J. Phys. Chem. 1981, 85,3599-3603
into account the So-SI geometry changes in the totally symmetric modes. Also, we have used local-mode arguments to estimate the S1 frequencies of the nontotally symmetric (b,) C-H-stretching and C-0 stretching motions. In the ground electronic state these modes are known to differ in frequency by less than 1% from their ag counterparts. This is con~istent’~ with the C-H and C-0 stretches being good local modes. Thus, we expect that in S1(where strong progressions in the b, modes are not available for determining the excited-state frequencies) ahCH = CJ CH and = cosco. In the case of for example, %is implies a So-S1 frequency change of almost 20%. Naaman et al., however, have chosen-in the absence of direct observation of uhCoand co CH in Sl-to put wb,S1 = wBsO. This procedure, along w i h the neglect of Sl-So geometry changes for the v;s, leads to a loss of much structure in the transient absorption spectrum. In any case, a possible conclusion suggested by the Naaman et al. experiment is that So anharmonicity must mix in many more vibrationally hot ground electronic states than are prepared initially by the internal conversion. Rather than the lack of time dependence, it is the structure in the probe (transient absorption) spectrum
3599
which is significant. As discussed in sections I11 and IV, the high-energy So* states are linear superpositions of large numbers of (zero-order) product states. The breakdown of mode separability corresponds to the mixture of the (- 100) S1 So* prepared So* levels with a much larger number of states which do not interact directly with SI. But, via the potential energy anharmonicity, the “new” So*%contribute to the So* S1* absorption. Accordingly, we must enumerate the So*-S1* Franck-Condon factors for all of these states, just as we did for those few which were initially prepared by the internal conversion. In particular-as discussed earlier in this section-we need to know how to partition the bath states into two seta, one which couples strongly with the S1 So* populated states and the other which is weakly coupled. But a quantummechanical calculation along these lines becomes unfeasible since too many transitions (Le., too many So*%) need to be included to account for the little bit of structure observed. Analysis of this experiment, then, remains an open challenge.
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Acknowledgment. This work was supported in part by
NSF Grants CHE79-02983 and CHE80-24270.
Static and Time-Resolved Fluorescence Quenching for the Study of Solubilization and Transmembrane Transport in Vesicles Mats Almgren Department of Physical Chemistry I, Chemical Center, University of Lund, S-220 07 Lund 7, Sweden (Received: June 12, 1981; In Flnal Form: August 6, 198 1)
Fluorescence spectroscopy, static and time-resolved fluorescence quenching, and fluorescence stopped-flow measurements have been utilized in the study of interactions between vesicle membranes and fluorescent molecules and/or quenchers. The fluorescence of pyrenebutyric acid in a vesicle with positive charge, didodecyldimethylammoniumbromide, is strongly quenched by a negative quencher like iodide ion. At low quencher concentrations quenching occurs only at the outer vesicle surface, leaving -30% of the probes protected. At high iodide concentrations also the inner surface was affected. Such an inside-outside discrimination was not evident in the quenching behavior for R~(bpy),~+, quenched by methylviologen in dicetylphosphate containing vesicles. Characteristic in this case was instead a site-competition behavior. Ru(bpy)BzCevidently binds to two different types of sites in these vesicles. One is ionic; at this site quenching, but also competition, occurs, so that at high quencher concentrations Ru(bpy)32+retreats to the second, more hydrophobic site, where it is well protected from the quencher. Stopped-flowmeasurements in this case revealed no measurable relaxation that could give evidence for a penetration of the membrane. Transmembrane transport was measured for cetylpyridinium chloride by using its quenching of pyrene fluorescence as monitor. The time for the “flip-flop” process was on the order of 100 s.
Introduction Photophysical and photochemical phenomena in aggregate systems have turned into a very active research area.’ Just as in the case of micelles, the interest for photoprocesses in vesicles derives both from the possible uses of these partially ordered structures in the control of photochemical processes-in particular, photochemical solar energy conversion2-and from the use of the photophenomena in elucidating the dynamic organization of the
We have for some time been interested in both of these interconnected aspects of photophenomena in vesicle sol u t i o n ~ .In ~ the course of these studies, we have noted on some occasion unusual types of fluorescence quenching. In some cases only part of the fluorescence was quenched although no protective action was expected; in other cases an expected protection was absent. The fluorescence quenching is more varied and more complex than in micelles. In this paper three different types of fluorescence quenching behavior will be exemplified and shown to yield
(1) The literature is overwhelming. General reference is here made only to some recent review~.~J (2) (a) Gratzel, M. Ber. Bunsenges. Phys. Chern. 1980,84,981-91. (b) Matauo, T.;Nagamura, T.; Itoh, K.; Nishijima, T.; Mern. Fac. Eng., Kyushu Uniu. 1980,40, 25-36. (c) Calvin, M. Int. J.Energy Res. 1979, 3, 73. (d) Thomas, J. K. Chern. Rev. 1980,80,283-99.
(3) (a) J. Fendler, “Membrane Mimetic Chemistry”;Wiley-Interscience, in press. (b) Radda, G. Methods Membr. Biol. 1975,4,97-188. (c) Sackmann, E. Ber. Bunsenges. Phys. Chern. 1978,82,891-909. (4) (a) Almgren, M. Phys. Chern. Lett. 1980, 71,539. (b) Almgren, M. J. Am. Chern. SOC. 1980, 107, 7882-7. (c) Almgren, M.;Thomas, J. K. Photochern. Photobiol. 1980, 31, 329.
structure^.^
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information on different aspects of the dynamic interaction of small molecules-quenchers or fluorescent probes-with the vesicles. Experimental Section Fluorescence spectra and intensities were measured with an Aminco SPF 500. Fluorescence decay curves were determined by a single-photon counting technique as described el~ewhere.~ The fluorescence stopped-flow method has also been d e ~ c r i b e d . * ~ ? ~ Vesicle solutions were prepared by quickly injecting an ethanol solution of the lecithins or dicetyl phosphate into a rapidly stirred water solution. Normally solutions containing 1-2 mM of the monomers in vesicular form and 4-8 vol % ethanol were obtained. Soybean lecithin (EPi, Epicuron 200, Lucas-Mayer), dicetyl phosphate (DCP, Sigma), and didodecyldimethylammonium bromide (DDAB, Eastman) we used as supplied. Pyrene and 1-pyrenebutyric acid (PBA), both from Eastman, were recrystallized several times from ethanol. Ruthenium 2,2’-bipyridine chloride (G. Frederick Smith), methylviologen (MV, Sigma), hexadecylpyridinium chloride (Schuchardt), cetyltrimethylammonium bromide (CTAB, BDH), and sodium dodecyl sulfate (SDS, BDH) were used as supplied. Results and Discussion Steady-State Fluorescence Quenching. The SternVolmer type of quenching
2.0
-
[ I ]-‘/(mM)-’ I
1.0
1
0
2
3
Figure 1. Quenching curve according to eq 2 for pyrenebutyric acid in Eplcuron-DDAB vesicles quenched by KI. Measured intensities are independent if PBA was added before (0)or after (A)the vesicle preparation.
io
4 -
3-
applies in vesicle solutions if all of the fluorescent molecules are equally accessible for the quencher, and the quencher is partitioned between water and vesicles as in a simple distribution equilibrium. The value of the Stern-Volmer constant depends then both on the rate of quenching at/in the membrane and on the partitioning. Ionic or polar quenchers are expected to be active at the membrane interfaces. If added to the outside solution, as is normally the case, a (temporal) protection against quenching would be given those fluorescent molecules that reside at the inner vesicle interface. The effectivity of this protection depends on the rates of transmembrane movements of both the fluorescent substance and the quencher, but in entirely different ways. For the fluorescence probe, it is enough that it remains on the inside for a time longer than the fluorescence lifetime-a requirement probably fulfilled by most fluorescent probes, at least those that are slightly polar-whereas the quencher must remain at the outside longer than the mixing time, i.e., at least on the order of a few milliseconds with stopped-flow techniques. Assuming a fraction, a,of the fluorescent molecules is protected, the remaining fraction being quenched according to eq 1, one obtains
With a independent of quencher concentration, a plot of lo/(Io - I ) vs. [&I-‘ would extrapolate to (1- a)-l at infinite concentration. Examples of quenching curves plotted according to eq 2 are given in Figures 1-3. At low quencher concentrations a linear plot is obtained which by extrapolation yields values of a = 0.25 for the quenching of PBA in 3:l EPiDDAB by I-, a = 0.25-0.35 for Ru(bpy)gP+ in 1:l EPi-DCP ( 5 ) Almgren, M.; Lofroth, J.-E. J. Colloid Interface Sci. 1981, 81, 486-99.
0 mole fraction DCF
2
0.5
1.0
)O [MV2+]-‘/(mM)-’
Figure 2. Quenching curves according to eq 2 for 25 pM Ru(bpy)z+ in 2 mM EPi-DCP vesicles at 25 OC. Insert: Quencher concentration at the minimum as a function of the mole fraction of dicetyl phosphate in the vesicle bilayer.
6
[-
100
Figure 3. Quenching curves according to eq 2 for pyrene in EPi-CTAB vesicles quenched by cetylpyridinium chloride: (a) 1.0 mM EPI, 25 pM pyrene, and 20 mM Trls buffer at 25 O C ; (b and c) 1.2 mM EPi, 0.05 mM CTAB, and 25 p M pyrene in 0.15 M NaCl at 25 OC. In b the ordinate values were calculated from the intensity measured a long time after mixing, and in c from the intensities immediately after mixing, obtained by extrapolation of time-resolved measurements.
quenched by methylviologen, and a = 0 for pyridinium chloride quenching of pyrene in EPi vesicles. The quencher was always added to the vesicle solutions (a “titration”). The quenching behavior was the same with probes added before and after vesicle preparation. At high quencher concentrations, deviations from the predicted behavior occur. In Figure 1 the experimental
Solubilization and Transmembrane Transport in Vesicles
The Journal of Physical Chernlstty, Vol. 85, No. 24, 198 1
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1counts
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't'
0
Flgure 4. N ~ t ~ ~ a l i fluorescence ied spectra of Ru(bpyh* in water, DCP vesicles, and Eplcuron-SDS vesicles (Epicuron = EPI in text). Ru(bpyh2+ concentration was 30 pM throughout. DCP vesicles: 2 mM DCP, 8 vol % ethanol. (0) Wtthout quencher. (1) 16.5 pM MV2+. (3) 240 pM MV2+. EPI-SDS vesicles: 2 mM Epicuron, 1 mM SDS. I n thls case no spectrum change occurred on quenching. The curves were normalized at the asterisk.
quenching curve dips down at quencher concentrations above -5 mM KI. A similar behavior was observed in a few cases with R ~ ( b p y ) in ~ ~DCP + vesicles (at 25 "C, far below the transition temperature). The behavior indicates that the vesicles start to "leak" a t these quencher concentrations, or may become destroyed. It is known that vesicular DDAB is very sensitive to salt concentration in general: and to I- in particular. The induced leakage at 5 mM I- is still an interesting feature. Whether the leakage occurs by a breakdown of the vesicle membrane or whether it is an induced transport of I- through the membrane remains to be explored. For Ru(bpy),2+ in DCP-containing vesicles, an increase in fluorescence intensity at higher quencher concentrations occurs (Figure 2). With Cu2+as quencher the fluorescence intensity course was in principle the same, but the minimum occurred at a much higher quencher concentration. In 2 mM 1:l DCP-EPi the minimum came at 20 X lo4 M for MV2+and 65 X lo4 M for Cu2+. This quenching behavior is similar to that observed by Meisel et a1.' for Ru(bpy)32+in poly(viny1sulfate) on Cu2+ quenching, although not quantitatively. In our case the minimum fluorescence intensity occurs at a concentration of quencher that is far less than the concentration of ionic sites; [Q] = 0.1 mM or less at [DCP] = 2 mM. Furthermore, the quencher concentration at a minimum depends quadratically on the mole fraction of DCP in the vesicle (insert of Figure 2). This indicates that the sites where the quenching and ion competition occur are formed by (a pair of) cooperating negatively charged phosphate head groups. The observation may be related to the formation of a (charge induced) domain structure of the type discussed by S a ~ k m a n n . ~ ~ Two of the quenching curves in Figure 3 show no signs of quenching protection; cetylpyridinium seems to quench pyrene fluorescence just as well on the inside as on the outside. The third curve was extrapolated from time-resolved measurements and will be discussed below. (6) McNeil, R.; Thomas, J. K. J. Colloid Interface Sci. 1980, 73,522. (7) (a) Meisel, D.; Matheson, M. S. J. Am. Chem. SOC.1977, 99, 6577-81. (b) Meisel, D.; Matheson, M. S.; Rabani, J. Ibid. 1978, 100, 117-22. (8) Tachiya, M.; Almgren, M. J . Chen. Phys. 1981, 75, 865. (9) Kornberg, R. D.; McConnel, H. M. Biochemistry 1971,10,1111.
500
1000
1500 2000
ns
Flgure 5. Fluorescence decay curves for Ru(bpy) in dicetyl phosphate vesicles at 30 O C at different quencher (MV") concentrations: 30 pM Ru(bpy)$+ introduced before vesicle preparation; DCP, 2 mM. Quencher concentrations: (0) 0, (1) 16.5, (2) 82.6,and (3) 243 MM.
TABLE I: Decay Times for the Fluorescence Decay Curves in Fi ure 5 As Fitted to a Two-Exponential t a,e-t/Tz Model: 106(MVZ+), M T , , ns ~ T ~ , " ns fZ 0 260 (0.11) 490 (0.89) 1.05 16.5 116 (0.79) 535 (0.21) 1.10 82.5 43 (0.59) 510 (0.41) 1.65 243 175 (0.35) 0.91 485 (0.66) a
Relative amplitude in parentheses.
Fluorescence Spectra. Fluorescence spectra of Ru(bpy):+ in water, DCP vesicles, and lecithin-SDS vesicles are displayed in Figure 4. There is an increasing red shift in the spectrum from water over DCP to EPi-SDS vesicles. In the latter the spectrum is similar to that of Ru(bpy)t+ in SDS micelles. Also shown is the renormalized fluorescence spectrum of Ru(bpy)gP+ in DCP vesicles quenched by MV2+. A component of the spectrum at a wavelength corresponding to the peak of the aqueous spectrum was removed on quenching already at low quencher concentrations. The fluorescence spectrum did not change on further increase of the MV2+concentration. The increase in fluorescence intensity at high concentrations of MV2+is thus not due to a release of Ru(bpy)l+ into the water, but must mean that Ru(bpy)32+retreats to other solubilization sites in the vesicle membrane, where it apparently is well protected toward attack from ionic quenchers. Time-Resolved Fluorescence Quenching Studies. In Figure 5 logarithmic fluorescence decay curves are presented for Ru(bpy)s2+ emission in pure DCP vesicles without quencher and with three MV2+concentrations, below, at, and above the intensity minimum. At low concentrations of quencher the decay curves look similar to those obtained in micellar systems with a rapid initial stage followed by a slow exponential decay. A two-exponential model was fitted to the data by a nonlinear least-squares procedure, as discussed e l s e ~ h e r e .The ~ resulting decay times (and reduced 5"! test) are presented in Table I. The results in Table I show that the reality is far more complex than the two-site model suggests. Two exponentials are already necessary to fit the data in the absence of quencher, and also the slow portion of the decay at the highest quencher concentration (in that case the initial rapid decay was excluded from the computations). However, it seems warranted to conclude that two main types
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of sites for Ru(bpy)2+ are important. The first is the ionic sites which seem to require a cooperation of two DCP molecules for their formation, since their abundance (as evidenced by the quencher concentration giving minimum intensity) depends quadratically on the fraction of DCP lipids in the vesicle. Quenching by MV2+(or Cu2+)occurs a t these sites, but also competition so that Ru(bpy)l+ is successively displaced by quencher ions, and retreats into the second type of sites. These sites give a red shift in the Ru(bpy),2+ fluorescence compared to water and the ionic sites, but far less than SDS micelles or SDS-containing vesicles. On these sites, R ~ ( b p y ) , ~is+well protected from quenching by the ionic quenchers; only a slight quenching may be noted from the fluorescence decay time reported in Table I at the highest quencher concentration (which was still quite low, only -0.24 mM as compared to the DCP concentration of 2.0 mM, or the I- concentrations of 1 mM and more used in the quenching of PBA). The remarkable feature is instead the strong quenching and binding of MV2+at the ionic sites. The strong red shift and the broadening of the fluorescence spectrum of Ru(bpy),2+in SDS micellar solution were discussed by Meisel et al.' They concluded, from comparisons with the effect of various alcohols (smaller red shift) and poly(viny1 sulfate) (no red shift) on the spectrum, that the red shift was indicative of an interaction with the hydrocarbon chains of the SDS monomer in the micelles. The changes on quenching in the spectrum from DCP vesicles are qualitatively in line with this: the ionic sites give a fluorescence spectrum similar to that in water or poly(viny1 sulfate), whereas the protected sites yield a red-shifted spectrum, albeit much less red-shifted than in SDS or SDS-containing vesicles. Ru(bpy),2+in the protected sites of DCP seems thus to be in contact with hydrocarbon chains to some extent. The much stronger shift in the presence of SDS indicates that both the head group and the hydrocarbon chains are important for the effect. Stopped-Flow Measurements. Stopped-flow experiments were performed to study the penetration of the vesicle membrane by the quencher and/or the fluorescent probe. With DCP-EPi vesicles, Ru(bpy),2+,and MV2+no relaxation signals were observed (in the time range from ca. 10 ms to 10 min). Several experiments were performed with varying compositions of the mixing solutions. Two examples: vesicles composed of 0.50 mM DCP, 1.2 mM EPi with 10 pM MV2+added afterwards, were mixed (1) with a solution containing Ru(bpy)l+ and DCP-EPi vesicles or (2) with a solution of only 30 KM Ru(bpy),2+ in water. In both cases the equilibration of Ru(bpy)t+ and MV2+on the outer surfaces of the vesicles would be expected to be too fast to be observed, but penetration into the interior by Ru(bpy)S2+or MV2+should be observable. The fact that no relaxation signal was seen is puzzling in view of the fact that the MY-quenching curve for Ru(bpy),*+ in DCP-EPi vesicles was the same irrespective of whether R ~ ( b p y ) ~was ~ +added before or after the preparation of the vesicles. It is possible that Ru(bpy)t+ is preferentially attached to the outer surface. Otherwise the experiment indicates a leakage through the membrane within the mixing time (a few milliseconds). An example of the relaxation curves observed in the study of cetylpyridinium chloride quenching of pyrene fluorescence in EPi-CTAB vesicles is given in Figure 6. The first part of the process was recorded on a fast time scale (30 ms per division) and the remainder at 15 s per division. In this experiment a vesicle solution formed from 1.2 mM soybean lecithin (EPi) and 0.1 mM CTAB were mixed with a vesicle solution of 1.2 mM EPi-0.1 mM ce-
Almgren
E
0
3
B
ii 0
30 60 90 120 150 time, milliseconds
10
25
40
55 70 seconds
85 100 115
Figure 6. Stopped-flow relaxation signal from 1.2 mM EPI-0.1 mM (CTAB) mixed with 1.2 mM EPi-0.1 mM cetylpyridinium chloride. Both solutions contained in addition 0.15 M NaCI, 5 vol % ethanol, and 15 FM pyrene. Excitation at 338 nm; emission above 400 nm measured. Observe break In time scale.
tylpyridinium chloride. Both solutions contained 0.15 M NaC1,5 vol 90ethanol, and 25 pM pyrene. The fluorescence intensity observed is mainly from pyrene excimers. Cetylpyridinium quenches this fluorescence much more effectively than CTAB. The rapid relaxation process is due to the equilibration over all vesicles of cetylpyridinium a t the outer surface. This process is not seen in experiments where EPi-CTAB vesicles with pyrene are mixed with a water solution of cetylpyridinium chloride. In this case only the slow process remains which seems to reflect the transmembrane passage of cetylpyridinium. The characteristics of these processes have not been studied in detail. The fast process is similar, as expected, to the transfer processes of pyrene and perylene studied previously.48bThe inverse relaxation time should be equal to 2k-, where k- is the rate constant for dissociation of a quencher from the vesicle."bi8 The process was not simply e ~ p o n e n t i a l .Mean ~ ~ relaxation times of 100 ms a t 15 "C, -40 ms at 30 "C, and 30 ms at 40 "C were determined. The slow process was studied in a series of experiments mixing 1mM EPi-CTAB (10%) in 0.15 M NaC1,5 vol % ethanol, 25 pM pyrene with cetylpyridinium in water, concentration from 15 to 115 pM, a t 30 "C. The reproducibility between different series of this kind was not good, but better within each series of measurements using the same vesicle solution. The relaxation time (the process was not exponential; a mean value is reported) decreased with increasing surfactant concentration, from 120 s at 15 pM to -60 s at 115 pM. By extrapolating the relaxation curve backward to time zero, one obtains an intensity value, I*, that corresponds to the situation where the fluorescence from the outside is quenched completely but that from the inside is left intact. Plots of both Io/(Io - I*) and Io(Io - I,) are shown in Figure 3. It is apparent that the I* plot indicates a considerable fraction of protected pyrene. The rate of the transmembrane movement of the cetylpyridinium is very high compared to the so-called flipflop times reported for phospholipids, -6.5 h for egg lecithing a t 30 "C and -8 h (at 50 "C) for pyrene-substituted lecithin in DPPC bilayers.'O However, much shorter times have been observed for simple amphiphilic molecules like pyrenenonanoic acid1' and pyrenedecanoic acid,1° where the flip-flop must be fast compared to the
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(10)Galla, J. J.; Theilen, U.;Hartmann, W, Chem. Phys. Lipids 1979, 23, 239-51. (11) Doody, M. C.;Pownall, H.J.; Kao, Y . J.; Smith, L. C . Biochemistry 1980, 19, 108-16.
J. Phys. Chem. 1981, 85,3603-3607
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transport between vesicles. For pyrenenonanoic acid the relaxation time for the transfer between dimyristoylphosphatidylcholine vesicles at 28 "C was reported as 0.185 s at pH 7.4 and 12.5 s at pH 2.8. The flip-flop rates in these cases are closer to the transfer rates of very small molecules like ~ a t e r and ~ ~ acetic J ~ acid.13 Systematic studies of the transfer rates of small molecules through a lipid bilayer remains to be done. Fluorescence stoppedflow methods based on excimer formation, energy transfer, or fluorescence quenching would then be very powerful t00ls.l~ Summary and Conclusions Fluorescence quenching techniques may be used to estimate the inside/outside distribution ratio of a vesicle-
bound fluorescent molecule. Protection against quenching does not necessarily mean, however, that the fluorescent molecule and the quencher are on opposite sides of the vesicle membrane. In the case of Ru(bpy)gP+in dicetyl phosphate containing vesicles, time-resolved measurement and spectral changes in the presence of quenchers show the presence of at least two different solubilization sites. Rapid transfer of the quencher through the vesicle bilayer makes the estimation of distribution ratios by static methods impossible. As shown in the case of cetylpyridinium quenching of pyrene in positively charged vesicles, the distribution ratio may be obtained from fluorescence stopped-flow measurements which also are powerful means for studying the dynamics of the transmembrane transport process.
(12)Lawaczeck, R. Ber. Bunsenges. Phys. Chem. 1978,82, 930. (13)Alger, J. R.;Prestegard, J. H. Biophys. J. 1979,28, 1-14. (14)Verkman, A. S.Biochim. Biophys. Acta 1980,599, 370-9.
Acknowledgment. This project has been supported by the Swedish Natural Science Research Council.
The 147-nm Photolyses of Tetramethylene Sulfide, Tetramethylene Sulfoxlde, and Tetramethyiene Sulfone A. A. Scala," Ismael Colon, and W. Rourke Department of Chemistty, Worcester Polytechnic Institute, Worcester, Massachusetts 0 1609 (Received: June 30, 198 1; In Flnal Form: August I 1, 198 I )
Tetramethylene sulfide, sulfoxide, and sulfone have been photolyzed with 147-nm radiation in the gas and condensed phases. Mechanisms which assume diradical intermediates are presented for the photodecomposition of each compound. The major reaction in all three cases is the elimination of ethylene by a 0-cleavage of the initially formed diradical: .XCH2CH2CH2CH2- .XCH2CH2.+ CzH4. The competing 0-cleavage produces X=CH2 and C3He This second 6-cleavage is less important in the sulfoxide and the sulfone than it is in the sulfide. The direct elimination of the sulfur atom, and any attached oxygen atoms, is of no importance in the sulfide and of increasing importance as the oxidation state of the sulfur atom increases. The similarity between the mass spectral fragmentation and the vacuum ultraviolet photochemistry of these compounds is noted and the possible involvement of triplet states is mentioned.
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Introduction Although there is a wealth of information on the photochemistry of heterocyclic organic compounds in the near-ultraviolet, there are few studies of simple heterocycles which absorb only in the vacuum ultraviolet. There are even fewer studies which examine the photochemistry of heterocycles as a function of the heteroatom. Sulfur heterocycles are of particular interest because of their stability in a variety of oxidation states. While the photochemistry of alicyclic sulfides has been examined to some extent,'+ the photochemistry of alicyclic sulfoxides10and sulfonesll has received little attention. This paper presents the results of an investigation of the vacuum ultraviolet photolyses of tetrahydrothiophene (THT), tetramethylene sulfoxide (TMSO), and tetra(1)W.E.Haines, G. L. Cook, and J. S. Ball, J. Am. Chem. Soc.. 78, 6213 (1956). (2)K.S. Sidhu, Can. J. Chem., 46,3305 (1968). (3)H. A. Wiebe and J. Heicklen, J.Am. Chem. Soc., 92, 7031 (1970). (4)S.Braslavskv and J. Heicklen. Can. J. Chem.. 49. 1316 (1971). (5) D. R. Dice &d R. P. Steer, J . khys. Chem., 77; 434 (1973). (6)D. R. Dice and R. R. Steer, J. Am. Chem. Soc., 96, 7361 (1974). (7)D. R. Dice and R. P. Steer, Can. J. Chem., 52, 3518 (1974). (8) D. R. Dice and R. P. Steer, Can. J. Chem., 53, 1744 (1975). (9)D. R. Dice and R. P. Steer, Can. J. Chem., 56, 114 (1978). (10)F. H.Dorer and K. E. Salomon, J. Phys. Chem., 84,1302 (1980). (11)A. A. Scala and I. Colon, J. Phys. Chem., 83,2025 (1979). 0022-3654/81 /2085-3603$01.25/0
1 THT
TMSO
0
\ /p
TMSO,
methylene sulfone (TMSOJ at 147 nm. The purpose of this study is not only to present the various reaction channels for each of these compounds but also to elucidate how changing the oxidation state of the sulfur atom affects the modes of fragmentation. Since the mass spectral behavior of heterocycles is often similar to their photochemistry in the vacuum ultraviolet, some discussion of the mass spectra of these compounds is of value. The mass spectrum of THT12 indicates that loss of a two-carbon fragment from the molecular ion is the predominant reaction channel, followed in importance by the loss of a three-carbon fragment. The only other significant processes involve the loss of HS and H2S. The mass spectrum of TMSO is drastically altered from that of the simple sulfide.13 The major fragmentation involves loss of HSO, which suggests that rearrangement (hydrogen (12)A. M.Duffield, H. Budzikiewicz, and C. Djerassi, J. Am. Chem. Soc., 87, 2920 (1965).
(13)R. Smakman and T. J. DeBoer, Org. Mass Spectrum., 3, 1561 (1970).
0 1981 American Chemical Society
