Synthesis, physicochemical properties and third-order optical nonlinearities of cadmium (II) dibromide l - Proline monohydrate for optical limiting application

Journal Pre-proof Ref: Publication of your article [MOLSTR_127457] in Journal of Molecular StructureSynthesis, Physicochemical Properties and Third-order optical nonlinearities of Cadmium (II) Dibromide L - Proline Monohydrate for optical limiting application

Rejeena V. Rajan, Lija K. Joy, D. Sajan, A.K. Thomas, S. Sathiskumar, T. Balakrishnan, G. Vinitha PII:

S0022-2860(19)31566-2

DOI:

https://doi.org/10.1016/j.molstruc.2019.127457

Reference:

MOLSTR 127457

To appear in:

Journal of Molecular Structure

Received Date:

28 September 2019

Accepted Date:

20 November 2019

Please cite this article as: Rejeena V. Rajan, Lija K. Joy, D. Sajan, A.K. Thomas, S. Sathiskumar, T. Balakrishnan, G. Vinitha, Ref: Publication of your article [MOLSTR_127457] in Journal of Molecular StructureSynthesis, Physicochemical Properties and Third-order optical nonlinearities of Cadmium (II) Dibromide L - Proline Monohydrate for optical limiting application, Journal of Molecular Structure (2019), https://doi.org/10.1016/j.molstruc.2019.127457

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Synthesis, Physicochemical Properties and Third-order optical nonlinearities of Cadmium (II) Dibromide L - Proline Monohydrate for optical limiting application Rejeena V. Rajana,b , Lija K. Joya, D. Sajan a*,A.K. Thomasb , S. Sathiskumarc, T. Balakrishnand, G. Vinithae, aCentre

for Advanced Functional Materials, Postgraduate and Research Department of Physics, Bishop Moore College, Mavelikara, Alappuzha, Kerala 690110, India bSt. Cyril's College, Department of Physics, Vadakkedathukavu P.O., Kilivayal, Adoor , Pathanamthitta 691529, India cPost Graduate Department of Physics, Srinivasan College of Arts and Science, Perambalur – 621 212, Tamil Nadu, India dCrystal Growth Laboratory, PG & Research Department of Physics, Periyar EVR College (Autonomous), Tiruchirappalli – 620 023, Tamil Nadu, India eDivision of Physics, School of Advanced Sciences, Vellore Institute of Technology (VIT), Chennai 600127, India

*Corresponding Author Email Id: [email protected] Tel.: +91 9495043765; Fax: +91 4792303230 Abstract Metal-semi organic NLO single crystal of Cadmium (II) dibromide L - Proline monohydrate (CBLPM) has been synthesized and the crystal was grown from solution by room temperature slow evaporation method. The crystal structure and cell parameters were refined by Rietveld refinement technique and confirmed that the metal-semi organic samples are in the orthorhombic system with space group P212121. The crystal structure of CBLPM is stabilized by intermolecular N-H….O, N-H…..Br, O-H…..O and O-H…..Br hydrogen bonds. The water molecules serve as donors for the weak O-H…O and O-H…Br hydrogen bonds which link adjacent chains forming a threedimensional structure. A Hirshfeld surface analysis suggests that the most significant contribution to the crystal packing is by H…Br contacts (26.2%).The complete vibrational features and electronic absorption spectra of the title compound were analyzed by FT-IR, FT-Raman and UV– visible spectra combined with density functional theory and time-dependent density functional computations. The second-order hyperpolarizability value of the molecule was also calculated at density functional theory method. The third-order nonlinear optical properties of the crystal were studied by Z-scan techniques using CW laser with wavelength 532 nm. The open aperture result exhibits the saturation absorption, which indicates that this material has potential candidate for optical limiting applications. Keywords: Crystal growth, DFT, Hirshfeld surface, UV–Visible, Z-scan, Optical limiting 1|Page

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1. Introduction Organometallic compounds with second-order, third-order nonlinear optical properties and luminescence are of growing interest as new molecular multifunctional materials, since they offer additional flexibility compared to organic chromophores by introducing active electronic charge transfer transitions between the metal and the ligand, which are tunable by virtue of the nature, oxidation state and coordination sphere of the metal center [1, 2]. As part of the design of novel NLO materials, much effort is being devoted to understand the origin of non-linearity in large systems and to correlate NLO responses with molecular structure and geometry [3].Metal-organic hybrids also offer interesting advantages with respect to both pure organics and pure inorganic materials in that they may be reliably designed by integrating highly predictable structural features, such as hydrogen bonds and coordination bonds, in each of these cases which are used jointly to achieve spatial and dimensional control in organic–inorganic hybrid [4]. Compounds containing αamino acids and inorganic salts became an alternative to popular inorganic crystals. Complexes of L – Proline with alkali metals [5, 6], alkaline earth metal [7] and transition metals [8-10] have been reported. Growth and its characterization of L – proline cadmium chloride monohydrate [11], dibromo bis (L – Proline) Cd (II)[12] , L - proline lithium bromide monohydrate [13] have been reported. Crystal structure of metal coordination compounds of cadmium (II) dibromide L -proline monohydrate (CBLPM) has been reported [14, 15].. Among various second order NLO materials, metal – organic coordination compounds have fascinated much more interest due to their potential of combining the advantage of both organic and inorganic materials, such as high NLO coefficients, stable physic – chemical properties [16, 17]. In metal organic compounds, metal centres can act as both donors and the bridging moiety in D – π – A system and the metal ligand bond is predictable to display large molecular hyper polarizability because of the transfer of electron density between the metal atom and the conjugated ligand system [18]. Additionally, in the case of metal – organic coordination complex, the group II B divalent d10 ions, Zn2+, Cd2+ and Hg2+ complexes have engrossed our significance for their matchless characteristics of pale colour [19]. However, literature survey reveals that neither quantum chemical calculations nor experimental studies of the CBLPM have been reported so far. The present work deals with the vibrational spectral studies aided by density functional theoretical (DFT) calculations to elucidate 2|Page

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the correlation between the molecular structure and NLO property by investigating the Intra molecular Charge Transfer (ICT) interaction, hydrogen bonds and static first and second order hyperpolarizability of the novel third order NLO material cadmium (II) dibromide L -proline monohydrate 2. Experimental methods The title salt CBLPM was synthesized by slow evaporation solution growth technique. Commercially available L- Proline (Sigma Aldrich) and cadmium bromide tetrahydrate (AlfaAesar) taken in equimolar ratio (1:1) were dissolved in double distilled water and the resultant solution was stirred for 3h using a magnetic stirrer. The filtered homogeneous solution was evaporated at room temperature which yields a tiny crystalline salt of CBLPM. The purity of the crystalline salt was improved by successive recrystallization process. Phase purity, homogeneity and structure were determined by X-ray powder diffraction measurements at room temperature using a Bruker D8 ADVANCE X-ray diffractometer with an incident Cu Kα radiation of 1.540598 Å in a range of Bragg’s angle (10º -80°) at a rate of 1°/min. The XRD data have been analysed by refining the experimental data using a standard Rietveld refinement technique using Full PROF software [20]. The FT-IR spectrum of the compound was recorded in the 4000 to 400 cm-1regions with a resolution of 1 cm-1 using Perkin-Elmer Spectrometer by KBr pellet technique. The FT-Raman spectrum was recorded in the region 3500-50 cm-1 using Bruker RFS 27: standalone FT-Raman spectrometer with resolution 2 cm-1. The UV-visible absorption spectrum of the compound was recorded in the range of 187-600 nm using JASCO spectrophotometer. The Z-scan measurement of the sample was carried out using a Semiconductor – continues wave laser beam (532 nm, 100 mW) which was focused by a lens with 3.5 cm focal length. A 1mm wide optical cell containing the solution of the sample was translated across the focal region along the axial direction. 3. Computational details Density functional theory (DFT) calculations were carried out using the Gaussian’09 suite of programs [21]. Initially, the structures of the CBLPM molecule were optimized, and then the vibrational wavenumbers were calculated with the B3LYP/ LANL2DZ level [22] basis sets. In order to avoid the differences between the experimental and computed wavenumber which is due to the neglection of anharmonicity and effectiveness of basis set, the calculated wavenumbers have 3|Page

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been scaled using Scaled Quantum Mechanical Method (SQM) procedure [23, 24].The potential energy distribution (PED) has been calculated by MOLVIB.7 program written bySundius [25]. Natural bond orbital (NBO) analysis has been performed by using the NBO 3.0program [26]. 4. Results and Discussion 4.1 Powder X-ray diffraction study The XRD data have been analyzed by refining the experimental data using a standard Rietveld refinement technique using Full PROF software [20]. The Rietveld method is a wellestablished technique for extracting structural details from powder diffraction data. The refined lattice parameters determined are a= 10.1496 Å b=13.5893 Å and c=7.5176 Å with the crystal system belongs to orthorhombic, P212121 space group, which are consistent with earlier reports [19]. The CIF file obtained through Rietveld refinement technique is deposited in the Cambridge Crystallographic data center (CCDC number CCDC 1863868).



Figure 1(a) Photograph of the grown CBLPM crystal and (b)Molecular structures of compounds with the atom labelling scheme (displacement ellipsoids at 50% probability level) Figure 1 (a) shows the photograph of the grown CBLPM crystal of dimension 2 ×2×1 mm3

and figure 1 (b) shows the Oak Ridge Thermal Ellipsoid Plot Program (ORTEP) plot drawn for the molecule at the 30% probability ellipsoid level with the atom numbering scheme. The plot of the molecule shows the coordination around Cd (II). The bond distances and bond angles for the compound were comparable to those reported earlier for the same [14]. In CBLPM molecule, the Cd (II) ion is coordinated by four bromido ligands and two carboxylate oxygen atoms of two 4|Page

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symmetry-related proline ligands, which exist in a zwitterionic form, in a distorted octahedral geometry. There is an intramolecular N-H….O hydrogen bond between the amino group and the carboxylate fragment. Each coordinating ligand bridges two Cd (III) atoms, thus forming infinite chains running along the c-axis direction. The water molecules of crystallization serve as donors for the weak intermolecular O-H….O and O-H…..Br hydrogen bonds that link adjacent polymeric chains, thus forming a three-dimensional structure. This N-H….O and N-H…..Br hydrogen bonds are clearly visible in the packing figure shown in figure 2.

Figure 2 The crystal packing of CBLPM viewed along a axis. Dashed lines denote intermolecular hydrogen bonds. 4.2 Hirshfeld Surface analysis Hirshfeld surface analysis provides the percentage contribution of the intermolecular interactions inside the unit-cell packing for the title compound. This is performed using Crystal Explorer 3.1[27]. The dnorm and de surfaces are presented in Fig. 3(a). All C-H…O contacts are recognized in the dnorm mapped surface as deep-red depression areas showing the interaction between the neighbouring molecules. Fig.3 (b) shows the 2D fingerprint plots, which are used to analyze all of the intermolecular contacts at the same time, revealed that the main intermolecular interactions in the compound were Br...Cd, H...Br, O...Cd and O...H/H...O.

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Figure 3. (a) Hirshfeld dnorm surfaces and (b) full fingerprint for CBLPM molecule Figure 4 shows the relative contributions to the percentage of Hirshfeld surface area for the various intermolecular contacts in the CBLPM molecule. It’s the characteristic features in the fingerprint plots analysis. The highest contribution occurs due to H….Br contacts (26.2 %).The O...H interactions are represented by a spike in the bottom left (donor) area of the fingerprint plot, which represents the water oxygen interacting with acid oxygen, forming a 2D network of hydrogen bonds. The H...O interactions (14.9 %)are represented by a spike in the bottom right region of fingerprint plot(acceptor). The proportion of Br...Cd interactions comprise 9.5% of the total Hirshfeld surfaces. Lower percentages are observed for the O...Cd (6.8%) contacts, with the last contact contribution corresponding largely to other interactions.

Figure 4. Relative contributions to the percentage of Hirshfeld surface area for the various intermolecular contacts in the CBLPM molecule

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4.3 Optimized Geometry Optimized Geometry of CBLPM is shown in figure 5. Optimized structural parameters of CBLPM calculated at B3LYP/LANL2DZ are listed in Table 1. In CBLPM, a hydrogen bond between water molecule and bromine is observed and this bond length is equal to 2.3577Å which is found to be shorter when compared with vander Walls radii. In proline molecule, intermolecular hydrogen bonding interactions from H6-O21is 1.61 Å and that of H9-O21 is 2.81 Å. CBLPM molecule has a distorted shape with nitrogen atom of pyrrolidine ring and carboxylic group pointing towards the central cadmium atom. O2-C7bond length is1.3009 Å which is quite less than the standard value of C-O bond length which is 1.43 Å [15]. From the reported values, the bond lengths of Cd-O of monohydrate molecules ranges 2.287-2.634 Å [15] but in CBLPM, O2- Cd19 bond length is less than the reported values (2.1506Å) which is due to the effect of hydrogen bonding interactions. In Cadmium Bromide, bromine atoms are bonded with Cadmium atom having the bond lengths of 2.6789 and 2.5551 Å. From these results, Br1- Cd19 bond length is longer than

Cd19 - Br20 bond length which is due to the O-H…Br intermolecular hydrogen bonding interactions.

Figure 5. Optimized Geometry of CBLPM

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Table 1

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4.4 Natural Bonding Orbital (NBO) Analysis The most important interactions between ‘filled’ (donors) Lewis-type NBO’s and ‘empty’ (acceptors) non-Lewis NBO’s are reported in Table 2. For each donor NBO (i) and acceptor NBO (j), the stabilization energy E(2) associated with delocalization or hyperconjugation is estimated using the following formula [28] The hyper conjugative interaction energy was deduced from the second-order perturbation approach:

E  2   n where  F 

2

 F





*

2

 

 n

Fij

2

E

or Fij2 is the Fock matrix element between i and j NBO orbitals, εσ and εσ* are the

energies of σ and σ* NBO’s and nσ is the population of the donor σ orbital. The NBO analysis clearly gives evidence for the formation of strong H-bonded interaction between oxygen lone electron pairs and π* (N–H) antibonding orbitals. Second-order stabilization energy

E(2)

associated

with

the

hyper

conjugative

interactions

n2(O21)π*(N4-H6);

n1(O3)π*(N4-H5)and n2(O3)π*(N4-H5) are 25.19, 1.94 and 7.89 kcal/mol respectively. This can be used as a measure of the intramolecular N-H-O hydrogen bonding interaction between the oxygen lone pair and the antibonding orbitals. Also the n4(Cd19)π*(O2-C7); n5(Cd19)π*(O3C7) ; n2(Br1)π*(C8-H9) and n3(Br1)π*[(C3-H7),(C8-H9), (C8-C10)] interactions shown in Table 2 corresponds to intra-molecular interactions due to the orbital overlap resulting in high stabilization energy which is responsible for conjugation of respective bonds in the pyrrolidine ring. These results explain the NLO activity of the molecule. The lone pair to antibonding orbital interactions, n3(Br1)→π*(O21-H22) and n1(Br1) → π*(O21-H22) have delocalization energies 8.75

Kcal/mol,

and 0.47 Kcal/mol respectively. The π(C13–C14) and π(C10–C15) conjugated with π*(C10–C15) and π*(C11–C12)respectively, results in a stabilization of 24.39 Kcal/mol and 22.09 Kcal/mol respectively, which is an evidence for charge transfer within the molecule, and is responsible for the better chemical activity of the molecule. 

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Table 2

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4.5 UV-Vis Analysis For understanding the nature of electronic transitions in terms of their energies and oscillator strengths, time-dependent DFT (TD–DFT) calculations involving configuration interaction between the singly excited electronic states were conducted on CBLPM. The experimental and simulated UV absorbance spectrum of the CBLPM crystal is recorded and is shown in Fig. 6. The excitation energy and oscillator strength for CBLPM calculated using TDCAM-B3LYP calculations are given in Table 3. From the experimental spectrum it is observed that the UV absorbance exhibits a strong absorbance peak at 204 nm. Maximum absorption peak predicts electronic transition at 204 nm by an oscillator strength f = 0.0234 showing good agreement with the experimental data, the characteristic peak that arises due to n* transition corresponds to associated with intermolecular hydrogen bonding [29]. The theoretical wavelength values are obtained at 209 and 203 nm. HOMO and LUMO are the main orbitals that take part in chemical stability. For the calculated absorption spectra, the wavelength 209 nm corresponds to the electronic transition from the HOMO to LUMO (98%), the wavelength 203 nm corresponds from HOMO-1 to LUMO+1 (99%) and HOMO-2 to LUMO (51%). The orbital from HOMO to LUMO involves intermolecular charge redistribution. The electron densities of LUMO mainly localized in tartrate anion moieties. The electron densities of HOMO are highly delocalized and spread on the entire molecule indicating that electrons can more readily move around the molecule which can lead to an improved intermolecular charge transfer interaction (ICT). The electronic transition that takes place from HOMO to LUMO show that a donor to acceptor charge transfer interaction provides NLO activity in CBLPM molecule. The energy gaps between the HOMO and LUMO for CBLPM are shown in Figure-7. HOMO – LUMO energy gap of 4.8261 eV, represents the optical gap which nearly fits with the energy gap of 4.9eV obtained from UV analysis.

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Table 3

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Figure.6 (a) UV-Vis spectrum of CBLPM at water and theoretical spectrums at gas phase and 6(b) HOMO-LUMO energy gap of CBLPM.

4.6 Nonlinear Optical Studies Molecular Electronic NLO Responses In order to understand the structure property relationship, theoretical investigations have played an important role in novel NLO material design. The static and dynamic dependent NLO processes such as static first-hyperpolarizability 𝛽(0;0,0), static-second-hyperpolarizability 𝛾 (0;0,0,0), the electro-optical Pockels effect 𝛽( ―𝜔;𝜔,0), the second-harmonic generation 𝛽 ( ―2𝜔;𝜔,𝜔), the dynamic second-hyperpolarizabilities for the quadratic electro optic Kerr effect 𝛾 ( ―𝜔;𝜔,0,0), and the electric-field-induced second harmonic generation 𝛾( ―2𝜔;𝜔,𝜔,0) for CBLPM were calculated at DFT/CAM-B3LYP/6-31G(d) level of theory. The dynamic dependence of hyperpolarizabilities for the molecule were calculated at the characteristic Nd:YAG laser wavelength 1064 nm (ħ𝜔 = 0.042823 𝑎.𝑢). The static first hyperpolarizability value (𝛽(0;0,0)) , electro-optical Pockels effect 𝛽 ( ―𝜔;𝜔,0), the second harmonic generation 𝛽( ―2𝜔;𝜔,𝜔), static-second hyperpolarizability, γ(0;0,0,0), second hyperpolarizability for the quadratic electro optic Kerr effect, γ( ― ω;ω,0,0), and DC electric-field-induced second-harmonic generation hyperpolarizability, γ( ― 2ω;ω,ω,0) 10 | P a g e

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γ(0;0,0,0), γ( ― ω;ω,0,0)andγ( ― 2ω;ω,ω,0)for CBLPM are 2.53584×10-30esu, 2.60917 ×1030esu,

2.80204 ×10-30esu, 6.25054 ×10-36esu, 6.55543 ×10-36esu and 7.2505 ×10-36esu, respectively.

Third order nonlinear optical study The closed and open aperture mode of Z-scan method is a powerful tool for the precise measurement of intensity dependent nonlinear refractive index (𝑛2), nonlinear absorption coefficient (𝛽) and third order nonlinear optical susceptibility (𝜒3) of CBLPM. This is done by thermal lens model developed by Sheik-Bahae [30].According to this model, the thermal nonlinearity materialize due to the localized rise in temperature when Continuous Wave (CW) laser is illuminated through the liquid sample. The increase in laser power upon the material creates a temperature difference which can causes thermal lensing, thermal birefringence and fracture [31]. The occurrence of nonlinear refraction (NLR) will be analyzed by closed aperture zscan, while the open aperture z-scan will be accurate towards the nonlinear absorption (NLA) response. The parameters of the z-scan setup used in the experiment are consolidated in Table 4. 

Table-4

The nonlinear absorption coefficient, (𝛽) , and imaginary part of third-order NLO susceptibility, 𝑙𝑚(𝜒3) can be obtained from [32, 33] the open aperture Z-scan data by the following relation

𝛽=

2 2∆𝑇 𝐼0𝐿𝑒𝑓𝑓

𝑙𝑚(𝜒 ) = 10 3

𝜀0𝑛20𝑐2𝜆𝛽

―2

4𝜋2

where 𝛥𝑇 is the one peak value at the open aperture Z-scan curve,𝐼0is the intensity of the laser beam at focus 𝑧 = 0, 𝐿𝑒𝑓𝑓 is the effective thickness of the sample and 𝑐 is the speed of light. The nonlinear refractive index (𝑛2) and real part of third-order NLO susceptibility, 𝑅𝑒 (𝜒3) are obtained from closed aperture data, which can be calculated as 𝑛2 =

𝜆∆𝜙0 2𝜋𝐼0𝐿𝑒𝑓𝑓 𝜀0𝑛20𝑐2𝑛2

𝑅𝑒 (𝜒3) = 10 ―4

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𝜋

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Second order hyper-polarizability and third-order nonlinear optical susceptibility(𝛾) describes the interaction of the incident photons with the permanent dipole moment of the molecules can be obtained as 𝛾=

𝑙𝑚[(𝜒3)] 𝑓4𝐶𝑚𝑜𝑙𝑁𝐴

𝑓 is the Lorentz field factor and is given by 𝑓 =

(𝑛2 + 2) 3

, 𝐶𝑚𝑜𝑙is the molar concentration, and 𝑁𝐴 is

the Avogadro constant.

Fig. 7. (a) Open-aperture and (b) Closed aperture Open aperture z- curve of CBLPM crystal Fig.7 (a) shows the open aperture (OA) Z-scan curve with symmetric valley which indicates the reverse saturation of absorption (RSA) with positive nonlinear absorption at the enhancement of intensity close with the focus. Maximum absorption occurs when the sample is at the focal point of the lens, since the axial peak irradiance 𝐼0 of the beam is the maximum. The mechanism of nonlinear absorption (NLA) in the CBLPM can occur via transition processes from S0 (ground state) to the S2 (2nd excited state) correspond to excited state absorption (ESA) process. Reverse saturable absorption is a phenomenon that happens only if thermally agitated molecules accumulated in the excited state decay directly back into the ground state resulting in focal point reduction and lower transmittance at the focus [33, 34]. For the CW excitation, the NLA usually originates from thermo-optics effect, which eventually enhance the excited state absorption (ESA) 12 | P a g e

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process. Thus, we can infer that ESA assisted RSA is the nonlinear mechanism of CBLPM molecules. Closed aperture (CA) Z-scan data in Fig. 7 (b) shows a peak which is followed by a valley in normalized transmittance which indicates the change in refractive index with strong selfdefocusing effect with negative value of𝑛2 [32].The self-focusing phenomena correspond to thermal in nature, resulting from absorption of laser beam at 532 nm. The localized absorption from the intense laser instantaneously creates spatial temperature profile across the beam waist. The dependence of temperature towards refractive index means that the phase of the transmitted beam is distorted accordingly attributed to thermal lensing effect. The thermal nature of the nonlinear refraction (NLR) was confirmed by measuring the peak to valley separation. The asymmetry curve about the principle focal point in CA suggests thermal nonlinearity dominating in the CW mode. Hence, thermal lens model has been used to fit the experimental CA curve shown in Fig.7 (b). The real part of the nonlinear susceptibility (Re  (3) ) should be larger for practical applications [33, 34]. Simultaneously, the nonlinear absorption coefficient (β) should be as small as possible. These two conditions collectively make NLO material with high value of  ( 3) . From Table.4it is observed that the very small values of β makes the Im  ( 3) values less for CBLPM crystals. The CBLPM molecules show optical nonlinearities due to the donor-acceptor intermolecular interactions through intermolecular hydrogen bonds [35, 36]. The electron delocalization and intermolecular charge transfer make the CBLPM molecules to possess large molecular hyperpolarizability and thus contribute to large third-order susceptibility (  ( 3) ) [36]. 4.7 Vibrational Spectral Analysis The computed vibrational wavenumbers, their IR and Raman activities and the atomic displacements corresponding to the different normal modes are used to identify the vibrational modes unambiguously. The experimental data refer to the solid state, whereas the DFT computations correspond to the molecule in vacuum and do not consider intermolecular interactions, causing the deviations of some of the experimental values from computed results. The SQM vibrational wavenumbers, measured infrared and Raman band positions and their tentative assignments are given in Table 5. The observed FT-IR and Raman spectra as well as the simulated theoretical spectra computed at B3LYP/ LANL2DZ level are given in Figs 8 and 9 for visual comparison. 13 | P a g e

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Table 5

Fig.8. (a)) Experimental FT-Raman spectra of CBLPM and (b) Simulated Raman spectra of CBLPM Pyrrolidine Ring Vibrations Pyrrolidine ring stretching modes, ring bending modes and symmetrical ring-breathing mode appear at 1100 - 800 cm-1andbelow800 cm-1 respectively [37]. In CBLPM, pyrrolidine ring stretching mode appears in IR as very strong bands at 1033, 918 cm-1 and in Raman as medium intensity band at 1036 and 915 cm-1. The bending mode is observed in IR at 634cm-1in Raman at 640 and 540cm-1which has a lower wavenumber due to this deformation modes related with CH2 and NH2group. Methylene Vibrations Asymmetric, symmetric, scissoring, and wagging of CH2 group vibrations appear in theregions 3000 ± 50, 2965 ± 30, 1455 ± 55 and 1350 ± 85 cm-1respectively [38-40]. In CBLPM, CH2 asymmetric stretching bands of the pyrrolidine ring is observed in IR at 2994 as a weak band and the counterpart in Raman at 2994 and 2968 cm-1 as strong bands.CH2Symmetric stretching bands of the pyrrolidine ringare observed as a weak band at 2881cm-1 and in Raman as strong 14 | P a g e

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bands at 2922 and 2910 cm-1.The rocking vibrations are observed in IR as a very strong band at 1325 cm-1and wagging is observed as a very strong in IR at 1171 cm-1and the counterpart in Raman appears as weak bands at 1238 and 1187 cm-1. The lowering of the wavenumbers and the increase in the intensity of CH2 stretching modes are due to back donation in the ring which is caused by the presence of nitrogen atom in the ring. The methylene group makes a red shift position and amends the intensity of CH stretching and bending modes. The back donation from the lone pair of nitrogen to the σ antibonding orbital of the CH bonds causes weakening of the bonds, Thus C-H force constants are increasing and results in the enhancement of IR band intensity of C-H stretching modes.

Fig.9. (a)) Experimental FT-IR spectra of CBLPM and (b) Simulated Infrared spectra of CBLPM Hydroxyl Group Vibrations In H2O group symmetric and asymmetric stretching vibrations are expected to occur in the region 3400-3500 cm-1[41]. These bands are observed as a strong band in IR at 3107 cm-1 and in 15 | P a g e

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Raman at 3212 cm-1. The red shifting depicts the intermolecular N-H…O and O-H…Br hydrogen bonding interactions. Here the strengthening of OH bond and its contraction on the basis of polarization and conjugation within the molecule exists which is significant in the enhancement of NLO activity of CBLPM. Amino Group Vibrations The asymmetric and symmetric stretching vibrations of aromatic NH2 groups are expected to occur in the region around 3500 cm−1and 3400 cm−1 respectively [38-40]. In CBLPMNH2asymmetric stretching vibrations are observed at 2973 and 2663 cm-1. The symmetric stretching vibrations are observed in IR as a medium intensity band at 2719 cm-1 and in Raman as a very strong band at 2663 cm-1. Lowering of this mode indicates the presence of N–H…O intermolecular hydrogen bonding. The NH2 bending modes are found in the range 1650–1580 cm-1 [42]. In the title molecule, the NH2 bending wavenumbers are observed around the wavenumber of 1665 cm-1. NH2scissoring mode generally appears in the region 1650–1580 cm−1[43] which is observed as a very strong IR band at 1539 cm-1 and as a weak Raman band at 1536 cm-1. The observed band at 1477 cm−1 in Raman spectra attributed to NH2 wagging vibrations. Carboxylate Group Vibration In CBLPM, the cadmium metal is bonded to carboxylate through ionic bonding interaction and produce asymmetric and symmetric stretching vibrations which predict the binding mode of the ligand. Asymmetric and symmetric stretching vibrations of carboxylate ion is expected to occur in the region1650-1550 cm-1and near 1400 cm-1[43-44].Asymmetric stretching vibration is observed as a very strong IR band at 1590 cm-1and its counterpart is observed as a weak band in Raman at 1612 cm-1 and symmetric stretching vibration is observed as a medium intensity band in IR at 1365cm-1 with its counterpart in Raman at 1356 cm-1.Lowering of these vibrations is owing to the interaction of the lone pair oxygen atom of carboxylate ion with the Cadmium of Cadmium Bromide as a consequence of metal coordination. Symmetric deformation vibration of hydrogen atom attached to bromine atom was observed as a very strong band at1569 cm-1in IR and as a weak band at 1536 cm-1 in Raman. 5. Conclusions Nonlinear optical single crystals CBLPM have been grown successfully by slow evaporation solution growth technique at room temperature. The crystal structure and cell 16 | P a g e

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parameters were refined by Rietveld analysis and confirmed that the metal-semi organic molecules crystalize in the orthorhombic system with space group P212121. The optimized geometry reveals that CBLPM has distorted shape with nitrogen atom of pyrrolidine ring and carboxylic group

pointing towards the central cadmium atom due to the presence of O-H…Br intermolecular hydrogen bonding interactions. The percentage of Br...Cd interactions comprise 9.5% of the total Hirshfeld surfaces. The vibrational spectral investigation predicts the red shifting of O-H and N-H stretching wavenumbers which are due to O-H⋯O and N-H⋯O intermolecular interactions, which is evident from NBO analysis also. Lowering of wavenumber and the increase in intensity of CH2 stretching modes due to the properties of back donation in the ring which is caused by the presence of the nitrogen atom in the ring. Strengthening of OH stretching bond and its contraction on the basis of polarization and conjugation within the molecule is significant in the enhancement of NLO activity of CBLPM. HOMO – LUMO energy gap of 4.8261 eV, represents the optical band gap which nearly fits with the energy gap of 4.9 eV obtained from UV analysis. The Z-scan technique has

confirmed the enhancement of nonlinear absorption coefficient (𝛽), third order refractive index (𝑛2 ), third order susceptibility (𝜒3) and second order molecular hyperpolarizability (𝛾) in the CBLPM crystal. Acknowledgments D.Sajan (DS) thanks the UGC-DAE Consortium for Scientific Research, Mumbai Centre,R-5 Shed, Bhabha Atomic Research Centre, Trombay, Mumbai 400085 INDIA for the financial support (U DCSR/MUM/CD/CRS-M-238/2017/1007 dt.16.01.2017). The authors (DS and MG) also acknowledge the DST-FIST program (SR/FST/College-182/2013, November 2013 & FIST No.393 dated 25- 09–2014) to the Bishop Moore College Mavelikara for providing the UV visible and Computational facilities. The author (DS) is highly grateful to Prof. T. Sundius for the Molvib program and fruitful discussions on the Normal Coordinate Analysis (NCA).

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Author Contribution

Rejeena V. Rajan- Do All Works Lija K. Joy- Co -supervisor D. Sajan -Supervisor A.K. Thomas- Phd Student do computational help S. Sathiskumar- Help for sample preparation T. Balakrishnan- Help for sample preparation , G. Vinitha- Z-scan experimental facility provided

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Rejeena V. Rajan- Do All Works Lija K. Joy- Co -supervisor D. Sajan -Supervisor A.K. Thomas- Phd Student do computational help S. Sathiskumar- Help for sample preparation T. Balakrishnan- Help for sample preparation , G. Vinitha- Z-scan experimental facility provided

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Highlights   

The Cadmium (II) Dibromide L - Proline Monohydrate single crystal was synthesized by slow evaporation technique. Detailed vibrational Spectral investigation was carried out using normal coordinate analysis following the scaled quantum mechanical force field methodology. Third-order nonlinear properties of open- and closed aperture were exhibited selffocus and self-defocus nonlinear behavior.

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Table 1 Optical parameters of CBLPM for B3LYP/LANL2DZ Bond length Br1- Cd19 O2-C7 O2- Cd19 O3-C7 N4-H5 N4-H6 N4-C8 N4-C16 H6…O21 C7-C8 C8-H9 C8-C10 C10-H11 C10-H12 C10-C13 C13-H14 C13-H15 C13-C16 C16-H17 C16-H18 Cd19 - Br20 O21-H22 O21-H23

Theoretical 2.679 1.301 2.151 1.269 1.048 1.063 1.53 1.514 1.618 1.554 1.092 1.557 1.093 1.096 1.551 1.094 1.096 1.541 1.095 1.093 2.555 0.994 0.974

Bond Angle C7 -O2- Cd19 O2- C7 -O3 Br1- Cd19- O2 O2- Cd19- Br20 H5- N4-C16 H6- N4-C8 H6 -N4-C16 C8 -N4-C16 N4- H6…O21 O3- C7-C8 N4-C8-C7 N4-C8-H9 N4- C8-C10 C7-C8-H9 C7- C8-C10 H9 -C8-C10 C8- C10-H11 C8- C10-H12 C8- C10-H13 C10-C13-H14 C10-C13-H15 C10-C13-C16 N4-C16-C13 H15-C13-C16 C13-C16-H17 C13-C16-H18 Br1- Cd19- O2 Br1- Cd19- Br20 O2- Cd19- Br20 N4-C16-H17 N4-C16-H18

Theoretical 135.436 128.098 99.8504 125.531 111.546 108.748 115.676 108.111 161.316 116.899 105.884 109.331 105.429 109.081 114.486 112.281 111.132 109.895 104.958 112.505 110.76 103.502 103.182 110.191 111.742 114.41 99.8504 134.285 125.531 108.156 109.791

Dihedral Angles Cd19-O2-C7-O3 Cd19-O2-C7-C8 C7-O2-Cd19-Br1 C7-O2-Cd19-Br20 H5-N4-H6…O21 C8-N4-H6…O21 C16-N4-H6…O21 H5-N4-C8-C7 H5-N4-C8-C10 H6-N4-C8-C7 H6-N4-C8-H9 H6-N4-C8-C10 C16-N4-C8-C7 C16-N4-C8-H9 C16-N4-C8-C10 H5-N4-C16-C13 H6-N4-C16-C13 C8-N4-C16-C13 N4-H6…O21-H22 N4-H6…O21-H23 O2-C7-C8-N4 O2-C7-C8-H9 O2-C7-C8-C10 O3-C7-C8-N4 O3-C7-C8-H9 O3-C7-C8-C10 C10-C13-C16-N4 C8-C10-C13-C16 C7-C8-C10-C13 N4-C8-C10-C13 N4-C8-C10-H11

Theoretical 94.652 -84.98 24.957 -160.9 -92.34 18.729 140.56 13.642 -108.1 -102.2 15.188 136.09 131.47 -111.1 9.766 81.493 -152.4 -30.24 5.9449 172.17 162.25 44.698 -82.06 -17.42 -135 98.272 38.667 -32.92 -101.4 14.522 136.57

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Table 2: Second order perturbation theory analysis of CBLPM Fock matrix in NBO basis.

π (C11–C12) π (C11–C12) π(C13–C14) π(C13–C14) π(C10–C15) π(C10–C15) n4(Cd19) n5(Cd19) n2(Br1)

ED(i) (e) 1.97451 1.97451 1.98116 1.98116 1.97487 1.97487 1.99953 1.99808 1.97067

n3(Br1)

1.95425

n3(Br1) n1(Br1) n2(O21) n1(O3) n2(O3)

1.95425 1.98962 1.95272 1.97017 1.86853

Donor (i)

Acceptor (j) π*(C10-C15) π*(C13-C14) π*(C10–C15) π*(C11–C12) π*(C11–C12) π*(C13–C14) π*(O2-C7) π*(O3-C7) π*(C8-H9) π*(C3-H7) π*(C8-H9) π*(C8-C10) π*(O21-H22) π*(O21-H22) π*(N4-H6) π*(N4-H5) π*(N4-H5)

E(2)a (kcal.mol-1) 17.61 19.39 24.39 20.15 22.09 15.73 0.19 0.07 0.06 0.26 0.07 0.12 8.75 0.47 25.19 1.94 7.89

ED (j) (e) 0.36394 0.29596 0.36394 0.31706 0.31706 0.29596 0.09058 0.31180 0.01604 0.31180 0.01604 0.02031 0.02556 0.02556 0.05171 0.01604 0.01604

E(j)-E(i)b (a.u) 0.28 0.29 0.27 0.27 0.28 0.30 1.19 0.72 0.87 0.40 0.94 0.74 0.89 1.08 1.14 1.19 0.79

F(i,j)c (a.u) 0.063 0.068 0.072 0.067 0.071 0.061 0.014 0.007 0.006 0.010 0.007 0.008 0.079 0.020 0.151 0.043 0.072

Table : 3 UV–vis excitation energy and oscillator strength for CBPLM calculated by TD-CAM-B3LYP Energy Wavelength(nm) (cm-1) Experimental Theoretical 47745 49042 49058

204

209 203 203

Osc. Strength

Symmetry

Major contributions

0.0012 0.0234 0.0216

Singlet-A Singlet-A Singlet-A

HOMO LUMO (98%) H-1LUMO (99%) H-2LUMO (99%)

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Table.4 Nonlinear optical parameters obtained from Z-scan measurement data for CBLPM crystal. Laser beam wavelength (𝜆)

532 nm

Focal length of Lens(f)

103 mm

Power of laser input

100 mW

Optical path distance (Z)

675 mm

Power of the output beam of laser

3.47 kW/cm2

Beam radius at the aperture(ωa)

3.5 mm

Radius of the laser beam (ωL)

1.25 mm

Radius of aperture (ra)

1.25 mm

Linear refractive index(n0)

1.32745

Linear absorption coefficient (α)

0.084

Nonlinear refractive index (n2)

6.68x10-10 cm2/W

Nonlinear absorption coefficient (β)

2.2422x10-5cm/W

Real part of third-order susceptibility [Re(χ3)]

2.53x10-7 esu

Imaginary part of third-order susceptibility [Im(χ3)]

1.33x10-7 esu

Third-order nonlinear optical susceptibility (χ3)

2.86x10-7 esu

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Table 5 Experimental FT-IR, Raman and theoretical wavenumbers in (cm−1) assignments and PED contributions of CBLPM crystal by NCA based on SQM force field calculations.

Wavenumbers (cm-1) Calculated (B3LYP/ LANL2DZ) 3426 3212 2994 2968 2922 2910 2909 2663 1665 1612 1536 1479 1425 1356 1324 1274 1254 1238 1224 1187 1159 1036 983 954 915 872 834 802 756 704 640 618 540 507 412

Infrared Raman 3107 s 2994 w

2881vw 2719 m 1590 vvs 1539 vvs 1428 vvs 1365 m 1325 vvs 1279 m

1171vs 1033 vvs 918 vs 832 vvs 766 s 634 s 470 vs

3212 vvs 2994 s 2968 s 2922 vs 2910 vvs

Band Assignments

1274 s 1254 w 1238 vw 1224 vs

νASOH( 68), νASOH( 32) νASOH ( 64), νASOH ( 36) νASCH ( 96) νASCH ( 98) νSSCH ( 98) νSSCH ( 99) νSSCH ( 99) νSSNH ( 89), νSSO...H ( 10) 4 ( 48), 5 ( 30), ωNH ( 18) COO ( 93) O...BrSD ( 68), NHSCI ( 18), COROCK ( 17) ωHN ( 43), ωCC ( 35), βNH ( 52), βNHO ( 32) βNH (41), COO ( 23), βNHO (18), βCHN (18), βCC ( 55), βCH rock ( 20), βCHN (13) βNH ( 18), βCN ( 17), βCHN ( 13), βNHO ( 10) ωCN ( 26), βCN ( 15), CCAR ( 14), βCC ( 11) βCHN ( 31), βCC ( 24), ωCH ( 21)βNH( 14) O...BrSD ( 49), ωCN ( 17), XCYDE ( 11)

1187 w 1159 m 1036 m 983 m 954 vw 915 w 872 vs 834 s 802 m 756 m 704 w 640 vw 619 m 540 w 507 s 412 m

CH ( 42), βCC βCC ( 26), ωCN ( 22), βCHN ( 10) CCring( 60), CNAR( 38), CCAR( 20), ωCH ( 13), RDEO3 ( 12), T4 ( 35), ωNH ( 21), T5 ( 20) CCring ( 35), CNAR( 11) CCring ( 64), CNAR ( 11) ωCHN( 25), T4( 15) ωCH ( 34), CNAR ( 22), ωCC ( 12) ωCC ( 32), XCYDE ( 16), CCAR ( 14), ωCHN ( 13), CCWAG ( 48), ωCN ( 13), COCd ( 10) RDEF3(28), T4 ( 15), CCWAG (14), RDEO3( 13) ωCN ( 22), XCYDE ( 15), CCAR ( 11) ωCH (35), RDEO3( 26), RDEF3(15), ωCC( 14), OCROCK ( 62), βNH ( 17) CCROC ( 13), XCYDE ( 12), CCWAG ( 11)

2663 vs 1665 vw 1612w 1536 w 1477 m 1424 vw 1356 m

υ: stretching, γ: scissoring, ω: wagging, τ: torsion, Γ: twisting, β: in-plane bending, δ: deformation, ρ:rocking, s: symmetric, as: antisymmetric, vs:very strong, s: strong, ms: medium strong, w: weak.