PHOTOIONIZATION OF THE LEUCOCARBINOLS OF MALACHITE GREEN
ion and oxygen gas. A mechanism analogous to the above will describe the radiolytic decomposition of
1037
KC103 by X-rays14 to form On(g), CIOz-, CIOz, C10-, and C1-.
The Effect of the Properties of Solvents of Various Dielectric Constants and Structures on the Photoionization of the Leucocarbinols and Leucocyanides
of Malachite Green, Crystal Violet, and Sunset Orange and Related Phenomena
by Edward 0. Holmes, Jr. Hughes Research Laboratories, Malibu, California (Received August 12, 1966)
Photoionization occurs only in solvents whose dielectric constant is greater than 4.7. Malachite green leucocyanide shows 100% photoionization in three straight-chain alcohols but less in chlorinated hydrocarbons; hence, a “chemical factor” is present. The photoionization of the leucocyanides of crystal violet and sunset orange in absolute alcohol are and absorbance ratios of malachite green leucocyanide were reported. The average A, determined in various solvents after various treatments. The anomalous photochemical behavior of malachite green carbinol in ethyl alcohol is interpreted on the basis of two forms, one of which is nonphototropic. The absorption bands of crystal violet, malachite green, and sunset orange ions are related to structure.
I. Historical The reason for the choice of this problem is that in 1957 the author1 published a paper on the degree of photo:onization of malachite green leucocyanide (MGLC) in solvents consisting of mixtures of two of the following : (1) n-hexane, (2) 1,2-dichloroethane, and (3) 1,l-dichloroethane. It was discovered that photoionization increased with increasing dielectric eonstant of the solvent. Very little photoionization occurred at values below 4.5, but above this value photoionization increased rapidly. Although the dipole moment of the solvent increased a t the same time, it was felt that the effect was due largely to increasing dielectric constant ( E ) . Spore? states that “the dielectric constant is not the sole criterion for photoionization. . . .” Hence, it seemed both interesting and worthwhile to undertake
an investigation to determine whether or not a “chemical factor,” also, might not be influential, and to learn something about its nature.
11. Qualitative Results To begin with, solutions of malachite green leucocyanide (MGLC) in solvents of various dielectric constants were made up and irradiated with ultraviolet light transmitted through a Pyrex filter. It was noted whether or not photoionization occurred by visually observing the appearance of the characteristic color of the carbonium (MG+) ion. It was found that photoionization began with solvents whose dielectric constants were in the range E = 4.5-4.7, except for diethyl ether where no photoioniza(1) E. 0. Holmes, Jr., J. Phys. Chem., 61,434 (1957). (2) A. H.Sporer, Trans. Faraday SOC.,57, 983 (1961).
Volume 70,Number 4
April 1966
1038
EDWARD 0. HOLMES, JR.
Table I : Qualitative Results on Photoionization of Malachite Green Leucocyanide in Solvents of Various Dielectric Constants Dielectric constant Name
(4
Cyclohexane n-Hexane Dioxane Carbon tetrachloride Tetrachloroethylene Benzene Diethyl ether
2.05 1.85 2.20 2.24 2.30 2.28 4.335
Trichloroacetic acid Chloroform
4.6 4.8
n-Butylamine Glacial acetic acid a-Chlorotoluene Chlorocyclohexane l12-Dichloroethane 1,l-Dichloroethane Chloroacetic acid 1-Hexanol Cyclohexanol &Butyl alcohol 1-Butanol Isopropyl alcohol Isopropyl alcohol Methyl ethyl ketone n-Propyl alcohol Acetone Ethyl alcohol Dimethylformamide Methanol Acetonitrile Water Ethylene carbonate Formamide n-Methylformamide Hexylamine'
5.3 6.15 7 7.6 9.9 10.23 12.3 13.3 15 15.8 17.8 18.3 18.3 18.3 20.1 21 24.3 36.7 37.2 38 78 85.1 109 182.4
Methyl sulfoxide' Trichloroacetonitrile"
...
Dipole moments, D.
0 0 0 0 0 0 1 . 2 7 in benzene 1.1 1.11in benzene 1.40 1.74
1.85 2.3 1.89 2.07 1.524 1.64 1.9 1.66 1.62 1.6 1.75 2.79 1.69 2.89 1.69 3.5 1.7 3.9 1.84 4.87 3.2
...
1.32-1.59 in benzene 4.3 2.0
+
Photoionization"
Rem ark s
Blue fluorescence
-
+ -
+ + + + + + + + + + + + + + + + + + + + + + +
Dimer Unusual yellow, fades Stable color Stable 25 days
Stable 25 days Fast reversal b
Color fades in 2 hr Slight yellow through Pyrex, ppt with quartz ultraviolet
O 0.98 ( N H ~ O mole , fraction of water). = occurs; - = does not occur. * See quantitative results where N H ~ = be positioned as complete data are not available.
tion occurred but a strong blue fluorescence was observed. Table I gives a complete list of the various solvents tried and the results obtained.
111. Preparations
A . Malachite Green Leucocyanide. It was found quite impossible to prepare malachite green leucocyanide directly free from contamination by the carThe Journal of Physical Chemistry
' These cannot
binol. Hence a new method of purification was devised that gave a pure product quickly and easily. The impure malachite green leucocyanide was prepared by treating a solution of 2 g of malachite green oxalate (Eastman) in 11. of water with another solution containing 2 g of potassium cyanide in an equal volume of water. After allowing the precipitate to settle for 2 hr, the crude leucocyanide was removed by filtration and washed with water.
PHOTOIONIZATION OF THE LEUCOCARBINOLS OF MALACHITE GREEN
It was then dissolved in benzene and shaken in a separatory funnel with a dilute solution of potassium cyanide, twice. to convert a maximum amount of oxalate to the leucocyanide. Then, the carbinol remaining as an impurity was extracted by shaking the benzene layer with a very dilute water solution of hydrochloric acid. This was repeated several times until the acid solution was colorless. Finally, the remaining acid was extracted from the benzene layer by shaking with water. The benzene layer was now bright yellow. This color was easily removed by treatment with animal charcoal. Following filtration, the benzene solution was evaporated on a hot plate until crystallization began. Nom three to four times the volume of nhexane was added and the resulting solution was evaporated slowly until white crystals started to form. On cooling, a large crop of pure white crystals, mp 188", resulted which was dried in a vacuum pistol at the temperature of boiling chloroform. Recrystallization from hexane showed no change in color or melting point. B. Malachite Green Carbinol. The crude carbinol was made similarly to the crude leucocyanide, using sodium hydroxide in place of potassium cyanide. In this case, purification by extraction with acid was not applicable owing to the ease of reaction of the carbinol with the slightest trace of acid. h great deal of effort was expended in the purification of the crude product to get colorless crystals with the same melting point. The melting points of crystals obtained by various methods varied from 102 to 121" and the crystal form from needles to hexagonal dendrites from the same n-hexane solution, depending solely on the rate of crystallization. Many of our preparations were light green. It was found that the color could be completely removed by dissolving these in ether. The solution was now materwhite. n-Hexane was added and the carbinol crystallized from this solvent. The crystals were dried as usual in a vacuum pistol in subdued light but frequently turned green again. Table V will show that the physical properties were the same regardless of crystal form and melting point. Others3 have reported this variation in melting point and crystal form. C. Preparation of Crystal Violet Leucocyanide and Carbinol. The cyanide was prepared in a manner similar to that of the malachite green leucocyanide and had a melting point of 283". In the preparation of the carbinol, we experienced the same type of difficulties as with malachite green carbinol but finally suc-
1039
ceeded in obtaining crystals that were very pale purple and melted with decomposition at about 189". D. The Preparation of Sunset Orange4 Carbinol and Leucocyanide. The preparation of the carbinol is adequately described by Walba and B r a n ~ h . ~ To prepare the leucocyanide, the carbinol was converted to the dye by dissolving in ether and shaking with dilute hydrochloric acid solution. After separating the layers, the water layer containing the dye was treated with an aqueous solution of potassium cyanide. As no precipitation occurred, a concentrated solution of sodium hydroxide in water was added drop by drop until a heavy, finely divided precipitate was formed and the solution became colorless. The precipitate, largely the leucocyanide, was extracted with ether several times. The ether layer was then extracted with dilute hydrochloric acid many times until both the ether layer and the water layer were colorless. Now the excess acid was removed by extracting the ether layer with water. It was found that the addition of an equal volume of n-hexane to the ether layer facilitated the extraction with acid. The ether-hexane layer that now contained the leucocyanide free from carbinol was evaporated to dryness at room temperature with the aid of a jet of dry nitrogen and yielded long brown needlelike crystals which, after drying at the temperature of boiling chloroform, gave a melting point of 160". This product, in solution in absolute alcohol, produced no color when treated with hydrochloric acid, showing that this leucocyanide, like those of crystal violet and malachite green, is not acid sensitive and, in addition, that it contained no carbinol.
IV. Procedure As our initial objective mas to ascertain whether or not there mas a "chemical factor" in addition to the dielectric constant of the solvent that affected the degree of photoionization, we proceeded in the following manner. Two series of solvents were chosen in which the variation in structure followed, as far as possible, a regular pattern, and all the members possessed a relatively high degree of purity. The first series consisted of ethyl, n-propyl, and n-butyl alcohols and the second of 1,6-dichlorohexane, 1,2-dichloroethane, 1,l-dichloroethane, and chloroform. Solutions of malachite green carbinol in the various (3) V. Villinger and E. Kopelschini, Ber., 45, 2916 (1912). (4) Name assigned by author for the N-dimethylaminotriphenylmethyl cation: E. 0. Holmes, Jr., J . Phvs. Chem., 62, 884 (1958). ( 5 ) H. Walba and G. E. K. Branch, J . Am. Chem. SOC.,73, 3347 (1951).
Volume 70, Number 4 April 1966
EDWARD 0. HOLMES, JR.
1040
Table 11"
--Solven-
Solute Malachite green carbinol
Original solution Corresponding x band on addition of excess HCl x band on max irradiation, no HCl % photoionization
Amax
A Amax
A Amax
A
EtOH
n-PrOH
n-BuOH
264 0.76 622 2.37 620 1.95 82
265 0.68 624 2.33 623 0.95 41
265 0.88 624 2.35 624 0.65 28*
272 0.91 622 2.38 100
272 1.06 623 2.35 100
272 1.05 625 2.30 98
1,6-Dichlorohexane
1,2-Dichloroethane
1,I-Dichloroethane
... ... 627 2.41 625 1.83 77
268 0.72 622 2.27 263 1.60 70
268 0.76 622 2.43 622 1.70 70
622 1.04 622 1.48 ?
... ...
... ...
625 1.6 66
630 2.1 92
274 0.67 622 0.48 20
274 0.96 610 0.08 8
CHCla
... ...
Malachite green leucocyanide
Original solution
Amax
A
a
x band, excess HCl, then
Xmax
max irradiation % photoionization
A
M . 'See section on detailed discussion.
All wavelengths are in millimicrons; all concentrations were converted to 2.17 X
pure solvents were made as nearly as possible to the M , for comparison same concentration (2.17 X with earlier work by the author at this concentration1p4) and the ultraviolet spectra were obtained on a Cary 14 spectrophotometer. Then a minute amount M ) was added of hydrochloric acid (about 20 X to convert all the carbinol into the ionic form (MG+), and now the visible as well as the ultraviolet spectrum was scanned. A second trace of hydrochloric acid was added, and the range was rescanned as a check on the first run. Another portion of the same solution was irradiated with a PEK-200 super high-pressure mercury arc lamp Table 111"
Original
Amax
A x band with excess HC1, then max irradiation x band with excess HCI, then max irradiation yo photoionization
Amax
A Amax
A
-Crystal Carbinol
violetLeucocyanide
-Sunset Carbinol
orangeLeucocyanide
264 ,.. 589 2.35
272 1.53
., . .. .
262 0.43 465* 0.29
269 0.49
... . .
589 2.37
.. .
...
101
...
,
,.
.
,
The Journal of Physical Chemistry
Table IV
Solutea
..
... 465b 0.31
All wavelengths are given in millimicrons; solutions are in absolute ethanol. * Considered by the author t o be the y band. Reverse reaction was so fast that it was impossible to measure (see section on flash photolysis). a
through a Pyrex filter and a layer of water to remove wavelengths below 290 mp and heat. The irradiations were repeated until maximum values were obtained for the peak of the strongest band (A,, 622 mp x band). By dividing this maximum value by that of a solution of the carbinol of corresponding concentration, which had been completely converted by acid into the carbonium ion (MG+), and multiplying by 100, the percentage photoionization was calculated. A similar radiation procedure was used with the leucocyanide except that previous to irradiation an excess of hydrochloric acid was added to prevent any reverse reaction. The same procedures were used in the cases of the carbinols and leucocyanides of crystal violet and sunset orange. The results are tabulated in Tables 11-IV.
Mole fraction of water
Malachite green leucocarbinol Malachite green leucocarbinol Malachite green leucocarbinol Malachite green leucocarbinol a
I n ethanol-water.
mp ( A ) , Dielectric after addition constant of acid (4
Amax,
Amax. mp ( A )
0
263 (0.76)
624 (2.37)
24.3
0.76
261 (0.59)
622 (2.25)
62'
0.98
255 (0.42)
618 (1.62)
75b
0.98
After 12-min irradiation A = 1.18; photoionization 737,
Computed assuming linearity.
PHOTOIONIZATION OF THE LEUCOCARBINOLS OF MALACHITE GREEN
V. Discussion of Results Table I1 presents the data necessary for the calculation of the percentage photoionization of both malachite green leucocarbinol and leucocyanide in various solvents according to the method described in part IV. It may be seen that the photoionization of the carbinol varies irregularly in all the solvents used. On the other hand, that of the cyanide is 100% or close to it (estimated error 2%) in the three alcohols used. However, in the four other solvents used (all chlorinated hydrocarbons) the leucocyanide shows a distinct “chemical factor” related to the positions of the chlorine atoms in the molecule. When the chlorine atoms are at the end of the chain, as in l12-dichloroethane, the photoionization is a maximum, namely, 92%, but when they are adjacent on the same carbon atom, as in 1,l-dichloroethane, it is only 20%. This is most interesting as the dielectric constants of the two are 9.9 and 10.23, and their dipole moments are 1.89 and 2.07, respectively-very close to each other. Earlier results by the author’ under quite different conditions of irradiation showed a difference in the same direction but not so great (3 :2). The value of 6674 for the photoionization in 1,6dichlorohexane may be attributed to the greater separation of the two chlorine atoms in the molecule although there remains the possibility that a lower dielectric constant (not yet determined) might be partially responsible. The low degree of photoionization in chloroform solution is the result of its dielectric constant being so very close to the marginal value ( E = 4.54.-7, see Table I). Table 111is similar in structure to Table I1 and gives the results of the photoionization of crystal violet leucocyanide (100%) and sunset orange leucocyanide (loo+%) in absolute alcohol. Nothing unusual was experienced in the case of the crystal violet. However, in the case of sunset orange the average value for the absorbance of the y band produced photochemically from the leucocyariide (0.31 at 465 mp, SO+, having no x band), came out larger than the corresponding value (0.29) for the same band from conversion of the same concentration of the carbinol into the colored ion (SO+) photochemically, an obviously impossible result. The source of trouble seemed to be in the acid conversion. In half of the runs, the absorbance values of the peaks for the y bands (465 mp) went through a maximum; in the other half, they reached a constant value as acid was added in the most minute amounts. In every case, the value was lower than that obtained with photoionization of the leucocyanide. Even here trouble was enhanced by a reverse reaction that could
1041
not be prevented by the addition of acid before irradiation, as was possible with crystal violet and malachite green. More knowledge, which was gained on the unusual behavior of sunset orange by flash photolysis, will be presented in a following section. I n order to determine the degree of photoionization in a transparent stable medium of high dielectric constant such as water, it was necessary to diIute alcoholic solutions of the solute by slowly adding water. In this way we were able to attain a mole fraction of water (equal to 0.98) so large that the solute molecules were in an environment closely approximating that of water. Table IV gives the results obtained showing the values of A, and absorbancy with changing environment. The first line in Table IV is taken from Table I1 to facilitate comparison. It is interesting to note and absorbancies change how much the values of A, as the proportion of water is increased. This is not surprising as the partial molal values of the physical properties of water-alcohol vary unevenly and to a considerable extent, and surely affect the optical properties. Table IV gives only one determination of the photoionization of malachite green leucocyanide-that in which the proportion of water was the greatest (NH20 0.98). This was made at standard concentration (2.17 X M ) . Allowance for change in volume of solution on mixing of the two components had to be made in all cases. The fact that the solution with the highest proportion of water which has a dielectric constant about three times as great as pure ethyl alcohol is only 73% photoionized is added evidence of the influence of a “chemical factor,” and considering all the results in the Tables II-IV, the author feels that he has established beyond the question of a doubt that “the dielectric constant is not the sole criterion for photoionization.”2 The Anomalous Behavior of Malachite Green Leucocarbinol. On account of the wide variation in the melting points and crystal forms of malachite green leucocarbinol (cf. part 111 B) and the unusual phototropic behavior of the low and high melting forms (Table V, part I), a special effort was made to interpret these differences. Table V contains the average values of several determinations with each of the forms. Infrared spectra on the solid forms in potassium bromide and also in liquid carbon tetrachloride solution showed no differences between them. In search for a criterion that would be most significant in revealing any differences in the photochromic properties, it was decided to determine the initial rate of increase of the absorbance of the x band peak Volume 70,Number $,
April 1966
EDWARD 0. HOLMES, JR.
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0.5
Table V"
Part I. Average Initial Mp, "C A Corresponding Xmax of peak, mp Corresponding x band peak after addition of acid, m p A of x band peak after addition of acid
Sample A
Sample B
Values 103 0.75 266 624
121 0.75 265 625
2.32
2.35
I
I
I
I
1
1
250
260
270
280
1
0.4
0.3
Part 11. Value after Irradiation to Pseudo-Stationary State I n Visible Region A,,= of x band peak, mp 623 622 yo photoionization based on absorb31 23 ancy increase of x band - 0 .008/0. 3 - 0.22/0.3 Fading rate of peak value ( A decrease/min av A of interval) (concn 1.55 X low6Ill) Initial photolysis rate at start (zero 0.95 0.57 time) increase in A of x band/ min at 2.17 X M)
0.2
0. I
I n Ultraviolet Region A at Xmsx at pseudo-stationary state 0.49 70photolysis based on above value 42 decrease in absorbance on addi33 tion of acid Xmax after irradiation and acid, mp 251 0.23 Corresponding A Portion of A attributable to h band 0.15 peak A that could be attributed t o di0.08 methylaniline if it is assumed that one molecule of malachite green leucocarbinol is photolyzed so as t o produce two molecules of DbIA (above)
0.42 41 32 254 0.23 0.15
0.08
a All values adjusted to a concentration of 2.17 X lo-& M of carbinol in n-butyl alcohol, unless otherwise stated.
(624 mp). Under these conditions, all the carbinol is present and there is no appreciable reverse reaction. To obtain these data, extrapolation methods were used. Preliminary results, although not precise, indicated a very large difference in the initial rates of the two forms. I n order to be sure that each sample received exactly the same amount of light, a simple apparatus was devised that we named the "spin-rotor," whereby the contents of four test tubes could be irradiated in a beam of light while being rotated at about 200 rpm. All solutions of malachite green leucocarbinol in nbutyl alcohol were diluted to exactly the same concentration. Checks using the same solution in all four test tubes gave excellent results. Measurements The Journal of Phyaical Chemistry
0
240
WAVELENGTH,
mp
Figure 1. Malachite green leucocarbinol in n-butyl alcohol (1.32 X 10-5 M): A, original solution; B, at pseudo-photostationary state; A-B, difference of above curves; C, after addition of excess HCl; and D, contribution of h band (computed).
in the visible spectrum (Table V, part 11) show the lower melting form to have about 50% greater rate of photoionization than the higher, which is so far beyond the limit of any possible experimental error that it was considered quite significant. The balance of the data in Table V was obtained in the following manner. To illustrate the various steps we are introducing Figure 1 which gives the absorbance curves of a typical run. To begin with, the absorbance and corresponding A,, , in the ultraviolet of the original solutions of the carbinols were measured immediately after the solutions were made up (curve A). Then a minute amount of hydrochloric acid was added to convert all the carbinol into the ionic form, and all absorbances were measured throughout the spectrum. More acid was added as a check on these values (Table V, part I, lines 4 and 5). Next, another portion of the same solution was irradiated successively
PHOTOIONIZATION OF THE LEUCOCARBINOLS OF MALACHITE GREEN
until what appeared to be a stationary state was reached. However, in nine cases out of ten the absorbance of the x band peak rose to a maximum and then started to decrease so that we termed the maximum to be a “pseudo-stationary state” (curve B, Figure 1). The rate of the reverse (thermal) reaction for this band peak was now determined by allowing the chart of the Cary to run while the instrument remained at a constant wavelength. As soon thereafter as possible, and absorbancies in the ultraviolet were read, the A, and frequently the recovery rate of the latter was determined. Finally, excess hydrochloric acid was added and all absorbancies and values were measured again (curve C, Figure 1).
VI. Interpretation of Results As our primary object is to find and interpret, if possible, any differences in photochemical behavior between the low- and high-melting forms of the carbinol, we will begin with the 50% (70% from preliminary runs) difference in the initial rate of formation of the carbonium ion MG+. Neither the ultraviolet nor the infrared spectra showed any evidence of impurities that might be responsible. Hence, it appears that in solution the carbinol molecules exist in two types of clusters, one having different photochromic properties than the other. Some basis for this assumption is the recent work of Ericks and others, who have discovered that triphenylmethyl perchlorate ionic crystals6* (and also those of p-roseaniline perchlorate6b) contain benzene rings surrounding the methyl carbon atom which resemble right-handed and left-handed overlapping propeller blades. Yow if this arrangement were present in the carbinol crystals and gave two different types of clusters on solution, one having, for example, more of the left-handed form than of right, a difference in photochemistry would undoubtedly be observed. A second phenomenon that seemed most unusual was the difference between the measured and the calculated rates a t the “pseudo-stationary state.” It was possible to measure the rate of decrease of absorbance of the peak of the x band (623 mp), as already mentioned. Xow a t the stationary state, the rate of decrease of absorbance and its rate of increase must be equal and opposite. From the initial rate of increase of the x band a t zero time of 0.95hAlmin (Table V, part 11, line 4) and the absorbancies of the MGOH a t the start and at the stationary state (0.75 and 0.49 for sample A), it should be possible to calculate the rate of formation of the carbonium ion MG+ assuming that the latter value (AmaX of curve B in Figure 1) truly represents the
1043
amount of photolyzable MGOH. Hence, the calculated rate of the stationary state is (0.49/0.75)0.95 = 0.62, which, corrected for concentration differences, is 0.44. This is 55 times as large as the measured rate (-0.008/0.3). This must mean that in the photostationary state there is not nearly so much photoionizable carbinol as one would assume. Although these figures are approximate, the difference is so enormous that it cannot be due to experimental error and suggests that there must be a form of carbinol that can not be photoionized but yet reacts with acid to form the MG+ represented by curve C (Figure 1) and the ultraviolet data in Table V, part 11. The possibility that its curve B might be due to the butyl ether of malachite green formed in the solution itself is ruled out by the fact that the ether can be photoionized to the extent of 72%. We checked our basic assumption by integrating the area under several absorption curves and found it proin the concentraportional to the absorption at A, tion range used above to within a few per cent. Curve A-B in Figure 1 might be considered the absorption curve of the photoionized carbinol. Its A, is slightly to the right of that of curve B and much to the right of curve C which leads us to surmise that curve A is a composite curve. The final results on samples A and B seem to be the same but this is not surprising as now there remains only the absorbance of the photoproducts and that due to the h band in this far-ultraviolet region (see part
VII). We regret that it is impossible to give an unequivocable interpretation of the rather conflicting data but are strongly inclined to believe that two forms of carbinol are present (possible three) with distinctly different photochromic properties, when dissolved in butyl alcohol. Moreover, the facts that crystals that are initially white and, while protected from light, heat, and water and carbon dioxide, eventually turn green and that the low-melting form converts into a higher melting form while its melting point is being taken lead us to believe that the ultimate stable form is a green ionic crystal and that the carbinol tends to go through the following transitions : low-melting form + high-melting form + green ionic crystal. Results with Flash Photolysis. (a) The reason that no photoionization is observed visually when our solutes are irradiated in a solvent of no dipole moment and low dielectric constant, such as hexane or dioxane, might be that there is a reverse reaction of extreme ( 6 ) (a) A. H. G . De Mosquita, C . H. Mac Gillarry, and K. Erioks, ~ c t aCryst., 18,437 (1965); (b) unpublished results.
Volume 70, Number 4
April 1966
EDWARD 0. HOLMES, JR.
1044
Table VI: Am,= and Absorbance Ratios for x, y, and g Bands of MG+ 7-Previoue
Solvent
HCl added initially
Ethyl alcohol Ethyl alcohol-water (mole fraction water 0.76) Ethyl alcohol-water (mole fraction water 0.98) n-Butyl alcohol 1,2-Dichloroethane Ethyl alcohol n-Propyl alcohol n-Butyl alcohol 1,2-Dichloroethane 1,l-Dichloroethane Chloroform Ethyl alcohol Ethyl alcohol n-Propyl alcohol n-Propyl alcohol n-Butyl alcohol Mean (12 best values)
treatment-
Irradiated
X X
HCl added after irradiation
X
X X X X X X
X X X X X
Absorbance ratios Band-----Y R
Y
g
317 319 320
1 1 1
0.19 0.19 0.20
0.18 0.18 0.19
X X
X
X
618
425
311
1
0.21
0.19
624 622 622 624 623 623 623 620
430 430 426 428 434 430 430 428
318 318 318 319 320 319 318 318
1 1 1 1 1 1 1 1
0.23 0.19 0.19 0.19 0.19 0.21 0.18 0.21
0.22 0.17 0.20 0.18 0.21 0.15 0.18 0.19
319 318 318 319 318 319.0
1 1 1 1 1 1
0.17 0.19 0.19 0.19 0.19 0.19
0.17 0.19 0.19 0.19 0.18 0.185
Malachite Green Cyanide 620 428 X 622 428 X 623 428 624 428 624 430 622.4 428.5
rapidity. To answer this question a solution of malachite green leucocyanide in n-hexane was examined by flash photolysis with a xenon lamp using a 360-joule input and a filter that transmitted in the range 230-400 mp. The result showed less than 0.1% conversion with a reverse reaction of about 4 sec, or virtually nothing. The same leucocyanide in p dioxane showed even less photoionization. (b) With malachite green leucocyanide dissolved in absolute ethyl alcohol we obtained 100% photoionization just as previously reported with successive irradiation from our mercury lamp. Flash photolysis is surely much quicker and more convenient. Malachite green carbinol gave 82% photoionizationexactly the same amount as with the slower and more cumbersome method of successive irradiations. (c) When the reverse reaction is very rapid, as with sunset orange leucocyanide and carbinol, flash photolysis is absolutely necessary. As was done in the former cases, the apparatus was standardized and checked with an acid solution of the carbinol whose absorbance was adjusted to unity, at maximum of the 465-mp band. A solution of the carbinol was flash irradiated using The Journal of Physical Chemistry
ma
Band-----
Malachite Green Carbinol 622 42 7 622 427 622 428
X X X
Amax,
,
a 106-joule discharge three times in succession a few seconds apart. The results showed 76, 76, and 70% photoionization. With still another portion of the solution, 180-joule inputs were used, and we waited after each successive flash until the absorbance returned to zero, which rquired only a few minutes. The results were 82.5, 52, and 26% photoionization. Hence, it is observed that, although there is very little decomposition of the carbinol owing to the irradiation, the reverse reaction forms a product that cannot be photoionized. Using a solution of sunset orange leucocyanide in absolute ethyl alcohol of the same concentration as above, successive flashes using 106 joules showed 103, 28, and 11% photoionization. The solution was allowed to fade completely before the last two.
VII. Comparison of A, of Band Peaks and Ratio of Absorbancies In addition to our search for a “chemical factor,” we were interested in the effects of various solvents on the position of the peaks of the various absorption bands and the ratios of the corresponding absorbancies of the peaks of the bands of the malachite green carbon-
PHOTOIONIZATION OF THE LEUCOCARBIROLS OF MALACHITE GREEN
ium ion. Data gathered from various runs are presented in Table VI which shows solvents, acid treatment, and irradiations used in each case. A remarkable constancy was found. The average of 12 determinations of the A,, of the three principal bands are 622.4, 428.5, and 319.0 mp with absorbancy ratios of 1, 0.19, and 0.185, regardless of the nature of the solvent, irradiation time, or added HCI. The greatest deviation from this average was exhibited by a solution of malachite green leucocyanide in nbutyl alcohol to which an excess of acid had been values were 624, 430, and 318 mp added. The A, and absorbance ratios, 1,0.23, and 0.22.
1045
Table VI1 Amax,
Ion
Band
mp
x y g h n
589
2.35
...
...
...
305 251
0.93 0.56
9.84 12.0
...
...
...
x
622 428 318 251 236
2.35 0.44 0.423 0.195 0.23
4.82 7.0 9.44 12.0 12.7
5.10
cv+
VIII. The Relation of the Absorbance Bands to Structure of the Carbonium Ions of Crystal Violet, Malachite Green, and Sunset Orange in Absolute Alcohol Considerable previous work7 has been done relating maximum absorbancies and corresponding wavelengths to structure. However, it does not appear that those of the three closely related ions, CV+, NG+, and SO+, have been considered in view of the new short-wave band (then band not previously reported). Table VI1 gives the data and frequency relations. Inasmuch as our interpretation will deviate somewhat from the previous, we have not adhered strictly to the older notation. S o attempt has been made to locate the positive charge on the ion, and the ring systems have been written in conventional manner fully realizing that the true structure of the ion is uncertain and must be represented by several resonance structures. Table T'II, in which the frequency ratios are compared, shows that the x, g, and h bands are closely related and belong to the same system. Naking the conventional assump tion, the band of lowest frequency, the x band, of the crystal violet ion (CV+) is the result of an induced dipole along the longest dimension of the molecule, namely. through ring 1 and either of the 2 rings- -the quinoid ring and a dimethylaniline ring. The correspondence of the peak frequencies of the g band and h band IS so close to those of dimethylaniline itself that me feel that dipoles induced in each of the 2 rings in the ion are responsible for these. Table VI1 reveals that the ratios of the frequencies of the x band and g band peaks for the ions CV+ and AIG+ (1) within each ion--1.95 and 1.95-and (2) between the two ions-1.05 and 1.04-are almost identical, so that these bands should result from the same induced dipoles as above-rings 1 and 2 for the x band, and ring 2 for the g band in the malachite green ion. Furthermore, the fact that the absorbance of the g band peak for the MG+ is so close to half its
Frequency M ) X 10-14
A (2.17 X
y
g h n MG
x
...
...
...
y g h n
463 345 264 255
0.29 0.07 0.18 0.14
6.48 8.70 11.4 11.8
sot Ion Ratios of Frequencies of Xmax for Same Ion 5
h
n
Band
X
g
Y
CV MG
1.93 1.93
1.22 1.27 1.31
1.81 1.82
+
so
+
...
+
...
Ion Ratios of Frequencies of Corresponding Xmax Compared between Different Ions X
CY
1.05
g
h
Y
1.04
1.oo
...
1.08
1.05
1.08
n
+
so +
MG
1.08
+
value in CV+ strongly suggests this relation. The same is true for the h band. The ion of sunset orange (SO+) has no x band because there is no dimethylaniline ring present. Table (7) G. N. Lewis and M. Calvin, Chem. Rev.,2 5 , 273 (1939); G. N. Lewis and D. Lipkin, J. Am. Chem. Soc., 64, 2801 (1942); G. N. Lewis and J. Bigeleisen, ibid., 65, 520 (1943); 65, 2170 (1943); H.Walba and G. E. K. Branch, ibid., 73, 3347 (1951).
Volume 70, Number 4
April 196%
1046
VI1 shows a close relation of the frequencies of the y, g, and n bands (all 1.08) of this ion to those of the MG+ ion. S o w the n band, that of highest frequency may be attributed to the dipoles induced in the benzene rings present (3 rings), as the peak frequency of this band is within the range of those of benzene itself. I n the three ions, the h and g bands do not appear to have such a close relationship. However, they appear in closer relationship in the CV+ and MG+ ions than in the 3iG+ and SO+ ions. Regarding the n band that is common to both ions, we feel that in the MG+ ion it is the result of the strong 250-mp band in the dimethylaniline structure, whereas in the SO+ ion it originates from a new dipole set up through the two benzene structures (3-3). To explain the presence of the very weak g band in the SO+ ion we must assume that there remains a small amount of resonance character of dimethylaniline associated with the quinoid ring itself.
The JOUTnal of Phyaical Chemistry
EDWARD 0. HOLMES, JR.
The identical values for the ratio of the frequencies of n and y bands in the MG+ and the SO+ ions (1.81 and 1.82) and the ratio of the frequencies of the corresponding values between the y and n bands between these ions (1.08 and 1.08) lend strong support to the arguments presented above.
Acknowledgments. The author desires to express his gratitude to Dr. George Smith, Director of the Hughes Research Laboratory, for his invitation to use the facilities of the laboratory for the investigations herein described, and to Dr. J. David Margerum, who proposed my association with the laboratory and made space, supplies, and equipment available. I n addition, he is further indebted to Dr. Margeruni for all flashphotolysis determinations and valuable suggestions and to Dr. Robert Brault for assistance and advice in the preparation of the pure organic compounds used and infrared determinations.
