J. Phys. Chem. 1901, 85, 2671-2676
The results are interpreted by taking into account the interaction between bound metal chelates. That is, the molecular plane of the ligand declines from the direction of the polymer axis due to the repulsion between the neighboring bound metal chelates (Figure lla). On the other hand, for large P/M, there is enough room for one bound metal chelate to be parallel to the polymer axis (Figure llb). Presently, it is not certain whether the bound metal chelate is still fixed in this direction at zero field or whether it rotates around the axis combining a Cu(I1) ion and a sulfonate group up to this direction under the electric field. The latter possibility is less probable, because the present electric field is not so strong as to make a small molecule like Cu(a- or PPAN)' orient in a definite direction. As for the conformation giving rise to negative dichroism, a polymer in a compact conformation is suspected of being responsible. For example, some part of a polyethylene chain is reported to be atactic if its conformation is helical.18 If PSS- takes such a conformation, the neighboring sulfonate groups are located closer to each other (ca. 5 A) than in the stretched-out form (ca. 7 A). Accordingly, the neighboring metal chelates suffer greatly electrostatic repulsion resulting in a greater decline of the ligand plane from the helical axis. This may explain the larger value for the negative dichroism. The persistence length, L, for the positive dichroisms of Cu(aPAN)+ is estimated to be shorter than 1400 A. Since the present PSS- chain is 5300 A long in its stretched-out form, it is concluded that the polymer does not disorient as a whole but that several subunits of the polymer chain move independently to return to the coiled state. In contrast to this, L for the negative dichroism is #J
2671
determined to be 6200 A, which is a magnitude similar to the whole chain length. This result is also consistent with the above assumption that helical conformation is responsible for the negative dichroism, because in such a conformation, the whole polymer is expected to behave as a rigid rod in the disorienting process. The results in Figure 6, a and b, show that the binding of a trivalent metal ion affects the binding state of Cu(@PAN)+remarkably. An added metal ion may bind with more than two ss- residues. If this kind of chelation occurs at two distant sites as in Figure l l c , the polyelectrolyte chain is no longer flexible. As a result, the Cu(PPAN)+ bound somewhere between the above sites cannot orient under the electric field. The results are consistent with the postulate by other workers concerning the trivalent metal effects on the catalytic activities of a metal-polyelectrolyte complex.1e In the present system, there is no essential difference between Cu(aPAN)+and Cu(PPAN)+. In contrast to this, however, it was recently found that the former binds with bovine serum albumin to exhibit positive or negative electric dichroism, depending on pH, while the latter hardly binds with the same macromolecule.20 In this respect, a polymer in biological systems is more competent in recognizing the properties of a small molecule than a synthetic one.
Acknowledgment. Thanks are due to Dr. S. Harada of Shizuoka Women's College for the gift of KPSS samples and to Dr. K. Nitta of Hokkaido University for his fruitful discussion. (19) E. Tsuchida, H. Nishide, and M. Takashita, Makromol. Chem., 175, 2292 (1974). (20) A. Yamagishi, Biopolymers, 20, 201 (1981).
(18) G. Natta and P. Corradini, Macromol. Chem., 39, 238 (1960).
Photochemistry of Thietane Excited to Its Second Excited Electronic Singlet State F. H. Dorer," M. E. Okarakl, and K. E. Salomon Department of Chemistv, San Francisco State UnlversRy, San Francisco, Callfornla 94 132 (Received: February 12, 198 7; In Final Form: May 15, 1981)
Thietane, excited to its S2state, undergoes fragmentationto ethylene and thioformaldehyde by one channel and a competing reaction, unique to the S2state, is decomposition to cyclopropane and sulfur atoms. Previous work on the S2 state of thietane has been extended to include a more detailed examination of the spectra and energy partitioning in the cyclopropane forming reaction; and, by photolyzing cis- and trans-3-ethyl-2propylthietane, we have followed the stereochemical course of both reaction channels. The products of both reactions largely retain the stereochemistry of the reactant. Energy partitioning indicates that S(3P)is the atomic fragment when cyclopropane is produced. A mechanism which accommodates the experimental results assumes that, once excited to its 'B2 electronic state, intersystem crossing to the 3B2state competes with C-S bond rupture that forms the 1,4-diradicalintermediate which yields the ring cleavage products. The 3B2state decomposes to cyclopropane and S(3P)by a mechanism that likely involves a singlet trimethylene diradical as an intermediate.
Introduction Over a period of time the photochemistry of three- and four-membered ring cyclic sulfides were under active investigation.14 Most of the work in the literature deals
with elucidating the photochemistry of the first excited singlet electronic state, and little work has been devoted to understanding the processes that occur after sulfides are excited to their S2 state. The photochemistry of
(1) A recent review in Braslavsky, S.;Heicklen J. Chem. Rev. 1977, 77, 473. (2) Strausz, 0. P.; Gunning,H. E.; Denes, A. S.;Csizmadia, I. G. J.Am. Chem. SOC.1972,94,8317.
(3) (a) Dice, D. R.; Steer, R. P. Can. J.Chem. 1975,53,1744. (b) Ibid. 1978,56,114. (c) Ibid. 1974,52,3518. (d) Dice, D. R.; Steer, R.P. J.Am. Chem. SOC.1974, 56,7361. (e) Dice, D. R.; Steer, R. P. J. Chem. SOC., Chem. Commun., 1973,106. (f) Dice, D. R.; Steer, R. P. J.Phys. Chem. 1973, 77, 434. (4) Wiebe, H. A.; Heicklen, J. J. Am. Chem. SOC.1970, 92, 7031.
0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 18, 1981
Dorer et al.
thietane, I, has been extensively studied by Dice and Steer
[I?s I
(DS).3 There is an initial study in the literature that deals with the chemistry of this system excited to its S2 state by Wiebe and Heicklen (WH);4however, there are still questions about the mechanism of the photochemistry of thietane excited to its second excited electronic singlet state. Photofragmentation from the S1state occurs by initial rupture of a C-S bond to form a 1,4-diradicalintermediate. The intermediate can recyclize or decompose to ethylene and thioformaldehyde. The quantum yield for ethylene formation approaches unity at higher temperatures and shorter photolysis wavelengths. The evidence is that initial C-S bond rupture has a quantum yield of unity even at longer wavelengths of excitation in the S1state. Excitation to the S2 state brings on photofragmentation to cyclopropane and sulfur atoms to compete with ethylene formation. This interesting reaction has not extensively investigated. Although polymer formation is also observed on photolysis to the S2 state, its quantum yield must be small since the total quantum yield for C2and C3product formation appears to be close to, if not equal to, unityS4 In this work we have extended the measurements of WH with a goal of understanding the photochemistry of thietane excited to its S2state. We have examined the spectra of thietane and extended the product composition measurements to lower pressure in order to learn how energy is partitioned in the cyclopropane forming reaction; and by examining the photolysis of a stereochemicalprobe, cisand trans-3-ethyl-2-propylthietane, we have characterized the stereochemical course of the olefin cleavage and cyclopropane forming reactions when thietane is excited to its S2 state. Finally, we offer a mechanism for thietane fragmentation that can accomodate the experimental results.
Experimental Section Materials. Thietane (trimethylene sulfide) was purchased from Aldrich Chemical. The material was purified on a polypropylene glycol GLC column and stored at -78 OC. 3-Ethyl-2-propylthietane was prepared by the method of Searles et and the cis- and trans- isomers were separated by gas chromatography. The identification of the reactant was aided by GLC-mass spectrometry. Calibration mixtures of the products were made by using commercial materials in order to identify them on the analytical GLC column. Although authentic samples of the 1-ethyl-2-propylcyclopropane isomers were not available, they could be easily identified by comparison to the known retention times of cis- and truns-1,2-dimethylcyclopropanes relative to each other and the corresponding olefin isomers. Photolysis of thietane was normally carried out to conversions of the reactant of between 2 and 10%; occassionally a run was carried out with up to 30% conversion of the reactant. The 214- and 229-nm photolysis experiments showed no consistent variation in product ratio as a function of the extent of photolysis in the region of reactant conversion for these experiments. Photolysis times were between 8 and 120 min, depending on the lamp and the pressure of the reactant. Polymer buildup on the windows was never great enough to diminish light transmission to the point of requiring special cleaning of the cells. (5) Searles, S.; Hays, H. R.; Lutz, E. F. J. Org. Chem. 1962, 27, 2828.
Mnm)
Flgure 1. Gas-phase absorption spectra (26 torr, 23 length) of thietane in the 240-340-nm region.
OC,
10-cm path
".U
200
220
210
230
240
Mnm)
Figure 2. Gas-phase absorption spectra (0.3 torr, 23 OC, 10-cm path length) of thletane in the 200-240-nm reglon.
Equipment. The spectrometers, photolysis, equipment, vacuum apparatus, and analytical procedures used in this work have been described in the literature.6
Results Spectra. The gas-phase absorption spectra of thietane in the wavelength region of 200-330 nm are illustrated in Figures 1and 2. The transitions of interest are the relatively weak (emax N 14 L mol-l cm-') transition a t lower energies with an onset at about 310 nm, and the more intense absorption (em= N 2200 L mol-' cm-l) with an onset at around 229 nm. The UV spectra of alkyl sulfides have been characterized by assuming local CZvsymmetry about the C-S-C c h r ~ m o p h o r e . ~The , ~ infrared spectra of trimethylene sulfide has also been a n a l y ~ e d . ~If we assume local C2"symmetry about the chromophore, the lowest energy, dipole forbidden (oscillator strength = 3 x lo4) absorption is lA2 lAl which results from a u* n transition. The higher energy, dipole allowed (oscillator absorption likely is lB2 lAl which strength N 3 results form a second u* n transition.'
-
-
-
+
(6) Dorer, F. H.; Salomon, K. E. J. Phys. Chem. 1980, 84, 3024. (7) Rosenfield, J. S.; Moscowitz, A. J.Am. Chem. SOC.1972,94,4797. (8) Clark, L. B.; Simpson, W. T. J. Chem. Phys. 1966, 43, 3666. (9) (a) Scott, D. W.; Finke, H. L.; Hubbard, W. N.; McCullough, 3. P.;
Katz, C.; Gross, M. E.; Messerly, J. F.; Pennington, R. E.; Waddington, G. J. Am. Chem. SOC.1953, 75, 2795. (b) Durig, J. R.; Lord, R. C. J. Chem. Phys. 1966,45, 61. (c) Borgers, T. R.; Straws, H. L. Ibid. 1966, 45, 947.
The Journal of Physical Chemistry, Vol. 85, No. 18, 198 1 2673
Second Excited Electronic Singlet State of Thietane
TABLE I: Thietane Product Ratios as a Function of Excitation Energy thietane (c-C,H, f , , A, C3H6/ c-C,H,/ C3H6)/ Press., nm torr C,H, C,H, C,H, 214 20 0.073 1.50 1.57 16.8 0.042 0.73 0.77 229 23Sb 14 0.11 0.11 240b 12 0.12 0.12 10 0.008 0.001 0.009 254a ~
18
< y =, Y
?
Experiments carried out Data taken from WH., with a 150-W xenon lamp and calibrated Bausch and Lomb monochrornotor. Irradiation times were 150-210 min. The band pass at half-peak height of the excitation radiation was 9 nm; consequently, some, if not all, of the c-C,H, results from direction excitation of the reactant to the 'B, state.
-
The vibrational progression observed in the higher energy 'B2 'Al transition has spacings of 140 15 cm-'. The fairly regular spacings and their relative intensities indicate that the low-frequency ring-deformation mode may be serving as the vibronic origin for this electronic transition. If thietane is corrupted to CZusymmetry, this mode would belong to the b2 irreducible representation, and its fundamental and overtones could be active in the 'B2 'Al electronic transition. The relative intensities, and slightly irregular spacings, near the onset of the 'B2 'Al transition indicate that the first few peaks are "hot band" transitions involving the ring-deformation mode. The true 0-0 onset for this electronic transition is likely -226.6 nm. The spectrum becomes continuous at energies of 1800 cm-' above the onset which could indicate that at higher energies of excitation the excited state lifetime becomes quite short s). At lower energies of excitation in the 'B2 'Al band the line width of the vibronic structure would still s. The allow an excited-state lifetime as short as calculated radiative lifetime of the 'B2state is -2.5 X lo4 s. Since we could not observe fluoroscenceemission in the gas phase on excitation of thietane into its 'B2 or 'A2 states, we determined the maximum emission quantum yield on excitation to the 'B2 state could be no greater than Consequently, the lifetime of the 'B2 state must be
