Green Synthesis of New Tetra Schiff Bases and Bis-Azo Bis-Schiff Bases Derived from 2,6-Diaminopyridine as Promising Photosensitizers

Nine new tetra Schiff bases (M2 M9) were prepared in moderate yields via the condensation of different aromatic amines and bis-Schiff base (M1) in microwave synthesizer. Also five new azo-Schiff bases (M16 M20) were prepared by the condensation of (M1) with the azo-salicylaldehyde (M11 M15) using the same method. The green synthesis by microwave irradiation was chosen as route due to its novelty, cleanliness, efficiency, time and solvent saving properties compared with the conventional methods which lack these advantages; such as time consume and wasting environment polluting organic solvents to achieve the same efficiency in synthesis. The prepared compounds which are believed by us to be competent as photosensitizers in photochemical systems were identified by IR and NMR spectroscopy besides elemental analysis.


Introduction
Schiff bases, by containing the azomethine group (C=N) as the central structural form, have been the subject of prevalent research in the various fields of industry, pharmaceuticals and the synthesis of biologically active organic compounds [1] [2] [3] [4]; besides corrosion inhibitors [5]. Recently Schiff bases became an important class of ligands in coordination chemistry [6]; Metal complexes the-reof have shown attractive properties such as antibacterial behavior [7] [8] and exhibit interesting magnetic characteristics as well as catalytic oxidation [9].
Various methods and routes have been developed for the synthesis of Schiff bases such as reflux in ethanolic solution, stirring, grinding in mortar, besides microwave irradiation methods [10]. The last one has received increasing interest from researchers [11] [12] [13]. Synthesis of some Schiff bases was reported in aqueous medium as a green alternative approach [14]. Azo-Schiff compounds are a new class of chemical compounds that are receiving increasing concern in scientific research [15]- [21]. In the present days, these derivatives display remarkable applications in each and every field [22]- [30]. An earlier study showed that the incorporation of Schiff bases into azo compound structure resulted in enhanced photostability property of the prepared azo-Schiff compounds compared with the azo precursors; whereas the azo linkage, as chromophor, assists in extending the absorption of the resulted compound to the longer waves of the solar spectrum [30]. This acquired property enables them to be potential photosensitizers in photochemical systems as a part of the recent strategy for environmentally benign solar energy conversion methods [31] [32] [33]. The aim of this work is to prepare some new tetra-Schiff bases and azo-Schiff bases as a part of a comprehensive plane to develop new photosensitizers adapting the advantageous concept of green methods in synthesis [6] [12].

Materials and Methods
All chemicals were obtained from sigma-Aldrich, Merck, and Scharlau, and used without further purification. Melting points were determined using a calibrated thermometer by electro thermal melting point apparatus Stuart-SMP11; and were uncorrected. Elemental analyses were performed by department of chemistry, Ibn-al Haitham College of Education, University of Baghdad, Baghdad/ Iraq. IR spectra were recorded in Ibn-al Haitham college of Education, University of Baghdad as KBr discs on a Nicolet 100 FT-IR spectrophotometer.
Uv-visible spectra were recorded on (UV-VIS) Spectrophotometer-PG +92. 1 H NMR spectra were recorded as solutions in DMSO on a NM Ready 60 pro, chemical shifts were referenced to Tetramethylsilane in the same university. Microwave irradiations were carried out in microwave synthesizer, BoMann.

General Procedure for the Synthesis of Tetra SCHIFF Bases (Green Route Method)
The same procedure which was used in preparing the precursor M 1 ; was applied in synthesizing the following compounds by mixing (1 mmol

Results and Discussions
Microwave radiation procedure were followed in the synthesis of tetra Schiff with the lower π-π* electronic transition of the azomethine group [36]. This band was also assigned by some workers to charge transfer transition International Journal of Organic Chemistry Scheme 1. The steps for tetraschiff-bases synthesis.
resulted from (Phc) as donating group, and the azomethine group as the accepting one [35] [36] [37]. In the prepared Schiff bases, the bands shifted to longer wavelengths (395 -420) nm, which may be attributed to both the bathochromic shifted π-π* transition due the effect of polar solvent (DMSO) and n-π* electronic transition of azomethine nitrogen atom [38]. Azo compounds, in general, are colored compounds so they absorb light in the visible part of the solar spectrum; but their main problem is photoinstabilities, so we aimed to incorporate the energy rich azomethine group in the body of azo compounds so as to protect them from photodegradation by dissipating the solar radiation by photoisomerization and other photodissipation mechanisms [39] [40]. Moreover; this participation will serve in preparing new colored photosensitizers which may find their way to application in the well-known photochemical conversion systems [30]. This will be tested in the upcoming works, keeping in mind that there is a long way and severe challenges associated with this mission such as matching of oxidation potential of the prepared sensitizers and reduction potential of the relay systems.

Conclusion
The newly prepared, colored and thermally stable tetra-Schiff bases as well as azo-Schiff bases using microwave activation as environmentally friendly method-depending on our humble knowledge and experience-are candidates to be efficient absorbers in solar liquid collectors, and also as photosensitizers in Dye Sensitized Solar Cells (DSSC) to produce cost effective electricity or in four components systems to produce photo hydrogen fuel from aqueous mediums.