PHOTOTROPY OF TETRACHLOROKETODIHYDRONAPHTHALENE 2449
Dec., 1962
Acknowledgment.-This research was supported in part by Bureau of Naval Weapons Contracts
NOas-59-6112-C, NOas-60-6105-C, and NOw-610587-d.
THE PHO~~OTROPY OF TETRACHLOROKETODIHYDRONAPHTHALENE AS A REVERSIBLE PHOTOCHEMICAL REACTION1 BY G. SCHEIBE AND F. FETCHTMAYR Institut f u r Physikalische Chemie der Technischen Hochschule, Munchen, Germany Received M a y 86, 1Q6B
The phototropy of the crystals of p-tetrachloro-1-ketodihydronaphthalene, which has been observed in the past, is shown to be a dissociation of the excited molecule into a chlorine atom and an aroxyl radical. The reaction is induced by light absorption in the keto group. The same reversible reaction also can be observed in suitable solvents (a) a t low temperature in a glass-like frozen state and (b) in carbon tetrachloride at room temperature. The rupture of the C-Cl bond is made possible by the gain in stabilization energy in the aroxyl radical. The anisotropy of light absorption observed by Weigert supporta this theory.
A. Introduction Many compounds are known which show a change in color when subjected to the action of light but revert to their original color in the dark. Marckwald2 was the first to recognize that these light reactions are reversible. For this reaction, which he first observed in the crystals of p-tetrachloro-1-keto-dihydronaphthalene,he introduced the term “phototropy.” The colorless crystals of p-tetrachloro-l-ketodihydronaphthalene (p-TKK) become amethyst colored when exposed to light. When placed in the dark, the crystals lose their coloration within a few hours. This reversible reaction can be repeated practically as often as desired. Hitherto it has not been possible to observe this photoreaction in solutions and melts of 0-TKX. Nor do the crystals of the isomer a-TKN show this phenomenon. [ncidentally, until quite recently, it was not dcfinntely known n-hich of the structures helongs to the a form and which belongs to the p form. 0 ’‘
a“0 ’‘
CI
CI
a-TKN m.1). 106”; p = 4 . 1 6 D
and chemical comparison^.^ The absorption spectra of both a- and p-TKN in ethanol and petroleum ether are shown in Fig. l a and lb. More recently, results obtained by Hoppe and Rauch5 in their X-ray investigations into the st’ructure of P-TKN by the “Faltmolekul” method and by the diffuse scattering m e t h ~ d ~ . ~ have shown that this opinion is correct. The dipole moments of a- and p-TKN, which have been determined recently in benzene s ~ l u t i o nare , ~ also in conformity with this opinion. B. General Observations on the Problem of Phototropy Before we discuss our investigations with pTKN, it may be useful to point out what explanations have been brought forward in the past to explain phototropy. After Marckwald discovered phototropy in pTKK, it was found that there are many other compoundss which show this phenomenon in the solid, crystallized state. In some cases it was possible to explain t,he mechanism, and it, was found that the new absorption was due t’o a system change which can be formulated chemically.8 In working with p-TKN, neither Marckwald ’ nor Stobbes were able to observe a chemical change of the system when exposed to light. It was assumed, therefore, that a physical change of the system was the cause of phototropy. This assumption was based on work carried out by Weigert.lo He subjected this compound in 1918 to an int’ensive crystallographic and spectroscopic investigat.ion and came to the conclusion that the phot’otropyof this substance was bound up with the crystal form. He was not able to explain the mechanism, but he did observe that t,he crystals, which are rhombic in ultraviolet light of relat’ively short wave length, are phototropically excited by polarized light, whose electric vector vibrates in
c1 c1
P-TKX
111.p. 116’; p = 3.17 D (phototropic)
The opinion t’hat the a and p forms are as shown in I and I1 originally was based on the results obt’ained by yon Dobrogoiski in his spectroscopic (1) Thesis of F. Feichtmayr, T H Miinchen, 1957; cf. F. Feichtmayr and G. Scheihe, 2. Naturforsch., lSb, 51 (1958); G.Soheibe, Chem.Ing.-Technilc, 31, 321 (1939). (2) W. Marckwald Z. physik. Chem., SO, 140 (1899); this paper also contains a report on the phototropic behavior of quinoquinoline. Marckwald assumed t h a t a physical change of t h e system was the cause of the phototropy. (3) The color change of 8-TKN when exposed to light had been observed by F. Zincke and 0 . Xegel when they first synthesized this compound, hut they did not investigate i t further (Ber., 21, 1027 (1888)). The term a- or 8-TKN was first used b y Zincke, who was able t o isolate these t w o isomers, independent of the method of preparation. The two isomers differ from each other in the melting point and the hchs.vior t o a a r d light, but they have muoh the same clieniicill behavior.
(4) Thesis of A.
V O I L Dobrogoiski, T H Munchen, 1954. (5) W. Hoppe and R. Rauch, Z. Krist., 116, 141 (1961). (6) Cf.W.Hoppe, Angew. C h m . , 69, 659 (1957). (7) IC. Feichtrriayr and 17. Wurstlin, unpublished results. ( 8 ) Examples: anils, hydrazones, fulgides. stilben derivatives, etc. % . survey is given by L. Chalkley, Chem. Rev., 6, 217 (1929); H. Stobbe, “Handw. Naturwissenschaften,” 2nd ed.. 1932, Val. 7, p. 999. (9) W. Marckwald. Z.Elektrochem., 24, 381 (1918). (10) F. R’cigert, i b i d . , 1 4 , 222 (1918).
G. SCHEIBE AND F. FEICHTMAYR
2450 500
Wave length, ma. 400 300
250
500
Vol. 66
Wave length, mp. 400 300
250
4
4
3
3 i
-
u;
M
bn
0
2
2
1
1
25 30 35 40 45 Wave number, cm.-1 X 10-9. Fig. la. a-Tetrachloro-1-ketodihydronaphthalene: in ethanol; - * - in petroleum ether.
25 30 35 40 Wave number, cm.-1 x 10-8. Fig. 1b.-&Tetrachloro-1-ketodihydronaphthalene: in ethanol; . - in petroleum ether. 20
20
--
the c axis. The absorption band which causes the red color(maximum about 525 p ) absorbs only light which vibrates in the a-b plane. This pleochroism of the excited crystals already had been observed by Marckwald.2 This behavior is no longer surprising to us today, because Hertel” and also Hoppe and Rauch6 have shown that the aromatic rings all lie parallel in the a-6 plane. In aromatic systems, the transition moment for the T+T* absorption lies in the ring plane,12and it would therefore appear logical to assume that the maximum excitation occurs in the a 4 plane. The longest wave length band must, however, correspond to the n e r * transition of the carbonyl group in view of the structure and intensity of this band and the dependence of its spectral position on the solvent13 (see Fig. 1). As the transition moment of this band is vertical to the C-0 axis and thus vertical to the aromatic p1ane,I4 it is not surprising that in long wave length ultraviolet light the excitation is particularly pronounced in the c axis. Incidentally, in a-TKN the T ~ ? T transition * already covers the n - d transition (see Fig. 1). (11) H. Hertel and K. Schneider, Z. Elektrochem., 57, 536 (1931). (12) G. Scheibe, St. Hsrtwig, and R. Muller. Z . Eleklrochem., 49, 372 (1943); F. Dorr and M. Held, Angew. Chem., 7 2 , 287 (1960). (13) G. Scheibe, Ber., 69, 2619 (1926): M. Kaaba. Discusaiona Faraday Soc., 9, 14 (1950). (14) Cj. J. W. Sidman. Chem. Rev., 68, 689 (1958): “Electronic Tranaitions due t o Nonbonding Electrons in Carbonyl, Azo-Aromatic, and other Compounds.”
-
48
---
The question arose whether Weigert was right in claiming that a physical changeI6 occurs with P-TKN, or a chemical change which can be expressed as a formula is responsible for the phototropic behavior.
C. Investigation of the Phototropy of p-TKN von Dobrogoiski recently has provided evidence* in our Institute that the phototropy of p-TKN is not connected with the crystal form. When a diluted, glass-like frozen solution of the compound in alcohol is irradiated, it is possible to observe a color change which is comparable to the color change in the crystal and which disappears again when the solution thaws. In Fig. 2 the absorption spectra of crystal and solution excited by light are compared with each other. The results show clearly that the phototropic condition is bound up with an individual molecule. von Dobrogoiski also was able to show that the (15) F. Weigert offered the following explanation of phototropy: when the cryetal is expoaed t o light, intermolecular optical influences are developed in the crystal by neighboring carbonyl groups approaching one another, and these optical influences can be observed a s coloration. H. Hertell1 assumed in view of the position of the molecules in the lattice t h a t colored “quinhydron-like molecule compounds” are formed in t h e crystal under the action of liglit. It should also be pointed out t h a t C. V. Gheorghiu in his paper (Bull. &ole polutech. Jassy, 1, 141 (1947)) assumed t h a t the “unexcited ground state” and the “light excited state” of p T K N correspond to various resonance structures. This assumption was based on a mistaken conception of resonance.
Dec., 1962
PHOTOTROPY OF TETRACHLOROKETODIHYDRONAPHTHALENE 245 1 Wave length, mp.
isomeric a-TKN in glass-like frozen solution undergoes a phototropic coloration. He found that the absorption spectra of the phototropic states of a- and p-TKN are identical, although the two compounds absorb differently when they are not excited. This observation was the first indication that thc phototropic change in p-TKN may be due to a chemical change, viz., a dissociation of the excited molecule into ra,dicals. It was assumed as a hypothesis for working that a chlorine atom is split off from the tetrahedral carbon when a- and pTKN are exposed to light. The resulting 2,3,4trichloronaphthoxyl radical must be regarded as responsible for the red coloration. The dark reaction is then a recombination of chlorine atom and aroxyl..
LI
c1
8-TKN colorless
ci
-
T I 2,3,4-Trichloronaphthoxyl radical; red
600 500
1.1
6
CI a-TKN yellow
0.8
After phototropy was recognized to be a property of the individual molecules, and phototropy seemed to be based on a dissociation of 0-TKK into a chlorine atom and a naphthoxyl radical, it was desirable to provide chemical proof for this hy(16) F. Dorr and F. Engelmann, Naturwiasenschaften, 39, 397 (1952). On the basis of the results obtained m t h fluorescein (G. N. Lewis, et ol., J . Chem. Phya., 17, 804 (1949)), the authom claim that the tnplet form is responsible for the paramagnetism. (17) H. S. Gutowsky, R. L. Rutledge, and J. 13. Ilunsberger, J. Chem. Phys., 29, 1183 (1958). (18) Acknowledgments are due to Professor G. Porter and Dr. D. J. Morantz for carrying out the flash spectroscopic investigations and for their useful suggestions.
I
1.0
0.9
D. Photochemistry of 8-TKN in Solution
300
I
!
'0'
This assumption made it possible to offer a plausible explanation for the paramagnetism of the excited crystals which had been measured by Dorr and Engelmann16 in 1952 with the magnetic balance. It must be mentioned, however, that subsequent investigations of the paramagnetic resonance absorption of the excited p-TKNlS4J7 gave negative results. An explanation for this has not yet been brought forward. The results of flash spectroscopic investigations carried out for us by Porter and Moranta's provided the first proof that our assumption is correct. In these investigations it was found that a transient intermediate product is formed in highly viscous paraffin oil even at room temperature during the photochemical decomposition of the pTKK; this intermediate product is absorbed a t approximately 500 p , and in a 1.5 X 10-3 molar second. solution it has a half period of 3 X This intermediate product also could be detected spectroscopically in the less viscous solvents hexane and alcohol. We shall deal in the next section with the photochemistry of p-TKN in solution.
400
. p i
15 20 25 30 35 Wave number, cm.-1 X 10-8. Fig. 2a.-,%Tetrachloro-l-ketodihydronaphthalene: absorption of a crystal, not exposed - -; exposed for 2.5
hr. -
- -.
-
pothesis. For this purpose it was necessary to investigate solutions which were not frozen. Proof of the Dissociation of Excited 6-TKN into Radicals.-When a benzefie solution of pTKN is irradiated at room temperature, no red coloration can be observed, but after some time the solution turns yellow and this change in color is irreversible. Flash spectroscopic investigationsls in hexane and alcohol also show distinctly that there is a decomposition reaction. It seemed likely that the yellow coloration is caused by compounds which are formed from the primary aroxyl radicals in subsequent reactions. We attempted to intercept these aroxyl radicals, which apparently have a short life, with other radicals. We succeeded in doing this with the yellow equilibrium solution of triphenylmethyl radicals in benzene containing p-TKN in dissolved form. In the dark no reaction occurs, but on exposure in the absorption range of p-TKN a complete decoloration occurs within approximately 10 sec. A benzene solution of triphenylmethyl radicals is also susceptible to light, but it takes a t least 10 min. before any decoloration commences. There are indications that both the aroxyl radical1g and the chlorine atomz" react with the triphenylmethyl. (19) St. Goldsobmidt and Ch. Steigerwald (Ann.. 438, 202 (1924). were able to show that the 9-chloro-1-phenanthroxyl radical reacts with triphenylmethyl instantly.
G. SCHEIBE AND E’. FEICHTMAYR
2452
I 0
-
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Wave length, inp. 500 400
600
Absorption of 0-TKN in Ether-Ethanol-18OOC - - - Brkm
Clpxur
Vol. 66
350
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7 7 !
.-.. drier rqmrurr
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C-7 W’m. d*2cm
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The results hitherto obtained prove the dissociation of excited cy- and p-TK?u‘ into radicals, but I I , they give no indication regarding the structure of ii cy‘ the molecule fractions. The experimental results I I I 5 to be discussed prove the decomposition of the I excited p-TKK into aroxyl and C1atom. i i Phototropic Behavior of p-TKN in Solution.i I The radicals which are formed on exposure to light I exist in hexane, benzene, and alcohol only for a fracI i tion of a second because there are numerous sub3 sequent reactions which they can undergo (rei‘ combination, dimerization, reaction with the soli I vent, etc.). It is understandable, therefore, that I i it has not been possible hitherto to observe the phototropic behavior of /3-TKN in solution. The use of carbon tetrachloride as solvent in photochemical reactions, however, made it possible to 15 20 25 30 achieve such a high stationary radical concentraWave number, em.-’ X 10-3. Fig. 2b.--rlbsorption of 8-TKN in ether-ethanol at - ISO”, tion that the “phototropy” of 0-TKN in solution c is 7 X 10-3 M , d is 2 em.: - . - before exposure; - - - could be observed. In this solvent, transfer reacafter exposure. tions or other reactions of the radicals with the solvent hardly ever occur, so that the lifetime of the The use of radicals as initiators in polymeriza- aroxyl radicals is increased markedly. The red tion reactions can be regarded as the best method color of the solution does not disappear until of detecting the presence of radicals. If a com- approximately 5 min. have elapsed. There is, pound which can be polymerized by radicals is however, a very light, bleaching, yellow dyeing used as a solvent for the substance whose radical which is just strong enough to be detected, and this decomposition is to be investigated, the “cage” indicates that a dimerization of the aroxyl radicals effect of the solvent, an effect which would favor a to quinol ether occurs in this system together with recombination of the radicals, is weak because the the recombination of aroxyl and ( “ 1 atom to pTKK. solvate sheath consists of the reactive molecules. The polymerizable solvent used for P-TKX was vinyl acetate or acrylonitrile. In order to ’9‘ determine whether polymerization in vinyl acetate had been initiated, 0.2 cc. of the irradiated solution was taken with a pipet after 15,30, and 60 minutes and dropped into 2 cc. of heptane. As the polymer is insoluble, the occurrence of turbidity indicates that polymerization is taking place. In acrylonitrile the photochemical polymerization is indicated by the instantaneous precipitation of thc high polymer on exposure to light. This method was used to investigate the dissociation of the excited molecule into radicals of several substances, some of which gave a positive result, including a-TKN (I), p-TKN (11), l1l-dibromo-2-ketodihydronapht halene (IT), tribromophenolbromine I
I I
I
I 8
1
,
/
L
(VI. (20) M. Gomberg (Ber , 36, 1822 (1902)) pointed out that the
til-
phenylnietliyl radiral reacted very rapidly nitti halogen. Reactions of triphen>lmetiiyl with simple radicals such as NO2 KO, H also proceed veiy rapidly (cf. J E Leffler, “The Reactire Intermediates of Organic Chemistry ” Interscicnce Publishers, Inc , New Yoik, PI’.
I-.,1956)
The colored solutions of the photochemically produced aroxyls in carbon tetrachloride now can be used for various tests t o investigate the chemical behavior of the naphthoxyl radicals. These tests show that the disappearance of thr red color is not influenced markedly by the prcserice of oxygen.
Dec., 1962
PHOTOTROPY OF TETRACHLOROKETODIHYDRONAPHTHALENE 2453
It therefore can be assumed that the naphthoxyl radical does not react with oxygen to form peroxide. As the tests carried out by E. Mullerz1show, aroxyls of the benzene series are susceptible to oxygen and form peroxides, whereas aroxyls of the phenanthrene series are stable to oxygen, as shown by St. Gold~chmidt.'~ If a certain amount of chlorine is added to the solution of p-TKN in carbon tetrachloride, no red coloration can be observed on exposure. One possible explanation for this is that the photochemical splitting of the chlorine molecule, which occurs a t the same time, causes the concentration of chlorine atoms to become so high that the recombination of aroxyl with chlorine atoms proceeds too rapidly to permit any accumul% tion of the colclred radical. In this connection, it should be mentioned that alcohol or an addition of hydroquinone decolorizes the solution immediately. The aroxyl character of the radical is proved quite clearly by the behavior of the colored solution when p-naphthol is added. The solution becomes colorless immediately; the reaction mechanism beingz2
toward radical~,?~ the spatial conditions in the crystal have a strong influence on the occurrence of phototropy. This is confirmed by the fact that the crystals of P-TKN no longer show phototropy on prolonged heating to approximately 100' and chilling.z As indicated by the Debye-Scherrer diagrams,' this is due to a change in structure in the crystal. This indicates clearly that it depends decisively on the arrangement of the molecules in the crystal lattice whether any accumulation of the aroxyls or chlorine atoms is possible. It has not yet been possible to determine whether this is because no dissociation into radicals a t all can occur when the crystal has a certain structure, or because the recombination reaction aroxyl chlorine atom to D-TKN proceeds far more rapidly. One interesting result obtained in these tests is that the paramagnetism which Dorr and Engelmann16 observed in the excited crystals, dies away a t normal temperature in accordance with the law of a monomolecular reaction. The only explanation for this unexpected result is that during the necessary bimolecular recombination of chlorine 2RO. IIi(_>H+ ROH O=R--ORl atom aroxyl to p-TKS (see 111) each chlorine atom combines again with the aroxyl from which aroxyl p-naphthol phenol quinol ether it has been split off by the action of light (decorresponding to layed primary recombination). It must be conthe aroxyl cluded that the C1 atoms in the crystal lattice of We also have observed that dissociation into p-TKK cannot move freely. Nothing definite radicals occurs with numerous quinol derivatives can be stated, however, until further details are (halogen compounds and quinol ether) of benzene, known regarding the crystal structure, e.g., of both modifications of ,8-TKK or of a-TKS. naphthalene, anthracene, and phenanthrene.' In a solution containing 8-TKN and tritertiary It also should be mentioned that the rate of the butylphenol we observed the blue color of the stable recombination reaction in the aroxyl is strongly phenoxy1 radical when the solution was exposed to influenced by the temperature. The exposed light. This probably is due to a dehydrogenation of crystals remain red for days at the temperature of the phenol by the chlorine atom, and is analogous to liquid air, but they become colorless after approxithe observation made by Illullerzl in dehydro- mately 24 hours at room temperature, or after only a few minutes a t 80". This means that the regenating tritertiary butylphenol with lead oxide. lcurthermore it was to be expected that when union of aroxyl and chlorine atom t o p-TI
