Synthesis of triazine based dialdehyde Schiff's base – new templates for Molecular Imprinting and study of their structural and photophysical properties

2.1 Materials and methods

Reagents and solvents were procured from S.D. fine chemicals (India) and were used without purification. Fractosil 500 was used for Molecular Imprinting. The reaction was monitored by TLC using on 0.25 mm E-Merck silica gel 60 F₂₅₄ precoated plates, which were visualized with UV light. The FT-IR spectra were recorded on Perkin–Elmer 257 spectrometer using KBr discs. ¹H NMR and ¹³C NMR spectra were recorded on VXR 400-MHz instrument using TMS as an internal standard. Mass spectra were recorded on Finnigan mass spectrometer. The visible absorption spectra of the compounds were recorded on a Spectronic Genesys 2 UV–Vis spectrophotometer. TGA measurements were performed on SDT Q 600 v8.2 Build 100 model of TA instruments Waters (India) Pvt. Ltd.

2.2 Synthesis of compounds

2.3 Molecular Imprinting of Fractosil 500 with DIPOD Schiff's base 7a

Fractosil 500 was refluxed in 5% HNO₃ for 1 h and then washed with H₂O thoroughly and dried in a vacuum oven (120 °C) for 5 h before use. Into a suspension of oven-dried Fractosil 500 in 20 mL of dry toluene was added DIPOD Schiff's 7a dissolved in a minimum amount of chloroform. The suspension was refluxed in a nitrogen atmosphere for 2 days, mixture was filtered and silica was washed thoroughly with toluene and then chloroform and dried under vacuum for 6 h. This modified silica gel was stirred in a solution of 5 N HCl in 5 mL 50% methanol for 5 h to remove DIPOD and separate out imprinted silica which contains free amine groups. After 5 h, the silica gel was filtered, washed with 5 mL methanol and 20% sodium bicarbonate solution followed by water (20 mL) to remove acidic content and dried under vacuum for 5 h. The filtrate was concentrated under reduced pressure to recover DIPOD and which was further confirmed by thin layer chromatography as well as with standard DIPOD samples. The imprinted silica gel was heated with 25% aqueous HF (40 mL) in 10 mL chloroform. After 1 h, 20 mL water was added and the organic layer was removed. The organic layer was washed with water and dried over sodium sulfate and concentrated up to dryness. The obtained amine was confirmed by TLC, UV and FT-IR analysis with standard amines.

3.1 Density functional theory calculation

Density functional theory calculations were performed to optimize the geometry of compound 5. Ground state geometry was optimized with B3LYP level of theory and 6–31/G(d) basis set using Gaussian 03 W package (Frisch et al., 2004).

The optimized geometry has energy −1563.23970820Hartree (−42537.93494877 eV) with PMS gradient norm approximately 0.00. The triazine ring and N,N-diethyl amino phenyl ring are in plane but due to the angular nature of C–O–C bonds (14–18–22) and (16–19–21) 4-formyl phenyl rings are out of plane. The distance between two formyl groups (C32–C34) is calculated to be 13.02846 Å and results are summarized in Table 1 and optimized structure is summarized in Fig. 2. The distance between incorporated groups on solid support depends on the distance between two formyl groups present in molecules, the calculated distance between two formyl groups in compound 5 is 13.02846 Å.

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Figure 2

Optimized structure of compound 5.

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3.2 Thermal stability

In order to examine the thermal stability of these DIPOD Schiff's bases, Thermo gravimetric (TG) and differential scanning colorimeter (DSC) analyses were carried out between 40 and 600 °C under a nitrogen atmosphere. The TG curves of the compounds are shown in Fig. 3. The TG results indicate that the frame work of the synthesized compounds is stable up to 300 °C. Above 300 °C the thermo gravimetric curve of the synthesized compounds showed a major loss in weight for compounds 7a–7c, but compound 7d did not decompose even up to temperature 600 °C. The comparisons of the T_d (decomposition temperature) showed that the thermal stability of the 7a–7d decreases in the order 7d > 7b > 7a > 7c. Thermal stability and their plausible degradation scheme are presented in, Fig. 3.

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Figure 3

Thermogravimetric analysis of compounds 7a–7d.

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3.3 Photophysical properties

The UV–Vis absorption and emission spectra of compounds 7a–7d were recorded in dimethyforamide (DMF), to find out the role of Schiff's base functionality for electron communication in DIPOD system between the electron donating and acceptor terminals on photophysical properties at room temperature and compound concentration was 1 × 10⁻⁶ M. The λ_max (absorbance) values of compounds 7a–7d were obtained as 372 (1.554), 375 (1.106), 372 (0.801) and 335 (0.421) nm, respectively and λ_max (emission) values of compounds 7a–7d were obtained as 434 (179), 428 (97), 428 (25), 411 (19) nm, respectively. As can be seen the absorption characteristics of compounds 7a–7d are nearly similar. The absorption and emission spectra of compounds 7a–7d are shown in Figs. 4 and 5 respectively. Electron donating group on Schiff's causes a blue shift.

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Figure 4

UV–Vis absorption spectra of compounds 7a–7d.

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Figure 5

Fluorescence emission spectra of compounds 7a–7d.

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Quantum yield of compounds 7a–7d were determined by using anthracene as standard. Absorption and emission characteristics of standard as well as unknown samples were measured at different concentrations of unknown samples and standard at 2, 4, 6, 8 and 10 ppm levels. Absorbance intensity values were plotted against emission intensity values. A linear plot was obtained. Gradients were calculated for each compound and for standards. All the measurements were done by keeping the parameters constant such as same solvent and constant slit width. Relative quantum yield of all synthesized DIPOD derivatives 7a–7d were calculated by using Formula (1) (Padalkar et al., 2012).

Relative fluorescence quantum yield

The fluorescence quantum yields of 7a–7d in ethanol at room temperature are obtained and presented in Table 2. It is observed that the quantum yield values for 7b–7d were much lower than those for 7a. Furthermore the fluorescence quantum yield of 7a is about 10 times higher than that of 7b and all compounds showed similar fluorescence behavior.