Multiple Hydrogen Bonds Promoted ESIPT and AIE-active Chiral Salicylaldehyde Hydrazide

Article abstract of DOI:10.1002/cjoc.201800115

The simpler, the better! A series of simple and highly fluorescent salicylaldehyde hydrazide molecules (41 samples) have been designed and prepared. Even though these soft materials contain a very small π-conjugated system, they can go through multiple intramolecular and intermolecular hydrogen bonds promoted excited-state intramolecular proton-transfer (ESIPT) to display strong blue, green, yellow, and orange aggregation-induced emission (AIE) with large Stokes shifts (up to 184 nm) and high fluorescence quantum yields (Ф up to 0.20). Unusual mechanochromic fluorescence enhancements are also found in some solid samples. Through coordination, hydrogen and halogen bonds, these flexible molecules can be used as Mg²⁺ (Ф up to 0.46) probes, universal anion (Ф up to 0.14) and unprotected amino acids (Ф up to 0.16) probes, and chiral diamine (enantiomeric selectivity and Ф up to 0.36 and 0.062, respectively) receptors. Combining their advantages of AIE and biocompatibility, these low cytotoxic dyes have potential application in living cell imaging. Furthermore, the effects of different functional groups on the molecule arrangement, ESIPT, AIE, probe, and chiral recognition properties are also examined, which provide a simple and bright paradigm for the design of multiple-stimuli-responsive smart materials.

Full text of DOI:10.1002/cjoc.201800115

Multiple Hydrogen Bonds Promoted ESIPT and AIE-active Chiral
Salicylaldehyde Hydrazide†
Man Wang‡a, Caiqi Cheng‡b,c Jintong Song,a Jun Wang,b Xiangge Zhou,a Haifeng Xiang*a, and Jin Liu*c
a College of Chemistry, Sichuan University, Chengdu, 610041, China. Fax: (+86) 28-8541-2291. E-mail: xianghaifeng@scu.edu.cn.
b State Key Laboratory of Oral Diseases, Department of Orthodontics, West China Hospital of Stomatology, Sichuan University, Chendu, Sichuan, China.
c Laboratory of Stem Cell Biology, State Key Laboratory of Biotherapy, West China Hospital, Sichuan University, Chengdu, 610064, China. E-mail: ljamelia@163.com
‡ These authors contributed equally to this work
ABSTRACT The simpler, the better! A series of simple and highly fluorescent salicylaldehyde hydrazide molecules (41 samples) has been designed and
prepared. Even though these soft materials contain a very small π-conjugated system, they can go through multiple intramolecular and intermolecular
hydrogen bonds promoted excited-state intramolecular proton-transfer (ESIPT) to display strong blue, green, yellow, and orange aggregation-induced
emission (AIE) with large Stokes shifts (up to 184 nm) and high fluorescence quantum yields (Ф up to 0.20). Unusual mechanochromic fluorescence
enhancements are also found in some solid samples. Through coordination, hydrogen and halogen bonds, these flexible molecules can be used as Mg2+
(Ф up to 0.46) probes, universal anion (Ф up to 0.14) and unprotected amino acids (Ф up to 0.16) probes, and chiral diamine (enantiomeric selectivity and
Ф up to 0.36 and 0.062, respectively) receptors. Combining their advantages of AIE and biocompatibility, these low cytotoxic dyes have potential
application in living cell imaging. Furthermore, the effects of different functional groups on the molecule arrangement, ESIPT, AIE, probe, and chiral
recognition properties are also examined, which provide a simple and bright paradigm for the design of multiple-stimuli-responsive smart materials.
KEYWORDS promoted excited-state intramolecular proton-transfer (ESIPT), aggregation-induced emission, anion hosts/probes, amino acids probes,
chiral diamine
1. Introduction
Organic luminescent (fluorescent or phosphorescent) materials1 11 are commonly polycyclic aromatic molecules with extended

π-electron conjugation, but these rigid materials might suffer some disadvantages, such as the synthesis difficulty, poor solubility,
fluorescence aggregation-caused quenching12,13 (ACQ). Moreover, organic fluorescent materials usually have a small Stokes shift, which
is harmful for the application of fluorescent bioimaging, because the fluorescence might be reduced by the self-absorption and inner
filter effect.14,15 It has been proved that the chemical modifications, such as π-conjugation, substituent effect, or donor–acceptor (DA)
system, is a very efficient way to tune the photophysical properties of a conventional organic fluorescent material.4,8 However, these
modifications are difficult to increase the Stokes shift, because the absorption and emission band would increase simultaneously.
Excited state intramolecular proton transfer (ESIPT)16-18 is a photochemical process that produces a tautomer with a different electronic
structure from the original excited form. The basic requirement for ESIPT is the presence of an intramolecular hydrogen bond between
the proton donor (OH and NH2) and the proton acceptor (N and CO) groups in close proximity to each other in a molecule. For
example, through intramolecular hydrogen bond (NHO), salicylaldehyde-based Schiff bases exist a tautomeric transformation from
the excited enol into to the excited keto form and consequently result in their most remarkable advantage of the large Stokes shift
(Figure. 1a). Therefore, organic aggregation-induced emission (AIE)-active materials19 26 with ESIPT-inspired large Stokes shifts are highly

desirable.
Up to now, most AIE-active materials have non-planar AIE cores, such as silole27,29 or tetraphenylethene (TPE).30 43 Our research

group has a longstanding interest in the synthesis, optical properties, and sensing applications of salicylaldehyde-based bis-Schiff base,
N,N’-bis(salicylidene) ethylenediamine2,34-41 and salicylaldehyde azine,13,42 and tri-Schiff base43 owing to its facile preparation, good
stability, and well-tuned red-green-blue (RGB) ACQ/AIE-active emission with high fluorescence quantum yields (Ф). For example,
π-conjugated Ph36,44,45 and SA13,42,46-50 and π-unconjugated Cn39, Cy, DiPh41, and TSB43 show strong RGB AIE, but πconjugated
CN13,36,37,51-53 is a well-known ACQ-active red emitter. Recently, π-conjugated salicylaldehyde-based mono-Schiff bases were reported to
emit strong AIE.54-56 For most AIE-active materials, they are π-conjugated molecules and stack closely with not only a short interplanar
distance (d, for plane molecules, such as SA) or strong intermolecular aromatic HAr interactions (for non-plane molecules, such as
π
silole and TPE) but also weak intermolecular face-to-face ππ interactions. The fo…rmer can ensure to eliminate the intramolecular
rotations (IRs), the later would prevent the formation of excimer. For non-conjugated AIE-active Cn, Cy, diPh, and TSB, however, the
restriction of CC and CN 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.
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Figure. 1 ESIPT of salicylaldehyde-based Schiff base (a). Promoted ESIPT of
SHs (b, c, and d). Intermolecular hydrogen bonds of SHs (e). Reaction between SH and analyte (f).
Amide group (NHCO), as the form of peptid bond, exists widely in all kinds of creatures. It is well known that the presence of
amide group would help to induce strong intermolecular hydrogen bonds to form self-assembled supramolecular gels.57-59 What would
happen if amide group is introduced to the salicylaldehyde-based Schiff bases? Whether can the intermolecular hydrogen bonds
improve the AIE properties of salicylaldehyde hydrazides (SHs, Figure. 2)? In the literature, however, there are only limited examples of
non-emissive SHs that were used as turn-on fluorescence probers for detecting ions, such as Mg2+,60,61 Zn2+,60,62 Al3+,62-67 F,68 and
CN,69,70 and showed polar solvent-induce fluorescence.71 Herein, we firstly reported a series of simple and soft SH materials (41 samples,
Figure. 2) that exhibit multiple intramolecular and intermolecular hydrogen bonds promoted ESIPT and AIE properties with large Stokes
shifts and can be used as multiple-stimuli-responsive smart materials for many applications in mechanochromism, universal probes,
chiral recognition, and living cell imaging. The purposes of the presence of amide group to salicylaldehyde-based Schiff bases are: 1) to
promote ESIPT properties by more intramolecular hydrogen bonds (Figure. 1b and 1c); 2) to extend π-conjugation system by the
transformation from NHC=O into N=COH and more intramolecular hydrogen bonds (Figure. 1c and 1d); 3) to enhance AIE properties
by intermolecular hydrogen bonds (Figure. 1e); 4) to reinforce reaction activity to an analyte (Figure. 1f); 5) to easily introduce a chiral
group for chiral recognition; 6) its low cytotoxicity and good biocompatibility.
Figure. 2 Fluorescent salicylaldehyde-based Schiff bases: SHs in this work.
2. Results and discussion
2.1. Synthesis and characterization
The target products of SHs can be prepared by two methods (Scheme 1). Both of them are two-step synthesis with all inexpensive
reagents and in favorable yields. In this work, however, we finally adopted method 1 to prepare SHs, because the by-product of bis-Schiff
base SA would be obtained in the first step in method 2. The by-product and product have similar chemical properties, and thus it is
difficult to purify the product by simple recrystallization. Moreover, to avoid the degradation during the purification step through
hydrolysis, it is better to purify Schiff bases by recrystallization (e.g., EtOH/hexane) than chromatography. Naph-SH-OH has a weak
solubility only in DMF and DMSO. Most other SHs have a bad solubility in petroleum ether, hexane, and water but a good solubility in
CH3CN, CH2Cl2, DMF, and DMSO. Most SHs either in solution or in solid state are stable within several months under air. For SHs,
good-quality single crystals can be obtained by the method of slow solvent diffusion/evaporation (CH2ClCH2Cl/hexane or EtOH/hexane).
For the purpose of comparison, the five reference substances Ref1Ref5 (Figure. 2) were prepared as well.
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Scheme 1 Synthesis of SHs.
3.1. Photophysical and AIE properties
The UV/visible absorption and fluorescence data of all synthesized SHs at room-temperature are listed Table S1, Figure. 37, and
S1S9 (in Supporting Information). To gain an insight into the nature of the excited states and transitions, density functional theory (DFT)
and time-dependent DFT (TD-DFT) were also carried out with the Gaussian 09 program package (B3LYP 6-31G(d,p)). The lower energy
absorption band of Naph-SH-C3 (abs = 353 nm in MeCN) is reproduced well using TD-DFT calculations, which predict one absorption
maximum at 354 nm (Figure.3). The lower energy absorption mainly corresponds to the highest occupied molecular orbital (HOMO) 
lowest unoccupied molecular orbital (LUMO) (100 %, oscillator strength, fosc = 0.4365). The energy level diagram and frontier molecular
orbitals of Naph-SH-C3 are also given in Figure. 3, which indicate that its π-conjugation system is efficient extended by the presence of
amide group (NHCO). The lower energy absorption of Naph-SH-C3 can mainly be assigned to the π  π* transition and n π*
transition from lone pair electrons of electron-donating O (OH) and N (NH) atom to the π-conjugated conjugation, but the
non-conjugated alkyl side chain has little contribution.
1.0
Experimental absorption
Computational absorption
Computational fosc
0.8
0.6
0.4
0.2
0.0
200 250 300 350 400 450 500 550 600
Wavelength / nm
Figure. 3 Computational and experimental absorption spectra (in MeCN), energy level diagram, and frontier molecular orbitals of Naph-SH-C3.
5
3
As examples, the absorption spectra of Naph-SH-R in pure organic solvent of MeCN (1.0 × 10 mol dm ) are given in Figure. 4.
Naph-SH-R (R = C1, C3, C5, and BnOH) containing a same Naph-SH structure and different non-conjugated side groups have similar
absorption intensity and bands (abs = ~353 nm), indicating that these non-conjugated side groups have little effect on their absorption.
Even though R group is changed into benzene ring, the absorption bands (abs = 358368 nm) of Naph-SH-Ph, Naph-SH-F, and
Naph-SH-CN show very small red shifts. This might be caused by the fact that the benzene ring can be rotate freely in dilute solution and
consequently can’t extend the π-conjugated conjugation efficiently. That is why the absorption band (abs = 373 nm) of 4-NEt2-SH-CN
can’t be red shifted obviously (Figure. S1), even if it has a donor (−NEt2)−acceptor (−CN) (DA) system. However, Naph-SH-OH shows the
most red-shifted absorption band (abs = 406 nm) with a shoulder at 428 nm, which can be contributed to the fact that the
intramolecular hydrogen bond =O…HN (Figure. 1c) would eliminate the rotation of benzene ring and consequently extend its
π-conjugated conjugation efficiently (see the later discussion).
5x104
Naph-SH-C1
Naph-SH-CN
Naph-SH-C3
Naph-SH-C5
Naph-SH-Ph
4x104
3x104
2x104
Naph-SH-F
Naph-SH-OH
Naph-SH-BnOH
1x104
0
250
300
350
400
450
500
Wavelength / nm
Figure. 4 Absorption spectra of Naph-SH-R in MeCN.
Since the absorption and AIE properties of (S), (R), and racemic R-SH-BnOH are similar, only racemic R-SH-BnOH are discussed in this
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section. As we expected, all SHs exhibit weak fluorescence in pure organic solvent of MeCN (Table S1). For example, Naph-SH-C3 shows
very weak blue fluorescence (em = 435 nm, Ф < 0.001) in the dilute MeCN, because the dynamic IRs of central NN and CC single bond
in Naph-SH-C3 (Figure. 1b) molecules provide a possible way to non-radiatively annihilate its excited states and result in the absence of
fluorescence consequently. However, if water is added into MeCN, the fluorescence of Naph-SH-C3 is strengthened (Figure. 5), because
Naph-SH-C3 molecules cannot be dissolved in water, which causes Naph-SH-C3 molecules to precipitate and aggregate. At the same time,
adding water would lead to a darker color and non-transparent solution (Figure. 5), which further confirms this aggregation. Its
green-yellow fluorescence reaches the maximum (em = 550 nm, Ф = 0.042) with a large Stokes shift of 97 nm, when the volume fraction
(f) of water is increased to 80 %. Furthermore, solid Naph-SH-C3 also displays strong green-yellow fluorescence (em = 561 nm, Ф = 0.11)
under 360 nm UV lamp. All these findings reveal the fact of its AIE nature. Such AIE properties of Naph-SH-C5 (Figure. S4), NaPh-SH-CN
(Figure. S5), and 4-NEt2-SH-CN (Figure. S6) in water were observed as well.
All SHs have stronger fluorescence in solid state or water than in organic solvents (Table S1). Most solid samples, such as crystals,
powders (Figure. 6), and casting films (Figure. S7), exhibit not only red-shifted but also much stronger fluorescence compared with water
samples (Table S1), and thus we will focus on their solid AIE characteristics. Different solid forms of AIE-active materials that have
different molecular arrangements might have different fluorescence properties. For most SHs, however, the different solid forms, such as
crystal, powder, and casting film, have similar fluorescence properties, except that Naph-SH-BnOH and 4-NEt2-SH-F powders show much
stronger fluorescence than their crystals (see the later discussion).
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
92%
450
500
550
600
650
700
Wavelength / nm
5
Figure. 5 Emission spectra and photographs (under room light and 360 nm UV light) of Naph-SH-C3 in MeCN-H2O with different f values (1.0 × 10 mol
3
dm ).
Figure. 6 Photographs (top: under room light, bottom: under 360 nm UV light) of SHs powders (1 = Naph-SH-C5, 2 = SH-OH, 3 =Naph-SH-OH, 4 =
3-NO2-SH-OH, 5 = SH-Ph-CN, 6 = 3-Cl-SH-CN, 7 = Naph-SH-CN, 8 = 4-NEt2-SH-CN,9 = 3-NO2-SH-CN, 10 =SH-Ph; 11 = Naph-SH-Ph, 12 = 3-NO2-SH-Ph, 13 =
Naph-SH-F, 14 = SH-Ph-F, 15 =SH-BnOH(S), 16 = SH-BnOH(R), 17 = 4-NEt2-SH-BnOH(S), 18 = 4-NEt2-SH-BnOH(R)).
As shown in Figure. 7, S8, S9, and Table S1, for solid SHs, different substituents have different effects on the emission peaks (em
:
380587 nm) and colors (blue, green, yellow, and orange). For R-SH-Ph, electron-donating NEt2 (4-NEt2-SH-Ph, em = 533 nm) or
π-extended system (Naph-SH-Ph, em = 519 nm) substituent would blue shift its fluorescence band (SH-Ph, em = 540 nm), while
electron-accepting NO2 (3-NO2-SH-Ph, em = 570 nm) substituent result in red-shifted emission (Figure. S8). For R-SH-CN, however,
these substituents totally lead to a blue shift in fluorescence band (Figure. S9). For Naph-SH-R’, the fluorescence bands of Naph-SH-CN
are affected by the length of alkyl side chain (em = 501, 561, and 547 nm for n = 1, 3, and 5, respectively) (Figure. 7). Moreover, the
fluorescence bands of Naph-SH-Ph (em = 556 nm) and Naph-SH-BnOH (em = 554 nm) are similar. Compared with fluorescence than
absorption data, it is clear that a substituent has much better obvious effects on fluorescence bands than absorption bands (Table S1),
indicating that the fluorescence transitions (ESIPT) might originate differently from the original excited form of absorption transitions
(Figure. 1a).
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Naph-SH-C1
Naph-SH-C3
Naph-SH-C5
Naph-SH-Ph
Naph-SH-OH
Naph-SH-F
Naph-SH-CN
Naph-SH-BnOH
450
500
550
600
650
700
Wavelength / nm
Figure. 7 Normalized emission spectra of some Naph-SH-R’ solids
All fluorescence quantum yields (Table S1) of the aggregated SHs in water and solid state were measured by the optical dilute
method of Demas and Crosby72 with a standard of quinine sulfate (Фr = 0.55, quinine in 0.05 mol dm3 sulfuric acid) and an integrating
sphere, respectively. Although SHs have a small π-conjugated system, some of their fluorescence quantum yields are unexpected high
(up to 0.20, Table S1). Our previous results revealed that chlorination and fluorination is a very efficient way to improve the fluorescence
quantum yields of π-unconjugated Cn,39 Cy, DiPh41, and TSB solids.34-41 However, this rule of chlorination and fluorination is out of work
for π-conjugated SHs. The presence of π-extended naphthalene ring (Naph-SH-C3, Ф = 0.12; Naph-SH-Ph, Ф = 0.081; Naph-SH-F, Ф =
0.089; Naph-SH-CN, Ф = 0.12; Naph-SH-BnOH, Ф = 0.079) would help to improve its fluorescence quantum yield, which is in agreement
with the results of π-conjugated SA.42 Naph-SH-OH with promoted ESIPT and extended π-conjugation system by the intramolecular
hydrogen bondHO…HN has a highest fluorescence quantum yield of 0.20 (Figure. 1c, see the later discussion).
4.1. Mechanism of AIE
The molecular structures and arrangements play a key role in AIE. In order to investigate the effect of molecule arrangements, the
x-ray single crystal structures of SH-Ph,73 Naph-SH-OH,74 SH-C1,75 Naph-SH-C3 (CCDC 1585254), Naph-SH-C5 (CCDC 1585255),
4-NEt2-SH-C5 (CCDC 1814156) 4-NEt2-SH-F (CCDC 1585256), 3-F-SH-Ph (CCDC 1585257), Naph-SH-Ph (CCDC 1585258), 4-NEt2-SH-Ph
(CCDC 1585259), Naph-SH-BnOH(R) (CCDC 1585260), 4-NEt2-SH-BnOH(R) (CCDC 1585261), and Naph-SH-BnOH(S) (CCDC 1585262) are
depicted in Figure. 912 and S10S20. Unlike step-like Cn,39 tripod-like TSBs,43 V-type Cy, and propeller-type diPh,41, these SHs (except
R-SH-BnOH) are plane molecules with a small dihedral angle (040.5o) between two benzene rings (Figure. 8), like SA42 molecules.
Figure. 8 Dihedral angle of some SHs.
As expected, one intramolecular N…HO hydrogen bond (2.0933 Å) is found in SH-Ph (Figure. 9a), which would eliminate free
rotations of phenol ring. However, the distance of O…HO is 3.1330 Å, which is too long to form another intramolecular hydrogen
bond (Figure. 1b). SH-Ph is a plane molecule with a small dihedral angle (23.3o, Figure. 8 and 9) between two panels of π-conjugated
benzene rings. SH-Ph molecules show a slipped face-to-face lamellar packing (Figure. 9b) with a very short interplanar distance of d 
3.19 Å (Figure. 9d). Moreover, intermolecular NH…OH hydrogen bonds (2.2164 Å), H…H (2.4185, 2.5784, 2.5921, 2.6131, and 2.8939
Å), and O…H (2.6471 and 2.7042 Å) interactions are found in the two closest SH-Ph molecules. These above intramolecular and
intermolecular interactions would help to restrain the IRs and lead to the presence of AIE consequently. Anyway, it is noteworthy that
some moderate intermolecular face-to-face π–π interactions between two neighbor SH-Ph molecules are still observed (Figure. 9d),
which might be one possible reason for the moderate Φ (0.079) for SH-Ph solid.
For 4-NEt2-SH-Ph, it should be noted that the methyl C atom in one of the –NEt2 groups might appear disorder in the single crystal
structures. Such disorder is quite common in terminal alkyl groups.76 As depicted in Figure. S10, The intramolecular N…HO hydrogen
bond and dihedral angle of 4-NEt2-SH-Ph molecule are 1.9245 Å (Figure. S10a) and 34.8o (Figure. 8), respectively. 4-NEt2-SH-Ph
molecules have a similar slipped face-to-face lamellar packing as SH-Ph molecules (Figure. S10b). Unlike that of SH-Ph molecules, the
intermolecular hydrogen bond of 4-NEt2-SH-Ph molecules is NH…O (2.1001 Å) (Figure. S10e) rather than NH…OH. There are only
one type of strong intermolecular interactions (O…H: 2.4942 Å and 2.9659 Å) in the two adjacent 4-NEt2-SH-Ph molecules (Figure. S10e).
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Furthermore, the interplanar distance of 4-NEt2-SH-Ph molecules is 3.58 Å (Figure. S10c). These data reveal that 4-NEt2-SH-Ph molecules
containing a big bulky NEt2 group have a looser packing and smaller π-conjugated system than SH-Ph molecules (dihedral angle/crystal
3
3
density of SH-Ph and 4-NEt2-SH-Ph are 23.3o/1.37 g cm and 34.8o/1.24 g cm , respectively), and thus 4-NEt2-SH-Ph solid (Φ = 0.039)
has weaker AIE than SH-Ph solid (Φ = 0.079), even if 4-NEt2-SH-Ph molecules have little intermolecular face-to-face π–π interactions
(Figure. S10c and S11d).
Figure. 9 X-ray single crystal structures and packing of SH-Ph molecules: (a) single molecule; (b) packing in a unit cell (H atoms are omitted); (c)
intermolecular interactions of the two closest molecules; (d) top and side view of the two adjacent molecules (H atoms are omitted).
Our previous work demonstrated that the introduction of F or Cl atoms is a very efficient way to improve the fluorescence quantum
yields of π-unconjugated Cn,39 Cy, DiPh41, and TSB solids,43 since F and Cl atoms would help to induce intermolecular halogen bonds. In
this work, however, there is no any halogen bond in 3-F-SH-Ph crystal (Figure. S11), due to the existence of strong intermolecular NH…
O hydrogen bond (2.0031 Å, Figure. S11d). Cross-stacking 3-F-SH-Ph molecules exhibit an edge-to-face herringbone packing motif
without any intermolecular face-to-face π–π interactions (Figure. S11c and S11d). The intramolecular N…HO hydrogen bond and
dihedral angle of 3-F-SH-Ph molecule are 1.8864 Å (Figure. S11a) and 40.5o (Figure. 8), respectively. This biggest dihedral angle would
interrupt the extension of π-conjugated system efficiently, resulting in no AIE for 3-F-SH-Ph solid.
It is easy to extend π-conjugation system through the presence of naphthalene ring in Naph-SH-Ph. The intramolecular N…
HOhydrogen bond and dihedral angle of Naph-SH-Ph molecule are 1.8650 Å (Figure. S12a) and 33.0o (Figure. 8), respectively. The
intermolecular hydrogen bond of Naph-SH-Ph molecules is NH…O (1.9639 Å, Figure. S12e). Naph-SH-Ph molecules exhibit a more
slipped face-to-face lamellar packing (Figure. S12b) than SH-Ph. Although the interplanar distance (d = 3.52 Å, Figure. S12c) of
Naph-SH-Ph molecules is longer than that of SH-Ph molecules, Naph-SH-Ph molecules are more π-conjugated with little face-to-face π–
π interactions (Figure. S12c and S12d). These above factors lead to Naph-SH-Ph solid (Φ = 0.081) has a bit bigger Ф than SH-Ph solid (Φ =
0.079). It is noteworthy that the solvent molecules are involved in crystals. For nonpolar solvent, ClCH2CH2Cl molecules have some weak
intermolecular interactions (O…H: 2.6361 Å and Cl…H: 2.9100 Å) with Naph-SH-Ph molecules (Figure. 10a and S12b). For polar solvent,
however, H2O molecules go through much stronger intermolecular hydrogen bonds (OH…O: 1.8845 Å and NH…OH2: 2.1114 Å) to
act with Naph-SH-Ph molecules (Figure. 10b), which results in the planar structure of Naph-SH-Ph in H2O (Figure. 10c).77 This planar
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structure would be more π-conjugated than the twisty structure, and thus it might be another reason for the unusual phenomenon of
polar solvent-induce fluorescence71 (Figure. S21).
Figure. 10 Intermolecular interactions between solvent (a: ClCH2CH2Cl; b,c: H2O) and Naph-SH-Ph molecules in X-ray single crystals.
As shown in Figure. 11, with the help of another OH group, Naph-SH-OH molecule has two types of intramolecular hydrogen bonds
(N…HO: 1.9004 Å and HO…HN: 1.9219 Å), and thus its dihedral angle is close to zero (Figure. 8). Naph-SH-OH has a similar cross
molecular stacking with Naph-SH-Ph (Figure. S13). The d (3.05 Å, Figure. 11b) and intermolecular OH…O hydrogen bond (1.8074 Å,
Figure. 11c) of Naph-SH-OH molecules are almost smallest. Moreover, planar Naph-SH-OH molecules almost have no intermolecular
face-to-face π–π interactions (Figure. 11b). Therefore, as shown in Table S1, with promoted double ESIPT processes, Naph-SH-OH solid
has the highest  up to 0.20. It must be pointed out that big π-conjugated and high stacked Naph-SH-OH molecules have a worst
solubility among all SHs molecules.
Figure. 11 X-ray single crystal structures and packing of Naph-SH-OH molecules: (a) single molecule; (b) top and side view of the two adjacent molecules
(H atoms are omitted); (c) intermolecular interactions of the two closest molecules.
As the previous discussion, it is strange that the fluorescence bands of Naph-SH-Cn (n= 3 and 5) solids are much different. Their X-ray
single crystal structures (Figure. S14 and S15) reveal that their molecular structures, arrangements, and intermolecular hydrogen bonds
are much different. Naph-SH-C5 molecules appear a slipped face-to-face lamellar packing (Figure. S14) with a bit long interplanar
distance (d = 3.44 Å) and strong intermolecular face-to-face π–π interactions, and thus the  (0.033) of Naph-SH-C5 solid is low. The
intermolecular hydrogen bond of Naph-SH-C5 molecule is NH…O= (1.9978 Å). The H (–NH) and O (–C=O) atoms in the above discussed
X-ray single crystal structures are trans-form. For Naph-SH-C3 molecule (Figure. S15), however, it is unusual that they are cis-form (the
same cis-form is also observed in SH-C1 molecule,75 Figure. S16). Unlike Naph-SH-C5 molecules, Naph-SH-C3 molecules show a parallel
and stratiform packing through double intermolecular hydrogen bonds (2.0046 Å) between the H (–NH) and O (–C=O) atoms (Figure.
S15d). The interplanar distance (d = 3.44 Å) of Naph-SH-C3 molecules is similar with that of Naph-SH-C5 molecules, but there is no
intermolecular face-to-face π–π interactions between Naph-SH-C3 molecules (Figure. S15c), which might be contributed to the fact that
Naph-SH-C3 solid (Φ = 0.10) has a higher  than Naph-SH-C5 solid (Φ = 0.033).
Since chiral Naph-SH-BnOH(S) and Naph-SH-BOH(R) are a pair of enantiomers, their molecular structures and arrangements are
similar and mirrored (Figure. 12).41 Like Naph-SH-OH molecule, Naph-SH-BnOH(S or R) molecules have two types of intramolecular
hydrogen bonds (=N…HO: ~1.89 Å and HO…HN: ~2.14 Å). Moreover, small intermolecular OH…O= hydrogen bond (~1.88 Å) and
interplanar distance (d = 2.96 Å) with little intermolecular face-to-face π–π interactions are found in their stratiform packing molecules
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(Figure. S17 and S18). Therefore, their solids emit AIE ( = ~0.064), even although the benzene rings are almost vertical to the
naphthalene rings. 4-NEt2-SH-BnOH(R) (Figure. S19) molecule has a similar molecular structure with Naph-SH-BnOH molecule, but only
one type of intramolecular hydrogen bond (N…HO: 1.9148 Å) is existed, which might result in no AIE for 4-NEt2-SH-BnOH solid. In the
polar solvent of EtOH, 4-NEt2-SH-BnOH(R) molecules acting as cages to catch EtOH molecules through two types of intermolecular
hydrogen bonds (EtOH…O=: 1.9060 Å and NH…OHEt: 2.0251 Å), which indicates that these soft and chiral SH molecules might act
with amino acids or other chiral molecules through intermolecular hydrogen bonds as well.
Figure. 12 X-ray single crystal structures (a) and packing (b) of Naph-SH-BnOH(S) and Naph-SH-BnOH(R) molecules.
The AIE of Naph-SH-C5 solid can be further improved by changing Naph group into 4-NEt2 group (4-NEt2-SH-C5: Φ= 0.11). Compared
with Naph-SH-C5 molecules, 4-NEt2-SH-C5 molecules (Figure. S20) have a similar interplanar distance (d = 3.45 Å) but little
intermolecular face-to-face  interactions. Moreover, the strongest intermolecular interactions of two closest 4-NEt2-SH-C5 molecules
are H…H (2.1538 Å) rather than intermolecular HO…HN hydrogen bond (2.1770 Å).
Finally, for the purpose of the mechanism evaluation, the photophysical properties of reference substances, Ref1Ref5 (Figure. 2),
were examined. All of them are non-emissive in solid and water at room temperature, indicating that both salicylaldehyde-based Schiff
base structure and amide group (NHCO) are of greatest importance for AIE. The former would provide not only big enough
-conjugation system and OH group for   * and n  * transition but also strong intramolecular N…HOhydrogen bond for
preventing the IRs of phenolic ring and enabling ESIPT process. The later might promote ESIPT and extend -conjugation system.
Moreover, the later would help to enhance strong intermolecular interactions to achieve AIE through intermolecular hydrogen bonds.
These SHs having small π-conjugated systems emit strong solid-state emission along with large Stokes shifts, which would enable us to
doubt that their emission does not originate from fluorescence but phosphorescence.78 However, time-resolved emission spectra (Figure.
S22) reveal that the emission decay lifetime of SH-BnOH, Naph-SH-C3, Naph-SH-OH, and SH-OH (powder) is 1.75, 2.56, 1.87, and 1.69 ns,
respectively. And thus, we can draw a conclusion that their emission originates from fluorescence rather than phosphorescence. These
bit long emission decay lifetimes also accord with the nature of ESIPT.
5.1. Mechanochromism properties of SHs
Since the fluorescence properties of AIE-active materials are highly dependent on their molecule arrangements, their fluorescence
often shows a red shift along with emission intensity decrease upon the transition from crystalline into amorphous states under external
stimuli,19-20,27 It is rare to encounter mechanochromic AIE-active materials with enhanced fluorescence.39,43,79 Among these SHs,
4-NEt2-SH-F, Naph-SH-BnOH(S), and Naph-SH-BOH(R) (Figure. S23-S24) are mechanochromic material with turn-on fluorescence. As
example, the pristine crystal solids of 4-NEt2-SH-F (Figure. 13) are weak emissive, but after grinding or fast evaporation of CH2Cl2 by
reduced pressure distillation, the resulted powders emit strong green emission under the irradiation of UV light. Their powder
wide-angle X-ray diffraction experiments (Figure. 13) reveal that all the solid samples of 4-NEt2-SH-F have intense and sharp reflection
peaks, which indicates that all the solid samples mainly are microcrystalline and there is no phase transition from a crystalline into an
amorphous phase. Crystal-defect-induced emission might be contributed to the fluorescence enhancement of ground and evaporated
4-NEt2-SH-F solids.39,43,79
evaporated solids
ground solids
pristine crystals
simulated pattern
10
20
30
40
50
2 / degree
Figure. 13 X-ray powder diffraction and photographs (under sunlight or 360 nm UV light) of 4-NEt2-SH-F solids.
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6.1. Ion probe properties
As the previous reports,60,61,63-67 SHs acting as an O^N^O terdentate ligand can coordinate with many metal ions, and thus they can
be used as metal ion probes. Among these SHs, Naph-SH-C3, SH-C3, and Naph-SH-Ph exhibit the best performances for detecting Mg2+,
as shown in Table S2. For example, Naph-SH-C3 has the biggest chelation-induced fluorescence enhancements (Φ up to 0.46) and good
selectivity to Mg2+ (Figure. S25) over many other metal ions. Its fluorescence intensity increases linearly upon the addition of 0−1.0
equivalent of Mg2+ (correlation coefficient R2 =0.978, n = 10) and reach saturation upon the addition of 1.0−2.0 equivalent of Mg2+
(Figure. S26 and S27), revealing that the stoichiometry of the resulted complex (Figure. 1f) is 1:1 (Figure. S27), which is consistent with
the former reported result.60,61
Our previous work41,43 demonstrated that salicylaldehyde-based Schiff bases can be used as anion probes. In this work, we also tried
to use SHs to detect anions under the same experimental conditions. Various sodium or potassium salts (1.0
were added to dilute dye DMSO solution (2.0 × 10−5 mol dm−3) by a microsyringe. The sensing performance would be much worse if

0.10 mol dm−3 in water)
other solvents, such as MeCN, EtOH, and MeOH, were used. As shown in Table S3 and Figure. S28S32, the selected SHs usually emit
2
4
3
2

strong anion-induced fluorescence (Φ up to 0.14) upon adding OH, CO3 , AC, P2O7 , PO4 , S , and F, which indicates that they can
be used as universal anion probes with turn-on fluorescence properties. As example, upon adding many anions, including OH, F, Cl,
2
3

2
2

4
3
2
Br, I, NO, CO3 , HCO , SO4 , S , SO32 , P2O7 , PO4 , and HPO4 , to dilute Naph-SH-C5 DMSO solution would turn on the


blue-green fluorescence (up to 60-fold enhancement) (Figure. 14a). OH (Figure. 14a and 14b) was used to investigate the concentration
effect. The fluorescence of Naph-SH-C5 exhibits a linear enhancement (R2 = 0.991, n = 11) upon the addition of 0−2.5 equivalent of OH
and then reaches saturation (Ф = 0.072). For Cy, DiPh,41 and TSB43 without amide group, 100 equivalent of OH should be added for
fluorescence saturation. This reveals that SHs have much stronger interactions with anions than Cy, DiPh, and TSB, which is in
accordance with the fact that SHs with the help of amide group might react with anions through triplex hydrogen or halogen bonds
(Figure. 1f).
7.1. Chirality and chiral recognition
The chirality properties of NaPh-SH-BnOH(R) and NaPh-SH-BnOH(S) are examined by circular dichroism (CD) spectra. As depicted in
Figure. 15, NaPh-SH-BnOH(R) shows a positive Cotton effect at 358 nm and further strong peaks at 250, and 212 nm. The CD spectrum
of NaPh-SH-BnOH(S) exactly mirrors that of NaPh-SH-BnOH(R), indicating that the two compounds are a pair of enantiomers.
HS

SO32
HCO3
probe
AC
(a)

2
NO3
SO42
HPO4


NO2
I
2
3
H2PO4
CO3
4
P2O7
PO4

HSO3
Br
F
OH
S2
Cl

0.8
1.1
1.3
1.5
1.8
2
0
0.2
0.5
2.2
2.5
3
3.5
4
4.5
5
450
500
550
600
650
700
Wavelength / nm
(b)
0
1
2
3
4
5
OH / eq.
Figure. 14 (a) Top: emission spectra of Naph-SH-C5 upon the addition of 2.5 equivalent of different anions. Bottom: emission spectra of Naph-SH-C5 upon
the addition of different equivalent of OH. (b) Plot of emission intensity of Naph-SH-C5 at 520 nm as a function of OH concentration. Insert:
5
photographs of Naph-SH-C5 + 2.5 equivalent of OH, F, D-Arg, and Mg2+under 360 nm UV light). The probe is 2.0 × 10 mol dm3 in DMSO (in MeCN for
Mg2+) and is excited at 400 nm.
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30
15
0
Naph-SH-BnOH(R)
Naph-SH-BnOH(S)
-15
-30
250
300
350
400
probe
Pro
Trp
Ser
Val
His
Ala
Gln
Arg
Leu
450
500
550
600
650
700
Wavelength / nm
5
3
Figure. 15 Top: CD spectra of NaPh-SH-BnOH(R) and NaPh-SH-BnOH(S) in EtOH (5.0 × 10 mol dm ). Bottom: Emission spectra of Naph-SH-BnOH(R or S)
(2.0 × 10 mol dm3 in DMSO, excited at 400 nm) upon the addition of different 100 equivalent of D- or L-amino acid.
5
First, we found that L- or D-amino acids can induce the fluorescence enhancements (Φ up to 0.16) of SH-C3, Naph-SH-C3,
Naph-SH-C5, and 4-NEt2-SH-CN, like anions (Table S4 and Figure. S33S36). Considering the problem of solubility in both water and
DMSO, we chose nine amino acids including Alanine (Ala), valine (Val), leucine (Leu), serine (Ser) proline (Pro), histidine (His), tryptophan
(Trp), arginine (Arg), and glutamine (Gln). However, these achiral probes have similar sensing responses to L- or D-type of same amino
acids, and thus they have no any chiral recognition. Then, chiral NaPh-SH-BnOH(R) and NaPh-SH-BnOH(S) were used to identity L- or
D-amino acids (Figure. 15 and Table S4). Unfortunately, NaPh-SH-BnOH(R) and NaPh-SH-BnOH(S) also did not show any chiral
recognition of L- or D-amino acids. Finally, we tried to use NaPh-SH-BnOH(R) and NaPh-SH-BnOH(S) for chiral recognition of the some
other chiral molecules, including L-/D-tartaric acid, (S,S)/(R,R)1,2-diaminocyclohexane(DACH), (S,S)/(R,R)1,2-diphenylethylenediamine
(DPEDA), and (S)/(R)1-(4-Methoxyphenyl)ethylamine (MEA) (Table S5 and Figure. S37S43). Upon adding L-/D-tartaric acid, there is no
any fluorescence enhancement for both NaPh-SH-BnOH(R) and NaPh-SH-BnOH(S), but three amine compounds induce strong
fluorescence enhancements (Φ up to 0.062). Combining the above result of amino acid-induced fluorescence enhancements, we could
draw a conclusion that SHs would have much stronger interactions with NH2 group than –COOH group. Both NaPh-SH-BnOH(R) and
NaPh-SH-BnOH(S) have similar fluorescence enhancements upon adding (S) or (R)MEA (Figure. S3841). Even although
NaPh-SH-BnOH(S) can’t identity the chirality of (S,S)/(R,R)DACH and (S,S)/(R,R)DPEDA (Figure. 16), it is fortunate that NaPh-SH-BnOH(R)
can discriminate the enantiomers of DACH and DPEDA. For example, adding 100 equivalent of (S,S) or (R,R)DACH to NaPh-SH-BnOH(R)
solution (R2 =0.988, n = 10), the fluorescence would turn on with a 50- and 112-fold enhancement, respectively. The selective
recognition of chiral molecular enantiomers is related to the enantiomeric fluorescence difference ratio, ef, according to ef =
(ISI0)/(IRI0), in which I0 represents the fluorescence emission intensity of receptor in the absence of a chiral substrate and IS and IR are
the fluorescence intensities in the presence of (S) and (R) substrates, respectively. NaPh-SH-BnOH(R) shows an good ef value of 0.4 for
enantioselective recognition of (S,S) and (R,R)DACH. The ef of NaPh-SH-BnOH(R) is 0.36 for enantioselective recognition of (S,S) and
(R,R)DPEDA.
Since it is too difficult to grow good-quality single crystals from the mixture of the probe and diamine compounds of DACH and
DPEDA in DMSO solution, we failed to get a clear insight into the chiral discrimination ability. We just guess that Naph-SH-BnOH(R) has
much stronger interactions with diamine compounds than monoamine compound of MEA and non-amine compound of tartaric acid.
Moreover, Naph-SH-BnOH(R) that is not a full π-conjugated molecule would be flexible and consequently might have a better ability to
form a cavity80 for the recognition of a tiny chiral structure difference between (S,S) or (R,R)diamine compounds.
NaPh-SH-BnOH(R)
(S,S)DACH
NaPh-SH-BnOH(S)
(S,S)DACH
(R,R)DACH
(R,R)DACH
300 eq.
5.0 eq.
NaPh-SH-BnOH(R)
(S,S)DPEDA
NaPh-SH-BnOH(S)
(S,S)DPEDA
(R,R)DPEDA
(R,R)DPEDA
500
600
700
Wavelength / nm
0
50
100
150
200
250
300
450 500 550 600 650 700
450 500 550 600 650 700
(R,R)DACH / eq.
Wavelength / nm
Figure. 16 Top: emission spectra of the probes upon the addition of 100 equivalent of different (S,S) or (R,R)DACH and DPEDA. Bottom: Emission spectra
5
3
and work line of Naph-SH-BnOH(R) upon the addition of different equivalent of (R,R)DACH. The probe is 2.0 × 10 mol dm in DMSO and is excited at
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400 nm.
8.1. Mechanism of anion, amino acid, and DACH probes
1
In order to investigate the possible interaction mechanism between the dye and anion/amino acid/DACH, H nuclear magnetic
resonance (1H NMR) analysis was done. As shown in Figure. 17a, the 1H NMR signal of reactive H5 and H3 (enol form, Figure. 1d) atoms in
Naph-SH-BnOH(R) appears at δ = 12.7 and 11.9 ppm, respectively, in the downfield region. If adding 100 equivalent of OH (in D2O,
Figure. 17b), F (in D2O, Figure. 17c), D-Arg (in D2O, Figure. 17d), or (R,R)DACH (in DMSO-d6, Figure. 17e), these downfield peaks would
disappear, indicating that these reactive OH groups change into O by strong intermolecular hydrogen bonds between the dye and the
1
analyte. The H NMR signal of OH1 group (δ = 6.6 ppm) at chiral carbon atom also shows obvious changes in both chemical shift and
peak pattern, which might reveal that the chiral group has strong intermolecular interactions with the analyte through hydrogen bonds
and consequently can has capacity of chiral recognition. Moreover, since the neighboring nitrogen and oxygen atoms have strong
1
intermolecular interactions with the analyte through hydrogen and halogen bonds, the H NMR signals of non-reactive H4 (δ = 9.6 ppm)
and H2 (δ = 5.2 ppm) atoms exhibit obvious changes as well.
1
Figure. 17 H NMR spectra of Naph-SH-BnOH(R) (a), Naph-SH-BnOH(R) + 100 equivalent of OH (b), Naph-SH-BnOH(R) + 100 equivalent of F (c),
Naph-SH-BnOH(R) + 100 equivalent of D-Arg (d), and Naph-SH-BnOH(R) + 100 equivalent of (R,R)DACH (e) in DMSO-d6.
The mechanisms of the aggregation- and anion/amino acid-induced fluorescence enhancement are totally different. For example,
the green-yellow AIE (em = ~555 nm) of Naph-SH-C3 is originated from the aggregated state (in water or solid) but its blue-green (em
=
~500 nm) anion/amino acid-induced fluorescence is originated from the monomer (2.0  10 mol dm3 in DMSO). As mentioned above,
anion/amino acid would lead to changing two reactive OH3 (enol form) and OH5 into O, and thus Naph-SH-C3 in form of O (Figure.
S44) is used to evaluate the nature of analyte-induced excited states and transitions by calculation. The lower energy absorption band of
Naph-SH-C3 (in form of O, abs = 400 and 464 nm for experiment and computation, respectively) is HOMO  LUMO. The π-functions of
HOMO and LUMO of Naph-SH-C3 in form of OH and O are similar, which reveals that anion/amino acid would not induce to change of
the way of transition (π  π* transition). This is much different from our previous work43 that anion would induce to change of the way
of transition from n  π* into π π* for TSB. Moreover, anion/amino acid would red shift the lower energy absorption band with an
increment of molar extinction co-efficient (Figure. S44). This phenomenon can be reasonably explained by the fact that electron-rich O
anions have resonance structures of benzoquinone.81 These resonance structures lead the -function frontier molecular orbitals to a
uniform distribution in the molecule skeleton, and consequently Naph-SH-C3 (in form of O) has not only a smaller energy gap of
transition but also an increment of molar extinction co-efficient. This also might be contributed to its high fluorescence quantum yield in
dilute DMSO solution. Since there is no anymore ESIPT for Naph-SH-C3 (in form of O), its fluorescence blue shifts into blue-green region.
5
9.1. Living cell imaging
These small conjugated AIE-active materials might have a lower cytotoxicity and better biocompatibility, and thus some of them
were used in cell imaging applications.82 The human gastric cancer cell lines (IM95m) were imaged by the dye using a standard
cell-staining protocol. As can be seen from Figure. 18, incubated with the dyes, about 95 % cells are live and grow similar as in the
control experiments in the absence of the dyes, indicating that these dyes have little toxicity on the living cells. The cells show negligible
background fluorescence (Figure. 18b). However, intense intracellular green, yellow, and orange AIE is observed after the cells are
incubated with Naph-SH-CN (Figure. 18c), 4-NEt2-SH-CN (Figure. 18d), 3-NO2-SH-Ph (Figure. 18e), and 3-NO2-SH-OH (Figure. 18f),
respectively. In order to reveal the location of the dye after it enters living cells, co-staining studies of 4-NEt2-SH-CN with a cell nucleus
dye of 4’6-diamidino-2-phenylindole (DAPI) were conducted as well. When excited by 380 nm, the co-stained cells emit strong blue
emission mainly from DAPI (Figure. 18g), but they emit strong green emission mainly from 4-NEt2-SH-CN when excited 460 nm (Figure.
18h). Figure. 18d, 18g, 18h, and 18i indicate that 4-NEt2-SH-CN is located in not only cytoplasm but also cell nucleus. Therefore, these
AIE-active materials may have potentially applications in probing or monitoring biologically important processes in vitro as well as in
vivo.
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Figure. 18 Brightfield (a: incubated without dye) and fluorescence images (b: incubated without dye; c: incubated with Naph-SH-CN; d: incubated with
4-NEt2-SH-CN; e: incubated with 3-NO2-SH-Ph; f: incubated with 3-NO2-SH-OH; g: incubated with DAPI and 4-NEt2-SH-CN excited at 380 nm; h: incubated
with DAPI and 4-NEt2-SH-CN excited at 460 nm; overlay of g and h) of human gastric cancer cell lines (IM95m).
3. Applications and Potentials
Electronic Supplementary Information (ESI) available: General, materials, computational details, crystallographic information files
(CIF), crystallographic data, absorption, and fluorescence emission data. CCDC: Naph-SH-C3: 1585254; Naph-SH-C5: 1585255;
4-NEt2-SH-C5: 1814156; 4-NEt2-SH-F: 1585256; 3-F-SH-Ph: 1585257; Naph-SH-Ph: 1585258; 4-NEt2-SH-Ph: 1585259; Naph-SH-BnOH(R):
1585260; 4-NEt2-SH-BnOH(R): 1585261; Naph-SH-BnOH(S): 1585262. See DOI: 10.1039/b000000x/.
4. Conclusions
In the present work, we report the unique multiple hydrogen bonds promoted ESIPT, AIE, mechanochromism, universal probes,
chiral recognition, and cell imaging properties of a series of simple and highly fluorescent salicylaldehyde hydrazide molecules. Their
molecular structures, -conjugated systems, and non-covalent intermolecular interactions are dependent on the intermolecular and
intramolecular hydrogen bonds. Moreover, hydrogen bonds are also susceptible to environmental changes, and consequently they can
be used as universal probes for many common chemicals. Therefore, these simple salicylaldehyde hydrazide molecules provide a simple
and bright paradigm in the design of multiple-responsive soft materials.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (no. 21372169) and Sichuan Provincial Foundation
(2008SG0021). We acknowledge the comprehensive training platform of the specialized laboratory of the College of Chemistry, Sichuan
University, for material analysis. We acknowledge the comprehensive training platform of the specialized laboratory of the College of
Chemistry, Sichuan University, for material analysis. We would like to thank the Analytical & Testing Center of Sichuan University for
CCD X-ray single crystal diffractometer work and circular dichroism CD spectrometer work. We are grateful to Daibing Luo and Yani Xie
for help with the single crystal and circular dichroism measurements.
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