J.-Phys. Chem. 1984, 88, 2950-2953
2950
ARTICLES Photoisomerization and Time-Resolved Raman Studies of 15,15’-cis-P-Carotene and 15,15’-trans-P-Carotene I. W. Wylie and J. A. Koningstein* The Ottawa- Carleton Institute for Graduate Studies and Research in Chemistry, Department of Chemistry, Carleton University, Ottawa, Ontario K1S 5B6,Canada (Received: June 20, 1983; In Final Form: January 3, 1984)
Monobeam pump-Raman pulsed laser studies of the vibrational spectrum in the electronic ground and excited states of 15,15’-cis-/3-caroteneand 15,15’-trans-P-caroteneare reported. These studies, in combination with the results of monochromatic transmission experiments and time-resolved Raman spectroscopy, show that photoisomerization of 15,15’-cis- to alltrans-0-carotene is favored over that of trans to cis in carbon disulfide while photoisomerization of cis in hexane does not occur.
Introduction There are several reports in the literature which deal with the vibrational Raman spectrum of 15,lS-cis- and all-trans-p-carotene (hereafter known as “cis” and “trans”, respectively). These reports can be divided into two areas: experimental data obtained with (1) continuous wave (CW) lasers and (2) pulsed lasers. Spectral data obtained with C W lasers include a study of the Raman excitation profiles for the resonance-enhanced intensity of the more prominent modes and a theoretical analysis of these pr~files.l-~Of particular importance to the present work is the position of the vibrational Raman shifts of cis- and trans-p-~arotene.~The difference between the cis and trans isomers can be seen in the spectral region above 1000 cm-’. A doublet at 1240 cm-’ is visible solely in the spectrum of the cis isomer. Furthermore, the u1 band of the cis isomer is at 1533 cm-’, while that of the trans isomer is shifted to 1525 cm-’. The uI mode has C=C stretch character while the v2 mode at 1157 cm-’ is primarily due to C-C stretching motion. This mode has the same shift in both isomers in the two solvents used in this work, carbon disulfide (CS,) and hexane. The second area of published data, obtained with pulsed lasers, includes Raman spectral data from electronic excited Information on the position of (1) F. Inagaki, M. Tasumi, and T. Miyazawaw, J. Mol. Spertrosc., 50,286 (1974). (2) S. Sufra, G. Dellipiane, G. Masetti, and G. Zerbi, J. Raman Specfrosc., 6, 267 (1977). (3) R. J. Thrash, H. L. B. Fang, and G. E. Leroi, J . Chem. Phys., 67,5930 (1977). (4) M. Lutz, I. Agalidis, G. Hervo, R. J. Cogdell, and F. Reiss-Husson, Biochim. Biophys. Acta, 503, 287 (1978). (5) A. Warshel and P. Dauber, J . Chem. Phys., 66, 5477 (1977); W. Siebrand and M. 2. Zgierski, ibid., 71,3561 (1979); M. Samcc, W. Siebrand,
D. F. Williams, E. G. Woolgar, and M. Z. Zgierski, J . Raman Spectrosc., 11, 369 (1981). (6) N. H. Jensen, R. Wilbrandt, P. B. Pagsberg, A. H. Sillesen, and K. B. Hansen, J . Am. Chem. Soc., 102, 7441-4 (1980). (7) R. Wilbrandt and N. H. Jensen, J . Am. Chem. SOC.,103, 1036-41 (1981). (8) R. Wilbrandt and N. H. Jensen, Ber. Busenges. Phys. Chem., 85, 508-11 (1981). (9) G. H. Atkinson, J. B. Pallix, T. B. Freedman, D. A. Gilmore, and R. Wilbrandt, J . Am. Chem. Soc., 103, 5069-72 (1981). (10) R. F. Dallinger, J. J. Guanci, W. H. Woodruff, and M. A. J. Rogers, J . Am. Chem. SOC.,101, 1355-7 (1979). (11) R. F. Dallinger, S . Farquharson, W. H. Woodruff, and M . A. J. Rodgers, J. A m . Chem. SOC.,103,7433-40 (1981). (12) L. V. Haley and J. A. Koningstein, J. Phys. Chem., 87, 621-5 (1983). (13) L. V. Haley and J. A. Koningstein, to be submitted for publication in Chem. Phys.
0022-3654/84/2088-2950$01.50/0
vibrational modes in the triplet and the extremely short-lived (-300 fs) singlet excited state points to a lowering of the frequency of the u I and v2 modes in the triplet.6-8 The high-resolution excitation profile for the resonance-enhanced intensity of the v1 mode at 1525 cm-’ for all-trans-pcarotene in CS2 shows12 four maxima at 19480, 19 795, 21 005, and 21 325 cm-I. The maximum at 19480 cm-’ of trans represents the 0-0 transition between the ground state (Sgtrans) and the first in this paper), and singlet excited state of the blue system (SItrans the maximum at 21 005 cm-’ is due to a vibronic sideband (SItrans) of the v1 mode. If a 4 X M solution of trans-0-carotene is exposed to pulsed laser radiation which is resonant with the energy of the vibronic sideband (at 469 nm), the SItrans excited state can become populated. This in turn permits the excitation of vibrational Raman scattering from Sltra”at high photon flux values.12J3 The bands of the excited-state vibrational Raman spectrum are lifetime b r ~ a d e n e d , ’and ~ a value of T ( S ~ ~ 300 ~ ~fs was ~ ~ob) tained. The frequencies of the v, and u2 modes in SItrans are equal (within experimental error) to those for v1 and v 2 in Sgtrans.On the other hand, if the above solution is exposed to laser radiation with a wavelength resonant with the maximum in the excitation profile at 21 325 cm-I, an electronic excited state at 19 795 cm-I becomes populated.13 Whereas the frequency of v 2 remains at 1160 cm-’, that of v I in this unknown electronic excited state shifts is 19480 cm-I above upward to 1538 cm-’. The energy of Sltrans the ground state, while the energy of Slcisis 19 794 cm-I above the ground state (see section 11). In the present paper, the Raman spectra of 15,15’-cis-0-carotene in hexane and CS, are compared with some of the data discussed above. The comparison yields information on the frequencies of excited-state vibrational bands and on photoisomerization. Time-resolved Raman studies of 0-carotene in CS2 are discussed in section IV.
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11. Experimental Section
Pulsed laser induced Raman and fluorescence spectra of the 0-carotene isomers were recorded with an instrument described e1~ewhere.l~The main components are a pulsed (4 ns) tunable dye laser, a double monochromator, and a fast detection system consisting of a photomultiplier tube (cooled RCA 8850) and a boxcar with a 2-11s gate. The preparation of the solutions and their transfer to Raman cells were performed in a drybox under a N2atmosphere; all components were free of 0,. The carotene (14) D. Nicollin, P. Bertels, and J. A. Koningstein, Can.J . Chem. 58, 1334 (1980).
0 1984 American Chemical Society
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 2951
15,15’-cis- and 15,15‘-trans-@-Carotene 400nm
I
I
,
600nm
500nrn
I
t
CIS
’
L
0
’
,
04
08
I 2 nw
I,
Figure 2. Pulsed laser monochromatic transmission studies at XI = 448 M solution of 15,15’-cis-@-carotene in hexane. An nm of a 6 X analysis of T vs. I , assuming the steady-state approximati~n’~ and an irradiated volume based on the diffraction-limitedcross section for the laser beam passing through a IO-cm lens, yields a 7(SIcisin hexane) of 3.1 ps.
1533
..
i
Figure 1. The absorption and pulsed laser induced (XI = 448 nm) fluorescence spectrum of a 6 X M oxygen-free solution of 15,15’cis-&carotene in hexane. The intensities are arbitrary. Also indicated is the position of SIabove So as obtained by the method outlined in ref
12. TABLE I: Spectroscopic Data (in cm-I) for 15,15’-cis- and a// - trans -@-Carotene
15,15’-cis in hexane 15,15’-cis in CS2 all trans in CS2
vibronic 0-0 electronic sidebands origins abs fluor 20830 22261 19400 19794 19480
20835
17860
u1 mode“
1533
SI 1529
1525 1533
1525 1529
So
Raman spectrum. samples were obtained from Sigma Chemical Co. Reverse-phase HPLC data on all-trans-p-carotene dissolved in acetone did not indicate impurities or other isomers. The cis sample was a gift from Hoffmann-La Roche Ltd. This sample contained a maximum of 5% of the all-trans isomer as determined by HPLC in the aforementioned laboratories. The fingerprint bands at 1540 cm-l ( V J and 1160 cm-’ (v2) of cis were used to check the purity of the solutions containing the cis isomer before and after the pulsed laser experiments. In addition, transmission studies at high and low laser light levels yielded valuable information on the composition of the samples13 and relative populations of ground and excited states. Absorption spectra of the carotene samples were recorded on a Varian DMS 90 UV-vis spectrophotometer. The solutions of cis in oxygen-free, spectral grade hexane in particular were extremely stable and were used for 3-5 days in the pulsed laser studies, while those of cis in CS2 remained stable for not more than 1 day; Le., cis is unstable with respect to trans in this solvent. 111. Raman Spectral Studies of cis- and trans-@-Carotenein CS2 and Hexane Raman and Fluorescence and Monochromatic Transmission Studies of cis-@-Carotenein Hexane. The pulsed laser induced fluorescence of a 6 X M solution of cis-p-carotene in hexane is shown in Figure 1. This spectrum was recorded with a Raman spectrometer (see section 11), which facilitated the detection of the short-lived and weak fluorescence of p-carotene. The absorption spectrum of p-carotene in hexane exhibits sharper features as compared12 to that of @-carotenein CS2,and all bands of the former are shifted by -30 nm toward shorter wavelength. The Stokes shift of cis in hexane is approximately equal to that of trans in CS2 (2990 cm-l vs. 2970 c d ; see Table I). Following the interpretation12 of absorption and fluorescence data of trans in CS2, the position of SIcisin hexane is calculated to be at 20 830 cm-’ (480.0 nm). It was observed that the green fluorescence of
Figure 3. Part of the Raman spectrum of 15,15’-cis-@-carotene in hexane. The upper trace was recorded under conditions of max photon density at the focused position of a 10-cm lens (XI = 448 nrn, I = 1 mW; see also Figure 2) impinging on a 1-mm cell. It is evident from the fwhm that compared to the Raman spectrum of the ground state (lower trace) another band is present. The dotted line represents a normalized substracted Raman band obtained by digitization of the upper and lower traces (Foc - 0.75Defoc) and is assigned to scattering from u i in SIci5.
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cis in hexane (SI So) was more intense than the corresponding fluorescence for all trans in CS2. Since the lifetime of the latter fluorescence is 300 fs and since larger. fluorescence yields indicate longer lifetimes, the conclusion is reached that 7(SlCis in hexane) > 300 fs. In order to obtain a better estimate of the lifetime of 7(Slc”), a study was made of the pulsed laser intensity dependent value of the transmission at XL = 448 nm (see Figure 2). Pumping at that wavelength creates Slclsexcited states from which other laser photons can be absorbed. The transmission values at XL = 448 nm suggest that the cross section of the S , S1 transition So at 448 nm is smaller than the cross section for the S1 transition. Assuming that the diameter of the laser beam inside the 1-mm quartz cell maintains a constant (diffraction limited) minimum value, one can a value of 7(Slck)I3 ps from the 1.2 X loL4photons per pulse required to effectively saturate the SlcIsstate of molecules which occupy the irradiated volume. This value represents a lower limit but is consistent with the remarks made above. The vibrational Raman spectrum of Slcisin hexane should be detectable if the spectrum is induced with XL = 448.0 nm at a N
-+
-
(15) B. Halperin and J. A. Koningstein, Can.J . Chem., 59, 2792 (1981). (16) M. Asano, J. A. Koningstein, and D. Nicollin, J . Chem. Phys., 73, 688 (1980); M. Asano, D. Mongeau, D. Nicollin, R. Sasseville, and J. A. Koningstein, Chem. Phys. Lett., 658 293 (1979).
2952 The Journal of Physical Chemistry, Vol. 88, No. 14, 1984
Figure 4. Part of the Raman spectrum of a 5 X M solution of in CS2. The lower trace depicts a typical spectrum of the cis-@-carotene u1 mode of Socis recorded with unfocused laser light. The upper trace,
recorded with focused laser light, shows a significant shift toward lower frequency. Digitization and subtraction of these two spectra yields a value of 1526 i 2 cm-' for the frequency of the underlying band. Since the frequencies of both SOfrans and SItfans are at 1525 cm-1,12the underlying band is assigned to scattering from both these states. power level of 1 mW (5 X photons s-l). On the other hand, is recorded at lower laser power levels, for the spectrum of SOCIS which the transmission value is at a minimum (as in the absorption spectrophotometer). The laser wavelength was in resonance with the position of the 0-1 vibronic sideband of the v l mode in SlCIs. Part of the Raman spectrum in the spectral region of the v1 mode is shown in Figure 3 for low and high photon flux levels. If the upper trace is corrected for scattering from the Socisground state by subtracting a percentage of this spectrum, then the frequency of the v1 mode is shifted from 1533 cm-' for Somto 1529 =t2 cm-' in SlCis.Photoisomerization of cis- to trans-p-carotene does not appear to occur in the hexane solution because a Raman band at 1525 cm-' which corresponds to the position of the v 1 mode in S y is not present. The spectrum of cis in hexane between 1 100 and 1300 cm-' was also investigated. The positions and the features of the Raman bands in this region remain unchanged if the sample is exposed either to high or low laser light levels. The fingerprint doublet at 1240 cm-' which is representative of cis remains observable, although weak. Raman Scattering of &@-Carotene in CS2 Part of the Raman spectrum of cis-@-carotene in CS, is shown in Figure 4. The ground-state vl spectral region is different from that of vibrational ground-state scattering of the cis isomer in hexane (Figure 3). In the case of the CS2 solution, one notes that, besides a band at 1533 cm-l, at least one other Raman band is present toward lower frequency. The v 1 mode of the trans isomer has a shift of 1525 cm-' and is a likely candidate for this lower frequency band. On the basis of the above and also on the fact that the other fingerprint band at 1240 cm-' of the cis isomer is more difficult to detect in CS2than in the hexane solutions, the conclusion is reached that, in contrast to the solution in hexane, cis undergoes photoisomerization into trans-@-carotenein CS2. This is substantiated by monochromatic transmission experiments whose results are shown in Figure 5. Saturation of the longer lived SlcB occurs first, after which a noted increase in transmission indicates population of Sltrans, newly formed through photoisomerization (see discussion below). Saturation of the longer lived SlcLs occurs first, after which a newly noted increase in transmission indicates population of Sltram, formed through photoisomerization (see discussion below). In our experiments, the wavelength of the pulsed laser which causes isomerization is in resonance with the 0-1 vibronic sideband due to v1 of SIm.Earlier light-scattering studies of trans-p-carotene in CS2 indicated that, besides the vibrational spectrum of Sltrans, the vibrational spectrum of an electronic excited state of another species was recorded (see Figure 3, ref 12). The v1 mode of the latter occurs at 1529 f 2 cm-'. This band did not show the lifetime broadening observed for the v, band of SItrans.As mentioned in hexane is -3 ps. From Figure 5 earlier, the lifetime of SICIS
Wylie and Koningstein
0'
0.2
0.4
0.6
08
mW It
Figure 5. Pulsed laser monochromatic transmission studies of a 1.1 X M solution of (0)cis-0-carotenein CS2 and a 9 X M solution of (0)a[/-trans-@-carotene in CS,. The experiments were performed by focusing the laser light with a 10-cm lens on a 1.0-mm cell, using two
rotatable Glan-Thompson balsam prisms to methodically reduce the laser intensity in an easily controllable fashion. From the observation that TCk becomes equal to Ttranrat high photon flux levels, isomerization of cis to trans takes place. Lifetime determinations (see ref 15) yield 7(Slcis)N 2.5 ps and 7(Sltrans E 1.2 ps. The latter estimated value of T has to be N 300 fs from lifetime-broadening measurecompared with 7(Sltranr) ments of Raman bands in SIfrans in CS (ref 13). it is obvious that T(SlCis) > T(Sltrans). In light of the results discussed above regarding the laser-induced photoisomerization of cis to trans in CS2, it is reasonable to assume that there is sufficient excitation energy in the laser beam to allow the reverse process to occur and to produce significant numbers of SlCis.We therefore assign the nonlifetime-broadened Raman band at 1529 f 2 crn-l, observed both in our earlier studies on all-trans solut i o n ~ and ' ~ in our present work on 15,15'-cis in hexane, to the v1 mode of the first singlet excited state of the 15,15'-cis isomer. This assignment is corroborated by the fact that the 15,15'-cis isomer is the next most stable isomer after all-trans-@-carotene." Shown in Figure 4 is the Raman spectrum of the v l region of cis-@-carotenein CS2. The lower spectrum shows a band centered around 1533 cm-' in agreement with the position of v1 in Socis. The experiment was performed with unfocused laser light (A, = 480.0 nm). The upper spectrum was recorded from a solution illuminated with focused laser light; a shift toward lower frequency is evident. This shift may be attributed to the presence of v 1 in S F at 1525 crn-l. It is found that the magnitude of the shift increases to a limit of 1525 cm-' when the intensity of the laser radiation is increased (sharper focusing). These spectral results indicate that photoisomerization is occurring in the beam at laser intensities sufficiently large to produce large numbers of excited-state molecules (see discussion below and Figure 5). Therefore, the conclusion is reached that the creation of SIcis plays a role in the photoisomerization of cis to trans. The frequency of the v1 mode (1529 cm-') in Slcisis not equal to that of vl (1533 cm-l) of Socis, while v1 is at 1525 cm-I in S1 trans and Sgtrans, The potential well of SItrans is shifted to a larger degree with respect to that of S p n sthan Seis to Sohs(see excitation profiles of Figure 1, ref 12); it could well be that the potential barrier between the ground states of cis and trans is significantly larger than that between Sltrans and SIcis.It follows that the isomerization process from cis to trans is favored over that from trans to cis since E(SICIS) - E(SltranS) = 315 cm-I (see Table I). The results of monochromatic transmission studies as a function of pulsed laser intensity shown in Figure 3 and 5 show even more clearly the nature of the photoisomerization process. Whereas the transmission-dependent intensity increases gradually in the case of cis-,?-carotene in hexane and trans-@-carotenein CS2,such is not the case for the cis-@-carotene solution in CS2. For low (17) G. P. Moss, Pure Appl. Chem., 51, 507 (1979).
15,l 5’4s- and 15,15’-trans-@-Carotene
Figure 6. Representative time-resolved Raman spectra in the u, region of a saturated solution of cis in CS2at to and to + 12 ns. These spectra were recorded with spherically focused pulsed laser light, with an average power of 4 mW, on a 10.0-mm cell. The solution had an optical density
too large to allow the passage of the laser light, and therefore the spectra were recorded near the leading edge of the cell with the beam focused near the cell wall. The scale of the to spectrum is 250 mV while that of the upper spectrum is 50 mV. The time and frequency shift of five different spectra recorded throughout the laser pulse are shown in the inset. The photon to molecule ratio of the lower trace (to) is 1.6 X 104:1 and -2 X 103:1for the upper trace vs. 1 X lo6:1 for the spectra in Figure 4. A cylindrically focused spectrum (not shown) yields a shift of 1533 cm-I, identical with that of the to + 12 ns or the to - 8 ns spectrum. pulsed laser intensity at 480.0 nm the picosecond-lived Slcisstate becomes populated. At a power level of -0.2 mW a sudden increase in the value of T occurs. At large power levels the transmission of the cis solution becomes equal to that of a solution of all-trans-0-carotene in CS,. Apparently, at a power level of >0.2 mW sufficient numbers of trans molecules have been created so that saturation of Slhmcauses a significant increase in the value of the transmission of the molecules occupying the irradiated volume. On the other hand, such a behavior is not found in the all-trans solution (see also ref 12), showing that at 480.0 nm this process is more effective for cis- to trans-@-carotenethan for transto cis-@-carotene in CS2.
IV. Time-Resolved Raman Spectroscopy All the preceding Raman spectra were recorded with the 2 4 s gate of the boxcar set at the time which the laser pulse intensity reaches a maximum (t = to). In Figure 6 , the less intense regions of the pulse were investigated by scanning the time gate of the boxcar to times before to and after to. The laser intensity decreases rapidly at these times as can be deduced from the change of scale from the upper to the lower trace (see Figure 6). The Raman shift of the to spectrum of cis in CS2 recorded with spherically focused laser light shows a marked shift away from
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 2953 1533 cm-’ toward lower frequency (photon to molecule ratio of 16OOO:l). The t < to and t > to spectra show a progressive return toward the 1533-cm-’ marker. The t = +12 ns and t = -8 ns spectra (boxcar times) have the same Raman shift as a “softly” focused spectrum recorded with a cylindrical lens which is not shown (photon to molecule ratio of -1OO:l). The position of the band in the tospectrum corresponds to the position of the SId band deconvoluted from the focused spectrum in hexane (Figure 3). The progressive shift toward this lower frequency as the laser pulse intensity increases toward the maximum at to is interpreted as arising from an increasing population of SICIS.The “softer” focusing of the cylindrical lens does not populate Slmeffectively, and therefore scattering was not observed from this level in this spectrum. The transmission curve in Figure 5 was performed on a 1.1 X lo4 M solution (photon to molecule ratio of 1 X 106:1at a power of 0.8 mW). The spectra reported in Figure 6 were performed on a saturated solution at 4-mW average power. This many-fold increase in concentration requires a much higher laser power to observe optical saturation. It is estimated that the position of this solution on the transmission curve in Figure 5 would be below the 0.2-mW sharp rise. Since this point is below the plateau at which isomerization would occur, it is not unexpected that no evidence of isomerization was observed in the spectra in Figure 6. In conclusion, the saturated solution used for this experiment was too concentrated to allow significant isomerization. The high photon density from the spherical lens populated Slcissufficiently to allow a shift toward its frequency at t = to. The less intense regions of the laser pulse populated SImto a lesser degree as shown by the return to the Soh mark at 1533 cm-l in relation to increasing absolute values of t.
V. Conclusion In this study, the frequency of the v1 mode of the S1 singlet excited state of 15,15’-cis-@-carotenewas determined to be at 1529 f 2 cm-’ through observation of excited-state Raman spectra. Pulsed monochromatic transmission experiments were performed with focused laser light that gave the energy ranges necessary to populate the excited states. These experiments also allowed calculation of approximate lifetime of these states in the solvents hexane and CS2 through computer modeling15of the upper state populations, assuming the steady-state condition. These transmission experiments also vividly portrayed the isomerization process that occurs in CS2 but not in hexane. Time-resolved Raman experiments showed exactly which powers were sufficient to populate Slcisthrough observation of the 1533-cm-’ band. Finally, time-resolved Raman experiments showed graphically through shifts in band position the changes of excited-state population as a function of time during the laser pulse. Acknowledgment. We wish to thank Dr. L. V. Haley for discussions and Dr. J. D. Harvey of hoffmann-La Roche Ltd. for providing us with the sample of 15,15’-cis-fi-carotene. This research was supported by the Natural Sciences and Engineering Research Council of Canada. Registry No. 15,15’-cis-&Carotene,19361-58-1;all-trans-p-carotene, 1235-40-7.
