Imino-pyridyl and PPh3 mixed ligand complexes of Cu(I)X (X: I, Br, and Cl): Synthesis, structure, DFT and Hirshfeld surface studies European Journal of Chemistry 11 (4) (2020) 334-341 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2020 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. http://dx.doi.org/10.5155/eurjchem.11.4.334-341.2037 European Journal of Chemistry View Journal Online View Article Online Imino-pyridyl and PPh3 mixed ligand complexes of Cu(I)X (X: I, Br, and Cl): Synthesis, structure, DFT and Hirshfeld surface studies Jahangir Mondal , Amit Kumar Manna and Goutam Kumar Patra * Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, 495009, India mailtobapi88@gmail.com (J.M.), amitmanna51@gmail.com (A.K.M.), goutam.patra@ggu.ac.in (G.K.P.) * Corresponding author at: Department of Chemistry, Guru Ghasidas Vishwavidyalaya, Bilaspur, Chhattisgarh, 495009, India. e-mail: goutam.patra@ggu.ac.in (G.K. Patra). 10.5155/eurjchem.11.4.334-341.2037 Received: 30 August 2020 Received in revised form: 10 October 2020 Accepted: 20 October 2020 Published online: 31 December 2020 Printed: 31 December 2020 Three new halide bridged copper(I)complexes [Cu2(µ-L)(µ-X)2)(PPh3)2]n {X: I (1), Br (2) and Cl (3)} have been synthesized by the reaction of Cu(I)X (X: I, Br and Cl) with PPh3 and the polydentate imino-pyridyl ligand L. Interestingly, copper(I) forms coordination polymers with the ligand L and the co-ligand PPh3. These complexes 1, 2 and 3 have been characterized by elemental analysis, IR, UV-Vis, and NMR spectroscopy. The crystal structure of the complex 2 has been determined by single-crystal X-ray analysis. Crystal data for complex 2: triclinic, space group P-1 (no. 2), a = 9.471(10) Å, b = 11.043(11) Å, c = 13.215(18) Å, α = 65.853(18)°, β = 69.94(2)°, γ = 67.350(14)°, V = 1135(2) Å3, Z = 2, T = 296.15 K, μ(MoKα) = 2.806 mm-1, Dcalc = 1.535 g/cm3, 4059 reflections measured (3.462° ≤ 2Θ ≤ 44.818°), 2639 unique (Rint = 0.0637, Rsigma = 0.1621) which were used in all calculations. The final R1 was 0.0700 (I > 2σ(I)) and wR2 was 0.2207 (all data). Hirshfeld surface analysis of the complex 2 showed H···H, N···H and Br···H interactions of 55.9, 14.4 and 4.1%, respectively. MEP of ligand L reflects the whole molecule is reddish yellow in color because of equally distributed electron density over the molecule. For this reason, the ligand is supramolecularly arranged via -{CuI2-µ-X2} rhomboid core in the complex 2. The ligand L is non-emissive at room temperature in dichloromethane, whereas the complexes 1, 2 and 3 are photoluminescent. DFT and Hirshfeld surface studies have also been performed for complex 2. DFT Imino-pyridyl ligand X-ray crystal structure Hirshfeld surface studies Bridged copper(I) complexes Copper(I) coordination polymer Cite this: Eur. J. Chem. 2020, 11(4), 334-341 Journal website: www.eurjchem.com 1. Introduction In recent years, crystal engineering has recognized the intermolecular covalent and non-covalent interactions in terms of self-assembly towards the design of novel functional materials [1-6]. Studies of copper(I) complexes with mixed ligand systems containing triphenyl phosphine and ligands having S and N donors have been increasing because of the flexibility of these ligands and the different steric charac- teristics of the phosphine ligands, which can modify the compound geometry [7,8]. Copper(I) halides have been used as the inorganic component in the building of novel coordination polymers with diverse structural motifs such as rhomboid fragment, cubane-like or chair-like fragment, one-dimensional ladder strand or zig-zag chain [9]. Cu(I)-complexes are well known for their enriched photoluminescence properties and their potential applications in solar energy conversion, light- emitting devices, luminescence-based sensors, and probes of biological systems [10-15]. Numerous Cu(I)-complexes such as discrete monomers, dimeric clusters and 1D coordination chains have been reported by several researchers [16-18]. Most of the complexes contain halide as the counter anion in combination with triphenylphosphine and N-heterocyclic molecules as the ligand. The electronic and structural characteristics of the N- heterocyclic ligands play an important role in determining the structure of the polynuclear complexes and consequently their photophysical properties [19-21]. In addition, the ancillary ligands such as halide and phosphine also have great effects on the emissive properties of the Cu(I)-complexes [22-25]. Analysis of the self-assembly for various intermolecular interactions like hydrogen bonding or π-π stacking is important to understand how molecules interact with their direct environment and focus on insight into crystal packing behavior. Hirshfeld surface-based tools appear as a novel approach to this end [26-31]. The central element in this method is the derivation of the Hirshfeld surface and immediately inter- pretable visualization of a molecule within its environment, and the decomposition of this surface to provide a directly accessible 2D map [32] as “molecular fingerprint”. In continuation of our studies on the Cu(I) coordination polymers of imino-pyridyl ligands, we have reported here the synthesis, structure, spectroscopic, photophysical, and theore- tical studies of halide-based Cu(I) complexes 1, 2 and 3 of a polydentate imino-pyridyl ligand (L) (Scheme 1). ABSTRACT RESEARCH ARTICLE KEYWORDS http://dx.doi.org/10.5155/eurjchem.11.4.334-341.2037 http://www.eurjchem.com/ http://dx.doi.org/10.5155/eurjchem.11.4.334-341.2037 mailto:mailtobapi88@gmail.com mailto:amitmanna51@gmail.com mailto:goutam.patra@ggu.ac.in mailto:goutam.patra@ggu.ac.in http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.11.4.334-341.2037&domain=pdf&date_stamp=2020-12-31 Mondal et al. / European Journal of Chemistry 11 (4) (2020) 334-341 335 2020 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.11.4.334-341.2037 N N NN L Scheme 1. Ligand (L) system used in this study. 2 CuX + 2 PPh3 Cu Cu X X PPh3Ph3P NN N NCu Cu X X PPh3 Ph3P (X: I, Br, Cl) CH2Cl2 RT (1) X: I (2) X: Br (3) X: Cl NN N N NN N N (L) 2:2:1 Scheme 2. Synthetic route of the complexes 1, 2 and 3. 2. Experimental 2.1. Materials and physical measurements All chemicals used in this study were purchased from Aldrich Chemical Company, USA and Acros Chemical Company, USA, and used without further purification unless otherwise mentioned. The melting point was determined by an electro- thermal IA9000 series digital melting point apparatus and is uncorrected. Microanalyses were carried out using a Perkin- Elmer 2400II elemental analyzer. Infra-red (IR) spectra and solution electronic spectra were recorded on Nicolet Magna IR (Series II) and Shimadzu UV-160A spectrophotometers, respectively. 1H NMR and 13C NMR and electro-spray ionization mass (ESI-MS) measurements were made using a Bruker Advance 400 MHz and Finnigan LCQ Decaxp MAX mass spectrometer, respectively. X-ray powder patterns are collected on a Philips PW-1710 automated diffractometer. Fluorescence spectra were recorded on a Perkin Elmer LS55 Luminescence Spectrometer. 2.2. Synthesis of the imino-pyridyl ligand (L) The imino-pyridyl ligand (L) has been prepared by following a reported procedure [33]. 1 mL (15 mmol) of distilled ethylenediamine and 2.86 (30 mmol) of freshly distilled 4-pyridinecarboxaldehyde were refluxed in 50 mL of anhydrous methanol for 6 h. On evaporating the solvent, a slightly yellow semi-solid was obtained which on recrystallization from n-hexane gave colourless needles suitable for X-ray analysis. (1E, 1'E)-N, N'-(ethane-1, 2-diyl) bis(1-(pyridin-4-yl)methan imine) (L): Color: Colorless. Yield: 61%. M.p.: 125-128 °C. FT-IR (KBr, ν, cm-1): 1648 (C=N). 1H NMR (400 MHz, CDCl3, δ, ppm):8.66 (d, J = 4 Hz, 4H, Ar-H), 8.25 (s, 2H, CH), 7.53 (d,J= 4 Hz, 4H, Ar-H), 4.03 (s, 4H, CH2). 13C NMR (200 MHz, CDCl3, δ, ppm): 160.84, 150.42, 142.63, 121.83, 61.27. Anal. calcd. for C14H14N4: C, 70.57; H, 5.92; N, 23.51. Found: C, 70.43; H, 5.92; N, 23.62%. UV/Vis (CH3OH, λmax, nm, (ε)): 236 (27.30). 2.3. Synthesis of [Cu2(µ-L)(µ-I)2(PPh3)2]n (1) To a solution of triphenylphosphine (0.262 g, 1 mmol) in CH2Cl2 (60 mL), solid cuprous iodide (0.19 g, 1 mmol) was added. The reaction mixture was stirred for 1 h to get a clear colorless solution. The 30 mL CH2Cl2 solution of the ligand, L (0.119 g, 0.5 mmol) was added with constant stirring for another 1h at room temperature to give a clear reddish solution. The solution was evaporated to dryness to obtain a reddish-yellow solid. It was washed with 5 mL of CH2Cl2 (Scheme 2). [Cu2(µ-(1E, 1'E)-N, N'-(ethane-1, 2-diyl)bis(1-(pyridin-4-yl)met hanimine))(µ-I)2 (PPh3)2]n (1): Color: Reddish yellow. Yield: 85%. M.p.: >200 °C. FT-IR (KBr, ν, cm-1): 1610 (C=N). Anal. calcd. for C50H44Cu2I2N4P2: C, 52.51; H, 3.88; N, 4.90. Found: C, 52.63; H, 4.13; N, 4.71%. UV/Vis (CH3OH, λmax, nm, (ε)): 370 (2.33). 2.4. Synthesis of [Cu2(µ-L)(µ-Br)2(PPh3)2]n (2) To a solution of triphenylphosphine (0.262 g, 1 mmol) in CH2Cl2 (60 mL), solid cuprous bromide (0.144 g, 1 mmol) was added. The reaction mixture was stirred for 1 h to get a clear orange solution. The 30 mL CH2Cl2 solution of the ligand, L (0.119 g, 0.5 mmol) was added with constant stirring for another 1h at room temperature to give a clear orange solution. The solution was evaporated to dryness to obtain orange solid. It was washed with 5 mL of CH2Cl2. Bright red single crystals of the complex, suitable for X-ray analysis were obtained by direct diffusion of hexane to the CH2Cl2 solution of the yellow solid (Scheme 2). [Cu2(µ-(1E, 1'E)-N, N'-(ethane-1, 2-diyl)bis(1-(pyridin-4-yl)met hanimine))(µ-Br)2(PPh3)2]n (2): Color: Bright red. Yield: 89%. M.p.: >200 °C. FT-IR (KBr, ν, cm-1): 1607 (C=N). Anal. calcd. for 336 Mondal et al. / European Journal of Chemistry 11 (4) (2020) 334-341 2020 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.11.4.334-341.2037 C50H44Cu2Br2N4P2: C, 57.21; H, 4.22; N, 5.34. Found: C, 56.98; H, 4.33; N, 5.27%. UV/Vis (CH3OH, λmax, nm, (ε)): 366 (2.32). 2.5. Synthesis of [Cu2(µ-L)(µ-Cl)2(PPh3)2]n (3) To a solution of triphenylphosphine (0.131 g, 0.5 mmol) in CH2Cl2 (30 mL), solid cuprous chloride (0.049 g, 0.5 mmol) was added. The reaction mixture was stirred for 1 h to get a clear reddish solution. The 20 mL CH2Cl2 solution of the ligand, L (0.60 g, 0.25 mmol) was added with constant stirring for another 1h at room temperature to give a clear red solution. The solution was evaporated to dryness to obtain a reddish-yellow solid. It was washed with 5 mL of CH2Cl2 (Scheme 2). [Cu2(µ-(1E,1'E)-N,N'-(ethane-1,2-diyl)bis(1-(pyridin-4-yl)met hanimine))(µ-Cl)2(PPh3)2]n (3): Color: Reddish Yellow. Yield: 80%. M.p.: >200 °C. FT-IR (KBr, ν, cm-1): 1619 (C=N). Anal. calcd. for C50H44Cu2Cl2N4P2: C, 62.50; H, 4.62; N, 5.83. Found: C, 62.48; H, 4.73; N, 5.74%. UV/Vis (CH3OH, λmax, nm, (ε)): 360 (2.32). 2.6. X-Ray crystallography X-ray single-crystal data are collected using MoKα (λ = 0.7107 Å) radiation on a BRUKER APEX II diffractometer equipped with CCD area detector. Data collection, data reduction, structure solution/refinement are carried out using the software package of SMART APEX [34]. The structures are solved by direct methods (SHELXS-97) and standard Fourier techniques, and refined on F2 using full-matrix least-squares procedures (SHELXL-97) using the SHELX-97 package [35] incorporated in WinGX [36]. Generally, non-hydrogen atoms are considered anisotropically. Whenever possible, the hydrogen atoms are located on a difference Fourier map and refined. In other cases, the hydrogen atoms are geometrically fixed. 2.7. Theoretical calculations Gaussian 09 program [37] has been used for the quantum chemical calculations. The possible ground state structures have been optimized with density functional theory (DFT) at B3LYP/6-311G**and B3LYP/LANL2DZ. GaussView 5 program [38] was used for the visualization of the studied systems. 2.8. Hirshfeld surfaces calculations For obtaining additional insight into the intermolecular interaction of molecular crystals, Hirshfeld surface analysis helps as a powerful set-up. The size and shape of Hirshfeld surface allow the qualitative and quantitative study and imagining of intermolecular close contacts in molecular crystals [39]. The Hirshfeld surface enclosing a molecule is defined by a set of points in 3D space where the contribution to the electron density from the molecule of interest is equal to the contri- bution from all other molecules. Molecular Hirshfeld surfaces are built based on electron distribution calculated as the sum of spherical atom electron densities [27,40]. Thus, an iso-surface is obtained, and for each point of the iso-surface, two distances can be defined: de, the distance from the point to the nearest atom outside to the surface, and di, the distance to the nearest atom inside to the surface. Furthermore, the identification of the regions of particular importance to intermolecular interactions is achieved by mapping normalized contact distance (dnorm), expressed as: dnorm = (di-rivdw)/rivdw+(de-revdw)/ revdw; where rivdw and revdw are the van der Waals radii of the atoms [26]. The value of dnorm is negative or positive when intermolecular contacts are shorter or longer than rvdw, respectively. The graphical plots of the molecular Hirshfeld surfaces mapped with dnorm employ the red-white-blue color scheme, where red color indicates the shorter intermolecular contacts, white color shows the contacts around the rvdw separation, and blue color is used to point out the longer contact distances. Due to the symmetry between de and di in the expression for dnorm, where two Hirshfeld surfaces touch, both will display a red spot identical in color intensity as well as size and shape [41]. The mixture of de and di in the form of a 2D fingerprint plot provides a summary of intermolecular contacts in the crystal and are in complement to the Hirshfeld surfaces [26]. The information about the intermolecular interactions in the immediate environment of each molecule in the asymmetric unit is achieved by such plots. In addition, the close contacts between particular atom types can be highlighted in so-called resolved fingerprint plots [28], which allow the facile assignment of an intermolecular contact to a certain type of interaction and quantitatively summarize the nature and type of intermolecular contacts. Two additional colored properties (shape index and curvedness) based on the local curvature of the surface can also be specified [42]. The Hirshfeld surfaces are mapped with dnorm, shape-index, curvedness and 2D fingerprint plots (full and resolved) reported in this manuscript were generated using Crystal-Explorer 3.1 [43]. 2.9. Molecular electrostatic potential (MEP) The molecular electrostatic potential at a given point around a molecule can be defined in terms of total charge distribution of the molecule and relates with dipole moments. It supplies a method to understand the electron density which is useful for determining the electrophilic reactivity and nucleophilic reactivity along with hydrogen-bonding interac- tions [44,45]. 3. Results and discussion 3.1. Syntheses of the complexes 1, 2 and 3 Imino-pyridyl ligand L is a 1+2 condensate of ethylene diamine and 4-pyridinecarboxaldehyde. The Cu(I) complexes 1, 2 and 3 have been synthesized in good yields by reacting L with Cu(I)X (X: I, Br and Cl) and PPh3 in 1:2:2 proportion in dichloromethane solvent at room temperature. The syntheses of the complexes 1, 2 and 3 are summarized in Scheme 2. These complexes are stable in the solid state for about 2-3 weeks in air. 3.2. Structural description of the complex 2 Single crystal XRD analysis unveils that complex 2 is crystallized in a triclinic P-1space group and it is a 1-D coordination polymer. The molecular diagram of the complex is shown in Figure 1. The crystallographic details of complex 2 have been summarized in Table 1, bond lengths and angles of complex 2 have been listed in Table 2 and 3, respectively. Compound 2 consists of two independent Cu(I) ions, two bromide anions, and two triphenylphosphine units with one imino-pyridyl ligand L. Each Cu(I) cation shows four coordinated distorted tetrahedral geometry. The three phosphorus-bound benzene rings make dihedral angles of 59.5(2), 70.3(2), and 85.5(2)° with each other. The Cu atom displays tetra-coordination, excluding the central Cu···Cu contact of 2.989 Å, and is coordinated by one phosphorus atom (Cu-P = 2.207(5) Å), one nitrogen atom (Cu-N = 2.041(12) Å) and two bromine atoms (Cu-Br = 2.495(3) and 2.514(3) Å) in a distorted tetrahedral geometry. The angles around Cu atom are 104.1(4)° (Br1-Cu2-N1), 102.5(3)° (Br1i-Cu2-N1), 109.53(15)° (P1-Cu2-Br1), 110.78(14) (P1-Cu2-Br1i), 122.0(4)° (P1-Cu2- N1) and (symmetry code: i 1-x, 1-y, -z). In the crystal packing, there is no intramolecular hydrogen bonding. There exists π-π stacking (distance 3.457 Å). Polymeric view of [Cu2(µ-L)(µ- Br)2(PPh3)2]n (2) has been shown in Figure 2. Mondal et al. / European Journal of Chemistry 11 (4) (2020) 334-341 337 2020 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.11.4.334-341.2037 Table 1. Crystallographic data and refinement parameters for complex 2. Compound [Cu2(µ-L)(µ-Br)2(PPh3)2]n Empirical formula C25H22BrCuN2P Formula weight 524.86 Temperature (K) 296.15 Crystal system Triclinic Space group P-1 a (Å) 9.471(10) b (Å) 11.043(11) c (Å) 13.215(18) α (°) 65.853(18) β (°) 69.94(2) γ (°) 67.350(14) Volume (Å3) 1135(2) Z 2 ρcalc (g/cm3) 1.535 μ (mm-1) 2.806 F(000) 530.0 Crystal size (mm3) 0.24 × 0.22 × 0.20 Radiation MoKα (λ = 0.71073) 2Θ range for data collection (°) 3.462 to 44.818 Index ranges -10 ≤ h ≤ 9, -10 ≤ k ≤ 11, -14 ≤ l ≤ 14 Reflections collected 4059 Independent reflections 2639 [Rint = 0.0637, Rsigma = 0.1621] Data/restraints/parameters 2639/0/259 Goodness-of-fit on F2 0.945 Final R indexes [I≥2σ (I)] R1 = 0.0700, wR2 = 0.1685 Final R indexes [all data] R1 = 0.1688, wR2 = 0.2207 Largest diff. peak/hole / (e Å-3) 0.98/-0.58 CCDC no. 1027695 Figure 1. Crystal structure of [Cu2(µ-L)(µ-Br)2(PPh3)2]n (2). Thermal ellipsoids are drawn at the 50% probability level. Figure 2. Polymeric view of [Cu2(µ-L)(µ-Br)2(PPh3)2]n (2). H atoms are omitted for clarity. 338 Mondal et al. / European Journal of Chemistry 11 (4) (2020) 334-341 2020 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.11.4.334-341.2037 Table 2. Bond distances (Å) for complex 2. Atom Atom Length (Å) Atom Atom Length (Å) Br1 Cu2 2.495(3) C8 C9 1.34(2) Br1 Cu21 2.514(3) C9 C10 1.36(2) Cu2 Br11 2.514(3) C10 C11 1.39(2) Cu2 Cu21 2.989(5) C12 C17 1.429(19) Cu2 P1 2.207(5) C12 C13 1.347(18) Cu2 N1 2.041(12) C17 C16 1.39(2) P1 C6 1.802(15) C13 C14 1.358(19) P1 C12 1.790(16) C15 C14 1.41(2) P1 C18 1.814(16) C15 C16 1.35(2) N1 C5 1.306(17) C20 C19 1.41(2) N1 C1 1.360(17) C20 C21 1.32(2) C5 C4 1.381(19) C18 C23 1.39(2) C2 C3 1.363(18) C18 C19 1.41(2) C2 C1 1.378(19) C23 C22 1.38(2) C3 C4 1.347(19) C21 C22 1.36(2) C3 C25 1.476(18) C25 N2 1.257(16) C6 C7 1.388(18) N2 C 1.439(16) C6 C11 1.394(18) C C2 1.48(2) 11-x, 1-y, -z; 2 3-x, -y, -z. Table 3. Bond angles (°) for complex 2. Atom Atom Atom Angle (°) Atom Atom Atom Angle (°) Cu2 Br1 Cu21 73.25(11) C7 C6 P1 127.6(12) Br1 Cu2 Br11 106.75(11) C7 C6 C11 116.7(14) Br11 Cu2 Cu21 53.08(7) C11 C6 P1 115.4(12) Br1 Cu2 Cu21 53.67(10) C9 C8 C7 119.2(15) P1 Cu2 Br1 109.53(15) C8 C7 C6 123.2(15) P1 Cu2 Br11 110.78(14) C8 C9 C10 120.6(17) P1 Cu2 Cu21 125.28(16) C9 C10 C11 120.9(16) N1 Cu2 Br1 104.1(4) C10 C11 C6 119.2(14) N1 Cu2 Br11 102.5(3) C17 C12 P1 121.7(14) N1 Cu2 Cu21 112.7(3) C13 C12 P1 121.1(14) N1 Cu2 P1 122.0(4) C13 C12 C17 117.2(15) C6 P1 Cu2 116.6(5) C16 C17 C12 119.8(15) C6 P1 C18 102.7(7) C12 C13 C14 124.6(15) C12 P1 Cu2 111.2(6) C16 C15 C14 121.4(17) C12 P1 C6 104.6(7) C13 C14 C15 117.2(16) C12 P1 C18 103.4(7) C15 C16 C17 119.8(16) C18 P1 Cu2 116.8(5) C21 C20 C19 119.9(18) C5 N1 Cu2 125.2(12) C23 C18 P1 123.8(13) C5 N1 C1 114.3(14) C23 C18 C19 119.8(15) C1 N1 Cu2 118.3(12) C19 C18 P1 116.4(14) N1 C5 C4 125.4(16) C22 C23 C18 119.4(18) C3 C2 C1 119.5(14) C18 C19 C20 118.2(16) C2 C3 C25 119.7(14) C20 C21 C22 122.6(18) C4 C3 C2 117.8(15) C21 C22 C23 120(2) C4 C3 C25 122.4(16) N2 C25 C3 122.0(15) N1 C1 C2 123.4(16) C25 N2 C 118.2(13) C3 C4 C5 119.2(16) N2 C C2 110.8(15) 11-x, 1-y, -z; 2 3-x, -y, -z. Figure 3. π-π Stacking in complex 2. The 1-D coordination chains are connected by supra molecular π···π interactions to form a 3D supramolecular structure (Figure 3). 3.3. Theoretical investigations B3LYP correlation function by the Gaussian 09 package using a DFT method has been performed to further understand the electronic structure of the ligand L. The optimized structure of ligand L has been displayed in Figure 4(a). The energies of some selected molecular orbits for L are shown in Figure 4(b). The energy gaps between HOMO-LUMO, (HOMO-1)-(LUMO+1), (HOMO-2)-(LUMO+2) are 8.2027, 8.3160 and 9.1338 eV for the ligand molecule. The HOMO-LUMO energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital for ligand L is 5.30 eV. Mondal et al. / European Journal of Chemistry 11 (4) (2020) 334-341 339 2020 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.11.4.334-341.2037 (a) (b) LUMO (E = -1.599 eV) HOMO (E = -6.800 eV) Figure 4. (a) Optimized molecular structure of the ligand L; (b) Surface plots of HOMO and LUMO of the ligand L. (a) (b) (c) Figure 5. Hirshfeld surfaces of complex 2. (a) 3D dnorm surface, (b) surface index, (c) curvedness. (a) (b) (c) (d) Figure 6. 2D fingerprint plots of complex 2. (a) standard full and (b) resolved into H···H and (c) resolved into N···H (d) Br···H contacts, showing the percentages of contacts contributing to the total Hirshfeld surface area of the molecule. This value is a useful quantity for examining the kinetic stability. A large energy gap corresponds to a high energy required for electron excitation. 3.4. Molecular Hirshfeld surfaces The Hirshfeld surface is a suitable tool for describing the surface characteristics of molecules. The molecular Hirshfeld surfaces of complex 2 was generated using a standard (high) surface resolution with the 3D dnorm surfaces mapped over a fixed color scale of -0.22 (red) to 1.4 Å (blue). The shape index mapped in the color range of -0.99 to 1.0, and curvedness was in the range of -4.0 to 0.4. The surfaces were shown to be transparent to allow visualization of the molecular moiety in a similar orientation for all of the structures around which they were calculated. The molecular Hirshfeld surface (dnorm, Shape index, and Curvedness) of complex 2 is shown in Figure 5. The Hirshfeld surface analysis of the complex 2 showed H···H, N···H, and Br···H interactions of 55.9, 14.4, and 4.1% respectively, which revealed that the main intermolecular interactions were H···H intermolecular interactions. The N···H and Br···H interactions were represented by a small area in the left side, whereas the H···H interactions were represented by the largest region in the fingerprint plot (Figure 6) and thus had the most significant contribution to the total Hirshfeld surfaces (55.9%). 340 Mondal et al. / European Journal of Chemistry 11 (4) (2020) 334-341 2020 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.11.4.334-341.2037 L 1 2 3 Figure 7. Molecular electrostatic potential (MEP) of ligand (a) L; (b) 1, 2 and 3. Figure 8. Emission spectra of complexes 1, 2 and 3 in CH2Cl2 at room temperature, on excitation at 380 nm. 3.5. Molecular electrostatic potential (MEP) The potential increases in the order red