Applicability of Reddish Orange Light Emitting Samarium (III) Complexes For Biomedical and Multifunctional Optoelectronic Devices

Six crimson samarium (III) complexes based on β-ketone carboxylic acid and ancillary ligands were synthesized by adopting grinding technique. All synthesized complexes were investigated via employing elemental analysis, infrared, UV-Vis, NMR, TG/DTG and photoluminescence studies. Optical properties of these photostimulated samarium (III) complexes exhibit reddish-orange luminescence due to 4 G 5/2 → 6 H 7/2 transition at 606 nm of samarium (III) ions. Further, energy band gap, color purity, CIE color coordinates, CCT and quantum yield of all complexes were determined accurately. Replacement of water molecules by ancillary ligands enriched the complexes (S2-S6) with decay time, quantum yield, luminescence, energy band gap and biological properties than parent complex (S1). Interestingly, these ecient properties of complexes may nd their applications in optoelectronic and lighting systems. In addition to these the antioxidant and antimicrobial assays were also investigated to explore the application in biological assays. Investigation of IR, UV-visible and NMR ( 1 H & 13 C) spectra of ligand and complexes in details conrm the formation of complexes. Besides, photoluminescence spectral studies shows that under 356 nm excitation, all synthesized complex exhibits the characteristics peaks at 566 nm, 606nm and 651 nm which can be credited to ( 4 G 5/2 → 6 H 5/2 ), ( 4 G 5/2 → 6 H 7/2 ) and ( 4 G 5/2 → 6 H 9/2 ) electronic transitions of Sm 3+ respectively. The energy band gap and refractive index of all complexes and ligand are determined precisely, which enables the applicability of them in semiconductor devices. Investigation of luminescence decay time in detail provide the homogenous environment around samarium (III) ion with excellent intrinsic lifetime (0.7 2-2.42) of all synthesized complexes. CIE colors coordinates, quantum yield (22.22-74.38) and color purity (98.01-99.04) of all S1-S6 complexes were determined accurately. According to CCT, these complexes are warm in appearance. Energy transfer dynamics validates the successfull transfer of energy from ligand (triplet energy state) to samarium (III) ion (resonating energy states). Biological activities of these complexes display the potential use of these complexes in pharmaceutical eld.


Introduction
From last few decades, lanthanide materials gathered the special attention due to their versatile applications in various elds like lasers [1], OLEDs [2], display devices [3], LED bulbs [4] and biological systems [5]. Hence, synthesis of lanthanides complexes with prominent high quantum yield, large stock shifts and long luminescence decay time has become a hot research topic at present time. Lanthanide complexes possess luminescence due to 4f-4f transitions, which are Laporte forbidden according to spin parity rule [6]. Hence, direct excitation in lanthanides is not bene cial as it results in low molar absorptivity and weak luminescence [7,8]. In order to overcome this catastrophe, a light harvesting chromophore was incorporated in coordination sphere which absorb energy from external source and transfer to lanthanides via antenna effect [9]. Generally, β-hydroxy ketone, β-keteocarboxylic acid and aromatic carboxylic acid are vigilantly studied due to their signi cant ability to transfer the absorbed energy from external source to lanthanides [10,11]. However, β-keteocarboxylic acid is an excellent chromophore to generated excellent reddish orange emission for display devices and other applications. Further, specially Eu (III), Tb (III) and Sm (III) having unique optical properties such as narrow emission bands, long decay time, large stock shifts and high luminescence due to electronic transitions in emission spectra [12]. Out of these, samarium (III) complexes grab signi cant attention due to their ability to emit reddish orange (606 nm) emission utilized in high quality display devices.
Currently, six scarlet samarium (III) complexes have been prepared by utilizing 1-cyclopropyl-6-uro-4-oxo-7-piperazin-1-ylquinoline-3-carboxylic acid (L), bathophenanthroline (batho), 1,10-phenanthroline (phen), neocuproine (neo), 2,2'bipyridyl (bipy) and 5,6-dimethyl-1,10-phenanthroline (dmph) ligands via grinding technique. All synthesized complexes were investigated via utilizing UV-Vis, 13 C-NMR, 1 H-NMR and IR spectroscopy. The information regarding elemental composition and thermal stability can be achieved by using elemental and thermogravimetric analysis. The photoluminescent spectra and decay time are executed effectively for reporting the photoluminescent aspects of samarium (III) complexes. The emitting color of complexes is con rmed by colorimetric parameters (color purity and CIE) of complexes. The sensitization of samarium ion by ligand can be well illustrated by investigation of energy transfer process in detail. To assess the antioxidant and antimicrobial activities, complexes are screened for DPPH and tube dilution techniques respectively. Experimental

Reagents and instruments
All solvents and chemicals were of analytical reagent grade with a stated purity 99% acquired from commercial source named Sigma Aldrich and used as such without further puri cation. The photoluminescence spectra were executed on spectrophotometer (Hitachi F-7000) equipped with xenon lamp. The elemental analyses enquired on Perkin Elmer 2400 CHN elemental analyser. Thermal stability was checked up to 800 o C temperature on SDT Q 600 analyzer with 20 o C/min in nitrogen atmosphere. 1 H and 13 C NMR spectra were performed on Bruker Avance II spectrophotometer in DMSO at 400 MHz frequency. IR spectra were obtained by perkin Elmer 400 spectrophotometer using KBr pellet in 4000 cm −1 to 400 cm −1 . The decay time value was determined via the FL solutions software F-7000 by monitoring 4 G 5/2 → 6 H 7/2 emission line of Sm 3+ in solid state. The UV-Vis absorption spectra were executed on Shimadzu-2450 spectrophotometer. DPPH and tube dilution technique were used to investigate the antioxidant and antimicrobial activities respectively.

Preparation of complexes
To synthesize the complex S1, ligand (0.498 g) and samarium nitrate hexahydrate (0.222 g) were placed in mortar and grinded properly. In order to mix all contents appropriately few drops of water added and pH of solution was adjusted at 7 with the help of NaOH (0.01 M) solution. Resulting paste was puri ed by dissolving in 10 mL water and centrifuged for 2 minutes, moreover, same procedure was repeated for 2-3 times and dried the resultant sample in oven at 50 o C. Same steps were repeated by adding ancillary ligands such as bipy (0.078 g) S 2, neo (0.104 g) S 3, dmph (0.104 g) S 4, batho (0.161 g) S 5 and phen (0.09 g) S 6 supplemented to the mortar having L and Sm(NO 3 ).6H 2 O. To portray the energy transfer dynamics triplet state of ligands were considered, for which same procedure repeated to prepare corresponding gadolinium complexes. The synthetic route and structure of all samarium complexes via utilizing grinding technique was displayed in scheme 1 [13].

Evaluation of biological Activities
2.3.1 Antioxidant activities DPPH (2,2-diphenyl-1-picrylhydrazyl) protocol was adopted to compute the antioxidant activities of all referred complexes. The stable free radical DPPH lost its violet color and change into pale yellow due to association with antioxidants moiety, which results a signi cant drop in absorption at 517 nm on spectrophotometer. In order to prepare numerous concentrations such as 25, 50, 75 and 100 µg/mL, dimethylsulfoxide (DMSO) was used as a solvent. Further, 1 mL DPPH solution was added to the 1mL solution of each test samples in their corresponding asks. The absorbance was recorded after 30 minute incubation in dark at 25 o C by taking ascorbic acid as a standard. All tests were performed in triplicate to get concordant values and the scavenging activity of DPPH is expressed in IC 50 (50% of maximum scavenging activity) values. The IC 50 value was determined by graph plotting between scavenging activity (SCA) and numerous concentrations of test samples. By using equation 1 the DPPH scavenging activity of all test samples was determined [14]: (1) Where A t and A c refers to the absorbance test samples and control reaction respectively.

Antimicrobial activities
Antimicrobial assays were executed for all title complexes by employing tube dilution protocol [15]. Antibacterial activities of these samples were carried out against the following in vitro gram negative bacteria: Pseudomonas aeruginosa (MTCC1688), Escherichia coli (MTCC 443) and in vitro gram positive bacteria: Streptococcus pyogenes (MTCC442), Staphylococcus aureus (MTCC 96). Antifungal activities of these complexes were screened against the fungal strain takes Candida albicans (MTCC 227), Aspergillus clavatus (MTCC 1323) and Aspergillus niger (MTCC 282). The reference drugs nor oxacin and greseofulvin were used for antibacterial and antifungal activities respectively.
All samples (reference + ligand + complexes) were dissolved in DMSO to give concentration of 200µg/mL. The incubation period for antibacterial activities is 24h at 37 o C and for antifungal activities is 7 days at 24 o C for Aspergillus avatus and Candida albicans but 48h at 37 o C for Aspergillus niger [16]. The zone of inhibition of antimicrobial activities has been recorded in MIC values. Table 1 displays the elemental analytic data from CHN elemental analyser, which had been estimated to be in   Figure 1 represents the infrared spectrum of complex S1 and free ligand (L), to evaluate the chelation site in bonding. The IR spectra of S 2-S 6 complexes show that all complexes have almost identical IR spectra as displayed in Figure   S1 in supplementary le. The position and intensity of some guide peaks are changed from ligand to complexes, which help to nd the binding site in complexes. The broad band scrutinized at 3410 cm −1 depicts the presence of aqua molecule in all synthesized (S1-S6) complexes [17]. In ligand spectra the band noted at 1711 cm −1 and 1625 cm −1 re ects the existence of carboxylic and ketonic group respectively. It is important to note that the band due to carboxylic acid (1711cm −1 ) completely disappeared in synthesized complexes, peak in all S1-S6 complexes signalize the chelation through carboxylato group. Additionally, bands due to ketonic group (1625 cm −1 ) shift in all S1-S6 complexes ( Table 2) revealing that the second chelation site is through ketonic oxygen. The asymmetric and symmetric vibrations of carboxylato group appears at 1585-1592 cm −1 and 1370-1375 cm −1 respectively which are absent in ligand spectra [18]. Further, carboxylate group is a bidentate ligand, hence, can bind either as unidentate or as bidentate by producing change in relative vibrational positions of asymmetric and symmetric stretching.

Spectral analysis
Complexes exhibits ∆ν > 200 cm −1 [∆ν = ∆ν as (COO − )− ∆ν s (COO − )], this demonstrates the unidentate linkage of carboxylic group [19]. The observed ∆ν values for all synthesized S1-S6 complexes are found to be 210-220 cm −1 as enlisted in Table2, suggesting the unidentate attraction of carboxylato group. Further, the new band appeared at 459-464 cm −1 , 539-548 cm −1 and 1481-1486 cm −1 in S2-S6 complexes assigned for stretching vibrations of ν Sm−O , ν Sm−N and ν C−N respectively in all synthesized complexes as elucidated in Table 2. Table 2 The characteristic IR bands (cm −1 ) for ligand and its samarium (III) complexes.  Figure 2 represents the 1 H-NMR spectrum of free ligand and synthesized complex (S1) carried out in DMSO as a solvent. It is evident from the gure that some signi cant changes take place when the spectrum of complex is compared with the ligand. The anisotropic property of samarium ions is responsible for up eld and down eld shifts in complexes [20]. The peak observed at 15 ppm manifest the presence of carboxylic group in ligand, which completely disappeared in synthesized complexes, con rming the chelation of ligand through deprotonated carboxylic acid [21]. In the ligand spectra the peaks observed at 7.56 -9.39 ppm (aromatic -CH 2 ), 3.26-3.98 ppm  up eld in corresponding samarium (III) complexes due to paramagnetism of Sm 3+ ion. It is evident from the gure that the peak of carboxylic group in ligand was completely disappeared in complex and the peak of ketonic group in ligand shifts by 7.74 -7.86 in all synthesized complexes, which depicts that chelation of L in complexes through ketonic (C=O) and carboxylic group (COO − ) respectively [23]. Figure 3 depicts the UV-Vis absorption spectra of all S1-S6 synthesized complexes and L (1×10 -5 mol/L) by taking DMSO as a solvent. Ligand acts as key absorption for all S1-S6 complexes because absorption displayed by these complexes is weak in 200-500 cm -1 region. It is evident from the gure that absorption maxima observed at 280 nm wavelength is assigned to π -π * electronic transitions [24]. Further, the encapsulation of ancillary ligand in S2-S6 complexes upsurge the absorbance as well as saturates the coordination framework.
All results observed from spectroscopic measurements are in good agreement with each other and certi es that the chelation of ligand with samarium (III) ions through carboxylato (COO − ) and ketone groups in the coordination. Figure 4 reveal the thermal decomposition pattern for S1 complex, since thermal decomposition pattern exhibited by all synthesized complexes are in similar nature, so, S1 was taken as a representative. The complex S1 exhibits 18% mass loss attributed to decomposition of 15 water molecules present outside of coordination sphere up to 88 o C. The next loss in mass is 71% accredited to collapsing of complex and removal of two water molecules and three ligand molecules up to 242 o C to 557 o C present in coordination sphere (i.e. complex start decomposing) [25]. At last, the oxide of samarium (III) ion remains as residue, which was further con rmed through analysis of DTG curves.

Thermal analysis
Theoretical and computed values are in good agreement with total loss of mass in complex. The complexes posses' optimum stability, hence, can be employed as luminescent materials in optoelectronic devices.

Optical band gap & refractive index
Where, S denotes scattering coe cient, K represents absorption coe cient and R ∞ refers to ratio of R normal to R standard . The optical band gap values for L and all synthesized samarium (III) complexes are determined by extrapolation of a tangent up to x-axis in Tauc plot. The observed values for all S1-S6 complexes and L are catalogued in Table 3. As the table illustrate that the value of energy band gap is less for complexes in comparison to the ligands, resulting a number of extra electronic states between samarium (III) ions, hence reinforcement of energy transfer increases, so photoluminescence also increase [27]. Therefore, the property of large energy band gap values makes them a promising candidate for semiconductor power appliance. Further, in order to nd out the refractive index (n) of all S1-S6 complexes precisely, the energy band values are employed by the following relation [28]: (n 2 − 1) Where, n represent the refractive index and E g signi es energy band gap values, the estimated value for all synthesized are epitomized in Table 3. The refractive index values enable these complexes as a promising candidate in optoelectronic devices. to excited state 4 F 9/2 , 4 D 5/2 , 6 P 7/2 , 4 F 5/2 + 4 I 13/2 , 4 G 7/2 , 4 I 7/2 + 4 M 15/2 in samarium (III) ions respectively [29,30] Figure 6(b) represents the three dimensional emission spectra of all S1-S6 complexes monitored at excitation wavelength (356 nm) in solid state. The luminescence spectra displays mainly three peaks at 566 nm, 606 nm and 651 nm which belongs to 4 G 5/2 → 6 H j (where j = 5/2, 7/2, 9/2) electronic transition of samarium (III) ions in all synthesized complexes respectively. Out of these, rst transition, 4 G 5/2 → 6 H 5/2 , is magnetic dipole transition, which follows the selection rule of ∆J = 0, where J is total angular momentum, hence intensity of this peak does not depend on coordination environment around Sm 3+ . 4 G 5/2 → 6 H 7/2 is mixed transition with (partly magnetic and partly electric dipole) but has dominating electric dipole character whereas 4 G 5/2 → 6 H 9/2 is purely electric dipole transitions. The most intense peak of spectra at 606 nm due to hypersensitive 4 G 5/2 → 6 H 7/2 transition is responsible for vermillion emission of complexes and makes them suitable for orange light emitting devices. This transition complies with the selection rule of ∆J = ±1, The electric dipole transition at 651 nm represent immensely asymmetrical surroundings around Sm3+ ion [31,32]. The upsurge the photoluminescence from S1 complex to S2-S6 complexes is explained by √ introduction of ancillary ligands in place of water which results increase in radiative rate by diminished the vibrational quenching caused by water molecules.

Luminescence decay curves and quantum yield
The average environment surrounding samarium (III) ion is investigated by observing decay time curves under 356 nm excitation and 606 nm emission wavelengths respectively as depicted in Figure 6(c). The luminescence decay time curves are well tted by monoexponential function and it is derived from the equation given [33]: Herein, τ represents the decay time for radiative transitions while I 0 and I represent the integrated intensity of peaks at time 0 and t, respectively. Luminescence decay time curves are best tted in mono-exponential function, which speci es the homogenous environment around samarium (III) ion in complexes. Further, the total decay time depends upon both radiative and nonradiative as given by the following relation: The  Table 4. The observed order for decay time in complexes is found to be S1<S2<S3<S4<S6<S5, the higher value of decay time in S2-S6 complexes than that of S1 was credited to extended conjugation by introduction of ancillary ligands. The decay time of complexes purses single exponential behaviour which is responsible for homogenous coordination environment and single luminescent centre around central metal ion. Quantum yield is an important parameter, utilized to observe the luminescence of samarium (III) ion in complexes [34].
It can be described as the ratio of numerical quantity of photons emitted to photons absorbed. However it can be calculated using following equation: Where, represent quantum yield, τ and τ rad represent the total decay time and decay time for radiative transition. The decay time for radiative transition (τ rad ) of samarium (III) complexes was found to be 3.24ms for transition 6 G 5/2 manifold for Sm 3+ . The observed values reveal increase in quantum yield for S2 -S6, relative to S1. This can be interpreted due to synergistic effect of ancillary ligand and lesser nonradiative transition which leads to increase in luminescence. It is noteworthy to emphasise that the luminescence decay time of the synthesised complexes is higher than most of the reported-[Sm(ligand) 3 .ancillary] complexes in literature, while life time was fetched by observing 4 G 5/2 → 6 H 7/2 emissive transition, as mentioned in Table 5.
Color purity (CP) of all complexes is ascertained by employing CIE color coordinates with respect to white light, which shows how actively a meticulous complex act as reddish orange color emitter. CP of all complexes was calculated by using the following relation: CP = (x s − x i ) 2 + (y s − y i ) 2 (x d − x i ) 2 + (y d − y i ) 2 (9) In aforementioned equation, (x s , y s ) represents color coordinates of S1-S6 complexes, (x i , y i ) refers to white light color coordinates (x i = 0.33, y i = 0.33) and (x d , y d ) represents the dominated color coordinates. CP determined by equation 9 is reported in Table 4, which specify that the color purity of S2-S6 is signi cantly higher as compared to S1. This fact √ Page 10/21 is explained on the basis of synergic effect of ancillary ligands in place of solvent molecules. Hence, these complexes are proved to be bright orange color emitting materials to be utilized in OLEDs.
Further, an important parameter CCT (correlated color temperature) helps to investigation the quality and nature of light emitted from a light source. Depending on their CCT values, the complexes are cool light source (above 4000K), warm light source (below 3200K) and neutral light source (3200-4000K) [34]. The CCT can be evaluated by applying the Mc-Camy equation: CCT = -437 n 3 + 3601n 2 -6861n + 5514.31 (10) Where n can be written as: n = x − x e y − y e Here, (x, y) displays the CIE color coordinates of all S1-S6 complexes and (x e , y e ) stands for chromaticity epicentre (0.3320, 0.1858). Observed values of CCT for all S1-S6 complexes are found to be below 3200K (Table 4) indicating the applicability of these complexes in home appliances as a warm light source.

Energy transfer dynamics
The photosensitization is a multistep phenomenon that manifest the excitation of L from ground state to singlet excited state, thereafter energy is transferred to triplet state by intersystem crossing and then to emitting levels of samarium (III) ions via nonradiative process. Further, energy transfer from emitting levels to ground levels of samarium (III) ions is responsible for luminescence as portrayed in Figure 8. It is important to note that there must be suitable energy gap between ligands and metal ions for effective sensitization, the smaller or larger energy gap leads to weak luminescence due to either back energy transfers or inadequate overlaps among acceptor energy levels. The overlap between the absorption spectrum of ligand and excitation spectrum of complex (S1) displayed in Figure S3 put in supplementary le, which indicates the effective sensitization of samarium (III) ions by the L. In order to investigate the energy transfer mechanism the singlet and triplet state of ligand and ancillary ligands are calculated by referring the edge wavelength of absorption spectrum and shortest emission of phosphorescence spectrum of gadolinium complexes respectively. The absorbance spectrum of dmph and neo is shown in Figure S4 and S5 in supplementary le and their inset represent phosphorescence spectra of gadolinium complexes respectively. The absorbance spectrum of L is displayed in Figure 9 and phosphorescence spectrum of gadolinium complex of ligand is shown in inset of this gures. The photoluminescence acquired by gadolinium complexes is 32,000 cm −1 (lowest possible excitation), which is much higher than the photoluminescence of ligands, hence the peak due to gadolinium complex is not observed in spectrum. The energy difference between singlet and triplet state of L is found to be 3320cm −1 , hence intersystem crossing is not much effective (Empirical rule), which results in internal conversion of energy from ligand to metal ion [40]. Further, Latva's rule state that the energy difference between ligand and lanthanides ions must be 2000-5000cm −1 for effective energy transfer [41]. Energy difference between singlet and triplet states of ligand and ancillary ligands are found to be 4719 cm −1 (L), 4433 cm −1 (phen), 5233 cm −1 (bipy), 3430 cm −1 (dmph), 4957 cm −1 (neo) and 3333 cm −1 (batho) as tabulated in Table 6 with respect to samarium ion, which as optimum to e cient transfer of energy.   Table 7. Larger value of SCA leads to lower value of IC 50 , results higher antioxidant capacity of synthesized complexes. The stable free radical of DPPH adopt diamagnetic character by accepting a proton from antioxidant moiety, hence show decrease in absorbance so scavenging activity increases [36]. The antioxidant activities of all S1-S6 complexes are good as compared to ligand, due to donation of electrons from L→Sm 3+ , which increase the capability of complexes to oxidise. Further, it is evident from the table that the antioxidant activities of S2 -S6 complexes are higher than S1, explained on the basis of synergic effect produced by ancillary ligands. These complexes have excellent antioxidant activities so can be used as antioxidant agent in pharmaceutical eld.   Table 8 embodied the results observed form antimicrobial (antibacterial and antifungal) activities of all test samples with respect to standard by using tube dilution technique [35]. The antimicrobial activities of all synthesized complexes are higher than that of ligand due to delocalization of π electrons, which results to increase in lipid attraction tendency of L towards Sm 3+ ion. So, Sm 3+ ion can access to deeper extent of microorganism cell and improve the antimicrobial activities by slow down the growth of microorganism [42]. On the basis of this fact, one can easily see that these complexes employed as good bactericidal and fungicidal agent in pharmaceutical eld. Conclusions Six ternary complexes of samarium (III) ions by utilizing ketocarboxylic acid and ancillary ligands were synthesized and characterized. The proposed composition of complexes is con rmed by elemental analysis and TGA/DTG investigation of complexes. Investigation of IR, UV-visible and NMR ( 1 H & 13 C) spectra of ligand and complexes in details con rm the formation of complexes. Besides, photoluminescence spectral studies shows that under 356 nm excitation, all synthesized complex exhibits the characteristics peaks at 566 nm, 606nm and 651 nm which can be credited to ( 4 G 5/2 → 6 H 5/2 ), ( 4 G 5/2 → 6 H 7/2 ) and ( 4 G 5/2 → 6 H 9/2 ) electronic transitions of Sm 3+ respectively. The energy band gap and refractive index of all complexes and ligand are determined precisely, which enables the applicability of them in semiconductor devices. Investigation of luminescence decay time in detail provide the homogenous environment around samarium (III) ion with excellent intrinsic lifetime (0.7 2-2.42) of all synthesized complexes. CIE colors coordinates, quantum yield (22.22-74.38) and color purity (98.01-99.04) of all S1-S6 complexes were determined accurately. According to CCT, these complexes are warm in appearance. Energy transfer dynamics validates the successfull transfer of energy from ligand (triplet energy state) to samarium (III) ion (resonating energy states). Biological activities of these complexes display the potential use of these complexes in pharmaceutical eld. Figure 1 The IR spectra of L and S1 complex.

Figure 2
The 1H-NMR spectrum of ligand L and its S1 complex.

Figure 3
Page 18/21 UV-Vis absorption spectra of ligand and all synthesized S1-S6 complexes. TG/DTG curve of S1 complex of samarium in nitrogen atmosphere. Figure 5 (a) The diffused re ectance spectrum of ligand and inset represent the corresponding re ectance spectrum. (b) The diffused re ectance spectra of S1-S6 complexes and inset displays its corresponding re ectance spectra. Figure 6 (a) The excitation spectra of all S1-S6 complexes monitored at 606 nm. (b) the emission spectra of all S1-S6 complexes monitored at 356 nm. (c) Luminescence decay curves for all S1-S6 complexes monitored at EM = 606nm and EX = 356nm.