In recent years a lot of Planar aromatic molecules have been designed and synthesized that have received considerable attention as organic semiconductors and optoelectronic materials, due to their high fluorescence in solution, and high thermal and chemical stability [1,2,3,4].
Up to now, rylene diimides are probably the most studied, modified, synthesized as π-conjugated system with aromatic core [5,6,7]. But the relatively easy synthesis of pyromellitic diimide compared with that of other rylene derivatives has made these molecules interesting systems to be explored . The pyromellitic diimide is one of a planar aromatic compound with π-conjugated system that could be conjugated into larger multicomponent assemblies through intercalation [9-13] and give derivatives that are used widely for molecular recognition , molecular sensors , molecular machines, chemical catalysis and drug delivery [16,17,18,19]. Pyromellitic diimide derivatives decorated with fluorinated side chains have been used as active layers in n-type organic thin-film transistors, which show field-effect mobilities up to 0.079 cm2 V−1 s−1 and Ion/Ioff ratios of 106 [20,21], suggesting that the moiety has high performance to other analogues due to the close π−π conjugated electron between the benzene molcules and diimide cores . The pyromellitic diimide also has been one order of magnitude lower than that of the naphthalene diimide analogue , due to the smaller size of the aromatic (benzene) core. The side chain of N,N′- substituents has strongly influenced the electrical characteristics, showing the importance of carrying out the current study on pyromellitic derivatives properties.
Synthesis of N, N'- bis amino pyromellitic diimide 
Pyromellitic dianhydride (1gm, 4 mmol) was added slowly to excess of a hydrazine hydrate 99% 10-20 times in ice bath, stirred for 4 hour and left overnight; a yellow precipitate was formed, dried and recrystallized by ethyl acetate.
Synthesis of N,N'- bis (2-amino acetyl chloride) pyromellitic diimide (2)[24,25]
N,N'- bis amino pyromellitic diimide (1.23 gm, 5mmol) was added to chloro acetyl chloride (1-2 mL) and refluxed on steam bath for 2-3 hrs.; after cooling yellow precipitate was formed, dried and recrystallized by ether.
Synthesis of N,N'- bis (2-amino acetyl azide) pyromellitic diimide (3)
Sodium azide (0.1625 gm, 2.50 mmol) was added to N,N'- bis (2-amino acetyl chloride) pyromellitic diimide (0.5 gm,1.25mmol.) in 250 mL of dimethyl formamide and refluxed for 6 hrs, then cooled to home temperature and precipitate was formed, dried and recrystallized.
Synthesis of N,N'- bis 1,2,3-triazole pyromellitic diimide [4-7]
N,N'- bis (2-amino acetyl azide) pyromellitic diimide (0.1 gm ,2.4 mmol) was added to α-β unsaturated aldehyde derivatives (4.8 mmol) in DMF, refluxed at 110 °C for 24 hrs., washed with ether; a brown precipitate was formed, dried and recrystallized.
Results and discussion
Scheme 1 shows the prepared compounds:
In the first reaction, we used excess of hydrazine to avoid condensation and polymerization; the mechanism is shown in Scheme 2.
Scheme 3 shows the triazole ring formed by 1,3- dipolar cycloaddition in side of 1,3-dipolar cycloaddition, and the azide with carbonyl; the double bond in α-β unsaturated aldehyde can be formed; and trizoline can be oxidized by aerobic oxygen to triazole .
All prepared compounds were characterized by physical properties (Table 1), FT-IR (Table 2), 1H-NMR (Table 3).
The FT-IR spectra showed stretching bands at 3031-3099 cm-1 for CH aromatic, while CH aliphatic appeared between 2896-2925cm-1. The carbonyl groups of imide gave strong stretching bands (1631-1654cm-1). The band in the region of 1400-1566 cm-1 was referred to as C=C aromatic while N=N appeared at 1325 cm-1. The N-H appeared at 3186-3286 cm-1. The detailed infrared spectral data is illustrated in Table 2 [28-30].
The 1H-NMR spectra showed singlet signals in the region 2.54 – 4.18 ppm for (CH2, 4H), and singlet signals in the region 0.87 – 2.04 ppm) for N-H, 2H, while Aromatic protons showed SINGLE signals at 7.55–7.95 ppm and multipalte in compound .
The optical properties of the prepared compounds [4-7] were analyzed by UV-Vis absorption spectroscopy. Absorption spectra showed similar characteristics and the maximum (λmax) appeared at uv range of 200-300 nm. The absorption band being responsible for π-π* transition in the amine group is covered by absorption of the pyromellitic units (Figures 1-5).
The optical energy band gap values (Eg) at about (≥ 4.7 eV) was calculated from the onset of UV–vis absorption band. Direct band gap energy can be calculated from tauc model, from the following Equation:
(αhυ)2 =A0 (hυ-Eg)
Where A0 is a constant, hυ is the photon energy, Eg is the optical band gap energy, and ⍺ is optical absorption coefficient deduced from the absorption data.
The energy band gap in transition state (˃ 4 ev.), showed in Figures 6-8, are calculated from tauc models.
An electrical property of a wide energy gap (Eg ≥ 3.3 eV) is critical for semiconductors to realize full transparency with excellent photostability in the visible range . The four compounds [4-7] show a good expected optical energy gap that can be used as n- type materials in solar cell although showing a good optical sensitive for UV wave length (near visible wave length in compound  and  that can be used as detector or chemical sensors for these wave length.
The authors would like to extend their sincere appreciation to the Deanship at Baghdad University College of Science, and I want to thank everyone who helped me to complete this research.
How to cite this article: Ahmed Khudhair Kadhim*, Muna Ismael Khalaf, Mohammed Kadhim. Synthesis, characterization, Band gap and optical properties of new pyromellittic diimide core dervatives. Eurasian Chemical Communications, 2021, 3(12), 872-880. Link: http://www.echemcom.com/article_138837.html
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