Salieylaldehyde azines as fluorophores of aggregation-induced emission enhancement characteristics

Article abstract of DOI:10.1021/jo802631m

A series of salicylaldehyde azine derivatives were found to exhibit interesting aggregation-induced emission enhancement (AIEE) characteristics. In good solvent, all these compounds displayed very weak fluorescence, while strong emission was observed when they were placed in poor solvent. Moreover, the AIEE color of these compounds varied from green to red depending on the substituents on azines. Their in situ formation also promises potential applications in fluorescence sensing of hydrazine.

Full text of DOI:10.1021/jo802631m

sion enhancement (AIEE)2 characteristics not only promise great
potential applications in optical sensors, light emitting diodes,
photovoltaic cells, and photoemitters, but also challenge the
current knowledge of the photoluminescence process and may
help us extend our understanding on fluorescence mechanisms.
However, most organic fluorescent dyes show fluorescence
quenching in their aggregate or solid states owning to the
formation of delocalized excitons or excimers, which may cause
enhanced nonradiative deactivation of the excited state.3 Only
a limited number of organic compounds have been reported to
display AIEE characteristics, such as siloles,2c,d,4 1-cyano-trans-
1,2-bis(4′-methylbiphenyl)ethylene (CN-MBE),2b,5 thienylazu-
lene,6 arylethene derivatives,7 cis-2,5-diphenyl-1,4-distyrylben-
zene, and 1,4-di[(E)-2-phenyl-1-propenyl]benzene (PPB).8
Therefore, exploration in new AIEE fluorophores is still of great
interest.
Salicylaldehyde Azines as Fluorophores of
Aggregation-Induced Emission Enhancement
Characteristics
Weixin Tang, Yu Xiang,* and Aijun Tong*
Key Laboratory of Bioorganic Phosphorus Chemistry &
Chemical Biology (Ministry of Education), Department of
Chemistry, Tsinghua UniVersity, Beijing 100084, People’s
Republic of China
tongaj@mail.tsinghua.edu.cn;
xiangyu00@mails.tsinghua.edu.cn
ReceiVed August 28, 2008
A design principle of AIEE molecules in published articles
was the connection of two or more conjugated/aromatic moieties
by rotatable C-C single bonds.2,4-8 Fluorophores of this type
exhibit free intramolecular rotation in the single molecule state,
but the rotation is inhibited in the aggregated state. Because
intramolecular rotation could be an effective mechanism for
fluorescence quenching,3 such fluorophores generally show
AIEE characteristics. However, although it is quite simple in
molecular design, the synthesis of these potential AIEE com-
pounds is often complicated.
In this paper, a series of facilely synthesized salicylaldehyde
azine derivatives9 were found to exhibit AIEE characteristics
(Scheme 1). They are different in molecular design compared
to most reported AIEE fluorophores: (1) two salicylaldimine
moieties are connected by rotatable N-N single bond rather
than C-C bond and (2) intramolecular hydrogen bonds therein
ensure intramolecular rotation allowed only for the N-N bond.
More interestingly, the AIEE color is dependent on the
substituents of these azine derivatives (from green (λmax ) 513
nm) to red (λmax ) 570 nm)). As one can monitor the color of
AIEE fluorescence simply by varying a single chemical
substitution, this research may help in developing multicolor
AIEE dyes. In aqueous solution containing 4% acetic acid, in
A series of salicylaldehyde azine derivatives were found to
exhibit interesting aggregation-induced emission enhance-
ment (AIEE) characteristics. In good solvent, all these
compounds displayed very weak fluorescence, while strong
emission was observed when they were placed in poor
solvent. Moreover, the AIEE color of these compounds
varied from green to red depending on the substituents on
azines. Their in situ formation also promises potential
applications in fluorescence sensing of hydrazine.
Organic dyes that exhibit strong fluorescent emission in their
aggregate or solid state have attracted increasing attention in
recent years.1 These fluorophores of aggregation-induced emis-
(3) (a) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (b) Friend,
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Tang, Y.; Ho¨ussler, M.; Tong, H.; Dong, Y.; Lam, J. W. Y.; Ren, Y.; Sung,
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10.1021/jo802631m CCC: $40.75
Published on Web 01/29/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 2163–2166 2163

FIGURE 1. Crystal structure of compound 2. Left: ORTEP drawing
with 35% probability ellipsoids. Right: A packing view along the b
direction.
SCHEME 1. Chemical Structures of Compounds 1-8
FIGURE 2. Absorption (a) and fluorescence excitation/emission spectra
(b) of 1-3 (10 µM) in “solution” (ethanol/water, 9:1, v/v) and
“aggregate” (ethanol/water, 1:9, v/v) states.
situ formation of these AIEE schiff bases is also successfully
applied in the sensing of lower ppm levels of hydrazine,10 which
is an energetic and hazardous compound.
Compounds 1-5 were synthesized by very simple procedures
of condensing corresponding salicylaldehyde derivatives and
hydrazine hydrate in ethanol or ethanol/dichloromethane. They
were characterized by NMR, ESI-MS, and elemental analysis.
The crystal structure of 2 is shown in Figure 1. Intramolecular
hydrogen bonds of salicylaldimine moieties and stacking of
molecules could be clearly observed as expected, suggesting
its potential as an AIEE fluorophore by inhibition of free
intramolecular rotation when changed from solution to aggregate
state.
The AIEE characteristics of 1-3 are investigated in ethanol/
water (from 9:1 to 1:9, v/v) buffered by 10 mM HEPES at pH
7.0 and shown in Figure 2. In a good solvent of 9:1 ethanol/
water, 1-3 were well dispersed and displayed structured
absorption spectra and weak fluorescence emission of their
“solution” state. However, when they were in a poor solvent of
1:9 ethanol/water, level-off tails in the visible region of their
absorption spectra (commonly observed in nanoaggregates
suspensions11) clearly suggested the formation of “aggregate”
FIGURE 3. Effect of ethanol volume fraction on the AIEE fluorescence
intensity (peaks in fluorescence spectra) of 1-3 (10 µM) in ethanol/
water containing 10 mM HEPES at pH 7.0. Excitation was performed
at 400 nm.
state, and strong fluorescence emission was observed conse-
quently. This AIEE effect could be explained by the blocking
of the nonradiative intramolecular rotation decay of excited
molecules through formation of J- or H-aggregates.2b,12 The
explanation is supported by the fluorescence enhancement of
1-3 as the viscosity of solvent (glycol/glycerin) increased
(Figure S1 in the Supporting Information), because high
viscosity would inhibit intramolecular rotation and induce the
AIEE effect. The quantum yields13 of 1-3 in their solution and
aggregate states were determined as 0.002, 0.002, 0.001, 0.13
(τ ) 3.09 ns), 0.11 (τ ) 3.05 ns), and 0.05 (τ ) 2.32 ns),
respectively. As illustrated in Figure 3, the AIEE effect occurs
at 30% volume fraction of ethanol for 1 and 2, and 60% for 3.
(10) (a) Thomas, S. W., III; Swager, T. M. AdV. Mater. 2006, 18, 1047. (b)
Guo, L.; Matysik, F.-M.; Gla¨ser, P.; Engewald, W. Electrophoresis 2005, 26,
3341. (c) Kempter, C.; Karst, U. Analyst 2000, 125, 433. (d) Chen, X.; Xiang,
Y.; Li, Z.; Tong, A. Anal. Chim. Acta 2008, 625, 41.
(11) Tang, B. Z.; Geng, Y.; Lam, J. W. Y.; Li, B.; Jing, X.; Wang, X.; Wang,
F.; Pakhomov, A.; Zhang, X. X. Chem. Mater. 1999, 11, 1581.
(12) Oelkrug, D.; Tompert, A.; Gierschner, J.; Egelhaaf, H.; Hanack, M.;
Hohloch, M.; Steinhuber, E. J. Phys. Chem. B 1998, 102, 1902.
(13) Calculated by using quinine sulfate in 0.1 M H2SO4 (excitation at 366
nm) as a standard (Φ ) 0.55) for solution. For solid, an integral sphere was
applied with BaSO4 white plates as a standard (Φ ) 1.0).
2164 J. Org. Chem. Vol. 74, No. 5, 2009

FIGURE 4. Effect of pH on the fluorescence intensity (peaks in
fluorescence spectra) of 1-3 (10 µM) in ethanol/water 1:9 (v/v)
containing 10 mM HEPES at various pH. Excitation was performed at
400 nm.
Lower ethanol volume fraction was found to enhance the AIEE
effect of 1-3, which was in agreement with the fact that more
aggregates should form in poorer solvents. Besides absorption/
fluorescence spectra, the formation of fluorescent aggregates of
1-3 in poor solvent are also supported by the results of dynamic
light scattering (DLS, Figure S2(a) in the Supporting Informa-
tion), solid state fluorescence (Figure S3 in the Supporting
Information), and SEM (Figure S4 in the Supporting Informa-
tion): (1) no particle (>5 nm) could be observed for 2 in 30%
ethanol (nonfluorescent), while particles of micro- or submi-
crometer sizes were detected when volume fraction of ethanol
was decreased to 20% or 10% (fluorescent); (2) the solid state
fluorescence of 1-3 (solids isolated by centrifugation from 100
µM 1-3 in ethanol/water (1:9, v/v) buffered by 10 mM HEPES
at pH 7.0) correlated well with their AIEE aggregate fluores-
cence in poor solvent; and (3) SEM images of the solids obtained
in (2) also suggested the formation of particles with similar sizes
revealed by DLS results.
The substituents of these aromatic azines play important roles
in both the availability and emission color of AIEE. Although
1-3 showed the AIEE effect when dispersed in poor solvent
compared to good solvent, no fluorescence or AIEE effect could
be observed for benzoaldehyde azine (4) or 2-methoxyben-
zoaldhyde azine (5) whether in 9:1 or 1:9 ethanol/water.
Absorption spectra of 4 and 5 showed no level-off tail in the
visible region even in 9:1 to 1:99 ethanol/water, indicating no
aggregate state was formed for these two compounds due to
their good solubility. More interestingly, when the electron
donating OH group is at the 4-position of azine 1, the color of
the AIEE was green (λmax ) 513 nm), while with Cl substitution
at the 5-position of azine 3, the color shifted to red (λmax ) 570
nm). Azine 2 without substitution showeda yellow (λmax ) 542
nm) AIEE effect.
The effect of pH on AIEE fluorescence is investigated for
1-3 in ethanol/water (1:9, v/v) buffered by HEPES at pH
4.0-10.0 (Figure 4). Increasing pH resulted in a slight decrease
in the AIEE effect of 2 and 3 but nearly eliminated that of 1
when pH reached 10.0. Actually, according to DLS result
(Figure S2(b) in the Supporting Information), no particle (>5
nm) was found for 1 in ethanol/water (1:9, v/v) at pH 10.0, but
micrometer-sized particles were detected when pH increased
to 7.0. This suggested that 1 might undergo deprotonation at
higher pH and negatively charged ions of 1 tended to release
from fluorescent aggregates to form weakly fluorescent monomers.
Because the corresponding substituted salicylaldehydes are
completely nonfluorescent and exhibit strong reactivity toward
hydrazine to form azines of AIEE characteristics, one might
apply these aldehydes as chemodosimeters for hydrazine detec-
FIGURE 5. Fluorescence spectra (a) and titration curves (intensity of
peaks in fluorescence spectra) (b) of 6-8 (2 mM) in the absence and
presence of various amounts of hydrazine in water containing 4%
(volume fraction) AcOH and 20% ethanol. Excitation was performed
at 400 nm.
tion by fluorescence enhancement. As shown in Figure 5,
4-hydroxylsalicylaldehyde (6), salicylaldehyde (7), and 5-chlo-
rosalicylaldehyde (8) were utilized to detect hydrazine at lower
ppm level in aqueous solution containing 4% AcOH and 20%
ethanol, respectively. Among these three candidates, 8 showed
the highest sensitivity, as could be seen from the time-dependent
fluorescence experiment for reaction kinetics (Figure S5 in the
Supporting Information). The high sensitivity of 8 is probably
because of the strong reactivity of 8 to hydrazine due to the
electron withdrawing effect of 5-chloro substitution. The reaction
products of 6-8 with hydrazine were isolated and confirmed
by ESI-MS and NMR to be 1-3 as expected, respectively. DLS
experiment (Figure S2(c) in the Supporting Information) also
revealed that the reaction of 8 and hydrazine yielded some
submicrometer-sized particles of 3 in solution, which led to the
fluorescence observed. Interestingly, hydroxylamine, 1,2-eth-
yldiamine, ammonia, methylamine, common metal ions, and
anions of various inorganic acids showed nearly no interference
to hydrazine sensing by 8, suggesting its high selectivity.
In conclusion, we described the AIEE fluorescence properties
of a series of salicylaldehyde azine derivatives 1-3 in this paper.
It is very interesting to note that the AIEE fluorescence color
of these azines could be monitored by the substituent on the
phenyl ring. This AIEE effect suggests the great potential of
sensing hydrazine in water by salicylaldehyde derivatives with
high sensitivity and selectivity.
Experimental Section
Synthesis of Compounds 1-5. General Procedure: In a 100
mL flask 7.0 mmol of a certain aldehyde was dissolved in 50 mL
of ethanol. Hydrazine hydrate (0.2 g, 3.4 mmol; 85%) was then
J. Org. Chem. Vol. 74, No. 5, 2009 2165

added dropwise with vigorous stirring at room temperature. After
the addition, the stirred mixture was allowed to stand at room
temperature overnight, and the resulting precipitation was filtered
and washed by 30 mL of ethanol or ethanol/water (1:1, v/v) 3 times.
After drying, azines were obtain in high yields (62-79%).
Recrystallization of the products was carried out in DMSO/ethanol
(1:1, v/v). If the aldehyde was not easily soluble in ethanol, ethanol/
dichloromethane (1:2, v/v) was used as solvent. The addition of
NaOH in DMSO-d6 was for a sufficient solubility of each compound
in 13C NMR measurement.
2H), 7.48 (d, 2H), 8.79 (s, 2H). 13C NMR (DMSO-d6, 2.5 equiv of
NaOH) δ (ppm) 112.6, 122.6, 124.2, 125.9, 131.5, 158.6, 171.4.
Elemental Anal. Found: C 54.1, H 3.41, N 8.89. Calcd for
C14H10Cl2N2O2: C 54.4, H 3.26, N 9.06.
Benzoaldhyde Azine (4). 4 was synthesized with benzoaldhyde
as a light yellow solid (yield 51%). ESI mass spectrometry: m/z
1
209.4 ([M + H] +); M+ calcd 208.1. H NMR (CDCl3) δ (ppm)
7.46 (m, 6H), 7.85 (dd, 4H), 8.68 (s, 2H). 13C NMR (CDCl3) δ
(ppm) 128.7, 128.9, 131.3, 134.2, 162.2. Elemental Anal. Found:
C 80.2, H 5.96, N 13.1. Calcd for C14H12N2: C 80.7, H 5.81, N
13.4.
4-Hydroxysalicylaldhyde Azine (1). 1 was synthesized with
2-Methoxybenzoaldhyde Azine (5). 5 was synthesized with
4-hydroxysalicylaldhyde as dark yellow solid (yield 65%). ESI mass
1
spectrometry: m/z 271.2 ([M - H] -); M- calcd 272.1. H NMR
2-methoxybenzoaldhyde as a light yellow solid (yield 61%). ESI
1
mass spectrometry: m/z 269.3 ([M + H] +); M+ calcd 268.1. H
(DMSO-d6) δ (ppm) 6.34 (d, 2H), 6.41 (dd, 2H), 7.43 (d, 2H),
8.78 (s, 2H), 10.22 (br, 2H), 11.41 (br, 2H). 13C NMR (DMSO-d6)
δ (ppm) 103.0, 108.7, 110.8, 133.5, 161.2, 162.3, 162.6. Elemental
Anal. Found: C 61.5, H 4.56, N 10.1. Calcd for C14H12N2O4: C
61.8, H 4.44, N 10.3.
NMR (DMSO-d6), δ (ppm) 3.89 (s, 6H), 7.05 (dd, 2H), 7.15 (d,
2H), 7.51 (dd, 2H), 7.99 (d, 2H), 8.95 (s, 2H). 13C NMR (DMSO-
d6) δ (ppm) 56.3, 112.5, 121.2, 122.2, 127.1, 133.5, 157.1, 159.3.
Elemental Anal. Found: C 71.5, H 6.23, N 10.2. Calcd for
C16H16N2O2: C 71.6, H 6.01, N 10.4.
Salicylaldhyde Azine (2). 2 was synthesized with salicylaldhyde
as a yellow solid (yield 75%). ESI mass spectrometry: m/z 239.3
1
([M - H] -); M- calcd 240.1. H NMR (DMSO-d6, 2.5 equiv of
Acknowledgment. We are very grateful for the financial
support from the National Natural Science Foundation of China
(NSFC, No. 20875054 and 90813014).
NaOH) δ (ppm) 6.08 (dd, 2H), 6.34 (d, 2H), 6.89 (dd, 2H), 7.48
(d, 2H), 8.83 (s, 2H). 13C NMR (DMSO-d6, 2.5 equiv of NaOH) δ
(ppm) 109.7, 121.8, 122.6, 127.9, 132.0, 159.8, 172.6. Elemental
Anal. Found: C 69.6, H 5.17, N 11.5. Calcd for C14H12N2O2: C
70.0, H 5.03, N 11.7.
Supporting Information Available: Experimental proce-
dures, characterization data, and additional figures for the
compounds described. This material is available free of charge
via the Internet at http://pubs.acs.org.
5-Chloro-salicylaldhyde Azine (3). 3 was synthesized with
5-chloro-salicylaldhyde as a yellow solid (yield 79%). ESI mass
1
spectrometry: m/z 307.3 ([M - H] -); M- calcd 308.1. H NMR
(DMSO-d6, 2.5 equiv of NaOH) δ (ppm) 6.32 (d, 2H), 6.84 (dd,
JO802631M
2166 J. Org. Chem. Vol. 74, No. 5, 2009