A study on fluorescence spectra of Schiff bases
Yang Huisen, Xia Xinfu
( Department of Pharmacy, Shaanxi College of Traditional Chinese medicine, Xianyang, 712083; Department of Chemistry , University Shihezi, Shihezi, 832003)
AbstractSchiff bases with the typical structure were synthesized, their fluorescence spectra were determined in solid state, and photochromism and thermochromism of the related Schiff bases were investigated. The result shows that mono-Shiff bases have symmetrical excitation and emission spectra, while the excitation and emission spectra are asymmetric for bis-Schiff bases. No photochromism is found with mono-Schiff bases, while both photochromism and thermochromism are obvious for bis-Schiff bases.
In view of the fact that information will be transmitted and processed on the optical medium , the development of compounds used for storing of information, optical driving, non-linear optical appliances and others has emphasized significance [1-3] . Schiff bases have photochromic and thermochromic properties, many of them have non-linear optical property, and all of these have been given great attention now. This paper reports fluorescence spectra of the Schiff bases with typical structure, and preliminarily studies on photochromism and thermochromism of the related Schiff bases. These results will be helpful for probing into the relationship between their spectra and structures.
1.1 Instruments and materials
Salicylaldehyde( A.R), benzaldehyde( A.R), aniline( A.R), diethylenetriamine( A.R), o-Phenylenediamine ( A.R), p-Phenylenediamine( A.R) absolute alcohol( A.R).
LS50B fluorescence /luminescence /phosphorescence spectrograph, ultraviolet lamp.
1.2 Synthesis of Schiff bases with different structures
An aldehyde of certain amount was added with 100ml of absolute alcohol in a three-necked round bottom flask, after the content was shake up, an amine of the calculated amount as the reactant proportion was added. The reaction mixture was stirred until yellow or jacinth crystals precipitated out. If no crystal formed, the reaction mixture was concentrated to remove the alcohol until the crystal precipitated. The powdered was filtered, washed and then recrystallized in alcohol. Finally, the obtained powdered was placed at a dark vessel. Take different aldehydes and amines and repeat the above-said procedure to synthesize other Schiff bases, the obtained compounds are identified as follows:
The elements analysis shows( the theory value in the bracket):
Compound 1, C:85.83%(86.19%), H: 6.01%(6.08%), N:7.69%(7.73%);
Compound 2, C:79.01%(79.29%), H: 5.46%(5.53%), N:8.81%(8.93%);
Compound 3, C:76.70%(76.92%),H:5.21%(5.13%), N:9.29%(8.97%);
Compound 4, C:76.68%(76.92%),H:4.91%(5.13%), N:9.20%(8.97%);
Compound 5, C:73.41%( 73.72%), H:6.80%( 7.17%), N:9.72%(9.56%).
These compounds all have characteristic absorption of C=N at 1610cm-1 in their IR spectra.
1.3 Spectral properties of the Schiff bases
Excitation and emission spectra of the synthesized Schiff bases with different structures were detected in LS50B fluorescence/luminescence/phosphorescence spectrometers .Relevant results showed in Figures 1-5.
Fig The excitation and emission spectra of benzalaniline
ex = 448 nm em= 607
2 RESULTS AND DISCUSSION
2.1 Relationship between the colors and structures of the synthesized products
There are two categories in the synthesized Schiff bases, one is the Schiff base poly condensed from salicylaldehyde and different amines, another is the Schiff base from benzaldehyde and different amines. A rule may be found with the both categories, i.e., the color of the former is darker than that of the latter. This is because there is a hydroxyl group on which an atom with unpaired electrons is as the auxochromic group in the molecule structure of salicylaldehyde. When the atom is introduced into a conjugated system, the unpaired electrons on the hydroxyl group also enter the conjugated system, the delocalization of-electrons in the molecule is enhanced, resulting in darker color of the compound.
2.2 Characteristics of fluorescence spectrum of the Schiff bases
Excitation and emission spectra of some synthesized Schiff bases are illustrated in Figures 1 – 5 respectively. And the detection of fluorescence spectra condition is reflected in the solid state of front surface.
Fig The excitation and emission spectra of salicylanilin
nmex= 450 nm lem= 624 nm
Fig. 3 The excitation and emission spectra of salicyl-o-diaminobenzene
ex392,414, 477nm em= 560nm
It may be seen from Figs.1 through 5 that both the excitation and emission spectra of mono-Schiff bases have more stronger symmetry. While the excitation spectra of bis-Schiff bases are more complex, their emission spectra are relatively simple, and both the spectra have more stronger asymmetry. For the bis-Schiff bases, only emission spectra with the same wavelength may be obtained regardless of different excitation wavelengths, and their spectra, no matter excitation or emission spectrum, have relatively less symmetry due to difficulty to conjugate, on the other hand, the excitation and emission spectra are more symmetrical for the mono-Schiff bases, which structures are readily subjected to conjugation
Fig .4 The excitation and emission spectra of salicyl-p-diaminobenzene
ex= 370nm lem= 500nm
Fig. 5 The excitation and emission spectra of salicyl diethylene triamine
ex= 385,412, 440nm lem= 576nm
Chemistry online has found in the detection of excitation spectra of Schiff bases after expose them to ultraviolet lamp for 1min each time that very obvious photochromism appears for salicylaniline(Fig 6), salicyl-o-diaminobenzene(Fig7), or salicyl diehylenetriamine (Fig 8) in solid state.
Fig.6The photochromism excitation spectrum of salicylaniline
Fig. 7 The photochromism excitation spectrum of salicyl-o-diaminobenzene
The same law may be seen in Figures 6, 7, and 8: the peak intensity is gradually increased upward, change of the fluorescence intensity is fairly sharp in the first five or six minutes, and after that the change becomes much gentle. The photochromism is found to be reversible to a certain degree in the test. The reason of fluorescence spectra change so may be that the original enol-imine structure changes into a keto-amine structure after radiation of light, i.e., the hydrogen atom that originally was on the phenol hydroxyl group transferred onto the nitrogen atom. Because the luminophor of the keto-amine structure has higher energy than that of the enol-imine structure, the keto-amine structure may change into the enol-imine structure after light radiation stops 
The condition is that we have heated the solid state of sample to 60ºC, then quickly detected them in the fluorescence spectrometer.
Less papers on thermochromism of Schiff bases are reported, we have dynamically detected thermochromism of salicyl-p-diaminobenzene (see Fig.9), and salicyl-o-diaminobenzene (see Fig. 10). It is found there is the same basic law for thermochromism as that for photochromism, and so we think a certain relationship must be between thermochromism and the molecular structure.
Fig. 8 The photochromism excitation spectrum of salicyl diehylenetriamine
Fig. 9 The thermochromism excitation spectrum of salicyl-p-diaminobenzene
Fig.10 The thermochromism excitation spectrumsalicyl-o-diaminobenzene
It is found in the test that after exposed to light or heat, the absorption spectra of the Schiff bases from salicylaldehyde and amines obviously change. No photochromism is found for the Schiff bases from benzaldehyde and amines. This seems to relate to the fact that the structure of the former is readily changed, while that of the latter is relatively stable. For organics, photochromism, or thermochromism is concerned with change of the molecular structure, e.g., tautomerizm, cis-trans isomerism, ring-opening or ring-closing reaction, on occasion, dimerization or oxidation-reduction reaction. Perhaps, there is a triangular relationship among thermochromism, photochromism, and the molecular structure.