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Synthesis and characterization of new Zn-phtalocyanine-based semi-conducting materials
⁎Corresponding author. Tel.: +216 22842695; fax: +216 71588233. touatisarah5@yahoo.fr (Sarra Touaiti)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
New Zn-phtalocyanines-based organic materials were synthesized and characterized. The optical properties of these π-conjugated systems were investigated by UV–visible absorption and photoluminescence (PL) spectroscopy. The electrochemical behavior was reported. The optical and electrochemical gaps were estimated. Those complexes, having low gap energy, have been identified as holding much promise for the development of electronic devices.
Keywords
Electrochemical gap
Optical gap
Voltage
Diode
1 Introduction
Metallophthalocyanines complexes, M(PCs), have attracted attention due to their structure rich in π-conjugation system owing to potential applications in various engineering and technological fields such as opto-electronic devices (Ghosh et al., 1974), gas sensors (Hamman et al., 1991; Wilson et al., 1991), static induction transistors (Joseph and Menon, 2002; Joseph et al., 2001; Kudo et al., 2001), and photoreceptor devices in laser beam printers and photocopiers (Borsenberger and Weiss, 1993).
Additionally, certain derivatives of phthalocyanines have shown promise as photodynamic reagents for cancer therapy and other medical applications (Henderson and Dougherty, 1992). But, the poor solubility of M(PCs) limits their applications in the field of gas sensing. To solve this problem, introducing certain substituent at their peripheral ring (Seelan et al., 2001; Szymczyk and Abramczyk, 2004) was a promising way.
A suitable functionalization of the M(PCs) has a strong influence on the π-electron conjugation of the macromolecule (Iglesias et al., 2002), since it makes salvation easier, decreasing thus the difference between HOMO and LUMO orbital (gap energy). Complexes with aromatic substituent have shown good potentials in a range of applications due to their interesting ground and exciting state properties. It is therefore vital to understand how to control the chemical properties of such compounds in order to be able to tune their photophysical properties according to application-specific requirements. While increasing the number of aromatic rings, we move the wavelength to highest values (red shift), making their use in diodes and photovoltaic cells indispensible.
In this paper we report the synthesis and characterization of two Zn(II) phthalocyanine derivatives substituted by either four quinoleinoxy (PC6) or four phenylphenoxy (PC7). PCs structures were characterized by Fourier Transform Infrared Spectrophotometry (FT-IR). We studied their electrochemical, optical (by UV–vis absorption and photoluminescence spectroscopy), and electrical properties.
2 Experimental
2.1 Materials and measurement
N,N′-dimethylformamide (DMF), dimethylsulfoxide (DMSO), diethylether, chloroform were freshly used, alcohols and initiator products were used as received from Aldrich. 4-nitrophtalonitrile was synthesized from phthalic anhydride in four steps, K2CO3was dried before use.
1H-NMR spectral data were obtained on a AV 300 spectrometer. FTIR spectra were acquired on a Perkin–Elmer BX FT-IR system spectrometer by dispersing sample in KBr pellets. UV–vis absorption spectra were recorded on a Cary 2300 spectrophotometer. Photoluminescence (PL) spectra were obtained on a Spectra-Physics 2017 spectrometer model 3900S.
2.2 Synthesis
The synthesis of the macrocycle zinc (II) phthalocyanines PC6 and PC7 begins with the nitration in position 4 of phthalimide followed by reaction of the amine hydroxide (Valentin et al., 2008), to form the 4-nitrophthalamide 2, the dehydration by the thionyl chloride in N,N-dimethylformamide (Michael, 2006) leads to 4-nitrophthalonitrile 3 (Scheme 1).
Dinitriles 4 and 5 were prepared by a nucleophilic substitution reaction of 4-nitrophthalonitrile with the corresponding alcohol in the presence of K2CO3 (Scheme 2).
The cyclo-tetramerization of dinitriles 4 and 5 with zinc (II) acetate in the presence of urea and ammonium molybdate was performed in nitrobenzene (Scheme 3).
2.2.1 Synthesis of 4-nitrophthalimide 1
After adding nitric acid (15.1 mL, 0.27 mol) to sulfuric acid (90.1 mL), the solution was stirred in an ice bath for 30 min. Then the solution was allowed to warm to room temperature. The phthalimide was added to acidic solution and heated at 35 °C with stirring until a colorless solution was achieved. The solution was stirred for 4 h. A white solid was precipitated from ice water, filtered and watched with deionized water. The title compound was dried under ambient conditions. 80% yield was obtained. RMN 1H(DMSO-d6): δ: 8.003 (d, J = 5.7 Hz,1H); 8.342 (s, 1H); 8.540 (d, J = 6.9 Hz, 1H); 11.761 (s, 1H). RMN 13C (DMSO-d6) δ: 118.16; 124.92; 129.87; 134.42; 137.66; 151.71; 167.61; 167.91.
2.2.2 Synthesis of 4-nitrophthalamide 2
Compound 1 was stirred in 32% ammonia solution (70 mL) for 24 h. The resulting deep yellow product was filtered and washed with cold water until the disappearance of excess of ammonia. The title compound was dried at 110 °C, a 95% yield. RMN 1H(DMSO-d6): δ: 7.636 (s, 2H); 7.704 (d, J = 8.1, 1H); 8.020 (s, 1H); 8.079 (s, 1H); 8.297 (dd, J = 2.4 Hz, J = 8.7 Hz, 1H); 8.338 (d, J = 2.1 Hz, 1H). RMN 13C (DMSO-d6) δ: 122.35; 124.38; 129.11; 137.19; 142;65; 147.02; 167.67; 168.67.
2.2.3 Synthesis of 4-nitrophthalonitrile 3
Under nitrogen and stirring, 20.87 ml of thionyl chloride (34 g, 0286 mol) was slowly added to 50 ml of N,N-dimethylformamide at a temperature lower than 5 °C. 15 g of 4-nitrophthalamide was added and the reaction was maintained for 18 h. The crude product was precipitated from a mixture of water–ice. The product was dried to yield 86% of pure product.
RMN 1H(DMSO-d6): δ: 8.439 (d, J = 8.7,1H); 8.685 (dd, J = 2.1 Hz, J = 8.4 Hz, 1H); 9.033 (d, J = 2.1, 1H). RMN 13C (DMSO-d6) δ: 114.55; 114.86; 116.59; 120.22; 128.52; 128.81; 135.27; 149.69.
FTIR: 3091 (C–H aromatic), 2242 (CN), 1534 (asymmetric N⚌O band), 1349 (symmetric N⚌O band), 853 (C–N).
2.2.4 Synthesis of dinitriles 4 and 5
4-(quinoleinoxy)phtalonitrile 4. To a mixture of 4-nitrophtalonitrile 3 (0.5 g, 2.88.10−3 mol) and 8-hydroxyquinoleine (0.418 g, 2.88.10−3 mol), in DMSO was added anhydrous K2CO3 (4.03.10−3 mol, 0.571 g). The mixture was stirred for a further 48 h. Then, the reaction mass was poured into water (25 mL) and the precipitate formed was filtered off, washed with water, and crystallized from EtOH–water to give the product as a yellow crystalline powder. Yield: 90% 1H-NMR (300 MHz, CDCl3, ppm): 7.2 (d, J = 8.7 Hz, 1H), 7.6 (t, J = 7.7 Hz, J = 2.5 Hz, 1H), 7.76–8 (m,5H), 8.5 (t, J = 8.5 Hz, J = 2.7 Hz,1H), 8.8 (d, J = 7.75 Hz, 1H). RMN 13C (DMSO-d6) δ: 108;118; 119; 122.51; 128; 136; 137; 148; 150; 163.
4-(phenylphenoxy)phtalonitrile 5. The reaction was performed as described above for compound 4, using 4-nitrophtalonitrile 3 (0.5 g, 2.93.10−3 mol), and 4-phenylphenol (0.508 g, 2.93.10−3 mol) to give dinitrile 5. Yield: 85% 1H-NMR (300 MHz, CDCl3, ppm): 7.85 (d, J = 7.7 Hz, 1H), 7.65 (d, J = 7.9 Hz, 1H), 7.42–7.58 (m, 5H), 7.23–7.38 (m, 5H).
2.2.5 Synthesis of phthalocyanines derivatives 6 and 7
Zn-tetra(4-quinoleinoxy)phtalocyanine PC6. A mixture of 4-(quinoleinoxy)phtalonitrile (0.2 mol), ZnAC2 (0.010 mol), molybdate sel (0.02) and urea (20 g) was refluxed in nitrobenzene for 7 h at 170 °C. Then methanol and water were added to induce precipitation. The solid was filtered and washed sequentially with water and methanol. The solid was dried under vacuum to give 45% of pure product as a green powder. The structure of the product was confirmed by UV–vis absorption, NMR and FTIR.
λmax (nm): 355,672,715 (DMF), NMR 1H(DMSO-d6) δ: 7.1–9.4 (m,36H), FTIR:(νmax/cm−1): 3058 (ArH), 2920, 1610, (–HC⚌N), 1574, 1485, 1469, 1393, 1278, 1230 (ArOAr).
Zn-tetra(4-phenylphenoxy)phtalocyanine PC7. According to the above procedure described for PC6, dinitrile 5 (0.2 mol) and zinc acetate dihydrate (0.01 mol) were used to yield 40% of pure PC7. The formation of phthalocyanine was confirmed by UV–vis NMR and FTIR.
λmax (nm): 345, 612, 682, (DMF) 1H-NMR (300 MHz, CDCl3, ppm): 2.48 (s,4H); 6.8–8.6 (m, 48H). FTIR (νmax/cm−1): 3058 (ArH), 2920, 1610 (–HC⚌N), 1574, 1485, 1469, 1393, 1278, 1230 (ArOAr), 690, 750 (C6H5).
2.3 Fabrication and characterization of diodes
Single-layer devices were elaborated as sandwich structure between an aluminum (Al) cathode and an indium tin oxide (ITO) anode. The phthalocyanines compounds solutions (3.10−2 M in DMF) were deep-coated onto ITO glass. A thin aluminum layer was deposited by thermal evaporation at 3.10−6 Torr.
3 Results and discussion
3.1 Optical properties
3.1.1 UV–vis absorption spectroscopy
Absorption spectrum seems to be the most important characteristic of PCs (Hacıvelioğlu, 2008) and related compounds.
As shown in Fig. 1, the absorption spectra of PC6 showed a very intense absorption band at 684 nm (682 for PC7) called Q-band, a vibronic band at 613 nm (613 for PC7) and a B or Soret band at 353 nm (352 nm for PC7). At the same concentration The Q-band of PC6 was more intense then Q-Band of PC7 which is probably due to the presence of a heteroatom on the benzene rings (Table 1).
| Compounds | λmax/nm | λonset/nm | Optical band gap Egap op (eV) | |
|---|---|---|---|---|
| B band | Q band | |||
| PC6 | 353 | 613, 684 | 708 | 1.75 |
| PC7 | 352 | 613, 682 | 705 | 1.76 |
The optical band gaps were estimated from the absorption onset of the phthalocyanines solutions. The calculated values were 1.75 and 1.76 eV for PC6 and PC7, respectively as shown in Table 1.(See Fig. 1)
3.1.2 Photoluminescence measurements
Photoluminescence (PL) measurements are used to show that a material is photo-active.
The luminescence properties of the materials were studied by analyzing the photoluminescence spectra of the material in the visible region from 400 to 800 nm. In solution PCs undergo strong hydrophobic interactions that lead to the formation of aggregated molecules. For this reason, samples for PL spectra were loaded on a powder sample holder. Fig. 2 show the photoluminescence spectra.
Samples on excitation with the wavelength of 488 nm, show emission in red region at 543 and 537 nm for PC6 and PC7, respectively. But PC7 shows very weak intensity in comparison with PC6.
3.2 Electrochemistry
3.2.1 Cyclic voltammetry
The cyclic voltammetry (CV) was employed to investigate the redox behavior of the phtalocyanine-based materials and to estimate their HOMO (Highest Occupied MolecularOrbital) and LUMO (Lowest Unoccupied Molecular Orbital) energy levels. The use of CV analysis is reliable as the electrochemical processes in OLEDs (Hiz et al., 2012).
The glass ITO was used as working electrode and scanned in LiClO4/dimethylformamide. The voltammograms obtained are shown in Fig. 3.
All processes are ring based since the central Zn metal is electroinactive (Arslanoglua et al., 2007; Wang et al., 2012). PC6 exhibits two irreversible oxidation pics at 0.83 and 1.28 V and a quazi-reversible reduction pic at −0.17 V. PC7 exhibits two irreversible oxidations at 0.78 and 1.33 V and a reversible reduction pic at −0.22 V (Fig. 3).
According to an empirical method and by assuming that the energy level of ferrocene/ferrocenium couple is 4.8 V below the vacuum level, the HOMO energy level (EHOMO), LUMO energy level (ELUMO), and the electrochemical gap (Eg-el) were calculated as follows:
| EHOMO | ELUMO | Electrochemical gap Egap el (eV) | |
|---|---|---|---|
| PC6 | −5.05 | −3.64 | 1.41 |
| PC7 | −5.06 | −3.6 | 1.46 |
After repetitive scans (100scan) we noticed an increase in the currents of the voltammogram envelop which indicates that a deposit was formed on the electrode (Fig. 4). Thus, different phthalocyanine film thickness can be obtained by controlling the number of the electropolymerizing scans.
3.3 Current–voltage measurements
The measurement of current-density (J)–voltage (V) characteristics is an important technique for characterizing the diode. Two single-layer devices with an ITO/PC/Al configuration were fabricated to investigate the current- voltage (I–V) characteristics of the phthalocyanine based materials.
As shown in Fig. 5, the I–V curves indicate typical diode behavior with relatively low turn-on voltages of 2.83 and 3.03 V for PC6 and PC7, respectively. Nevertheless, no electroluminescence could be recorded for these simple devices. The reason is probably unbalanced charge injection which increases the probability of radiation less excitement quenching at the electrode/phthalocyanine material interface (Hiz et al., 2011; Ndayikengurukiye et al., 1997). Therefore, we consider that the devices turn-on voltages indicate the threshold of a hole-governed unipolar injection. In fact, work is in progress to build electroluminescent multilayer devices.![Current–voltage curves for [ITO/PC6/Al] and [ITO/PC7/Al] devices.](/content/184/2017/10/2_suppl/img/10.1016_j.arabjc.2013.05.023-fig8.png)
4 Conclusion
Two aromatic Zn-phtalocyanines semi-conducting materials (PC6 and PC7) were synthesized and characterized by UV and IR spectroscopy. The two compounds show red emissions. The PC6 compound has higher fluorescence quantum efficiency. The voltammetric responses led to the formation of poly PC6 and poly PC7 on ITO electrode upon 100 scan. The I–V characteristics of devices with an (ITO/PC/Al) configuration demonstrate typical diode behavior with relatively low turn-on voltages. All these features make these compounds promising active materials for phthalocyanine-based OLEDs.
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