CHAPTER III SYNTHESIS, CHARACTERIZATION AND PHOTO-LUMINESCENT PROPERTIES OF SCHIFF BASE METAL COMPLEXES

Transcription

1 CHAPTER III SYNTHESIS, CHARACTERIZATION AND PHOTO-LUMINESCENT PROPERTIES OF SCHIFF BASE METAL COMPLEXES Schiff bases have been playing an important role in the development of coordination chemistry. Schiff base metal complexes have been studied extensively because of their wide range of applications in various fields. These are widely applicable for their catalytic activity in a large number of homogeneous and heterogeneous reactions. Schiff base was reported first of all, by Hugo Schiff in 1864 [1]. Schiff bases have a general formula with azomethine group as RHC=NR where R and R are alkyl, aryl, cyclo alkyl or heterocyclic groups which may be variously substituted. These can be prepared by condensation reaction of primary amines and carbonyl compounds in different solvents with the elimination of water molecules and resulting in formation of imines with a characteristic C=N double bond. The presence of dehydrating agents normally favours the formation of Schiff bases. MgSO 4 or Na 2 SO 4 are commonly employed as a dehydrating agent. Schiff bases are stable solids, though care should be taken in the purification steps as it undergoes degradation. Chromatographic purification of Schiff bases on silica gel is not recommended as they undergo hydrolysis. In such cases it is better to purify the Schiff base by crystallization methods. The nitrogen atom of azomethine group is sp 2 hybridized containing a lone pair of electrons. These are of chemical importance and impart excellent chelating ability when used in combination with one or more donor atoms close to the azomethine group. The chelating ability of Schiff bases combined with an ease of preparation and flexibility in varying chemical environment about the C=N group makes it an interesting ligand in co-ordination chemistry [2, 3]. The Schiff base compounds of salicylaldehyde derivative with diamine are N 2 O 2 compounds and so called as salen ligands. This term is originally used for salicyldehyde and ethylenediamine and now is used in the literature to describe the class of (O, N, N, O) tetradentate bis Schiff base ligands. Most common Schiff bases have NO or N 2 O 2 -donor atoms but the oxygen atoms can be replaced by sulphur, nitrogen, or selenium atoms. Stereogenic centres can also be introduced in the synthetic design of Schiff bases macrocyclic and supramolecular chemistry [4, 5]. Schiff base ligands form stable metal complexes with most transition metal ions

2 prepared by treating metal salts with Schiff base ligands under suitable experimental conditions. Cozzi [6] have reported various synthetic routes commonly employed for preparation of Schiff base metal complexes. There are numerous literature reviews on the synthesis and characterization of metal complexes [4, 7-9]. H 2 O H H O H 2 N R' N R' R R Aldehyde or ketone Primary amine Schiff base Figure 3.1 Formations of Schiff-Bases H R O R' R O N H R"NH 2 H R O N H N H H R" R"NH 2 R' R" R' R" H R N R H O H N R" H N R" H R OH N H R' R" R' H R' R" R"NH 2 Figure 3.2 Mechanism of Schiff base synthesis Mechanism of condensation of carbonyl compounds with amines [5] Organic electroluminescent devices are useful in flat panel displays since Tang and Van Slyke reported on high performance organic electronic devices [10]. Their discovery was based on employing a multilayer device structure containing an emitting layer and a carrier

3 transport layer of suitable organic materials. Organic dyes, chelate metal complexes and polymers are the major categories of materials used in the fabrication of organic EL devices. Transition-metal complexes have been increasingly used in the design of functional molecular materials. In this regard, phosphorescent d 6 and d 8 metal complexes containing p- conjugated ligands with N and/or C donor atoms have been extensively studied and used in the development of high-performance organic light emitting diodes (OLEDs) [11-13]. Materials applications of transition-metal Schiff base complexes are less developed. The Schiff base ligands can be easily prepared and structurally modified. They have been demonstrated to have immense practical applications, such as in the development of metal catalysts for highly enantio-selective organic transformation reactions [14-19]. Schiff base complexes are now-a-days used as electroluminescent materials for flat panel displays [20, 21]. Schiff base complexes with transition and non-transition metals are used as promising materials for optoelectronic applications in flat panel displays due to their outstanding photo and electroluminescent (PL and EL) properties, and the ease of synthesis that readily allows structural modification for optimization of material properties [22-30]. Metal complexes offer many attractive properties, such as displaying a double role of electron transport and light emission, higher thermal stability and their ease of sublimation. Moreover, an attractive feature of these complexes is the ability to generate a much greater diversity of tunable properties and their color emission by virtue of the coordinated metal centre or by modifying the backbone substituents of ligands [20]. The introduction of different substituent resulted in tuning of the optical properties of the Schiff base complexes [31-34]. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are associated with the electron and hole transport properties of the substances. The highest occupied molecular orbitals (HOMO) are mainly localized on the oxygen atoms and lowest unoccupied molecular orbital (LUMO) are located on the nitrogen moieties of the salicylidene moiety of salen ligands. The energy gap between HOMO s and LUMO s changes by addition of different substituent on the ligands that is useful for color tuning purposes. Metal zinc complexes of Schiff base became attractive for their interesting fluorescent properties in particular; the salicylideneamine-zinc (II) complexes exhibit photoluminescence as well as electroluminescence [22-29]. Aluminum complexes with 8-hydroxy-quinoline and its derivatives as Alq 3 are excellent metal-chelate complexes widely used as emitting materials and electron transporting materials in OLEDs [10, 35-37].

4 Salicylaldehyde schiff bases are similar to 8-hydroxyquinoline in structure in which they have at least one hydroxyl group, a coordination nitrogen atom, and a delocalised π conjugated system [38]. Therefore, organic metal-chelate complexes of salicylaldehyde schiff base ligands also exhibit good luminescent properties [39 41]. Kim et. al. [40] had reported the photoluminescence (PL) and EL of the aromatic bridged azomethine metal complexes with beryllium, magnesium and zinc. The results indicated that some of the complexes have complicated structures which exist as dimer or dinuclear complexes due to its rigid conformation of the aromatic bridged structures. These complexes exhibit strong blue or blue-green emission, but they were insoluble in common organic solvents except for dimethylsulfoxide which has higher boiling point (b. p. 189 C), and most of them were difficultly sublimated in vacuum. Most of the complexes were difficultly used for fabricating EL devices by thermal vacuum deposition or spin-coating method. To improve the properties of solubility, stability and electron transporting capability of schiff base zinc complexes, Yu et. al. [42, 43] reported schiff base complexes in which the ligands were condensated from conjugated aromatic aldehydes and oxa-alkyldiamines (diglycolamine or triglycolamine). These complexes contained hetero atoms in flexible alkyl chain, and provide coordination atoms for metal ions, but also increase the polarity of the molecules. So the ligand backbone may play an important role in improving stability of the complexes compared with longer alkyl chain schiff base ligands. 2-Hydroxy-1-naphthaldehyde was used to increase the conjugated system of the complexes which can improve their electron transport ability. Photo physical properties of zinc (II) Schiff base complexes containing hetero atoms in flexible alkyl chain bridge have been reported by Yu et. al. [42, 43]. These complexes displayed excellent blue fluorescence both in solution and in the solid state. In the past several years, phosphorescent platinum (II) emitters were reported as electro-phosphorescent dopant materials [44-48]. The photo-physical and electroluminescent properties of platinum (II) Schiff base complexes were revealed which showed that they can be useful candidates for electro-phosphorescent high-performance OLED applications [49]. The three primary color red, green and blue are used for full color displays. The blue emitting materials are one of the color for the development of full color displays based either on the color changing medium technology or the RGB filtered white emission [50-56]. The efficiency, durability and high driven voltages for blue materials lead to poor performance of organic electro luminescent devices (OLEDs). It was much more difficult to obtain highly efficient blue-light emission due to its intrinsic characteristic of the large band gap of the emitting material. This problem could be overcome by choosing selective organic

5 compounds, dyes or polymers such as oxadiazole [57], cyclopentadiene derivatives [58], distylpyrazine [59], polyalkylfluorene [60], poly (p-phenylene) [61], polypyridines [62] which emit blue light. Many blue fluorescent dyes had been explored which exhibited excellent performance, such as distyrylarylene derivatives and the common way to improve efficiency and stability of OLEDs was to use the fluorescent dye doping technique [59, 63, 64]. However, the doping method was rather inconvenient for device fabrication because it was difficult to control the deposition rate in co-evaporation process and the doping ratio. In particular organic metal complexes have attracted a lot of attentions for OLEDs applications. However, the major disadvantages of metal-chelate complexes were the lack of suitable blue emitting complexes. The effect of conjugation enhancement, introduction of electron donating substituent, and complexation on the photo-luminescent and thermal properties of tetradentate Schiff bases of salicylaldehyde and o-vanillin derivatives and their zinc (II) complexes were well studied by Kotova et. al [65]. Peculiarities of the synthesis of the zinc (II) complexes with Schiff bases were considered. From density functional calculations it was confirmed that the luminescence of these Schiff base compounds was due to the π π* transition between orbitals of the organic ligand and enhancement of conjugation of the chain and introduction of electron donating substituents lead to a decrease of the energy gap and results in a bathochromic shift of the emission maximum. Molecular geometries of salen zinc complexes had been studied extensively [66-68]. The variability of PL and EL properties of Zn salen complexes could originate from the molecular conformational change [69]. Mixed ligand complexes of aluminium and gallium with tridentate ligands of Schiff bases were reported and these had better properties than Alq 3. Salicylidene-o-aminophenolato (8-quinolinato) aluminum had high efficiency as a host material for red light emitting devices. [70]. Binuclear gallium complex with mixed ligands based on tridentate ligands of a Schiff base and 8-hydroxyquinoline, bis(salicylidene-o-aminophenolato)-bis(8-quinolinato)- bis-gallium (III), exhibited a combination of both intra molecular and intermolecular interactions of the quinoline ligands, which was different from Gaq 3. The binuclear complex possessed higher luminescent efficiency and excellent thermal stability, therefore, had great potential as an active material for OLEDs [71]. Gallium complexes with N-salicylidene-oaminophenol (saph 2 ) Schiff bases, acac and alcohol acting as complex-stabilizing agents were prepared for their use to fabricate small molecules OLED devices. Here alcohol molecules might be lost during thermal evaporation but this did not affect functioning of the complexes as emitting layer in the construction of SMOLEDs. The crystallographic data of the complex

6 showed that the geometry of the complex was planar while PL data showed a dramatic enhancement of the emission intensity. Substitution of Methyl (EDG) or Br (EWG) saph 2 shifts the EL emission to larger or smaller wavelength, respectively, and both PL and EL intensities were enhanced by substitution [72]. Kawamoto and his co-workers reported the synthesis of Zn (II) and Cd (II) complexes with 2-substituted benzothiazolines. Zn (II) afforded tetrahedral mononuclear complexes, [Zn(R-Ph-C (H) =N-C 6 H 4 -S) 2 ] with a N 2 S 2 donor set and a distorted tetrahedral geometry. It has been found that the electronic properties of the substituents, as well as their positions on the pendent phenyl rings of the Schiff base ligands affect the electronic absorption spectra of the complexes. All the complexes were luminescent in CH 2 Cl 2 /toluene glass at 77 K [73]. Recently Wang et. al reported Schiff base zinc complexes with N-(2-hydroxybenzidene)-p-aminodimethylaniline and N-(2-hydroxy-1- naphthidene)-p-aminodimethylaniline, these complexes exhibited good solubility in organic solvents, excellent thermal stabilities and were suitably used for the fabrication of OLEDs. Keeping this in view, the use of metal complexes for the fabrication of small molecular organic light emitting devices and the suitability of Schiff base complexes for their use in these devices as emitting layer, we have synthesized some Schiff base ligands with salicylaldehyde and diamines. The metal complexes of these ligands with zinc and beryllium metals were prepared and characterized. The photo-physical properties of these metal complexes were studied and are presented in this chapter. 3.1 Synthesis of the ligands: The ligands Schiff bases were synthesized by the standard procedure by the reaction of salicylaldehyde with diamines under reflux in methanol [75]. The metal complexes were synthesized using general procedure. Schiff bases were synthesized using salicylaldehyde and amine derivatives as shown in scheme 3.1 to Bis(salicylidene)ethylene-1, 2-diammine [salen] The Schiff base was synthesized by the reaction of salicylaldehyde and ethylenediamine in 2:1 molar ratio in methanol. A solution of Salicylaldehyde (2 mmol) was taken in 20 ml methanol then a methanolic solution of ethylene-1, 2-diamine (1 mmol) was added dropwise with stirring to the reaction mixture. The mixture was stirred for 2 h at 60 o C.

7 Progress of the reaction was monitored by thin layer chromatography. The yellow colored precipitates were formed after completion of the reaction. The precipitates formed were filtered and washed with deionised water. The product was then purified by re-crystallisation with methanol and then dried in oven at 75 o C temperature. The yield of the product was very high ~ (87 %). The synthetic route is given below (Scheme 3.1): 2 CHO OH H 2 N(CH 2 ) 2 NH 2 Methanol, 60 o C 2 h CH=N OH N=HC HO Scheme 3.1 Synthetic route of [salen] Bis(salicylidene)propylene-1,3-diammine [salpen] A solution of Salicylaldehyde (2 mmol) was taken in 20 ml methanol then a methanolic solution of propylene-1, 3-diamine (1 mmol) was added dropwise with stirring to the reaction mixture. The mixture was stirred for 2 h at 60 o C. Progress of the reaction was monitored by thin layer chromatography. The yellow colored precipitates were formed after completion of the reaction. The precipitates formed were filtered and washed with deionised water. The product was then purified by re-crystallisation with methanol and then dried in oven at 75 o C temperature. The yield of the product was very high ~ (86 %). The reaction process is given below (Scheme 3.2): 2 CHO OH H 2 NCH 2 CH 2 CH 2 NH 2 Methanol, 60 o C 2 h CH=N OH N=HC HO Scheme 3.2 Synthetic route of [salpen] Bis(salicylidene)butylene-1,4-diammine [salbutene]

8 The Schiff base was synthesized as according to the process mentioned earlier. To a methanolic solution of Salicyldehyde (2 mmol) and methanolic solution of butylene-1, 4- diammine (1 mmol) was added dropwise with stirring. After 2 h of stirring at 60 o C yellow colored precipitates were formed. The precipitates formed were filtered and washed with deionised water. The product was then purified by re-crystallisation with methanol and then dried in oven at 75 o C temperature. The yield of the product was ~ (84 %). (CH 2 ) 4 2 CHO OH H 2 N(CH 2 ) 4 NH 2 Methanol, 60 o C 2 h CH=N OH N=HC HO Scheme 3.3 Synthetic route of [salbutene] Bis(salicylidene)hexylene-1, 6-diammine [salhexene] The Schiff base was synthesized by the reaction of salicylaldehyde and hexylene-1,6- diamine in 2:1 molar ratio in methanol. A solution of Salicylaldehyde (2 mmol) was taken in 20 ml methanol then a methanolic solution of hexylene-1, 6-diamine (1 mmol) was added dropwise with stirring to the reaction mixture. The mixture was stirred for 2 h at 60 o C. Progress of the reaction was monitored by thin layer chromatography. The yellow colored precipitates were formed after completion of the reaction. The precipitates formed were filtered and washed with deionised water. The product was then purified by re-crystallisation with methanol and then dried in oven at 75 o C temperature. The yield of the product was ~ (84 %). Scheme is shown below: (CH 2 ) 6 2 CHO OH H 2 N(CH 2 ) 6 NH 2 Methanol, 60 o C 2 h CH=N OH N=HC HO Scheme 3.4 Synthetic route of [salhexene] Bis(salicylidene)heptylene-1,7-diammine [salheptene]

9 In similar way Schiff base was synthesised by adding drop wise solution of heptylene- 1,7-diamine (1 mmol) in absolute methanol to a methanolic solution of salicylaldehyde (2 mmol) in a conical flask with magnetic stirring at 60 o C for 2 h. The product formation was checked by thin layer chromatography. The yellow colored precipitates formed were filtered and washed with deionised water. The compound was recrystallized with methanol and dried in oven. The yield of the product was ~ (83 %). Scheme is shown below here (Scheme 3.5): (CH 2 ) 7 2 CHO OH H 2 N(CH 2 ) 7 NH 2 Methanol, 60 o C 2 h CH=N OH N=HC HO Scheme 3.5 Synthetic route of [salheptene] 3.2 Synthesis of the zinc metal complexes: The zinc metal complexes were prepared with the above synthesized Schiff bases Bis(salicylidene)ethylene-1,2-diaminatozinc(II) [Zn(salen)] The metal complex was prepared by reaction of ligand bis (salicylidene) ethylene-1, 2-diamine (salen) with zinc acetate (ligand and metal) at 1:1 molar ratio in methanol. The Schiff base ligand (1 mmol) was taken in 50 ml methanol and heated on a magnetic stirrer at 60 o C for 1 hour. The aqueous solution of zinc acetate (1 mmol) was added drop wise to the flask with magnetic stirring. The mixture was kept at 60 o C temp for 2 hour on magnetic stirrer. After 2 h of stirring a cream colored precipitate of the complex separated from the reaction mixture which were filtered, washed with deionised water, ethanol and dried at 100 o C. The cream colored metal chelate gave bluish green light under UV lamp excitation source. CH=N OH N=HC HO Zn(ac) 2 60 o C, 2 h stirring CH=N O Zn N=HC O Scheme 3.6 Synthetic route of [Zn(salen)] Bis(salicylidene)propylene-1,3-diaminatozinc(II)

10 [Zn(salpen)] The metal complex was prepared by reaction of ligand bis (salicylidene) propylene-1, 3-diamine (salpen) with zinc acetate (ligand and metal) at 1:1 molar ratio in methanol. The Schiff base ligand (salpen) (1 mmol) was taken in 50 ml methanol and heated on a magnetic stirrer at 60 o C for 1 hour. The aqueous solution of zinc acetate (1 mmol) was added drop wise to the flask with magnetic stirring. The mixture was kept at 60 o C temp for 2 h on magnetic stirrer. After 2 h of stirring cream colored precipitate of the complex separated out from the reaction mixture which were filtered, washed with deionised water and ethanol and then dried at 100 o C. The cream colored metal chelate gave bluish green light under UV lamp excitation source. CH=N OH N=HC HO Zn(ac) 2 60 o C, 2 h stirring CH=N O Zn N=HC O Scheme 3.7 Synthetic route of [Zn (salpen)] Bis(salicylidene)butylene-1,4-diaminatozinc(II) [Zn(salbutene)] The metal complex was obtained by reaction of ligand and zinc acetate in methanol as above procedure. The Schiff base ligand (salbutene) (1 mmol) was taken in 50 ml methanol and heated on a magnetic stirrer at 60 o C for 1 hour. The aqueous solution of zinc acetate (1 mmol) was added drop wise to the flask with magnetic stirring. The mixture was kept at 60 o C temp for 2 h on magnetic stirrer. After 2 h of stirring cream colored precipitate of the complex separated out from the reaction mixture which were filtered, washed with deionised water and ethanol and then dried at 100 o C. The cream colored metal chelate gave bluish light under UV lamp excitation source Bis(salicylidene)hexylene-1,6-diaminatozinc(II) [Zn(salhexene)] The Schiff base ligand (salhexene) (1 mmol) was taken in 50 ml methanol and heated on a magnetic stirrer for at 60 o C for 1 hour. The aqueous solution of zinc acetate (1 mmol) was dropwise added to the flask with magnetic stirring. The mixture was kept at 60 o C temp for 2 h on magnetic stirrer. After 2 h of stirring cream colored precipitate of the complex

11 separated out from the reaction mixture which were filtered, washed with deionised water and ethanol and then dried at 100 o C. The cream colored metal chelate gave bluish light under UV lamp excitation source. (CH 2 ) 4 (CH 2 ) 4 CH=N N=HC Zn(ac) CH=N N=HC 2 60 o Zn C, 2 h stirring OH HO O O Scheme 3.8 Synthetic route of [Zn(salbutene)] (CH 2 ) 6 (CH 2 ) 6 CH=N OH N=H HO Zn(ac) 2 60 o C, 2 h stirring CH=N O Zn N=HC O Scheme 3.9 Synthetic route of [Zn(salhexene)] Bis(salicylidene)heptylene-1,7-diaminatozinc(II) [Zn (salheptene)] The Schiff base ligand (salheptene) (1 mmol) was taken in 50 ml methanol and heated on a magnetic stirrer at 60 o C for 1 hour. The aqueous solution of zinc acetate (1 mmol) was dropwise added to the flask with magnetic stirring. The mixture was kept at 60 o C temp for 2 h on magnetic stirrer. After 2 h of stirring cream colored precipitate of the complex separated out from the reaction mixture which were filtered, washed with deionised water and ethanol and then dried at 100 o C. The cream colored metal chelate gave bluish light under UV lamp excitation source. (CH 2 ) 7 (CH 2 ) 7 CH=N OH N=HC HO Zn(ac) 2 60 o C, 2 h stirring CH=N O Zn N=HC O Scheme 3.10 Synthetic route of [Zn(salheptene)] Characterization of ligands and metal complexes 3.3 Structural characterization

12 The ligands and zinc metal complexes were characterized with CHN analysis, NMR, FTIR spectral techniques [salen] CHN Analysis: The C, H, N analysis of the compound indicated the formula of the compound to be bis(salicylidene)ethylene-1,2-diamine (C 16 H 16 N 2 O 2 ) (found: C, 71.60; H, 5.98; N, 10.47; calc.: C, 71.64; H, 5.97; N, 10.45%) FTIR Analysis: The broad characteristics peak at 3050 cm -1 represented the O-H stretching vibration. This peak was at the lower side due to internal hydrogen bonding. The peak centred at 2955 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1678 cm -1 represented the C=N vibrational absorption. The peaks at 1219 cm -1 represented the C-O stretching vibrations. The characteristics peaks from 600 to 800 cm -1 showed the presence of benzene rings. 1 HNMR: The 1 HNMR spectral studies of bis(salicylidene)ethylene-1,2-diamine taken in CDCl 3 showed the peaks for hydrogens were present at (s 2H) for hydroxyl protons; 8.33 (s 2 H) for azomethine protons and (m 8H), 3.90 (s 4H) [salpen] CHN Analysis: The C, H, N analysis of the compound indicated the formula of the compound to be bis (salicylidene)propylene-1,3-diamine (C 17 H 18 N 2 O 2 ) (found: C, 72.28; H, 6.40; N, 9.96; calc.: C, 72.34; H, 6.38; N, 9.93%) FTIR Analysis: The broad characteristics peak at 3049 cm -1 represented the O-H stretching vibration and showed that there was internal hydrogen bonding. The peak cantered at 2944 cm -1 was assigned to the stretching vibration of C-H bond in the aromatic ring. The peak at 1656 cm -1 showed the C=N vibrational absorption. The peaks at 1209 cm -1 represented the C-O stretching vibrations and the characteristics peaks of benzene rings from 600 to 800 cm -1 showed the presence of benzene rings.

13 1 HNMR The 1 HNMR spectral studies of bis(salicylidene)propylene-1,3-diamine taken CDCl 3 showed the peaks for hydrogens were present at (s 2H) for hydroxyl protons; 8.34 (s 2H) for azomethine protons and (m 8H), 3.69 (m 4H), 2.13 (m 2H) [salbutene] CHN Analysis: The C, H, N analysis of the compound indicated the formula of the compound to be bis (salicylidene)butylene-1,4-diamine (C 18 H 20 N 2 O 2 ) (found: C, 72.93; H, 6.78; N, 9.49; calc.: C, 72.97; H, 6.76; N, 9.45%) FTIR Analysis: The broad characteristics peak at 3051 cm -1 represented the O-H stretching vibration, it was internally hydrogen bonded. The peak centred at 2944 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1672 cm -1 represented the C=N vibrational absorption. The peaks at 1210 cm -1 represented the C-O stretching vibrations. The characteristics peaks of benzene rings were present from 600 to 800 cm HNMR The 1 HNMR spectral studies of bis(salicylidene)butylene-1,4-diamine taken in CDCl 3 showed the peaks for hydrogens were present at (s 2H) for hydroxyl protons; 8.36 (s 2H) for azomethine protons and (m 8H), 3.71 (m 4H), 2.14 (m 4H) [salhexene] CHN Analysis The C, H, N analysis of the compound indicated the formula of the compound to be bis (salicylidene)hexylene-1,6-diamine (C 20 H 24 N 2 O 2 ) (found: C, 74.03; H, 7.43; N, 8.67; calc.: C, 74.07; H, 7.41; N, 8.64%) FTIR Analysis The broad characteristics peak at 3066 cm -1 represented the O-H stretching vibration, this was internally hydrogen bonded. The peak centred at 2964 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1633 cm -1 represented the C=N stretching vibration. The peaks at 1214 cm -1 represented the C-O stretching vibrations.

14 The characteristics peaks of benzene rings from 600 to 800 cm -1 showed the presence of benzene rings. 1 HNMR The 1 HNMR spectral studies of bis(salicylidene)hexylene-1,6-diamine taken in CDCl 3 showed the peaks for hydrogens were present at (s 2H) for hydroxyl protons; 8.42 (s 2H) for azomethine protons and (m 8H), 3.60 (s 4H), 2.55 (m 4H), 1.70 (m 2H), 1.44(m 2H) [salheptene] CHN Analysis The C, H, N analysis of the compound indicated the formula of the compound to be bis (salicylidene) heptylene-1, 7-diamine (C 21 H 26 N 2 O 2 ) (found: C, 74.53; H, 7.71; N, 8.30; calc.: C, 74.56; H, 7.69; N, 8.28%) FTIR Analysis: The broad characteristics peak at 3070 cm -1 represented the O-H stretching vibration was hydrogen bonded. The peak centered at 2984 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1646 cm -1 represented the C=N vibrational absorption. The peaks at 1212 cm -1 represented the C-O stretching vibrations. The characteristics peaks from 600 to 800 cm -1 showed the presence of benzene rings. 1 HNMR The 1 HNMR spectral studies of bis(salicylidene)heptylene-1,7-diamine taken in CDCl 3 showed the peaks for hydrogens were present at (s 2H) for hydroxyl protons; 8.45 (s 2H) for azomethine protons and (m 8H), 3.63 (s 4H), 2.58 (m 4H), 1.73 (m 2H), 1.48(m 2H), 1.38 (m 2H) [Zn(salen)] CHN Analysis The C, H, N analysis of the complex indicated the formula of the complex to be Bis (salicylidene)ethylene-1,2-diaminato zinc (II) (C 16 H 14 N 2 O 2 Zn) (found: C, 57.89, H, 4.25, N, 8.47, cal: C, 57.93, H, 4.22, N, 8.44% ) 1 HNMR

15 The 1 HNMR spectral studies of the complex taken in DMSOD 6 showed that the peaks for aromatic hydrogens were present at 3.91 (m 4H), (m 8H), 8.34 (s 2H). The singlet peak due to hydroxyl proton at 13.2 (s 2H) {which was present in bis(salicylidene) ethylene-1,2-diamine} were absent in the 1 HNMR spectra of the complex and confirmed the formation of the complex. The results showed that the peaks shifted significantly downfield against the 1 HNMR peak values of the ligand itself, indicated the formation of the reported complex. FTIR Analysis: The broad characteristics peak at 3051 cm -1 was absent in the spectra which confirmed the formation of the complex. The peak centered at 2904 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1620 cm -1 represented the C=N vibrational absorption. There was shift in the stretching frequency due to donation of electron density as compared to ligand. The peaks at 1197 cm -1 represented the C-O stretching vibrations, it also shifted to low value as compared to ligand. The characteristics peaks of benzene rings from 600 to 800 cm -1 showed the presence of benzene rings. This confirmed the formation of the zinc complex [Zn(salpen)] CHN Analysis: The C, H, N analysis of the complex indicated the formula of the complex to be Zn (salpen) (C 17 H 16 N 2 O 2 Zn) (found: C, 59.01; H, 4.65; N,8.12; calc.: C, 59.06; H, 4.63; N, 8.106%) 1 HNMR The 1 HNMR spectral studies of the complex taken in DMSOD 6 showed that the peaks for aromatic hydrogens were present at 2.56 (m 2H), 3.68 (m 2H), 3.83 (m 2H), (m 8H), 8.32 (s 2H). The singlet peak due to hydroxyl proton at (s 2H) {which was present in bis(salicylidene)propylene-1,3-diamine} were absent in the 1 HNMR spectra of the complex and confirmed the formation of the complex. The results showed that the peaks shifted significantly downfield against the 1 HNMR peak values of the ligand itself, indicated the formation of the reported complex. FTIR Analysis: The broad characteristics peak at 3051 cm -1 were absent in the spectra which confirmed the formation of the complex. The peak centred at 2904 cm -1 was attributed to the

16 stretching vibration of C-H bond in the aromatic ring. The peak at 1622 cm -1 represented the C=N vibrational absorption. There was shift in the stretching frequency due to donation of electron density as compared to ligand. The peak at 1195 cm -1 represented the C-O stretching vibrations also shift to low value. The characteristics peaks from 600 to 800 cm -1 showed the presence of benzene rings [Zn(salbutene)] CHN Analysis The C, H, N analysis of the complex indicated the formula of the complex to be Zn (salbutene) (C 18 H 18 N 2 O 2 Zn) (found: C, 60.06, H, 5.02, N, 7.75, cal: C, 60.10, H, 5.0, N, 7.79% ) 1 HNMR The 1 HNMR spectral studies of the Bis(salicylidene)butylene-1,4-diaminatozinc(II) taken in DMSOD 6 showed that the peaks for aromatic hydrogens were present at 2.15 (m 4H), 3.72 (m 4H), (m 8H), 8.35 (s 2H). The singlet peak due to hydroxyl proton at (s 2H) {which was present in Bis(salicylidene)butylene-1,4-diamine} were absent in the 1HNMR spectra of the complex and confirmed the formation of the complex. The results showed that the peaks shifted significantly downfield against the 1 HNMR peaks values of the ligand itself, taken from literature indicated the formation of the reported complex. FTIR Analysis The broad characteristics peak at 3051 cm -1 were absent in the spectra which confirmed the formation of the complex. The peak centred at 2912 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1622 cm -1 represented the C=N vibrational absorption. There was shift in the stretching frequency due to donation of electron density as compared to ligand. The peaks at 1186 cm -1 represented the C-O stretching vibrations also shift to lower value. The characteristics peaks from 600 to 800 cm -1 showed the presence of benzene rings [Zn(salhexene)] CHN Analysis The C, H, N analysis of the complex indicated the formula of the complex to be Zn (salhexene) (C 20 H 22 N 2 O 2 Zn) (found: C, 61.92, H, 5.69, N, 7.25, cal: C, 61.95, H, 5.67, N, 7.22% )

17 1 HNMR The 1 HNMR spectral studies of the complex taken in DMSOD 6 showed that the peaks for aromatic hydrogens were present at 1.45 (m 2H), 1.71 (m 2H), 2.56 (m 4H), 3.60 (m 4H), (m 8H), 8.41 (s 2H). The singlet peak due to hydroxyl proton at (s 2H) {which was present in bis(salicylidene)hexylene-1,6-diamine} were absent in the 1 HNMR spectra of the complex and confirmed the formation of the complex. The results showed that the peaks shifted significantly downfield against the 1 HNMR peak values of the ligand itself, indicated the formation of the reported complex FTIR Analysis The broad characteristics peak at 3049 cm -1 were absent in the spectra which confirmed the formation of the complex. The peak centred at 2943 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1624 cm -1 represented the C=N stretching vibration. There was shift in the stretching frequency due to donation of electron density as compared to ligand. The peaks at 1186 cm -1 represented the C-O stretching vibrations also shifted to low value. The characteristics peaks from 600 to 800 cm -1 showed the presence of benzene rings [Zn(salheptene)] CHN Analysis The C, H, N analysis of the complex indicated the formula of the complex to be Zn (salheptene) (C 21 H 24 N 2 O 2 Zn) (found: C, 62.74, H, 5.99, N, 7.00, cal: C, 62.78, H, 5.97, N, 6.97% ) 1 HNMR The 1 HNMR spectral studies of the complex taken in DMSOD 6 showed that the peaks for aromatic hydrogens were present at 1.39 (m 2H), 1.47 (m 2H), 1.72 (m 2H), 2.59 (m 4H), 3.68 (m 4H), (m 8H), 8.45 (s 2H). The singlet peak due to hydroxyl proton at (s 2H) {which was present in bis(salicylidene)heptylene-1,7-diamine} were absent in the 1 HNMR spectra of the complex and confirmed the formation of the complex. The results showed that the peaks shifted significantly downfield against the 1 HNMR peak values of the ligand itself, indicated the formation of the reported complex. FTIR Analysis

18 The broad characteristics peak at 3049 cm -1 were absent in the spectra which confirmed the formation of the complex. The peak centred at 2931 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1625 cm -1 represents the C=N stretching vibration. There was shift in the stretching frequency due to donation of electron density as compared to ligand. The peaks at 1185 cm -1 represented the C-O stretching vibrations also shift to lower value. The characteristics peaks from 600 to 800 cm -1 showed the presence of benzene rings. 3.4 Solubility All the zinc Schiff bases were insoluble in common organic solvents. They are sparingly soluble in methanol. However the complexes showed good solubility in DMSO solvent. 3.5 Thermal characterization (TGA) Thermo-gravimetric analysis (TGA) of the samples was carried out to investigate the thermal stability of the metal organic framework. The TGA of zinc metal complexes was done over a temperature range from o C at a scan rate of 10 o C/min in nitrogen atmosphere [Zn(salen)] Curve of fig. 3.3 corresponds to TGA of Zn(salen) in nitrogen atmosphere. It can be seen from the TGA data that complex exhibited good thermal stability. The onset temperature of weight loss was 320 o C and 10% weight loss occurred at 350 o C. At 380 o C the complex weight loss occurred and decomposed completely [Zn(salpen)] Curve of fig. 3.4 corresponds to TGA of Zn(saplen) in nitrogen atmosphere. It can be seen from the TGA data that complex exhibited good thermal stability. The onset temperature of weight loss was 380 o C and 14% weight loss occurred at 420 o C. At 460 o C the complex weight loss occurred and decomposed completely [Zn(salbutene)] Curve of fig. 3.5 corresponds to TGA of Zn(sabutene) in nitrogen atmosphere. It can be seen from the TGA data that complex exhibited good thermal stability. The onset temperature of weight loss was 230 o C and 10% weight loss occurred at 350 o C. At 370 o C the complex weight loss occurred and decomposed completely.

19 Percentage Wt loss Percentage Wt loss Percentage Wt loss TGA Curve Temperature ( o C) Figure: 3.3 TGA graph of Zn(salen) TGA Curve Temperature ( o C) Figure: 3.4 TGA graph of Zn(salpen) TGA Curve Temperature ( o C) Figure: 3.5 TGA graph of Zn(salbutene) [Zn(salhexene)]

20 Percentage Wt loss Percentage Wt loss Curve of fig. 3.6 corresponds to TGA of Zn(salhexene) in nitrogen atmosphere. It can be seen from the TGA data that complex exhibited good thermal stability. The onset temperature of weight loss was 320 o C and 5% weight loss occurred at 368 o C. At 380 o C the complex weight loss occurred and decomposed completely [Zn(salheptene)] Curve of fig. 3.7 corresponds to TGA of Zn(salheptene) in nitrogen atmosphere. It can be seen from the TGA data that complex exhibited good thermal stability. The onset temperature of weight loss was 350 o C and 10% weight loss occurred at 380 o C. At 390 o C the complex weight loss occurred and decomposed completely TGA Curve Temperature ( o C) Figure: 3.6 TGA graph of Zn(salhexene) TGA Curve Temperature ( o C) Figure: 3.7 TGA graph of Zn(salheptene) 3.6 UV-Visible absorption and photoluminescence (PL) characterization

21 Intensity(a.u.) The UV-visible absorption bands of the zinc complexes match closely with the protonated ligand precursor. The electronic spectra of zinc complexes show metal perturbed ligand centered transitions. The metal complexes show absorption peaks due to n π and π π electronic transitions. The band gap energy was measured from the absorption spectrum. Upon excitation at absorption wavelengths these materials fluoresced in the visible region of the spectra. The complexes showed good luminescence in solid state as well as solution state. As there are no d-d transitions in zinc complexes and the emission of light is assigned as relaxation from higher energy level to the lower energy level due to ligand centered transitions [Zn(salen)] Excitation and emission spectra were recorded on Horiba Jobin YVON Fluolog Model No FL 3-11 spectro fluorometer. The excitation and emission spectra of the complex were taken in methanol solvent. Fig. 3.8 shows the excitation and emission spectra of the complex. Curve (a) and Curve (b) are the excitation and emission spectra of the complex respectively. Two absorption peaks one at 271 nm and another at 360 nm were observed. It implies clearly that low energy peak belongs to the n π transitions localized on the aromatic ring of the ligand and a high energy peak at 271 nm belongs to the ligand centred π π transitions UV-vis Wavelength(nm) Figure 3.8 UV-vis absorption and photo-luminescent emission of [Zn(salen)]

22 Absorbance [Zn(salen)] Optical band gap Energy(eV) Figure 3.9 Square of absorption vs energy curve of [Zn(salen)] The optical band gap of the complex was calculated by plotting graph between absorbance 2 Vs energy [76] as shown in fig. 3.9, it was found 3.03 (ev). Complex exhibited strong fluorescence at 447 nm in the spectrum upon excitation at 271 nm and 360 nm wavelengths. In emission spectrum, broad peak at 447 nm resulting in a bright blue emission had a considerable intensity as shown in curve B of fig The emitted color of photoluminescence was blue having CIE (1931) color coordinates at x = 0.15, y = [Zn (salpen)] The excitation and emission spectra of the complex were taken in methanol solvent. Fig shows the excitation and emission spectra of the complex. Curve (a) and Curve (b) are the excitation and emission spectra of the complex respectively. Two absorption peaks one at 263 nm and another at 349 nm were observed. It implies clearly that low energy peak at 349 nm belongs to the n π transitions localized on the aromatic ring of the ligand and a high energy peak at 263 nm belongs to the ligand centred π π transitions. The optical band gap of the complex was calculated by plotting graph between absorbance 2 Vs energy, it was found 3.14 (ev) as shown in fig Complex exhibited strong fluorescence at 440 nm in the spectrum upon excitation at 267 nm and 354 nm wavelengths. In emission spectrum, broad peak at 440 nm resulting in a bright blue emission had a considerable intensity as shown in fig The emitted color of photoluminescence was blue having CIE (1931) color coordinates at x = 0.15, y = 0.11.

23 Absorbance 2 Normalized Intensity(a.u.) UV-Vis Wavelength(nm) Figure 3.10 UV-vis and photo-luminescent [Zn(salpen)] [Zn(salpen)] Optical band gap Energy(eV) Figure 3.11 Square of absorption vs energy curve of [Zn(salpen)] [Zn(salbutene)] The excitation and emission spectra of the complex were taken in methanol solvent. Fig shows the excitation and emission spectra of the complex. Curve (a) and Curve (b) are the excitation and emission spectra of the complex respectively. Two absorption peaks one at 267 nm and another at 354 nm were observed. It implies clearly that low energy peak at 354 belongs to the n π transitions localized on the aromatic ring of the ligand and a high energy peak at 267 nm belongs to the ligand centred π π transitions. The optical band gap of the complex was calculated by plotting graph between absorbance 2 Vs energy shown in fig. 3.13, it was found 3.15 (ev).

24 Absorbance 2 Normalized Intensity(a.u.) UV-vis Wavelength(nm) Figure 3.12 UV-visible and PL of [Zn(salbutene)] Complex exhibited strong fluorescence at 436 nm in the spectrum upon excitation at 267 nm and 354 nm wavelengths. In emission spectrum, broad peak at 436 nm resulting in a bright blue emission had a considerable intensity as shown in fig The emitted color of photoluminescence was blue having CIE (1931) color coordinates at x = 0.15, y = [Zn(salbutene)] Optical band gap Energy(eV) Figure 3.13 Square of absorption vs energy curve of [Zn(salbutene)] [Zn(salhexene)] The excitation and emission spectra of the complex were taken in methanol solvent. Fig shows the excitation and emission spectra of the complex. Curve (a) and Curve (b) are the excitation and emission spectra of the complex respectively. Two absorption peaks one at 261 nm and another at 347 nm were observed. It implies clearly that low energy peak at 347 nm

25 Absorbance 2 Normalized Intensity(a.u.) belongs to the n π transitions localized on the aromatic ring of the ligand and a high energy peak at 261 nm belongs to the ligand centred π π transitions. The optical band gap of the complex was calculated by plotting graph between absorbance 2 Vs energy shown in fig. 3.15, it was found 3.74 (ev). Complex exhibited strong fluorescence at 433 nm in the spectrum upon excitation at 261 nm and 347 nm wavelengths. In emission spectrum, broad peak at 433 nm resulting in a bright blue emission had a considerable intensity as shown in fig The emitted color of photoluminescence was blue having CIE (1931) color coordinates at x = 0.15, y = UV-Vis Wavelength(nm) Figure 3.14 UV-vis absorption and PL emission spectra of [Zn(salhexene)] [Zn(salhexene)] Optical band gap Energy(eV) Figure 3.15 Square of absorption vs energy curve of [Zn(salhexene)]

26 Normalized Intensity(a.u.) [Zn(salheptene)] The excitation and emission spectra of the complex were taken in methanol solvent. Fig shows the excitation and emission spectra of the complex. Curve (a) and Curve (b) are the excitation and emission spectra of the complex respectively. Two absorption peaks one at 268 nm and another at 353 nm were observed. It implies clearly that low energy peak at 353 nm belongs to the n π transitions localized on the aromatic ring of the ligand and a high energy peak at 268 nm belongs to the ligand centred π π transitions UV-Vis Wavelength(nm) Figure 3.16 UV-vis and PL of [Zn (salheptene)] The optical band gap of the complex was calculated by plotting graph between absorbance 2 Vs energy, it was found 3.98 (ev) shown in fig Complex exhibited strong fluorescence at 430 nm in the spectrum upon excitation at 261 nm and 347 nm wavelengths. In emission spectrum, broad peak at 430 nm resulting in a bright blue emission had a considerable intensity as shown in fig The emitted color of photoluminescence was blue having CIE (1931) color coordinates at x = 0.15, y = 0.13.

27 Normalized Intensity(a.u.) Absorbance [Zn(salheptene)] Optical band gap Energy(eV) Figure 3.17 Square of absorption vs energy curve of [Zn(salheptene)] Table: 3.1 Photo-physical properties of zinc metal complexes Compound UV-vis absorption, λ max (nm) PL π- π *, n- π *, λ max (nm) CIE X/Y Color of Emitted Light [Zn(salen)] x=0.15, y=0.11 Blue [Zn(salpen)] x=0.15, y=0.11 Blue [Zn(salbutene)] x=0.15, y=0.07 Blue [Zn(salhexene)] x=0.15, y=0.09 Blue [Zn(salheptene)] x=0.15, y=0.13 Blue [Zn(salen)] [Zn(salpen)] [Zn(salbutene)] [Zn(salhexene)] [Zn(salheptene)] Wavelength(nm) Figure 3.18 Combined UV-visible spectra Schiff base zinc complexes

28 Normalized Intensity(a.u.) [Zn(salen)] [Zn(salpen)] [Zn(salbutene)] [Zn(salhexene)] [Zn(salheptene)] Wavelength(nm) Figure 3.19 Combined photo-luminescent spectra of zinc Schiff base complexes Discussion- Schiff bases are known for their complex formation with various metal ions and for their applications as emissive materials in organic light emitting diodes. In the present work schiff base complexes of salicylaldehyde and diamines had synthesized and their photophysical properties were studied. A series of Schiff base ligands of salicylaldehyde and different diamines had synthesized, zinc complexes had been prepared with these ligands. This was done to tune the color for the fabrication of full color displays for use of these metal complexes as emissive layer in OLEDs. These complexes emit blue light under UV-visible radiations absorption. [Zn(salen)] complex emit at 447 nm, [Zn(salpen)] emit at 440 nm, [Zn(salbutene)] emit at 436 nm, [Zn(salhexene)] emit at 433, [Zn(salheptene)] emit at 430 nm. All these metal complexes had high thermal stability. On increasing the number of alkyl groups in bridging chain in Schiff base ligands, there was blue shift in emission wavelengths of complexes, this might be due to slight decrease in conjugation chain. Synthesis and Characterization of Beryllium complexes 3.7 Synthesis The beryllium metal complexes were synthesized with above synthesized Schiff bases Bis(salicylidene)ethylene-1,2-diaminatoberyllium(II) [Be(salen)]

29 The metal complex was prepared by reaction of ligand Bis (salicylidene) ethylene-1, 2-diamine (salen) with beryllium sulphate (ligand and metal) at 1:1 molar ratio in methanol. The ligand (salbutene) was prepared by adding dropwise solution of ethylene-1, 2-diamine (1mM) in absolute methanol to a methanolic solution of salicylaldehyde (2mM) with magnetic stirring at 60 o C for 2 h. The beryllium metal complex was then prepared by drop wise addition of aqueous beryllium sulphate (1mM) to the above solution with stirring at 60 o C. After 2 h of stirring a cream colored precipitate of the complex was separated from the reaction mixture which was filtered and dried at 100 o C Bis(salicylidene)butylene-1,4-diaminatoberyllium(II) [Be(salbutene)] The metal complex was obtained by reaction of ligand Bis (salicylidene) butylene-1, 4-diamine (salbutene) with beryllium sulphate (ligand and metal) at 1:1 molar ratio in methanol. The ligand (salbutene) was prepared by adding drop wise solution of butylene-1, 4- diamine (1mM) in absolute methanol to a methanolic solution of salicylaldehyde (2mM) with magnetic stirring at 60 o C for 2 h. The beryllium metal complex was then prepared by drop wise addition of aqueous beryllium sulphate (1mM) to the above solution with stirring at 60 o C. After 2 h of stirring a cream colored precipitate of the complex was separated from the reaction mixture which was filtered and dried at 100 o C. CH=N N=HC BeSO 4 CH=N N=HC OH HO 60 o C, 2 h stirring O Be O Scheme 3.11 Synthetic route of [Be(salen)] (CH 2 ) 4 (CH 2 ) 4 CH=N OH N=HC HO BeSO C,2 hrs stirring CH=N O Be N=HC O Scheme 3.12 Synthetic route of [Be(salbutene)]

30 3.8 Structural Characterization [Be(salen)] CHN Analysis The C, H, N analysis of the complex indicated the formula of the complex to be Be (salen) (C 16 H 14 N 2 O 2 Be) (found: C, 69.75, H, 5.12, N, 10.20, cal: C, 69.81, H, 5.09, N, 10.18% ) FTIR Analysis The broad characteristics peak at 3052 cm -1 were absent in the spectra which confirmed the formation of the complex. The peak centred at 2927 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1620 cm -1 represented the C=N stretching vibration. There was shift in the stretching frequency due to donation of electron density as compared to ligand. The peaks at 1206 cm -1 represented the C-O stretching vibrations. The characteristics peaks from 600 to 800 cm -1 showed the presence of benzene rings. 1 HNMR The 1 HNMR spectral studies of the complex taken in CDCl 3 showed that the peaks for aromatic hydrogens were present at 3.92 (m 4H), (m 8H), 8.33 (s 2H). The singlet peak due to hydroxyl proton at 13.2 (s 2H) {which was present in bis(salicylidene) ethylene- 1, 2-diamine} were absent in the 1 HNMR spectra of the complex and confirmed the formation of the complex. The results showed that the peaks shifted significantly downfield against the 1 HNMR peak values of the ligand itself, indicated the formation of the reported complex [Be(salbutene)] CHN Analysis The C, H, N analysis of the complex indicated the formula of the complex to be Be (salbutene) (C 18 H 18 N 2 O 2 Be) (found: C, 71.18, H, 5.96, N, 9.28, cal: C, , H, 5.94, N, 9.24% ) 1 HNMR The 1 HNMR spectral studies of the Be(salbutene) taken in CDCl 3 showed that the peaks for aromatic hydrogens were present at 1.50 (m 2H), 2.16 (m 2H), 3.53 (m 2H), 3.82 (m 2H),

31 (m 8H), 8.22 (s 2H). The singlet peak due to hydroxyl proton at 13.2(s 2H) {which was present in Bis(salicylidene)butylene-1,4-diamine} were absent in the 1 HNMR spectra of the complex and confirmed the formation of the complex. The results showed that the peaks shifted significantly downfield against the 1 HNMR peaks values of the ligand itself, taken from literature indicated the formation of the reported complex. FTIR Analysis The broad characteristics peak at 3049 cm -1 were absent in the spectra which confirmed the formation of the complex. The peak centred at 2943 cm -1 was attributed to the stretching vibration of C-H bond in the aromatic ring. The peak at 1626 cm -1 represented the C=N stretching vibration. There was shift in the stretching frequency due to donation of electron density as compared to ligand. The peaks at 1208 cm -1 represented the C-O stretching vibrations. The characteristics peaks from 600 to 800 cm -1 showed the presence of benzene rings. 3.9 Thermal characterization (TGA) Thermo-gravimetric analysis (TGA) of the samples was carried out to investigate the thermal stability of the metal organic framework. The TGA of beryllium metal complexes was done over a temperature range from o C at a scan rate of 10 o C/min in nitrogen atmosphere [Be(salen)] Curve of fig.3.20 corresponds to TGA of Be(salen) in nitrogen atmosphere. It can be seen from the TGA data that complex exhibited good thermal stability. The onset temperature of weight loss was 210 o C and 10% weight loss occurred at 315 o C. At 330 o C the complex weight loss occurred and decomposed completely [Be(salbutene)] Curve of fig.3.21 corresponds to TGA of Be(salbutene) in nitrogen atmosphere. It can be seen from the TGA data that complex exhibited good thermal stability. The onset temperature of weight loss was 200 o C and 10% weight loss occurred at 300 o C. At 330 o C the complex weight loss occurred and decomposed completely.

32 Percentage Wt loss Percentage Wt loss TGA Curve Temperature ( o C) Figure: 3.20 TGA graph of Be(salen) TGA Curve Temperature ( o C) Figure: 3.21 TGA graph of Be(salbutene) 3.10 UV-visible and photo-luminescent characterization [Be(salen)] The excitation and emission spectra of the complex were taken in methanol solvent. Figure 3.22 shows the excitation and emission spectra of the complex. Curve (a) and Curve (b) are the excitation and emission spectra of the complex respectively. Two absorption peaks one at 268 nm and another at 347 nm were observed. It implies clearly that low energy peak at 347 nm belongs to the n π transitions localized on the aromatic ring of the ligand and a high energy peak at 268 nm belongs to the ligand centred π π transitions.

33 Absorption 2 Normalized Intensity(a.u) UV-vis Wavelength(nm) Figure 3.22 UV-visible and PL of [Be(salen)] The optical band gap of the complex was calculated by plotting graph between absorbance 2 Vs energy shown in fig. 3.23, it was found 3.12 (ev). Complex exhibited strong fluorescence at 425 nm in the spectrum upon excitation at 268 nm and 347 nm wavelengths. In emission spectrum, broad peak at 425 nm resulting in a bright blue emission had a considerable intensity as shown in fig The emitted color of photoluminescence was blue having CIE (1931) color coordinates at x = 0.30, y = [Be(salen)] Optical band gap Energy(eV) Figure 3.23 Square of absorption vs energy curve of [Be(salen)] [Be(salbutene)] The excitation and emission spectra of the complex were taken in methanol solvent. Figure 3.24 shows the excitation and emission spectra of the complex. Curve (a) and Curve

34 Absorbance 2 Normalized Intensity(a.u.) (b) are the excitation and emission spectra of the complex respectively. Two absorption peaks one at 268 nm and another at 353 nm were observed. It implies clearly that low energy peak at 353 belongs to the n π transitions localized on the aromatic ring of the ligand and a high energy peak at 268 nm belongs to the ligand centred π π transitions. The optical band gap of the complex was calculated by plotting graph between absorbance 2 Vs energy shown in fig. 3.25, it was found 3.14 (ev). Complex exhibited strong fluorescence at 425 nm in the spectrum upon excitation at 268 nm and 353 nm wavelengths. In emission spectrum, broad peak at 425 nm resulting in a bright blue emission had a considerable intensity as shown in fig The emitted color of photoluminescence was blue having CIE (1931) color coordinates at x = 0.16, y = UV-Vis Wavelength(nm) Figure 3.24 UV-vis absorption and PL of [Be(salbutene)] [Be(salpen)] Optical band gap Energy(eV) Figure 3.25 Square of absorption vs energy curve of [Be(salbutene)]