Synthesis and Characterization of New Photoresponsive, Ortho and Para Oriented Azomethine Polymers

Five new azomethine polymers having aliphatic-aromatic moieties were synthesized by polycondensation reaction of dialdehydes and diamines. The dialdehyde monomers differ only in the orientation of the aromatic ring (ortho or para) and were synthesized by condensation reaction between aromatic aldehyde and 1,6-dibromohexane. The molecular mass of the monomers was recorded through E.I mass spectrum. The polymers structures were confirmed by elemental microanalysis, FT-IR, 1HNMR and UV-Vis Spectroscopy. The morphology of monomers and polymers was evaluated by scanning electron microscopy (SEM). All the polymers were soluble in DMSO (on heating) and somewhat in other solvents. Thermal stability of polymers was analyzed by thermogravimetry (TG) and differential thermal analysis (DTA), all the polymers showed good thermal stability higher than their corresponding monomers. The TG of polymers indicated maximum rate of weight loss (Tmax) within 412–708 °C. Fluorescence emission spectra of polymers were recorded and the results indicated that all the polymers were photo-responsive and indicated 1 to 4 emission bands with maximum within 349–606 nm. The limit of detection of polymers was within 0.625–1.25 μg/ml. The polymers were also examined for their antimicrobial activities against bacteria and fungi.


Introduction
The conjugated azomethine polymers also called Schiff base polymers are reported and studied since last several decades 1 .3][4] However, an interest in preparation of new Schiff base polymers and their applications in different field keep on increasing. 5The researchers are focusing their attention toward conjugated azomethine polymers during recent years 6,7 because of their useful properties such as electrical conductivity, optoelectronic and thermal stability. 8,91][12][13][14] Poly(azomethines) containing (-N=C) functional group have been applied successfully to some extent as transporting materials in organic solar cells [15][16][17] and their application. 18They can act as antimicrobial agents and these are proving interesting, because these are nonvolatile and thermally stable and cannot penetrate through human skin. 19,20They could protect losses through skin by volatilization. 21,22The conjugated polyazomethines indicate fluorescence properties, they can be applied in the manufacture of chemical sensors, photoluminescence devices and light emitting diodes. 23,24The Schiff base polymers derived from aromatic aldehydes with ortho-hydroxy group (salicylaldehyde) can act as chelate polymers with transition metal ions for their removal from industrial contaminated and waste water. 25The polymeric Schiff bases having aliphatic-aromatic groups indicate better thermal stabilities, but they are difficult to process as they have high melting/ decomposition points and are insoluble in common organic materials. 269][30] Flexible spacers have also been introduced to enhance their solubility without affecting their thermal stability. 31In the present work five new photo-responsive polyazomethines were synthesized, they differ in orientation of ether groups attached with the aromatic rings and also various types of aromatic or alicyclic rings were incorporated in the polymer chain, the purpose of these structural modifications was to investigate their effects on the properties (solubility, thermal stability and Qureshi et al.: Synthesis and Characterization of New Photoresponsive, ... fluorescence) of polymers.The monomers and their polymers are characterized by different spectroscopic techniques, thermal analysis, scanning electron microscopy (SEM), solubility, spectrofluorimetry and antimicrobial activities.

3. Synthesis of Polymers
The polymers were synthesized by slightly modified general procedure as reported 26,31 as under: A 250 ml round bottom flask equipped with condenser and magnetic stirrer was charged with equimolar mixture (5mmol) of different diamines and dialdehydes, both were dissolved separately in DMF solvent, then p-toluenesulfonic acid was added as catalyst.The mixture was refluxed under nitrogen with continuous stirring for 6h.The mixture was poured into 250 ml of water and allowed to form precipitate.The product was collected by filtration, washed with ethanol and dried.

4. Analysis of Monomers and Polymers.
The elemental microanalysis of polymers was performed by elemental microanalysis Ltd, Devon, United Kingdom.E.I mass spectra of the monomers were recorded on JEOL JMS 600 mass spectrometer (USA) at HEJ Research Institute of Chemistry, University of Karachi, Sindh-Pakistan.UV-Vis spectra of monomers and polymers were recorded in DMSO solvent within 500-200 nm on Perkin Elmer double beam Lambda 35 spectrophotometer (Perkin Elmer, Singapur) using dual 1 cm quartz cuvette.Spectrophotometer was controlled by the computer with software.FT-IR spectra of the synthesized compounds were recorded within 4000-600 cm -1 on Nicolet Avatar 330 FT-IR with Attenuated total reflectance (ATR) accessory (smart partner) (Thermo Scientific, USA). 1 HN-MR spectra of the compounds were recorded on Bruker AVANCE-NMR spectrophotometer (UK) at 400 MHz using tetramethylsilane (TMS) as internal standard and DM-SO as solvent at HEJ Research Institute of Chemistry, University of Karachi Sindh-Pakistan.Fluorescence measurement was performed on Spectrofluorophotometer RF-5301 PC Series (Shimadzu Corporation, Kyoto, Japan) using 1cm quartz cuvette.Thermogravimetry (TG) and Differential thermal analysis (DTA) were performed at Centralized Resource Laboratory, University of Peshawar, Peshawar-Pakistan on thermogravimetric thermal analyzer Pyris Diamond TG/DTA (Perkin Elmer, USA) in nitrogen atmosphere with a flow rate of 50 ml/min and heating rate of 20 °C /min from 50 °C to 800 °C using 5 to 9 mg of sample placed on ceramic pan.In order to determine the morphologies of polymers they were also characterized by Scanning electron microscopy using JEOL JSM-6490LV Scanning Electron Microscope (USA) at Center for Pure and Applied Geology, University of Sindh, Jamshoro, Sindh-Pakistan.The accelerating voltage for taking images was 15 KV.
The antibacterial activity of the polymers was measured through 96 well plate method by using microplate alamar blue assay.The antibacterial activity was tested against bacterial species: Escherichia coli, Shigella flexenari, Staphylococcus aureus, and Pseudomonas aeruginosa using standard drug Ofloxacin.For measuring antifungal activity of the polymers agar tube dilution method was used.The antifungal activity was tested against fungal species: Trichphyton rubrum, Candida albicans, Aspergillus nigar, Microsporum canis, Fusarium lini, Canadida gla-Scheme 1. Reaction scheme (a) synthesis of para oriented polymers (b) synthesis of ortho oriented polymers.brata using standard drug Amphotericin B for Aspergillus nigar and drug Miconazole for other species.Percent inhibition of the polymers was compared with the percent inhibition of the standard drug.For antibacterial assay 2 mg of polymer was dissolved in DMSO solvent to get concentration of 50 µg/ ml.For antifungal assay the concentration of polymers was 200 µg/ ml in DMSO.Incubation period was 7 days at 28 °C ± 1 °C.

1. Synthesis of Monomers and Polymers
The dialdehyde monomers (p-HOB or o-HOB) were prepared by condensation of p-hydroxybenzaldehyde or o-hydroxybenzaldehyde with 1,6-dibromohexane.The monomers were obtained in good yield, p-HOB = 92% and o-HOB=81%.The aliphatic spacers of n-hexane are common in both the (dialdehyde) monomers.The variation is only in the ortho and para linkages.The polycondensation of an equimolar mixture of dialdehyde (p-HOB or o-HOB) with diamines (1,5-naphthalenediamine, 1,4-phenylenediamine, 1,2-cyclohexanediamine, 2,6-diaminopyridine) results into polymers (PpHOBND, PpHOBPD, PpHOBCy, PoHOBPD or PoHOBP) containing aliphatic-aromatic groups in the main chain following the reaction Scheme 1.The polymers were also obtained in good yield (76-79%).The structure of the polymers was confirmed by different techniques and the results supported their formation.Salih İlhan et al. have reported the formation of Schiff base by the condensation of monomer o-HOB with 2,6-diaminopyridine. 34 Similar reactants were used for the synthesis of polymer PoHOBP, the melting/decomposition point of the polymer (PoHOBP) was above 360 °C while the reported Schiff base decomposed at 280 °C, the mass spectrum of the polymer (PoHOBP) obtained through E.I mass spectroscopy did not show the mass corresponding to Schiff base, the polymer (PoHOBP) had higher mass than the reported Schiff base which supported the formation of the polymer (PoHOBP).

2. Solubility of Monomers and Polymers
The solubility of monomers and polymers is summarized in Table 1.The monomers were soluble in organic solvents and insoluble in water.The polymers were soluble in DMSO on heating but the PpHOBCy was soluble in DMSO without heating also.The better solubility of PpHOBCy is because of the presence of more flexible cyclohexane ring while other synthesized polymers have rigid aromatic rings.

E.I Mass Spectrum of Monomers
The mass spectrum of p-HOB is already reported 31

4. FT-IR of Monomers and Polymers
FT-IR of p-HOB is reported 31 and the FT-IR of o-HOB also agreed with the reported values 34 .The comparative FT-IR of p-HOB and o-HOB showed as under: monomer p-HOB and o-HOB showed strong band at 1685 cm -1 and 1678 cm -1 for υ C=O respectively, p-HOB shows bands at 1595, 1507 cm -1 and o-HOB at 1595, 1484 cm -1 for υ C=C aromatic rings.The p-HOB showed bands at 1250, 1069 cm -1 and o-HOB at 1244, 1072 cm -1 for C-O-C vibrations.The polymers PpHOBND, PoHOBPD and Po-HOBP showed weak band while PpHOBCy indicated medium intensity band within 1668-1682 cm -1 due to υ C=O of end on groups but this band was not visible in PpHOB-PD.The polymers indicated band of strong to medium intensity within 1596-1640 cm -1 due to υ C=N.One to two bands were visible within 1601-1482 cm -1 due to aromatic rings of the polymers.The polymers show band within 1233-1249 due to C-O-C asymmetric vibrations and a band within 999 to 1021 due to C-O-C symmetric vibrations.The polymers spectra showed number of band within 980-646 cm -1 due to in plane and out of plane C-H vibration of aromatic rings as shown in Figure 2. Similar assignments have been indicated for FT-IR of polyazomethines 8 .

5. 1 HNMR Spectroscopy of Monomers and Polymers
The 1 HNMR of monomer p-HOB 31 and 13 C-NMR of o-HOB 34

6. UV-Vis Spectroscopy of Monomers and Polymers
UV-Vis spectra of monomers and polymers were obtained in DMSO.The monomer p-HOB shows a broad band at 283 nm and its molar absorptivity was 3.2 × 10 4 L.mole -1 cm -1 .The monomer o-HOB shows two bands at 258 nm and 322 nm with molar absorptivities 1.5 × 10 4 and 8.8 × 10 3 L mole -1 cm -1 .The polymers PpHOBND, PpHOBPD and PoHOBP showed two bands while polymer PoHOBPD showed three bands (Figure 4), the increase in the number of bands in the absorption spectra is due to π-π* transition in conjugated azomethine with naphthalene, phenyl and pyridine rings.The polymer PpHOBCy showed only one band because extension of conjugation was not possible with cyclohexane ring.The results are summarized in Table 2.

7. Fluorescence Spectroscopy of Monomers and Polymers
The monomers and polymers contained conjugated chromophoric groups which could indicate fluorescence intensity within UV-Vis region.Choi et al. 24 have reported fluorescence from poly(azimethines).Fluorescence emission of the monomers and polymers were examined in DMSO solvent.The monomer p-HOB showed a emission band at 378 (at excitation 275 nm) and o-HOB shows two emission bands 354 nm ( excitation 258 nm) and 374 nm ( excitation 322 nm).The polymers indicated two to four emission bands (Figure 5), except PpHOBCy which indicated one emission band at 349 nm (excitation 275 nm).The results of spectrofluorometric studies are summarized in Table 3, and the results showed that all the monomers and the polymers were fluorescence materials.The polymer PpHOBND indicated highest fluorescence intensity and PpHOBCy indicated lowest intensity.There was a shift in wavelength of emission and excitation of the polymers as compared to their corresponding monomers p-HOB and o-HOB due to polymerization.The number of emission bands observed were higher (3 and 4) for the polymers PoHOBPD and PoHOBP derived from o-HOB as compared to the polymers PpHOBND, PpHOBPD and PpHOBCy (1 to 2 emission bands) derived from the mon- omer p-HOB due to ortho group effect.All the polymers showed 1 or 2 color emissions except PpHOBCy, the emission colors include violet, blue, green and red.The limit of detection (LODs) of the polymers in DMSO were calculated, at least signal to noise ratio 3:1 at the emission band of higher sensitivity and were observed within 0.625-1.25 µg/ml.

8. Thermal Analysis of Monomers and Polymers
Thermal behavior of monomer and polymers was evaluated by TG (Thermogravimetry) and DTA (Differential thermal analysis) in nitrogen atmosphere.TG and DTA of monomer p-HOB is reported. 31TG of o-HOB showed three stages of weight loss with 73% weight loss within 216-465 °C, 6% weight loss within 466-542 °C and 15% weight loss within 543-625 °C with maximum rate of weight loss (T max ) at 357 °C, DTA showed melting endotherm at 93 °C, followed by vaporization/decomposition exotherms at 403, 464 and 532 °C and large decomposition exotherm at 603 °C.TG of PpHOBND showed four stages of weight loss with 6% weight loss within 300-426 °C, 37% weight loss within 427-520 °C, 6% weight loss within 521-605 °C and 48% weight loss within 606-795 °C, T max indicated at 708˚C, DTA showed two exotherms at 416 and 466˚C due to vaporization/decomposition and large decomposition exotherm at 714˚C.TG of PpHOBPD showed two stages of weight loss with 28% weight loss within 363-500 °C and 66% weight loss within 501-705 °C with T max at 628 °C, DTA showed two large decomposition exotherms at 398 and 615 °C.TG of PpHOBCy showed four stages of weight loss with 22% weight loss within 280-425 °C, 40 % weight loss within 426-500 °C, 8 % weight loss within 501-555 °C and 22% weight loss within 556-626 °C with T max at 469 °C (the lower T max value was may be due to the presence of cyclohexane ring), DTA showed two decomposition exotherms at 366 and 470 °C, and a large decomposition exotherm at 596 °C.TG of PoHOBPD showed three stages of weight loss with 44% weight loss within 346-477 °C, 9% weight loss within 478-558 °C and 38% weight loss within 559-674 °C, T max value showed at 412 °C, DTA indicated three vaporization/decomposition exotherms at 395, 463 and 517 °C, followed by large decompo-  with T max at 655 °C, DTA showed three vaprization/decomposition exotherms at 358, 448 and 514˚C and a large decomposition exotherm at 684˚C.The TG/DTA graphs of all the polymers are given in Figure 6.The polymers indicated high thermal stability as compared to monomers because their T max values were higher than their corresponding monomers.The thermal analysis results are given in Table 4.

9. Biological Activities of Polymers
The polymers were tested for their biological activities against bacteria and fungi.The polymer PpHOBND showed 40% antifungal activity against Aspergillus nigar, PpHOBPD showed 30% inhibition against Fusarium Lini, PpHOBCy indicated 20% inhibition against Candida albicans, PoHOBPD showed 15% inhibition against Microsporum canis while the polymer PoHOBP did not showed inhibition against fungi, the results of antifungal activities are summarized in Table 5.The polymer PpHOBCy indicated 22% antibacterial activity against staphylococcus aureus and 3.24% inhibition against Escherichia Coli, Po-HOBPD showed 18.6% inhibition against staphylococcus aureus, PpHOBND showed 11% inhibition against Escherichia Coli and 9% inhibition against staphylococcus aureus, PpHOBPD showed 7.18% inhibition against Escherichia Coli and 4.53% inhibition against staphylococcus aureus and the polymer PoHOBP showed 8.86% inhibition against Escherichia Coli, the results of antibacterial activities are summarized in Table 6.

10. Scanning Electron Microscopy of Monomers and Polymers
The SEM images of the monomers and polymers were recorded at 100, 50, 20 and 10 µm.The polymer PpHOBND and PpHOBPD had sponge like morphology (Figure 7c and 7d).The polymer PpHOBCy had fibrous like clusters with porous surface (Figure 7e).The morphology of polymer PoHOBPD was agglomerated and this agglomerated structure was due to inter-particle attraction of monomers (Figure 7f).PoHOBP had nanoscale roughness (Figure 7g) while the reported Schiff base derived from o-HOB 34 had agglomerated clusters (Figure 7h).The monomer p-HOB had seeds like morphology (Figure 7a) and the monomer o-HOB had leaves like appearance (Figure 7b).The results support that the morphology of the polymers was different from their corresponding monomers.

Conclusion
Five new photo-responsive polyazomethines with flexible spacers of n-hexane were synthesized by one step polycondensation between dialdehydes and diamines.The polymers were characterized by elemental microanalysis, UV-Vis, fluorescence, FT-IR, 1 HNMR, TG/DTA and SEM.The polymers indicated fluorescence emissions within visible region with LODs of polymers at 0.625-1.25 µg/ml levels and high thermal stabilities within the range of 412-708˚C.The polymers were also tested for their antimicrobial activities against bacteria and fungi, the polymer PpHOBND indicated moderate antifungal activity against Aspergillus nigar.

Figure 1 .
Figure 1.E.I mass spectrum of the monomer o-HOB are reported.The comparative 1 HNMR (DM-SO) spectra of monomers indicated δ ppm for p-HOB at 9.850 and o-HOB at 10.375 for CHO, p-HOB indicated two doublets at 7.840 and 7.103 while o-HOB indicated multiplet at 7.641, doublet at 7.208, and triplet at 7.044 due to C-H aromatic protons.The p-HOB indicated triplet at 4.089 while o-HOB indicated triplet at 4.130 for O-CH 2 groups.p-HOB indicated triplet at 1.746 and singlet at 1.482 while o-HOB indicated doublet at 1.803 and

Figure 2 .
Figure 2. FT-IR spectrum of polymer PoHOBPD, conditions as experimental

Figure 4 .Figure 5 .
Figure 4. UV/Vis spectrum of the polymer PoHOBPD conditions as experimental

Table 4 .
Thermal analysis data of monomers and polymers