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THE
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The Relation between the Absorption Spectra and the Chemical Constitution of Dyes. XXX. Photoisomerization of Azo Dyes in Aqueous Solution1!2 BY MAYN. INSCOE, JOHN H. GOULD AND WALLACE R. BRODE RECEIVEDMARCH16, 1959 Although it has been possible to detect little or no phototropism in solutions of p-aminoazo and p-hydroxyazo compounds in hydroxylic solvents up to this time, i t has been found t h a t phototropic effects (which undoubtedly stem from photochemical cis-trans isomerization) can readily be observed with p-alkoxyazo compounds. A number of water-soluble methoxyazo compounds have been prepared. The cis isomers of these compounds are relatively stable in water, and all of t h e compounds show marked spectral changes when their aqueous solutions are irradiated. I n contrast t o t h e behavior of t h e corresponding hydroxyazo compounds, the rate of the thermal c i s to trans reaction of some methoxy compounds is more rapid in alcohol or acetone t h a n in water.
Introduction Previous studies on the reversible photochemical cis-tram isomerization of p-hydroxyazo and paminoazo compounds have indicated that the phototropic effects are suppressed by ethan01.~ The effect of this hydroxylic solvent would appear to be chiefly a result of an increase in the rate of the thermal cis to trans isomerization, for Hartley has shown that the addition of traces of methanol to benzene solutions of p-hydroxyazobenzene or paminoazobenzene greatly reduces the half-lives of the cis isomer^.^ It is therefore not surprising that aqueous solutions of most typical water-soluble azo dyes show no change in spectrum upon irradiation. However, one well-known dye, Chrysophenine (the sodium salt of 4,4'-bis-(4-ethoxyphenylazo)stilbene-2,2'-disulfonic acid, (2.1.24895),6 has been reported to show marked phototropic effects in aqueous solution^.^ Studies with this dye indicate
Chrysopiic~iinc
that these changes in spectrum are likewise a result of cis-trans isomerization about the azo linkages,* and that the ethylenic linkage is not involved. Chrysophenine differs from other common azo dyes by the presence of ethoxyl groups rather than the auxochromic hydroxyl or amine groups more commonly found in these dyes. Irradiation of water solutions of the dye Brilliant Yellow (C.I. 24S90), which differs from Chrysophenine only by ( 1 ) Presented a t the 134th National Xleeting o f the American Chemical Society, Chicago, Ill., September, 1938. ( 2 ) This research was supported in part by the United States Air Force through t h e Air Force Office of Scientific Research of the Air Research and Development Command under contract S o . CS-6803 7 - 3 . Reproduction in whole or in part is permitted for any purpose of the L-niied States Government. (3) LT,R . Brode, J. H. Gould and G . 51. Wyman, T H I S J O u R N A r l , 7 5 , 1856 (1803). ( I ) G. .4 Hartley, J . Chem. S o ' . , (333 (1938). (.i) G . hI. \\'yman, Chem. Rer,s., 66, 625 (1QS53), ( l i ) C . I numbers are taken from the "Culuur Index," 2nd edition, 'l'he Society of Dyers and Colourists, Bradford, England, l!&ij. ( 7 ) E I. Stearns, J . O p l . SOC.A m c r . , 32, 282 (1942); H . F. \\'illis, .I 0. Warwicker. H. A . Standing a n d A R Urquhart. T r n n s . F n v n d a y .hf>r.,41, 506 (194.5). (8) E. Aiherton and Ii. H . Peters, "Recent Advances in the Chemi i t r ) o f Ci>lrliiring Xlatters." Special F'riblication T u . 1, T h e Chemical h:i?ty I . < i n d i 8 1 1 [ ) 17 ,l!):ti]
possessing hydroxyl groups in place of the ethoxyl groups, produces no spectral changes. Furthermore, in benzene or acetone solutions, the cis isomer of p-methoxyazobenzene has been reported to be very much more stable (the half-life is 600 times longer) than that of p-hydroxyazobenzene.4 The relative stability of the cis isomers of 9-alkoxyazo compounds (as compared to the corresponding hydroxy- or aminoazo derivatives) suggested the desirability of investigating aqueous solutions of a number of methoxyazo compounds for possible phototropic effects. Experimental ( a ) Preparation of Methoxyazo Compounds.-The hydroxyazo compounds used as starting materials were either commercial dyes of known composition or were prepared by conventional methods of diazotization and coupling. Methyl sulfate was used as the methylating agent, and the solvent was water, acetone or a n acetone-water mixture. Small portions (1 to 2 ml.) of methyl sulfate and of a 20% sodium hydroxide solution were added alternately to a well-stirred solution of the hydroxyazo dye, keeping the mixture basic a t all times. No external heating was required. After 5 or 10 minutes the reaction mixture became noticeably warmer, and in some cases precipitation of the methylated dye was observed. Addition of the methyl sulfate and sodium hydroxide was continued until no color change was observed when a small sample of the reaction mixture was diluted and acidified. The total reaction time varied from one to several hours, and a considerable excess of methyl sulfate was required. Stirring was continued for a n hour after the reaction appeared to be complete. Additional sodium hydroxide then was p u t in and the reaction mixture was warmed to rcmove excess methyl sulfate. The crude product obtained by concentrating the reaction mixture was recrystallized several times from water or alcohol, the choice of solvent depending on the dye. When working with hydroxyazo compounds derived from salicylic acid, it was observed t h a t some methylation of the carboxyl groups occurred. Before recrystallization, the5c methyl groups were removed by refluxing the product with ZOyo sodium hydroxide solution for a n hour. The dyes prepared for this investigation are iiited in Table I. ( b ) Measurement of Absorption Spectra.-The absorption spectra were determined by means of a Cary recording spectrophotometer (model 14) using matched fused quartz absorption cells with the solvent as the reference standard. ( c ) Irradiation of Solutions.-For irradiation, a 500-watt projection lamp, placed between two concave mirrors, was used. Light from the source was condensed by one mirror, (9) We have found this dye t o be somewhat phototropic in acetone. T h e spectral changes obtained are very similar t o thuse observcd with Chrysophenine, b u t are not as extensive. We did nut o b h r r \ c any increase in intensity above 450 mw as reported for this dye in pyridinewater solution by A . S. Dunn ("Recent Advances in the Chemistry o f Colourinl: Matters," Special Publication So,1, 'The Chemical S n c i e t y , London, 11. 45 (195~3)). Such a n increase in intensity might be a rcsult of an indicator change, for this d y e has an absorption maximum a t k73 m p in 1,;isic solutions
Nov. 5 , 1039 TABLE I . b 3 S O K P T I O S S P E C r R A U A T A O F A ~ E T H O X Y A Z OCOMPOUNDS I N LVATER
--Effect
_______ I v n m Form5------fnlax
X
A,,,,,,
Compound, acidn
m-(~-llethoxyphen!-lazo)-benzenesulfonic
p-(4-Methoxyphenylaz~)-benzenesulfonic p - ( 2,4-Dimethoxyphenylazu)-benzenesulfonic ?-Methoxy-3-( 2,4-dimetliox~-phenylazo)-benzenesulfonic 4,4'-Dimethosyazobenzene-3,3'-dicarb~xylic
348 352 375 382 362 352
10-d
25 26 16 16 21 40
A?
C?
241 241 240 252 253 230
9
Filter usedC
A'd
348-386 348-386 352-415 368-394 342-395 342-403
365 367 385 385 370 395
X
13 11 8 10 11 12
of irradiati
