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) Received Sep. 9, 2002. Abstract Schiff 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 EXPERIMENTAL1.1 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: 1 2 3 4 5 The elements analysis
shows( the theory value in the bracket): 2 RESULTS AND DISCUSSION nm lex= 450 nm lem= 624 nm Fig. 3 The excitation and emission spectra of salicyl-o-diaminobenzene lex=392, 414, 477nm lem = 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.
lex= 385,412, 440nm lem= 576nm 2.3 Photochromism 2.4 Thermochromism 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.
lem= 500nm Fig. 9 The thermochromism excitation spectrum of salicyl-p-diaminobenzene lem = 576nm Fig.10 The thermochromism excitation spectrum salicyl-o-diaminobenzene lem = 560nm 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. REFERENCES กก |
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