J. Phys. Chem. 1985, 89, 407-410
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ARTICLES Transient Resonance Raman Study on the Trans-Cis Photoisomerization of N-Methylthioacetamide Chihiro Kato, Hiro-o Hamaguchi,* and Mitsuo Tasumi Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo- ku, Tokyo 113, Japan (Received: January 5, 1984; In Final Form: August 7, 1984)
The trans to cis photoisomerization of N-methylthioacetamide (NMTAA) has been studied by transient resonance Raman spectroscopy. It has been revealed that the time of formation of the cis form is less than 5 ns. No long-lived excited triplet state is likely to be involved in the photoisomerization process.
1. Introduction Previously, resonance Raman',* and matrix-isolation infrared3 studies of the trans-cis photoisomerization of N-methylthioacetamide (NMTAA) were reported from this laboratory. Both trans- and cis-NMTAA exhibit a strong absorption in the 230290-nm region, which originates from the lowest '?r* + ?r transition of the W - N chromophore. Photoexcitation of the trans conformer in this wavelength region leads to trans to cis isome r i ~ a t i o n l -and ~ the reverse process also occur^.^ It is therefore clear that the initial step of the photoisomerization is the excitation to the lowest ' P * P state. To obtain further information on the mechanism of the photoisomerization process, time-resolved dynamic approaches are necessary. Since the NMTAA molecule shows no fluorescence at all, time-resolved fluorescence spectroscopy is not applicable. The present paper reports a transient resonance Raman study of the photoisomerization of NMTAA. This method utilizes the fact that both the trans and cis bands can be detected separately in the resonance Raman spectrum'**and that comparison of their intensities with a standard band gives the relative number of the NMTAA molecules existing in either the trans or cis form. These advantages, which are characteristic of vibrational spectroscopy such as Raman, have proved useful in dynamic studies of molecular processes. 2. Experimental Section The sample of NMTAA was commercially obtained from Chiba Kagaku Kenkyu-sho and was purified by recrystallization from benzene. The block diagram of the apparatus used in the present experiment is shown in Figure 1. Two pulsed-laser systems were used as the light source. The first system (system I) consisted of an N E C GLG-3300 Ar+ laser, a Spectra Physics 3448 cavity dumper, and an Inrad 5-15 second harmonic generator. It delivered -4 mW at 257.3 nm (15-ns pulse width, 4-MHz repetition, 1 nJ/pulse energy) and was used for measurements with low energies and a high repetition rate. The experiment with highenergy pulses was carried out with the second system (system 11) employing a Quanta-Ray DCR-2A Nd:YAG laser and a fourth harmonic generator (266 nm, 5 ns, 15 Hz, -2 mJ/pulse). In the double-pulse experiment, in which the effect of photoexcitation by a preceding pulse was monitored by the following one, the
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(1) Sugawara, Y.;Hamaguchi, H.; Harada, I.; Shimanouchi, T. Chem. Phys. Lett. 1977, 52, 323-326. (2) Harada, I.; Tasumi, M. Chem. Phys. Lett. 1980, 70, 279-282. (3) Atah, S.; Takeuchi, H.; Harada, I.; Tasumi, M. J. Phys. Chem. 1984, 88,449-45 1.
0022-3654/85/2089-0407$01.50/0
output of system I1 was separated into two beams, one delayed by about 20 ns from the other. These two beams were again spatially superimposed and focused on the sample. A jet-flow sampling technique was adopted. Raman scattered light was analyzed with a Spex 1877 Triplemate polychromator and detected with a PAR 1420-2 intensified silicon photodiode array detector. The detector was operated in the gated mode ( 5 4 s gate) in the double-pulse experiment. Otherwise it was used in the C W mode. The spectral data were transferred to a DEC MINC- 11 minicomputer and displayed on a X-Y plotter after appropriate processing.
3. Results 3.1. Ultraviolet Absorption Spectra. The ultraviolet absorption spectrum of trans-NMTAA in an aqueous solution shows two characteristic features? a strong band at 256 nm (e = 12400 cm-l 60 cm-' M-'). By M-I) and a weak shoulder at 315 nm (e analogy with the spectrum of thioacetamide (TAA),e6 the former is assigned to the lowest %*?r state and the latter to 3?r*ns,where ns indicates the nonbonding orbital localized on the sulfur atom. Only data for a chloroform solution have been reported for cis= NMTAA;' Amax = 282 nm and emax = 12600 cm-' M-' (A,, 266 nm, emax = 13500 cm-' M-' for the trans form in chloroform). Upon irradiation with ultraviolet light from a low-pressure Hg lamp (dominated by the 253.7-nrn line), the spectrum of NMTAA in an aqueous solution changed as shown in Figure 2a-e. The peak at 256 nm shifted toward the longer wavelength side and a concurrent decrease of the absorbance was observed. If the irradiated solution was left in the dark, the absorption spectrum gradually changed to the reverse direction. After 24 h in the dark, the peak position recovered to 256 nm as shown in Figure 2e, while the absorbance remained at about 84% of the original value. No further change occurred even if the solution was allowed to stand for longer time. The existence of a clear isosbestic point at 261.5 nm indicates that the spectral changes in the dark are due to the conversion between two different species, of which one must be trans-NMTAA which is characterized by the peak at 256 nm. The other species, which is photolytically formed from transNMTAA and converted back to it in the dark, and which has an absorption peak at the longer wavelength side with a smaller absorption coefficient, is most probably the cis form of NMTAA.
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(4) Hosoya, H.; Tanaka, J.; Nagakura, S . Bull. Chem. Soc. Jpn. 1960,33, 850-860. (5) Kjellin, G.; Sandstram, J. Acta Chem. Srand. 1973, 27, 209-217. (6) Barrett, J.; Deghaidy, F. S. Spectrochim. Acta, Part A 1975, 31, 707-7 13. (7) Walter, W.; Schaumann, E. Chem. Ber. 1971, 104, 3361-3377.
0 1985 American Chemical Society
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The Journal of Physical Chemistry, VoJ. 89, No. 3, 1985
Kat0 et al.
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RAMAN SHIFTIcm-1 Figure 3. Transient resonance Raman spectra of NMTAA in aqueous M for (a) and (b), 1.01 X low2M for (c), (d), and solutions (1.06 X (e)): (a) obtained with system I, jet flow; (b) system I, stationary, (c) system 11, jet flow, 0.079 mJ/pulse; (d) system 11, jet flow, 0.49 mJ/ pulse; (e) system 11, jet flow, 1.2 mJ/pulse. The cis bands are marked
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WAVELENGTH I n m Figure 2. Photoinduced and subsequent reverse changes of the ultraviolet absorption spectrum of NMTAA in an aqueous solution (3.7 X lo4 M) (a) before irradiation; (b) after the irradiation with a low-pressure Hg lamp for 1 min; (c) after 45 min in the dark (d) after 3 h in the dark (e) after 24 h in the dark.
It is known that the trans form of NMTAA is much more stable than the cis; the enthalpy difference between the two forms is 2.3 kcal/mol in chloroform.' This means that only 2% of the NMTAA molecules exists in the cis form at 300 K. The activation energy for cis to trans conversion depends on the solvent and the M) concentration. The value in a chloroform solution (9 X is 21 kcal/mo17 and the corresponding rate constant is 1 X s-'. It is likely that the activation energy is much larger in an aqueous solution because of the stronger solvent effects including hydrogen bonding. Then, the rate for the cis to trans conversion can be as small as what was observed in the present study. The decrease in the absorbance in the spectrum in Figure 2e indicates that a significant amount of photodecomposition occurs as well as isomerization. In fact, the longer wavelength side of the absorption band in Figure 2e is slightly different from that in the original spectrum, showing the existence of some extra species which are not converted back to trans-NMTAA. The photodecomposition of TAA with the ~ T * Texcitation has already been reported.* ( 8 ) Larson, D. B.; Arnett, J. F.; Seliskar, C. J.; McGlynn, S. P. J . Am. Chem. Sor. 1974, 96, 3370-3380.
by arrows. 3.2. Transient Resonance Raman Spectra. The transient resonance Raman spectra of NMTAA in aqueous solutions are shown in Figure 3. Spectra in parts a and b of Figure 3 were obtained with system I and those in parts c, d, and e with system 11. The jet flow sampling technique was adopted in parts a, c, d, and e of Figure 3, while 3b was measured from a stationary aqueous solution of NMTAA. The estimated number of laser pulses incident on a sample molecule was about 24 for system I (the speed of the jet flow being 17 m/s and the area of focused laser beam 0.008 mm2), while irradiation by just one pulse was experienced by a molecule when system I1 was used (5.7 m/s, 0.8 mm2). All the observed bands in Figure 3a were assigned to the trans form of NMTAA by Sugawara et a1.l on the basis of a normal coordinate calculation. In spectrum 3b, on the other hand, two extra bands were found at 720 and 1461 cm-'. It is readily expected from the results in section 3.1 that an ultraviolet laser irradiation of a stationary sample solution can result in the photoformation and accumulation of the cis-NMTAA molecule in the area of the focused laser beam. On this ground, Harada and Tasumi2 assigned the 720-cm-' band to photolytically formed cis-NMTAA. They also extended the normal coordinate analysis to the cis form using the force constants of Suzukig and Sugawara.1° The normal and N-deuterated species were included in the analysis. The 720-cm-' band (715 cm-' in N-deuterated (9) Suzki, I. Bull. Chem. SOC.Jpn. 1962, 35, 1456-1464 (10) Sugawara, Y.,unpublished work.
The Journal of Physical Chemistry, Vol. 89, No. 3, 1985 409
Cis-Trans Photoisomerization of NMTAA TRANS
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0 a05 0.1 0.2 0.5 1.0 2.0 LASER E N E R G Y l m J Figure 5. Laser power dependence of the intensities of the trans band (679 em-') and the cis band (720 cm-l) of NMTAA in an aqueous solution. The solid curve was obtained by fitting the three points in the low-energy region to a theoretical curve, N = No exp(7x) with y = 0.24 ( x is proportional to the laser energy). See text.
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E 3 RAMAN S H I F T Icm-' Figure 4. Laser power dependence of the transient resonance Raman spectrum of NMTAA in an aqueous solution (8.3 X lo4 M, containing 9.5 M CH3CN as an internal intensity standard). System I1 was used for excitation: (a) 0.13 mJ/pulse; (b) 0.32 mJ/pulse; (e) 0.79 mJ/pulse; (d) 1.94 mJ/pulse.
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NMTAA) was assigned to the C=S stretching of the cis form, while the corresponding band at 679 cm-l (670 cm-') was ascribed to the same mode of the trans. The calculation reproduced the trend that the cis C=S stretching frequency was about 40 cm-' higher than the corresponding trans frequency. This assignment was further supported by Ataka et ala3in matrix isolation infrared studies. The relative intensity of the 722- (corresponding to 720 cm-l in an aqueous solution) and 701-cm-l (679 cm-l) bands in an Ar matrix at 20 K changed reversibly by the alternate irradiation with a low-pressure H g lamp (253.7 nm, mainly excited the trans form) and with a Xe lamp (longer wavelength than 280 nm, mainly excited the cis form). A concurrent reversible change was also observed in the ultraviolet absorption spectrum, in which irradiation with the Hg lamp produced a spectral change similar to that observed in an aqueous solution. These results eliminate the possibility that the 720-cm-' band originates from any photodecomposition products. It seems beyond doubt that the 720cm-' band is due to the cis form of NMTAA. The 1461-cm-I band should also be ascribed to this form. The spectral changes in parts a-e of Figure 3 then indicate that (i) no appreciable photoisomerization occurred with system I (the cis bands were observed only from a stationary sample solution in which accumulation of the photolytically formed cis molecule could take place); (ii) a significant amount of cis-NMTAA was formed and Raman probed within the same laser pulse when system I1 was used; (iii) the number of the cis molecules increased relatively to the trans as the energy of the laser power was increased. An order-of-magnitude estimate gives a photon flux of 1.1 X 1021cm-2 s-l for a pulse from system I (1 nJ/pulse at the sample point) and 3.4 X cm-* s-' for system I1 (1 mJ). By combining these values with the absorption cross sections of trans-NMTAA (4.7 X lo-'' cm2 at 257 nm and 3.2 X lo-'' cm2 a t 266 nm), the rate of photoexcitation is estimated to be 5.2 X
104 s-l for system I and 1.1 X lo9 s-I for system 11. These numbers mean that only a negligibly small portion ( 1.9 X 10-2 for 24 pulses) of the trans-NMTAA molecules were photoexcited with system I but that one molecule could be excited five times oh an average when irradiated by a 1 mJ pulse from system 11." The fact that photolytically formed cis-NMTAA was detected only with system I1 and not with system I (when jet flow sampling was used) is thus reasonably explained. Concerning results (iii), the laser power dependence of the intensities of the NMTAA bands was examined with a reference band (918 cm-' band of CH3CN which was added to the sample solution). The observed spectral changes are shown in Figure 4. With increasing laser power, both the intensities of the trans (679 cm-') and the cis (720 cm-I) bands decreased relative to the reference band. The former showed a more rapid decrease than the latter, which resulted in an apparent relative increase of the latter. This trend can be seen more clearly in Figure 5, where the intensities of these two bands are plotted against the energy of the laser pulse. In order to account for the observed decrease of the NMTAA band intensities, the double-pulse experiment was carried out. In this experiment, the preceding pulse, the energy of which was varied in the range 0.052-1.94 mJ/pulse, was used for photoexcitation of the sample and the following pulse with 20-11s delay was used to probe the resonance Raman scattering from the same sample region which was irradiated by the preceding pulse. The energy of the following pulse was lowered to 0.02 mJ/pulse so that the effect of photoexcitation within this pulse was made as small as possible. The detector was operated in the gated mode with a 5 4 s gate synchronous to the probing pulse, so that only the Raman scattering occurring in this gate period could be detected. A decrease in the intensities of the NMTAA bands was observed in the double-pulse experiment as well when the energy of the preceding laser pulse was increased. No appreciable difference was found between the laser power dependence in the single- and double-pulse experiments. This means that the decrease of the band intensities and hence the decrease of the number of the NMTAA molecules in the ground electronic state does not recover within 20 ns of photoexcitation. 4. Discussion
Result (ii) indicates that there exists at least one pathway for trans-cis photoisomerization of NMTAA in which the time of (1 1) The estimation assumes that the photoexcited molecule relaxes to the ground state within a time scale much shorter than 5 ns. This assumption seems to be valid for NMTAA as is discussed in section 4.
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formation of the cis form is less than 5 ns. This contradicts the previous conclusion by Harada and Tasumi,2 who estimated a lower limit for the time of formation as 0.5 ms from a C W flow experiment.I2 In a C W excitation measurement, the rate of photoexcitation is very small (7.8 X lo2 s-I for a 1-mW C W excitation of trans-NMTAA at 257.3 nm) and accordingly it takes relatively long time for a significant portion of the sample molecules to be photoexcited (1.3 ms for excitation of trans-NMTAA once on an average). If the flow rate is large enough so that the majority of the sample molecules do not experience photoexcitation while they are irradiated, the concentration of the photolytically produced species remains too low to be detected by Raman scattering. Thus, the apparent lower limit for the time of formation determined by the C W flow method can be incorrect if the rate of photoexcitation is small. This is expected to be the case with NMTAA. Only little has been studied on the photophysics of thioamides. It is known that a freshly prepared solution of TAA shows neither fluorescence nor phosphorescence from a low-temperature glassy solution.s We could not detect any emission from NMTAA either. The nonemissive property of the %*a state suggests the existence of an unusually fast radiationless process that suppresses the fluorescence. This process is presumed to be intersystem crossing to the %r*nS on the following grounds. Intersystem crossing between the 'A** and 37r*ns states is expected to be efficient from the selection rules given by El-SayedI3 and from the fact that these two states are close in energy. Large spin-orbit interaction due to the sulfur atom may further enhance the efficiency. In fact, the T S absorption to 37r*ns has a high extinction coefficient (c 60 cm-I M-I), implying that a significant singlet character is being mixed into this triplet state. It has already been shown by Sugawara et al.' that photoexcitation to the 37r*ns state by the irradiation of a 325.0-nm laser line also produced the cis form of NMTAA. The combination of this fact with the discussion in the preceding paragraph leads to the presumption that trans to cis conversion in the photoisomerization process of NMTAA takes place in the triplet manifold of 37r*ns or in subsequent relaxation processes. Nothing is known about the location and the lifetime of the 37r*astate which is expected to lie below 3a*ns and which may be involved in the relaxation process of 37r*ns. However, we are able to derive semiquantitative information about the overall rate of this relaxation process from the laser power dependence of the Raman intensities. The double-pulse experiment described in section 3.2 revealed that the decrease in the number of the trans-NMTAA molecules
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(12) Woodruff. W. H.; Spiro, T. G. Appl. Spectrosc. 1974, 28, 74-75. (13) El-Sayed, M. A. J. Chem. Phys. 1963, 38, 2834-2838.
Kat0 et al. in Figure 5 did not recover within 20 ns of photoexcitation. From the results in section 3.1, this decrease must partially be accounted for by the photodecomposition of trans-NMTAA. There is also the possibility that the depletion of the ground state is caused by an excited electronic state, say %*A, which has much longer lifetime than 20 ns. Such a long-lived excited state, however, is unlikely to be involved in the main pathway of the relaxation process of 37r*nSfor the following reasons. If backconversion from the cis form is neglected (this is valid for excitation with low-energy pulses in which the cis concentration remains low), the number of the trans molecule N depends on the laser power as N = No exp(-yx), where No is the number of the trans molecule before irradiation, x is the overall probability of photoexcitation during the period of the laser pulse, and y is the total quantum efficiency for the isomerization, decomposition, and formation of the supposed long-lived excited state. The three data points in the lowenergy region can be fitted to a theoretical curve with y = 0.24 as shown in Figure 5, where the laser energy in the abscissa is proportional to x. This implies that about 24% of the trans molecules photoexcited to IT*T are subject to the above three processes. Of these the isomerization must make a contribution since a significant number of the cis molecules (about 15% if the scattering cross section is assumed to be the same for the trans and cis bands) are produced at a pulse energy of 0.2 mJ/pulse where trans-NMTAA is excited once on an average. It is also estimated from the results in section 3.1 that the quantum yield for decomposition is comparable with that of isomerization. Therefore, the observed decrease of the number of the trans molecules in Figure 5 may well be accounted for by the isomerization and the decomposition. N o long-lived excited state is likely to be involved in the main route of the relaxation processes of photoexcited trans-NMTAA. This is in accord with the fact that we could not observe any transient absorption in the wavelength region 750-385 nm and in the time range 0 ns-10 ps after photoexcitation. There seems to exist no isomerization pathway which involves a long-lived excited electronic state. In conclusion, the present transient resonance Raman study has revealed that photoisomerization of NMTAA occurs within 5 ns of photoexcitation. The isomerization is likely to take place after the photoexcited molecule relaxes from the 'a*rto the %*nS state by efficient intersystem crossing, although the possibility of direct isomerization from 'a*a is not enitrely eliminated. N o long-lived excited state is involved in the main photoprocess pathway of NMTAA. Acknowledgment. This work was supported by a Grant-in-Aid for Special Distinguished Research (No. 56222005) from the Ministry of Education, Science, and Culture. Registry No. NMTAA, 5310-10-1.
