2446
I?. I. METZ,W. C.SERVOSS, AND F. E. WELSH
This is a minimal set; there are in some cases a number of modifications of each of these species.
Vol. 66
We note in Table I the possible natures of A, B, and C for various systems,
PHOTOCHROMIC BEHAVIOR OF SYDNONES BY F. I. METZ,W. C.SERVOSS, AND F. E. WELSH Midwest Research Institute, Kansas City, Missouri Reeeiaed May 86, 1968
Seven different sydnones (N-benzylsydnone; N-p-meth3lbenzylsydnone; N-3,4-dimethylbenzylsydnone; N-p-chlorobenzylsydnone; N,N’-ethylene-bis-sydnone; N,N‘-tetramethylene-bhydnone; and N-3-pyridyls dnone) have been examined for photochromic behavior. Although all of the compound sydnones exhibited some degree orphotochromism in the solid state, the response of the group of materials was inconsistent and could not be related to structural parameters. N-3Pyridylsydnone (N3PS) was selected for more extensive study on the basis of the extent and spectral location of the photochromic change which it undergoes. The material waa investigated in the solid state aa well as in solution. A mechanism for the photochromic behavior of N3PS is suggested on the basis of experimental evidence presented.
TABLE I 1. Introduction SYDNONES EXAMINED FOR PHOTOCHROMISM Increasing interest has been shown during the past few years in photochromism and photochromic Compound Formula materials. Various applications such as devices for temperature control of satellites, computer (11 memory elements, and optical filters with control- N-Benzylsydnone lable, variable absorbancies have been proposed. htidwest Research Institute has been engaged in the (11) study of photochromic materials and photochro- N-p-Methylbensyl CH3 sydnone mism for several of these end uses. A group of anhydro-compounds prepared by Earl and his ~ollaborators2~~ a t the University of Sydney (111) C H ~ have been termed “sydnones.” These compounds N-3,PDimethylbenzylsydnone CH3 have been shown to be neutral, highly crystalline, stable, and fairly soluble in most organic solvents ~ C H r ~ - - & j ’ H including benzene. Baker, Ollis, and P001e4 have (IV) N-p-ChlorobensylN, ,eo proposed a mesoionic hybrid structure of aromatic sydnone 0 type. HC----N--CH,-CHZ-N--( I1 The purpose of this paper is to report the results (VI !@ I 1 0 1 of solid state and solution studies of photochromism N,N‘-Ethylene-bisoc, N, N, ,(O 0 0 in sydnones, with particular emphasis on N-3-pyrisydnone The sydnones investigated are listed dylsydnone. I~C-N-CH?CH2(’~12C~I?-~-CH (VI) in Table I. The degree of photochromism (reversi- N,N’-Tetramethyloc,10N, I 21 ,0 1 ,(O 0 ble color transformation in the visible region ex0 ene-bis-sydnone hibited by solids or solutions upon exposure to exciting radiation) of each of these materials is (N3PS) presented in Table 11. N-3-Pyridylsydnone was N-3-Pyridylsydnone the most photochromic sydnone of the eight compounds investigated, and was therefore examined TOLEII1 in greater detail. ULTRAVIOLET EXCITATION CXWUCTERISTICS OB SYDNONE 11. Experimental Procedures and Results FILMS A. Preparation of Materials.-Compounds I throu h VI (Table I ) were made available for this study by t i e Cancer Chemotherapy National Service Center, National Institutes of Health, through the courtesy of Dr. C. C. Cheng, Head of the Cancer Chemotherapy Section, Midwest Research Institute. N-3-Pyridylsydnone (N3PS) was prepared by the methods of Tien and Hunsberger.6 Purification of all the compounds waa by successive recrystallization. B. Solution Studies.-In solution, the various sydnones were not photochromic, however, non-reversible color formation did occur in some instances. This type of change ( 1 ) G . H. Brown and W. G. Shaw, Rep, Pure A p p l . Chem., 11, 2 (1961). (2) J. C. Earl and A. W. Mackey, J . Chsm. Sac., 899 (1935). (3) R . A. Eade and J. C. Earl, ibid., 591 (1948). (4) W. Baker, W. D. Ollie, and V. D. Poole, ibid., 307 (1949). (5) J. M. Tien and I. M. Hunsberger, J . Am. Chem. Soc., 77, 6605
(1955): 83,181 (1961).
-
-
Sydnone
Optical exoitation characteristics ARb Wave length (%) (%I (mid
I I1 I11
+3.3 +1.9 -3.0
ATme,xa
+3.5
360
+0.4 +5.5 +4.8
360
500
IV -0.6 360 V -1.4 +0.2 400 VI +1.2 +4.0 360 -4.8 +5.0 600 N3PS 0 Maximum change (plus or minus) in transmission, after 2-min. exposure to ultraviolet irradiation; ultraviolet Bource: GE H-100 FL-P mercury projection bulb with a Coming D CVX RDL filter to screen the visible light. Recovery-change in transmission after 5 min. expoaure to infrared irradiation; infrared source: standard 250-watt infrared bulb; all values represent mean values obtained from several experiments.
PHOTOCHROMIC BEHAVIOR OF SYDNONES
Dec., 1962
was studied in dilute (10 mM) solutions of N3PS. Water and propylene carbonate solutions slowly turned brown and exhibited a new absorption peak near 380 mp. Benzene solutions turned brown and deposited a brown, watersoluble residue. A brown precipitate was also formed in cyclohexane. In water solution, the color formation was completed in a few days. The rocess was more rapid if nitrogen was bubbled through t i e solution. Oxygen did not cause a similar effect. In propylene carbonate, the color change required several weeks. Nitrogen caused an increase in color, ;wobserved in the aqueous solutions. However, in this solvent, the absorption returned to the original value in 30 min. following the treatment with nitrogen. Comparisons of the solution spectra of the various sydnones indicated that the absorptions a t ca. 300 mp are due to n +T* transitions.6 N-Benzyl-, N-p-chlorobenzyl-, N-pmethylbenzyl-, and N-3-pyridylsydnone were examined as 0.25-0.35 mM solutions in cyclohexane and in water. The results are summarized in Table 111. I n each case, a blue shift of 10-20 mp was observed in going from the former to the latter solvent. In addition, conductance measurements were made on solutions of K3PS in cvclohexane and water. No conductance (cu. 1 pa. could have been detected) was observed when 0.5-1 v./cm. was applied to 1.5 mM solutions.
2447
TABLE IV WAVELENQTHSUSSD IN FATIGUE STUDIES Ultraviolet range
Unfiltered ultraviolet lamp Filtered ultraviolet lamp Cerium oxide (thin film)
(w.4 200-400 300-400 350-400
Several films were prepared and stored in the dark. The effect of ultraviolet irradiation was determined daily b comparing transmission spectra taken before and after eacg irradiation. Fatigue appeared after fewer irradiations when 5-min. periods were used than when irradiation lasted for only 1 min. In the latter series, the specimens recovered completely in all instances. The optimum (greatest change in per cent transmission) ultraviolet range for the shorter irradiation time appeared to be 300-400 mp; however, a smaller energy range (350-400 mp) seemed to produce less fatigue over a greater number of cycles. These results indicate that fati ue of the sydnone film might be diminished by the use of a cerium oxide film as a filter. D. Infrared Studies.-Infrared spectra were taken to determine if the effect of photochromic changes extended into the infrared region. A Beckman IR-4 spectrophotometer equipped with sodium chloride optics wa8 used to obtain spectra in the region from 660 to 5000 cm.-l. SpecTABLE I11 tra of N-benzglsydnone (I), N-p-methylbenzylsydnone (11), EXTINCTION COE~FFICIENTS OF SYDNONE SOLUTIONS N-3,4-dimethylbenzylsydnone (111), N-p-chlorohenzylsydnone (IV), N,N’-ethylene-bis-sydnone (V), and N,N’-tetraSolvent methylene-bis-sydnone (VI) were obtained on thin films --Cyclohexane--Watervacuum evaporated onto salt plates. Thin films and potas(mr) log t h(mr) log c Sydnone sium bromide pellets of N3PS were studied. N-Benzyl300 3.80 287 3.95 The frequency of an infrared absorption is primarily 307(sh) determined by the mechanical motions in the molecule, while 320( sh) the intensity of an absorption is mainly dependent upon the electrical properties of the molecule. Thus, it would be exN-p-Chlorobenzyl253 3.60 ca. 215 cu. 4. pected that a structural alteration accompanying a photo300 3.04 287 3.94 chromic change would appear as a difference in the position 308(sh) and intensity of certain absorptions in the spectrum. If a 322( sh) change occurred it would be most pronounced for N3PS, since this compound exhibits photochromism to a greater N-p-Methylbenzyl300 3.75 289 3.93 extent than do the other sydnones studied. Compounds I 308(sh) to IV were evaporated onto salt plates. Their infrared 320(sh) spectra were obtained and compared with the spectra taken N-( 3-Pyridy1)238 3.67 232 4.04 after 2 min. irradiation with ultraviolet light. The two spectra of each compound were essentially identical, al.. 265 3.42 though a slight decrease in the intensity of the absorption 325 3.15 305 3.74 bands of the irradiated material was noted. The similarity the infrared spectra of compounds I to IV before C. Solid State Studies.-Thin films of each of the syd- between after ultraviolet irradiation is an indication that there none8 listed in Table I were prepared by vacuum vaporiza- and are no structural alterations associated with photochromic tion, and their response to ultraviolet and infrared irradia- changes under the conditions of our experiments. tion was determined in the spectral range 300 to 700 mp. The infrared spectrum of N3PS wa8 obtained before and Compounds I, 11, and VI exhibited an increase in trans- after irradiation with ultraviolet. The blue form of N3PS mission after exposure t o ultraviolet irradiation, whereas was caused to revert to the colorless form by irradiation the other s dnones showed a decrease in transmission. an infrared lamp before taking the next spectrum. The N35S can be returned to its colorless state by ir- with spectra differed only by slight changes in band intensity. radiation with infrayed light for periods of 1 to 5 min., or by The sample was then alternately irradiated with ultraviolet storage in the dark for several days. A more rapid re- The infrared. After each exposure the spectrum was recovery of the films can be induced by the application of an and corded in the region in which the greatest change in band e.m.f. for periods of 1 min. or less. had occurred. A progressive slight decrease in After prolonged or repeated ultraviolet exposure, thin intensity band intensity was observed. This decrease in band infilms of N3PS showed fatigue or an inability to return to tensity is due to a partial decomposition of the their original state. Although infrared irradiations or an samples,probably rather than to structural changes. applied e.m.f. were effective in hastening the recovery of All of the sydnones studied absorb in the 3100 t o 3200 sydnone films, four or five alternate exposures to ultra- cm.-1 range. Several authors’-0 have assigned peaks in this violet and infrared radiation dwtroyed the photochromism range to the sydnone ring C-H stretching mode. A specof N3PS. The observed fatigue can be attributed to one trum of N-p-methylbenzyl-C-bromosvdnone was taken in or more factors: thermal decomposition of the N3PS film order to provide more eonclusive evidence for assignment of during infrared radiation or e.m.f. application: alteration these bands. A spectrum of this compound showed no of electronic configuration as a result of ultraviolet radia- absorption the above mentioned region. In addition, tion; or a combination of several such factors which re- the band ain t 3150 cm.-l in N3PS was not present after sulted in decomposition of the films. The first factor was bromination. common absorption of the sydnones in eliminated by allowing the sydnone to recover in the dark. this range, plusThe the disappearance of the absorption in the The second factor was examined by narrowing the range of brominated materials, make plausible the assignment of a wave lengths during ultraviolet radiation. If an optimum energy range for excitation-recovery exists, the excitation(7) C. Greco, “Synthesis of Some Substituted Pyridyl Sydnones,” recovery cycles for the selected energy ranges should differ, i.e., possibly show a greater or lesser degree of fatigue. Table Doctoral Dissertation, Fordham University, New York, N. Y., 1960. (8) J. M. Tien and I. M. Hunsberger. J . Am. Chcm. Soe., 7 7 , 6604 ‘CV lists the filters used in these studies. e . .
( 5 ) G. 8. Brealey and hI. Kasha, J . A m . Chem. Soc., 77,4462 (1955).
(1955). (9) D. J. Voaden, Dissertation, Oxford University, England, 1957.
2448
F. I. METZ,W. C. SERVOSS, AND F. E. WELSH
band in the 3100 to 3200 cm.+ region to the sydnone C-H stretching vibration. The assigned frequencies, accurate t o within 20 cm.-*, are listed in Table V.
TABLE V SYDNONE C-H STRETCHING FREQUENCIES Cm.-I
N-Ueneylsgdnone S-p-hfethylbenzylsycinone S.3,4-I)imethylbenzylsydnone N-p- Chlorobenzyhydnone K,N '-Ethylene-bis-sydnone N,N '-Tetramethylene-bis-sydnone N3PS
3135 3120 3125 3120 3115 3160 3150
Vol. 66
crystal lattice forces on its inter-ring resonance cannot be estimated unambiguously. The formation of a metastable sydnone intermediate by ultraviolet excitation can be postulated to decrease the number of contributing resonance forms in the case of compounds I, 11, and VI; and to increase the number of contributing forms in compounds 111, IV, V, and N3PS. In the case of N3PS, the most significant electronic difference is the lack of a -CH2-- shielding group between the sydnone and pyridine rings. The entire molecule therefore contributes to the metastable intermediate, and the slight photochromic behavior of the benzylsydnones is understandable. Analytically, one would predict that bisydnonyl
111. Discussion and Summary Photochromism in solid organic materials has HC -N-N -c €1 been explained by isomerism, free radical formation, 101 I @ l OC, /N N, ,CO molecular aggregation, and triplet state excitation. 0 0 The slight photochromic response observed in the benzylsydnones indicates that the behavior may be would be highly photochromic. On the other hand, a general property of the mesoionic ring. One might decreased photochromism would be expected in 3expect photochromism to be enhanced in the bis- picolylsydnone sydnones, but such behavior has not been observed. An analysis of the experimental evidence obtained does not permit an absolute elucidation of a mechanism for the photochromic behavior of N3PS. However, the observed fatigue and the diminution of several infrared absorption bands after ultraThe metastable intermediate postulated as the violet excitation do support the formation of a blue form of K3PS may involve hydrogen bonding metastable intermediate. between the sydnone hydrogen of one molecule and On the basis of the hybrid structure given by t'he carbonyl oxygen of a second sydnone ring; , ~ substituted nitrogen atom in the however, the infrared data indicate only a slight Baker, et ~ l . the sydnone ring would be expected to bear a large diminution of the C-H band, not a shift as would be fractional positive charge, and the negative end of expected if hydrogen bonding existed. the sydnone dipole would be (diffusely) directed The eventual stabilization of the intermediate toward the carbonyl grouping. Stabilization of the (blue) form is not accomplished by repeated ultrasydnone ring requires the ring to be substituted; violet radiation. The "fatigued" material is such substitution will, of course, influence the con- thought to be the result of decomposition. Such tributing resonance forms. The number of energy decomposition may be enhanced by the presence of levels increases with the number of possible con- impurities. Sine preparations of S3PS of differing figurations, while the average separation between impurity content were examined. These preparalevels decreases. The increase in the number of tions showed no major difference in response to possibilities of resonance for a molecule is associated ultraviolet irradiation, but recovery of the "impure" with a displacement of the first absorption band specimens was generally slow and fatigue quite toward longer wave lengths. The material thus rapid. Comparison of infrared spectra of these can undergo a change of color, if the metastable samples showed that the largest differences occurred state has more (or less) contributing forms than the in the 800-900 cm.-' region. The group of bands ground state of the molecule. If resonance between in this region is the same group which exhibits such forms is restricted, as would be the case where some variation in intensity during the photochromic two conjugated electron systems are separated by change. Specific assignments for these bands are one or more -CH2- groups, the observed spectrum not yet available. is only ;t superposition of the separate parts. A recent study12of the photochromism and elecSteric inhibition of the resonance leads to band tron spin resonance absorption of S-5-pyridylsydshifts toward shorter wave lengths. The molecular none has indicated that the photochromism results structure of several crystalline sydnones has been from color centers analogous to those of the alkali examined by Schmidt'O and Barnighausen, et c ~ 1 . l ~ halides. This mode of energy storage had preSchmidt concluded that in N-phenyl- and N-p- viously been suggested as one of several possibilities tolylsydnone the benzene ring and the 5-membered by Gutowsky, Rutledge, and Hunsberger. l 3 The sydnone ring system were approximately coplanar. mesoionic character of the sydnone ring lends supHowever, Barnighausen, et al., indicate that the port to a metastable state characterized by color two ring planes in E-p-bromophenylsydnone con- center formation; however, further verification is tain a dihedral angle of 27". KO such studies on required. S8PS have been reported. Therefore, the effect of (10) G. M.J. Schmidt, Bull Res CounczZlsraeZ, 1, (1951-1952). (11) 11. Rain~gliausen,F. Jellinek, and A. Vas, Proc. Chem. Soc., 110,
(1961).
(12) T. Mill, A. Van Hoggen, and C. F. Wakly, J. Chem. Phys., 34, 1139 (1961). (13) H. S. G u t o n \ k y , R. L. Rutledge, and 1. h1. Hunsberger, z h d . , 29, 1183 (1958).
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. SCHEIBEAND 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’otropy of 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).
