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# Open Chemistry

### formerly Central European Journal of Chemistry

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Volume 15, Issue 1

# Synthesis, characterization, second and third order non-linear optical properties and luminescence properties of 1,10-phenanthroline-2,9-di(carboxaldehyde phenylhydrazone) and its transition metal complexes

• Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthinilayam, India, 515134
• Other articles by this author:
/ S.S.P. Prashanth
• Department of Physics, Sri Sathya Sai Institute of Higher Learning, Prasanthinilayam, India, 515134
• Other articles by this author:
/ S. Srivastava
• Department of Physics, Sri Sathya Sai Institute of Higher Learning, Prasanthinilayam, India, 515134
• Other articles by this author:
/ G. Nageswara Rao
• Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthinilayam, India, 515134
• Other articles by this author:
/ D. Rajesh Babu
• Corresponding author
• Department of Chemistry, Sri Sathya Sai Institute of Higher Learning, Prasanthinilayam, India, 515134
• Email
• Other articles by this author:
Published Online: 2017-12-13 | DOI: https://doi.org/10.1515/chem-2017-0036

## Abstract

The requirement for materials which exhibit good second and third order non-linear optical properties and also for materials which could sense metals in trace quantities has kindled renewed investigations. Organometallics and coordination compounds show a lot of promise as new NLO materials combining the variety of organic moieties with the strength and variable oxidation states of metals. Especially ligands which selectively detect industrial pollutants like Cd and biologically significant metals like Zn are necessary. In the current work the ligand 1,10-phenanthroline-2,9-di(carboxaldehyde phenylhydrazone) (L) and its Ni2+, Co2+, Fe2+, Zn2+, Cd2+ and Ir3+ complexes were synthesized. These were characterized by UV-Vis, FT-IR, 1H NMR, MS and CHN microanalysis techniques. The complexes were shown to have the formula [ML]2+. The second and third order NLO of the ligand and its complexes were recorded These new compounds were found to have same order of third order nonlinear optical susceptibility as that of CS2 and their second hyperpolarizability was an order of magnitude greater than that of C60. Furthermore the ligand also displays selective luminescence sensing of metals ions Fe2+ and Ir3+ even in the presence of other metal ions.

This article offers supplementary material which is provided at the end of the article.

## 1 Introduction

The development of the first laser by Miamen in 1960 laid the cornerstone for the development of the field of nonlinear optics as we know it now [1]. There is a great demand for organic molecules which exhibit nonlinear optical (NLO) properties due to the large hyperpolarizability and inherent freedom of synthesis which they offer [2]. The presence of push and pull π-conjugation confers upon organic molecules high values of second order polarizability [3]. It has to be noted that SHG is exhibited only by noncentrosymmetric crystals. A few organic compounds which crystallize in noncentrosymmetric structures give good optical quality crystals [4] but the presence of only organic moieties lack the mechanical strength that is required for practical purposes [5]. Thus with the objective of building closely-knit crystalline structures, the crystal engineering strategy of combining organic and inorganic moieties was introduced [6].

There have been several examples in these hybrid organic-inorganic moieties which exhibited good NLO properties [7,8,9,10]. Among these materials, organometallics and coordination compounds have been promising due to the variations they introduce in the nature, the oxidation state and coordination sphere of the metal ion and also due to the changes the metal ion brings about in the coordinating ligands [11,11,12,13,14,15,16,17,18]. Thus, the coordination of a metal ion to a ligand with conjugated π-system may increase the hyperpolarizability γ as a result of very extensive delocalization of electrons and polarizability. In this context, the ligand 1, 10-phenanthroline (phen) (1) is a classic bidentate ligand. This hydrophobic ligand chelates mainly with transition metal ions and has the advantage of being rigid and planar on account of its conjugated system which is electron poor and hence acts as a π-acceptor [19]. The advantages conferred by these structural features have led to the active study of the phen complexes for their catalytic, redox, photo-chemical, and photo physical properties [20,21,22,23,24]. Recent studies have shown that some of these metal complexes might exhibit nonlinearity of varying degrees.

1

The various methods available for recording the different degrees of nonlinearity need mention due to the fact that they all have varying degrees of sensitivity. The measurement of powder SHG is carried out using the method developed by Kurtz and Perry [25], which measures the efficiency of new materials with respect to reference materials urea or potassium dihydrogen phosphate (KDP).

Apart from the Z-scan method, DFWM is also used to measure the third-order nonlinear optical susceptibility of materials. In this method, three laser beams irradiate the same part of the sample thus significantly reducing the errors induced due to sample imperfections. As there is no contribution to the signal by the linear scattering, DFWM is a better choice to characterize the nonlinearity of those samples possesseing linear scattering. Also all the components of χ(3) tensor are determined by introducing half wave plates at 45o into the various input beams [26].

Iron plays a very important role in human biology. It acts as a carrier of oxygen to and from the tissues. Additionally it plays a major role in the functioning of several enzymes in the muscle tissues [27,28,29,30].Deficiency of iron would lead to anemia and reduced stamina as the oxidative metabolism in the muscles is disturbed [31,32,33]. Moreover iron deficiency leads to impaired brain functionality [34,35,36]. The use of Ir as catalytic converter in automobiles in combination with Pt, Pd and Rh has led to the pollution of the environment with traces of this metal ion [37]. This has particularly driven the concentration of Ir high in water bodies next to roadways with high traffic. The average concentration of Ir is at 1µgm/ltr in liquid samples and 50µgm/kg in solid samples [38,39]. This calls for probes to sense these elements at micromolar concentration ranges. Luminescent sensors which offer good sensitivity and resolution with ease of usage at low cost are preferable to other probes.

In the present work we report the synthesis, characterization, SHG, third order non-linear optical properties and fluorescence properties of the ligand 1,10-phenanthroline-2,9-di(carboxaldehyde phenylhydrazone) and its Ni2+, Co2+, Fe2+, Zn2+, Cd2+ and Ir3+ complexes.

## 2 Experimental

All the reagents used were of analytical grade procured from Sigma-Aldrich and SD-Fine and all the solvents were purified by distillation. The CHN elemental analysis was carried out on Elementar Vario EL III instrument. Infrared spectra (FT-IR, 4000-400 cm-1) were collected on a Thermos FT-IR spectrophotometer at 25°C as KBr pellets. Ultraviolet-visible (UV-Vis) spectra were recorded on a Shimadzu UV-2450 double-beam spectrophotometer using quartz cells with a path length of 10 mm at room temperature. NMR spectra were recorded on Bruker Avance III, 400MHz instrument with TMS as internal standard. The mass spectra were obtained on Varian 1200 L Single Quadrupole instrument. Luminescence spectra were recorded on a Perkin-Elmer fluorescence spectrophotometer at room temperature (25°C). Melting points were determined in liquid paraffin bath and are uncorrected.

## 2.1 Synthesis

1,10-Phenenthroline-2,9-dicarboxaldehyde: a mixture of neocuproine (3 g) and selenium dioxide (7.5 g) was refluxed in 200 mL of 96:4 dioxane, water system for two hours and filtered through celite while hot [40]. The crude product was recrystallized from hot acetone to remove the remnant selenium. This is then passed through a column of silica gel 60-120 mesh CHCl3: MeOH (1% Acetic acid) 92:8 to collect the pure compound (yield 30%), m.p-231-232oC. UV: λmax-268, 231 nm. IR (KBr) cm-1: 3060, 2856, 1701, 1616, 1594 and 1554. ESI-MS: m/z 236[M]+, 259[M+Na]+, PMR (CDCl3):δ10.35(s, 2H), δ8.30 (d, 2H), δ8;.78 (d, 2H) and δ8.28(s, 2H). 13C NMR: δ120.08, δ131.41, δ138.37, δ129.20, δ152.15, δ145.23 and 193.69.

1,10-Phenanthroline-2,9-di(carboxaldehyde phenylhydrazone): a mixture of phen dialdehyde (118 mg; 0.5 mmoL) and phenyl hydrazine (163 μltrs ; 1 mmoL) in 25 mL of absolute ethanol was refluxed for six hours and filtered while hot to collect the product as an orange solid (yield 70%). m.p-215o-2160C. UV: λ max (DMSO) – 400(1.8), 348(1.3), 300(0.9), 258(1.5) nm. IR (KBr) cm-1:3328, 3050, 3035, 2953, 1601, 1574, 1498, 1361, 1157, 854, 756. ESI-MS: m/z 417(M+H+), 433(M++NH4+). PMR(DMSO d6): δ15.68 (s,1H), δ11.10 (s,1H), δ8.31(s,1H), δ8.33(d, J=8.3Hz,1H), δ8.59(d, J=8.3Hz,1H), δ8.03(d, J=8.8Hz1H), δ7.97(d, J=8.8Hz1H), δ8.46(d, J=8.3Hz,1H), δ7.91(d, J=8.3Hz,1H), δ7.56 (s,1H),δ11.10 (s,1H), δ7.30(d, J=7.0Hz,4H), δ7.50(m,4H), δ6.91(t, J=7.0Hz,1H) and δ7.08(t, J=7.0Hz,1H). 13C NMR: δ156.62, δ137.57, δ127.59, δ144.46, δ120.24, δ128.63, δ8128.56, δ119.03, δ143.51, δ127.45, δ145.32, δ156.16, δ144.96, δ 113.26, δ113.59, δ129.72, δ130.34 and δ112.92.

Synthesis of metal complexes: To a solution of ligand (41.6 mg; 0.1 mmol) in dioxane, methanolic solution of metal salt (0.1 mmol) was added and stirred at room temperature for 30-45 mins. The product was collected as solid by centrifugation and recrystallized from methanol. Various salts used were NiCl2.6H2O, CoCl2.6H2O, FeCl2.4H2O, Zn (OAc)2, Cd(NO3)2 and IrCl3.

Cd(II) complex: PMR (DMSO d6): δ8.06(d, J=8.3Hz, 2H), δ8.47(d, J=8.3Hz, 2H), δ7.65(s,2H), δ8.20(s,2H), δ10.82(s,2H), δ6.15 (d, J=7.3Hz, 4H), δ6.24(dd, J=7.8Hz, 7.3Hz, 4H) and δ6.11(t, J=7.8Hz,2H). 13C NMR: δ141.81, δ126.13, δ140.27, δ129.07, δ135.73, δ125.19, δ149.85, δ139.86, δ113.28, δ127.85, δ121.07. Anal/ Calc. (C, H, N) for C26H20N10O8Cd: 43.69, 2.73, 15.65/ 43.80, 2.83, 15.77.

Zn(II) complex: PMR (DMSO d6): δ8.58(d, J=8.3Hz,2H), δ8.53(d, J=8.3Hz,2H), δ8.12(s,2H), δ8.45(s,2H), δ11.66(s,2H), δ7.18 (d, J=7.8 Hz, 4H), π47.28(dd, J=7.8Hz, 6.3Hz, 4H) and δ6.95(t, J=6.3Hz,2H). 13C NMR: δ144.93, δ126.74, δ144.02, δ126.12, δ152.3, δ121.9, δ8153.87, δ137.79, δ113.88, δ129.88 and δ121.1. Anal/ Calc. (C, H, N) for C30H28N6O4Zn: 63.18, 4.27, 14.65/ 63.22, 4.95, 14.74.

Ni(II) complex: PMR (DMSO d6) : δ8.02 (d, J=8.8Hz, 1H), δ8.48 (d, J=8.8Hz, 1H), δ8.44 (d, J=8.3Hz, 1H), δ8.58 (d, J=8.3Hz, 1H), δ87.90 (d, J=8.3Hz, 2H), δ7.55 (s,2H), δ8.30 (s,1H), δ11.11(s,1H), δ15.67(s,1H), δ7.31(d, J=8.3 Hz, 4H), δ7.50(dd, J=8.3 Hz, 7.8Hz, 4H), δ6.88(t,1H, J=6.8 Hz, 6.3Hz), δ6.99(d, J=8.3 Hz, 1H). Anal/ Calc. (C, H, N) for C26H20N6Ni: 65.69, 4.24, 17.65/ 65.72, 4.24, 17.69.

Co(II) complex: PMR (DMSO d6) : δ8.16(,2H), δ8.49(2H), δ7.78(2H), δ7.84 (1H), δ7.81 (1H), δ10.23(1H), δ 11.14(1H), δ7.23(4H), δ7.49(4H) δ6.80(1H), δ6.90(1H). Anal/ Calc. (C, H, N) for C28H28Cl2N6O2Co: 55.15, 4.57, 13.64/55.09, 4.62, 13.76.

Fe(II) complex: PMR (DMSO d6) : δ7.93(,2H), δ8.40(2H), δ8.26 (2H), δ7.54 (2H), δ10.96(2H), δ7.20(4H), δ7.27(4H), δ6.83(2H). Anal/ Calc. (C, H, N) for C28H27ClN6O2Fe: 58.14, 4.75, 14.76/ 58.91, 4.77, 14.72.

Ir(III) complex: PMR (DMSO d6) : δ8.26 (s,2H), δ8.56 (d,2H, J=8Hz), δ8.37 (d,2H, J=8Hz), δ7.97 (s,2H), δ11.69(s,2H), δ7.24(d,4H, J=7.3 Hz), δ7.31(t,4H, J=7.3 Hz), δ6.93(t,2H, J=7.3 Hz). Anal/ Calc. (C, H, N) for C26H20Cl3N6Ir: 42.57, 2.33, 11.21/ 43.49, 3.23, 11.70.

Comparative UV-Vis and mass data is given in Table 1 and IR analysis is given in Table 2.

Table 1

Comparative UV-Vis, IR and MS of L and its complexes.

Table 2

IR spectra of the ligand and its complexes.

## 2.2.1 Powder SHG Measurement

The experimental setup in the Kurtz method consists of Q-switched laser; filters; photomultiplier tube; oscilloscope; and beam splitter. The photomultiplier tube detects the second harmonic generated when the unfocussed laser beam falls on the sample. The filters remove the fundamental and display the generated second harmonic on the oscilloscope. The reference beam that is used to generate second harmonic by the reference sample is obtained by the placement of beam splitters in front of the sample. This is used to monitor the signals of both the reference and sample by simultaneously displaying them on a dual-beam microscope [41] . Figure 3 shows the schematic of the experimental setup.

Figure 1

Second Harmonic Generation (Kurtz-Perry Method).

Figure 2

Degenerate Four Wave Mixing.

Figure 3

Fluorescence spectrum of L.

The SHG measurement was made using 8 ns pulses of 1064 nm from Nd:YAG laser working at a repetition rate of 10Hz. Laser pulses of energy 1 mJ/pulse were smeared on to the microcrystalline powdered samples taken in a glass capillary. The second harmonic wave of 532 nm produced from the sample was detected by a photomultiplier tube and the resultant signal was fed into an oscilloscope. KDP crystals were used as reference material.

## 2.2.2 Degenerate four-wave mixing

These were used to measure the third order NLO. Figure 4 shows the schematic of the experimental setup.

Figure 4

Fluorescence spectra of L and Its complexes. (a) Fe(II) complex; (b) Ir(III) complex; (c) Co(II) complex; (d) Ligand; (e) Cd(II) complex; (f) Zn(II) complex.

The SHG output of the laser (532 nm) is reflected successively at mirrors M1 and M2 (high reflectance (HR) at 532 nm) which is incident on a glass plate B1 whose reflectance(R) and transmittance (T) is 22% and 70%, respectively. The reflected beam from B1 is again incident on a glass plate B3 with R= 26% and T= 74%. The reflected beam is the probe which is incident on the sample. The transmitted beam from B1 hits the 50:50 beam splitter B2 (actual R= 53% and T= 42%). Both these beams are reflected by M3 and M4 (HR at 532 nm) and then are incident on the sample, the transmitted beam from B2 is called Pump1 and the reflected beam from B2 is called Pump 2.

The three beam pumps 1, 2 and the probe interact in the non-linear medium (sample) and as a result a fourth beam is generated, the Phase Conjugate signal whose propagation is in the opposite direction of the probe but the path is along the probe beam. The third order non-linear susceptibility is found by measuring the intensity of this signal.

Ethical approval: The conducted research is not related to either human or animals use.

## 3.1 1,10-Phenanthroline-2,9-dicarboxaldehyde

Phen dialdehyde (2) was synthesized by the method from the literature [40]. The selenium dioxide serves as an oxidizing agent and turns into selenium by the end of the reaction forming a red mass at the bottom of the vessel. The original procedure does not report any purification procedure and the yield they report matches the crude yield. However selenium dioxide over oxidizes the reactant to give 1,10-phenanthroline-9-al-2-carboxylic acid besides phen dialdehyde. In the present work the product was purified by column chromatography and the two compounds were characterized by spectral studies. The presence of aldehyde groups was confirmed by the strong band at 1701 cm-1c=0) and 2856 cm-1 peak due to aldehyde C-H stretch in the IR spectrum. In the PMR spectrum the peak at δ10.35(s, 2H) is assigned to the aldehyde C-H, H-3 and H-8 are seen at δ8.30 (d,2H), the peak at δ 8.78(d,2H) corresponds to H-4 and H-7; H-5, H-6 are observed at δ 8.28(s,2H). In the mass spectrum the peaks at m/z 236 (M+) and m/z 259 (M+Na) are also indicative of this structure. The absorption in the UV spectrum at 268nm is attributed to the π→π* transition to the lowest-energy excited singlet state. At lower pH there is a bathochromic shift of 10nm which is attributed to charge transfer nature of π→π* transition [42].

## 3.2 1,10-Phenanthroline-2,9-di (carbaldehyde phenylhydrazone)

This ligand (3) was prepared by the reaction of phenyl hydrazine with 1,10-phenanthroline-2,9-dicarboxaldehyde under reflux conditions in absolute ethanol. The ligand has a maroon color and the structure is supported by the IR data. The formation of the ligand is confirmed by the absence of the C=O stretch and also by the presence of N-H stretch at 3328 cm-1, the presence of peaks at 1601 cm-1 and 1361 cm-1 which correspond to C-N-H bending and C-NH stretch vibrations. Also the C=N stretch at 1574 cm-1confirm the formation of ligand [43].

Scheme 1

Synthesis of Ligand.

The PMR spectrum recorded in DMSO (d6) also supports the suggested structure. The ligand shows signals at δ8.31 (1H, s, H-10) and δ7.56 (1H, s, H-11) for the two imine hydrogens. The compound also shows two signals at δ15.68 (1H, s) and δ11.10 (1H, s) for the amine hydrogens which are exchangeable with D2O confirming the formation of ligand. The H-5 and H-6 which are ought to be equivalent, give two separate signals, each a doublet with a coupling constant of 8.8Hz. Similarly the H-4, H-7 and H-3, H-8 also though ought to be equivalent give doublets each with a coupling constant of 8.3 Hz. The signals of the protons of the phenyl rings are also different indicating that the ligand is not symmetrical and hence not planar. The assignment of the PMR signals is given in the Figure S1. The 13C NMR spectrum also supports this theory about the non-planar structure of the ligand by giving separate signal for each carbon in the ligand. The 13C NMR assignment is given in the Figure S2.

The UV-Vis spectrum of the ligand showed peaks at 400, 348, 300 and 258 nm. The peak at 300 nm is attributed to the π-π* transition which has undergone bathochromic shift when compared to the aldehyde. The UV-Vis spectrum of the ligand is given in Figure S3. The mass spectrum also confirms the formation of the ligand as evidenced by the generation of [M+H]+ ion at m/z 417 and [M+NH4]+ at m/z 433.

## 3.3 Transition metal complexes of the Ligand

The complexes were prepared by stirring methanolic solutions of metal salts with the solution of the ligand in dioxane. The products were collected as precipitates. The general structure of the complexes is given below (4). The shifts in the UV-Vis spectra indicate the formation of the complexes. The coordination to metal ion leads to the disappearance of the band at 348 nm. Also the complexes exhibit intense colors due to metal to ligand charge transfer (MLCT).

4

The UV-Vis spectra of the ligand and the complexes are compared in Figure S4. The formation of the complexes was also confirmed by the decrease in the C=N stretch from 1574 cm-1 (in the ligand IR) to 1550-1515 cm-1 due to coordination to the metal ion. The IR spectrum of the complexes showed N-H stretch at 3328 cm-1, C-H aromatic stretch bands at 3050 cm-1 and 3035 cm-1. The increase in the C-N-H bend from 1601 cm-1 to 1620 cm-1 also indicated the coordination of imine nitrogen [44] (Table 2).

Normal PMR and 13C NMR spectra of the Cd(II) and Zn(II) complexes were recorded and Paramagnetic NMR spectra were recorded for Ni(II), Co(II), Fe(II) and Ir(III) complexes. The PMR spectrum of Cd(II) complex showed only ten signals indicating the structure to be symmetrical and hence the is suggestive of the planar structure of the complex. The exchangeable protons on the amine group gave singlet at δ8.20. The protons H-5, H-6 are equivalent and gave a singlet at δ7.65. The protons H-4 and H-7 gave a doublet at δ8.47 which coupled with H-3 and H-8 (δ8.06) with a coupling constant of 8.3Hz. The phenyl rings also showed expected coupling with the ortho protons(δ6.15(d,4H)) coupling with the meta protons (δ6.24(dd,4H)) with a coupling constant of 7.3Hz and the para protons (δ6.11(d,2H)) also coupling with the meta protons with a coupling constant of 7.8Hz. The 13C NMR spectrum of the Cd(II) complex also showed only thirteen signals which supports the symmetrical planar structure of the complex. Similarly the PMR spectrum of the Zn(II) complex also showed only ten signals indicating the structure to be symmetrical and hence the phen ligand is planar in the complex which is supported by its 13C NMR spectrum showing thirteen signals. The proton NMR spectra of Fe(II) and Ir(III) complexes also showed ten signals each thus indicating that the ligand in these complexes is also symmetrical and planar.

Whereas the PMR spectrum of Ni(II) complex showed different signals for the imine protons H-10 (δ7.55(s,1H)) and H-11 (δ8.30(s,1H)). Also the exchangeable protons attached to the nitrogen atoms of the phenyl hydrazone system showed different signals at δ11.11(s, 1H) and δ15.67(s, 1H). The complex also showed different signals for H-5 and H-6 at δ8.02 and δ8.48 respectively with a coupling constant of 8.8Hz.The protons H-4 and H-7 also showed different signals at δ8.44(d, 1H) and δ8.58(d, IH) respectively coupling with protons H-3 and H-8 with a coupling constant of 8.3Hz both of which gave signal at δ7.90(d, 2H). The Co (II) complex also showed similar signals and thus indicated the unsymmetrical and nonplanar structure of the phen ligand in these complexes.

The NMR spectra of the complexes have their signals shifted to lower δ values compared to those of the ligand which indicates a metal to ligand charge transfer. The assignments of the signals of PMR and 13C NMR of Cd (II) and Zn (II) are given in Figure S5 and Figure S6, respectively. The paramagnetic assignments of Ni (II), Co (II), Fe (II) and Ir (III) are given in Figures S7, S8, S9 and S10, respectively.

The mass spectra of the complexes showed molecular ion peaks for all the complexes. The Ni (II), Co(II) and Fe(II) complexes showed (ML+H)+ peaks at m/z 473, 474 and 473 units, respectively. The Ni(II) and Co(II) complexes also showed peaks at m/z 356 and 357, respectively which is attributed to the fragmentation [ML-Ph—NH=N—CH-]+. The Zn(II) complex exhibited molecular ion peak at [ML+Na]+=m/z 494 and also at [M-2Ph+K]+=m/z 357. The Cd(II) complex has its molecular ion peak at [ML+K]+=m/z 567. It also gave a peak corresponding the fragmentation [ML-Ph+Na]+ at m/z 474. Even though the Ir(III) complex did not show any molecular ion peak the fragmentation patterns [ML-2Ph+K]+ and [ML-Ph–NH-]+ have been identified at m/z 493 and 439, respectively. The mass spectral analysis establishes the general formula of the complexes to be [ML]2+ where M=Ni(II), Co(II), Fe(II), Zn(II), Cd(II) and Ir(III).

The acetate ion was determined as the counter ion for the Cd(II) complex indicated by the IR spectrum which showed characteristic strong absorptions at 1545 cm-1 and 1384cm-1 characteristic for the acetate salts [45] and further confirmed by a singlet at δ1.65 in the PMR spectrum of the complex. Similarly the counter ion for the Zn(II) complex has also been indicated as the nitrate ion by the IR spectrum which showed a strong absorption for the asymmetric N=O stretch at 1552 cm-1 and a weaker symmetric absorption at 1391 cm-1 which are characteristic of nitrate ion [46,47,48].

## 3.4.1 Third order NLO

A reference sample CS2 was used to measure the third order NLO susceptibility Χ(3) of the samples by comparison. For the analysis of these samples, we used 7 ns pulses at 532 nm from the second harmonic output of a Q-switched Nd:YAG laser. This beam is divided by beam splitters so that the two counter propagating pump beams have the same energy (5mJ) and the probe beam has 0.8 mJ. The c(3) measurements were made using the reference sample CS2. The magnitude of the third order nonlinearity were calculated using the equation 1 [49].

$χsample(3)=(n0nref)(LrefL)IPC(sample)IPC(ref)12αLexp⁡(αL2)1−exp⁡(αL)χref(3)$(1)

The refractive index of CS2 was taken as 1.6 and its third order nonlinear susceptibility (χ(3)ref)) was taken to be 2.8 X 10-20 m2/V2. The sample was made with concentration of 1mg/ml with methanol. The refractive index of each of these samples was taken as 1.34 as the refractive index of the dilute solutions is almost equal to that of solvent. The effect of linear absorption was taken into account as shown in the above formula. The second hyperpolarizability was also calculated using the equation 2 [50].

$γ=χeff(solution)(3)−χeff(solvent)(3)Nf4$(2)

N: the number density (m-3) was calculated by using the concentration and the molecular weight of each of these samples. The results are given in Table 2.

The γ value of phen is 1.76 X 10-31 esu [51] on account of its planarity and rigid conjugation system which gets polarized with light induction. The current complexes show hexadentate nature with four nitrogen donors two from phen ring and two imine nitrogens. All the metal ions have diffuse d-shells which can interact with the π-electron system of the ligand and thus causing charge transfer reactions. These charge transfers lead to polarizability which in turn result in good NLO properties. The extent of NLO properties exhibited would depend upon the extent of interaction of the d-orbital the π-system. Both Ni(II) and Fe(II) have open d-shells which lead to better interaction and hence increased charge transfer leading to good NLO effects. In the case of Cd(II) and Zn(II) complexes the metal to ligand charge transfer is expected to lead to the results shown. Also the atomic radii for Cd(II) and Zn(II) are 95 pm and 74 pm, respectively. The larger size of Cd (II) leads to more polarizability leading to better NLO properties.

Table 3

Results of third order NLO studies.

The new complexes were found to have high third order nonlinear optical susceptibility of the same order as that of CS2. The second hyperpolarizability was found to be an order of magnitude greater than that of C60. The third order nonlinear optical susceptibility of these complexes was two orders of magnitude greater than that of the complexes reported by Zhi-Bin Cai et al. [19].

## 3.4.2 Second harmonic generation

In general, second order NLO is shown by those compounds which have extensive π-conjugation. This is enhanced by the presence of both electron donating and electron withdrawing groups. The current set of compounds studied has their π-electrons in conjugation around the system which is symmetrical. The SHG measurements were carried out using Kurtz powder method at input beam energy of 1mJ/pulse. The SHG signal of the reference material KDP at this energy is 21.2 mV. Among the compounds studied, the ligand and the Fe2+ complex showed SHG values similar to that of KDP with 20 mV and 21 mV, respectively. The Cd2+ and Zn2+ complexes showed values of 12 mV and 19 mV, respectively, whereas Ni2+, Co2+ and Ir3+ complexes did not show any SHG activity.

## 3.5 Fluorescence Studies

The UV-Vis spectra of the ligand and its complexes when recorded in MeOH exhibited absorption at 235 nm. Thus the fluorescence emission spectra of the ligand as well as its complexes at one micromolar concentration were recorded by exciting them at 235 nm. The emission spectrum of the ligand has shown strong emission at around 300 nm (Figure 1).

Only the emission spectrum of Fe(II) complex and Ir(III) complex showed variation by emitting at 370 nm and 350 nm, respectively. All the other complexes have emission spectra similar to of that the ligand. The spectra are shown in the Figure 2. Thus the ligand 1,10-phenanthroline-2,9-di(carbaldehyde phenylhydrazone)(L) can be used as a sensing agent for Fe(II) and Ir(III) ions even in the presence of these other metal ions. Also, the emission gets quenched in DMSO. The fluorescence can be explained by the fact that chelation to the metal ion would make the system rigid and hence would allow the easy flow of electrons in the π-band [52, 53]. Further the internal charge transfer transitions help in the strong emissions by these complexes [54, 55].

## 4 Conclusion

The compound 1,10-phenanthroline-2,9-dialdehyde was synthesized and purified. The ligand 1,10-phenanthroline-2,9-di(carboxaldehyde phenylhydrazone) and its transition metal complexes were synthesized, characterized and were studied for their NLO and luminescence properties. The complexes of 1,10-phenanthroline-2,9-di(carboxaldehyde phenylhydrazone) were found to have high third order nonlinear optical susceptibility and their second hyperpolarizability was found to be an order of magnitude greater than that of C60. The third order nonlinear optical susceptibility of these complexes was two orders of magnitude greater than that of the complexes reported by Zhi-Bin Cai et al. These results suggest the compounds as plausible candidates for use in optical gates and switches.

Fluorescence spectra of the ligand and its complexes were recorded and were found to have good emission spectrum. However among its complexes only Fe(II) complex showed difference in the emission spectrum from the ligand. Hence, the ligand can be used as a sensing agent for Fe(II) even in the presence of other metal ions.

## Acknowledgements

The work is dedicated to Bhagawan Sri Sathya Sai Baba, Founder Chancellor of Sri Sathya Sai Institute of Higher Learning. Financial assistance from the CSIR (No. 01 (2286)/08/EMR – II) is gratefully acknowledged. Thanks are also due to Sai Siddhartha and Sai Manohar for their due help in the preparation of the manuscript.

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Accepted: 2017-09-26

Published Online: 2017-12-13

Conflict of interest: Authors state no conflict of interest.

Citation Information: Open Chemistry, Volume 15, Issue 1, Pages 283–292, ISSN (Online) 2391-5420,

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