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The CC stretching Raman lines of [β-carotene isomers in the S1 state as detected by pump-probe resonance Raman spectroscopy
CHEMICAL
Volume 154, number 4
PHYSICS LE-JTERS
27 January
1989
THE C=C STRETCHING RAMAN LINES OF It-CAROTENE ISOMERS IN THE S, STATE AS DETECTED BY PUMP-PROBE RESONANCE RAMAN SPECTROSCOPY Hideki HASHIMOTO
and Yasushi KOYAMA
Faculty of Science, Kwansei Gakuin University, Uegahara. Nishinomiya 662. Japan Received 2 September
1988: in !inal form 27 October
1988
A Raman line due to the S, state of isomeric S-carotene (at 1777 cm-’ for all-trans and at 1760 cm-’ for 15&s) has been detected by means of transient, resonance Raman spectroscopy using < 100ps,355 nm pump and 532 nm probe pulses, the latter being rigorously resonant with the S.+S, absorption. The Rarnan line isassigned to a C-C stretching vibration in the 2 ‘A._Fstate by inference based on shorter polyenes.
1.
Introduction
Carotenoids in photosynthetic systems have the functions of harvesting, transferring and dissipating the light energy. In order to reveal the mechanisms of the above functions, the lowest energy level of the carotenoids should be determined. Carotenoids are practically non-fluorescent, and the energy gap of the optically allowed, ‘B,f -+ IA; (ground) transition has been considered to be too large for efficient energy transfer to chlorophyll or bacteriochlorophyll through coupling with their Q,cground transition. Involvement of the optically forbidden, 2 ‘A; +‘A; transition in the energy transfer was proposed [ 11, and [ 1 ] and both Fiirster’s dipole-dipole mechanism Dexter’s exchange mechanism [2 ] were discussed. In particular, Thrash et al. [ 3 ] showed, by means of the resonance Raman excitation profile, the pres ence of the hidden 2 ‘4 state below the ‘BT state for all-trans-pcarotene. However, the characterization of the S, state of &carotene is still controversial: Dallinger et al. [ 4,5 ] recorded the Raman spectrum using 35 ps, 532 nm pulses, but no Raman lines inherent to the S, state were found, the S, lifetime was concluded to be shorter than 1 ps. Haley and Koningstein [ 6 ] and Wylie and Koningstein [ 7 ] estimated the S, lifetime, for both all-trans- and 1S-cis@carotene, to be FZI 300 fs from the broadening of the Raman lines, but to be a few ps from the transmission changes. On the other hand, Wasielewski and 0 009-2614/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
Kispert [ 81 estimated the S, lifetime of all-trans-& carotene to be 8.4 ps from the recovery of the Sl+So absorption. Recently, we recorded the S,+S, absorption of all-trans- and 15_cis+-carotene [ 91 and determined the S, lifetime to be 14 ps (by using 0.5 ps pulses; excitation at 355 nm) [ lo]. Since it is well established for shorter polyenes that the S, state is actually the 2 ‘A,- state and that the state is characterized by the abnormally high C=C stretching frequency, we applied a pump-probe, transient resonance Raman technique to g-carotene isomers in search of a C=C stretching line above 1750 cm-‘.
2. Experimental AILtrans-p-carotene was purchased from Wako Pure Chemical Ind. and recrystallized from benzene. 15-cis-P-carotene was a gift from Dr. Niels-Henrik Jensen at Riss National Laboratory, Denmark. The purity determined by HPLC was 98% for the former and 95% for the latter; isomerization during the Raman measurements was negligible. Benzene solution (50 ml) of IO-’ M all-trans- or 10v4 M 15-cis-P_carotene was squirted (in the air) from a nozzle as a flat jet stream (thickness x400 pm, linear speed 1.4 m/s) and circulated. The mode-locked (76 MHz) and Q-switched (800 Hz) pulse trains (duration +Z100 ps) of THG (355 nm, 50 mW) and of SHG B.V.
321
Volume 154. number 4
CHEMICAL PHYSICS LETTERS
(532 nm, 20 mW)
from a Nd:YAG laser (Quan416) were used for pumping and probing the S, state, respectively; the pump and probe beams were co-axially focused onto the jet stream through a quartz lens (f= 50 mm). Each probe pulse was delayed optically with reference to the pump pulse. The zero delay time was not determined experimentally; the delay time giving the highest S, population is denoted as “zero”. The Raman detecting system was described previously [ 111. Since the pump pulses caused very high fluorescence background, the background data with pump pulses alone were subtracted from those with pump and probe pulses for the pump-probe recording. From the recording, the ground state recording without pump pulses was subtracted in order to obtain the spectrum of the S, state. Spectral data for 13 min (fig. 2) or 1.66 min (fig. 3) were accumulated, on which no smoothing was performed.
tronix
3. Results Fig. la shows the pumping conditions man measurements for (1) all-trans and p-carotene. Using the e at 355 nm (8x trans and 3.1 x lo4 for 15-cis) and the (b)
in the Ra(2) 15-c& lo3 for allpath length
; I
II
300
400
500 Wavelength
500 I
I
600
I
700
nm
Fig. 1. The pumping and @robing conditions in the Raman measuremenfs of the S, state: Electronic absorption spectra of the (a) S,CS,, and (b) S.tS, transitions for (I ) all-tram- and (2) 15-cis-fiarotene in n-hcxane solution. Concentrations: (a) 1 x lWJ M, (b-l ) 1.3~ lo-* M, and (b-2) 3.2~ 10m5 M. Path length I cm. The wavelengths of the pump and probe pulses are indicated with broken lines.
322
27 January 1989
(400 pm), the absorbance by the jet stream at 355 nm was calculated to be 0.32 for all-trans and 0.12 for lS-cis. Assuming the energy of the maximum mode-locked pulse in a Q-switch envelope, 4 pJ, the number of photons delivered to the sample was estimated to be w 10” for both isomers. On the other hand, the number of molecules in the beam ( 100 pm diameter and 400 Frn thick) was estimated to be x IO’’for 15-cis ( x 10” for all-trans). Judging from the recovery of the ground state, 8.4 ps [ 81, one molecule can be excited z 10 times within a pulse of 100 ps duration. Therefore, it turns out that the number of photons exceeded the number of molecules approximately by one order of magnitude for the maximum mode-locked pulse in the case of the 1%cis isomer. (The former agreed approximately with the latter in the case of all-trans.) Fig. 1b shows the probing conditions. The S,tS, absorptions [9] were recorded for the samples (Abs355 = 1.0; path length 1 cm) using a pair of 2025 ps, pump (355 nm) and probe pulses. The Lmax is 560 nm for all-trans, and 567 nm for lS-cis. The absorbance by the jet stream at 532 nm was estimated to be 0.14 for all-trans and 0.04 for lS-cis. However, the 90” scattering geometry through the jet stream (the angle of incidence 45’ ) was employed in order to reduce the fluorescence background. Fig. 2 shows the recordings of the Raman scattering for (a) all-trans and (b) 1Scis; ( 1) probe pulses only, (2) pump and probe pulses, and (3) the 10 times expansion of (2) minus ( 1). Here, a weak but distinct Raman line is detected for each isomer, i.e. at 1777 cm-’ for all-trans and at 1760 cm-’ for 1 Scis. (The different frequencies are regarded as evidence showing that those lines are not due to an experimental artifact.) The Raman lines are ascribed to the lowest excited singlet (S,) state due to the above experimental conditions. (Since the 355 nm pump pulse sits in wavelength at the edge of the ‘A,+ c ‘A8- transition (cis peak), there should be electronic relaxation down to the S, state (see below)_ Each Raman line is assigned to a C=C stretching mode from its frequency. Fig. 3 shows the rise and decay of the SI Raman line for the all-trans isomer. The changes can be explained in terms of the St lifetime ( 14 ps) and the pulse duration ( < 100 ps). The depletion of the So
Volume 154, number 4
CHEMICAL PHYSICS LETTERS
27 January 19g9
Raman line (at 1525 cm-‘) which has the same time course as the appearance of the S, Raman line, was detected in addition to the broadening of the latter (data not shown). (The above observations are regarded as another evidence for the S, Raman lines.) The maximum population in the S, state was estimated to be in the range of 5-10% from the bleaching of the So Raman line.
4. Discussion 2000 Raman
I cm-’
Shift
Fig. 2. The C-C stretching Raman line in the S, (2 ‘A; ) state for (a) all-trans- and (b ) 1S-&+-carotene in benzene solution (concentrations; (a) 1.0X lo-’ M and (b) 1.0x10-’ M). (1) Probe pulses only (the Sostate), (2) pump and probe pulses, and (3) [(2)-(l)]xlO(theS,state).Hatchedlinesarcduetothe solvent, benzene.
500 ps
100 ps
50
ps
20 ps
Assignment of the S, state to the 2 ‘4 state has been established for shorter polyenes [ 121. Experimentally, one-photon and two-photon absorption as well as fluorescence excitation spectroscopies were used in this assignment for octatetraene [ 131, 2,l Odimethylundecapentaene [ 141 and 2,1 Zdimethyltridecahexaene [ 15 1. Theoretically, the calculations of CI including triple and quadruple excitations were necessary to obtain the proper order of the low-lying ‘B: (S,), 2 ‘4 (S,) and ‘A; (S,) states and the energy gaps between them [ 16,171. The same order of the energy levels was also anticipated for longer polyenes. In particular, the strongest C=C stretching line in the 2 ‘& state is characterized by its abnormally high frequency. Recently, Kasama et al. [ 18,191 detected, by means of resonance CARS, the corresponding C-C stretching Raman line for diphenyloctatetraene and diphenyldecapentaene. The extremely high C=C stretching frequencies have been ascribed to a vibronic coupling between the 2 ‘& and the ‘4 states through an A,, C=C stretching vibration [ 15,191, since theoretical calculations prediced that the C-C stretching frequency in the 2 ‘A; state should be lower than that in the ‘& state
WI. 0
.I50
2ooo ~omon
PS
ps
1700 Shift
I
cm-l
Fig. 3. The rise and decay of the C-C stretching Raman line in the S, state for all-trans+carotene. The relative delay times are indicated (see section 2).
Fig. 4 compares the Raman lines of the S, (solid line) and S, (broken line) states for the above shorter polyenes with those for the present isomers of &carotene; the Raman line with the highest frequency and intensity is chosen in each case. The frequencies of the S,, C=C stretching Raman lines of B-carotene lead to the assignment of its S, state to the 2 ‘A; state. Comparison of the C=C stretching frequencies with respect to the length of the polyene chain reveals the following trends (the two compounds at the top are excluded from this comparison because of the con323
Volume 154, number 4
CHEMICAL PHYSICS LETTERS
I I
I 8
(7) I 1800
I
I
1700
1600
Raman
Shift
1 1500
I cm-l
Fig. 4. Comparison of the C-C stretching Raman lines in the S, (solid line) and So (broken line) states for polyenes with different chain lengths; ( 1) diphenyloctatetraene, (2) diphenyldecapentaene, ( 3) octatetraene, (4) 2,1 Qdimethylundecapentaeaene, (5) 1,12_dimethyltridecahexaene., (6) all-trans-wrotene and (7) 15-cis-P-carotene. The Raman line with the highest frequency and intensity is chosen in each case.
jugated phenyl groups at both ends) : ( I ) The So, C=C stretching frequency decreases when the chain length increases. (2) The S,, GC stretching frequency is almost constant irrespective of the chain length. Trend ( 1) can be explained in terms of a decrease in the C=C bond order due to an increased conjugation for longer polyenes. Trend (2) indicates stronger vibronic coupling for longer polyenes. Since the strength of the vibronic coupling is inversely proportional to the energy gap between the 2 ‘A; and ‘Ag state which is expected to decrease for longer polyenes [ 17,201, the stronger coupling for longer polyenes is naturally expected. Thus, the S1, C-C stretching frequency for the isomeric @carotene is considered to reflect a very strong vibronic coupling. However, the strength of the vibronic coupling (the separation of the S, and So frequencies) is found not to be a simple function of the chain length; the fluctuation may be due to differences in the form of normal modes, which can cause different interaction between the vibration and the electronic motion. 324
27 January 1989
The present results have provided additional experimetital evidence for the presence of the 2 ‘A; state as the lowest excited singlet state; its lifetime is in the range of 10 ps. (The frequencies of the Raman lines were indicative of the 2 ‘A,- state, and their lifetimes, of the S, state detected by transient absorption spectroscopy (lifetime 14 ps). ) The roles of the 2 ‘& state in the transfer and dissipation of the light energy in the photosynthetic systems are expected to be threefold: ( 1) Its energy (lower than that of the ‘B: (S,) state) provides much better overlap with the Q, level of (bacteria) chlorophylls, a fact which facilitates more efficient energy transfer [ 11. (2) Its lifetime, which is much longer than that of the ‘B: state ( z 300 fs [ 71) due to the forbidden character of the 2 ‘x -+I$- transition, provides an energy pool for efficient energy transfer. (3 ) The 4, C-C stretching vibrational motion, which is responsible for the vibronic interaction, can provide a pathway of energy dissipation by enhancing the 2 ‘Ai + ‘A; deactivation.
Acknowledgement The authors are indebted to Dr. Hiroyoshi Nagae of Kobe City University of Foreign Studies for discussions. This work has been supported by a grant from the Private Schools Promotion Foundation of Japan.
References [ 1] R.J. Thrash, H.L.-B. Fang and G.E. Leroi, Photochem. Photobiol. 29 (1979)
1049.
[ 21 K.R. Naqvi, Photochem. Photobiol. 31 (1980) 523. [ 3 ] R.J. Thrash, H.L.-B. Fang and G.E. Leroi, J. Chem. Phys. 67 (1977) 5930. 14,I R.F. Dallinger, S. Farquharson, W.H. Woodruff and M.A.J. Rodgers, J. Am. Chem. Sot. IO3 ( 198 1) 7433. 15 R.F. Dallinger, W.H. Woodruff and M.A.J. Rodgers, Photochem. Photobiol. 33 (1981) 275. [6 ‘I L.V. Haley and J.A. Koningstein, Chem. Phys. 77 ( 1983) 1. (7 I. W. Wylie and J.A. Koningstcin, I. Phys. Chem. 88 ( 1984 ) 2950. [8 M.R. Wasielewski and L.D. Kispert, Chem. Phys. Letters 128 (1986) 238. [9 Y. Koyama, Rev. Laser Eng. 15 (1987) 983.
Volume 154, number 4
CHEMICAL
PHYSICS LETTERS
[IO] Y. Hirata, N. Mataga, H. Hashimoto and Y. Koyama, unpublished. [ 111 H. Hashimoto and Y. Koyama, J. Phys. Chem. 92 ( 1988) 2101. [ 12] B.S. Hudson, B.E. Kohler and K. Schulten, in: Excited states, Vol. 6, ed. E.C. Lim (Academic Press, New York, 1982) p. 1. [ 131 M.F. Granville, G.R. Holtom and B.E. Kohler, J. Chem. Phys. 72 ( 1980) 467 I. [ 141 R.L. Christensenand B.E. Kohler, J. Chem. Phys. 63 (1975) 1837. [ 1S] R.A. Auerbach, R.L. Christensen, M.F. Granville and B.E. Kohler, J. Chem. Phys. 74 ( 198 1) 4.
27 January
1989
[ 161 K. Schulten, I. Ohmine and M. Karplus, J. Cbem. Phys. 64 (1976) 4422. [ 171 P. Tavan and K. Schulten, J. Chem. Phys. 70 ( I979 ) 5407. [ 18 ] A. Kasama, M. Taya, T. Kamisuki, Y. Ada&i and S. Maeda, in: Time-resolved vibrational spectroscopy, eds. A. Laubereau and M. Stockburger (Springer, Berlin, 1985) p, 166. [ 191 A. Kasama, M. Taya, T. Kamisuki, Y. Adachi and S. Maeda, in: Time-resolved vibrational spectroscopy, ed. G.H. Atkinson (Gordon and Breach, New York, 1987) p. 304. [20] A.C. Lasaga, R.J. Aemi and M. Karplus, J. Chem. Phys. 73 (1980) 5230.
325
Volume 154, number 4
PHYSICS LE-JTERS
27 January
1989
THE C=C STRETCHING RAMAN LINES OF It-CAROTENE ISOMERS IN THE S, STATE AS DETECTED BY PUMP-PROBE RESONANCE RAMAN SPECTROSCOPY Hideki HASHIMOTO
and Yasushi KOYAMA
Faculty of Science, Kwansei Gakuin University, Uegahara. Nishinomiya 662. Japan Received 2 September
1988: in !inal form 27 October
1988
A Raman line due to the S, state of isomeric S-carotene (at 1777 cm-’ for all-trans and at 1760 cm-’ for 15&s) has been detected by means of transient, resonance Raman spectroscopy using < 100ps,355 nm pump and 532 nm probe pulses, the latter being rigorously resonant with the S.+S, absorption. The Rarnan line isassigned to a C-C stretching vibration in the 2 ‘A._Fstate by inference based on shorter polyenes.
1.
Introduction
Carotenoids in photosynthetic systems have the functions of harvesting, transferring and dissipating the light energy. In order to reveal the mechanisms of the above functions, the lowest energy level of the carotenoids should be determined. Carotenoids are practically non-fluorescent, and the energy gap of the optically allowed, ‘B,f -+ IA; (ground) transition has been considered to be too large for efficient energy transfer to chlorophyll or bacteriochlorophyll through coupling with their Q,cground transition. Involvement of the optically forbidden, 2 ‘A; +‘A; transition in the energy transfer was proposed [ 11, and [ 1 ] and both Fiirster’s dipole-dipole mechanism Dexter’s exchange mechanism [2 ] were discussed. In particular, Thrash et al. [ 3 ] showed, by means of the resonance Raman excitation profile, the pres ence of the hidden 2 ‘4 state below the ‘BT state for all-trans-pcarotene. However, the characterization of the S, state of &carotene is still controversial: Dallinger et al. [ 4,5 ] recorded the Raman spectrum using 35 ps, 532 nm pulses, but no Raman lines inherent to the S, state were found, the S, lifetime was concluded to be shorter than 1 ps. Haley and Koningstein [ 6 ] and Wylie and Koningstein [ 7 ] estimated the S, lifetime, for both all-trans- and 1S-cis@carotene, to be FZI 300 fs from the broadening of the Raman lines, but to be a few ps from the transmission changes. On the other hand, Wasielewski and 0 009-2614/89/$ ( North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division )
Kispert [ 81 estimated the S, lifetime of all-trans-& carotene to be 8.4 ps from the recovery of the Sl+So absorption. Recently, we recorded the S,+S, absorption of all-trans- and 15_cis+-carotene [ 91 and determined the S, lifetime to be 14 ps (by using 0.5 ps pulses; excitation at 355 nm) [ lo]. Since it is well established for shorter polyenes that the S, state is actually the 2 ‘A,- state and that the state is characterized by the abnormally high C=C stretching frequency, we applied a pump-probe, transient resonance Raman technique to g-carotene isomers in search of a C=C stretching line above 1750 cm-‘.
2. Experimental AILtrans-p-carotene was purchased from Wako Pure Chemical Ind. and recrystallized from benzene. 15-cis-P-carotene was a gift from Dr. Niels-Henrik Jensen at Riss National Laboratory, Denmark. The purity determined by HPLC was 98% for the former and 95% for the latter; isomerization during the Raman measurements was negligible. Benzene solution (50 ml) of IO-’ M all-trans- or 10v4 M 15-cis-P_carotene was squirted (in the air) from a nozzle as a flat jet stream (thickness x400 pm, linear speed 1.4 m/s) and circulated. The mode-locked (76 MHz) and Q-switched (800 Hz) pulse trains (duration +Z100 ps) of THG (355 nm, 50 mW) and of SHG B.V.
321
Volume 154. number 4
CHEMICAL PHYSICS LETTERS
(532 nm, 20 mW)
from a Nd:YAG laser (Quan416) were used for pumping and probing the S, state, respectively; the pump and probe beams were co-axially focused onto the jet stream through a quartz lens (f= 50 mm). Each probe pulse was delayed optically with reference to the pump pulse. The zero delay time was not determined experimentally; the delay time giving the highest S, population is denoted as “zero”. The Raman detecting system was described previously [ 111. Since the pump pulses caused very high fluorescence background, the background data with pump pulses alone were subtracted from those with pump and probe pulses for the pump-probe recording. From the recording, the ground state recording without pump pulses was subtracted in order to obtain the spectrum of the S, state. Spectral data for 13 min (fig. 2) or 1.66 min (fig. 3) were accumulated, on which no smoothing was performed.
tronix
3. Results Fig. la shows the pumping conditions man measurements for (1) all-trans and p-carotene. Using the e at 355 nm (8x trans and 3.1 x lo4 for 15-cis) and the (b)
in the Ra(2) 15-c& lo3 for allpath length
; I
II
300
400
500 Wavelength
500 I
I
600
I
700
nm
Fig. 1. The pumping and @robing conditions in the Raman measuremenfs of the S, state: Electronic absorption spectra of the (a) S,CS,, and (b) S.tS, transitions for (I ) all-tram- and (2) 15-cis-fiarotene in n-hcxane solution. Concentrations: (a) 1 x lWJ M, (b-l ) 1.3~ lo-* M, and (b-2) 3.2~ 10m5 M. Path length I cm. The wavelengths of the pump and probe pulses are indicated with broken lines.
322
27 January 1989
(400 pm), the absorbance by the jet stream at 355 nm was calculated to be 0.32 for all-trans and 0.12 for lS-cis. Assuming the energy of the maximum mode-locked pulse in a Q-switch envelope, 4 pJ, the number of photons delivered to the sample was estimated to be w 10” for both isomers. On the other hand, the number of molecules in the beam ( 100 pm diameter and 400 Frn thick) was estimated to be x IO’’for 15-cis ( x 10” for all-trans). Judging from the recovery of the ground state, 8.4 ps [ 81, one molecule can be excited z 10 times within a pulse of 100 ps duration. Therefore, it turns out that the number of photons exceeded the number of molecules approximately by one order of magnitude for the maximum mode-locked pulse in the case of the 1%cis isomer. (The former agreed approximately with the latter in the case of all-trans.) Fig. 1b shows the probing conditions. The S,tS, absorptions [9] were recorded for the samples (Abs355 = 1.0; path length 1 cm) using a pair of 2025 ps, pump (355 nm) and probe pulses. The Lmax is 560 nm for all-trans, and 567 nm for lS-cis. The absorbance by the jet stream at 532 nm was estimated to be 0.14 for all-trans and 0.04 for lS-cis. However, the 90” scattering geometry through the jet stream (the angle of incidence 45’ ) was employed in order to reduce the fluorescence background. Fig. 2 shows the recordings of the Raman scattering for (a) all-trans and (b) 1Scis; ( 1) probe pulses only, (2) pump and probe pulses, and (3) the 10 times expansion of (2) minus ( 1). Here, a weak but distinct Raman line is detected for each isomer, i.e. at 1777 cm-’ for all-trans and at 1760 cm-’ for 1 Scis. (The different frequencies are regarded as evidence showing that those lines are not due to an experimental artifact.) The Raman lines are ascribed to the lowest excited singlet (S,) state due to the above experimental conditions. (Since the 355 nm pump pulse sits in wavelength at the edge of the ‘A,+ c ‘A8- transition (cis peak), there should be electronic relaxation down to the S, state (see below)_ Each Raman line is assigned to a C=C stretching mode from its frequency. Fig. 3 shows the rise and decay of the SI Raman line for the all-trans isomer. The changes can be explained in terms of the St lifetime ( 14 ps) and the pulse duration ( < 100 ps). The depletion of the So
Volume 154, number 4
CHEMICAL PHYSICS LETTERS
27 January 19g9
Raman line (at 1525 cm-‘) which has the same time course as the appearance of the S, Raman line, was detected in addition to the broadening of the latter (data not shown). (The above observations are regarded as another evidence for the S, Raman lines.) The maximum population in the S, state was estimated to be in the range of 5-10% from the bleaching of the So Raman line.
4. Discussion 2000 Raman
I cm-’
Shift
Fig. 2. The C-C stretching Raman line in the S, (2 ‘A; ) state for (a) all-trans- and (b ) 1S-&+-carotene in benzene solution (concentrations; (a) 1.0X lo-’ M and (b) 1.0x10-’ M). (1) Probe pulses only (the Sostate), (2) pump and probe pulses, and (3) [(2)-(l)]xlO(theS,state).Hatchedlinesarcduetothe solvent, benzene.
500 ps
100 ps
50
ps
20 ps
Assignment of the S, state to the 2 ‘4 state has been established for shorter polyenes [ 121. Experimentally, one-photon and two-photon absorption as well as fluorescence excitation spectroscopies were used in this assignment for octatetraene [ 131, 2,l Odimethylundecapentaene [ 141 and 2,1 Zdimethyltridecahexaene [ 15 1. Theoretically, the calculations of CI including triple and quadruple excitations were necessary to obtain the proper order of the low-lying ‘B: (S,), 2 ‘4 (S,) and ‘A; (S,) states and the energy gaps between them [ 16,171. The same order of the energy levels was also anticipated for longer polyenes. In particular, the strongest C=C stretching line in the 2 ‘& state is characterized by its abnormally high frequency. Recently, Kasama et al. [ 18,191 detected, by means of resonance CARS, the corresponding C-C stretching Raman line for diphenyloctatetraene and diphenyldecapentaene. The extremely high C=C stretching frequencies have been ascribed to a vibronic coupling between the 2 ‘& and the ‘4 states through an A,, C=C stretching vibration [ 15,191, since theoretical calculations prediced that the C-C stretching frequency in the 2 ‘A; state should be lower than that in the ‘& state
WI. 0
.I50
2ooo ~omon
PS
ps
1700 Shift
I
cm-l
Fig. 3. The rise and decay of the C-C stretching Raman line in the S, state for all-trans+carotene. The relative delay times are indicated (see section 2).
Fig. 4 compares the Raman lines of the S, (solid line) and S, (broken line) states for the above shorter polyenes with those for the present isomers of &carotene; the Raman line with the highest frequency and intensity is chosen in each case. The frequencies of the S,, C=C stretching Raman lines of B-carotene lead to the assignment of its S, state to the 2 ‘A; state. Comparison of the C=C stretching frequencies with respect to the length of the polyene chain reveals the following trends (the two compounds at the top are excluded from this comparison because of the con323
Volume 154, number 4
CHEMICAL PHYSICS LETTERS
I I
I 8
(7) I 1800
I
I
1700
1600
Raman
Shift
1 1500
I cm-l
Fig. 4. Comparison of the C-C stretching Raman lines in the S, (solid line) and So (broken line) states for polyenes with different chain lengths; ( 1) diphenyloctatetraene, (2) diphenyldecapentaene, ( 3) octatetraene, (4) 2,1 Qdimethylundecapentaeaene, (5) 1,12_dimethyltridecahexaene., (6) all-trans-wrotene and (7) 15-cis-P-carotene. The Raman line with the highest frequency and intensity is chosen in each case.
jugated phenyl groups at both ends) : ( I ) The So, C=C stretching frequency decreases when the chain length increases. (2) The S,, GC stretching frequency is almost constant irrespective of the chain length. Trend ( 1) can be explained in terms of a decrease in the C=C bond order due to an increased conjugation for longer polyenes. Trend (2) indicates stronger vibronic coupling for longer polyenes. Since the strength of the vibronic coupling is inversely proportional to the energy gap between the 2 ‘A; and ‘Ag state which is expected to decrease for longer polyenes [ 17,201, the stronger coupling for longer polyenes is naturally expected. Thus, the S1, C-C stretching frequency for the isomeric @carotene is considered to reflect a very strong vibronic coupling. However, the strength of the vibronic coupling (the separation of the S, and So frequencies) is found not to be a simple function of the chain length; the fluctuation may be due to differences in the form of normal modes, which can cause different interaction between the vibration and the electronic motion. 324
27 January 1989
The present results have provided additional experimetital evidence for the presence of the 2 ‘A; state as the lowest excited singlet state; its lifetime is in the range of 10 ps. (The frequencies of the Raman lines were indicative of the 2 ‘A,- state, and their lifetimes, of the S, state detected by transient absorption spectroscopy (lifetime 14 ps). ) The roles of the 2 ‘& state in the transfer and dissipation of the light energy in the photosynthetic systems are expected to be threefold: ( 1) Its energy (lower than that of the ‘B: (S,) state) provides much better overlap with the Q, level of (bacteria) chlorophylls, a fact which facilitates more efficient energy transfer [ 11. (2) Its lifetime, which is much longer than that of the ‘B: state ( z 300 fs [ 71) due to the forbidden character of the 2 ‘x -+I$- transition, provides an energy pool for efficient energy transfer. (3 ) The 4, C-C stretching vibrational motion, which is responsible for the vibronic interaction, can provide a pathway of energy dissipation by enhancing the 2 ‘Ai + ‘A; deactivation.
Acknowledgement The authors are indebted to Dr. Hiroyoshi Nagae of Kobe City University of Foreign Studies for discussions. This work has been supported by a grant from the Private Schools Promotion Foundation of Japan.
References [ 1] R.J. Thrash, H.L.-B. Fang and G.E. Leroi, Photochem. Photobiol. 29 (1979)
1049.
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