Investigations on structural, photo-physical and photometric parameters of metal based quinoline complexes for OLEDs

This paper includes the synthesis of bluish-green light emitting metal based hydroxyl quinoline complexes namely, Mgq2, Baq2 and MgxBa(1-x)q2 (Mg: Magnesium, Ba: Barium, q: 8-hydroxy quinoline, x = 0.1, 0.2,….0.9) by simple precipitation method at different values of pH and stoichiometry. It’s noteworthy to state that we are the first one to report novel complexes based on Ba (Baq2 and Mg0.5Ba0.5q2). The inquisition of these complexes was carried out to probe structural, photo physical and photometric parameters by FTIR, photoluminescence (PL) spectra and 1931 CIE colour calculator, respectively. These investigations reveal that pH value and stoichiometry have feeble effects on the emission wavelength and intensity. Investigations further reveal that among all the synthesized complexes, Baq2 (at pH 6.5) emits intense blue light in various environments. Thus, barium based quinoline complexes have the potential to sustain their emission features in solid state as well as in acidic and basic medium-one of the characteristic features which are highly essential for the fabrication of OLEDs by vacuum as well as solution techniques.


Introduction
Light-emitting devices (LEDs) have created in the field of light emitting optoelectronic devices nearly 50 years ago; nevertheless, during the past one and half decade they were just popular as an indicator in traffic signals, electronic appliances and many more [1,2]. These LEDs slowly emerged in numerous wavelengths including ultraviolet, visible (including violet, pink and white, apart from red, green and blue) and infrared regions of electromagnetic spectrum. The energy band gap (Eg) dictates the emissive wavelength of the diode, based on relation E=hc/λ. This functional feature of LEDs validated them into numerous diverse areas including displays, sensor applications, automotive uses, medical applications, as illuminators, in horticulture, solid state lighting technology medicine [3] and even for decorative purpose to name a few. LED bulbs-a combination of point source LEDs, in spite of being mercury free, long lasting and energy saving, they are restricted by technology, they shift colour in due course of time and temperature, also cause light pollution [4]. Hence, one of the newest kinds of LED lighting and displays emerged as organic light emitting diodes (OLEDs), which make use of organic phosphor IOP Publishing doi: 10.1088/1742-6596/1913/1/012022 2 capable of emitting light when facilitated with current [5]. These diodes/displays are popular sources, flexible with remarkable visual appeal. Organic phosphor that is used as emissive material in OLEDs have the potential to emit light in various environments (solid state, either in acidic or basic media) without much alteration in its emission wavelength. Based on these facts, volumes and volumes of research have been carried out since 1987 by C.W. Tang on aluminium based quinoline namely, tris(8hydroxyquinoline) aluminium (III) (Alq3) complex which emit green light and he even succeeded in developing a single layer OLED out of Alq3 [6]. Since then, two-layer, three layer and later multilayer OLEDs emerged and now they are playing a leading role in small as well as large panel displays that can be either rigid or flexible, based on the application [7][8][9][10][11][12]. OLEDs based on this organic metal chelates have gained momentum due to their elevated thermal stability, ability to transport electrons and emissive properties of the material [13,14]. Therefore in the present study, 8-hydroxyquinolate metal complexes, namely Mgq2, Baq2 and MgxBa(1-x)q2 were synthesized by economical synthesis procedure and the investigations have been carried out under various environments.

Experimental
Metal based quinoline complexes namely, Mgq2, Baq2 and MgxBa(1-x)q2 were synthesized by cost effective and less time consuming precipitation technique at room temperature.

Precursors
Following precursors of analytical grade (AR) were employed during the synthesis of metal based quinoline complexes are tabulated in Table 1. 3 milky white solution was obtained (say solution II). Solution I and II were mixed for 10 min and then NH4OH solution was added drop by drop to this mixture with continuous stirring till yellowish green precipitate was obtained. After the formation of precipitate, pH of the solution was noted. The precipitate was collected at various pH values at an interval of 0.5 from pH 5.5 to 8.0 so as to study the role of pH on the intensity of the obtained precipitate. Precipitate was then washed with double distilled water for 3 to 4 times and the precipitate was set aside in an oven at 45 0 C for 45 min so as to evaporate the left-over water molecules or any other solvents or moisture if any. By following the same procedure and by considering the same stoichiometry of magnesium and barium, MgxBa(1-x)q2 was synthesized [15].

Result and discussion
The Fourier transform infrared (FT-IR) spectrum of metal based quinoline complexes were carried out on SHIMADZU Model 8101A infrared spectrophotometer, photo-luminescence spectra was carried out on HITACHI F-4000 spectrofluorometer and 1931 Commission International de l'Eclairage (CIE) system software was used to explore CIE coordinates and Colour correlated temperature (CCT).

Fourier Transform Infrared (FT-IR) spectra
FTIR is an analytical technique, which authenticates the molecular structure of the synthesized complexes by portraying the sharp characteristic absorption peaks (in the infrared region, as the name suggests) of the sample under investigation. These absorption peaks help to identify the empirical molecular structure of the synthesized complexes, namely, Mgq2 Baq2 and MgxBa(1-x)q2 (at x = 0.5). In the present investigation, FTIR spectra was carried out in the range of 4000-800 cm -1 .

Figure 3. FTIR spectra of Mgq2
All the three spectra clearly portray scattering below 800 cm -1 , which may be due to crystalline nature of the complexes, hence not recorded. The FTIR spectra of Mgq2 reveal OH-stretching vibration inbetween 3600-3000 cm -1 . The absorption peaks at 2387.87 and 1951.96 cm −1 can be allocated to the stretching modes of methylene groups in Mgq2 [16]. A broad peak range 2400-2000 revels the presence of strong O=C=O stretching. Vibrations at 1604.77, 1577.77 and 1500.62 cm −1 can be assigned to quinoline group of the synthesized complex. The bands at 1469.76 and 1423.47 cm −1 can be ascribed to pyridyl and phenyl groups. Aromatic amine resonances (C-N-C)) can also be observed in-between 1370-1250 cm -1 [17]. The peaks at 742.59 and 790.81 cm −1 are allied with in-plane ring deformation as portrayed in figure 3, confirming the formation of Mgq2 complex. No major differences were found in FTIR spectra of Baq2 and MgxBa(1-x)q2 (at x = 0.5) in comparison with the FTIR spectra of Mgq2 complex.     Figure 6 shows the emission and excitation (photo-luminescence) spectra of Mgq2 at various values of pH, ranging in-between 5.5 to 8.0, at an interval of 0.5. Excitation spectra of Mgq2 at different values of pH displays broad excitation peak with a weak shoulder. At pH 5.5, Mgq2 complex shows excitation at 422nm with a weak shoulder at 389nm. Similarly, the complex at pH 6.0, 6.5, 7.0, 7.5 and 8 displays a sharp peak at 427, 427, 430, 428 and 427 nm with a weak shoulder at 384,382, 381, 380 and 380nm, respectively. Mgq2 complex emits blue light, which falls in the visible spectrum of electromagnetic spectrum at 484, 484, 485, 488, 488 and 486 nm, for pH 5.5, 6.0, 6.5, 7.0, 7.5 and 8, respectively. Maximum excitation as well as emission wavelength was observed in the complex synthesized at pH = 7.5, which may be due to hydrogen ion concentration in the solution that is in coherence with the quinoline containing magnesium. Even with the variation in pH, the nature and position of bands remained almost unaltered. However, a slight decrease in intensity was observed when the pH of the complex was gradually incremented or decremented by 0.5 starting from pH 5.5 till 8.0(excluding pH=7.5), suggesting its sensitivity to pH at one particular value of pH [18]. The variation of intensity with pH is observed as 7.5 > 8.0 > 6.5 > 6.0 > 7.0 > 5.5.

Figure 6. PL spectra of Mgq2 at various values of pH
Baq2 complex also maintained the same trend, however at pH 6.5, it shows maximum excitation wavelength at 395 nm with a weak shoulder at 348 nm. Even at pH 7.5, the peak position remained at 395 and 348 nm. Surprisingly, the shoulder disappears at pH 5.5, 6.0, 7.0, and 8, while the maxima peak displays at 472, 473, 470 and 471 nm, respectively. Baq2 complex also emits blue light, which falls in the visible spectrum of electromagnetic spectrum at 472, 473, 460, 470, 472 and 471 nm, for pH 5.5, 6.0, 6.5, 7.0, 7.5 and 8, respectively. Maximum excitation as well as emission wavelength was observed in the complex synthesized at pH = 6.5, which may be due to hydrogen ion concentration in the solution that is in coherence with the quinoline containing barium. The variation of intensity with pH is observed as 6.5 > 7.5 > 7.0 > 5.5 > 6.0 > 8.0 as shown in figure 7 PL spectral data of Mgq2 and Baq2 at various values of pH are tabulated in Table 2.

Photo-luminescence spectra with varying stoichiometry
We also made an attempt to vary stoichiometry of the combination of magnesium and barium nitrate during the synthesis of binuclear Mgx Ba(1-x)q2 complex at pH 7 [19]. The reason for selecting pH 7 is due to the fact that Mgq2 exhibited its maximum intensity at pH 7.5, while Baq2 exhibited its maximum intensity at pH 7.5. The average of both, which is going to be 7 is preferred. The composition of the chemical constituents considered during the synthesis of Mgx Ba(1-x)q2 complexes are tabulated in Table  3. Table 3. The composition of the chemical constituents for the synthesis of Mgx Ba(1-x)q2 complexes. Ba(1-x) The photo-luminescence spectra of Mgx Ba(1-x)q2 complexes displays the fact that when the stoichiometry of both the complexes is same i.e. when x = 0.5, the excitation and emission spectra registered maximum intensity due to the relaxation of an excited electron from the S1-S0 level [20]. However, for various x values of magnesium nitrate and barium nitrate quinoline based complexes, their PL spectra registers slightly different excitation and emission wavelengths and divergent intensities as shown in Fig.8. Thus, it can be inferred that when x < 0.5, the intensity was greater as compared to the intensity when x > 0.5. However, every value of x portrays emission in blue region of visible spectrum. The earlier reports state the fact that the emission intensity of quinoline samples decreased with noticeable shift with an increase in atomic number of the centre metal ion. The electron cloud between the centre metal and ligands would influence the luminescence intensity, however independent of the emission wavelength.   Table 4. Spectral data of the synthesized complexes at different stoichiometry

Photo-luminescence spectra in formic acid
The PL spectra of solvated metal based quinoline complexes were recorded in formic acid at molar concentration of 10 -3 mol/L. It was observed that the PL spectra of the solvated complex is hypsochromically shifted by around 20-25 nm as that of in solid state. This can be attributed to the solvent-solute interaction in the solution. Spectral data of PL spectra is tabulated in Table 5. Mgq2 displays blue emission at 465 nm, when excited at 423 nm as shown in figure 9 (i). Upon excitation of Baq2 at 275 nm, the emission spectra display a sharp emission peak at 450nm as shown in figure 9 (ii). figure 9 (iii) shows, that PL spectra of Ba0.5 and Mg0.5. at excitation of 425nm and emission at 482 nm [22]. Photo-luminescence data of the synthesized complexes solvated in formic acid are tabulated in Table 5. Thus, the luminescence wavelength is tuned to lower wavelengths, however lie in blue-bluish green region. These results prove the potential of the synthesized complexes to sustain in various environments like in solid state, acidic and basic environment with slight variations in emission wavelength and intensity. Hence, these complexes can be used in fabrication of OLEDs by solution techniques rather than the conventional vacuum technique, which is laborious and costly offence. Table 5. Photo-luminescence data of the synthesized complexes solvated in formic acid

Photometric evaluation
The colour emitted by the light source is generally analysed in terms of commission International de I' Eclairage (CIE) system by using chromatic coordinates (x, y) [23,24]. Figure 10 portrays the CIE coordinates of Mgq2 and Baq2 at different values of pH, while figure 11 and 12 displays the CIE coordinates of solvated complexes in formic acid and for different stoichiometry of Mg and Ba, respectively. The determined coordinates of CIE and colour correlated temperature (CCT) are tabulated in Table. 6, which reflect the fact that their emission lies in bluish green region, that is in coherence with the PL spectral data.