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The Carbon Tetrachloride Sensitized Photooxidation of Leuco Ethyl Crystal Violet
by A. MacLachlan Contribution No. l Y S from the Central Research Department, Experimental Station, E. I . du Pont de Nemours and Company, Wilmington, Delaware (Received July 19, 1966)
Based on direct observation of many of the reaction intermediates, a mechanism for halocarbon-sensitized oxidation of leuco ethyl crystal violet to the corresponding dye is presented. Sensitization occurs out of the leuco dye's singlet and triplet states and leads to a radical-ion intermediate rather than the triphenylmethyl radical directly. Rate constants of most of the reactions are given. Solvent effects on the various steps are studied.
Introduction
Experimental Section
In the 1 9 4 0 ' ~G. ~ N. Lewis and his associates demonstrated a number of stepwise photooxidation reactions of organic molecules contained in rigid glasses. They were able to distinguish electron ejection, triplet-state excitation, and bond-dissociation The luminescence phenomena noted4 were further investigated by Linschitz5 in a study of the origin of the delayed luminescence as it applied to electron ejection and trapping by the solvent and added solutes. Convincing evidence for the ejection of an electron into lithium diphenylamide and other compounds was obtained by recording the absorption spectrum of the solvated electron. In a second paper, Linschitzs and co-workers identified conclusively the radical ion formed by photoejection of an electron from N,N'diphenyl-p-phenylenediamine. The technique of time-resolved fluorescence spectroscopy and the flash photolysis technique' offer the possibility of examining these same reactions and intermediates in fluid solvents even at room temperature. This paper reports the application of these highspeed techniques to the study of the mechanism of halocarbon-sensitized photooxidation of tris(p-N,N-diethylaminopheny1)methane known as leuco ethyl crystal violet (LECV). Halocarbon-sensitized oxidation of amines is a well-known and very general reaction.s LECV was selected for study because of its easily characterized oxidation product, the strongly absorbing ethyl crystal violet dye.
A . Equipment. Fluorescence yield measurements were made using 2 X M solutions of LECV in ethanol. The 2537-A line of mercury was used as the exciting wavelength. Samples were placed in 1-cm quartz cells. Fluorescence quantum yields were made by comparison of the integrated fluorescence spectrum of the LECV with the fluorescence emission of a quinine bisulfate solution whose fluorescence quantum yieldQ is taken as 0.51. Fluorescence lifetime values were obtained by P. C. Hoe11 of this laboratory using a subnanosecond phase fluorimeter. This instrument consists of an exciting light, modulated at 10 Mc, which is absorbed by the sample and reemitted as coherently modulated fluorescence. A phase shift occurs, the magnitude of
The Journal of Physical Chemist?'?,
(1) G . N. Lewis, D. Lipkin, and T. T. Magel, J . Am. Chem. SOC., 63, 3005 (1941). (2) G . N. Lewis and M. Kasha, ibid., 66, 2100 (1944). (3) G . N. Lewis and D. Lipkin, ibid., 64, 2801 (1942). (4) G . N. Lewis and J. Bigeleisen, ibid.. 65, 2414 (1943). (5) (a) H. Linschitz, M. L. Berry, and D. Schweitzer, ibid., 76, 5833 (1954); (b) J. Eloranta and H. Linschitz, J . Chem. Phys., 38, 2214 (1963). (6) H. Linschitz, J . Rennert, and T. M. Korn, J . Am. Chem. Soc., 76, 5839 (1954). (7) G . Porter, Proc. Roy. SOC.(London), AZOO, 284 (1950). (8) (a) R. H. Sprague, H. L. Fletcher, and E. Wainer, Phot. Sei. Eng., 5 , 9 8 (1961); (b) D. P. Stevenson and G. M. Coppinger, J . Am, Chem. Soc., 84,149 (1962). (9) W. H. Melhuish, J . Phys. Chem., 64, 762 (1960); 65,229 (1961).
PHOTOOXIDATION OF LEUCOETHYLCRYSTAL VIOLET
u
FILTER AND
SHUTTER 1-
A
-1-
II L
FILTERS
Figure 1. Photolyzing monochromator with continuous analysis.
which can be related to the lifetime of the fluorescence state by the expression tan (ut')
=
719
Results A . Quantum Yields. It is readily demonstrated that LECV photooxidizes in the presence of a wide variety of materials (halocarbons, quinones, and other electron-deficient molecules) and yields the intensely colored cationic dye,,A,( 5900 A, E 110,000l./mole-cm). Halocarbons are an especially convenient series of sensitizers since they may be obtained pure and their structures may be varied. The major part of this study will be concerned with CC1, as a sensitizer. A4 solution The absorption spectrum of a 2 X of LECV in ethanol, in the presence of 0.1 M CC14, is given in Figure 2. No obvious effects of CC14 were observed on the LECV spectrum. However, photolysis with 3200-A light, thus photolyzing into the leuco dye, yields the dye cation only when CC1, is present. Quantum yields (ac) are given in Table I. There is a marked effect of CC1, concentration when oxygen is present and a negligible effect over the range studied when it is absent.
WT
where t' is the phase delay, w is 2irf (f = 10 Mc), and T is the lifetime of the fluorescing state. At small angles, the phase delay is equal to the lifetime. Quantum yield measurements of color (ac) were made on an apparatus designed by P. C. Hoell. Figure 1 presents a block diagram of the experimental arrangement. The sample to be irradiated is placed in a 1cm path length quarts cell, the photolyzing wavelength is selected by adjustment of the monochromator, and then the shutter is opened. Optical density is followed by a double-beam spectrometer wherein OD is presented to the x axis of an x-y recorder. The total dose is monitored by deflecting a small fraction of the light leaving the monochromator to a phototube and integrating circuit. Integrated dose is then fed to the y axis of the recorder. Graphs of OD vs. dose are obtained, the slopes of which are directly proportional to Quantum yields were placed the quantum yield, 9 ~ . on an absolute basis by taking advantage of the known quantum yield of crystal violet leuco nitrile (a = 1 in ethanol).1° The color produced is the same species as the oxidized LECV and thus it is merely a matter of comparing slopes. B. Flash Photolysis. The flash photolysis apparatus was similar to that described by Porter.' The flash duration has a half-width of 4 psec. The analyzing lamp was operated from a 6-v storage battery. Samples are deoxygenated by flushing for 10 min with purified argon. Argon pressure was used to drive the sample through the flash photolysis cell.
Table I : Effect of CCl4 Concentration and Oxygen and the Color Quantum Yield in Ethanol LECV, .If
x x x x 2x
2 2 2 2 2 2 2 2
x
x x x
CCla, ,M
10-4 10-4 10-4 10-4 10-4 10-4 10-4 10-4 10-4
0.005 0.01 0.03 0.05 0.1 0,001
0,005 0.03 0.1
0 2
present?
*C
Yes Yes Yes Yes Yes NO No No
0.36 0.52 0.61 0.65 0.68 1 09 1.14 1.17 1.16
KO
An important clue to the reason for the variation in
a, values can be obtained by examining the singletstate reactions of LECV. The reactions to be considered are rate
+ hv +DH* DH* +hv' + heat + DH DH* heat + DH
ki (DH*)
(2)
1~1
(DH *)
(3)
DH* +T
h(DH*)
(4)
+ Q +product
~ Q ( D H *(Q) )
(5)
DH
DH*
(1)
where DH is LECV, with D representing the complete (10) A. H. Sporer, Trans. Faraday SOC.,57, 983 (1961).
Volume 71,Number 3 February 1967
A. MACLACHLAN
720
r
3
ABSORPTION / t O.IM C C L 4
30c
4
2IO
0
FLUORESCENCE
T
I
1
I
I
I
I
I
I
I
1
0.01 0.02 003 004 005 0.06 0.07 O B 00s 0.10 CCl4
(M)
Figure 3. Plot of l/+fus. ccl4 for LECV in ethanol.
28
36
44
x
52
60
x lo-*
Figure 2. Spectral properties of LECV in ethanol.
molecule except for the single hydrogen atom (H) on the triphenylmethyl central carbon atom, DH* is the first-excited singlet state, T is the triplet, Q is an added quencher molecule (CCL), and kr, k ~ k,T , and k~ are the rate constants for fluorescence, internal conversion, conversion to a triplet state, and quenching, respectively. The fluorescence and phosphorescence spectra of LECV are shown in Figure 2. By measureM LECV ment of fluorescence yields for 2 X as a function of CCl, concentration, the curve given in Figure 3 was obtained. Since all the processes for loss of DH" are first order (pseudo first order for quenching), the slope of the line as plotted in Figure 3 is given by the Stern-Volmer equation. Furthermore, a plot of
the reciprocal lifetime ( T ) of the state DH* vs. CC1, concentration yields at 2 X lo-* M LECV a straight line (Figure 4) whose slope is kQ. From the values ob4
1
tained in Figures 3 and 4,there are two independent means of obtaining kf. If T ~ the , mean lifetime, and @fO, the fluorescence yield of DH* in the absence of Q, are used, kr is found to be 12 X lo7 sec-I. Using the slopes of Figures 3 and 4,kQ = 15.9 X lo9 hi!-' sec-l and kl = 7.1 X lo7 sec-I showing satisfactory agreement. From the curves of DH* quenching, it is seen that at concentrations of CCl, lower than 0.005 M, there is no probability of reacting with DH* during its lifetime. However, the @C reported in Table I demonstrates efficient oxidation at t h e CC14 concentrations in the absence of oxygen. This must mean that DH* converts to another species, either an intermediate free radical or the triplet state. Furthermore, these The Journal of Physicd Chemistry
I " ' " ' p
0.01
a02
0.07 0.01
a05 CC14
a06
0.07
a00
0.09 0.10
(MI
Figure 4. Plot of 1/r us. C c 4 in ethanol.
states then react with CCl, to produce D f . Oxygen strongly inhibits this reaction. B. The Triplet State. Two test sfor the partieipation of the triplet state were used. (1) The phosphorescence curve in Figure 2 places the triplet of LECV a t 4200 A and, thus, if triplettriplet quenching is to be observed, a molecule must be used with a triplet emission to a longer wavelength. Naphthalene" phosphoresces at 4700 A and should be an excellent quencher if triplet states are involved. Table I1 presents the results of naphthalene quenching on ipc. Photolysis was carried out with 3400-A light, a wavelength where all of the energy is being absorbed by the LECV. Carbon tetrachloride was added at a concentration such that no singlet-state quenching attributable to it could occur. (2) While it is possible that the previous results can be explained by arguing that efficient radical scavenging by naphthalene would yield the same experimental effect, a direct observation of the triplet state of naphthalene, produced by the quenching reaction, would be unequivocable evidence. This evidence was obt,ained by the flash photolysis technique. (11) G. Porter and F. Wilkinson, Trans. Faraday SOC.,57, 1686 (1961).
PHOTOOXIDATION OF LEUCO ETHYL CRYSTAL VIOLET
Table I1 : Naphthalene Quenching of *cn LECV, M
CClk, M
Naphthalene, M
Relative quantum yield
10-8
10-3 10-8 10-3
... 10-8 3 x 10-8
1 0.8 0.35
10-8
10-8
' All runs were made in the absence of oxygen. Filters and concentration were adjusted so that practically all the photolyzing light was absorbed by the LECV. In the absence of LECV, 0.003 M naphthalene in ethanol yields only an indication of the triplettriplet absorption at 4100 A. When 2 X M LECV is added, the 4100-A transient increases markedly in intensity. Combining this direct observation with the quantum yield data in Table I1 serves to identify the triplet state of LECV as the major product of the unquenched singlet state. C. Reaction Intermediates. Investigation of the rate of D + formation with flash photolysis revealed some interesting solvent effects. Observation of D + at 5900 A as a function of time after the photolysis flash (half-width 4 psec) demonstrated that in nonviscous polar solvents (methanol, ethanol) the D + is formed with appreciable delay, while in viscous (glycerol) or nonpolar solvents (ccl4, cyclohexane), the D + formation is instantaneous. Examination of the kinetics of D + formation reveals that good first-order kinetics are obeyed over the entire range of the slow part of the traces. Table I11 presents the first-order rate constants determined for a number of runs and sensitizers.
Table I11 : Slow Rate of D + Formation Sensitizer
Carbon tetrachloride Carbon tetrachloride Carbon tetrachloride Hexachloroethane Chloroform Carbon tetrachloride Cardon tetrachloride
M
Solvent
10-9
10-3
Ethanol Methanol Ethanol Ethanol
(20%) 0.04
Ethanol Ethanol
2 X 10-8 5 X lo-*
k, 8ec -1
29 34 24 19 (1) 29 (1) 23 20,20,22
The transient curves in ethanol and methanol are always composed of two distinct parts. A fraction of the D + is formed with a rate that follows the flash and the remainder is generated in the slow first-order
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process. Solvent viscosity is not the sole characteristic feature that determines the presence or absence of the two reactions because glycerol also suppresses the slow part. The mechanism below is consistent with all of the data.
+ cc4-+ DB+ + cci4DQ+ + CC4D + + HC1 + :CClz + C1DH + ccI4- ( -CCla + Cl-) + D H + + CC1,- + C1DH+ 1J D . + H + DH+ + D . --+ D + + DH DH*
(6)
(7) (8) (9) (10)
Reaction 6 is an electron-exchange reaction from either singlet or triplet DH, which yields the Wurstertype ion radical, DH+. The CC14- formed is still capable of oxidizing DH+ to D + and does so to the extent dictated by its proximity to DH+ and the rate of reaction. Reaction 7 will be favored by an increase in viscosity as was observed with glycerol. The prior formation of the ion radical DH+ is favored over a direct formation of D because of the dramatic solvent effect. At high dielectric constants, as in ethanol and methanol, the ion would be expected to have greater stability than in a solvent of lower dielectric constant, like CC14 or cyclohexane. The intermediate DH+ formed from the sensitized oxidation of DH was easily observed at -85" in methanol through the employment of esr spectroscopy. The single-line spectrum observed by photolysis of a 5% Ccl4-95% MeOH solution of M DH could not be resolved to obtain hyperfine structure. On warming, in the absence of further photolysis, the radical disappeared and the characteristic 5900-A absorption of D + appeared. Reaction 8 expresses the further possibla reactions of the reduced CC4, either in the form of CC14- or .CC18. It is necessary to invoke these additional oxidizing species because of the quantum yield data in Table I. It can be seen from the reactions leading to D + that a quantum yield of one would be expected. The precursor to D + can be observed at 4800 A. Because of its much lower extinction coefficient, meaningful kinetics could not be obtained ; however, the lifetime for decay corresponded to the lifetime for D + formation. There are no protonated forms of D + that have an absorption maximum at 4800 A. The intermediate DH+ must attain the final oxidation state, D+, a t the rate and kinetic order found for the slow part of the D + formation. Reactions 9 and 10 represent the most probable manner in which this is accomVolume 71, Number 3 FebTuaTy 1967
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A. MACLACHLAN
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plished. The first-order kinetics observed for D f formation dictate a prior equilibrium process as in (9) and furthermore make the D . formation the ratedetermining step. Addition of lo-* M HC1 to the solution increased the lifetime of D H + to 100 msec in agreement with the equilibrium nature of reaction 9. However, with all the other potentially protonating sites on LECV, this cannot be taken as proof for reaction 9.
Summary Reactions 1-9 represent a plausible reaction scheme for the halocarbon-sensitized oxidation of LECV. The identity of the actual sensitizing steps in the reaction with the processes studied by Lewis and coworker~’-~and L i n s c h i t ~in~ ~the ~ solid state a t low temperatures suggests that many of these previously studied reactions can be examined at room temperature using the flash photolysis technique.
The Journal of Physieal Chemistry
This work also supplies direct proof that both the singlet state and triplet state can interact with the sensitizers. Whether the singlet state is converted first to the triplet state by the sensitizer and then reacts cannot be ascertained from these data.
Acknowledgments. The author is deeply indebted to P. C. Hoe11 for his measurement of the fluorescence lifetimes of LECV and for all the instruments he has made available to our research staff, a number of which are mentioned in the Experimental Section. Thanks are also due to R. G. Bennett for many helpful discussions concerning fluorescence lifetimes, to V. F. Hanson for providing the flash photolysis apparatus, to C. Yembrick and R. Dessauer for getting the author interested in the problem, and finally to J. M. White for his assistance in carrying out many of the experiments.
