactions
(Fig. 5a, b, and S5†) are found between any neighboring Cy
molecules. On the contrary, many other strong intermolecular
interactions including H/H (2.4578, 2.5302, 2.6280, 2.7014,
˚
2.7544, 2.7770, 2.7896, 2.8042, 2.8886, 2.9240, and 2.9792 A),
˚
˚
C/H (2.8077 and 2.8296 A), and O/H (2.8111 A) interactions
are found in the two adjacent Cy molecules (Fig. S5†). These
above intramolecular and intermolecular interactions would
help to restrain the IRs and lead to the presence of AIE conse-
quently. Owing to the existence of chiral carbon atoms, two (S,S)
and (R,R) enantiomers (1 : 1) are found in the single crystals of
racemic Cy (Fig. 5a and b). The R/S-chirality-induced interac-
tions between two enantiomers (Fig. S5†) would help Cy mole-
cules arrange in a tight head-to-tail orientation packing.
Similar two (S,S) and (R,R) enantiomers (1 : 1) are also found
in the single crystals of racemic 3,5-Cl-Cy (Fig. 5c, S6, and S7†),
but 3,5-Cl-Cy molecules exhibit a different tail-to-tail orienta-
tion from Cy molecules (Fig. 5c). The R/S-chirality of two
enantiomers induce their phenol rings to pack together with
Fig. 6 X-ray single crystal structures and packing of 3-F-Cy, 3-F-(S,S)
Cy, and 3-F-(R,R)Cy molecules: (a) (S,S) and (R,R) enantiomers; (b) and
(c) packing in a unit cell (H atoms are omitted); (d) intermolecular
interactions of the two closest molecules (H atoms are omitted).
˚
a d of 3.46 A (Fig. S6a†) and little intermolecular face-to-face p–
p interactions (overlaps in two benzene rings). At the same
time, many other strong intermolecular interactions including
˚
H/H (2.6687, 2.6899, 2.8377, 2.8587, and 2.8978 A), O/H
˚
(2.4244, 2.6160, 2.6374, 2.6849 A), and Cl/H (2.8689 and
S5–S18.† Unlike step-like R-Cn (ref. 14g) and tripod-like TSBs15
molecules, these Cys and diPhs molecules are V- and propeller-
type, respectively.
˚
2.8843 A) interactions are found in the two adjacent 3,5-Cl-Cy
molecules (Fig. S6c, d, and S7†). Moreover, strong Cl/Cl
interactions (3.2848 A, Fig. S7c†) are observed, because two
˚
As expected, two intramolecular N/H hydrogen bonds
benzene rings are packed together. The F and Cl atoms would
help to induce intermolecular F/H, F/F, Cl/O, Cl/H, or
Cl/Cl interactions (halogen bond24) that is one possible reason
why chlorination or uorination can improve uorescence
quantum yield (see later discussion). All these data are consis-
tent with the fact that solid 3,5-Cl-Cy and its enantiomers have
the highest V values (0.32–0.35).
In order to evaluate the effects of different chiral (R,R) and
(S,S) bridges on the molecule arrangement, molecular packing
of 3-F-Cy, 3-F-(S,S)Cy, and 3-F-(R,R)Cy single crystal structures
are shown in Fig. 6 and S8–S15.† Like 3,5-Cl-Cy molecules,
racemic 3-F-Cy molecules exhibit a tight R/S-chirality-induced
tail-to-tail orientation packing with little intermolecular face-
˚
(1.6776 and 1.6348 A) between H (–OH) and N (C]N) atoms are
found in Cy (Fig. 5a), which would eliminate free rotations of
phenol rings. The dihedral angles between two panels of p-
conjugated phenol rings are about 56.6ꢂ, resulting in its V-type
molecular conguration and no intramolecular face-to-face p–p
interactions in one Cy molecule. In dilute organic solution, it is
obvious that the rotatable C–N and C–C single bonds in the
central cyclohexane bridge afford a possible way to non-
˚
to-face p–p interactions (d ¼ 3.54 A, Fig. 6a, b, S8, and S9†).
Many other strong intermolecular interactions including H/H
(2.6159, 2.6679, 2.6804, 2.7083, 2.8033, 2.8395, 2.8640, 2.8837,
˚
˚
and 2.9128 A), C/H (2.7592, 2.8238, and 2.9546 A), O/H
˚
(2.4532, 2.4701, 2.6238, 2.6691, 2.7395, and 2.8609 A), and F/H
˚
(2.7008, 2.7167, 2.7771, 2.8422, and 2.8584 A) interactions are
found in the two adjacent 3-F-Cy molecules. The photophysical
and AIE properties of 3-F-Cy, 3-F-(S,S)Cy, and 3-F-(R,R)Cy are
similar, but 3-F-Cy has a totally different molecule arrangement
from 3-F-(S,S)Cy and 3-F-(R,R)Cy (Fig. 6b and c). The molecule
arrangements of 3-F-(S,S)Cy and 3-F-(R,R)Cy are similar and
mirrored. The two closest molecules in 3-F-(S,S)Cy and 3-F-(R,R)
Cy single crystals overlap to form a triplex tail-to-tail, head-to-
tail, and head-to-tail arrangement. No intermolecular p–p
Fig. 7 X-ray single crystal structures and packing of 3-F-diPh (a, b) and
3-Cl-diPh (c, d) molecules: (a) and (c) (S,S) and (R,R) enantiomers; (b)
and (d) packing in a unit cell (H atoms are omitted).
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interactions but many similar strong intermolecular interac-
Anion probe properties
˚
˚
tions (H/H: 2.4496–2.9179 A, C/H: 2.8408–2.9800 A, O/H:
As shown in our previous work,15 TSBs (Fig. 1a) are tripod-like
side-single-opening cages with three exible arms of phenol
ring and can be used as anion hosts through hydrogen bond. In
this work, the two phenol rings of these salen ligands are cis
form with a small dihedral angles, and thus they acting as
clamps might react with anion through hydrogen bonds and
detect anion consequently.
In general, (S,S), (R,R), and racemic salen ligands have
similar anion responses, and thus racemic salen ligands are
discussed in this section. The anion probe properties of Cy, 3-F-
Cy, 3-Cl-Cy, 3,5-Cl-Cy, diPh, 3-F-diPh, 3-Cl-diPh, and 3,5-Cl-diPh
are demonstrated in Table S3, Fig. 8, and S20–S28.† As example,
upon adding many anions, including OHꢁ, Fꢁ, Clꢁ, Brꢁ, Iꢁ,
NO3ꢁ, CO32ꢁ, HCO3ꢁ, SO42ꢁ, S2ꢁ, SO32ꢁ, P2O74ꢁ, PO43ꢁ, and
HPO42ꢁ, to dilute 3,5-Cl-diPh DMSO solution would turn on the
blue uorescence (up to 41.1-fold enhancement) of 3,5-Cl-diPh
˚
˚
2.3803–2.8672 A, HAr/p: 2.54–2.57 A, and F/H: 2.4464–
˚
2.9903 A) are found in the two adjacent 3-F-(S,S)Cy or 3-F-(R,R)
Cy molecules (Fig. S10–S15†). However, there is no intermo-
lecular F/F interactions in 3-F-Cy, 3-F-(S,S)Cy, and 3-F-(R,R)Cy,
which would be contributed to the fact that their V values
(0.059–0.062) are lower than that of 3,5-Cl-Cy (0.32).
Our previous work demonstrated that the introduction of
strong electron-accepting –NO2 substituents would be an effi-
cient way to red shi uorescence bands of non-conjugated AIE-
active materials.14g,15 In this work, 3-NO2-Cy also shows a red-
shied uorescence band up to 565 nm (Fig. 4). We failed to
grow good-quality single crystals for X-ray analysis in the
previous and present work. It is fortunate that the X-ray single
crystal structure of 3-NO2-(R,R)Cy is obtained in the literature.23
Unlike the above results, there are strong intermolecular face-
to-face p–p interactions in 3-NO2-(R,R)Cy (d
˚
¼
3.31 A,
Fig. S16†), according with the fact that of 3-NO2-Cy has a red-
shied uorescence band with a not-so-high V value (0.051).
As shown in Fig. 7, two phenol rings and two benzene rings
in 3-F-diPh and 3-Cl-diPh molecules are cis form rather than
trans form, due to the existence of two chiral (R,R) or (S,S)
carbon atoms. Two phenol rings and two benzene rings are
separated each other to form their propeller structures, just like
another well-known AIE-active material of TPE.9,13,20 Therefore,
˚
˚
3-F-diPh (2.4888–2.8510 A) and 3-Cl-diPh (2.3507–2.8279 A)
molecules can tightly packed without any face-to-face p–p
interactions in their single crystals (Fig. S17 and S18†). It should
˚
be noted that strong intermolecular F/F (3.2922 A) and F/O
˚
(3.1760 A) interactions are found in 3-F-diPh and 3-Cl-diPh
molecules, respectively. All these factors enable solid 3-F-diPh
and 3-Cl-diPh have a high V value of 0.12 and 0.16, respectively.
Having small p-conjugated systems, these pure organic
salen ligands emit strong solid-state emission along with
large Stokes shis (up to 186 nm). This would enable us to
doubt that their emission originates from phosphorescence25
rather than uorescence. Time-resolved emission decay
spectra (Fig. S19†) reveal that the emission decay lifetime of 3-
F-Cy, 3-F-(S,S)Cy, 3-F-(R,R)Cy, 3-F-diPh, 3-F-(S,S)diPh, and 3-
F-(R,R)diPh powders is 0.70, 0.66, 0.66, 1.32, 1.72, and 0.93
ns, respectively. These data are in the range of ns, indicating
that their emission is not phosphorescence but uorescence.
The large Stokes shis might be contributed to the intra-
molecular hydrogen bonds which would lead to keto and enol
tautomers through an ultrafast excited state intramolecular-
proton transfer (ESIPT) process.26 With the same R substit-
uent, R-Cy and R-diPh have the similar uorescence proper-
ties, because the low-energy transition is mainly be assigned
to the p / p* transition involving molecular orbitals
essentially localized on the iminomethylphenol units with
little contribution from the non-conjugated cyclohexane and Fig. 8 (a) Top: emission spectra of 3,5-Cl-diPh upon the addition of
100 equivalent of different anions. Bottom: emission spectra of 3,5-Cl-
1,2-diphenylethane bridges (see the later discussion). More-
diPh upon the addition of different equivalent of Fꢁ. (b) Plot of emis-
over, our previous work revealed that both the multi-Schiff
sion intensity of 3,5-Cl-diPh at 468 nm) as a function of Fꢁ concen-
base structure and –OH groups are of the greatest impor-
tration. Insert: photographs of 3,5-Cl-diPh, 3,5-Cl-diPh
+ 100
tance for AIE.14g
equivalent of Fꢁ under 360 nm UV light). In all cases, the probe is 1.0 ꢀ
10ꢁ5 mol dmꢁ3 in DMSO and is excited at 380 nm.
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(Fig. 8a). Alkaline OHꢁ (Fig. S20 and S21†) and neutral Fꢁ exactly mirrors that of 3-F-(S,S)diPh, indicating that the two
(Fig. 8) were used to investigate the concentration effect. The compounds are a pair of enantiomers. Moreover, racemic 3-F-
uorescence of 3,5-Cl-diPh exhibits a linear enhancement diPh has little CD signals. Similar phenomena are observed for
(correlation coefficient R2 ¼ 0.970, n ¼ 15) upon the addition of 3-F-Cy, 3-F-(S,S)Cy, and 3-F-(R,R)Cy as well. Moreover, 3-F-(S,S)
0–40 equivalent of Fꢁ and then reaches maximum upon the Cy and 3-F-(S,S)diPh (or 3-F-(R,R)Cy and 3-F-(R,R)diPh) exhibit
addition of 100 equivalent of Fꢁ (V ¼ 0.24). If adding Fꢁ further, similar CD spectra, revealing that they have the same chiral
the uorescence would reduce and nally reach saturation (V ¼ conguration and energy transition.
0.21). A similar uorescence enhancement is observed upon
The uorescence responses towards various L- or D-amino
adding OHꢁ to dilute 3,5-Cl-diPh solution (V up to 0.29). The acids were investigated under the similar experimental condi-
probe performance would be much worse if other solvents, such tions with anion detection (Tables S4–S10, Fig. 10, and S31–
as MeCN, EtOH, and MeOH, are used.15
S63†). Considering the problem of solubility in both water and
Cy, 3-F-Cy, 3-Cl-Cy, 3,5-Cl-Cy, diPh, 3-F-diPh, and 3-Cl-diPh DMSO, we chose ten amino acids including alanine (Ala), valine
have similar anion effects with 3,5-Cl-diPh that they are very (Val), leucine (Leu), serine (Ser), proline (Pro), histidine (His),
sensitive to Fꢁ, OHꢁ, S2ꢁ and PO4
(Table S3†). On the tryptophan (Trp), arginine (Arg), glutamate (Glu), and gluta-
3ꢁ
contrary, step-like C4 (ref. 15) and C2 have much less anion mine (Gln). Upon adding amino acids, the uorescence
effect on uorescence (Fig. S29†). Our previous work15 revealed enhancements of Cy and diPh (Table S4 and Fig. S31†) are lower
that the reaction between TSBs and Xꢁ is not a normal chemical than those of 3-F-Cy (Table S5 and Fig. S32–S37†), 3-Cl-Cy (Table
reaction but intermolecular hydrogen bonding (–O–H/Xꢁ). S6 and Fig. S38–S43†), 3,5-Cl-Cy (Table S7 and Fig. S44–S49†),
The two phenol rings of Cy, 3-F-Cy, 3,5-Cl-Cy, 3-F-diPh, and 3-Cl- 3-F-diPh (Table S8 and Fig. S50–S55†), 3-Cl-diPh (Table S9 and
diPh are cis and diagonal form with a small dihedral angle of Fig. S56–S61†), and 3,5-Cl-diPh (Table S10, Fig. 10, S62, and
56.6, 51.5, 53.8, 54.6, and 47.5ꢂ, respectively. The intramolecular S63†). Since all nature amino acids are L-type ones, rstly we
distance O/O of Cy, 3-F-Cy, 3,5-Cl-Cy, 3-F-diPh, and 3-Cl-diPh used 3,5-Cl-(S,S)diPh and 3,5-Cl-(R,R)diPh to probe different
˚
is 6.081, 6.494, 6.352, 5.342, and 5.602 A, respectively. However, L-amino acids. As shown in Fig. 10a and Table S10,† like adding
C4 and C2 are step-like and their two phenol rings are parallelly anion, adding different L-amino acids to 3,5-Cl-(S,S)diPh or 3,5-
separated with much longer intramolecular distance O/O of Cl-(R,R)diPh DMSO solutions would turn on their uorescence
8.314 and 10.52 A, respectively.14g It means that, like TSBs,15 Cy, (V up to 0.26), indicating that they might be used as universal
˚
3-F-Cy, 3,5-Cl-Cy, 3-F-diPh, and 3-Cl-diPh are easier to act as amino acid probes (Fig. 10c, S62, and S63†). On the whole, 3,5-
tetradentate chelating agents (see the later discussion) and Cl-(S,S)diPh and 3,5-Cl-(R,R)diPh have a similar response that L-
consequently react with an anion more tightly than C4 and C2 Arg, L-Pro, and L-Ala would induce stronger uorescence
(Fig. S30†).
enhancements than other amino acids. Moreover, chiral 3,5-Cl-
(S,S)diPh and 3,5-Cl-(R,R)diPh could also discriminate the
enantiomers of some amino acids (Fig. 10b and Table S10†). For
example, adding D- or L-Gln to 3,5-Cl-(S,S)diPh solution, the
uorescence of 3,5-Cl-(S,S)diPh would turn on with a 36.2- and
5.82-fold enhancement, respectively. The selective recognition
of chiral molecular enantiomers is related to the enantiomeric
uorescence difference ratio, ef, according to ef ¼ (ID ꢁ I0)/(IL ꢁ
I0), in which I0 represents the uorescence emission intensity of
receptor in the absence of a chiral substrate and ID and IL are
the uorescence intensities in the presence of D- and
L-substrates, respectively. 3,5-Cl-(S,S)diPh shows an good ef
value of 6.22 for enantioselective recognition of D- and L-Gln. For
other amino acids, His (ef ¼ 2.71), Arg (ef ¼ 2.27), and Trp (ef ¼
2.13) enantiomers could be discriminated by the 3,5-Cl-(S,S)
diPh. 3,5-Cl-(R,R)diPh can identify Gln (ef ¼ 2.93) and His (ef ¼
2.46) enantiomers.
Chirality and chiral recognition
The chirality properties of 3-F-Cy, 3-F-(S,S)Cy, 3-F-(R,R)Cy, 3-F-
diPh, 3-F-(S,S)diPh, and 3-F-(R,R)diPh are examined by circular
dichroism (CD) spectra. As depicted in Fig. 9, 3-F-(S,S)diPh
shows a positive cotton effect at 326 nm and further strong
peaks at 270, and 249 nm. The CD spectrum of 3-F-(R,R)diPh
In addition, 3-F-(S,S)diPh has an excellent ef value of 0.11 for
enantioselective recognition of D- and L-Val (Table S8 and
Fig. S54†). 3-F-(R,R)diPh can discriminate D- and L-Arg (ef ¼
4.89) (Table S9†). 3-Cl-(S,S)diPh can efficiently discriminate Arg
(ef ¼ 5.48) and Gln (ef ¼ 0.21) enantiomers, on the other hand,
Arg (ef ¼ 4.43) and Trp (ef ¼ 0.18) enantiomers can be can be
distinguished by 3-Cl-(R,R)diPh (Table S9†). 3-F-Cy (Table S5†),
3-Cl-Cy (Table S6†), and 3,5-Cl-Cy (Table S7†) generally exhibit
worse ef values than 3-F-diPh (Table S8†), 3-Cl-diPh (Table S9†),
and 3,5-Cl-diPh (Table S10†). However, since it is too difficult to
grow good-quality single crystals from the mixture of the dye
Fig. 9 CD spectra of 3-F-Cy, 3-F-(R,R)Cy, 3-F-(S,S)Cy, 3-F-diPh, 3-F-
(R,R)diPh, and 3-F-(S,S)diPh in MeCN (3.0 ꢀ 10ꢁ5 mol dmꢁ3).
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bridge but not a circular bridge of cyclohexane, hence,
compared with Cys, they would be more exible and conse-
quently might have a better ability to form a cavity4a for the
recognition of a tiny chiral structure difference between D- and
L-amino acid.
Mechanism of anion and amino acid probes
In order to investigate the possible interaction mechanism
between the dye and anion/amino acid, 1H nuclear magnetic
resonance (1H NMR) analysis was done. As shown in Fig. 11a,
1
the H NMR signal of H1 atoms in 3,5-Cl-diPh appears a small
broad peak at downeld (d ¼ 14.5 ppm) due to the existence of
the intramolecular hydrogen bonds (–OH/N, Fig. 5).15,27 If
adding 100 equivalent of OHꢁ (Fig. 11b), Fꢁ (Fig. 11c), or L-Arg
(Fig. 11d) (in D2O), this downeld peak would disappear and
reform a new small sharp peak at upeld (d ¼ 10.1–10.2 ppm),
which might be assigned to the ArOH without intramolecular
hydrogen bonds.27,28 This upeld shi indicates that the strong
intramolecular hydrogen bonds are destroyed by adding anion/
amino acid. At the same time, the reduction (about 1/2–2/3) of
peak area in new peak reveals the formation of new intermo-
lecular hydrogen bonds. These intermolecular hydrogen bonds
would increase the molecular electron density through-bond
effect and lead to upeld shis of other H atoms conse-
1
quently.29 Moreover, the H NMR signal of H3 at chiral carbon
atoms also shows an obvious upeld shi from d ¼ 5.2 ppm into
d ¼ 4.7, (5.0 and 4.2), and 3.9 ppm upon adding OHꢁ, Fꢁ, and
L-Arg, respectively, which might indicate that the neighboring
nitrogen atoms have strong intermolecular interactions with
anion and amino acid through hydrogen and halogen bonds.
Fig. 10 (a) Emission spectra of 3,5-Cl-(S,S)diPh and 3,5-Cl-(R,R)diPh
upon the addition of 100 equivalent of different L-amino acids. (b)
Emission spectra of 3,5-Cl-(S,S)diPh and 3,5-Cl-(R,R)diPh upon the
addition of 100 equivalent of different L-and D-amino acids. (c) Plot of
emission intensity of 3,5-Cl-(R,R)diPh at 440 nm as a function of L-Arg
concentration. Insert: emission spectra of 3,5-Cl-(R,R)diPh upon
the addition of different equivalent of L-Arg. In all cases, the probe is
1.0 ꢀ 10ꢁ5 mol dmꢁ3 in DMSO and is excited at 380 nm.
and amino acid in DMSO solution, we failed to get a clear
insight into the chiral discrimination ability (see the later
Fig. 11 1H NMR spectra of 3,5-Cl-diPh (a), 3,5-Cl-diPh + 100 equiv-
alent of OHꢁ (b), 3,5-Cl-diPh + 100 equivalent of Fꢁ (c), and 3,5-Cl-
discussion). We just guess that diPhs have a 1,2-diphenylethane (R,R)diPh + 100 equivalent of L-Arg (d) in DMSO-d6.
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Furthermore, the blank experiments (Fig. S64 and S65†)
1
revealed that the H NMR peak at 10.1–10.2 was not caused by
an impurity.
The mechanisms of the aggregation- and anion/amino acid-
induced uorescence enhancement are totally different. For
example, the green AIE of 3-F-Cy is originated from the aggre-
gated state (in water or solid) but its blue anion/amino acid-
induced uorescence is originated from the monomer (1.0 ꢀ
10ꢁ5 mol dmꢁ3 in DMSO). To gain insight into the nature of the
excited states and transitions, gas-phase density functional
theory (DFT) and time-dependent-DFT (TD-DFT) were also
carried out for 3-F-Cy with the Gaussian 09 program package
(B3LYP 6-31G(d,p)). The energy absorption bands of 3-F-Cy
(labs ¼ 316, 253, and 211 nm in MeCN) are reproduced well by
the computation, which predict three absorption peaks at 318,
248, and 213 nm (Fig. 12). The lower energy absorption is
mainly contributed to the highest occupied molecular orbital
(HOMO) / lowest unoccupied molecular orbital (LUMO)
(318 nm, oscillator strength, fosc ¼ 0.0814). The energy level and
orbital isosurfaces diagrams of 3-F-Cy (Fig. 12) reveal that its
HOMO and LUMO are mainly made up of the p-functions of
iminomethylphenol units rather than the cyclohexane bridge.
Therefore, the lower energy absorption can mainly be assigned
to the p / p* transition involving molecular orbitals essen-
tially localized on the iminomethylphenol units with little
contribution from the non-conjugated cyclohexane bridge. As
mentioned above, Fꢁ, OHꢁ, or L-Arg would lead to changing
–OH into Oꢁ, and thus 3-F-Cy in form of Oꢁ is used to evaluate
the nature of the anion/amino acid-induced excited states and
transitions by calculation. For optimized structure, 3-F-Cy (in
form of Oꢁ, Fig. 12) has a much bigger dihedral angle (79.0ꢂ)
between two phenol rings than 3-F-Cy (in form of OH, 51.5ꢂ).
The lower energy absorption band of 3-F-Cy (in form of Oꢁ,
labs ¼ 365 and 350 nm for experiment and computation,
respectively) are mainly composed of three transitions,
including HOMO / LUMO (360 nm, fosc ¼ 0.150), HOMOꢁ1 /
LUMO+1 (344 nm, fosc ¼ 0.175), and HOMO / LUMO+1
Fig. 13 Brightfield (a) incubated without dye and fluorescence images
(b) incubated without dye; (c) incubated with 3,5-Cl-Cy; (d) incubated
with 3-NO2-Cy of HeLa cells.
(338 nm, fosc ¼ 0.127). The p-functions of HOMO, HOMOꢁ1,
LUMO and LUMO+1 of 3-F-Cy in form of OH and Oꢁ are similar,
which reveals that anion/amino acid would not induce to
change of the way of transition (p / p* transition). This is
much different from our previous work15 that anion would
induce to change of the way of transition from n / p* into p /
p* for TSBs. Moreover, anion/amino acid would red shi the
lower energy absorption band with an increment of molar
extinction co-efficient (Fig. 12 and S66†). This phenomenon can
be reasonably explained by the fact that electron-rich ArOꢁ
anions have resonance structures of benzoquinone.28 These
resonance structures lead the p-function Frontier molecular
orbitals to a uniform distribution in the molecule skeleton, and
consequently 3-F-Cy (in form of Oꢁ) has not only a smaller
energy gap of transition but also an increment of molar
extinction co-efficient and oscillator strength. This also might
be contributed to its high uorescence quantum yield in dilute
DMSO solution. It is strange that dilute 3-F-Cy (in form of Oꢁ)
DMSO solution having a red-shied absorption spectrum shows
a blue-shied uorescence spectrum, but AIE of 3-F-Cy (in form
of OH, in solid or water) is green-yellow. There is no anymore
ESIPT for 3-F-Cy (in form of Oꢁ), which might be contributed to
its blue uorescence.
Living cell imaging
These non-conjugated AIE-active materials might have a lower
cytotoxicity and better biocompatibility, and thus green-light-
emitting 3,5-Cl-Cy and red-light-emitting 3-NO2-Cy were used
in cell imaging applications. The HeLa cells were imaged by the
dye using a standard cell-staining protocol. As can be seen from
Fig. 13, incubated with 3,5-Cl-Cy or 3-NO2-Cy, the HeLa cells
grow similar as in the control experiments in the absence of the
dyes, indicating that both dyes have little toxicity on the living
cells. HeLa cells show negligible background uorescence.
However, intense intracellular green and red AIE is observed
aer HeLa cells are incubated with 3,5-Cl-Cy or 3-NO2-Cy,
respectively. Therefore, these AIE-active materials may have
potentially applications in probing or monitoring biologically
important processes in vitro as well as in vivo.
Conclusion
Fig. 12 Computational (gas-phase) and experimental absorption
spectra and Frontier molecular orbitals of 3-F-Cy (top, in form of OH)
and 3-F-Cy + 100 equivalent of OHꢁ (bottom, in form of Oꢁ) MeCN
solution.
In the present work, we demonstrate the unique AIE, chirality,
anion/amino acid probe, and cell imaging properties a series of
non-conjugated and cyclohexane/1,2-diphenylethane-linked
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salen ligands. These V- or propeller-type salen ligands with
small p-conjugated systems emit strong blue, green, and red
AIE through little p–p interactions and strong non-covalent
intermolecular interactions, such as O/H, H/H, C/H, F/
H, F/F, Cl/H, Cl/O, and Cl/Cl. Moreover, combine with the
advantages of non-conjugation and chirality, these so mate-
rials have multiple and strong interactions with anions and
amino acids by hydrogen and halogen bonds, and thus they
might be used as universal anion probes and chiral receptors of
unprotected amino acids with a good enantiomeric selectivity.
Therefore, we believe that these simple salen ligands provide
a new paradigm in the design of chiral and non-conjugated
uorescent materials for developing advanced organic opto-
electronic devices, orescent bio-probes, and cell imaging, and
so on. Further studies on circularly polarized luminescence30 of
these salen ligands are currently underway in our laboratory.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
cid¼home_promo.
This work was supported by the National Natural Science
Foundation of China (no. 21372169). We acknowledge the
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