Highly Selective and Sensitive Dual Channel New Schiff base Chemosensor based on 5-bromo-2-hydroxybenzaldehyde and its Co(II), Ni(II), Cu(II) and Zn(II) Complexes - Synthesis, Spectral and Theoretical characterization and Pharmacological applications

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In recent times many research activities have been carried out to prove the metal ion detection ability of Schiff bases. [13,14] In the present study, a new Schiff base has been synthesized from 5-bromo-2- hydroxybenzaldehyde and naphthalene-1,8-diamine and it is complexated with Co(II), Ni(II), Cu(II) and Zn(II) ions. The biological application of all the compounds has been analyzed and metal ions recognition property of the Schiff base has also been studied.

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Nội dung Text: Highly Selective and Sensitive Dual Channel New Schiff base Chemosensor based on 5-bromo-2-hydroxybenzaldehyde and its Co(II), Ni(II), Cu(II) and Zn(II) Complexes - Synthesis, Spectral and Theoretical characterization and Pharmacological applications

Cite this paper: Vietnam J. Chem., 2023, 61(4), 429-444 Research article DOI: 10.1002/vjch.202200150 Highly Selective and Sensitive Dual Channel New Schiff base Chemosensor based on 5-bromo-2-hydroxybenzaldehyde and its Co(II), Ni(II), Cu(II) and Zn(II) Complexes - Synthesis, Spectral and Theoretical characterization and Pharmacological applications Sambamoorthy Santhi1*, Rajalingam Renganathan2, Subbiah Amala1, Gobalakrishnan Suganya1, Karikalan Abinaya1 1 PG and Research Department of Chemistry, Seethalakshmi Ramaswami College Tiruchirappalli 620 002, Tamil Nadu, India 2 School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, India Submitted August 14, 2022; Revised November 7, 2022; Accepted May 3, 2023 Abstract A new Schiff base N,N’-Bis(5-Bromosalicylidine)-1,8-diaminonaphthalene (BBSDN) and its Co(II),Ni(II),Cu(II) and Zn(II) complexes were synthesized. All the compounds were characterized by various spectral studies (IR, UV, 1H NMR, 13C NMR and EI-Mass) and DFT calculations. Conductance measurements, magnetic study and metal estimation studies were performed for the complexes to establish the structure. Thermal studies were done to account for the coordination of water molecules. Redox property of the synthesized complexes was established by Cyclic Voltammetry studies. Synthesized compounds were screened against fungal and bacterial pathogens and in most of the cases the complexes were found to be more active than the Schiff base. MIC value for copper and zinc complexes against the fungal pathogen Candida albicans are determined as 25 ppm and 10 ppm respectively. Metal ion pertinency of the Schiff base was investigated by absorption and fluorescence methods and was found to be highly selective and sensitive for the detection of Cu(II) ion. Mode of interaction has been proved through 1H NMR titrations and the interaction is explained as due to CHEQ mechanism. Stability constant of the complex formed during the detection process was determined by Benesi-Hildebrand plot. Detection limit was computed as 1.46410-6M. The chemosensor property of the Schiff base is applied in bioimaging studies on HeLa cell lines. Keywords. Schiff base, chemosensor, bioimaging, coordination chemistry, transition metal complexes. 1. INTRODUCTION with selectivity and sensitivity is an active field of supramolecular chemistry for biological as well as Coordination chemistry may be considered as an analytical and environmental problems.[10,11] In the interdisciplinary arena involving inorganic present scenario, researchers do exhibit profound chemistry, organic chemistry and structural penchant in the construction of fluorescent probes for chemistry, overcoming boundaries in between them. the detection of metal ions as well as anions.[12] In Schiff bases, condensation products of amines and recent times many research activities have been reactive aldehydes, are important compounds with carried out to prove the metal ion detection ability of various potential applications.[1] Schiff base Schiff bases.[13,14] In the present study, a new Schiff compounds are one of the most extensively used base has been synthesized from 5-bromo-2- ligands in the field of coordination chemistry hydroxybenzaldehyde and naphthalene-1,8-diamine because of structural flexibility and their application and it is complexated with Co(II), Ni(II), Cu(II) and in various fields.[2,3] A large number of Schiff bases Zn(II) ions. The biological application of all the and their complexes have been studied for their compounds has been analyzed and metal ions important biological properties like antidiabetic,[4] recognition property of the Schiff base has also been antioxidant,[5] antimicrobial,[6] antifungal,[7] studied. [8] anticancer and complexing ability towards some 2. MATERIALS AND METHODS toxic metals.[9] The design and synthesis of fluorescent probes 2.1. Materials 429 Wiley Online Library © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Sambamoorthy Santhi et al. 5-bromo-2-hydroxybenzaldehyde (Sigma-Aldrich) Perkin Elmer spectrometer provided with quartz and naphthalene-1,8-diamine (Sigma-Aldrich) were cells using DMSO as solvent. The electron impact used as such. Cobalt, nickel, copper and zinc Mass spectra of all the synthesized compounds were chlorides are used for the synthesis of complexes. recorded in Q-T of Mass spectrometer, Department Commercially available rectified spirit was dried of Chemistry, IIT-Madras, Chennai. The ESR over calcium oxide, filtered and distilled before use spectra were recorded using JEOL model JES FA (boiling point 78ºC). Methanol used in this study is 200 ESR spectrometer in liquid nitrogen purchased from Merck Chemical Co. For metal ion temperature. The TGA/DTA analysis was done by detection study all the metal ions are taken in the Siint TG/DTA 6200. Electrochemical behaviour was form of their chloride salts and lead is used in its measured at room temperature in air tight three acetate form. electrode cell by using glassy carbon electrode as working electrode, a platinum wire served as the 2.2. Synthesis of Ligand N,N’-Bis(5- counter electrode and a Ag/AgCl in a saturated KCl Bromosalicylidine)-1,8-diaminonaphthalene solution as reference electrode. The absorption (BBSDN) studies of the Schiff base receptor were performed in the Shimadzu UV Spectrophotometer (UV-1800). A mixture of 5-Bromo salicylaldehyde (0.4 mmol) The fluorescence measurements were carried out in and 1,8 diamino-naphthalene (0.15 mmol) was a quartz cell, in a JASCO Spectrofluorometer (FP- ground well in a mortar for thirty minutes using a 8200). Density functional theory (DFT) calculations pestle and the chocolate colour paste obtained was were executed using the GAUSSIAN03 program kept aside for a day and washed with pet ether. A package using B3LYP level with 6-31G(d,p) as the light brown colour powder was obtained. The basis set. The optimized structure of the Schiff base compound was soluble in Ethanol, Methanol, was visualized by the help of the GAUSSVIEW 5.0 DMSO and DMF. The melting point is 162°C. molecular visualization program. 2.3. Synthesis of Metal Complexes 3. RESULTS AND DISCUSSION The Schiff base ligand BBSDN (1 mol) and the 3.1. IR spectra metal salt (1 mol) (cobaltous chloride, nickel chloride, cupric chloride, zinc chloride) were taken The band at 1597.06 cm-1 corresponding to >C=N in 70 ml of ethanol and refluxed for 4 hours. The stretching vibration confirms the formation of Schiff complexes formed were filtered, washed with base.[15] The sharp bands at 3348.48 cm-1 and ethanol and dried over anhydrous calcium chloride. 3288.63 cm-1 indicate the presence of phenolic –OH All the complexes are soluble in DMSO, methanol groups[16] and the bands at 1244.09 cm-1 and 1112.93 and DMF. cm-1 are due to C–O stretching vibrations of Metal estimation studies were done by using the phenolic groups.[17] The bands at 630.72 cm-1, standard methods namely pyrolytic method (for 717.73 cm-1 and 759.95 cm-1 evidence the presence cobalt), colorimetric method (for nickel & copper) of C-Br bond.[18] and volumetric method (for zinc). The electrical The C=N stretching frequency observed at conductivity of the complexes isolated was 1597.06 cm-1 in the ligand is shifted to 1600.92- measured in 10-3 M solution of DMSO using an 1633.71 cm-1 in the complexes indicating the ELICO conductivity bridge and a dip type coordination of the azomethine nitrogen to the conductivity cell. The magnetic susceptibility of the metal.[19] The metal nitrogen coordination is further complexes at room temperature were measured by supported by the emergence of a new band ranging Gouy method. Infrared spectral measurements were from 455.20 cm-1 to 530.42 cm-1 in the complexes made for the free ligand and their complexes in KBr corresponding to M-N bonds.[20] The disappearance medium using FT-IR spectrophotometer (model: of the sharp bands at 3348.42 cm-1 and 3288.63 cm-1 Shimadzu IR Affinity). 1H NMR and 13C NMR in the complexes authenticates the coordination of spectra were taken using Bruker 300 Avance II phenolic oxygen to the metal after deprotonation. (SASTRA University, Tanjore) and Bruker A VIII The coordination of phenolic oxygen is further 500 MHz NMR spectrometer (IIT Madras Chennai). evidenced by the appearance of a new band at The NMR spectra were recorded employing TMS as 536.21 cm-1, 628.79 cm-1, 624.94 cm-1 and 580.57 internal reference and DMSO as solvent at ambient cm-1 corresponding to metal oxygen bonds in the IR temperature. The UV spectra of the ligand and spectra of the Co(II), Ni(II), Cu(II) and Zn(II) complexes in UV visible region were taken using complexes respectively.[21] The shift in the band due © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 430
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Highly Selective and Sensitive Dual Channel… to C-O stretching vibration of the phenolic group to nm is attributed to 2B1g→2Eg, 2B1g→2B2g and lower frequency to an extent of 16 cm-1 in all the 2 B1g→2A1g transitions of Cu(II) in square planar synthesized complexes further substantiates the geometry.[32] The magnetic moment of 1.43 BM coordination of phenolic oxygen. further confirms the square planar geometry.[33] The electronic spectrum of the Zn(II) complex 3.2. 1H NMR spectrum shows two absorption bands around 338 nm (29585 cm-1) and 413 nm (24213 cm-1). The band at 413 nm The multiplet which extends from 5.62 ppm to 7.68 is attributed to the charge transfer transition, which ppm is assigned to the protons of the aromatic corresponds to the complex possessing tetrahedral rings.[22] The signal at 7.72 ppm due to two protons structure.[34] The Zn(II) complex is diamagnetic.[35] corresponds to the protons on the two azomethine linkages of the Schiff base. The phenolic protons 3.5. Mass spectral studies appear at 10.13 ppm and 11.00 ppm respectively.[23] 1 H NMR spectrum of BBSDN is shown in figure S1. The mass spectra of the Schiff base and all the synthesized complexes are found to be well 3.3. 13C NMR spectrum conforming to the molecular weight corresponding to the proposed structures. The highest intense peak All the possible carbon peaks are observed in the 13C observed at 525.9054 in the mass spectrum of the NMR spectrum. The azomethine carbon appears at Schiff base is due to the M+1 peak of the Schiff base 161.0 ppm.[24] The phenolic carbons appear at 190.1 (Calculated mass = 524.2026). ppm.[25] The chemical shift of aromatic carbons For cobalt complex, the peak at 616.1618 appears in the range from 105.0 ppm to 160.3 corresponding to M-1 peak confirms the proposed ppm.[26] The signal due to C-Br carbon atoms structure of six coordination with two coordinated appears at 60.45 ppm.[27] 13C NMR spectrum of water molecules. The nickel complex exhibits BBSDN is shown in figure S2. molecular ion peak[36] at 581.0106 confirming square planar geometry. The molecular ion peak at 3.4. UV spectra 585.8295 observed in the mass spectrum of copper complex authenticates the formation of the tetra The electronic spectra of the ligand and its coordinated copper complex. The presence of complexes are given in figures S3-S7. Two bands molecular ion peak at 587.2354 substantiates the are observed in the electronic spectrum of the Schiff formation of tetra coordinated of Zn(II) complex. base. The band at longer wavelength 327 nm (30581 The mass spectra of the ligand BBSDN and its cm-1) is due to the n-π* transitions occurring in the complexes are given in figures S8-S12. imine group. The shorter wavelength band at 271 nm (36900 cm-1) is assigned to the π-π* transitions of 3.6. EPR spectrum the aromatic rings. The electronic spectrum of the Co(II) complex In liquid nitrogen temperature, the [Cu(BBSDN)] exhibits spin allowed transition at 603 nm (16583 shows well defined peaks. The g tensor values of cm-1) assignable to 4T1g(F)→4T2g, 4T1g(F) →4A2g(F) Cu(II) complex can be used to derive the ground and 4T1g(F)→4A2g(P) transitions[28] and the magnetic state. In square planar complexes, the unpaired moment of 4.52 BM is in conformity with octahedral electron lies in the dx2 – y2 orbital giving 2B1g as the arrangement around Co(II) ion.[29] The other two ground state with g|| > g⊥. For [Cu(BBSDN)], g|| > g⊥ bands observed at 336 nm (29762 cm-1) and 343 nm (2.31 > 2.07) which indicates that the unpaired (29155 cm-1) are assigned to intra ligand charge electron is predominantly present in dx2 – y2 orbital[37] transfer transitions. rather than dz2. The screening effect by the dz2 The Ni(II) complex shows two bands. The first electrons leads to elongation (Jahn teller distortion) band at 327 nm (30581 cm-1) is due to intra ligand and finally detachment of two orbitals from the charge transfer transition and another band at 401 metal ion, resulting in square planar geometry. The nm (24919 cm-1) is assigned to the transition EPR spectrum of the complex is shown in figure 1. 1 A1g→1A2g[30] corresponding to the square planar geometry around Ni(II) and it is diamagnetic.[31] 3.7. TGA/DTA analysis The Cu(II) complex exhibits two bands, one at 353 nm (26313 cm-1) and another at 679 nm (14716 In thermo gravimetric analysis of cm-1). The first band is suggested for intra ligand [Co(BBSDN)(H2O)2], the first weight loss (observed charge transfer transitions. The second band at 679 © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 431
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Sambamoorthy Santhi et al. 5.65%, calculated 5.84%) noticed in the temperature complexes suggesting the absence of coordinated range of 90-203oC is due to the elimination of two water molecules. The weight loss, observed in the coordinated water molecules. The other weight loss thermal analysis of [Ni(BBDNS)], [Cu(BBDNS)] is due to the decomposition of the ligand moiety. and [Zn(BBDNS)] may be due to the decomposition The presence of two coordinated water molecules of the ligand moiety.[38] TGA/DTA curves are shown gave an octahedral geometry to the cobalt complex. in figures S13-S16. Such water loss is not observed in the case of other Figure 1: EPR Spectra of [Cu(BBSDN)] 4. METAL ESTIMATION STUDIES Amount of copper (10.45%) was determined by colorimetric method. The values obtained are in Metal estimation studies were done by using the agreement with the value calculated for the proposed standard methods.[39,40] structure of the complex. The amount of cobalt (9.89%) was estimated by Amounts of nickel (10.68) and zinc (10.12) were pyrolytic method which is in good agreement with ascertained by volumetric method. Here also the the calculated value for the proposed structure of the calculated values are well conforming to the complex. experimental values (table 1). Table 1: Physical and analytical data of the ligand BBSDN and its complexes [Co(BBSDN)(H2O)2], [Ni(BBSDN)], [Cu(BBSDN)], [Zn(BBSDN)] Melting/decomposition Molar % of metal eff BM No. Compound/Complex Color temperature conductance calculated calculated C -1cm2mol-1 (observed) (observed) 1 BBSDN Purple 162 -- -- -- 2 [Co(BBSDN)(H2O)2] Brown >250 50 9.54 (9.89) 3.87 (4.52) 3 pale 10.15 [Ni(BBSDN)] >250 44 - Brown (10.68) 4 [Cu(BBSDN)] Blackish 10.85 >250 30 1.73 (1.43) Green (10.45) 5 [Zn(BBSDN)] Brownish 10.79 >250 10 - orange (10.12) 5. CONDUCTANCE MEASUREMENTS 50 -1cm2mol-1 indicating their non-electrolytic nature (table 1). The molar conductance of all the four complexes in The tentative structure of the complexes is DMSO at room temperature was found to be below shown in figures 2 and 3. © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 432
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Highly Selective and Sensitive Dual Channel… two reduction peaks (Ered = -0.9009 V and -0.0539 V). This evinced the occurrence of one quasi reversible process and another irreversible process [41] . The cyclic voltammograms are shown in figures 4, 5, 6 and 7. Figure 2: [Co(BBSDN)(H2O)2] Figure 4: Cyclic voltammogram [Co(BBSDN)(H2O)2] Figure 3: [M(BBSDN)] M = Ni(II) / Cu(II) / Zn(II) 6. CYCLIC VOLTAMMOGRAMS In the cyclic voltammogram of [Co(BBSDN)] complex an irreversible redox process is observed which may be due to short lived oxidized (or) reduced state of metal ion as the cyclic voltammogram has two reduction peaks[41] (Ered = -0.9254V and -0.1143V). The cyclic voltammogram of [Ni(BBSDN)] exhibits two well defined quasi-reversible peaks. First reduction peak is observed at Ep value of - Figure 5: Cyclic voltammogram of [Ni(BBSDN)] 0.9655 V which is associated with an oxidation peak placed at Epa = -0.999 V. The second reduction peak appeared at Epc = -0.3390 V and is associated with an oxidation peak at Epa = 0.8918 V. The value of ΔEp is 0.0335 V and 0.5528 V for the first and second redox couples respectively and the ratio between oxidation and reduction peak current is indicative of simple quasi-reversible one electron redox processes.[42] The cyclic voltammogram of [Cu(BBSDN)] complex exhibits one oxidation peak (Eox = -0.1796 V) and the corresponding reduction peak is (Ered = -0.0256 V). This couple is quasi-reversible and the ratio between oxidation and reduction current Figure 6: Cyclic voltammogram of [Cu(BBSDN)] suggests the process to be simple one-electron 7. ANTIMICROBIAL STUDIES OF LIGAND transfer.[43] BBSDN AND ITS COMPLEXES The cyclic voltammogram of [Zn(BBSDN)] comprises one oxidation peak (Eox = -0.5978 V) and BBSDN and its complexes were dissolved in DMSO © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 433
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Sambamoorthy Santhi et al. and tested on bacterial pathogens such as complexes show maximum sensitivity against the Staphylococcus aureus, Klebsiella aerogenes and fungal pathogen Candida albicans. The fungal pathogens such as Candida albicans, antimicrobial activity images are shown in figure 8. Aspergillus niger. The biological activity of Schiff base ligand may be due to the presence of -OH group which may play an important role in the antibacterial activity, as well as the presence of imine group which imports in illuminating the mechanism of transformation reaction in biological systems.[44,45] The Schiff base and its complexes studied may be considered as sensitive towards all pathogens as they are having zone of inhibition greater than 12 mm. The diameter of zone of inhibition shown by all the complexes and ligand towards these organisms is listed in table 2. Photo copies reflect the sensitivity of compounds against pathogenic organisms. The complexes were found to be more sensitive than ligand towards almost all pathogens studied. Both the copper and zinc Figure 7: Cyclic voltammogram of [Zn(BBSDN)] Table 2: Diameters of zone of inhibition shown by BBSDN, [Co(BBSDN)(H2O)2], [Ni(BBSDN)], [Cu(BBSDN)] and [Zn(BBSDN)] towards various microorganism of concentration 100 g/ml S. Name of the Zone of inhibition (mm) No. Micro organism BBSDN [Co(BBSDN)(H2O)2] [Ni(BBSDN) [Cu(BBSDN)] [Zn(BBSDN) Standard 1 Bacterial Staphylococcus 16 20 18 15 18 35 aureus (NCIM 2079) 2 Bacillus subtilis 18 18 16 20 18 30 (NCIM 2063) 3 Fungal Candida albicans 18 17 20 21 24 32 (NCIM 3102) 4 Aspergillus niger 15 20 16 15 14 35 (NCIM 105) (a) (b) (c) (d) Figure 8: (a) Activity against S. aureus; (b) Activity against K. aerogenes; (c) Activity against C. albicans; (d) Activity against A. niger Minimum Inhibitory Concentration Studies. concentration required for their activity was Since copper and zinc complexes are found to have determined. Copper complex is found to be considerable activity against the fungal pathogen moderately active in 10 ppm and more active in 25 Candida albicans, minimum inhibitory ppm, while zinc complex is found to be active even © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 434
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Highly Selective and Sensitive Dual Channel… in 10 ppm concentration. The observed MIC values be fine-tuned and developed into good antifungal prove the potency of the complexes to act as a potent agents (figure 9). antifungal agent. Hence these two complexes may (a) (b) Figure 9: (a) MIC of Cu BBSDN; (b) MIC of Zn BBSDN 8. STUDY OF MATAL ION SENSING performing UV-Visible titration which involved PROPERTY sequential addition of CuCl2 to a solution of Schiff base from 0.2 equivalent to 2.0 equivalents (figure The metal ion sensing competency of the 10(b)). During these additions, the new peak synthesized Schiff base was investigated by observed around 400 nm gradually increased in its absorption and emission methods. All the studies absorbance and reached the maximum at 1.4 were performed in aqueous methanol medium. The equivalent addition. The interaction evidences the concentration of Schiff base was maintained at 5×10- formation of complex between the Schiff base and 5 M and that of metal ions at 3×10-3M. The Schiff Cu(II) ion and is further ratified by the presence of base chemosensor in methanol medium was made to isosbestic points at 312 nm and 351 nm interact with two equivalent of various metal ions The interaction leading to complex formation such as Na(I), Mg(II), Ca(II), Mn(II), Fe(III), Co(II), between the Schiff base and Cu(II) ion may involve Ni(II), Cu(II), Zn(II), Sr(II), Cd(II), Ba(II), Hg(II), two azomethine nitrogen atoms and two phenolic Pb(II) and Al(III) in aqueous medium. oxygen atoms in a deprotonated fashion. The formation constant of the formed complex is Absorption studies. The UV-Visible spectrum of calculated as 3.783103 by following Benesi– the Schiff base exhibited three predominant bands at Hildebrand plot method.[46] 233 nm, 332 nm and 342.5 nm. When the Schiff base receptor was made to interact with two Emission Studies. The metal ion detecting ability of equivalents of various metal ions, successful the synthesized Schiff base was also explored by interaction was observed only with copper(II) ion. Fluorescence spectroscopic studies. The This was ensured by the appearance of a new band fluorescence spectrum of the Schiff base receptor at 439 nm. A blue shift was also caused to the showed emission at 387.5 nm with an intensity of receptor band at 322 nm to an extent of 41 nm which 9867 a.u. when the excitation wavelength was kept was also accompanied by hyperchromic effect at 233 nm. In the presence of Cu(II) the emission is authenticating the interaction (figure 10(a)). The quenched considerably along with a shift in emission band emerged at 439 nm corresponds to d - d wavelength by 12 nm. Such a great amount of transitions which confirms the formation of quenching was not caused by any other metal ions coordination complex between the Schiff base and (figure 11). Quenching of emission may be caused the metal ion during the interaction. During this by intersystem crossing, electron exchange, and coordination the lone pair of electrons present on photoinduced electron transfer. The present azomethine nitrogen is transferred to the metal ion quenching may be due to the complex formation and so is not available for conjugation with the between Cu(II) ion and the Schiff base fluorophore aromatic ring. This process is corroborated by the caused by static PET process.[47-50] Quenching may blue shift that has occurred in the band at 322 nm. also be explained as due to intersystem crossing The interaction was further established by occurring in the Schiff base in the presence of © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 435
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Sambamoorthy Santhi et al. paramagnetic Cu(II) ion. Here the Schiff base non-fluorescent, which on excitation returns to the fluorophore acts as the electron donor. The metal ion ground state without emission of photons which is Cu(II) with vacant d-orbitals acts as the electron called as quenching. Since some amount of acceptor. During the interaction there is fluorophore is also present in the mixture, intensity transfer(donation) of electron from the donor atoms of emission is only quenched by complex formation of the Schiff base to Cu(II) ion resulting in the and does not become zero. formation of coordination complex. This complex is (a) (b) Figure 10: (a) UV-Vis spectra of receptor BBSDN with different metals; (b) UV-Vis titration of receptor BBSDN with Cu(II); Inset: Variation of absorbance at 402 nm in the presence of Cu(II) (a) (b) Figure 11: (a) Fluorescence spectra of receptor BBSDN with different metals; (b) Fluorescence titration of receptor BBSDN with Cu(II); Inset: Variation of emission at 402 nm in the presence of Cu(II) Selectivity. The selective detection of Cu(II) by the of intersystem crossing from S1 state to T1 state is receptor was established by recording the enhanced by the presence of paramagnetic natured fluorescence spectrum of receptor-Cu(II) (2 Cu2+ ion (d9 ion). As a result, the excited state equivalents) in the presence of 2 equivalents of each undergoes non-radiative decay leading to quenching of the other metal ions. The absence of interference of fluorescence through chelation enhanced by other metal ions clearly proves the selectivity of quenching mechanism (CHEQ)[52]. The chelate the receptor for Cu(II) ion[51] (figure 12). formation between the Schiff base and the metal ion was ascertained to occur through -OH group of two Mode of Interaction. In the fluorophore the process phenyl rings present in the Schiff base in a © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 436
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Highly Selective and Sensitive Dual Channel… deprotonated fashion and two imine nitrogens and is the presence of Cu(II), the band due to the >C=N proved by performing IR titrations. bond of the Schiff base appears at 1656.26 cm-1. In the free Schiff base, the above band appears at 1657.80 cm-1. This shift in the band proves the coordination between the azomethine nitrogen and the Cu(II) ion. The nitrogen coordination is further substantiated by the new band at 404.15 cm-1 of M-N bond. Another coordination site is phenolic oxygen in the ortho position of the azomethine group. This is evidenced by the new band present at 599.73 cm-1 corresponding to the M-O bond. This phenolic oxygen coordination is also confirmed by the shift to an extent of 3 cm-1 in the band due to -OH group. From the above discussion it is clear that there is formation of coordination complex between the Cu(II) ion and the Schiff base through the Figure 12: Fluorescence intensity of the receptor nitrogen and oxygen atoms[53] (figure 13). Therefore, BBSDN with 2 equiv. of Cu(II) the feeble fluorescence of sensor 1-Cu2+ complex and 2 equiv. of other metal ions stated could also be explained by the PET principle, that is the electron transfer occurred from N,N’-Bis(5- IR titrations. IR titrations were performed in order Bromosalicylidine)-1,8-diaminonaphthalene to establish the mode of interaction between the (BBSDN) fluorophore to the bound Cu2+ center Schiff base receptor (figure 9) and the Cu(II) ion. In causing fluorescence quenching. (a) 100 1601.71cm-1 898.96cm-1 2590.07cm-1 90 2322.23cm-1 2094.00cm-1 3923.20cm-1 1992.62cm-1 80 600.02cm-1 2222.90cm-1 70 1657.80cm-1 669.37cm-1 60 1311.60cm-1 2911.86cm-1 2995.60cm-1 700.56cm-1 50 %T 1407.21cm-1 1436.37cm-1 40 3435.98cm-1 954.46cm-1 30 20 1046.95cm-1 10 0 4000 3500 3000 2500 2000 1500 1000 500 400 cm-1 Name Description BRSADAN1- (b) 100 599.73 3923.08cm-1 90 2344.16cm-1 2093.90cm-1 1993.29cm-1 404.15 80 898.70cm-1 1656.26cm-1 668.80cm-1 70 1311.50cm-1 2995.98cm-1 2912.17cm-1 700.21cm-1 60 1407.15cm-1 1436.54cm-1 50 %T 954.23cm-1 3438.69cm-1 40 30 20 1 0 4 7 .3 5 c m - 1 10 0 4000 3500 3000 2500 2000 1500 1000 500 400 cm-1 Name Description BRSANDAN2- Figure 13: (a) IR spectrum of receptor BBSDN; (b) IR spectrum of receptor BBSDN with Cu2+ © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 437
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Sambamoorthy Santhi et al. ≈ 8000 a.u Chelate Cu2+ ≈ 10000 a.u Quenching due to Stoichiometry and Detection Limit CHEQ effect[52] The stoichiometry of the complex formed during the detection process was evaluated as 1: 1 by Job’s plot method [54] (Figure 14). The detection limit was manipulated as 1.464 X 10-6M [55]. Schiff base sensor Figure 14: Job’s plot of receptor BBSDN with Cu(II) Fluorescence activity towards HeLa cell lines. CuCl2.[47] The fluorescence image of Schiff base 5105 cells/ml of HeLa cells were seeded into the with Cu2+ ion evidences its penetration into the cover slip containing 6 well tissue culture plates and living cell and its readiness to get bound with the incubated at 37oC at 5% CO2 incubator for 24 hours. intracellular Cu2+ ion. The quenching mainly occurs After incubation, 50 μg/ml of BBSDN was added through non emissive-exciplex with electron transfer and reincubated for 30 minutes at 37oC at 5% CO2 from the exited BBSDN to metal ions (figure 15). incubator. Finally, 50 μg/ml of CuCl2 was directly added to the cover slip culture and evaluated DFT Analysis of N,N’-Bis(5-Bromosalicylidine)- immediately by fluorescent microscope using a 1,8-diaminonaphthalene (BBSDN). In general, fluorescent filter (Ex. 233 nm and Em. 387.5 nm). DFT analysis is done to support the structure The HeLa cells were treated with BBSDN ligand assigned to a compound. The structure of the Schiff followed by stimulation with CuCl2. After base BBSDN is optimized by using DFT stimulation no strong fluorescence emission was analysis.[56,57] DFT analysis is carried out with observed on the HeLa cells, indicating that the respect to Mulliken Charge Analysis, HOMO - fluorescence is well quenched after the addition of LUMO energy gap and MEP (figure 16). © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 438
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Highly Selective and Sensitive Dual Channel… Figure 15: Cell images of HeLa cells treated with the mixture of BBSDN and CuCl2 (a) Bright field; (b) Fluorescent image (blue filter); (c) Fluorescent image (green filter); (d) Fluorescent image (red filter) ascertained. The electron rich center is represented by red colored region.[58] In the Schiff base BBSDN the phenolic groups and azomethine groups are surrounded by red region indicating their availability for electrophilic attack. Electron delocalization is evidenced by MEP, ESP and total density maps (figure 19). The contour diagram shows the movement of electrons in the Schiff base. The contour of the Schiff base is mapped with various parameters such as electrostatic potential, total density, HOMO and LUMO (figure 20). Figure 16: Optimized structure of BBSDN Mulliken Charge Analysis. The Mulliken analysis helps for predicting the atomic charge distribution of various atoms present in a molecule. Mulliken Charges of the Schiff base BBSDN lie between - 0.559 & 0.559. The high negative charges are mainly located on nitrogen of azomethine groups and oxygen of phenolic groups. This observation authenticates the nitrogen and oxygen atoms as the coordination sites of the Schiff base. Mulliken charge transfer is represented by different colors (figure 17). Mulliken charge analysis proves that the electron rich centers are azomethine nitrogen and phenolic oxygen atoms. This provides evidence for the coordination of the above mentioned atoms to the metal. Frontier Molecular Orbitals. The energy gap Figure 17: Mulliken charge transfer of BBSDN between HOMO and LUMO is computed as 0.1238 a. u (3.3688 eV), which indicates the occurrence of MEP study also proves that the electron rich charge transfer transition. The smaller energy gap centers in the Schiff base molecule are azomethine proved the softness and reactivity of the Schiff base nitrogen and phenolic oxygens. (figure 18). 8. CONCLUSION Molecular Electrostatic Potential. From the MEP the electron rich center of the molecule can be A new Schiff base namely N,N’-Bis(5- © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 439
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Sambamoorthy Santhi et al. Calculations. The geometry of the complexes has been established from magnetic and thermal studies. The redox property of all the complexes and antibacterial and antifungal properties of both the Schiff base and the complexes have been explored. The complexes are found to be more active towards most of the organisms studied. The minimum inhibitory concentration for the most active complexes namely Cu(II) and Zn(II) has been estimated. The chemosensor competency of the synthesized Schiff base has also been evaluated and the Schiff base is found to be detecting Cu(II) ion selectively among the various other 15 metal ions without any hindrance. The mode of interaction has been proved through 1H NMR titrations and the interaction is explained as due to static PET process and CHEQ mechanism. The limit of detection is found to be in micromolar concentration. The metal ion sensing ability has been applied in bioimaging studies for HeLa cell lines. Acknowledgement. The authors are thankful to the Director, SAIF, IIT Madras, Chennai, St. Joseph’s College, Tiruchirappalli, SASTRA University, Thanjavur, Periyar College of Pharmaceutical Sciences, Tiruchirappalli and Trichy Research Figure 18: HOMO–LUMO energy gap of BBSDN Institute of Biotechnology Private Limited, Thillai Nagar, Tiuchirappalli for providing analytical Bromosalicylidine)-1,8-diaminonaphthalene support. The authors wish to express thanks to the (BBSDN) and its Co(II), Ni(II), Cu(II) and Zn(II) Managing Trustee, the Secretary, the Principal and complexes were synthesized and characterized by faculty members of Department of Chemistry, various spectral and analytical studies. The structure Seethalakshmi Ramaswami College, Tiruchirappalli, of the Schiff base and its highly electronegative Tamil Nadu, India for providing laboratory facilities coordinating areas are further proved by DFT and support. MEP ESP TOTAL DENSITY Figure 19: Molecular surfaces of BBSDN © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 440
25728288, 2023, 4, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/vjch.202200150 by Readcube (Labtiva Inc.), Wiley Online Library on [01/05/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Vietnam Journal of Chemistry Highly Selective and Sensitive Dual Channel… CONTOUR (TOTAL DENSITY) CONTOUR (ESP) CONTOUR (LUMO) CONTOUR (HOMO) Figure 20: Contour maps of BBSDN REFERENCES Venkatesh, S. Kaya, G. Serdaroğlu. Experimental and theoretical analysis of molecular structure, 1. H. Kargar, R. Behjatmanesh-Ardakani, V. Torabi, M. vibrational spectra and biological properties of the Kashani, Z. Chavoshpour-Natanzi, Z. Kazemi, V. new Co(II), Ni(II) and Cu(II) Schiff base metal Mirkhani, A. Sahraei, M. N. Tahir, M. Ashfaq, K. S. complexes, Journal of Molecular Structure, Munawar. Synthesis, characterization, crystal 2021, 1233, 130097. structures, DFT, TD-DFT, molecular docking and 5. K. Buldurun, N. Turan, E. Bursal, A. Aras, A. DNA binding studies of novel copper(II) and zinc (II) Mantarcı, N. Çolak, F. Türkan, I. Gülçin. Synthesis, complexes bearing halogenated bidentate N,O-donor characterization, powder X-ray diffraction analysis, Schiff base ligands, Polyhedron, 2021, 195, 114988. thermal stability, antioxidant properties and enzyme 2. G. A. Al‐Hazmi, K. S. Abou‐Melha, N. M. inhibitions of M(II)-Schiff base ligand El‐Metwaly, I. Althagafi, F. Shaaban, R. Zaky. Green complexes, Journal of Biomolecular Structure and synthesis approach for Fe(III), Cu(II), Zn(II) and Dynamics, 2021, 39(17), 6480-6487. Ni(II)‐Schiff base complexes, spectral, 6. R. Reshma, R. Selwin Joseyphus, D. Arish, R. J. conformational, MOE‐docking and biological Reshmi Jaya, J. Johnson. Tridentate imidazole-based studies, Applied Organometallic Chemistry, 2020, Schiff base metal complexes: Molecular docking, 34(3), e5403. structural and biological studies, Journal of 3. H. Kargar, A. A. Ardakani, M. N. Tahir, M. Ashfaq, Biomolecular Structure and Dynamics, 2022, 40(18), K. S. Munawar. Synthesis, spectral characterization, 8602-8614. crystal structure and antibacterial activity of 7. J. Saranya, S. Jone Kirubavathy, S. Chitra, A. nickel(II), copper(II) and zinc(II) complexes Zarrouk, K. Kalpana, K. Lavanya, B. Ravikiran. containing ONNO donor Schiff base ligands, Journal Tetradentate Schiff Base complexes of transition of Molecular Structure, 2021, 1233, 130112. metals for antimicrobial activity, Arabian Journal for 4. R. V. Sakthivel, P. Sankudevan, P. Vennila, G. Science and Engineering, 2020, 45, 4683-4695. © 2023 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 441
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