A large number of mono-para-substituted
1,3,5-triphenyl-2-pyrazolines and some related compounds has been synthesized,
and their absorption and fluorescence properties in cyclohexane and in methanol
measured. The effect of substituent on these properties and on the fluorescent
whitening ability of the 1,3,5-triphenyl-2-pyrazolines in general is discussed.
The presence of electron-withdrawing substituents, viz., cyano, methylsulfonyl
or methoxycarbonyl groups in the para-position
of the phenyl rings, has a dramatic effect on the absorption and the
fluorescence properties. Such substituents located (a) in the l-phenyl ring give pyrazolines exhibiting brilliant blue
fluorescence both in methanolic and cyclohexane solution, (b) in the 3-phenyl ring give highly fluorescent yellow compounds
which can be considered as dyes and (c)
in the 5-phenyl ring cause drastic quenching of the fluorescence.
The effect of substituents
on the absorption and fluorescence properties of a number of di-and tri-para-substituted
1,3,5-triphenyl-2-pyrazolines, several substituted 1,3,5,5-tetraphenyl- and trans-l,3,4,5-tetraphenyl-2-pyrazolines,
and some 1,3-diphenyl-trans-3a,4,5,6,7,7a-hexahydro-4,7-
methano-1H-indazoles in cyclohexane and in methanol has been studied. Certain
electron-with-drawing substituents, namely, cyano, methylsulfonyl or methoxycarbonyl,
have a marked effect. Such substituents located (�) in the 1-phenyl ring give pyrazolines exhibiting blue fluorescence which is quenched
if a cyano group is also present in the 5-phenyl
ring, (B) in the 3-phenyl ring give fluorescent yellow compounds which can be
considered as dyes, and (c) in the 5-phenyl ring cause quenching of the
fluorescence (unless an electron-withdrawing group is also present in the
3-phenyl ring).
Several 1,3-diphenyl-and 1,3,5-triphenyl-2-pyrazolines containing a heteroaromatic substituent (viz. imidazol-1-yl, pyrazol-1-yl and 1,2,4-triazol-1-yl) have been synthesized. When compared with their unsubstituted analogues, these compounds (a) absorbed more intensely at a longer wavelength, (b) exhibited greater fluorescence in a polar solvent (methanol) and (c) possessed similar stability to light.Large-scale irradiations of two 1,3,5-triphenyl-2-pyrazolines (viz. unsubstituted and that containing the 1,2,4-triazol-1-yl substituent) in methanolic solution revealed that, although dehydrogenation of the 2-pyrazoline ring was the major photochemical reaction, significant oxidation to the corresponding chalcone also occurred. In addition, phenol was detected among the irradiation products.Several sulphonated analogues of the abovementioned pyrazolines were prepared. When applied to wool, these compounds exhibited excellent fluorescent whitening properties.
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