Pyrene Scaffold as Real-Time Fluorescent Turn-on Chemosensor for

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Pyrene Scaffold as Real Time Fluorescent Turn-on Chemosensor for Selective Detection of Trace Level Al(III) and Its Aggregation Induced Emission Enhancement Milan Shyamal, Prativa Mazumdar, Samir Maity, Gobinda Prasad Sahoo, Guillermo Salgado-Moran, and Ajay Misra J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b09107 • Publication Date (Web): 20 Dec 2015 Downloaded from http://pubs.acs.org on December 26, 2015

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The Journal of Physical Chemistry

Pyrene Scaffold as Real Time Fluorescent Turnon Chemosensor for Selective Detection of Trace Level Al(III) and Its Aggregation Induced Emission Enhancement Milan Shyamal,† Prativa Mazumdar,† Samir Maity,† Gobinda P. Sahoo,† Guillermo SalgadoMorán‡ and Ajay Misra†,* †

Department of Chemistry and Chemical Technology, Vidyasagar University, Midnapur 721102, W.B, India

‡

Departamento de Ciencias Qumıcas, Facultad de Ciencias Exactas, Universidad Andres Bello, Sede Concepcion, Concepcion, Chile

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ABSTRACT: A pyrene based fluorescent probe, 3-methoxy-2-((pyren-2yl-imino)methyl)phenol (HL) was synthesized via simple one pot reaction from inexpensive reagents. It exhibits high sensitivity and selectivity towards Al3+ over other relevant metal ions and also displayed novel aggregation-induced emission enhancement (AIEE) characteristics in its aggregate/solid state. Upon binding with Al3+ in 1:1 mode, a significant fluorescence enhancement with a turn-on ratio of over ~200-fold was triggered via chelation enhanced fluorescence (CHEF) through sensor complex (Al-L) formation and amusingly excess addition of Al3+, dramatic enhancement of fluorescence intensity over manifold through aggregate formation was observed. The 1:1 stoichiometry of the sensor complex (Al-L) was calculated from Job’s plot based on UV-Vis absorption titration. In addition, the binding site of sensor complex (Al-L) was well established from the 1H NMR titrations and also supported by the fluorescence reversibility by adding Al3+ and EDTA sequentially. Intriguingly, the AIEE properties of HL may improve its impact and studied in CH3CN-H2O mixtures at high water content. In order to gain insight into the AIEE mechanism of the HL, the size and growth process of particles in different volume percentage of water and acetonitrile mixture were studied using time resolved photoluminescence (TRPL), dynamic light scattering (DLS), optical microscope and scanning electron microscope (SEM). The molecules of HL are aggregated into ordered one-dimensional rod shaped micro-crystals that show obvious optical waveguide effect.


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INTRODUCTION Exploring practical chemosensors to detect toxic metal ions1−5 such as Al3+, Hg2+, Pb2+ and Cu2+ and proficient light emitters in the aggregate and solid states to the development of advanced optical and biomedical devices, such as organic light emitting diodes (OLEDs)6−10 is an important research subject due to their potential impact. Aluminum is the third most abundant and prevalent element (8.3% by weight) after oxygen and silicon in the earth’s crust.11−13 It is the common sense that increasing the free Al3+ from soil by acid rain and human activities is poisonous to growing plants.14 Aluminum is found in its ionic form Al3+ in most animal and plant tissues and in natural waters. The general population is exposed to aluminum from its widespread use in water treatment, food additive, aluminum based pharmaceuticals, occupational dusts, aluminum containers, packaging materials, electrical equipment, computers and many others.15−18 According to a World Health Organization (WHO) report, the average daily human intake of aluminium is approx. 3-10 mg per day. Tolerable weekly aluminium intake in the human body is estimated to be 7 mg kg-1 body weight.15 Excessive exposure of Al3+ to the human body leads to lots of diseases such as microcytic hypochromic anemia, gastrointestinal problems, decreased liver and kidney function, memory loss, speech problems etc. The toxicity of aluminum causes damage of the central nervous system and is suspected to be involved in neurodegenerative diseases such as Alzheimer’s and Parkinson’s19,20 and is responsible for intoxication in hemodialysis patients.21 As a result of the close relationship between aluminum and human health, the investigation of Al3+ detection attracts more and more attention. The chemosensors for real-time sensing of biologically important ions have become important tools in various fields of modern science.22,23



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To date, some conventional methods for Al3+detection are atomic absorption spectrometry, atomic

emission

spectrometry,

inductively

coupled

plasma

mass

spectrometry

and

electrochemical methods.24 The methods are relatively complex and involve expensive sophisticated instrumentation, making them unsuitable for onsite detection. Simple, low cost and accurate analytical methods with high selectivity and sensitivity for the rapid detection of Al3+ therefore need to be exploited. The fluorescent chemosensors have recently attracted significant interest in the context of sensing of biological and environmentally relevant metal ions.25 It can provide several advantages over conventional methods, such as the low cost, easy sample preparation, rapid, convenient and sensitive detection, and biological imaging applications in vivo/vitro samples. Until now, in the field of aluminium sensing, several fluorescence turn-on sensors have been described, wherein the mechanism of sensing based on photoinduced electron transfer (PET),26 chelation enhanced fluorescence (CHEF),27 fluorescence resonance energy transfer (FRET),28 metal-ligand charge transfer (MLCT),29 C=N isomerization.30 Among these, ICT caused both intensity changes and spectral shifts, whereas PET exhibited various changes of emission intensities with some or no spectral shifts, and CHEF also provided fluorescence enhancements with or without any spectral changes. The fluorescent chemosensors for selective Al3+, compared to other transition metal ions are limited, due to its poor coordination ability and lack of spectroscopic characteristics.31-35 It has been found that as a hard-acid Al3+ prefer hard coordination sphere of N and O donor atoms.32 Therefore, it is highly demanding to develop easily synthesizable fluorescent chemosensors for rapid and selective recognition of Al3+. During the last decade, molecules with aggregation induced emission (AIE) or aggregationinduced enhanced emission (AIEE) properties have drawn considerable attention due to their


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great potential in real-world applications.36 AIEE active probes are luminescent in solution and become more emissive in the aggregated state. AIEE active luminogens have potential application in opto-electrical device, stimuli-responsive nano materials and active layers in the construction of efficient organic light emitting diodes (OLEDs).6−10 An important issue in finding the possible mechanism in understanding the AIEE process is the restriction of intramolecular rotation (RIR).37 The RIR mechanisms proposes that the emission of luminogens in the solution is quenched by the dynamic intramolecular rotations of aromatic rings, whereas the aggregate formation effectively suppresses the molecular motions, blocks the non-radiative energy dissipation channels, and opens up the radiative decay pathway. Moreover, some functional groups introduced into AIEE molecules favor new development of chemo- or biosensors for detecting metal cations, bio-molecules etc.38,39 So, AIEE active molecules are attractive candidates to construct fluorescent sensors with robust and quantitative sensing of various target analytes.40 Only a limited number of fluorescent probes have been reported to display AIEE characteristics. Hence, the designed synthesis of the fluorescent probes, having AIEE properties and also less synthetic difficulties has received intense attention of the chemists. Schiff bases39-44 are recognized as having simple synthetic steps and are also applied to many optical sensors, as well as in AIEE applications. However, to develop Schiff bases with sensor and AIEE properties, the presence of strong fluorophores are required.45 Pyrene derivatives have been found to be excellent fluorophores and they have been widely used in the fluorescent sensors because of their long fluorescence lifetime, high charge carrier mobility, and chemical stability.46 Therefore, pyrene derivatives have become an attractive choice for the designed fluorescent sensors and also AIEE properties. And up to now, scarce of examples of pyrene



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based chemosensor have been reported for the detection of Al3+ as well as performance of AIEE properties.47 Considering the above fact, we successfully designed and synthesized new pyrene based Schiff base, 3-methoxy-2-((pyren-2yl-imino)methyl)phenol (HL) with potential hard N, O donor atoms having simple one step synthetic process. HL can be chosen as an ideal fluorescent chemosensor due to its highly conjugated aromatic structure, excellent photostability, significant fluorescent behavior in the visible region and ability to act as a donor as well as an acceptor. Herein, we describe this HL as a fluorescent chemosensor for first time to highly recognize selective Al3+ ion via CHEF. The chemosensor HL contains pyrene moiety as a hydrophobic tail which decreases its water solubility and makes it suitable for forming highly fluorescent aggregate. Astonishingly, present results of our pyrene-based fluorophore HL showed another special aggregation induced emission enhancement (AIEE) properties upon increasing water content. Hence HL was proved to be a highly sensitive, real time and fluorescent “turn on” sensor for Al3+ and also useful for aggregation induced emission enhancement (AIEE) (Scheme 1).

EXPERIMENTAL SECTION Materials. All of the materials for synthesis were obtained commercially and used without further purification. All the solvents used were of analytical grade. Freshly prepared de-ionize water was used throughout the experiment. The stock solutions of metal ions were prepared from their nitrate salts and the solutions of anions were prepared from their sodium salts. All the titrations were carried out at room temperature. Buffer solutions of different pH were prepared using Gomori protocol and adjusted by a pH meter. 6 ACS Paragon Plus Environment

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Synthesis of 3-methoxy-2-((pyren-2yl-imino)methyl)phenol (HL). 1 equiv. of orthovanillin was added to 1 equiv. of amino pyrene in 50 ml of methanol with constant stirring and then refluxed for 7 hrs. The reaction was monitored by TLC. After completion, the reaction mixture was cooled and bright yellow colored crystalline solid product was filtered and washed thoroughly with methanol and then dried in a desiccator. Yield: 82%. Anal. Calcd. for C24H17NO2: C, 82.03%; H, 4.88%; N, 3.99%; Found: C, 82.18%; H, 4.97%; N, 3.86%. IR (cm– 1

, KBr): ν(C=N) 1628 cm-1. 1H NMR (400 MHz, CDCl3) δ: 14.35 (H, s (OH)), 8.94 (H, s (-

CH=N)), 8.60-6.96 (12H, ArH), 4.03 ppm (OCH3, s);

13

C NMR (400 MHz, CDCl3) δ: 163.11,

151.57, 148.60, 141.66, 131.42, 131.25, 130.36, 128.10, 127.34, 127.15, 126.32, 125.54, 125.43, 125.34, 125.24, 125.11, 124.64, 123.87, 122.31, 119.66, 118.75, 115.35, 114.81, 56.23 ppm; HRMS: MS-ES+ (m/z): [M+H]+: Calculated: 352.13, Found: 352.16. Physical Measurements. 1H and

13

C NMR spectra were recorded on a Bruker ASCEND

spectrometer operating at 400 MHz in CDCl3 and DMSO-d6. The chemical shifts were recorded in parts per million (ppm) on the scale. The ESI-MS was recorded on Qtof Micro YA263 mass spectrometer. The Schiff base (HL) was recognized and the purity of the HL was obtained using NMR (1H &

13

C) and mass (ESI-MS) spectroscopy. UV-Vis spectroscopic measurements were

carried out in a 1cm quartz cuvette with a Shimadzu UV-1800 spectrophotometer. Fluorescence spectra were recorded using Hitachi F-7000 Fluorescence Spectrophotometer. Fluorescence lifetimes were determined from time-resolved intensity decay by the method of time-correlated single-photon counting (TCSPC) measurements using a picoseconds diode laser (IBH) and the signals are collected at magic angles (54.7°). The instrument response function of the instrument is ~90 ps. The fluorescence decay data were collected on a Hamamatsu MCP PMT (R3809) and



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were analyzed by using IBH DAS 6 software. Nano LED at 375 nm was used as the excitation source. All pH measurements were made with an ORION VERSASTAR. The Fourier transform infrared (FT-IR) spectra were obtained in the range of 4000-400 cm-1 using KBr pellets on a Perkin Elmer Spectrum-Two FTIR spectrometer. Particle size distribution analysis was measured by dynamic light scattering (DLS) experiments on a Malvern Zetasizer Nano ZS90 instrument. Optical fluorescence microscopy images were taken using an NIKON ECLIPSE LV100POL upright microscope equipped with CCD camera (model no. DS- Fil). The morphologies of HL aggregates obtained in mixed aqueous mixtures were observed by ZEISS EVO 18 scanning electron microscope (SEM). Sample for the SEM study was prepared by placing a drop of the aqueous suspension of particles on a small glass slide followed by solvent evaporation under vacuum. Ground-state geometry of HL were optimized using the density functional theory (DFT) with B3LYP hybrid functional at the basis set level of 6-31+G(d, p). All the theoretical calculations were performed using Gaussian 09 package program. Dissociation constant has been found out from non-linear fittings by suitable computer-fit equation. Sample Preparation for UV-Vis and Fluorescence Spectroscopic Studies. A stock solution of HL (1.0 mM) was prepared in CH3CN. Stock solutions of various metal ions (1.0 mM, Na+, K+, Ca2+, Mg2+, Mn2+, Fe3+, Co2+, Cu2+, Zn2+, Hg2+, Ag+, Cd2+, Pb2+, Al3+) were prepared in deionized water. In titration experiments, a quartz optical cell of 1 cm path length was filled with a 2.0 ml solution of HL to which the stock solutions of metal ions were gradually added using a micropipette. For counter ion effect on Al3+ sensor responses, solution (1.0 mM) of acetate, nitrate, nitrite, chloride, hydroxide and sulphate salts of sodium were prepared using deionized water. In selectivity experiments, the test samples were prepared by placing


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appropriate amounts of the cations and anions stock into 2.0 ml of HL solution (20 µM). For fluorescence measurements, excitation was provided at 364 nm, and emission was acquired from 374 nm to 550 nm. Fluorescence Quantum Yield in Solution. Fluorescence quantum yield was determined in spectroscopic grade CH3CN using optically matching solutions of quinine sulfate (Φr = 0.546 in 1N H2SO4) as standard at an excitation wavelength of 364 nm. The quantum yield is calculated using the following equation.48 Φs = Φr (ArFs/AsFr) (ηs2/ηr2) where, As and Ar are the absorbance of the sample and reference solutions, respectively at the same excitation wavelength, Fs and Fr are the corresponding relative integrated fluorescence intensities and η is the refractive index of the solvents. Detection Limit. The detection limit was calculated on the basis of the fluorescence titration. The fluorescence emission spectrum of HL as a function of its increasing concentration was measured five times, and the standard deviation of blank measurement was calculated. To obtain the slope, the fluorescence emission intensity at 426 nm was plotted against the concentration of Al3+. The detection limit was calculated using the following equation,34 Detection limit = 3σ/k where σ is the standard deviation of blank measurement, and k is the slope of the calibration curve obtained from linear dynamic plot of fluorescence intensity vs. [Al3+].

RESULTS AND DISCUSSION The synthesis of chemosensor HL is depicted in Scheme S1 in the Supporting Information (SI). Synthesis details are described in experimental section, i.e. one pot condensation of 19 ACS Paragon Plus Environment


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amino pyrene and ortho-vanillin with high purity and satisfactory yield of 82% was obtained. It was fully characterized by physicochemical and spectroscopic analysis. HL is soluble in common polar organic solvents and absolutely insoluble in water. 1H NMR (Figure S1),

13

C

NMR (Figure S2), mass (ESI-Mass) (Figure S3) and IR (Figure S4) spectral analysis were used for HL characterization. The above spectral study revealed that the characteristic stretching frequency and proton signal of HL (in CDCl3) appear at 1628 cm-1 and 8.60 ppm, indicates the formation of

imine bond (>C=N) and azomethine proton signal (-CH=N), respectively.

Aromatic protons for HL appear within 8.60 to 6.96 ppm region whereas it’s OCH3 and phenolic (-OH) proton signals appear at 4.03 ppm and 14.35 ppm, respectively. Photophysical Properties. The photophysical properties of HL were investigated with absorption and fluorescence studies. The emission spectrum of free HL shows a band with maxima located around 426 nm on excitation at 364 nm in CH3CN. It is quite surprising to find that HL exhibits aggregation induced emission enhancement (AIEE) effect when volume percentage of water is increased in the binary solvent mixture (H2O/CH3CN). Moreover, the non-aggregation in acetonitrile-water solutions with low water fraction allows us to use HL as a fluorescent probe to detect trivalent metal ion Al3+ that have acceptable solubility in aqueous medium. Keeping in mind, to distinguish the sensory and AIEE behaviors of HL, we performed UV-Vis/PL sensor titration of HL in CH3CN, by adding metal ions in pure H2O. Similarly, 1H NMR titration was performed in DMSO-d6 by adding Al3+ in DMSO-d6. 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, these materials must have some probes that can provide the selectivity towards the specific analyte in organic solvents. Since pyrene derivative has the p-type semiconducting property, the utilization of HL


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as Al3+ sensor in organic solvents and its insolubility in water was considered as advantage for sensor device in the near future. To better understand the photophysical properties including AIEE nature of HL, we performed the quantum chemical computations using DFT/ B3LYP/6-31G (d,p) level of theory. We have optimized the energy of HL to find out the distribution of electron density within aromatic rings. The optimized structure of HL (Figure 1A) clearly reveals that the phenyl ring of HL is found to be deviated from the plane of 2-((pyren-2yl-imino) group plane with an angle 141.67°. So, HL can adapt twisted conformation i.e have a less planar structure which is clearly understandable from the side view of the optimized geometry of HL. The less planar structure of HL could be due to the steric repulsions between the hydrogen atoms of the phenyl ring and the hydrogen atoms of C2 and O6. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy of HL are shown in Figure 1B. It reveals that the HOMO electron densities are localized within pyrene and imine group, whereas the LUMO electron clouds are delocalized within the imine group, phenyl and pyrene rings. This delocalization of excited electronic energy to the freely rotating phenyl groups is responsible for opening up the non-radiative deactivation channels of excited HL in its isolated form in solution. The calculated HOMO and LUMO energy gap is found to be 3.30 eV, which is lower than that of the calculated HOMO-LUMO gap of pyrene group alone (3.84 eV). The decrease in the HOMO-LUMO energy band gap of HL compare to pyrene is due to the extended conjugation through imino-phenol group even though HL has the non-planar structure. These information are helpful in understanding the nature of transition as well as AIEE character of the probe HL. UV-Vis Absorption Studies on Al3+ Sensor. HL displayed well-defined absorption bands in acetonitrile solution at room temperature. Three absorption bands (234, 281 and 381 nm) were



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observed on the spectrum of HL (Figure S5), among them two strong bands at 234 and 281 nm were assigned to π→π* transition, while the broad one at 381 nm was accredited to the n to π* electronic transition. The binding affinity of HL in CH3CN towards Al3+ in water was investigated by UV-Vis absorption spectroscopy at room temperature. The UV-Vis absorption spectrum of HL obeys the Beer’s Law below a concentration of 0.1 mM, showing a good linearity between the absorption intensity and the concentration of HL. Thus, the concentration of HL was fixed at 20 µM for titration with Al3+. The absorption titration for the interaction of HL with Al3+ reveals that there is a gradual decrease in absorption intensities of HL on gradual addition of Al3+ in the range 0 20 µM, keeping the concentration of HL fixed at 20 µM (Figure S6). At the same time, the absorbance band at 381 nm becomes narrow and slightly blue shifted (~17 nm) to 364 nm. The result suggests a decrease in electron-donating ability of imine (-CH=N) nitrogen conjugated to the aromatic ring of pyrene, induced by Al3+ binding to form a chelate complex along with two hetero nitrogen (N) and oxy gen atoms (O). Such blue shift was also observed on some complex and ascribed to ligand-metal charge transfer (LMCT) process.49 To ensure the metal-to-ligand ratio of the Al-L complex, Job’s plot was established from the absorption spectra between the mole fraction (XM) and absorption maximum changes of HL in presence of Al3+. The maximum was obtained at molar fraction of ca. 0.523 and the result strongly suggests the formation of 1:1 ligand-to-metal complex as shown in Figure S6 (inset). Steady State and Time Resolved Emission Studies on Al3+ Sensor. The emission spectra of HL and fluorescence titration with Al3+ were investigated in acetonitrile solution at room temperature. Titration was carried out by keeping CH3CN:H2O ratio, 95:5 (v/v). Probe HL (20 µM) exhibited practically weak fluorescence and emission band is centered at around 426 nm


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with a low quantum yield (Φ = 0.002) at room temperature. This weak fluorescence is assigned to the photo-induced electron transfer (PET) caused by the electron transmission from the nitrogen atom of imino (-C=N) electron-donor to the large π conjugation system of pyrene fluorophore. Whereas significant monotonically enhancement of emission intensity at 426 nm was observed after gradual addition of Al3+ (0-20 µM with an equal span of 2 µM in H2O) meanwhile no obvious shift and change in the shape of emission maxima are observed14, resulting 200 -fold fluorescence enhancement on excitation at 364 nm, as shown in Figure 2. The fluorescence quantum yield of HL (Φ = 0.002) is also enhanced 160-fold (Φ = 0.305) in the presence of one equivalent of the Al3+ ions. Intriguingly, it was observed that the emission intensity at 426 nm increases linearly up to addition of excess Al3+ ions (>2 equivalents) i.e. fluorescence titration failed to evaluate the stoichiometry (1:1) of complex formation and the inset of Figure 2 demonstrate the relative increase of fluorescence intensity as a function of Al3+ concentrations. Initially, upon complexation with chelating agent HL through -OH and imine (CH=N) groups to Al3+, induces rigidity in the resulting molecule and tends to produce a large CHEF effect by suppressing the PET quenching process.42,50 Again the low solubility of Al-L in CH3CN:H2O (95:5, v/v), Al-L form small aggregates in the solution. Thus the resulting large enhancement of PL intensity is the combined effect of CHEF and aggregation induced emission enhancement (AIEE). After formation of complex, excess addition of Al3+ initiated the formation of larger aggregates in the solution, therefore an astoundingly enhancement of fluorescence intensity was observed. This statement was established by dynamic light scattering (DLS) measurement (Figure 3). The average particle size increased from 232 nm to 444 nm to 1753 nm upon varying the Al3+ concentration from 1.0 equivalent to 2.5 equivalents to 10 equivalents. The AIEE properties of HL in CH3CN/H2O mixtures were discussed in detail later. Therefore,



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the possible sensing mechanism based on CHEF and AIEE-activation was proposed as noted in Scheme 2. The fluorogenic sensing ability of probe HL could be observed by the naked eye when acetonitrile solutions of HL in the presence of Al3+ were illuminated with 265 nm UV-light via the enhancement of a clearly observed blue emission, shown in the inset of Figure 4A. The time resolved photoluminescence (TRPL) measurements were carried out further to understand the mechanism of the turn-on sensor responses of HL towards Al3+ ion as shown in Figure S7. The fluorescence lifetime (τ) of HL in acetonitrile is 1.92 ns whereas a longer average fluorescence lifetime 3.23 ns was detected for sensor complex (Al-L) (1:1). We have calculated the radiative rate constants (kr) and the total non-radiative rate constants (knr) of HL and sensor complex (Al-L).51 All these photophysical parameters are tabulated in Table S1. The data suggest that there is small change in knr values whereas almost ~102 times enhancement have been observed in kr values for sensor complex (Al-L). Strong chelation occurring between Al3+ ions and HL in sensor complex is responsible for such high value. It has been suggested that fluorescence enhancement in complex is due to unavailability of lone pair of electrons on the N atom of the chemosensor HL. This lone pair on free ligand system is responsible for photoinduced electron transfer (PET). Metal Ions Competition Studies. Interestingly, HL also displays excellent specificity and selectivity toward Al3+ in single and multi-component systems. A detailed analysis of the UVVis absorption and fluorescence emission spectrum of HL (20 µM in CH3CN) in the presence of other metal cations (50 µM) in H2O was carried out. We performed absorption and emission spectrum of different metal cations in three categories. (i) Metal cations of the same period in the periodic table of elements such as Mn2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+. (ii) Some hazardous heavy metal ions such as Pb2+, Cd2+, Hg2+ and Ag+. (iii) A few metal cations in the IA, IIA and


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IIIA groups such as Na+, K+, Ca2+, Mg2+, and Al3+. Surveying these data, we observed that HL shows excellent selectivity with strong enhancement of the emission intensity upon addition of Al3+, whereas other relevant competing metal ions had no significant change in the emission spectra. This clearly demonstrates the preference of HL towards Al3+ over the others as shown in Figure 4A. In order to establish the specific selectivity of HL to Al3+, we performed the single and dual metal competitive analysis, as shown in Figure 4A and Figure 4B, respectively. In a single metal system , all the metal (Ag+, K+, Na+, Ca2+, Mg2+, Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+ and Al3+ in H2O) concentrations were kept as 50 µM towards HL. However, for the dual-metal (blue bars) studies, two equal amounts of aqueous solutions of Al3+ and other metal ions (50 µM+50 µM) were combined. The photographs of HL with different metal ions (under UV light irradiations) were well verified its sensitivity by strong blue fluorescence, as depicted in Figure S8. The above deliberations point out that the oxophilic nature and high charge density of Al3+ favored strong complexation with pyrene based non-fluorescent probe HL leading to the formation of the complex and thereby resulting an intense fluorescence enhancement through CHEF and after complex(Al-L) formation, excess addition of Al3+ significantly increasing the emission intensity due to aggregate formation. Thus HL can be utilized as a ion-selective chemosensor i.e. selectively recognize and distinguish Al3+ ions in presence of other competing metal ions. Counter Ions Effect on Sensor. Most of the sensor responses are affected by the presence of counter ions. Therefore, we have performed the sensor titration of HL towards Al3+ in the presence of excess of other biologically relevant anions (CH3COO-, OH-, NO3-, NO2-, SO42- and Cl-). It has been found that there have no or negligible enhancement of fluorescence intensity by



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these anions as shown in Figure S9. Hence, it is concluded that the Al3+ sensor responses of HL are not affected really in the presence of different counter ions. Binding Sites and Reversibility. To gain a better understanding of the stoichiometry and binding sites of HL with Al3+, 1H NMR titration experiment was carried out in DMSO-d6 as depicted in Figure S10. Upon addition of 0.5 and 1.0 equivalent of Al3+, the signal of the -OH proton is gradually disappeared, strongly suggests the-involvement of heteroatom’s (O and N) and their chelation to form the Al-L complex in the sensing mechanism. There were no appreciable changes in the position of proton signals on further addition of Al3+, which supported the 1:1 metal-to-ligand ratio of the Al-L complex concluded from the Job’s plot. Furthermore, complexation behavior was well supported by the reversibility of Al-L. Reversibility is one of the significant features to satisfy the demand of a novel chemosensor mainly for improving practical real time applications of a probe. To inspect the reversible binding of HL with Al3+, we used EDTA as a strong chelating ligand in identical reaction conditions. The fluorescence intensity of HL in the presence of Al3+ at 426 nm decreased immediately when excess EDTA was added due to demetalation of Al3+ from the corresponding complex. (Figure S11), i.e. sensor complex (Al-L) was found to be reversible to its original state. Detection Limit (LOD) and Dissociation Constant (Kd).

In order to determine the

sensitivity and selectivity of HL towards Al3+, the calculation of limit of detection (LOD) was calculated by the 3σ method34 as shown in Figure S12. The emission intensity of HL without Al3+ was measured as a function of increasing concentration of HL and the standard deviation of blank (without Al3+) measurements was determined. A good linear relationship between the relative fluorescence intensity and the concentration of Al3+ were obtained to calculate the detection limit as low as 8.64 nM and this is well below the permissible level of Al3+ in drinking


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water according to the United State Environmental Protection Agency (USEPA). As a result our fluorescent probe HL displayed high sensitivity toward Al3+ ions. Furthermore, to confirm the better selectivity of HL towards Al3+, we determined the dissociation constant of sensor complex (Al-L). Assuming a 1:1 complex formation, the dissociation constant (Kd) was calculated on the basis of the titration curve of the sensor HL with Al3+ by non-linear least squares fitting of the emission intensity at 426 nm, shown in Figure S13.24 The Kd value of Al-L was estimated as ~4.0 µM (R2 = 0.998), which clearly demonstrates the potent binding affinity of HL towards Al3+ ions. pH Dependence and Time Course of Fluorescence Response.

It is well-known that

fluorescence sensors based on the electron donor/acceptor ends are usually hampered by the concentration of protons in the medium for recognition of the metal ions. Thus for many practical applications, it is very essential whether the sensors are active or not at the appropriate pH conditions for successful operation. Therefore, we measured the fluorescence intensity of HL between pH 2 to pH 11 in presence and absence of Al3+ ions as depicted in Figure 4C and Figure S14 respectively. The fluorescence intensity of both HL and Al-L remain in a turn on state with no change in PL intensity in the pH region 6.0 - 9.0 suggesting maximum complexation in this pH region. It is also found that the emission intensity of HL in the presence of Al(III) ions is higher than that in the absence of Al(III) ions. The fluorescence intensity decreases in the acidic (pH9) region has been explained due to ICT which hindered the complexation.33 Hence, the emission intensity is stable over this wide range of pH (6.0 - 9.0) and well suitable for applications under physiological pH conditions.



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A short response time is necessary for fluorescent sensor to monitor Al3+ ions in practical applications. Assuming a 1:1 complex formation, the time-dependent fluorescent analysis of HL (20 µM) in CH3CN was performed by adding 20 µM of Al3+ in H2O, as shown in Figure 4D. The result revealed that the relative fluorescence intensity increases and reaches the maximum CHEF value at 426 nm which is identical as shown in Figure 2, i.e. recognition event occur completely almost 7 minutes upon mixing. Aggregation-Induced Emission Enhancement (AIEE) of HL. The AIEE property of HL was investigated in CH3CN by varying different volume percentage of water and the results are shown in Figure 5A. Acetonitrile was used as the good solvent and water as the poor solvent for HL. It was observed that the aggregation of HL started in the CH3CN-H2O mixtures with higher water contents. The final concentrations of HL were kept constant at 3 µM. As shown in Figure S15, in dilute acetonitrile solution, HL shows structured absorption spectra and emits very weakly at 426 nm upon photo-excitation at 364 nm. As water content increased from 0% to 90%, HL shows broad absorption peak with decreasing absorption with blue shift in the range of 300450 nm. Also increasing the water content, the fluorescence intensity of HL was dramatically enhanced with spectral red shift of 10 nm (Figure 5A). This observation suggests that HL is an AIEE-active compound. However, the fluorescence intensity of HL was reached the maximum value at 70% water content and also visualized by the photographs (Figure S16). Even in the CH3CN-H2O (10/90, v/v) mixture, the fluorescence intensity was still much higher than that in pure CH3CN. In addition, AIEE nature of HL has been investigated by the TCSPC technique. Measurement of fluorescence lifetime is more robust than measurement of fluorescence intensity, because it depends neither on the intensity of excitation nor on the concentration of the fluorophore. Figure


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5B shows the fluorescence decay of HL in pure CH3CN and CH3CN-H2O (30/70) mixture where decay constant value of 5.18 ns in CH3CN-H2O (30/70) was increased in contrast to 1.92 ns for HL in pure CH3CN. Mechanism for AIEE. The origin of such AIEE phenomena of HL could be explained similar to those reported in the previous literatures.52,53 The single bond rotation is mainly responsible for the dominant non radiative decay. So, the restriction of intramolecular rotation (RIR)37 of HL plays a crucial role in the AIEE characteristics. HOMO-LUMO electron distribution (Figure 1B) in the ground state of the molecule HL suggests that free intramolecular rotation of HL quenches emission intensity via non-radiative decay more freely in the solution state, while the rotation is completely restricted in the aggregated state and the emission enhancement with spectral red shift was probably originated due to the molecule HL attains relatively planar geometry and the suppression of twisted intramolecular charge transfer (TICT).54-56 The above explanation was also well supported by the similar reports42,43,50 available in the literature. Generally, absorption with blue shift and absorption with red shift are due to Htype and J-type of aggregation, respectively. However, in the present case, decreasing absorption with blue shift was observed upon increasing water content, which clearly reveals that H-type aggregates57-61 are formed (i.e., face to face interaction) and there is a noticeable color change observed from colorless to bluish color under the UV light illumination upon addition of 90% of water (Figure S16). This clearly indicates that HL molecule is AIEE active, and this phenomenon is verified by employing both steady-state and time-resolved fluorescence spectroscopy. As illustrated in Figure 5A, the AIEE effect increased rapidly up to 70% water content and also the lifetime (Figure 5B) was increased and determined to be 5.18 ns. Also, we have calculated the radiative rate constants (kr) and the total non-radiative rate constants (knr) of HL in



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pure acetonitrile and CH3CN-H2O (30/70) mixture (Table S1).51 The data suggest that there is a small change in knr values whereas almost ~40 times enhancement have been observed in kr values for CH3CN-H2O (30/70) mixture. The observed high radiative rate constant value is due to the suppression of ICT process due to hydrogen bonding interaction of imine donor with water caused by aggregation of the HL molecules as a result HL molecule is more emissive. The proposed mechanism is clearly visualized in Scheme 3. It was noted that other Schiff base systems43,44 show similar type of emission behavior, enlightening that those molecules are AIEE active. However, the maximum fluorescence intensity of the aggregated hydrosol was obtained at 70% water content, further increase in water percentage led to a decrease in fluorescence intensity due to decreasing the solvating power of the aqueous mixture which hinders the formation of crystallization process and favors to proceed the formation of amorphous aggregates of smaller sizes.62 Thus, the above spectral results disclose that HL possesses AIEE property. In addition, the effect of solvent polarity and solvent viscosity on the fluorescence properties of HL was also investigated. HL showed solvatochromic fluorescence and the value of red shifts were observed to increase with increasing solvent polarity (e.g., λmaxem = 412 nm and 438 nm in cyclohexane and in DMF, respectively; Figure S17). The emission intensity of HL was also measured in viscous solvents prepared from methanol-glycerol mixtures (Figure 6).56,63 It has been observed that the increasing viscosity (Figure 6A) and decreasing temperature (Figure 6B) of the solution of HL enhances its emission intensity. At room temperature, the fluorescence intensity of a dilute solution of HL in a viscous glycerol/methanol mixture (9:1 v/v) is much higher than that in pure methanol. As the solution temperature is decreased from 25 °C to -23 °C, the fluorescence intensity of HL is increased. These phenomena occur because high viscosity and


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low temperature can hamper intramolecular rotation, leading to the closure of the non-radiative decay channel and thus enhanced fluorescence emission. The results indicate that the restriction of intramolecular rotation plays a very important role in inducing the dye to emit. Besides absorption, steady state and time resolved fluorescence emission spectra, the phenomenon is also supported by the results of dynamic light scattering (DLS) (Figure 7). The DLS results show that the sizes of HL (20 µM) aggregates (mean diameter 116 nm) obtained in CH3CN-water mixtures with 10% volume fractions of water is much smaller than that in 70% water content (mean diameter 387 nm). The size of the aggregates decreased to 201 nm after the water fraction was increased to 90% as previously reported in many other AIEE systems.64 SEM (Figure 8) and optical fluorescence (Figure 9) microscopic study were done to get an idea about the morphology of the aggregated structures. No obvious morphology of the particles was observed when the aggregation was obtained at low concentration of HL. While, morphology of the microstructures obtained from centrifugation of 0.3 mM HL in aqueous mixture of 70% water content. Both optical and SEM study revealed that the morphology of microstructures was rod like shape. The intense solid state luminescence from HL microstructures was observed by optical microscopy. Upon UV excitation, HL shows rod shaped micro-particles with yellow emission. Dazzling light emissions are observed at the ends of the micro-crystals (highlighted by the green circles in the Figure 9A), indicating that an optical waveguide effect is operating in the light transmission process of the micro-crystals. Increasing the concentrations of HL increases the number of micro-particles with strong yellow light emission, shown in Figure 9. Thus, the efficient luminescence in the solid state makes it hopeful material for use in the fabrication of organic light emitting diode (OLED).



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CONCLUSIONS In summary, we have been able to develop herein, the synthesis and characterization of a new simple and inexpensive fluorescent probe that selectively exhibits a “turn-on” fluorescence response towards Al3+ at nano-molar (nM) range and also be utilized for aggregation induced emission enhancement (AIEE). About ~ 200 fold and ~160 fold increases in fluorescence intensity and quantum yield of HL, respectively, after the addition of 1 equivalent Al3+ might be attributed due to the combination of CHEF and AIEE effect during the chelation of HL toward the Al3+ ion in a 1:1 complex mode. Amusingly excess addition of Al3+, dramatically enhances the fluorescence intensity over manifold due to Al3+ induced aggregation. It is also noteworthy that the choice of the probe HL allows selectively the optical detection of Al3+ in terms of detection limit, as here the LOD (8.6 nM) is the first lowest than the earlier reported in literatures.34,35 Whereas the probe is almost silent in the presence of other relevant competing metal ions.

In addition, we successively performed 1H NMR titration and fluorescence

reversibility by adding Al3+ ions and EDTA to achieve better understanding of the binding site of the HL with Al3+. More importantly, sensor complex (Al-L) was found to be active in wide ranges of pH (2-11) and also effective w.r.t time (0-7 minutes). Similar to the sensor properties, HL exhibits novel AIEE properties and emission intensity increases maximum at 70% water content. Particularly, HL imparted a high degree of fluorescence sensitivity toward polarity and viscosity. Furthermore, the sensor and AIEE properties of HL were also well supported by various studies including DLS, SEM and optical fluorescence microscope. It is of great implication from fundamental and practical perspect that the astonishingly high efficiency of solid-state emission with the brilliant yellow-color emission and also optical wave-guiding effect are achieved from the compact π-conjugated system of HL. These characteristics make the probe 22 ACS Paragon Plus Environment

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promising candidate for real-time detection of toxic Al3+ ions as well as suitable for organic emitters that allow the construction of advanced photonic devices such as OLED. Hence, such a fluorescent probe with novel AIEE property has a huge prospect of application in chemo-sensing and bio-sensing fields. Further studies on the design of novel fluorescent probes for sensing metal ions with AIEE property are underway in our laboratory.

ASSOCIATED CONTENT Supporting Information Additional details of a complete description of chemicals, materials, method, quantitative determination results and also the detail characterization data to establish the sensory and AIEE behavior of HL are available in the supporting information and contain Scheme S1, Figures S1S17 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Fax: +91 3222 275329 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT



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We gratefully acknowledge the financial support received from CSIR (Ref. No. 01(2443)/10/EMR-II), New Delhi and UGC innovative research program, Vidyasagar University for carrying out this research work. M.S. thanks RGNF, UGC, New Delhi, India for his fellowship. P.M. and S.M. also thanks CSIR, New Delhi, India for their fellowship. Departmental instrumental facilities from DST FIST and UGC SAP programs are gratefully acknowledge. We gratefully acknowledge the help provide by USIC, Vidyasagar University for doing spectroscopic measurements and optical microscopic measurements.

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Figure 1. (A) Optimized structure of HL. (B) Frontier molecular orbital plots of HOMO and LUMO energy levels of HL calculated by using B3LYP/6-31 G (d,p) as implemented on Gaussian 09.

Figure 2. Fluorescence spectra of HL (20 µM) in CH3CN (λex = 364 nm) with 0-50 µM of Al3+ in H2O (with an equal span of 2 µM); insets: relative fluorescence intensity changes with respect to Al3+ concentration. (Titration was carried out by keeping CH3CN:H2O ratio, 95:5, v/v)



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Figure 3. DLS-based particles size analysis upon addition of 1.0, 2.5 and 10 equivalents of Al3+ ions. a) HL in 95:5 v/v CH3CN:H2O ratio, Z-avg. = 69 nm, b) 1 equiv. Al3+, Z-avg. = 232 nm, c) 2.5 equiv. Al3+, Z-avg. = 444 nm, d) 10 equiv. Al3+, Z-avg. = 1753 nm.


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Figure 4. (A) Sensor HL (20 µM) in CH3CN (λex = 364 nm) responses towards relevant competitive metal ions (note: each metal selectivity was measured with 2.5 equiv. of metal ions). Inset: Visual color change observed with addition of Al3+ to acetonitrile solution of HL as seen under UV light illumination (365 nm). (B) Relative fluorescence intensities of HL (20 µM) in CH3CN (λex = 364 nm) with 50 µM of Al3+ in H2O in the presence of competing metal ions. Pink bar: HL (20 µM) in CH3CN with 50 µM of Al3+ in H2O. Blue bar: HL (20 µM) in CH3CN with 50 µM Al3+ + 50 µM of stated metal ions in H2O. (C) PL spectral changes of sensor complex (Al-L) (20 µM) in CH3CN upon the addition of pH (2-11) buffers in H2O. (D) Time dependent fluorescence spectral changes of HL (20 µM) in CH3CN (λex = 364 nm) with 20 µM Al3+ in H2O (each measurement carried out at equal span of 20 seconds). Inset: relative fluorescence intensity 35 ACS Paragon Plus Environment


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changes of HL, as a function of time (0-7 minutes). (All titration were carried out by keeping CH3CN:H2O ratio 95:5, v/v)

Figure 5. (A) Fluorescence spectra of HL (3 µM) in CH3CN, upon increasing the concentration of water from 0% to 70% (Note: the spectra were taken after 22 hours). (B) Time resolved fluorescence spectra of HL (0% water; violet), HL (70% water; pink) and Lamp (black).


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Figure 6. (A) Fluorescence spectra of HL (20 µM) in MeOH–Glycerol mixtures measured with different weight fractions of Glycerol: 0 (green), 10 (orange), 30 (violet), 50 (magenta), 70 (blue), 90 wt% (red). (B) Fluorescence spectra of HL (20 µM) in a glycerol/methanol mixture with 90% glycerol at -23 °C, 0 °C and 25 °C. Data for HL in methanol (20 µM) at 25 °C is shown for comparison.



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Figure 7. Particle size distribution of HL (20 µM) in mixed aqueous media of (a) 10% water content, (b) 70% water content and (c) 90% water content

determined by dynamic light

scattering (DLS) measurements.

Figure 8. Scanning electron microscopy (SEM) image of HL micro particles (scale bar = 10 µm).


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Figure 9. Optical fluorescence microscopy images (solid state) of (A) 0.3 mM (B) 0.4 mM of HL with UV light excitation (scale bar = 20 µm).

Scheme 1. Schematic representation of sensor and aggregation induced emission enhancement (AIEE) of fluorophore (HL).



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Scheme 2. Possible proposed mechanistic pathway for sensing Al3+ ions based on complexation and Al3+ induced aggregation.

Scheme 3. Proposed mechanistic pathway for aggregation induced emission behavior of HL.


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Pyrene Scaffold as Real-Time Fluorescent Turn-on Chemosensor for

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