Keto-salicylaldehyde azine: Asymmetric substituent effect on their optical properties via electron-donating group insertion

Article abstract of DOI:10.1039/c9tc05822b

Organic fluorescent probes have attracted increasing attention owing to their high sensitivity and recognition ability, structurally adjustable flexibility and manifestation in the form of visualization. Salicylaldehyde azine (SAA) and its derivatives exhibit a great potential for practical applications, but the synthesis of SAA asymmetric compounds in low yields, serving multiple functions, is cumbersome. As an alternative building block, Keto-Salicylaldehyde Azine (KSA) was developed by our group for constructing various AIEgens via an excited state intramolecular proton transfer (ESIPT) process to detect some cellular organelles and specific metal ions. For illuminating the structure-property relationship of the two asymmetric sides in a KSA unit, DPAS was employed as a model and the N,N-diethyl group with electron-donating effects was introduced on its salicylaldehyde side (S-terminal) and diphenylketone side (K-terminal). Three new derivatives were obtained and showed different photophysical properties, particularly in AIE performance and metal ion responsiveness, implying that their intrinsic electronic structure can be easily affected by the asymmetric substitution effect. Therefore, this study is a meaningful and valuable reference for KSA modification on expanding fluorescent probes with various functions via the purposeful regulation of their chemical structure, by either modifying the K-terminal or the S-terminal.

Full text of DOI:10.1039/c9tc05822b

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Keto-salicylaldehyde azine: asymmetric
substituent effect on their optical properties
Cite this:DOI: 10.1039/c9tc05822b
via electron-donating group insertion†
Jialin Tong, ‡a Kexin Zhang,‡b Jing Wang,‡a Hui Li,a Fan Zhou,b
abc
bc
Zhiming Wang,
*
Xiaojuan Zhang*a and Ben Zhong Tang
*
Organic fluorescent probes have attracted increasing attention owing to their high sensitivity and recognition
ability, structurally adjustable flexibility and manifestation in the form of visualization. Salicylaldehyde azine
(SAA) and its derivatives exhibit a great potential for practical applications, but the synthesis of SAA
asymmetric compounds in low yields, serving multiple functions, is cumbersome. As an alternative building
block, Keto-Salicylaldehyde Azine (KSA) was developed by our group for constructing various AIEgens via
an excited state intramolecular proton transfer (ESIPT) process to detect some cellular organelles and
specific metal ions. For illuminating the structure–property relationship of the two asymmetric sides in a
KSA unit, DPAS was employed as a model and the N,N-diethyl group with electron-donating effects was
introduced on its salicylaldehyde side (S-terminal) and diphenylketone side (K-terminal). Three new
derivatives were obtained and showed different photophysical properties, particularly in AIE performance
and metal ion responsiveness, implying that their intrinsic electronic structure can be easily affected by the
asymmetric substitution effect. Therefore, this study is a meaningful and valuable reference for KSA
modification on expanding fluorescent probes with various functions via the purposeful regulation of their
chemical structure, by either modifying the K-terminal or the S-terminal.
Received 24th October 2019,
Accepted 26th November 2019
DOI: 10.1039/c9tc05822b
rsc.li/materials-c
ions or anions in various environments is one of the most
typical and familiar uses, relevant in our daily lives.3 However,
1. Introduction
Organic fluorescent probes, as important analytical and detection most organic fluorescent probes with conjugated structures,
materials, have attracted increasing attention.1 Owing to their such as those containing aromatic rings, will inevitably exhibit the
tunability in chemical structures, they can be designed and aggregation-caused quenching (ACQ) phenomenon in aqueous
constructed into ideal probes according to our various practical environments, which is not suitable for their practical applica-
needs; and their structures can be constantly optimized in to tions.4 The discovery of aggregation-induced emission (AIE) has
increase their selectivity and sensitivity.2 For example, their fundamentally overcome this shortcoming to a large extent,5 and
application in monitoring the nature and concentration of metal the development of AIE-based fluorescent probes has gradually
become a key research theme.6
Salicylaldehyde azine (SAA) is a typical building-block capable
a School of Petrochemical Engineering, Shenyang University of Technology (SUT),
of excited-state intramolecular proton transfer (ESIPT),7 synthe-
Liaoyang 111003, China. E-mail: 1664392140@qq.com
b Key Laboratory of Luminescence from Molecular Aggregates of Guangdong
sized from hydrazine and salicylaldehyde Schiff-base. SAA deri-
vatives exhibit obvious AIE behaviors with highly distinguishable
fluorescence performance and have been widely applied as
chemosensors, bioprobes, bioimaging scaffolds, etc.8 These
symmetric compounds can be easily synthesized from salicyl-
aldehyde derivatives and hydrazine hydrate at room tempera-
ture with high yields (Scheme 1A). Tong et al. have given a great
contribution to the application of SAA-based derivatives.9 How-
ever, the synthesis of asymmetric SAA derivatives is difficult.
Not only an accurate control of the monomers’ feed ratio is
required, but also the reaction intermediates are easily con-
verted to their symmetric products due to the autocatalytic
Province, State Key Laboratory of Luminescent Materials and Devices,
Center for Aggregation-induced emission, South China University of Technology,
Guangzhou 510640, China. E-mail: wangzhiming@scut.edu.cn
c Department of Chemistry, Hong Kong Branch of Chinese National Engineering
Research Center for Tissue Restoration and Reconstruction, Department of
Chemical and Biological Engineering, The Hong Kong University of Science and
Technology, Kowloon, Hong Kong, China. E-mail: tangbenz@ust.hk
† Electronic supplementary information (ESI) available: The synthesis of com-
pounds, 1H NMR spectra of N-DPAS, 2N-DPAS and 3N-DPAS; Abs spectra of the
AIE properties of N-DPAS and 3N-DPAS; I/I0 emission plots of N-DPAS and 2N-
DPAS; The histogram analysis of N-DPAS, 2N-DPAS and 3N-DPAS dots upon
addition of the mixture of different ions. See DOI: 10.1039/c9tc05822b
‡ Jialin Tong, Kexin Zhang and Jing Wang contributed equally to this work.
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protonic acid (acetic acid), the diaryl ketones reacted with
excessive hydrazine hydrate in ethanol to form ketone hydrazone
derivatives, which were then cooled to precipitate their crystals,
and washed with water to remove excess hydrazine hydrate. The
crude products obtained without further purification could
directly react with the corresponding salicylaldehyde derivatives
to obtain the target structures. Compared to the synthesis route
of common fluorescent probes, the preparation cost of these KSA
derivatives was lower, which is very meaningful for future
practical applications.12
Scheme 1 (A) SAA synthetic route; (B) the synthetic route to obtain KSA.
Here, the N,N-diethyl group, a classic electron-donating group,
was introduced on KSA’s two substitution sides for observing the
effects of asymmetry on the luminescence behaviour. In N-DPAS,
the N,N-diethyl group was inserted para to the position of the
aldehyde group in salicylaldehyde, which could enhance its
electron-donating effects owing to resonance with the newly formed
azine core in N-DPAS. In order to prevent the effect of cis-trans
isomerization on the electronic structure due to the change of the
conjugated form of the hydrazine center, two N,N-diethyl groups
were introduced at the K-terminal to form 2N-DPAS. Combining
the structural characteristics of the two compounds above,
3N-DPAS was also prepared. Their chemical structures were
characterized via NMR, high-resolution mass spectroscopy
(HRMS), and satisfactory results were obtained. The three
derivatives all showed a similar typical intramolecular hydro-
gen bond formation to DPAS, which highly favors the ESIPT
process. Moreover, the diaryl structure at the K-terminal had a
certain degree of freedom and rotation ability, which provided
a possibility for the restriction of rotation from solution to the
aggregate state. All these factors suggest that the three com-
pounds possibly have AIE abilities. In terms of electronic
structure distribution, the three derivatives are significantly
different from DPAS. In general, the introduction of electron-
donating groups leads to the enhancement of the intra-
molecular CT effect, which usually leads to the quenching of
fluorescence in the aggregated state.13 In addition, as KSA has a
typical dentate chelating structure, the change in electron distribu-
tion on the phenolic hydroxyl group and the nitrogen atoms in the
azine group can also lead to a change in their target recognition
ability. In fact, for these positive and negative effects, which one is
more beneficial to improve the performance of KSA fluorescent
probes is our real concern (Fig. 1).
action (H+) of salicylaldehyde. In order to break through the bottle-
necks of SAA-based derivative syntheses and efficiently prepare more
functional asymmetric fluorescence probes, we have constructed
and developed a new type of fluorescence probe building-block
named keto-salicylaldehyde azine (KSA).10 Compared to the SAA
unit, KSA is easily prepared with fairly high yields from salicylalde-
hyde derivatives and diaryl-ketone hydrazones, and the latter is
either commercial available or easily prepared from the reaction
(Scheme 1B) between hydrazine hydrate and diaryl ketone deriva-
tives, without any purification via column chromatography. For
describing these structures more vividly, we defined the two sides
of the azine centre as the K-terminal (from the diaryl ketone end)
and the S-terminal (from the salicylaldehyde end). Employing the
Friedel–Crafts acylation and the phenolic hydroxyl reaction with
urotropine, different diaryl ketone and salicylaldehyde derivatives
could be obtained as expected. By changing the peripheral recogni-
tion group, these different KSA-based materials showed target
imaging and fluorescence switching ability.10,11
DPAS was previously reported by our group for lipid droplets-
targeting and metal ions probing, and one of its derivatives, named
2M-DPAS, could target lysosomes after introducing two basic
morpholine groups linked with six-carbon chains.10a Unfortunately,
the fundamental electronic effect of this asymmetric KSA structure
has not been discussed in detail. In this study, the N,N-diethyl group
was employed as an electron-donating group to introduce on the
S-terminal or K-terminal, and three compounds, N-DPAS, 2N-DPAS,
and 3N-DPAS, were synthesized; moreover, the effects of intra-
molecular charge-transfer (CT) and the ESIPT process on their
photophysical behaviours were discussed. Fortunately, the
asymmetric substitution effect, originating from their intrinsic
electronic structure, was observed to affect their functions,
particularly the AIE performance and metal ion responsiveness.
Hence, we present a meaningful and a valuable review for KSA-
modification on expanding fluorescent probes with various
functions via the purposeful regulation of chemical structures
in the K-terminal or the S-terminal.
2.2 Solvation effects
The solvation effect is the most distinctive feature for molecules
capable of ESIPT because the nature and polarity of the solvent
can directly influence the formation of intramolecular hydrogen
bonding.14 Herein, we chose six solvents to test the absorption (Abs)
2. Results and discussion
2.1 Molecule design
As described in Scheme S1 (ESI†), commercial and cheap diaryl
ketone and salicylaldehyde derivatives were employed as the
raw materials. Under the catalysis of a small amount of a
Fig. 1 Chemical structures of DPAS N-DPAS, 2N-DPAS, and 3N-DPAS.
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the K-emission, while the intramolecular CT effect might play an
important role in the weakening of ESIPT when introducing
electron-donating groups at the S-terminal. If both conditions
existed at the same time, there would be a competitive
relationship. This situation happened precisely in 3N-DPAS, as
shown in Fig. 2F, and its PL spectra became visibly very compli-
cated, and shows the sensitivity of the species to the solvent
polarity through a red-shift and widening of the peaks, which
was very difficult to discuss in detail their every peak source, but
it might be very beneficial to selective sensing behaviours in a
specific environment with little changes.
2.3 AIE performance
According to our previous reports, the rotations and vibrations of
DPAS would be restricted from solution to the aggregate state,
leading to the easy formation of intramolecular hydrogen bonds to
reduce enol–keto conversion barrier, the RIM and ESIPT process are
activated to exhibit AIE performance at K-emission location.10a
In order to discuss how the aggregation process is affected by the
substitution with electron-donating groups and by molecular
asymmetry, the absorptivity and PL spectra of N-DPAS, 2N-DPAS
and 3N-DPAS were recorded, and are presented in Fig. 3.
Fig. 2 Abs and PL spectra of N-DPAS (A and B), 2N-DPAS (C and D) and
3N-DPAS (E and F) in different solvents (10À5 M).
and photoluminescence (PL) abilities of our compounds, and
the results are illustrated in Fig. 2. In a similar way to most of the
ESIPT-based compounds,15 the shifts of their absorption peaks
were not significant, suggesting that the polarity of the external
environment did not have a significant effect on their ground
states. However, these changes became noticeably very compli-
cated in the fluorescence spectra. Unlike their model compound
DPAS, with a short-wavelength emission at 425 nm, originating
from the E form emission (E* - E) and a long-wavelength
emission at 550 nm, assigned to the emission of the K
form (K* - K),10a N-DPAS and 3N-DPAS did not show a clear
proportional switching trend between E-emissions and
K-emissions. However, in 2N-DPAS, the K-emission at 545 nm
seemed to have absolute advantage than the E-emission at about
440 nm (Fig. 2D), implying that the energy barrier for the
conversion from E* to K* might be relatively small and easily
crossed, even in low-polarity solvents, when the N,N-diethyl
groups are only inserted on the K-terminal. For N-DPAS, the
E-emission seemed to be dominant, and the red-shift from
440 nm to 460 nm might be due to the intramolecular charge
transfer effect caused by the introduction of the electron-
donating group at the S-terminal (Fig. 2B). Comparing the two,
we speculated that the introduction of electron donors at the
K-terminal could enhance the ESIPT process with a benefit on
Fig. 3 The Abs spectra of N-DPAS (A) in THF–water mixtures with
different water fractions (fw); The PL spectra of N-DPAS (B), 2N-DPAS (C)
and 3N-DPAS (D) in THF–water mixtures with different fw; E, K (I/I0
@440 nm and 545 nm) and E/K emission plots of 2N-DPAS (E and F),
where I0 is the PL intensity in pure THF solution.
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As shown in Fig. 3A, the absorption band at long wavelength
Journal of Materials Chemistry C
increased in absorption spectra of N-DPAS at fw 4 60%,
implying the formation of aggregate structures. Regrettably,
this enhanced emission was not significantly visible by naked
eye observation under a UV-lamp. As seen in the PL spectra, its
E-emission exhibited obvious ACQ in the aggregated state, and
its K-emission peak at 530 nm was enhanced very little even for
fw = 80% (the maximum fw), which indicates that the ESIPT
process is at a disadvantage relative to the dominant CT
process. This result was consistent with its solvation behaviour.
For 2N-DPAS, as the water content increased, the intensity of
the E-emission peak at 440 nm gradually decreased (ACQ), and
the intensity of the K-emission peak at 545 nm gradually
increased (AIE), implying that the ESIPT process and the
restriction of intramolecular rotation (RIR) became protagonists.
When fw = 90%, the spectrum was completely originating from
the K-emission, with higher intensity compared to other fw
(Fig. 3C). Based on its photophysical behaviour discussed above,
2N-DPAS showed the phenomenon of enol–keto conversion,
which indicates that the introduction of electron-donating
groups at the K-terminal makes the E* - K* process easier,
and increases the occurrence of ESIPT. Combining ESIPT with the
effect of RIM, its significant AIE performance can be explained.
Additionally, as shown in Fig. 3E, the signal contrast would be
improved by processing mathematically using the classic dual
fluorescence-changing process (K-emission/E-emission). 3N-DPAS
combined the characteristic of N-DPAS and 2N-DPAS, and its PL
spectra were complicated by many factors, such as solvent polarity
and aggregation (Fig. 3D). However, in careful comparison, its
Fig. 4 The changes of PL spectra of N-DPAS (A), 2N-DPAS (C) and 3N-DPAS
(E) in the mixture (fwN-DPAS = 80%; f2wN-DPAS = 100%; fw3N-DPAS = 80%) upon
E-emission located at about 465 nm, and the K-emission was at addition 5 eq. of respective ions; changes in PL spectra of N-DPAS (B) upon
addition of various Hg2+ (from 0 to 20 eq.), 2N-DPAS (D) upon addition of
around 521 nm. Although CT in 3N-DPAS was larger than in
2N-DPAS, the ESIPT process was more active than for N-DPAS.
Under the synergistic effect of the RIM process, 3N-DPAS exhibited
various Cu2+ (from 0 to 0.15 eq.) and 3N-DPAS (F) upon addition of various
Hg2+ (from 0 to 15 eq.) in different water fraction (fw).
a weak AIE activity (Fig. S5B, ESI†). In a word, standing at these
results, the introduction of electron-donating groups only to the hence they were used with the fw that achieved the relative
K-terminal of KSAs is very beneficial to constructing AIEgens, higher emission intensity: f Nw-DPAS = 80%, f w2N-DPAS = 100%, and
which is very important to develop KSA-based fluorescent f 3wN-DPAS = 80% (defined as blank). By measuring the PL spectra
probes, that can be exploited in their aggregate state.
in ion mixtures, the three probes showed some selectivity to
Cu2+ via fluorescence quenching (‘‘turn-off’’ mode) like DPAS,
but there was a significant difference in the degree of chelation
2.4 Metal ion selectivity
As typical Schiff base structures, KSA and its derivatives usually at the same ion/probe concentrations, illustrating that these
have a certain metal ion chelating ability, and they could asymmetric substitutions do affect the chelating ability of the
achieve the selective recognition of metal ions by controlling probes. As a typical AIE probe, 2N-DPAS showed a quenching
their molecular and electronic structure, and changing the effect similar to DPAS, owing to the electron cloud density
solvent.10b As a model compound, DPAS exhibited good selec- increment over the N atoms in the azine group when
tivity to Cu2+ as a fluorescence quenching (‘‘turn-off’’ mode) N,N-diethyl groups are inserted on the K-terminal. After the
probe with excellent limits of detection (LOD) in aqueous fluorescence titration process shown in Fig. 4D, the fluorescence
solutions;10b but in the three new derivatives, the ion-selective intensity of the K-emission at 548 nm gradually decreased with
behaviour was expected to change, because the electronic the continuous addition of Cu2+. Based on the curves of the
structure of their chelating sites is affected by the different fluorescence quenching (r2 = 0.9867), 2N-DPAS showed a limit of
effects of the electron-donating groups. Therefore, 16 metal detection of 0.11 mM, which is lower than that of common Cu2+
À
ions (using their ClO4 salts) were employed to explore the probes with similar structures (like SAA and its derivatives).8a,9c
probes’ selectivities (Fig. 4): Cu2+, Hg2+, Ag+, Li+, Na+, Zn2+, Simultaneously, the aggregates of 2N-DPAS exhibited a certain
Co2+, Pb2+, Mn2+, Ni2+, Ba2+, Cd2+, Mg2+, Al3+, Ca2+, and Fe3+
.
anti-interference ability for Hg2+ interference in ‘‘turn-on’’ mode,
Considering their practical application value, the aggregates as shown in Fig. S6B (ESI†), which was better than that of DPAS.
employed were prepared based on the AIE discussion above, The important photophysical data was shown in Table 1.
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Table 1 The important photophysical data of three compounds
lem (nm)
labs (nm)
PLQY (%)
Compound
Solnc
E-emissiona
K-emissiona
Add Hg2+b
Add Cu2+b
Aggregation ( fw = 95%)
Soln (THF)
Solid
N-DPAS
2N-DPAS
3N-DPAS
406
395
417
441
440
465
530
545
521
535
—
558
—
548
—
0.1
1.7
0.8
0.4
0.7
0.4
0.9
1.3
0.7
a
b
c
Max peak locations in the aqueous environment. Max peak locations in the metal ion selectivity. Max peak locations in the THF.
When the N,N-diethyl group was introduced on the S-terminal, substitution, 2N-DPAS gave a good AIE performance with
the sensitivity of the probe to Cu2+ decreased obviously, while the higher selectivity to Cu2+ than DPAS, without Hg2+/Fe3+ inter-
selectivity to Hg2+/Fe3+ increased significantly. In N-DPAS, the ference. Combining the spectral analysis of the asymmetric
fluorescence intensity increased with the addition of Hg2+ (as a substitution effect, the role of the different KSA modifications
model), which indicated fluorescent detection in ‘‘turn-on’’ mode. for various functions became very clear, which is very important
As shown in Fig. 4B, a good linear relationship (r2 = 0.9931) with a when developing function-oriented KSA probes.
LOD of 6.02 mM was obtained by a titration experiment. Although
the LOD for Hg2+ was still not ideal, such a simple composite
probe, having a fluorescent ‘‘turn-on’’ ability, is still an important
Conflicts of interest
tool for rapid detection in practical applications. Certainly, its
The authors declare no competing financial interest.
selectivity needs further improvement in distinguishing Hg2+ and
Fe3+ via other means. 3N-DPAS was observed to be less sensitive, in
accordance with the discussion on the solvation effect and the AIE Acknowledgements
performance. 3N-DPAS combined the characteristics of the other
two compounds, giving very little response to Cu2+ (‘‘turn-off’’
This work was financially supported by the National Natural Science
Foundation of China (21788102, 51673118 and 21975077), the
mode) and Hg2+/Fe3+ (enhanced mode), which is not of much
Science & Technology Program of Guangzhou (201804010218 and
practical value. Interestingly, some valuable red-shifts in the PL
201804020027), the Innovation and Technology Commission of
spectra appeared when Hg2+/Fe3+ were added, and the change
Hong Kong (ITC-CNERC14SC01), the Fundamental Research
could be observed by the naked eye from bright green emission
Funds for the Central Universities (2019ZD04) and the Fund of
to red one under UV-lamp. Employing titration experiments, the
Key Laboratory of Luminescence from Molecular Aggregates of
Guangdong Province (2019B030301003).
LOD for Hg2+ was estimated at 7.07 mM (r2 = 0.9773), similar to that
of N-DPAS. However, the fluorescence-induced colour change dis-
plays the effects in a more intuitive way. More importantly, the
significance of the simultaneous substitution on both terminals
was revealed. Comparing the three compounds’ performances in
Notes and references
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