Novel pyrene- and anthracene-based Schiff base derivatives as Cu 2+ and Fe3+ fluorescence turn-on sensors and for aggregation induced emissions

Article abstract of DOI:10.1039/c2ta00574c

Novel pyrene- and anthracene-based Schiff base derivatives P1 and A1 were synthesized via a one-pot reaction and utilized as fluorescence turn-on sensors towards Cu²⁺ and Fe³⁺ ions, respectively, and for aggregation induced emissions (AIEs). P1 in CH₃CN and A1 in THF illustrated the fluorescence turn-on sensing towards Cu²⁺ and Fe ³⁺ ions, respectively, via chelation enhanced fluorescence (CHEF) through excimer (P1-P1* and A1-A1*) formation. The 2:1 stoichiometry of the sensor complexes (P1 + Cu²⁺ and A1 + Fe³⁺) were calculated from Job plots based on UV-Vis absorption titrations. In addition, the binding sites of sensor complexes (P1 + Cu²⁺ and A1 + Fe ³⁺) were well established from the ¹H NMR titrations and supported by the fluorescence reversibility by adding metal ions and PMDTA sequentially. The detection limits (LODs) and the association constant (K _a) values of P1 + Cu²⁺ and A1 + Fe³⁺ sensor responses were calculated by standard deviations, linear fittings and from their fluorescence binding isotherms. More importantly, P1 + Cu²⁺ and A1 + Fe³⁺ sensors were found to be active in wide ranges of pHs (1-14 and 2-14, respectively). Moreover, the time effect along with the enhancements of quantum yield (Φ) and time resolved photoluminescence (TRPL) decay constant (τ) towards sensor responses were investigated. Similarly, P1 in CH ₃CN and A1 in THF showed AIEs by increasing the aqueous media concentration from 0% to 90%, with altered fluorescence peak shifts (red and blue shifts, respectively). As well as τ value enhancements, the Φ values of 0.506 and 0.567 (with 630- and 101-fold enhancements) were acquired for P1 in CH₃CN:H₂O (20:80) and A1 in THF:H₂O (40:60), respectively. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ta00574c

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Materials Chemistry A
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PAPER
Novel pyrene- and anthracene-based Schiff base
derivatives as Cu2+ and Fe3+ fluorescence turn-on
sensors and for aggregation induced emissions†
Cite this: J. Mater. Chem. A, 2013, 1,
1310
Muthaiah Shellaiah, Yen-Hsing Wu, Ashutosh Singh, Mandapati V. Ramakrishnam
*
Raju and Hong-Cheu Lin
Novel pyrene- and anthracene-based Schiff base derivatives P1 and A1 were synthesized via a one-pot
reaction and utilized as fluorescence turn-on sensors towards Cu2+ and Fe3+ ions, respectively, and for
aggregation induced emissions (AIEs). P1 in CH3CN and A1 in THF illustrated the fluorescence turn-on
sensing towards Cu2+ and Fe3+ ions, respectively, via chelation enhanced fluorescence (CHEF) through
excimer (P1–P1* and A1–A1*) formation. The 2 : 1 stoichiometry of the sensor complexes (P1 + Cu2+
and A1 + Fe3+) were calculated from Job plots based on UV-Vis absorption titrations. In addition, the
binding sites of sensor complexes (P1 + Cu2+ and A1 + Fe3+) were well established from the 1H NMR
titrations and supported by the fluorescence reversibility by adding metal ions and PMDTA sequentially.
The detection limits (LODs) and the association constant (Ka) values of P1 + Cu2+ and A1 + Fe3+ sensor
responses were calculated by standard deviations, linear fittings and from their fluorescence binding
isotherms. More importantly, P1 + Cu2+ and A1 + Fe3+ sensors were found to be active in wide ranges of
pHs (1–14 and 2–14, respectively). Moreover, the time effect along with the enhancements of quantum
yield (F) and time resolved photoluminescence (TRPL) decay constant (s) towards sensor responses were
investigated. Similarly, P1 in CH3CN and A1 in THF showed AIEs by increasing the aqueous media
concentration from 0% to 90%, with altered fluorescence peak shifts (red and blue shifts, respectively).
Received 5th October 2012
Accepted 2nd November 2012
DOI: 10.1039/c2ta00574c
As well as
s value enhancements, the F values of 0.506 and 0.567 (with 630- and 101-fold
www.rsc.org/MaterialsA
enhancements) were acquired for P1 in CH3CN : H2O (20 : 80) and A1 in THF : H2O (40 : 60), respectively.
also provided uorescence enhancements with or without any
spectral changes.
Introduction
Among all metal ions, Cu2+ is a signicant metal pollutant
due to its widespread use,8 but it is also required as a cofactor in
nearly 20 enzymes9 and is an essential micro-nutrient for all
known life forms. Moreover, long-term exposure to high levels
of Cu2+ has been reported to induce liver and kidney damage.10
In addition, most of the copper-selective sensors suffer from the
interfering effect11 of cations, such as Zn2+, Hg2+, Pb2+, Fe3+ and
Ag+. Therefore, the development of highly selective uorescence
probes12 for copper ions in the presence of a variety of other
metal ions has received great interest. Similarly, the Fe3+ ion is
essential for the proper functioning of all living cells, but it is
detrimental when present in excess.13 Most of the iron present
in biological systems is tightly associated with enzymes, as well
as in specialized transport and storage in proteins.14 Further-
more, iron homeostasis is an important factor involved in
neuroinammation and the progression of Alzheimer's
disease.15 Many sensor reports16 are available for Cu2+ and Fe3+
ions along with synthetic difficulties, but only a few of them
were accounted as turn-on sensors. Hence, we tend to develop
the specic sensory materials for Cu2+ and Fe3+ detections with
optical turn-on responses via simple synthetic pathways.
Owing to the biological and environmental importance of metal
ions, numerous sensory reports are available for different metal
ions with diverse mechanisms.1 Among the available detection
methods, chemosensors based on ion-induced uorescence
changes are predominantly attractive in terms of sensitivity,
selectivity, response time and local observation (e.g., uores-
cence imaging spectroscopy).2 Due to the uorescence
quenching effects3 of biologically important ions, the develop-
ment of uorescence turn-on sensors still remains a chal-
lenging task. Several molecular turn-on sensors4 were reported
for a variety of cations and anions based on photoinduced
electron transfer (PET), internal charge transfer (ICT), chelation
enhanced uorescence (CHEF) and deprotonation mecha-
nisms. Among them, PET5 exhibited various changes of emis-
sion intensities with some or no spectral shis, whereas ICT6
caused both intensity changes and spectral shis, and CHEF7
Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu 30049, Taiwan (ROC). E-mail: linhc@mail.nctu.edu.tw
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2ta00574c
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Similar to sensor applications, aggregation induced emis-
Journal of Materials Chemistry A
sion (AIE) has become more popular and has also been applied
to many areas of science17 by Tang et al. during the last decade.
Molecules with AIE characteristics have been found to serve as
chemosensors, bioprobes, stimuli-responsive nanomaterials
and active layers in the construction of efficient organic light-
emitting diodes.18 However, many of these molecules also have
synthetic difficulties, so we try to develop molecules that could
exhibit the AIE effects with less synthetic difficulties.
With these considerations, Schiff bases19 are recognized as
having simple synthetic steps and are also applied to many
optical sensors, as well as in AIE applications. However, to
develop Schiff baseswith sensor and AIE properties, the presence
of strong uorophores are required.20 Pyrene and anthracene
derivatives were evidenced as excellent uorophores and widely
used in the developments of uorescence (FL) sensors because of
their excellent photoluminescence properties and chemical
stabilities. Furthermore, pyrene and anthracene uorescent
probes self-assembled to form dimeric structures upon the
addition of certain metal cations to give P–P* and A–A* excimer
uorescence and also provided the AIE characteristics by tuning
the solvent conditions as reported previously.21 Therefore, we
developed the novel pyrene- and anthracene-based Schiff bases
and evaluated their sensor and AIE properties (Fig. 1).
Scheme 1 Synthesis of P1 and A1.
the HOMO of P1 positioned in the imine group and the LUMO
in the pyrene ring. Similarly, the HOMO electron clouds of A1
were located in the imine group and the LUMO electron clouds
were in both phenyl and anthracene rings.
Photophysical properties of Schiff bases and sensor
complexes
The photophysical properties of P1 and A1 and sensor
complexes (P1 + Cu2+ and A1 + Fe3+) are shown in Table 1. Both
Schiff bases P1 and A1 were excited at 395 nm and showed PL
emission maxima at 459 and 499 nm, respectively. Since excess
additions of water to P1 in CH3CN and A1 in THF caused the
aggregation induced emissions, we performed UV-Vis/PL sensor
titrations of P1 and A1 in CH3CN and THF, respectively, by
adding metal ions in pure H2O. Similarly, 1H titrations were
performed in CD3CN and [D8]-THF, respectively, by adding
metal ions in D2O. The quantum yield (F) enhancements of the
sensor complexes (P1 + Cu2+ and A1 + Fe3+) were observed as
shown in Table 1. In general, sensor devices based on organic
semiconducting materials should not be dissolved in water and
also should possess p- or n-type semi-conducting properties.
Herein, we report novel pyrene- and anthracene-based Schiff
base derivatives (P1 and A1) for the rst time as Cu2+ and Fe3+
turn-on sensors via CHEF and excimer formations and these
were also utilized for aggregation induced emissions (AIEs) by
increasing the concentration of H2O (0–90%) with huge
quantum yield (F) enhancements.
Results and discussion
Synthesis
As shown in Scheme 1, P1 and A1 were synthesized via a one pot Moreover, those materials must have some probes that can
aldehyde and amine condensation in methanol with 98 and provide the selectivities towards the specic analytes in organic
71% yields, respectively.
solvents.23 Since pyrene and anthracene derivatives have the
p-type semiconducting properties, the utilization of P1 and A1
as metal ion sensors in organic solvents and their insolubilities
in water were considered as advantages for sensor devices in the
near future. At the same time, to discriminate the sensory and
AIE properties of P1 and A1, all the metal ions were taken from
1 mM concentration rather than 1 ꢀ 10ꢁ4 M and hence the
Computational analysis
Computational analyses of P1 and A1 were carried out via the
semi-empirical (AM1) method,22 as evidenced in Fig. 2. It reveals
Fig. 1 Schematic representations of sensors and aggregation induced emissions
(AIEs) of Schiff bases.
Fig. 2 Computational analysis of HOMO and LUMO levels of P1 and A1
(semi-empirical AM1 method).
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Table 1 Photophysical properties of Schiff bases and sensor complexes
Association
Quantum yield
Detection limits (LODs)c Quantum yieldd (F) (F) enhancements se (ns)
Compounds
CHEFa
constantb (Ka)
P1 (lex ¼ 395 and lem ¼ 459 nm)
A1 (lex ¼ 395 and lem ¼ 499 nm)
NA
NA
NA
NA
NA
NA
0.0009
0.005
0.284
0.125
NA
NA
315 fold
25 fold
1.30
10.60
2.05
P1 + Cu2+ (lex ¼ 395 and lem ¼ 455 nm) 591 fold 1.96 ꢀ 106 Mꢁ1 9.72 ꢀ 10ꢁ7
M
M
A1 + Fe3+ (lex ¼ 395 and lem ¼ 495 nm) 35 fold 1.88 ꢀ 105 Mꢁ1 2.95 ꢀ 10ꢁ6
12.46
a
b
I/I0 values were reported as Chelation enhanced uorescence (CHEF). Association constants were calculated from the slopes of binding
c
d
isotherms. Detection limits (LODs) were obtained from their uorescence binding isotherms. Quantum yields were calculated using 9,10-
diphenylanthracene (F ¼ 0.9) as a reference standard. e Obtained from time resolved uorescence measurements.
differences between sensory and AIEs of P1 and A1 were well selectivities were conrmed further by single and dual metal
established.
studies as follows. In order to establish the specic selectivities
of P1 and A1 to Cu2+ and Fe3+, respectively, we performed the
single and dual metal competitive analysis, as shown in Fig. 4.
In a single metal system (black bars), all the metal (Li+, Ag+, K+,
Na+, Cs+, Ni2+, Fe3+, Co2+, Zn2+, Cd2+, Pb2+, Ca2+, Cr3+, Mg2+,
Cu2+, Mn2+, Hg2+, Fe2+ and Ag2+ in H2O) concentrations were
kept as 50 mM towards P1. However, for the dual-metal (white
bars) studies, two equal amounts of aqueous solutions of Cu2+
and other metal ions (50 mM + 50 mM) were combined. During
the single metal analysis, the Cu2+ effect at 100 mM was taken.
Similarly, those metal ions concentrations were kept as 60 mM
towards A1 in the single metal system (black bars) and for dual-
metal (white bars) studies, two equal amounts of aqueous
solutions of Fe3+ and other metal ions (60 mM + 60 mM) were
combined. Furthermore, in the single metal analysis, the Fe3+
effect at 120 mM was investigated. In addition to the selectivity
of A1 towards Fe3+ ions, Fe2+ ions also showed little selectivity
(Fig. 3) with 15 fold of CHEF and 7 fold quantum yield
enhancements. However, further sensing enhancements of A1
towards Fe2+ in CHEF and F values w.r.t ratiometric titrations
(0–5 equiv.) was not evidenced, so the better selectivity of A1 to
Fe3+ ions was conrmed. Moreover, the specic selectivities of
P1 and A1 by single and dual metal studies also conrmed their
respective selectivities towards Cu2+ and Fe3+ ions, as shown in
Fig. 4. The photographs of P1 + Cu2+ and A1 + Fe3+ (under UV-
light irradiations) well demonstrated their sensitivities by
strong blue and green uorescences, respectively, as depicted in
Fig. 5a and b. On the other hand, modest selectivity of A1 to
Cu2+ and Fe2+ was also visualized by the photograph shown in
Fig. 5c.
Fluorescence titrations on metal ions
Initially, P1 (20 mM) in CH3CN and A1 (20 mM) in THF were
investigated towards 60 mM (3 equiv.) of metal ions (Li+, Ag+, K+,
Na+, Cs+, Ni2+, Fe3+, Co2+, Zn2+, Cd2+, Pb2+, Ca2+, Cr3+, Mg2+,
Cu2+, Mn2+, Hg2+, Fe2+ and Ag2+) in H2O. As noticed in Fig. 3, P1
and A1 show better selectivities to Cu2+ and Fe3+ ions, respec-
tively, upon treatment with 2.5 and 3 equiv. of metal ions,
respectively. Furthermore, the CHEF for P1 + Cu2+ and A1 + Fe3+
were found to be 591 and 35 fold, respectively and with 315 fold
(P1 + Cu2+; F ¼ 0.284) and 25 fold (A1 + Fe3+; F ¼ 0.125) of
quantum yield enhancements. In addition, the above
Fluorescence titrations on Cu2+ and Fe3+ sensors
By increasing the concentrations of Cu2+ (0–50 mM with an equal
span of 2 mM in H2O) the sensitivity of P1 (20 mM) in CH3CN
towards Cu2+ ions was clearly observed in Fig. 6a. The uores-
cence spectrum of P1 (lem ¼ 455 nm) showed the rapid turn-on
responses and the inset illustrated the relative uorescence
intensity changes as a function of Cu2+ concentration. On the
other hand, upon the addition of Fe3+ (0–60 mM with an equal
span of 2 mM in H2O) the sensitivity of A1 (20 mM) in THF
towards Fe3+ ions was evidenced in Fig. 6b. The uorescence
spectrum of A1 (lem ¼ 495 nm) exhibited turn-on responses
Fig. 3 Sensor responses of (a) P1 (1 ꢀ 10ꢁ5 M) in CH3CN (lex ¼ 395 nm) and (b)
A1 (1 ꢀ 10ꢁ5 M) in THF (lex ¼ 395 nm) towards metal ions (note: each metal
selectivity was measured with 2.5 and 3 equiv. of metal ions, respectively).
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Fig. 6 Fluorescence spectra of (a) P1 (20 mM) in CH3CN (lex ¼ 395 nm) with 0–50
mM of Cu2+ in H2O (with an equal span of 2 mM) and (b) A1 (20 mM) in THF (lex
¼
395 nm) with 0–60 mM of Fe3+ in H2O (with an equal span of 2 mM); insets: relative
fluorescence intensity changes with respect to Cu2+ and Fe3+ concentrations.
rapidly and the inset demonstrated the relative uorescence
intensity changes as a function of Fe3+ concentration. Further-
more, the CHEF values of P1 + Cu2+ and A1 + Fe3+ sensor
responses increased 591 and 35 folds, respectively, due to their
strong uorescence natures. Similarly, the quantum yield (F)
values of P1 + Cu2+ and A1 + Fe3+ also increased 315 and 25 fold,
respectively, as shown in Table 1.
Fig. 4 Relative fluorescence intensities of (a) P1 (20 mM) in CH3CN (lex ¼ 395
nm) with 50 mM of Cu2+ in H2O and (b) A1 (20 mM) in THF (lex ¼ 395 nm) with 60
mM Fe3+ in H2O in the presence of competing metal ions. Black bar; P1 (20 mM) in
CH3CN and A1 (20 mM) in THF with 50 and 60 mM of stated metal ions in H2O,
respectively. White bar; P1 (20 mM) in CH3CN with 50 mM Cu2+ + 50 mM of stated
metal ions and A1 (20 mM) in THF with 60 mM Fe3+ + 60 mM of stated metal ions in
H2O. (100 and 120 mM of Cu2+ and Fe3+ were taken for Cu2+ and Fe3+ effects,
respectively).
Stoichiometries24 and binding sites
To ensure the binding sites of sensor responses of P1 and A1,
the stoichiometries of P1 + Cu2+ and A1 + Fe3+ were calculated
through Job's plots as shown in Fig. S7 and S8 (ESI†) the stoi-
chiometries of P1 + Cu2+ and A1 + Fe3+ were established by Job's
plots between the mole fraction (XM) and absorption maximum
changes at 395 and 346 nm, respectively. Upon the addition of
0–30 mM of Cu2+ or Fe3+ (with an equal span of 3 mM), the
absorption maxima of P1 and A1 were quenched rapidly up to
10 mM, aerwards they were found to be restored again.
Therefore, the Job's plots were plotted between XM and
absorption changes at 395 nm (P1 + Cu2+) and 346 nm (A1 +
Fe3+), where they went through maxima at molar fractions of ca.
0.412 (P1 + Cu2+) and 0.370 (A1 + Fe3+), respectively, as shown in
Fig. S7b and S8b (ESI†), representing their 2 : 1 stoichiometric
complexes. As shown in Fig. 7, the stoichiometries were further
Fig. 5 Photographs of (a) P1, P1 + Cu2+, (b) A1, A1 + Fe3+ and (c) A1 + Cu2+, A1 +
Fe2+ visualized under UV-light irradiations (l ¼ 365 nm).
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1
supported by H NMR titrations25 along with binding sites
conrmation. In addition to stoichiometry, the total disap-
pearance of the –OH peaks upon the addition of 0.5 equiv. of
metal ions (Cu2+ and Fe3+ ions) conrmed the involvement of
hetero atoms (O and N) and their chelation to form the excimers
(P1–P1* and A1–A1*) in the sensing mechanism. Moreover, the
binding site and the excimers mechanism were well supported
by the reversibilities of P1 + Cu2+ and A1 + Fe3+ sensor
complexes. Both sensors (P1 + Cu2+ and A1 + Fe3+) were found to
be reversible to their original state upon the addition of 2 drops
of 50% pentamethyl diethylene triamine (PMDTA)26 in CH3CN
or THF and can be reused for up to 4 cycles as established in
Fig. S10 (ESI†). Therefore, the possible sensing mechanism
based on the excimer formation was proposed as noted in Fig. 8.
Detection limits (LODs) and association constants (Ka)
In order to prove the selectivities of P1 and A1 towards Cu2+ and
Fe3+, respectively, the calculations of detection limits (LODs)27
were performed through standard deviations and linear ttings
Fig. 8 Possible proposed binding mechanism of (a) P1 and (b) A1, towards Cu2+
and Fe3+ ions.
as shown in Fig. S9 (ESI†). By plotting the relative uorescence
intensity (I/I0) changes as a function of concentration the
detection limits of P1 + Cu2+ and A1 + Fe3+ were calculated as
9.72 ꢀ 10ꢁ7 and 2.95 ꢀ 10ꢁ6 M, respectively. Assuming a 2 : 1
complex formation, the association constant (Ka) was calculated
on the basis of the titration curves of the sensor P1 and A1 with
Cu2+ and Fe3+. As shown in Fig. S13 (ESI†), the association
constants28 were determined by a linear least square tting
of data with [Cu2+] ¼ 1/2Ka[L]T. x/(1 ꢁ x2) + [L]T/2. x and [Fe3+] ¼
1/3Ka[L]T. x/(1 ꢁ x3) + [L]T/3. x equations, respectively. Where Ka
is the complex association constant; [Cu2+] and [Fe3+] are the
concentrations of Cu2+ and Fe2+ ions; [L]T is the initial
concentration of the sample; x ¼ I ꢁ I0/Imax ꢁ I0; I, I0 and I is the
absorption intensity at 455 nm (P1) or 495 nm (A1) of the
respective species, free ligand and the absorption intensity at
455 nm (P1) or 495 nm (A1) upon the addition of Cu2+ and Fe3+,
respectively. The Ka values of P1 + Cu2+ and A1 + Fe3+ were
estimated as 1.96 ꢀ 106 and 1.88 ꢀ 105 Mꢁ1, respectively.
Furthermore, to conrm the better selectivities of P1 and A1
towards Cu2+ and Fe3+, respectively, the pH and time effects of
their sensor responses are evaluated as explained below.
pH and time effects29
The P1 + Cu2+ and A1 + Fe3+ sensor responses were veried
between pH 0 and 14, maintained by the respective buffers (100
mM). In contrast to separate titrations (Fig. S12, ESI†) of pHs
(0–14) solutions (100 mM) to P1 and A1, the P1 + Cu2+ and A1 +
Fe3+ sensors were active in wide ranges of pHs (1–14 and 2–14,
respectively) in both cases, as shown in Fig. S11 (ESI†). The
above observation conrmed that P1 and A1 can also be utilized
as pH sensors to distinguish acidic and basic pHs. Assuming a
2 : 1 complex formation, the time-dependent uorescent anal-
yses of P1 (20 mM) in CH3CN and A1 (20 mM) in THF were carried
out by adding 10 mM (0.5 equiv.) of metal ions in H2O, as shown
Fig. 7 NMR spectral changes of (a) P1 (20 mM) in d-acetonitrile and (b) A1 (20
mM) in d-tetrahydrofuran titrated with 0–16 mM of Cu2+ and Fe3+ in D2O.
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in Fig. S14 (ESI†). Both P1 + Cu2+ and A1 + Fe3+ sensor responses
in time dependent analyses demonstrated identical results as
shown in Fig. 6. As noticed in Fig. S14c and d,† the relative
uorescence intensity changes also reached the maximum
CHEF values aer 10 minutes. The time resolved photo-
luminescence (TRPL) measurements were proceeded further to
reconrm the turn-on sensor responses of P1 and A1 towards
Cu2+ and Fe3+ ions, respectively.
Counter ion effect on sensor responses30
Since many sensor responses are affected by the presence of
counter ions, we performed the sensor titrations of P1 and A1
towards Cu2+ and Fe3+, respectively, with different counterion
salts. As evidenced in Fig. S15a (ESI†), the Cu2+ sensor response
was found to be increased from 591 fold to 620 fold in the
presence of hydroxide (OHꢁ) as a counter ion. However, the
sensor response in the presence of acetate (CH3COOꢁ) provided
just 553 fold CHEF enhancement. In the same way, the CHEF
ꢁ
values of Cu2+ sensor responses of other counter ions (NO3
,
Clꢁ, Brꢁ, ClO4ꢁ, IO3ꢁ and SO42ꢁ) lies between 560 and 591 fold.
Similar to the Cu2+ sensor response of P1, the A1 sensor
response towards Fe3+ (Fig. S15b, ESI†) also indicated the CHEF
value increased from 35 to 42 fold in the presence of hydroxide
(OHꢁ) as a counter ion. However, the presence of other counter
ions (CNꢁ, NO3ꢁ, Clꢁ, Fꢁ, ClO4ꢁ, IO3ꢁ and SO42ꢁ) produced the
CHEF values between 30 and 35 fold. Hence, it was concluded
that both the Cu2+ and Fe3+ sensor responses of P1 and A1 were
not affected incredibly in the presence of different counter ions.
Fig. 9 Time resolved fluorescence spectra of (a) P1 (black) and P1 + Cu2+ (gray);
(b) A1 (black) and A1 + Fe3+ (gray).
Time resolved photoluminescence spectra (TRPL)31
As reported in the literature and from our results (Fig. 9a and b),
we found that the uorescence decay constants (s) were affected
typically by turn-on sensor responses as summarized in Tables 1
and S1 (ESI†). From the TRPL signals without any sensor
responses the uorescence life time values of P1 and A1 were
1.30 and 10.60 ns, respectively. During the P1 + Cu2+ and A1 +
Fe3+ sensing processes, the faster decay components (A1) of P1
and A1 (35.58% and 14.31%) were increased to 96.16% and
23.51%, respectively, along with decreased values of longer
decay components (A2), as shown in Table S1 (ESI†). Based on
single exponential decay ttings, the average uorescence life
time values of P1 + Cu2+ and A1 + Fe3+ were estimated as 2.05,
and 12.46 ns, respectively. The increased decay constant (s)
values of P1 + Cu2+ and A1 + Fe3+ veried the turn-on sensor
responses of P1 and A1 through CHEF and excimer formations.
Aggregation induced emissions (AIEs) by Schiff bases
More interestingly, while preparing the stock solutions of P1 in
CH3CN and A1 in THF by varying the water concentrations for
sensor titrations, the uorescence spectra of P1 and A1 were
enhanced with spectral shis w.r.t time (0–18 hours) due to
their aggregated nature. Hence, we proceeded the aggregation
induced emission (AIE) analysis further. As shown in Fig. 10, the
PL intensities were enhanced by increasing the concentrations
of H2O (up to 90%), the PL emissions of P1 (in CH3CN) and A1
(in THF) were shied from 455 and 499 nm to 471 and 495 nm,
Fig. 10 Fluorescence spectra of (a) P1 (2 mM) in CH3CN (b) A1 (2 mM) in THF,
upon increasing the concentration of water from 0% to 90%. (Note: the spectra
were taken after 18 hours).
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respectively, in both extremes. Furthermore, as noticed in Table
2, the F values of 0.567 and 0.506 for P1 in CH3CN : H2O
(20 : 80) and A1 in THF : H2O (40 : 60) were acquired with 630
and 101 fold enhancements, respectively. Upon increasing the
concentration of H2O (0–90%) in P1 (in CH3CN) and A1 (in
THF), the absorbance spectra (Fig. S16, ESI†) and the quantum
yields (Fig. 11 and S17 (ESI†)) changes without certain trends
w.r.t. time (0–18 hours), which also well proved the AIE char-
acteristics of P1 and A1. The uorescence changes arose from
AIE were visualized by the photographs (Fig. 11 (insets)) of P1
(0 and 80%) and A1 (0 and 60%). In addition, the TRPL spectra
(Fig. S18, ESI†) and decay constant (s) values (Tables 2 and S1
(ESI†)) of P1 in CH3CN : H2O (20 : 80) and A1 in THF : H2O
(40 : 60) also conrmed their AIE nature. For P1 in
CH3CN : H2O (20 : 80) and A1 in THF : H2O (40 : 60), the faster
decay components (A1) of P1 and A1 (35.58% and 14.31%) were
increased and decreased to 71.31% and 8.40%, respectively,
along with decreased and increased values of longer decay
components (A2) as shown in Table S1 (ESI†). As shown in Table
2, the single exponential tting decay constant (s) values of 4.13
and 22.30 ns for P1 in CH3CN : H2O (20 : 80) and A1 in
THF : H2O (40 : 60) were both increased in contrast to 1.30 and
10.60 ns for P1 (in CH3CN) and A1 (in THF), respectively.
Mechanism of AIEs by Schiff bases
Fig. 11 Quantum yield (F) changes of (a) P1 and (b) A1, upon increasing the
concentration of water (0–90%), measured after 18 hours (note: quantum yield
(F) calculations were carried out based on UV-Vis and PL spectral changes); insets:
photographs of (a) P1 in CH3CN with 0% and 80% water and (b) A1 in THF with
0% and 60% of water, visualized after 18 hours.
As reported by previous literature,32 the AIEs mechanism of
both P1 and A1 possibly arose from the restriction of intra-
molecular rotation (RIR). Since, the single bond rotation is
mainly responsible for the dominant nonradiative decay, the
RIR effect might be the cause for the AIE nature of P1 and A1.
The hydrogen-bonded formation, suppression of PET process
and suppression of charge transfer (CT) or intramolecular
charge transfer (ICT) are the other mechanistic approaches for
AIEs of P1 and A1. However, our observations on the PL spectra
of AIEs for P1 and A1 suggested that both the red and blue shis
by aggregation were probably originated from the suppression
of twisted intramolecular charge transfer (TICT). The above
justication was also well supported by the similar reports33
available in the eld of AIEs. As evidenced in Fig. 10 and 11, the
PL spectra and F values of P1 (in CH3CN) and A1 (in THF) were
increased rapidly up to 80% and 60% of the water content arose
from the suppressions of TICT processes. However, the PL
spectra and F values of P1 in CH3CN : H2O (10 : 90) and A1 in
THF : H2O (30 : 70 to 10 : 90) were found to be decreased due to
the solvent effect on AIEs.33
Conclusions
In conclusion, novel pyrene- and anthracene-based Schiff base
derivatives P1 and A1 were synthesized via a one-pot reaction
and utilized as Cu2+ and Fe3+ turn-on sensors, respectively, and
their preliminary AIE characteristics via increasing the
concentration of H2O (0–90%) were also established. The 2 : 1
stoichiometry of sensor complexes P1 + Cu2+ and A1 + Fe3+ were
calculated from Job plots based on UV-Vis absorption titrations.
In addition, the binding sites of sensor complexes P1 + Cu2+ and
A1 + Fe3+ were well established from 1H NMR titrations and
supported by the uorescence reversibility by adding metal ions
and PMDTA sequentially. Hence, the possible sensing mecha-
nisms through excimer formations were proposed. Further-
more, by standard deviations and linear ttings the detection
Table 2 Quantum yields (F) and TRPL decay constants (s) values of P1 (0%
water), A1 (0% water), P1 (80% water) and A1 (60% water)
Quantum yield
(F) enhancements
Compounds
Quantum yielda (F)
sb (ns) limits (LODs) of P1 + Cu2+ and A1 + Fe3+ were calculated as
9.72 ꢀ 10ꢁ7 and 2.95 ꢀ 10ꢁ6 M, respectively. Similarly, based on
P1 (0%)
A1 (0%)
P1 (80%)
A1 (60%)
0.0009
0.005
0.567
0.506
NA
NA
630 folds
101 folds
1.30
10.60
4.13
22.30
uorescent binding isotherms the association constant (Ka)
values of P1 + Cu2+ and A1 + Fe3+ were estimated as 1.96 ꢀ 106
and 1.88 ꢀ 105 Mꢁ1, respectively. More importantly, P1 + Cu2+
and A1 + Fe3+ sensors were found to be active in wide ranges of
a
Quantum yields were calculated using 9,10-diphenylanthracene (F ¼
b
pHs (1–14 and 2–14, respectively) and also effective w.r.t time
(0–10 minutes). Similar to the sensor properties, P1 in
0.9) as
a
reference standard.
Obtained from time resolved
uorescence measurements.
1316 | J. Mater. Chem. A, 2013, 1, 1310–1318
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Journal of Materials Chemistry A
CH3CN : H2O (20 : 80) and A1 in THF : H2O (40 : 60) showed
greater AIE properties with shied PL emission peaks. In both
applications, the huge enhancements of quantum yield (F)
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Acknowledgements
The nancial supports of this project provided by the National 13 S. K. Sahoo, D. Sharma, R. K. Bera, G. Crisponi and
Science Council of Taiwan (ROC) through NSC 99-2113-M-009-
006-MY2 and NSC 99-2221-E-009-008-MY2 are acknowledged.
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