TRANSMISSION SPECTRA OF DYES I N T H E SOLID STATEr BY
w. c. HOLMES‘ AND
A. R.
PETER SON^
Practically no data are available in the literature on the transmission of light by dyes in the solid state. The method herein employed consisted in depositing a thin film of “air dried” dye upon a glass slide and interposing it at right angles to the path of one of the parallel beams of light of a visual spectrophotometer. The dye films were obtained by the evaporation of aqueous or alcoholic solutions of the dyes. With many dyes it is difficult or impossible to obtain continuous films in this manner without resorting to the employment of gelatine or similar agents which it was desired to exclude. It was found possible to obtain deposits of a reasonably satisfactory character, however, with numerous azo, triphenylmethane, and xanthene dyes. I n such instances the only difficulty involved was that of regulating the thickness of the dye film within limits wherein its light absorption could be measured advantageously. Typical absorption measurements obtained with such films are recorded in Table I. The dyes investigated were representative samples of commercial grade which were, in nearly all instances, recrystallized before use. The numbers recorded with them are their list numbers in the Colour Index of the Society of Dyers and Colourists (Bradford, England, 1924). Aqueous solutions were employed in obtaining these films of azo dyes and alcoholic solutions in obtaining those of dyes of other classes. The tabulated values are Bunsen extinction coefficients. Discussion Although the spectra of solid dyes frequently differ considerably from the spectra of their dilute solutions, they exhibit no features which are exclusively characteristic of the solid state. The outstanding modifications in spectra which occur when dyes are dissolved are ( I ) a shifting in the spectral location of the absorption band, attended by ( 2 ) evidences of increasing molecular dispersion and, frequently, by (3) evidences of tautomeric alteration in the dye. The first of these phenomena, however, also occurs in passing from one solvent to another, and evidences of increasing dispersion and of tautomerism which are of the same type, if not of equal degree, are often observed upon the mere dilution of aqueous dye solutions.* ~
* Contribution No. 205 from the Color and Farm Waste Division, Bureau of Chemistry
and Soils, U. S. Department of Agriculture, Washington, D. C. Senior Chemist, Color and Farm Waste Division, Bureau of Chemistry and Soils, C . S. Department of Agriculture, Washington, D. C. * Reaearch Asaociate, Commission on Standardization of Biological Stains. Holmes: Ind. Eng. Chem., 16, 35 (1924).
TRANSMISSION SPECTRA O F DYES IN T H E SOLID STATE
I249
TABLE I Extinction Coefficients (E) of Light transmitted by Dyes in the Solid State Part Janus Green B
Colour Index No. 133
460 mp 480 ” 490 ”
309
E
-
-
-
_.
0.57
500
’I
510
”
520
”
530
”
0.48
-
-
-
-
-
o
0.62
1.38 1.42
0.66
1.45
0.69
1.48
0.55
0.72
0.75 0.77
1.50 1.50
0.78 0.;7
0.78 0.83 0.86 0.89 0.92 0.94 0.9; 0.96 0.9; 0.92 0.90
540
”
550
”
560
”
0.63 0.69
570
”
0.72
0.75 590’’ 0 . 7 8 600 ” 0 . 8 1 ”
610 ” 620 ” 630 ” 640 ” 650 ” 660 ” 670 ” 680 ” 690 ” 700 ”
I-AZO Dyes
Niagara Pontamine Niagara Pontamine Pontamine Blue 3RD Blue AX Blue RW Sky Blue Sky Blue 6BX jBX 468 502 jIZ jI8 520
0.42 0.42 0.43
580
Dianil Blue R
o.;6j 0.76
1.49 1.48 1.46 1.43 1.40 1.36 1.32
0.56 0.62 0.69 0.76 0.84 o 94 I .03 1.09 1.16
70
0.74
0.83
0.74
0.84
0.72
0.82
0.69 0.67
1.25
0.79
1.18
0.87 0.85
0.74
0.64
1.10
0.68
0.62
0.63
0.j9
0.58
0.56
0.52
0.54
0.45
0.51
1.03 0.95 0.88 0.84 0.81
1.21
1.27
1.32
0.56 0.66 0.77
0.90 1.03 1.15 1.27
1.36 1.45 1.50 1.50 1.44
0.65 0.73 0.82 0.92 1.02
1.09 1.16 1.23
1.28 1.32
1.36 1.40 1.42
1.35 1.33
1.35
1.25
1.17
1.39 1.35
0.87
1.21
1.11
1.32
0.87 0.80
I
1.0;
I
0.73 0.67 0.62
1.24 1.16 0.97
,
,19
1.22
1.23
0.95 0.83
,30 1.29 1.30
0.70
1.30
0.57
1.27
Part 2-Triphenylmet hane Dyes Victoria Brilliant Alphazur- Xylene Pararos- Crystal Ethyl Green Green ine FG Blue VS aniline Violet Violet Colour Index No. 657 662 671 672 676 681 682
460 mp 480 ” 490 ” 500 ”
510
”
520
”
-
-
-
-
-
0.28 0.16 0.10 0.30 0.25 0 . 1 4 0.36 0.12
530’’ 0 . 4 0 540 ” 0.60 550 ” 0.80 560 ” 0.99
0.07
0.21
0.44
0.35
0.54
0 .52
0.73 0.98
0.71
E
-
0.72
0.77
0.77
Acid Fuchsine 692
-
0.80
-
1.10
0.62
1.18 1.23
0.15
0.82
0.22
0.87 0.91 0 . 8 0 1.23 0.89 1.07 0.95 1 . 2 0 0.89 1.18 1.08 1 . 1 7 0.88 1.22 1.19 I . I ~ 0.87 1.19 1 . 2 7 1.10 0.85 1.13 1.30 1 . 0 4
0.30
0.46 0.68 0.89 1.28
W. C. HOLMES AND A. R. PETERSON
1250
TABLE I (continued) Extinction Coefficients (E) of Light transmitted by Dyes in the Solid State Victoria Brilliant Alphssur- Xylene Pararos- Crystal Green Green ine F G Blue VS aniline Violet
Colour IndexNo. 657
”
1.13 I .23 I . 29 1.31 I .29 I .24
’I
1.22
’I
1.22
”
I .20
”
I .
”
I
” ”
0.90 0.65
”
0.41
570
”
580
”
590 600 610 620 630 640 650 660 670 680 690
”
700
” ”
16
.os
662
0.86 0.98 I .06 1.11
.09 I .06 I .06 I .07 I .03 0.96 0.86 0.75 0.61 0.58 I
67 I
.29 .60 1.77 I .89 1.93 I .96 I I
672
E 1.53 1.77 1.95 2.07
2 .I O
2.12
2.02
2.12
2.03
2.19 2.29 2.36 2.32 2.26
2.02
1.95 I
.Bo
1.29 0.69 0.37
2.07 I
.90
Part 3-Xanthene Ponta- Victoria Pyromine Violet Blue B nine G C4B Colour Index No. 698 739 729
460mp 480 ” 490 ” 500 ” 0 . 2 5 5’0 0.34 520 0.48 530 ” 0.67 540 ” 0 . 8 1 550 ” 0 . 8 9 560 ” 0 . 8 8 570 ” 0.81 580 ” 0.75 590 ” 0 . 6 8 600 ” 0.64 610 ” 0.60 620 ” o,jj 630 ’’ 0.47 640 ” 0.36 650 0.27
::
-
__ -
0.23 0.36
Pyronine B 74’ E
-
I
0.95 I .OI
I .48 1.59 I .66
I
.04
I
.os
0 . 50
I
.04
0.66 0.83 0.96 I .06
I .02
I
1.15 1.13
0.95 0.89 0.84 0.80 0.76 0.70 0.63
I .IO
0.57
.09 .06
0 . 50
1.58 1.45 I .40 I .30 I . 13 0.86 0.59 0.33 0.19
1.12
1.15
I I
I .OI
0.96
0.45 0.40
-
.70
0.11
-
68 I
676
0.83 0.81 0.75 0.67 0.61 0.54 0.46 0.40 0.35 0.32 0.30 0.29 -
Ethyl Acid Violet Fuchsine
I .06 0.97 0.90 0.83 0.78 0.73 0.68 0.63 0.56 0.48 0.38
0.28
0.21 0.16
I
682
692
.30
0.96 0.84 0.70 0.58 0.46 0.35
I .27
1.19 1.15 1.12
I
.08
I
.oo
-
0.88 0.72
0.56 0.41 0.27
-
-
Dyes Rhoda- Rhoda- Rhoda- Violamine B mine G mine 3B mine B 749
750
-
-.
-
-
0.35 0.44 0.53 0.61 0.62 0.62 0.66 0.68 0.67 0.60 0.36 0.18 0.11
-
-
0.56 0.69 0.86 0.95 0.95 I .oo I
.07
1.12 I .06 0.84 0.52
0.23
-
75’
0.33 0.45 0.60 0.71 0.79 0.82 0.83 0.85 0.87 0.87 0.83 0.76 0.60 0.42 0.31 0.25
-
-
-
757
0.37 0.48 0.67 0.83 0.95 I .02 I
.oj
I
.oo
0.93 0.82 0.66 0.54 0.40 0.27
0.19 0.12
TRANSMISSIOK SPECTRA O F DYES I N THE SOLID STATE
1251
TABLE I (continued) Extinction Coefficients (E) of Light transmitted by Dyes in the Solid State Ponta- Victoria mine Violet Blue B C4B Colour Index No 698 729
660 670 680 690
” ” ”
o o o o
o 18 o 13 o IO -
-
700
91 83 73 62
0.51
Pyronine G
Pyronine B
739
741
719
750
751
E -
-
-
-
-
-
-
-
-
Rhoda- Rhoda- RhodaViolamine B mine G mine 3B mine B
__ -
-
4-Xanthene Violamine RR Colour IndexNo. 758
460 mp 480 490 ” 0.60 500 ”
510
’l
520 ”
530 540 550 560 570
’’ ”
I’
”
580 ” 590 ’’ 600 ” 610 ” 620 ’’ 630 ” 640 ” 650 660 670
680 690 700
” I’
I’
’’
Violamine
G 759
768
0.30
0.27
0.60 0.83
0.40
0.53 1.02 0.67 0.81 I .23 1.33 0.92 1.42 0.95 1.40 0.89 1.30 0.78 0.60 1.15 0.92 0.38 0.69 0 . 2 1 0.47 0.31 0.19 __ -
-
-
0
0.95 0.89 0.86 0.85 0.72
-
-
-
-
-
Dyes
Ethyl Eosine B Eosine 770
771
0.44 0.43 0.70 0.58 0.97 0 . 7 2 1.19 0.90
Erythrosine 773
Phloxine Rose B Bengal B 778
0.68
-
0.52
0.88
0.71
0.93 0.97
0.43 0.67 0.90
1.00
1.10
0.92 I .06
1.00
1.05 I .02 1.00 1.09 1.06 0.99 0.97 1.03 1.04 0.91 1.04 1 . 1 5 1.13 1.27 0.95 0 75
1.13 1.17 1.11
0.92 0.67 0.38
0.82
0.68 0.52
-
0.23
-
-
-
0.11
-
-
-
-
0.21
1.10
0.32
0.10
779
1.20
0.53
0.17
07
-
E
-
0.78
-
Eosine
757
0.38 0.16 0.26 0.17
-
0.53 0.33 0.19 0.11
-
-
-
-
1.23
1.13
1 . 3 5 0.80 1.28 0.45 0.75 0.18 0.38 0.08 0.23
-
-
-
W. C. HOLMES AND
1252
A.
R. PETERSON
Fig. I affords a comparison of the absorption spectrum in the solid state with spectra of dilute alcoholic and aqueous solutions of several of the dyes investigated. 1*
$00
a o
SUO
s o
Violamrnc
si0
400
,,,,
RR
SO1
s 01 1.4
/.a. 1.0
0.f 0.L
A-dlcohoke r d B - a o u e o u s sol.
0.4
,0.z
A - a l c o h o l i c sol. E - a y u c o u r sol
c - solld dy R
I
_I
sa0
$90
s(0
sm
COO
&lo
+
SOD
Szo
Syo
S60
5#0
600
FIG.I
A comparison of the spectra of solid dyes with the spectra of their aqueous and alcoholic solutions indicates that the effect of solution upon the hue of the dye is intrinsically hypsochromic. The practical effect, however, may frequently be the reverse. This anomaly arises from the fact that the hue of many dyes is a composite hue of two isomeric forms, and that the solution
TRANSMISSION SPECTRA O F DYES IN T H E SOLID STATE
I253
of such dyes is attended by an alteration in the equilibrium between the tautomers which exerts an effect upon the composite hue which is opposite in type and greater in degree than is the direct effect of solution. Thus, Crystal Violet exists in two tautomeric forms with decidedly different hues, of which the lighter predominates in the solid state. The hue of each dye form becomes lighter when the dye is dissolved, but solution is attended by such extensive conversion of the lighter dye form into the deeper that the hues of solutions are decidedly deeper than that of the dye. The general effect of solution, however, is to displace absorption bands in the direction of shorter wave length, and dye solutions are lighter in hue than are the dyes from which they are prepared when tautomerism does not exert a preponderant effect. In general, the absorption bands of solid dyes are decidedly broader in proportion to their height than are those of their solutions. The depression of band maxima and the broadening of their slopes are criteria of increasing molecular aggregation4 and would be anticipated in passing from solution to the solid stat'e. I n the instance of Brilliant Green, the phenomenon of increasing molecular aggregation was observable in the solid state. Freshly prepared dry films of dye absorbed light in the manner indicated by the tabulated measurements, but the absorption bands became distinctly broader and less well defined within a few hours. Previous investigation of aqueous solutions of dyes has demonstrated that many dyes undergo tautomeric alterations of one type or another. The data of Table I are of interest because of the further evidence which they afford for that conclusion. Numerous indications of tautomerism in aqueous solutions of azo dyes of diverse character have been noted,3 and Brodej has recently established the tautomeric nature of even a very simple type of azo dye. The measurements with solid azo dyes indicrite a type of tautomerism in disazo dyes prepared from dianisidine which appears characteristic for that dye type. The type of taut'omerism illustrated in the instance of Ethyl Eosine (Fig. I ) has been observed previously in fluoresceine and in its halogenated derivatives substituted within the phthalic anhydride residue.3 The examination of solid dyes has now shown that it also occurs in the more important dyes of the same group in which substitution occurs within the resorcin residues. Similar results to those reported have also been obtained with several kindred dyes of laboratory preparation, and it may now be stated that all the dyes of the group which are substituted exclusively by halogens exist in two tautomeric forms. This tautomerism may be suppressed, however, by other types of substitution. Neither Eosine B (Table I) nor mercurochrome give evidence of tautomerism. I t appears very probable that' tautomerism in this dye group must originate in changes in the character of the quinoid oxygen bonding. The most obvious explanation which might be suggested is that of tautomerism between an anhydrous dye form in which the oxygen is typically quinoid and a hydrated Pihlblad: Z. physik. Chem., 81, 417 (1912). 51, 1204 (1929). Brode: J. Am. Chem. SOC.,
I254
W. C. HOLMES AND A . R. PETERSON
dye form in which it is replaced by two hydroxyl groups in the hemiquinoid arrangement. Although the typically quinoid structure has been assigned to the dyes of this group for many years, it has recently been shown that they retain the elements of a molecule of water with great tenacity and that their structure is probably hemiq~inoid.~,’It is by no means clear, however, that actual tautomerism occurs between quinoid and hemiquinoid structure. The previous study of dye solutions has demonstrated that aminated dyes of the triphenylmethane, xanthene and quinonimide classes undergo a striking tautomeric alteration with the dilution of their aqueous solutions.a This phenomenon has been observed with a very large number of such dyes, including many of those which are of great importance in scientific applications. I n general a decided degree of tautomeric alteration could be noted with any considerable variation in the dye concentration of aqueous solutions. With basic and acid fuchsines and with diamino derivatives in the triphenylmethane dye group, however, the degree of tautomeric alteration observable was very small, and the violamines in the xanthene dye group appeared stable. The recorded series of measurements on solid dyes indicate very clearly that .basic and acid fuchsines, and that diamino derivatives of the triphenylmethane series in general, undergo decided degrees of tautomeric modifications, differing from other triphenylmethane dyes only in respect to the variation in conditions which is required to render them manifest. On the other hand, the measurements on violamines in the solid state give no indication whatever of the presence of more than one dye form. It now appears, accordingly, that the violamines probably constitute the sole exception to the general rule that aminated triphenylmethane, xanthene and quinonimide dyes exist in two tautomeric forms. This fact seems significant. A hypothesis has been advanced which refers this type of tautomerism to valence rearrangements, occurring within the salt-forming amino group, which involve a tautomerism between trivalent and pentavalent bonding of the nitrogen atom.8 The violamines are unique among the dye classes referred to in respect to the fact that they do not allow a pentavalent amino arrangement. They are structurally incapable, accordingly, of the type of tautomerism postulated and would be expected to prove stable on the basis of the hypothesis in question. Summary Transmission spectra are recorded of thirty-two azo, triphenyl(I) methane and xanthene dyes in the solid state. (2) Solution is found to displace the absorption bands in the direction of shorter wave length. (3) The data indicate a high degree of molecular aggregation in dry dyes. (4) New evidence is afforded on the occurrence and degree of tautomerism in dyes. Gomberg and Tabern: J. Ind. Eng. Chem., 14,1115(1922). ‘Holmes and Scanlan: J. Am.Chem. Soc., 49, 1594 (1927). 8Holmea: Stain Tech., 1, 116 (1926).
a
