Novel pyrene- and anthracene-based Schi
ff base
derivatives as Cu
2+and Fe
3+fluorescence turn-on
sensors and for aggregation induced emissions
†
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 asfluorescence turn-on sensors towards Cu2+and Fe3+ions, respectively, and for aggregation induced emissions (AIEs). P1 in CH3CN and A1 in THF illustrated thefluorescence 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 the1H NMR titrations and supported by thefluorescence 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, linearfittings 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 alteredfluorescence peak shifts (red and blue shifts, respectively). As well as s value enhancements, the F 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.
Introduction
Owing to the biological and environmental importance of metal ions, numerous sensory reports are available for different metal ions with diverse mechanisms.1Among 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 effects3of biologically important ions, the
develop-ment of uorescence turn-on sensors still remains a chal-lenging task. Several molecular turn-on sensors4were 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, PET5exhibited various changes of
emis-sion intensities with some or no spectral shis, whereas ICT6
caused both intensity changes and spectral shis, and CHEF7
also provideduorescence enhancements with or without any spectral changes.
Among all metal ions, Cu2+is a signicant metal pollutant due to its widespread use,8but 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 effect11of cations, such as Zn2+, Hg2+, Pb2+, Fe3+and
Ag+. Therefore, the development of highly selectiveuorescence probes12for 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.13Most 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 neuroinammation and the progression of Alzheimer's disease.15Many sensor reports16are 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 specic sensory materials for Cu2+and Fe3+detections with
optical turn-on responses via simple synthetic pathways. Department of Materials Science and Engineering, National Chiao Tung University,
Hsinchu 30049, Taiwan (ROC). E-mail: [email protected]
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ta00574c
Cite this:J. Mater. Chem. A, 2013, 1, 1310 Received 5th October 2012 Accepted 2nd November 2012 DOI: 10.1039/c2ta00574c www.rsc.org/MaterialsA
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Similar to sensor applications, aggregation induced emis-sion (AIE) has become more popular and has also been applied to many areas of science17by 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.18However, 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 bases19are recognized as
having simple synthetic steps and are also applied to many optical sensors, as well as in AIE applications. However, to develop Schiff bases with sensor and AIE properties, the presence of stronguorophores are required.20Pyrene and anthracene
derivatives were evidenced as excellentuorophores and widely used in the developments ofuorescence (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.21Therefore, we
developed the novel pyrene- and anthracene-based Schiff bases and evaluated their sensor and AIE properties (Fig. 1).
Herein, we report novel pyrene- and anthracene-based Schiff base derivatives (P1 and A1) for therst 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 aldehyde and amine condensation in methanol with 98 and 71% yields, respectively.
Computational analysis
Computational analyses of P1 and A1 were carried out via the semi-empirical (AM1) method,22as evidenced in Fig. 2. It reveals
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. Moreover, those materials must have some probes that can provide the selectivities towards the specic analytes in organic 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 1 104 M and hence the
Fig. 1 Schematic representations of sensors and aggregation induced emissions (AIEs) of Schiff bases.
Scheme 1 Synthesis of P1 and A1.
Fig. 2 Computational analysis of HOMO and LUMO levels of P1 and A1 (semi-empirical AM1 method).
differences between sensory and AIEs of P1 and A1 were well established.
Fluorescence titrations on metal ions
Initially, P1 (20mM) in CH3CN and A1 (20 mM) in THF were
investigated towards 60mM (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 H
2O. 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
selectivities were conrmed further by single and dual metal studies as follows. In order to establish the specic 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 50mM towards P1. However, for the dual-metal (white bars) studies, two equal amounts of aqueous solutions of Cu2+ and other metal ions (50mM + 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 60mM 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 (60mM + 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 andF values w.r.t ratiometric titrations
(0–5 equiv.) was not evidenced, so the better selectivity of A1 to Fe3+ions was conrmed. Moreover, the specic selectivities of P1 and A1 by single and dual metal studies also conrmed 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 greenuorescences, 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 Cu2+and Fe3+sensors
By increasing the concentrations of Cu2+(0–50 mM with an equal span of 2mM in H2O) the sensitivity of P1 (20mM) in CH3CN
towards Cu2+ions was clearly observed in Fig. 6a. The
uores-cence spectrum of P1 (lem¼ 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. Theuorescence spectrum of A1 (lem¼ 495 nm) exhibited turn-on responses
Table 1 Photophysical properties of Schiff bases and sensor complexes
Compounds CHEFa
Association
constantb(Ka) Detection limits (LODs)c Quantum yieldd(F)
Quantum yield
(F) enhancements se(ns)
P1 (lex¼ 395 and lem¼ 459 nm) NA NA NA 0.0009 NA 1.30
A1 (lex¼ 395 and lem¼ 499 nm) NA NA NA 0.005 NA 10.60
P1 + Cu2+(l
ex¼ 395 and lem¼ 455 nm) 591 fold 1.96 106M1 9.72 107M 0.284 315 fold 2.05
A1 + Fe3+(l
ex¼ 395 and lem¼ 495 nm) 35 fold 1.88 105M1 2.95 106M 0.125 25 fold 12.46
aI/I
0values were reported as Chelation enhanced uorescence (CHEF).bAssociation constants were calculated from the slopes of binding isotherms.cDetection limits (LODs) were obtained from theiruorescence binding isotherms.dQuantum yields were calculated using 9,10-diphenylanthracene (F ¼ 0.9) as a reference standard.eObtained from time resolveduorescence measurements.
Fig. 3 Sensor responses of (a) P1 (1 105M) in CH
3CN (lex¼ 395 nm) and (b)
A1 (1 105M) in THF (l
ex¼ 395 nm) towards metal ions (note: each metal
selectivity was measured with 2.5 and 3 equiv. of metal ions, respectively).
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 stronguorescence 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.
Stoichiometries24and 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 3mM), the
absorption maxima of P1 and A1 were quenched rapidly up to 10 mM, aerwards 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. 4 Relativefluorescence intensities of (a) P1 (20 mM) in CH3CN (lex¼ 395
nm) with 50mM of Cu2+in H
2O and (b) A1 (20mM) in THF (lex¼ 395 nm) with 60
mM Fe3+in H
2O in the presence of competing metal ions. Black bar; P1 (20mM) in
CH3CN and A1 (20mM) in THF with 50 and 60 mM of stated metal ions in H2O,
respectively. White bar; P1 (20mM) in CH3CN with 50mM Cu2++ 50mM of stated
metal ions and A1 (20mM) in THF with 60 mM Fe3++ 60mM of stated metal ions in
H2O. (100 and 120mM of Cu2+and Fe3+were taken for Cu2+and Fe3+effects,
respectively).
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).
Fig. 6 Fluorescence spectra of (a) P1 (20mM) in CH3CN (lex¼ 395 nm) with 0–50
mM of Cu2+in H
2O (with an equal span of 2mM) and (b) A1 (20 mM) in THF (lex¼
395 nm) with 0–60 mM of Fe3+in H
2O (with an equal span of 2mM); insets: relative
fluorescence intensity changes with respect to Cu2+and Fe3+concentrations.
supported by 1H NMR titrations25 along with binding sites
conrmation. 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) conrmed 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)26in CH
3CN
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 linearttings
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 107and 2.95 106M, 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; x¼ I I0/Imax I0; I, I0and 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 M1, respectively. Furthermore, to conrm 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 veried 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 conrmed 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-dependentuorescent anal-yses of P1 (20mM) in CH3CN and A1 (20mM) in THF were carried
out by adding 10mM (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.
Fig. 8 Possible proposed binding mechanism of (a) P1 and (b) A1, towards Cu2+
and Fe3+ions.
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 aer 10 minutes. The time resolved photo-luminescence (TRPL) measurements were proceeded further to reconrm 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 (OH) as a counter ion. However, the sensor response in the presence of acetate (CH3COO) provided
just 553 fold CHEF enhancement. In the same way, the CHEF values of Cu2+ sensor responses of other counter ions (NO3,
Cl, Br, ClO4, IO3and SO42) 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 (OH) as a counter ion. However, the presence of other counter ions (CN, NO3, Cl, F, ClO4, IO3and SO42) 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. Time resolved photoluminescence spectra (TRPL)31
As reported in the literature and from our results (Fig. 9a and b), we found that theuorescence 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 theuorescence 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 decayttings, 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+ veried 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 shis 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 shied from 455 and 499 nm to 471 and 495 nm,
Fig. 9 Time resolvedfluorescence spectra of (a) P1 (black) and P1 + Cu2+(gray);
(b) A1 (black) and A1 + Fe3+(gray).
Fig. 10 Fluorescence spectra of (a) P1 (2mM) in CH3CN (b) A1 (2mM) in THF,
upon increasing the concentration of water from 0% to 90%. (Note: the spectra were taken after 18 hours).
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. Theuorescence 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 conrmed 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 exponentialtting 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
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 shis by aggregation were probably originated from the suppression of twisted intramolecular charge transfer (TICT). The above justication was also well supported by the similar reports33
available in theeld of AIEs. As evidenced in Fig. 10 and 11, the PL spectra andF 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 andF 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 theuorescence 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 linearttings the detection limits (LODs) of P1 + Cu2+ and A1 + Fe3+ were calculated as
9.72 107and 2.95 106M, respectively. Similarly, based on uorescent binding isotherms the association constant (Ka)
values of P1 + Cu2+and A1 + Fe3+were estimated as 1.96 106 and 1.88 105M1, respectively. More importantly, P1 + Cu2+ and A1 + Fe3+sensors were found to be active in wide ranges of pHs (1–14 and 2–14, respectively) and also effective w.r.t time (0–10 minutes). Similar to the sensor properties, P1 in 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)
Compounds Quantum yielda(F)
Quantum yield (F) enhancements sb(ns) P1 (0%) 0.0009 NA 1.30 A1 (0%) 0.005 NA 10.60 P1 (80%) 0.567 630 folds 4.13 A1 (60%) 0.506 101 folds 22.30
aQuantum yields were calculated using 9,10-diphenylanthracene (F ¼ 0.9) as a reference standard. bObtained from time resolved uorescence measurements.
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.
CH3CN : H2O (20 : 80) and A1 in THF : H2O (40 : 60) showed
greater AIE properties with shied PL emission peaks. In both applications, the huge enhancements of quantum yield (F) values were conceived in the analyses of the sensor (increased 315 and 25 folds) and AIE (increased 630 and 101 folds) measurements. Moreover, the sensor and AIE properties of P1 and A1 were also well supported by their TRPL spectra and decay constant (s) values.
Acknowledgements
Thenancial supports of this project provided by the National Science Council of Taiwan (ROC) through NSC 99-2113-M-009-006-MY2 and NSC 99-2221-E-009-008-MY2 are acknowledged.
Notes and references
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