A robust fluorescent chemosensor for aluminium ion detection based on a Schiff base ligand with an azo arm and application in a molecular logic gate

Saikat Banerjeea, Paula Brandãob and Amrita Saha*a
aDepartment of Chemistry, Jadavpur University, Kolkata-700032, India. E-mail: asaha@chemistry.jdvu.ac.in; amritasahachemju@gmail.com; Tel: +91-33-24572941
bDepartamento de Química, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal

Received 23rd August 2016 , Accepted 18th October 2016

First published on 19th October 2016


Abstract

In this present work we have reported the synthesis and structural characterisations of a N2O2 donor Schiff base chemosensor with an azo arm (H2L). Various spectroscopic tools like single crystal X-ray, NMR, UV-vis, FTIR, ESI-mass spectrometry etc. have been deployed to develop the present work. In recent years a number of azo derived chemosensors have been reported by different research groups. This is first time we are reporting the design and properties of an azo derived chemosensor (H2L) for the detection of aluminium ions in semi aqueous medium. It has been found that it selectively senses Al3+ ions in semi aqueous solution. Here, the sensing process is mainly based on a chelation enhanced fluorescence process (CHEF). It has very high selectivity over other metal ions and anions. A detailed literature survey has been carried out and compared with this work. It has an appreciably low detection limit i.e. 6.93 nM. 1H NMR titration was carried out to support the plausible complexation process. 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometry binding between the chemosensor and Al3+ ions has been confirmed from Job's plot. An inhibition molecular logic gate has been constructed using chemosensor (H2L), where Al3+ and EDTA act as inputs and fluorescence emission is the output. The structural and electronic parameters of the chemosensor (H2L) and complex [AL(L)]NO3 have been studied in detail using theoretical tools like DFT and TDDFT.


Introduction

The design and construction of powerful and highly selective fluorescent chemosensors have attracted considerable attention in biomimetic chemistry.1 Among various heavy metal ions aluminium is an essential element due to its abundance and use in every sphere of life in utensils, electronic and electrical components of different gadgets, building equipment, different packaging items, water treatment, food additives, pharmaceutical products, occupational dust etc. Aluminum is a known neurotoxin to organisms1 and is believed to cause Alzheimer's disease,2 osteomalacia3 and breast cancer.4 Toxicity, due to the presence of excess aluminium, in human health may arise as it inhibits several essential elements of similar size and charge like Mg2+, Ca2+ and Fe3+. It is also responsible for retarded growth of plants5 and oxidative damage of the cell membrane.6 In spite of its many drawbacks, the enormous use of Al in daily life causes accumulation of Al3+ and hence toxicity towards human health and the environment. The WHO recommend an average weekly human body dietary intake of Al3+ of around 7 mg kg−1 body weight.7 Thus, it is of utmost urgency to detect Al3+ ions in the environment in trace levels i.e. in ppb, ppm or nano levels. Unfortunately, the determination of Al3+ is complicated mainly due to its poor coordination ability, strong tendency to hydration, and lack of suitable spectroscopic characteristics.8 Some analytical methods are available for detection of Al3+ such as graphite furnace atomic absorption spectrometry, inductively coupled plasma atomic emission spectrometry, atomic absorption spectra,9 electrochemistry,10 mass spectrometry,11 and 27Al NMR technology.12 Most of them are expensive and time consuming. Optical detection particularly fluorescence spectroscopic technique is advantageous over the other techniques because of its operational simplicity, low cost, low detection limit, real-time detection, selectivity, time saving and environmental friendly property. Therefore design and synthesis of new chemosensor for the detection of Al3+ has received considerable attention. Recently various research groups have reported different organic probes which can selectively detect Al3+.13,14 Although example of such Al3+ detecting chemosensors are still less compared with other metal ions detecting chemosensors like Zn2+ mainly due to less coordinating ability of Al3+ compared to other metal ions. Therefore, more improvement in this field is still required. Current literature survey (Table S1) reveals that preparation of most of the chemosensors for detection of Al3+ involves expensive stating materials, multiple reaction steps, use of different solvent mixtures and maintenance of drastic reaction conditions i.e. very high or low temperature, presence of catalyst etc. G. Das and et al. in a recent work14n have reported an Al3+ sensing organic probe viz. (E)-N′-((E)-3-(4-(dimethylamino)phenyl)allylidene)picolinohydrazide using starting materials picolinohydrazide and 4-(dimethylamino)cinnamaldehyde. Preparation of picolinohydrazide involves multiple steps. Preparation of another chemosensor viz. 2-((naphthalen-6-yl)methylthio)ethanol involves inert atmosphere and finally the product has been purified through column chromatography with 65% yield.14e S. Goswami and coworkers have reported a spirobenzopyran-quinoline (SBPQ) based sensor which selectively detects Al3+ along with Fe3+ and Cr3+. Preparation of this type of chemosensors also involves multiple steps.14o It is also important to mention that 2-hydroxynaphthaldehyde based organic probes are mostly used for fluorescence sensing of Al3+.14p

Different mechanistic pathway like intramolecular charge transfer (ICT),15 photo induced electron transfer (PET),16 chelation-enhanced fluorescence (CHEF),17 metal–ligand charge transfer (MLCT),18 excimer/exciplex formation, imine isomerization,19 intermolecular hydrogen bonding,20 excited-state intramolecular proton transfer,21 displacement approach,22 and fluorescence resonance energy transfer23 are adopted to explain the action of different fluorescence probe. Among different types of chemosensors, Schiff bases incorporated with different functionalized moiety are widely used mainly due to their ease of synthesis and low cost.

In this work an azo based salen-type of Schiff base ligand (E)-6,6′-((1E,1′E)-(propane-1,3-diylbis(azanylylidene))bis(methanylylidene))bis(2-methoxy-4-((E)-phenyldiazenyl)phenol) (H2L) is structurally characterized and it exhibit high selectivity towards Al3+ (3.31 × 103 M−1) with low detection limit (6.93 nM). Azo derivatives have prevalent use in different fields like pharmaceuticals, optical data storage, non-linear optics, photo switching devices, textile industry dye-sensitized solar cells and they also act as fluorescent chemosensors.24–30 Our work is important and novel in this aspect because although various azo containing chemosensors are reported in literature but to the best of our knowledge in this work we are first time reporting one such sensor which can selectively detect Al3+. Our organic probe possesses some extra advantages. We have prepared H2L using easily available starting materials. The azoaldehyde was prepared from well known easy diazotization process with very high yield. The azoaldehyde upon reaction with a simple diamine produced H2L with >90% yield. We have successfully isolated both the azoaldehyde and Schiff base H2L in highly crystalline form. The ligand is characterized by different techniques including X-ray crystallography. The composition of the Al3+–ligand complex (complex 1) has been established by different spectroscopic data like IR, Mass, Job's plot and 1H NMR. The DFT computation of optimized geometry of H2L and the complex [Al(L)]+ has been used to support the electronic spectral properties.

Experimental

Materials and physical measurements

All reagent or analytical grade chemicals and solvents were purchased from commercial sources and used without further purification. Elemental analysis for C, H and N was carried out using a Perkin-Elmer 240C elemental analyzer. Infrared spectra (400–4000 cm−1) were recorded from KBr pellets on a Nicolet Magna IR 750 series-II FTIR spectrophotometer. Absorption spectra were measured using a UV-2450 spectrophotometer (Shimadzu) with a 1 cm-path-length quartz cell. Measurements of NMR spectra were conducted using a Bruker 300 spectrometer in DMSO-d6 and CDCl3 respectively. Emission was examined by LS 55 Perkin-Elmer spectrofluorimeter at room temperature (298 K) in HEPES buffer at pH = 7.4 solution under degassed condition. Fluorescence lifetimes were measured using a time-resolved spectrofluorimeter from IBH, UK.

Synthesis of (E)-5-(2-phenyldiazenyl)-2-hydroxy-3-methoxybenzaldehyde

4 mL conc. HCl was added to 15 mL water and kept in an ice bath keeping the temperature 0 °C. To it, aniline (4.0 mmol, 0.372 g) was added. To this mixture an aqueous solution of sodium nitrite (6.0 mmol, 0.414 g) was drop wise added over a time of 30 min. The mixture was stirred for 1 h at 0 °C. Then the resultant solution was added to the alkaline o-vanillin (4.8 mmol, 0.732 g) which is kept in another ice bath. The mixture was further stirred for 1 h at 0 °C. The solution was neutralised using dilute HCl and pH was maintained around 7.0. The mixture was extracted with chloroform and evaporated to result orange colored crystals of the aldehyde.

Yield: 0.953 g (94%). Anal. calc. for C14H12N2O3: C 64.65%; H 5.79%; N 16.34%. Found: C 64.36%; H 5.31%; N 16.17%. IR (cm−1, KBr): ν(N[double bond, length as m-dash]N) 1459 s; ν(C–H) 762 s. ESI-MS (positive) in MeOH: the base peak was detected at m/z = 279.00, corresponding to [M + 23]+.

1H NMR (CDCl3, 300 MHz) δ ppm: 4.01 (–OCH3) (s, 3H), 7.25–7.35 (Ar-H) (m, 2H), 7.74–7.86 (Ar-H) (m, 5H), 10.02 (–CH[double bond, length as m-dash]O) (s, 1H), 11.05 (–OH) (bs, 1H).

Synthesis of Schiff base ligand (H2L)

A mixture of (E)-5-(2-phenyldiazenyl)-2-hydroxy-3-methoxybenzaldehyde (2.0 mmol) and 1,3-diaminopropane (1.0 mmol, 0.074 g) was heated to reflux for 4 h in methanol–chloroform solvent mixture (1[thin space (1/6-em)]:[thin space (1/6-em)]1, v/v). Upon slow evaporation of the solvent mixture deep red colored crystals were obtained.

Yield: 0.5282 g (96%). Anal. calc. for C29H26N6O2: C 67.62%; H 5.49%; N 15.26%. Found: C 66.86%; H 5.31%; N 14.97%. IR (cm−1, KBr): ν(C[double bond, length as m-dash]N) 1648 s; ν(N[double bond, length as m-dash]N) 1455 s; ν(C–H) 764 s. ESI-MS (positive) in MeOH: the base peak was detected at m/z = 551.21, corresponding to [M + 1]+. UV-vis, λmax (nm), (ε (dm3 mol−1 cm−1)) in acetonitrile: 290 (4533) and 388 (5736).

1H NMR (CDCl3, 300 MHz) δ ppm: 2.25 (–CH2) (m, 1H), 3.82–3.84 (–CH2) (bs, 2H), 4.01 (–OCH3) (s, 3H), 7.42–7.90 (Ar-H) (m, 7H), 8.43 (–CH[double bond, length as m-dash]N) (s, 1H).

1H NMR (DMSO-d6, 300 MHz) δ ppm: 2.08 (–CH2) (m, 2H), 3.74–3.82 (–CH2, –OCH3) (bs, 10H), 7.30–7.48 (Ar-H) (m, 8H), 7.66–7.74 (Ar-H) (m, 6H), 8.66 (–CH[double bond, length as m-dash]N) (s, 2H).

13C NMR (DMSO-d6, 75 MHz) δ ppm: 31.46 (–CH2), 56.34 (–CH2), 118.22, 122.58, 126.93, 127.41, 129.08 and 130.47 (Ar-C), 165.38 (–CH[double bond, length as m-dash]N).

Synthesis of [Al(L)](NO3) complex

A 2 mL methanolic solution of aluminium nitrate nonahydrate (1.0 mmol, 0.375 g) was added drop wise to 20 mL methanolic solution of H2L (1.0 mmol, 0.551 g) followed by addition of triethylamine (2.0 mmol, ∼0.4 mL) and the resultant reaction mixture was stirred for 4 h under nitrogen atmosphere. Then the resultant mixture was dried to powdered form and further characterizations were carried out.

Yield: 0.6246 g (98%). Anal. calc. for AlC29H24N7O5: C 67.62%; H 5.49%; N 15.26%. Found: C 66.86%; H 5.31%; N 14.97%. IR (cm−1, KBr): ν(C[double bond, length as m-dash]N) 1646 s; ν(N[double bond, length as m-dash]N) 1545 s; ν(C–H) 770 s. ESI-MS (positive) in MeOH: the base peak was detected at m/z = 575.25, corresponding to [Al(L)]+. UV-vis, λmax (nm), (ε (dm3 mol−1 cm−1)) in acetonitrile: 286 (5083) and 375 (5949).

1H NMR (DMSO-d6, 300 MHz) δ ppm: 2.16 (–CH2) (bs, 2H), 3.89–3.95 (–CH2, –OCH3) (bs, 10H), 7.54–7.81 (Ar-H) (m, 14H), 9.24 (–CH[double bond, length as m-dash]N) (s, 2H).

X-ray crystallography

Single crystal X-ray data of azoaldehyde and Schiff base ligand (H2L) were collected on a Bruker SMART APEX-II CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature. Data processing, structure solution, and refinement were performed using Bruker Apex-II suite program. All available reflections 2θmax were harvested and corrected for Lorentz and polarization factors with Bruker SAINT plus.31 Reflections were then corrected for absorption, inter-frame scaling, and other systematic errors with SADABS.32 The structures were solved by the direct methods and refined by means of full matrix least-square technique based on F2 with SHELX-1997 and SHELX-2013 software package.33 All the non-hydrogen atoms were refined with anisotropic thermal parameters. C–H hydrogen atoms were inserted at geometrical positions with Uiso = 1/2Ueq. to those they are attached. Crystal data and details of data collection and refinement are summarized in Table 1.
Table 1 Crystal parameters and selected refinement details for H2L
Compound Azoaldehyde H2L
Empirical formula C14H12N2O3 C31H28N6O4
Formula weight 256.26 548.59
Temperature (K) 155(2) 150(2)
Crystal system Monoclinic Triclinic
Space group Cc p-1
a (Å) 10.7683(4) 8.648(2)
b (Å) 13.4401(5) 11.051(3)
c (Å) 8.4892(3) 13.816(4)
α (°) 90.00 91.833(9)
β (°) 93.755(2) 97.710(9)
γ (°) 90.00 92.902(8)
Volume (Å3) 1225.98(8) 1305.8(6)
Z 4 2
Dcalc (g cm−3) 1.388 1.395
Absorption coefficient (mm−1) 0.100 0.095
F(000) 536 576
θ range for data collection (°) 2.43–26.40 2.326–23.559
Reflections collected 4910 12[thin space (1/6-em)]084
Independent reflections/Rint 2204/0.0313 3806/0.0726
Observed reflections [I > 2σ(I)] 1980 2308
Data/restraints/parameters 2204/2/175 3806/0/373
Goodness-of-fit on F2 1.029 1.752
Final indices [I > 2σ(I)] R1 = 0.0331 R1 = 0.1611
wR2 = 0.0812 wR2 = 0.4280
R indices (all data) R1 = 0.0378 R1 = 0.2089
wR2 = 0.0842 wR2 = 0.4619
Largest diff. peak/hole (e Å−3) 0.150/−0.120 0.968/−0.620


Computational method

All computations were performed using the GAUSSIAN09 (G09)34 software package. Coordinates obtained from single crystal X-ray data were used for optimization of structure of ligand (H2L). For optimization we used the density functional theory method at the B3LYP level35,36 and the standard 6-31+G(d) basis set for C, H, N and O atoms37,38 and the lanL2DZ effective potential (ECP) set of Hay and Wadt39–41 for aluminum atom have been chosen for optimization.

TDDFT calculation was performed with the optimized geometry to ensure only positive eigen values. Time-dependent density functional theory (TDDFT)42–44 was performed using conductor-like polarizable continuum model (CPCM)44–47 and the same B3LYP level and basis sets in aqueous solvent system. GAUSSSUM48 was used to calculate the fractional contributions of various groups to each molecular orbital.

Results and discussion

Synthesis and characterization

The azoaldehyde 5-(2-phenyldiazenyl)-2-hydroxy-3-methoxybenzaldehyde is synthesized by first diazotization of primary aromatic amine (aniline) followed by coupling with an aromatic alcohol (o-vanillin). Azoaldehyde reacted with 1,3-diamino propane in 2[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio at ambient temperature in methanol to generate the chemosensor H2L (Scheme 1). The yield of both the aldehyde and H2L is >90%. Both of them are crystallized from slow evaporation of methanol–chloroform (3[thin space (1/6-em)]:[thin space (1/6-em)]5) mixture. They are well characterized by 1H NMR, 13C NMR, mass and IR spectroscopy. In IR spectrum, azo (N[double bond, length as m-dash]N) band of both azoaldehyde and H2L appear at 1459 cm−1 and 1455 cm−1 respectively (Fig. S1). Other important stretching vibrations of H2L are 1648 (s, ν(C[double bond, length as m-dash]N)); 3100 cm−1 (ν(–OH)); 764 cm−1 (ν(C–H)) respectively. Similar characteristic stretching frequencies of [Al(L)]NO3 are obtained at 1646 (s, ν(C[double bond, length as m-dash]N)); 1530 cm−1 (ν(N[double bond, length as m-dash]N)); 770 cm−1 (ν(C–H)), respectively (Fig. S2). In complex 1, nitrate ion is present as a counter anion. The characteristic stretching frequencies are obtained at 765 cm−1 (planar rock), 815 cm−1 (NO, deformation) and 1375 cm−1 (NO, asymmetric stretch), respectively, which are comparable with that of previously reported literature values.49 In 1H NMR spectrum of azoaldehyde (CDCl3 solvent), aldehyde proton (–CH[double bond, length as m-dash]O) appeared at 10.02 ppm whereas –OCH3 and phenolic –OH protons appeared at 4.01 ppm and 11.05 ppm respectively. Azoaldehyde contains two aromatic rings. Aromatic protons came as multiplate at two different regions i.e. from 7.25 to 7.35 ppm and from 7.74 to 7.86 ppm respectively. In case of the H2L (CDCl3 solvent), aromatic protons appeared around 7.74–7.30 ppm whereas azomethine proton, –OCH3 protons and aliphatic protons appeared at 8.43, 4.01, 3.95 and 2.25 ppm respectively. Here, phenolic –OH proton is so labile that it becomes very difficult to arrest in both CDCl3 and DMSO-d6 solvent system. In 13C spectra of H2L (DMSO-d6 solvent), aliphatic carbon signals appeared at 31.46 ppm and 56.34 ppm respectively, whereas aromatic carbons appeared in the region of 118 ppm to 130 ppm. The signal at 165 ppm is the characteristic signal for imine carbon ([double bond splayed left]C[double bond, length as m-dash]N) (Fig. S3–S6). Mass spectral analysis of starting azoaldehyde and H2L exhibit m/z peaks at 279.00 and 551.21, respectively (Fig. S7–S9). Mass spectrum of H2L in the presence of Al3+ shows peak m/z at 575.25 which is corresponding to [Al(L)]+ (Fig. S10).
image file: c6ra21217d-s1.tif
Scheme 1 The route to the syntheses of H2L.

The structure of both azoaldehyde and H2L have been established by single crystal X-ray diffraction measurement. Crystals of both the organic compound are obtained from slow evaporation of CHCl3–CH3OH solvent mixture. The ORTEP plot of azoaldehyde and H2L are shown in Fig. 1 and 2 respectively and the selected bond parameters are listed in Table 2. The azoaldehyde crystallizes with a monoclinic space group Cc. It has almost planar structure where the azo nitrogen atoms and the aromatic ring containing –CHO, –OCH3, –OH groups belong in the same plane. The remaining aromatic ring deviates from the plane to a small extent (10.71°). The N[double bond, length as m-dash]N and C[double bond, length as m-dash]O (–CHO) bond distances are 1.252 Å and 1.224 Å respectively which are similar with other reported values.50 Aromatic C–C bond distances observed around 1.368–1.404 Å. In solid state it form 2D network through intermolecular hydrogen bonding between the phenoxy O-atom and aromatic hydrogen atom and edge to edge π–π stacking interaction between two phenyl rings. The distances are 2.704 Å and 3.70 Å respectively (Fig. S11).


image file: c6ra21217d-f1.tif
Fig. 1 Ortep view of azoaldehyde. Atoms are shown as 30% thermal ellipsoids. H atoms are omitted for clarity.

image file: c6ra21217d-f2.tif
Fig. 2 Ortep view of the ligand (H2L). Atoms are shown as 30% thermal ellipsoids. H atoms are omitted for clarity.
Table 2 Selected bond lengths (Å) and bond angles (°) for azoaldehyde and H2L
Azoaldehyde H2L
  X-ray Calculated   X-ray Calculated
C3–O2 1.340 1.3450 N1–N2 1.22(1) 1.2645
C8–O1 1.224 1.2264 N5–N6 1.267(9) 1.2647
N1–N2 1.252 1.2611 C15–N3 1.29(1) 1.2853
C2–O3 1.362 1.3631 C17–N4 1.28(1) 1.2845
  C15–C16–C17 114.4(7) 115.79
C1–O1 1.259 1.275
C2–O2 1.286 1.297


The Schiff base ligand (H2L) crystallizes with a triclinic space group P[1 with combining macron]. The azo –N[double bond, length as m-dash]N– bond distances are 1.217 Å and 1.268 Å respectively. The average C[double bond, length as m-dash]N– bond distance is 1.287 Å. The C–C bond distances of the phenyl rings found around 1.342–1.472 Å, whereas aliphatic C–C bond distances appear around 1.511–1.531 Å. The propylenic part of the Schiff base, N3–C15–C16–C17–N4, is to some extent puckered due to the sp3 hybridization of the saturated portion of the ligand. The bond angle (C15–C16–C17), 114.35(6)° deviates appreciably from its ideal value. OH group of the both of phenyl ring are placed 180° apart from each other to minimize the dipole–dipole interaction. In the Schiff base ligand each azo nitrogen atoms, imine atoms and the aromatic ring containing –CHO, –OCH3, –OH groups belong in the same plane. The remaining aromatic ring deviates from the plane significantly by an angle of 83.03°. The molecule dimerizes in solid state through intermolecular H-bonding between the two –OH groups as well as between the –OH group and imine nitrogen atom. –O–H⋯O bond distance is 3.879 Å whereas –O–H⋯N (imine) bond distance is 2.357 Å (Fig. S12).

Structural parameters are again calculated from optimized structure using DFT where bond distances are marginally elongated by 0.0023–0.0435 Å and bond angles varies within the range of 0.05–1.39° (Table 2) compare with the X-ray crystallographic data. Therefore, the optimized results are helpful to explain the electronic structure and electronic properties of the ligand H2L.

Absorption study

The UV-vis spectrum of the chemosensor H2L was recorded at 25 °C in aqueous buffer–acetonitrile solution (1[thin space (1/6-em)]:[thin space (1/6-em)]100 v/v, HEPES buffer at pH 7.4) which exhibited well-defined bands at 290 and 388 nm respectively. In order to study the binding property of H2L toward Al3+ ion, UV-vis spectra of H2L (10 μM) in the presence of various concentrations of Al3+ (0–10 μM) were recorded at room temperature, as shown in Fig. 3. It has been observed that old peaks disappear and two new absorption bands emerge at 286 and 375 nm respectively, and its absorbance gradually increases with the gradual addition of Al3+ and it gets saturated upon addition of 1.0 equivalents of Al3+ keeping the concentration of H2L fixed at 10 μM. These observations indicate the coordination of H2L with one eq. of Al3+ (Fig. 3).
image file: c6ra21217d-f3.tif
Fig. 3 Absorption titration of H2L (10 μM) with gradual addition of Al3+, 0–10 μM in MeCN/HEPES buffer at pH 7.4.

Al3+ ion sensing by fluorescence studies

The fluorescence property of H2L was also investigated in HEPES buffer solution at pH 7.4 at room temperature. The ligand, H2L, emits weakly at 510 nm when excited at 388 nm and the fluorescence quantum yield is (Φ = H2L) 0.00939. Upon excitation at 290 nm we observed similar emission at 510 nm. Low fluorescence intensity of free H2L probably due to photo induced electron transfer process (PET) caused by electron delocalization from the two phenoxido oxygen atoms to the π-conjugated system of two aromatic rings, [double bond splayed left]C[double bond, length as m-dash]N– group and azo (–N[double bond, length as m-dash]N–) group. Here presence of extended delocalization through azo group (–N[double bond, length as m-dash]N–) initiates appearance of fluorescence pick at 510 nm. After gradual addition of Al3+ ion with various concentrations (0–10 μM), a significant changes in emission spectra have been noticed. In presence of metal ion the emission band of H2L is blue shifted to 478 nm. This result also confirms a high sensitivity of the receptor towards Al3+. A plot of fluorescence intensities at 478 nm (I478) vs. concentration of aluminum has been given in Fig. 4: inset. It shows that sensing character of H2L (I478) increases with the increasing concentration of Al3+ and a clear bend of the curve was observed at 1.0 equivalent of added Al3+ which prove 1[thin space (1/6-em)]:[thin space (1/6-em)]1 stoichiometry of the [Al(L)]+. Such type of sigmoid curve reflects nature of interaction between the organic probe and Al3+ ion. Upon addition of 1–3 μM of Al3+ to 10 μM solution of chemosensor, very small change of fluorescence intensity of the probe at wavelength 478 nm has been observed. Upon gradual addition of Al3+ from 4–9 μM fluorescence intensity increases from 300–1400 a.u. which indicates strong interaction between Schiff base ligand and Al3+. Maximum increase of fluorescence intensity has been observed up to 10 μM addition of Al3+. This clearly indicates a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 binding between H2L and Al3+. The curve became a plateau with further addition of Al3+, when further increment in fluorescence intensity has not been observed.
image file: c6ra21217d-f4.tif
Fig. 4 Fluorescence titration of H2L (10 μM) in HEPES buffer at pH = 7.4 by gradual addition of Al3+ (0–10 μM) with λem = 478 nm (5/5 slit). Inset: non-linear plot of fluorescence intensity vs. concentration of Al3+ ion.

Enhancement in fluorescence intensity of H2L in presence of Al3+ probably due to elimination of photoinduced electron transfer (PET) in free H2L followed by chelation enhancement effect (CHEF) through the co-ordination of azomethine-N and phenolic-O to metal ion (Scheme 2). Hence, upon complexation a large CHEF effect is observed because of the rigid framework and thereby obstructs the PET process. Thus, upon coordination with Al3+ ion, the PET process within the ligand system would become weak by reduction of either the electron-accepting ability of the π-conjugated system of two aromatic rings, [double bond splayed left]C[double bond, length as m-dash]N– and azo (–N[double bond, length as m-dash]N–) groups or the electron-donating ability of the phenoxido oxygen atoms.


image file: c6ra21217d-s2.tif
Scheme 2 Schematic diagram for photoinduced electron transfer (PET) in free H2L and chelation enhanced fluorescence effect (CHEF) in [Al(L)]+.

Fluorescence intensity of H2L has been examined with various cations (Cd2+, Co2+, Cr3+, Cu2+, Fe3+, Hg2+, K+, Mn2+, Na+, Ni2+, Sn2+, Zn2+, Ag+, Fe2+ and Ca2+) using their nitrate salt (Fig. S13). Fig. S13 shows that only Al3+ can induce significant enhancement of fluorescence intensity. Interestingly, upon addition of different metal ions the intensity of the fluorescence either remains unchanged or weakened. In presence of Co2+, Cr3+, Cu2+, K+, Mn2+, Na+, Ni2+, Fe2+ and Ca2+ metal ions emission intensity remain almost unchanged and in presence of Fe3+, Cd2+, Sn2+, Ag+, Hg2+, and Zn2+ metal ions emission intensity slightly increased (Fig. S13). A competition assay of H2L in the presence of Al3+ and other metal ions have also been studied and presented in Fig. 5, which prove that fluorescence enhancement due to Al3+ nullify the possible interference of other metal ions. Upon addition of different anions viz. S2O32−, S2−, SO32−, SO42−, SCN, N3, AsO43−, PO43−, ClO4, AcO, Cl, NO2, NO3 and SCN in HEPES buffer at pH 7.4 (Fig. 6) chemosensor H2L showed no significant fluorescence enhancement. Thus, the probe is an excellent example of fluorescence chemosensor towards Al3+ in the presence of different metal ions.


image file: c6ra21217d-f5.tif
Fig. 5 Relative fluorescence intensity profile of [H2L–Al3+] system in the presence of different cations in HEPES buffer at pH 7.4. H2L (10 μM) + Al3+ (10 μM) + Mn+ (500 μM), where Mn+ = ((1) – Cd2+, (2) – Co2+, (3) – Cr3+, (4) – Cu2+, (5) – Fe3+, (6) – Hg2+, (7) – K+, (8) – Mn2+, (9) – Na+, (10) – Ni2+, (11) – Sn2+, (12) – Zn2+, (13) – Ag+, (14) – Fe2+, (15) – Ca2+).

image file: c6ra21217d-f6.tif
Fig. 6 Relative fluorescence intensity profile of H2L (10 μM) in the presence of various common anions (50 μM) in HEPES buffer at pH 7.4. (1) – H2L + Al3+, (2) – 14H2L + anions, anions = (2) – S2O32−, (3) – S2−, (4) – SO32−, (5) – SO42−, (6) – SCN, (7) – N3, (8) – AsO43−, (9) – PO43−, (10) – ClO4, (11) – OAc, (12) – Cl, (13) – NO2, (14) – NO3.

To find out the binding ability of our chemosensor H2L with Al3+ ions, binding constant was calculated using Benesi–Hildebrand equation (eqn (1)) involving fluorescence titration curve.

 
image file: c6ra21217d-t1.tif(1)
where, Fmax, F0 and Fx are fluorescence intensities of H2L in the presence of Al3+ at saturation, free H2L and any intermediate Al3+ concentration at λmax = 478 nm. K is the dissociation constant of the complex. Concentration of Al3+ ions is represented by C and slop n is measured with Hill plot. Then binding constant (Ka) of the complex has been determined using the relation, Ka = 1/Kd. A plot of image file: c6ra21217d-t2.tif vs. image file: c6ra21217d-t3.tif provides the apparent binding constant value as 3.31 × 103 M−1 (Fig. S14) (n = 1.0).14e The Job's plot suggests that the ligand H2L forms 1[thin space (1/6-em)]:[thin space (1/6-em)]1 complex with Al3+ ions (Fig. S15 and S16). To determine the binding stoichiometry of H2L and Al3+ fluorescence intensity is plotted against different mole fractions of Al3+ while volume of solution has remained constant (Fig. S16). Maxima in this plot has been obtained at 0.5 mole fraction, which suggests about 1[thin space (1/6-em)]:[thin space (1/6-em)]1 complex formation of H2L and Al3+. The formation of the complex further confirmed from ESI mass spectroscopy. Mass spectrum of H2L in the presence of Al3+ shows peak m/z at 575.25 which is corresponding to cationic part of the complex 1 [Al(L)]+ (Fig. S10). The simulated pattern shows well agreement with that of experimental values (Fig. S17). Experimental findings (absorption spectra and fluorescence spectra of Al3+ and other cations and various anions with chemosensor H2L) clearly revealed that H2L has strong affinity towards Al3+. Here Al3+ acts as strong Lewis acid which can simply binds with two phenoxido oxygen atoms and two imine nitrogen atoms of the Schiff base ligand. Thus the chemosensor acts as a tetradentate N2O2 donor ligand and form a distorted tetrahedral complex with Al3+. Comparable size of the inner-cavity of the chemosensor and Al3+ i.e. suitable size and high charge density of Al3+ permits strong interaction between H2L and the metal ion.

Fluorescence decay behavior of H2L and complex 1 has been studied. Decay curves have been given in ESI (Fig. 7). Fluorescence life time of H2L is found to quite low (1.324 ns). Fluorescence life time of H2L in presence of one eq. of Al3+, increases up to 2.058 ns. Quantum yields of H2L and complex 1 are calculated using eqn (2) and found to be very low. These have been measured to be 0.00939 and 0.2133, respectively (Table S1). It is to be noted that although quantum yield of the complex is low but it is almost ∼22 folds greater compared to free H2L.

 
image file: c6ra21217d-t4.tif(2)
where Fx, Fs are the wavelength integrated emission intensities of the samples and reference (here reference is coumarine); As, Ax are the optical densities at their corresponding wavelengths of excitation.


image file: c6ra21217d-f7.tif
Fig. 7 Time-resolved fluorescence decay curves (logarithm of normalized intensity vs. time in ns) of H2L in the absence (image file: c6ra21217d-u1.tif) and presence (image file: c6ra21217d-u2.tif) of Al3+ ion, (image file: c6ra21217d-u3.tif) indicates decay curve for the scattered (λex = 478 nm).

Free ligand upon excitation gives peak at 510 nm. In presence of Al3+ there is a slight blue shift ∼32 nm of intensity and emission peak observed at 478 nm. Upon gradual addition of Al3+ to H2L a steady increase in emission intensity at 478 nm has been observed.

Sensitivity of H2L towards Al3+ has been checked by determining limit of detection (LOD) value. The detection limit of the chemosensor was calculated using 3σ method51 and it is found to be 6.93 × 10−9 mol L−1.14e Low LOD value clearly indicates high sensitivity of H2L towards Al3+ ion. In Table S1 some recently published chemosensors for Al3+ ion has been reported along with their LOD values.14a,e,g,i–o The reported chemosensor has some advantages over the others while it has some draw backs also. Our probe is synthesized by easy single step Schiff base condensation process. LOD value is significantly low but not the lowest among all.

We also examined fluorescence intensity of H2L in absence and presence of Al3+ ion at various pH values. We have maintained the pH range from 3 to 10. The solution concentration of both H2L and Al3+ is 10 μM respectively. It has been observed that the chemosensor in free condition exhibit very little fluorescence in the acidic condition (pH = 3 to pH = 7). Fluorescence intensity remains unchanged within this pH range. Upon increasing pH from 7 to 10 a slight increase in fluorescence intensity has been observed. In presence of Al3+, in acidic condition (pH = 3 to pH = 5) fluorescence intensity of the chemosensor remain unchanged due to protonation of phenolic hydroxyl group of the ligand, therefore nullifying chelation ability with Al3+. Al3+ ion sensing abilities were exhibited by the ligand when pH was increased from 5.0 to 8.0. Thus, H2L exhibited good fluorescence sensing ability towards Al3+ ion over a wide range of pH (Fig. 8). Based on the experimental results we can conclude that our chemosensor can be utilized as a selective fluorescent probe to recognize and distinguish Al3+ ion in presence of other metal ions in biological system under physiological condition.


image file: c6ra21217d-f8.tif
Fig. 8 Fluorescence intensity of H2L (black box; 10 μM) in the absence and presence of Al3+ ion (red circle, 10 μM) ion at various pH values in HEPES buffer.

DFT study

Geometry optimization of both H2L and [Al(L)]+ has been performed using DFT/B3LYP method. Mass spectral data analysis has confirmed that the composition of the complex ion is [Al(L)]+. The energy minimized structure of both H2L and [Al(L)]+ are shown in Fig. 9. Some selected bond distances and bond angles of the optimized geometries of [Al(L)]+ are listed in Table 3. DFT optimized structure reveals that Al3+ is tetra-coordinated and distorted tetrahedral in geometry (Fig. 9) where the chemosensor H2L act as a N2O2 donor center and binds to Al3+ through phenolic-O atom and imine-N. Theoretically calculated Al–N (imine) and Al–O (phenolato) distances are around 1.906 Å and 1.326 Å, respectively and have been found comparable with similar structure.14e It is interesting to mention that upon complex formation the C(15)–N(3), C(17)–N(4), C(1)–O(1) and C(20)–O(2) bond distances have been significantly elongated compared to free ligand (Table 3). Theoretical calculations show that electron density in both HOMO and LUMO of H2L is mainly distributed on the one part of the aromatic moiety. In metal complex ion i.e. [Al(L)]+ electron density in both HOMO and LUMO is allocated over the entire ligand system. Energy of some selected M.O. of both H2L and [Al(L)]+ are given in Table S3. Mulliken charge distribution shows positive charge on aluminium ion i.e. 1.227343. Mulliken charge distribution of [Al(L)]+ is provided in Table S4.
image file: c6ra21217d-f9.tif
Fig. 9 DFT optimized structures of H2L and [Al(L)]+. H-Atoms are omitted for clarity.
Table 3 Selected bond lengths (Å) and bond angles (°) of DFT optimized structure of [Al(L)]+. (B3LYP/6-31+G(d) basis set)
  Calculated   Calculated   Calculated
N1–N2 1.26019 C1–O1 1.32767 Al–O1 1.7584
N5–N6 1.26320 C2–O2 1.32668 Al–O2 1.7553
C15–N3 1.31355 Al–N3 1.90639  
C17–N4 1.31661 Al–N4 1.92372
C15–C16–C17 115.79 C15–C16–C17 116.50435


TDDFT study

For better understanding of electronic transition, TDDFT calculations were performed using B3LYP/CPCM method using same basis sets in acetonitrile. The calculated electronic transitions are given in Table 4. H2L shows intense absorption band for ligand based n–π and π–π* transitions around 290 nm and 388 nm respectively. The band at 290 nm is due to the contribution of HOMO−4 → LUMO+1 and HOMO → LUMO+2 transitions, whereas the band at 388 nm is due to the contribution of HOMO−1 → LUMO, HOMO−1 → LUMO+1 and HOMO → LUMO transitions. For the L–Al species the intense absorption band around 375 nm corresponding to HOMO → LUMO+1, HOMO → LUMO+2, HOMO → LUMO+3 and HOMO−2 → LUMO transitions (Fig. 10). The spectra calculated for the ligand and L–Al species were found to be compatible with those obtained experimentally. It is important to mention that in case of [Al(L)]+ species all transitions are ligand based.
Table 4 Electronic transition calculated by TDDFT using B3LYP/CPCM method in acetonitrile solvent for ligand (H2L) and [Al(L)]+
  Eexcitation (eV) λexcitation (nm) Osc. strength (f) Key transition Character
Ligand (H2L) 3.60 344.43 0.1608 HOMO−4 → LUMO+1 (42%) π(L) → π*(L)
3.57 346.63 0.0469 HOMO → LUMO+2 (29%) π(L) → π*(L)
3.19 388.28 0.902 HOMO−1 → LUMO (56%) π(L) → π*(L)
3.14 394.53 0.6101 HOMO−1 → LUMO+1 (48%) π(L) → π*(L)
3.019 410.64 0.0149 HOMO → LUMO (71%) π(L) → π*(L)
[Al(L)]+ 3.17 390.79 0.2576 HOMO−1 → LUMO+1 (86%) π(L) → π*(L)
3.15 393.75 0.0254 HOMO → LUMO+2 (82%) π(L) → π*(L)
3.02 411.01 0.0011 HOMO−1 → LUMO (86%) π(L) → π*(L)
2.95 419.85 0.6153 HOMO → LUMO (89%) π(L) → π*(L)



image file: c6ra21217d-f10.tif
Fig. 10 Pictorial representation of key transitions of H2L and [Al(L)]+.

In order to have better understanding of the emission spectrum we have optimized the triplet state (T1) of both ligand (H2L) and ([Al(L)]+) using same basis set. The emission wavelength obtained from the computation is very much comparable with that of experimental data. The data i.e. emission wavelength, emission energies and the nature of the transition obtained from the computation are given in Table 5. The emission band of the chemosensor (H2L) at 510 nm was theoretically obtained at 509.84 nm with major key transitions of HOMO(α) → LUMO(α)+1 (58%), HOMO(α) → LUMO(α)+2 (10%) and HOMO(β)−1 → LUMO(β) (11%) respectively. Whereas for the [Al(L)]+ the emission band at 478 nm was theoretically obtained at 476.93 with major key transitions viz. HOMO(β) → LUMO(β)+2 (22%) and HOMO(α)−1 → LUMO(α) (66%) respectively.

Table 5 Emission spectrum calculated by TDDFT using B3LYP basis set for ligand (H2L) and [Al(L)]+
  Eexcitation (eV) λemission (nm) Excited state Osc. strength (f) Key transition
Ligand (H2L) 2.43 509.84 5 0.0116 HOMO(α) → LUMO(α)+1 (58%)
HOMO(β)−1 → LUMO(β) (11%)
HOMO(α) → LUMO(α)+2 (10%)
[Al(L)]+ 2.17 476.93 8 0.2476 HOMO(α)−1 → LUMO(α) (66%)
HOMO(β) → LUMO(β)+2 (22%)


NMR studies

Firstly, the chemosensor was well characterized in CDCl3 solvent. In CDCl3, imine proton appeared at 8.43 ppm. Aromatic, aliphatic and methoxy protons appeared at 7.42–7.90 ppm, 3.82–3.84 ppm, 2.25 ppm and 4.01 ppm respectively. Then, we have performed 1H NMR studies of both free H2L and H2L + Al3+ in 1[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio in DMSO-d6 solvent (Fig. 11). The free chemosensor (H2L) exhibit clear peaks for different protons. Imine proton appears at 8.66 ppm. All aromatic protons appear as multiplate at around 7.74–7.30 ppm. Aliphatic protons appear as multiplate at 3.74 and 2.06 ppm respectively. –OCH3 protons appear around 3.74 ppm. In NMR spectrum signal of aliphatic one –CH2 got merged with that the signal of –OCH3. Unfortunately we could not identify signal for phenolic OH proton due to extensive hydrogen bonding with solvent molecules. Upon addition of Al3+ to the chemosensor peak positions of different protons have changed significantly. Imine protons have shifted notably from 8.66 ppm to 9.24 ppm indicating coordination of imine nitrogen with aluminium ion. In case of aromatic and aliphatic protons broadening and overlapping of signals observed. Therefore, formation of complex 1 in the presence of Al3+ is strongly evidenced from the NMR spectral studies.
image file: c6ra21217d-f11.tif
Fig. 11 1​H NMR titration of H2L in presence of 1 eq. of Al3+ ion.

Molecular logic gates

The property of this chemosensor encourages us to construct a molecular logic gate with two binary inputs. The two inputs are Al3+ and Na2EDTA where the output is monitored as change in fluorescence emission spectrum at 478 nm. In absence of both of the inputs there is no significant change in emission band which implies that the gate is ‘OFF’. On addition of Na2EDTA to the chemosensor there is no significant changes, rather the emission band remained unchanged. So, in presence of Na2EDTA the output was considered to be zero. On the other hand, upon addition of Al3+ ion solution alone to the chemosensor shows significant enhancement of the fluorescence intensity at 478 nm and now the output signal is 1 i.e. the gate is ‘ON’. However in presence of both of the inputs the fluorescence intensity is significantly quenched and the output is zero implying that the gate is ‘OFF’ again. These studies suggest that this molecular gate is acting as INHIBIT (INH) logic gate where the inputs and output are Al3+, Na2EDTA and change in fluorescence intensity at 478 nm, respectively. It is basically an ‘AND’ gate where one of its inputs is negated. The input which is negated, acts to inhibit the gate. We can say that the gate will behave like an ‘AND’ gate only when the negated input is set at a logic level ‘0’. Here the negated input is Na2EDTA. The respective truth table and pictorial diagram of the INH logic gate is depicted in Fig. 12 and Table 6.
image file: c6ra21217d-f12.tif
Fig. 12 Pictorial representation of logic gate.
Table 6 Truth table
IN1 (Al3+) IN2 (EDTA) Output (emission at 478 nm)
0 0 0
0 1 0
1 0 1
1 1 0


Conclusion

In summary, an azo based low cost, simple, easy to prepare, chemosensor H2L has been successfully prepared which is capable of recognizing Al3+ ion in presence of large number of other metal ions in HEPES buffer solution (1[thin space (1/6-em)]:[thin space (1/6-em)]100 v/v, HEPES buffer at pH 7.4) at 25 °C. The chemosensor has been structurally characterized. Fluorescence intensity of the probe has enhanced by ∼61 fold in presence of Al3+. The low detection limit of H2L for Al3+ (6.93 nM) suggests that the chemosensor could be a good choice for efficient monitoring of the Al3+ in real samples. H2L forms 1[thin space (1/6-em)]:[thin space (1/6-em)]1 complex with Al3+ which has been established by 1H NMR, MS studies and further supported by DFT calculations. H2L exhibited good fluorescence sensing ability towards Al3+ ion over a wide range of pH, therefore, H2L could be successfully applied to living cells for detecting Al3+. We also establish molecular logic gates (INH) based on two inputs (Al3+ and EDTA) and one output. Thus, this probe could be considered as a potential candidate for sensing Al3+ in less organic solvents.

Acknowledgements

A. S. gratefully acknowledges the financial support of this work by the DST, India (Sanction No. SB/FT/CS-102/2014, dated – 18.07.2015). The authors are thankful to Prof. Mahammad Ali and Mr Pravat Ghorai, Department of Chemistry, Jadavpur University, India for their scientific suggestions. The authors also acknowledge the use of the DST-funded National Single Crystal X-ray Diffraction Facility at the Department of Chemistry, Jadavpur University, Kolkata-700032, India for X-ray crystallographic studies.

References

  1. M. G. Sont, S. M. White, W. G. Flamm and G. A. Burdock, Regul. Toxicol. Pharmacol., 2001, 33, 66–79 CrossRef PubMed.
  2. (a) D. R. Crapper, S. S. Krishnan and A. J. Dalton, Science, 1973, 180, 511–513 CAS; (b) D. P. Perl and A. R. Brody, Science, 1980, 208, 297–299 CAS; (c) E. House, J. Collingwood, A. Khan, O. Korchazkina, G. Berthon and C. J. Exley, J. Alzheimer's Dis., 2004, 6, 291–301 CAS.
  3. G. C. Woodson, Bone, 1998, 22, 695–698 CrossRef CAS PubMed.
  4. P. D. Darbre, J. Inorg. Biochem., 2005, 99, 1912–1919 CrossRef CAS PubMed.
  5. R. W. Gensemer and R. C. Playle, Crit. Rev. Environ. Sci. Technol., 1999, 29, 315–450 CrossRef CAS.
  6. (a) A. Budimir, Acta Pharm., 2011, 61, 1 CrossRef CAS PubMed; (b) P. Nayak, Environ. Res., 2002, 89, 101–115 CrossRef CAS PubMed; (c) P. Zatta, Coord. Chem. Rev., 2002, 228, 271–284 CrossRef CAS.
  7. (a) J. Barcelo and C. Poschenrieder, Environ. Exp. Bot., 2002, 48, 75–92 CrossRef CAS; (b) B. Valeur and I. Leray, Coord. Chem. Rev., 2000, 205, 3–40 CrossRef CAS; (c) Z. Krejpcio and R. W. P. J. Wojciak, Environ. Stud., 2002, 11, 251–254 CAS.
  8. K. Soroka, R. S. Vithanage, D. A. Phillips, B. Walker and P. K. Dasgupta, Anal. Chem., 1987, 59, 629–636 CrossRef CAS.
  9. A. Zioła-Frankowska, M. Frankowski and J. Siepak, Talanta, 2009, 79, 623–630 CrossRef PubMed.
  10. F. Zheng and B. Hu, Spectrochim. Acta, Part B, 2008, 63, 9–10 CrossRef.
  11. F. Thomas, A. Maslon, J. Y. Bottero, J. Rouiller, F. Montlgny and F. Genevrlere, Environ. Sci. Technol., 1993, 27, 2511–2516 CrossRef CAS.
  12. Y. H. Ma, R. Yuan, Y. Q. Chai and X. L. Liu, Mater. Sci. Eng., 2010, 30, 209–224 CrossRef CAS.
  13. K. Soroka, R. S. Vithanage, D. A. Phillips, B. Walker and P. K. Dasgupta, Anal. Chem., 1987, 59, 629–636 CrossRef CAS.
  14. (a) L. Wang, W. Qin, X. Tang, W. Dou, W. Liu, Q. Teng and X. Yao, Org. Biomol. Chem., 2010, 8, 3751–3757 RSC; (b) K. K. Upadhyay and A. Kumar, Org. Biomol. Chem., 2010, 8, 4892–4897 RSC; (c) F. K. Hau, X. He, W. H. Lam and V. W. Yam, Chem. Commun., 2011, 47, 8778–8780 RSC; (d) A. Sahana, A. Banerjee, S. Das, S. Lohar, D. Karak, B. Sarkar, S. K. Mukhopadhyay, A. K. Mukherjee and D. Das, Org. Biomol. Chem., 2011, 9, 5523–5529 RSC; (e) A. Banerjee, A. Sahana, S. Das, S. Lohar, S. Guha, B. Sarkar, S. K. Mukhopadhyay, A. K. Mukherjee and D. Das, Analyst, 2012, 137, 2166–2175 RSC; (f) D. Karak, S. Lohar, A. Sahana, S. Guha, A. Banerjee and D. Das, Anal. Methods, 2012, 4, 1906–1908 RSC; (g) D. Karak, S. Lohar, A. Banerjee, A. Sahana, I. Hauli, S. K. Mukhopadhyay, J. S. Matalobos and D. Das, RSC Adv., 2012, 2, 12447–12454 RSC; (h) A. Sahana, A. Banerjee, S. Lohar, S. Das, I. Hauli, S. K. Mukhopadhyay, J. S. Matalobos and D. Das, Inorg. Chim. Acta, 2012, 12, 12–13 Search PubMed; (i) A. Sahana, A. Banerjee, S. Lohar, B. Sarkar, S. K. Mukhopadhyay and D. Das, Inorg. Chem., 2013, 52, 3627–3633 CrossRef CAS PubMed and references cited herein; (j) D. Maity and T. Govindaraju, Inorg. Chem., 2010, 49, 7229–7231 CrossRef CAS PubMed; (k) J. Ma, W. Shi, L. Feng, Y. Chen, K. Fan, Y. Hao, Y. Hui and Z. Xie, RSC Adv., 2016, 6, 28034–28037 RSC; (l) B. Sen, M. Mukherjee, S. Banerjee, S. Pala and P. Chattopadhyay, Dalton Trans., 2015, 44, 8708–8717 RSC; (m) J. Kumar, M. J. Sarma, P. Phukan and D. K. Das, Dalton Trans., 2015, 44, 4576–4581 RSC; (n) C. Kar, S. Samanta, S. Goswami, A. Ramesh and G. Das, Dalton Trans., 2015, 44, 4123–4132 RSC; (o) S. Goswami, K. Aich, S. Das, A. Kumar Das, D. Sarkar, S. Panja, T. K. Mondal and S. Mukhopadhyay, Chem. Commun., 2013, 49, 10739–10741 RSC; (p) A. Roy, S. Dey and P. Roy, Sensors and Actuators B, 2016, 237, 628–642 CrossRef CAS and references cited herein.
  15. (a) Z. Xu, Y. Xiao, X. Qian, J. Cui and D. Cui, Org. Lett., 2005, 7, 889 CrossRef CAS PubMed; (b) J. B. Wang, X. F. Qian and J. N. Cui, J. Org. Chem., 2006, 71, 4308–4311 CrossRef CAS PubMed.
  16. (a) T. Gunnlaugsson, A. P. Davis, J. E. O'Brien and M. Glynn, Org. Lett., 2002, 4, 2449–2452 CrossRef CAS PubMed; (b) D. H. Vance and A. W. Czarnik, J. Am. Chem. Soc., 1994, 116, 9397–9398 CrossRef CAS; (c) S. K. Kim and J. Yoon, Chem. Commun., 2002, 770–771 RSC.
  17. (a) N. C. Lim, J. V. Schuster, M. C. Porto, M. A. Tanudra, L. Yao, H. C. Freake and C. Bruckner, Inorg. Chem., 2005, 44, 2018–2030 CrossRef CAS PubMed; (b) S. Guha, S. Lohar, A. Banerjee, A. Sahana, A. Chaterjee, S. K. Mukherjee, J. S. Matalobos and D. Das, Talanta, 2012, 91, 18–25 CrossRef CAS PubMed; (c) S. Das, A. Sahana, A. Banerjee, S. Lohar, S. Guha, J. S. Matalobos and D. Das, Anal. Methods, 2012, 4, 2254–2258 RSC.
  18. (a) P. D. Beer, Acc. Chem. Res., 1998, 31, 71–80 CrossRef CAS; (b) M. J. Kim, R. Konduri, H. Ye, F. M. MacDonnell, F. Puntoriero, S. Serroni, S. Campagna, T. Holder, G. Kinsel and K. Rajeshwar, Inorg. Chem., 2002, 41, 2471–2476 CrossRef CAS PubMed.
  19. (a) S. Nishizawa, Y. Kato and N. Teramae, J. Am. Chem. Soc., 1999, 121, 9463–9464 CrossRef CAS; (b) J.-S. Wu, J.-H. Zhou, P.-F. Wang, X.-H. Zhang and S.-K. Wu, Org. Lett., 2005, 7, 2133–2136 CrossRef CAS PubMed; (c) B. Schazmann, N. Alhashimy and D. Diamond, J. Am. Chem. Soc., 2006, 128, 8607–8614 CrossRef CAS PubMed; (d) A. Banerjee, A. Sahana, S. Guha, S. Lohar, I. Hauli, S. K. Mukhopadhyay, J. S. Matalobos and D. Das, Inorg. Chem., 2012, 51, 5699–5704 CrossRef CAS PubMed; (e) A. Sahana, A. Banerjee, S. Lohar, S. Guha, S. Das, S. K. Mukhopadhyay and D. Das, Analyst, 2012, 137, 3910–3913 RSC.
  20. J.-S. Wu, W.-M. Liu, X.-Q. Zhuang, F. Wang, P.-F. Wang, S.-L. Tao, X.-H. Zhang, S.-K. Wu and S.-T. Lee, Org. Lett., 2007, 9, 33–36 CrossRef CAS PubMed.
  21. (a) A. Sahana, A. Banerjee, S. Guha, S. Lohar, A. Chattopadhyay, S. K. Mukhopadhyay and D. Das, Analyst, 2012, 137, 1544–1546 RSC; (b) S. Lohar, A. Sahana, A. Banerjee, A. Banik, S. K. Mukhopadhyay, J. S. Matalobos and D. Das, Anal. Chem., 2013, 85, 1778–1783 CrossRef CAS PubMed; (c) S. Das, S. Guha, A. Banerjee, S. Lohar, A. Sahana and D. Das, Org. Biomol. Chem., 2011, 9, 7097–7104 RSC.
  22. X. Peng, Y. Wu, J. Fan, M. Tian and K. Han, J. Org. Chem., 2005, 70, 10524–10531 CrossRef CAS PubMed.
  23. (a) J. M. Serin, D. W. Brousmiche and J. M. J. Frechet, J. Am. Chem. Soc., 2002, 124, 11848–11849 CrossRef CAS PubMed; (b) A. E. Albers, V. S. Okreglak and C. J. Chang, J. Am. Chem. Soc., 2006, 128, 9640–9641 CrossRef CAS PubMed; (c) S. H. Lee, S. K. Kim, J. H. Bok, S. H. Lee, J. Yoon, K. Lee and J. S. Kim, Tetrahedron Lett., 2005, 46, 8163–8167 CrossRef CAS; (d) W. R. Dichtel, J. M. Serin, C. Edder, J. M. J. Frechet, M. Matuszewski, L.-S. Tan, T. Y. Ohulchanskyy and P. N. Prasad, J. Am. Chem. Soc., 2004, 126, 5380–5381 CrossRef CAS PubMed; (e) M. Suresh, S. Mishra, S. K. Mishra, E. Suresh, A. K. Mandal, A. Shrivastav and A. Das, Org. Lett., 2009, 11, 2740–2743 CrossRef CAS PubMed; (f) P. Mahato, S. Saha, E. Suresh, R. D. Liddo, P. P. Parnigotto, M. T. Conconi, M. K. Kesharwani, B. Ganguly and A. Das, Inorg. Chem., 2012, 51, 1769–1777 CrossRef CAS PubMed; (g) K. Sreenath, J. Allen, R. M. W. Davidson and L. Zhu, Chem. Commun., 2011, 47, 11730–11732 RSC; (h) R. J. Wandell, A. H. Younes and L. Zhu, New J. Chem., 2010, 34, 2176–2182 RSC; (i) S. Lohar, A. Banerjee, A. Sahana, A. Banik, S. K. Mukhopadhyay and D. Das, Anal. Methods, 2013, 5, 442–445 RSC.
  24. (a) A. Natansohn and P. Rochon, Chem. Rev., 2002, 102, 4139–4176 CrossRef CAS PubMed; (b) A. S. Matharu, S. Jeeva and P. S. Ramanujam, Chem. Soc. Rev., 2007, 36, 1868–1880 RSC; (c) T. Ikeda, J. Mater. Chem., 2003, 13, 2037–2057 RSC; (d) S. Kubo, Z. Z. Gu, K. Takahashi, A. Fujishima, H. Segawa and O. Sato, J. Am. Chem. Soc., 2004, 126, 8314–8319 CrossRef CAS PubMed; (e) C. Dohno, S. N. Uno and K. Nakatani, J. Am. Chem. Soc., 2007, 129, 11898–11899 CrossRef CAS PubMed; (f) N. Kano, F. Komatsu, M. Yamamura and T. Kawashima, J. Am. Chem. Soc., 2006, 128, 7097–7106 CrossRef CAS PubMed; (g) A. Archut, F. Vogtle, L. De Cola, G. C. Azzellini, V. Balzani, P. S. Ramanujam and R. H. Berg, Chem.–Eur. J., 1998, 4, 699–706 CrossRef CAS.
  25. A. Abbotto, L. Beverina, N. Manfredi, G. A. Pagani, G. Archetti, H. G. Kuball, C. Wittenburg, J. Heck and J. Holtmann, Chem.–Eur. J., 2009, 15, 6175–6185 CrossRef CAS PubMed.
  26. S. Kawata and Y. Kawata, Chem. Rev., 2000, 100, 1777–1788 CrossRef CAS PubMed.
  27. Y. T. Li, C. L. Chen, Y. Y. Hsu, H. C. Hsu, Y. Chi, B. S. Chen, W. H. Liu, C. H. Lai, T. Y. Lin and P. T. Chou, Tetrahedron, 2010, 66, 4223–4229 CrossRef CAS.
  28. X. C. Chen, T. Tao, Y. Ge Wang, Y. X. Peng, W. Huang and H. F. Qian, Dalton Trans., 2012, 41, 11107–11115 RSC.
  29. (a) M. E. Moustafa, M. S. McCready and R. J. Puddephatt, Organometallics, 2012, 31, 6262–6269 CrossRef CAS; (b) M. E. Moustafa, M. S. McCready and R. J. Puddephatt, Organometallics, 2013, 32, 2552–2557 CrossRef CAS; (c) G. C. Hampson and M. Robertson, J. Chem. Soc., 1941, 409–413 RSC.
  30. A. Kumar, A. Kumar and D. S. Pandey, Dalton Trans., 2016, 45, 8475–8484 RSC.
  31. G. M. Sheldrick, SAINT, Version 6.02, SADABS, Version 2.03, Bruker AXS Inc., Madison, Wisconsin, 2002 Search PubMed.
  32. G. M. Sheldrick, SADABS: Software for Empirical Absorption Correction, University of Gottingen, Institute fur Anorganische Chemieder Universitat, Gottingen, Germany, 1999-2003 Search PubMed.
  33. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008, 64, 112–122 CrossRef CAS PubMed.
  34. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, GAUSSIAN09, Revision D.01, Gaussian Inc., Wallingford, CT, 2009 Search PubMed.
  35. A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652 CrossRef CAS.
  36. C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter Mater. Phys., 1988, 37, 785–789 CrossRef CAS.
  37. P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 270–283 CrossRef CAS.
  38. W. R. Wadt and P. J. Hay, J. Chem. Phys., 1985, 82, 284–298 CrossRef CAS.
  39. P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 299–310 CrossRef CAS.
  40. G. A. Petersson, A. Bennett, T. G. Tensfeldt, M. A. Al-Laham, W. A. Shirley and J. Mantzaris, J. Chem. Phys., 1988, 89, 2193–2218 CrossRef CAS.
  41. G. A. Petersson and M. A. Al-Laham, J. Chem. Phys., 1991, 94, 6081–6090 CrossRef CAS.
  42. R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 1996, 256, 454–464 CrossRef CAS.
  43. R. E. Stratmann, G. E. Scuseria and M. J. Frisch, J. Chem. Phys., 1998, 109, 8218–8224 CrossRef CAS.
  44. M. E. Casida, C. Jamorski, K. C. Casida and D. R. Salahub, J. Chem. Phys., 1998, 108, 4439–4449 CrossRef CAS.
  45. V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102, 1995–2001 CrossRef CAS.
  46. M. Cossi and V. Barone, J. Chem. Phys., 2001, 115, 4708–4717 CrossRef CAS.
  47. M. Cossi, N. Rega, G. Scalmani and V. Barone, J. Comput. Chem., 2003, 24, 669–681 CrossRef CAS PubMed.
  48. N. M. O'Boyle, A. L. Tenderholt and K. M. Langner, J. Comput. Chem., 2008, 29, 839–845 CrossRef PubMed.
  49. M. T. Wheeler and F. Walmsley, Thermochim. Acta, 1986, 108, 325–336 CrossRef CAS.
  50. G. Ozkan, M. Kose, H. Zengin, V. McKee and M. Kurtoglu, Spectrochim. Acta, Part A, 2015, 150, 966–972 CrossRef CAS PubMed.
  51. A. B. Pradhan, S. K. Mandal, S. Banerjee, A. Mukherjee, S. Das, A. R. K. Bukhsh and A. Saha, Polyhedron, 2015, 94, 75–82 CrossRef CAS.

Footnote

Electronic supplementary information (ESI) available. CCDC 1478332 and 1478333 contain the supplementary crystallographic data for azoaldehyde and H2L. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra21217d

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