PHOTOCHROMISM calculated spectrum in this Figure (8D) is one in which the relative signs were taken to be the same for the H-F couplings, as in Figure 8B, but with J p H F = +OB5 Hz. The effect of this choice was merely to invert the entire spectrum. Hence, it seems reasonable to conclude that the pura H-F couplings are of opposite sign from that of the other H-F couplings and that the ortho and metu H-F couplings have the same sign relative to the H-H couplings. I n summary, the H-F coupling constants reported here exhibit rather wide ranges of values which are consistent with the ranges previously reported for other derivatives. Unlike the corresponding H-H couplings in the monosubstituted benzenes, all of the H-F couplings in the fluorobenzenes are significantly affected by the substituent. The correlations found between the H-F couplings and substituent electronegativity are interesting, but not readily explicable. It is possible that these couplings may b e influenced not only by inductive factors, which seem largely responsible for variations in aromatic H-H couplings, but also
997 by some combination of mesomeric and geometric variations. Previous theoretical studies have indicated that the interpretation of H-F couplings presents, in general, a more difficult task than is the case for H-H coupling^,^^^^^ and there are further complications in aromatic systems. The present status of this problem indicates a need for additional investigations, both theoretical and experimental. Acknowledgments. This research was supported in part by a grant from the National Institutes of Health. The authors also wish to express their appreciation to R9r. Bill Jankowski, Analytical Instruments Application Laboratory, Varian Associates, Pittsburgh, Pa., for the fluorine spectra of o-chlorofluorobenzene and m-difluorobenzene at 94.1 1IHz. The proton spectrum of o-fluoroiodobenzene at 100 1SHz was provided by Jeolco, Medford, Mass. (36) J. A. Pople, Mol. Phys., 1, 216 (1958). (37) G. A. Williams and H. S . Gutowsky, J. Chem. Phys., 30, 717 (1959).
Photochromism: Spectroscopy and Photochemistry of Pyran and Thiopyran Derivatives by Ralph S. Becker and Jaroslav Kolc Department of Chemistry, University of Houston, Houston, Texaa '7'7004 (Received September. 1, 1967)
Several pyran and thiopyran derivatives, including an indolinospirobenzothiopyran, exhibit photochromic behavior. It is possible to sensitize coloration of the latter compound in a rigid matrix. The substitution of sulfur for oxygen causes dramatic red shifts in the absorption bands of the colored but not of the colorless compound. Structures for the colored products have been assigned. The photochemical process appears t o be highly efficient.
Introduction As part of our continuing investigation to evaluate the various spectroscopic parameters of molecules, energy transfer, and the importance of these in photochemistry, we have investigated several new molecular systems. One particular aspect worthy of clarification is the site of photochemical activity in the chromenes and indolinospirobenzopyrans. Another important area is the evaluation of the effects of substitution of sulfur for oxygen. Of particular concern are the spectroscopic properties of both the uncolored and colored forms, as well as the photochemical behavior of the uncolored form.
We wish to report reversible photochemical behavior for four new molecular ring systems: 2H-pyran, 2H-thiopyranJ 2H-thiochromeneJ and indolinospirobenzothiopyran.
Experimental Section All experiments were carried out in 3-methylpentane a t 77"K, unless otherwise noted. The absorption spectra were determined by means of a Cary Model 1.5 recording spectrophotometer with quartz cells 2 mm and 10 mm in path length. Concentrations were approximately fM. For production of the photocolored forms, a 1-kW Hg-Xe lamp with a Corning Volume 7% Number 3 March 1968
RALPHS. BECKERAND JAROSLAV KOLC
998
L 400
600 Wavelength, mr.
600
700
Figure 1. Absorption spectrum of the colored form of ZH-thiochromene in 3-methylpentane at 77°K.
300
400
600
500
700
Wavelength, mp,
Figure 2. Absorption spectra of 2,2-diphenyl-ZH-thiochrornene in 3-methylpentane at 77°K: colorless form; - - - -, colored form.
no. 9863 glass filter was used. Conversion from the colored back to the colorless forms was achieved by warming the sample to room temperature. The 2,4,6-triphenyl-2-benzyl-2H-pyranand 2,4dimethyl-2,6-diphenyl-2H-pyranwere gifts from Prof. J. Dreux of Ecole Superieure de Chimie Industrielle, Lyon, France. The 2,4,6-triphenyl-2-benzyl-2H-thiopyran was given to us by Prof. K. Dimroth, Chemisches Institut der Universitat Marburg, Marburg, Germany. The 6,ll-dihydro [lIbenzothiopyrano [4,3-b]indole, its 2-methyl and 2-chloro derivatives, and 5,ll-dihydro [2]benzothiopyrano [4,3-b]indole were received from Prof. N. P. Buu-Hoi, Institut de Chimie des Substances Naturelles du C.N.R.S., Gif-sur-Yvette, France. All compounds were checked for purity on thin-layer chromatography. Where purification was necessary, liquidcolumn chromatography was used. The 2H-thiochromene was prepared, but the principal product was not thiochromene but thiochromane. The Journal of Physical Chembtry
-,
This was confirmed by high-resolution nmr (100 Mc) and mass spectral analysis. Nonetheless, the mixture showed reversible photocoloration with the production of a blue-red compound. Thiochromane has no visible absorption nor does it exhibit photochemistry. The absorption spectrum of the colored form of 2H-thiochromene is shown in Figure 1. The 2,2-diphenyl-2Hthiochromene was prepared from 1,l-diphenylethylene and o-mercaptobenzaldehyde following a parallel preparation of the analogous oxygen compound.2 Purification was accomplished using solid-liquid chromatography. High-resolution nmr (100 M c ) spectral analysis showed absorption corresponding to 14 aromatic, 2 olefinic, and no other protons. This conforms to that expected for the desired compound. The 1’,3’,3’-trimethylspiro [2H-l-benzothiopyran(1) E. Luttringhaus and N. Engeihard, Ber., 93, 1525 (1960). (2) R. Wizinger and H. Wenning, Helv. C h i n . Acta, 23, 247 (1940).
999
PHOTOCHROMISM
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2,2'-indoline] was prepared by adding 2.1 g (15 mmol) of o-mercaptobenzaldehyde3 in 5 ml of ether to 1.7 g (10 mmol) of %methylene- 1,3,3-t rimet hylindoline in 30 ml of absolute methanol and refluxing. The mixture was then evaporated to dryness and chromatographed on a silica-gel column with ligroin as a solvent. The desired product was recrystallized from absolute ethyl alcohol and, after drying, had a melting point of 91-92'. Anal. Calcd for C19H19?;S (293.2): C, 77.76; H, 6.53; S, 4.78; S,10.93, Found: C, 77.51; H, 6.46; N, 4.72; S, 11.23. High-resolution nmr (100 ]IC) analysis gave singlets of C-methyls (7 8.88, 3H; and 7 5.62, 3H), singlet of N-methyl ( r 7.42, 3H), olefinic proton doublets (7 4.34, 1H; and T 3.73, l H ) , and aromatic proton multiplet ( T 2.9-3.5, SH). These data confirm the structure of the compound.
b ]indole do not exhibit photochromic behavior under the same conditions. Thus, it appears that the additional pyrrole ring imposes restrictions on the rearrangement of double bonds which is necessary for creation of the colored form.
6
I11
In addition, we synthesized the previously unknown 1',3', 3 '-trimethylspiro [2H-1-be n z o t hi o p y r a n-2,2'-indole] (IV) and found it to be photochromic. The hv
S
Results and Discussion Irradiation of a substituted 2H-pyran and a 2Hthiopyran resulted in the formation of yellow photoproducts. More will be said concerning these shortly. These results prompted us to investigate the sulfur analogs of the chromenes and indolinobenzospiropyrans. The 2,2-diphenyl-2H-thiochromene (I) gave a green product which is assigned as the o-thioquinoneallide (11). This is based on two principal facts. The ab-
I,R=Ph
q-0 Me Me
K
11, R = Ph
sorption spectrum of the photoproduct, Figure 2, is parallel to that product from the oxygen analog. Further, the photocolored products from the oxygen analogs, the 211 chromenes, have been shown to be o-quinoneallidea by subsequent reactions. In the contrast to these results, some more complicated thiochroxnenes as 6,ll-dihydro [llbenzothiopyran0 [4,3-b]indole (111), its 2-methyl and 2-chloro derivatives, and 5,ll-dihydro [2]benzothiopyrano[4,3-
Me IV
Me V
green form, assigned as structure V, can be produced by irradiation and can be eradicated by warming. The assignment of structure V as the photocolored form is based on the identification of the colored products from the chromenes4 and indolinobenzospiropyrans4s6 and the parallel spectral characteristics compared with the colored forms of the chromenes, thiochromenes, and indolinobenzospiropyrans.6s6 The absorption spectrum of the colored form, Figure 3a and b, is quite unique regarding the fact that the longest wavelength band extends into the near infrared to approximately 10,500 8. In addition, the shorter wavelength absorption band (maximum -4420A) is unusually intense compared with the other compounds (3) I?. Friedlander and E. Lenk, Ber., 45, 2083 (1912). (4) J. Kolc and R. Becker, J . Phys. Chem., 71, 4045 (1967). (5) R. Heiligman-Rim, Y . Hirschberg, and E. Fischer, J . Chem. SOC.,158 (1961); J . Phys. Chem., 66, 2465 (1962). (6) N. Tyer and R. Becker, unpublished data.
Volume 78,Number 9 March 1968
1000
RALPHS. BECKERAND JAROSLAV KOLC
2.0
d
'a a
3 1.0 0
200
300 Wavelength, ma.
400
Figure 4. .Absorption spectra of 2-benzyl-2,4,6-triphenyl-2H-pyranin 3-methylpentane at 77°K: , colorless form; - - - -, colored form.
that have been studied; for example see Figures 1 and 2 and indolinobenzospiropyran.~~~ The substitution of sulfur for oxygen causes only minor red shifts in the spectrum of the uncolored molecule but dramatic red shifts for the colored compound. The same is true for the thiochromene system. n transition in Although it is known that the R* thiobenzophenone is at considerably longer wavelength than in benzophenone, the longest wavelength band referred to in our case does not correspond to a a* + n R transition. This is primarily based on but a R* the vibrational spacings within the transition. There appears to be little or no spectral considerations of aromatic thioketone systems, probably because of their general rarity. Attempts were made to observe emission from all of the compounds discussed. However, no emission characteristic of the compounds was detected. Although this does not imply that there is no emission, it is at least extremely weak. Therefore, photochemistry (and any concomitant internal conversion) must be highly efficient. A 10-2 11 solution of this compound in rigid EPA at 77°K showed no coloration after irradiation for 20 min at 3900 A. However, a solution of M thioM thioxanthone irradiated at 3900 sopiran plus 2 X A showed coloration in 2 min. Thus triplet --+. triplet energy transfer occurred. Although earlier results' indicated that the pyran ring itself exhibited reversible photochemistry, the compound was a rather complex natural product, flindersine. Irradiation of a simply substituted pyran, 2-benzyl-2,4,6-triphenyl-2H-pyran(VI) produces a light yellow product. The structure of the colored product is assigned as VII, primarily based upon analogy with the o-quinoneallide produced from the chromene~.~ Furthermore, the absorption spectrum, Figure 4,is consistent with what would be expected for
200
300 Wavelength, mp.
400
Figure 5. Absorption spectra of 2-benzyl-2,4,6-triphenyl-2H-thiopyran in 3-methylpentane at 77°K: , colorless form; - - - -, colored form.
such a structure. The absorption spectra of both the pyran and the photoproduced ketone are shown in Figure 4. The reaction can be reversed by warming. Ph
Ph
+
+
The Journal of Physical Chemistry
Ph
Ph VI,X-0 VIII, x = s
x\
Ph CH2Ph
VII,X=O IX,X=S
However, some irreversible reaction(s) occurs as well, since the band maximum at approximately 350 mp does not return to its original intensity. The 2,4dimethyl-2,6-diphenyl-2H-pyranshows still less reversibility, indicating that aromatic substituents apparently play an important role in the degree of reversibility for pyran dienone transformation. The nature of the irreversible product(s) is not known. Irradiation of 2-benzyl-2,4,6-triphenyl-2H-thiopyran (VIII) produces a strongly yellow form assigned as thioketone (IX). The bases for the assignment are parallel to those given above for the pyran. The absorption spectra of VI11 and I X are shown in Figure 5. Reversibility is accomplished as for the pyran. It is worthwhile noting that cycling experiments show that the amount of irreversible character in this case is considerably less than for the pyran. Photocoloration is best accomplished by irradiation of a low-temperature viscous solution rather than a rigid matrix at 77°K. Considerably more colored product is produced at a faster rete under the former environmental condition. This obviously indicates a viscosity barrier is present and thus that a notable geometrical rearrangement occurs in the formation of the colored product.
+
(7) R. Backer and J. Michl, J. Am. Chem. Soc., 88, 5931 (1966).
1001
NMRSTUDIES (OFPROTON-EXCHANGE KINETICS
contain a pyran or thiopyran ring system, the photochemical change originates in such a ring and results from C-0 or C-S bond breakage. This would imply similar behavior for other molecules with parallel structural features.
I n conclusion, photochromic behavior has been established for some new compounds. Some of these compounds show unique spectral properties when in the colored forms. Furthermore, these results establish the fact that for all complex molecules studied that
Nuclear Magnetic Resonance Studies of Proton-Exchange Kinetics of N-Methylaccetamide and N-Acetylglycine N-Methylamidel by James E. Bundschuh and Norman C. Li Department of Chemistry, Duquesne Univereity, Pittsburgh, Pennsylaania 16219
(Received September 1 I 1967)
Proton-exchange kinetics of compounds with peptide-like bonds, N-methylacetamide (NMA) and N-acet, 1glycine N-methylamide (AGMA), have been studied by the nmr method. XM.4 has one proton capable of exchanging, while AGMA has two. For AGMA, CHICONHCH~COXHCHJ, the amino proton adjacent to the methylene group is labeled site A, while the amino proton adjacent to the N-methyl group is labeled site B. The specific rate of protolysis of KMA is proportional to hydrogen ion concentration, while there is a 1.3-1.4 power dependence of the specific rate on hydrogen ion concentration for both sites A and B of AGMA. The activation energies of protolysis for NMA and AGMA sites A and B were found to be 16.4,16.2, and 17.7 kcal/mol, respectively.
Introduction Nmr provides an effective method of elucidating the kinetics of rapidly exchanging protons. Grunwald, et aLj2 studied the collapse of the methyl spin-spin quartet of the methylammonium ion in aqueous solution in determining the rate of N H exchange as a function of pH. This technique was soon applied to di- and trimethylammonium ions3 by observing the collapse of the methyl spin-spin triplet and the collapse of the methyl spin-spin doublet, respectively. Protolysis kinetics of amino acids have been studied extensively.4-7 Methylamides have also been studied because of their peptide-like bonds.*-1° This is a report of our work evaluating the protonexchange kinetics of the amino protons of N-methylacetamide (ISMA) and N-acetylglycine N-methylamide (AGMA). The rates of exchange have been found to be accelerated by the addition of HC1 to the aqueous solutions. The rates are reported as a function of hydrogen ion concentration and temperature; rate constants are calcdated and the activation parameters are reported. Although the exchange rates for NMA have been reported by previous workers,s-10 it is felt that further studies were warranted for the comparison of the results and to fill a gap left by their investigations. Our investigation of NMA as a proton-exchanging
molecule has at least one common axis with each of the previous works and provides some interesting comparisons. AGMA has two amino protons in separate environments labeled in the manner
0
It
0
/I
CH3CNHCHZCNHCH3 A B The mean lifetime, 7,of a proton on a particular site can be determined by observing how much it disturbs the nmr signal of an adjacent site. Considering NMA, (1) This investigation was supported by PHS Research Grant No. GM-10539-05 from the National Institute of General Medical Sciences, Public Health Service. Abstracted in part from the Ph.D. Thesis of J. E. Bundschuh, Duquesne University, Pittsburgh, Pa., 1967. (2) E. Grunwald, A . Loewenstein, and 5 . Meiboom, J . Chem. Phys., 27, 630 (1957). (3) A . Loewenstein and S. Meiboom, ibid., 27, 1067 (1957). (4) M. Scheinblatt and H. S. Gutowsky, J . Am. Chem. SOC.,86, 4814 (1964). (5) M. Scheinblatt, J . Chem. Phya., 39, 2005 (1963). (6) M. Scheinblatt, J . Am. Chem. SOL,88, 2123 (1966). (7) M. Scheinblatt, ibid., 87, 572 (1965). (8) A. Berger, A. Loewenstein, and S. Meiboom, ibid., 81,62 (1959). (9) M. Takeda and E. 0. Stejskal, ibid., 82, 25 (1960). (10) A. Saika, ibid., 82, 3540 (1960). Volume 78. Number S
March 1968
