- Email: [email protected]
Vibrational relaxation in the 1Bu+ state of carotenoids as determined by Kerr-gate fluorescence spectroscopy
Chemical Physics Letters 400 (2004) 7–14 www.elsevier.com/locate/cplett
Vibrational relaxation in the 1Bþ u state of carotenoids as determined by Kerr-gate fluorescence spectroscopy Ryosuke Nakamura a, Ritsuko Fujii b,1, Hiroyoshi Nagae c, Yasushi Koyama b,*, Yasuo Kanematsu a a
JST-CREST, Venture Business Laboratory, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan b Faculty of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda, Hyogo 669-1337, Japan c Kobe City University of Foreign Studies, Gakuen Higashi-machi, Kobe 651-2187, Japan Received 5 August 2004; in final form 7 October 2004 Available online 6 November 2004
Abstract Subpicosecond time-resolved fluorescence spectra of all-trans-neurosporene and spheroidene, upon excitation above the þ 1Bþ u (v = 2) and 1Bu (v = 3) vibronic levels, respectively, were recorded by means of Kerr-gate spectroscopy. The spectral data matrices were analyzed by singular-value decomposition followed by global-fitting by the use of a sequential model. A pair of speciesþ associated difference spectra thus obtained in each carotenoid was ascribed to fluorescence from the 1Bþ u (v = 1) and 1Bu (v = 0) þ þ vibronic levels down to the ground 1Ag (v = 0, 1 and 2) levels. The lifetimes of the 1Bu (v = 1) and 1Bu (v = 0) states were determined to be 50 and 260 fs in neurosporene and 25 and 260 fs in spheroidene. 2004 Elsevier B.V. All rights reserved.
1. Introduction All-trans-carotenoids (Cars) are selectively bound to the antenna (LH1 and LH2) complexes from purple photosynthetic bacteria [1], and play an important role of light harvesting, which includes absorption of photons by Car followed by singlet-energy transfer to bacteriochlorophyll (BChl). The C2h symmetry of the all-trans conjugated chain gives rise to low-lying singlet-excited states including the 1Bþ u ; 3Ag ; Bu and 2Ag states; the optically forbidden 3Ag and 1Bu states were recently identified by measurement of resonance-Raman excitation profiles (RREPs) for Cars having the number of conjugated double bonds, n = 9–13 [2]. The RREP measurement determined the state ordering of *
Corresponding author. Fax: +81 795 65 9077. E-mail address: [email protected] (Y. Koyama). 1 Present address: Department of Physics, Graduate School of Science, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan. 0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.069
(ground) for Cars having 1Bþ u > 1Bu > 2Ag > 1Ag n = 9 and 10, and that of 1Bþ u > 3Ag > 1Bu > 2Ag > 1Ag for Cars having n = 11–13 (see Fig. 5a of [2]). The RREP measurement showed also that all the state energies decrease when n increases as functions of 1/(2n + 1). Those singlet-excited states that are located close-by facilitate efficient internal conversion in Car, and provide plural channels of singlet-energy transfer from Car to BChl [3–5]. This is most probably the reason for the natural selection of the all-trans configuration by Cars in the antenna complexes. Subpicosecond time-resolved, near-infrared absorption spectroscopy determined two different pathways of internal conversion in accord with the above state ordering: 1Bþ u ! 1Bu ! for Cars with n = 9 and 10, þ and 1Bu ! 3Ag ! for Cars with n = 11–13 [6]. The sudden drop in the efficiency of Car-to-BChl singlet-energy transfer on going from n = 10 to n = 11, which was determined by subpicosecond time-resolved absorption spectroscopy, was explained in terms of the relative energies between Car and BChl; the 1B u state becomes
8
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
lower than the Qx state and the 2A g state becomes too close to the Qy state. As a result, both the 1B u -to-Qx and the 2A g -to-Qy channels become closed, and the overall singlet energy-transfer efficiency drops from 84% down to 51% in the LH2 complex [7] and from 75% down to 46% in the LH1 complex [8]. In order to obtain a clearer picture of internal conversion in Cars and singlet-energy transfer from Car to BChl, it is absolutely necessary to determine the processes of vibrational relaxation. Here, we define the term Ôinternal conversionÕ more strictly as quasi-isothermal transformation from a vibrational level of one electronic state to a vibrational level of the other electronic state. (In the above description, we used this word in a broader sense, implying both processes of internal conversion and vibrational relaxation.) In many cases, vibrational relaxation takes place much faster than internal conversion. However, this is not necessarily the case in Cars, where internal conversion reactions take place very rapidly among the singlet-excited states that are located close-by. There have been some examples in which vibrational relaxation takes place not necessarily faster than, but in the same time scale or even slower than, internal conversion: In the 2A g state of all-trans-lycopene (n = 11), picosecond time-resolved absorption spectroscopy identified vibrational relaxation which took place in a time scale slower than that of internal conversion. The timeresolved spectra were explained in terms of vibrational structures originating from the C@C (m1) and C–C (m2) stretching modes, and the rates of the lowest vibrational relaxation ðk v1 and k v2 Þ relative to that of internal conversion (k) were determined to be k v1 =k and k v2 =k around 0.6 and 0.3 in quinoline and 0.2 and 1.6 in CS2 [9]. In the 1Bþ u state of neurosporene (n = 9), subpicosecond time-resolved absorption spectroscopy showed that the relative intensities of the vibrational progression peaks in stimulated emission changed depending on þ excitation either to the 1Bþ u ð0Þ level or to the 1Bu ð1Þ level, suggesting that vibrational relaxation did not take place faster than the 1Bþ u ! 1Ag internal conversion [10]. In the 1Bþ u and the 2Ag states of neurosporene, the high-frequency-shifts of the C@C stretching Raman line, reflecting vibrational relaxation in an anharmonic potential, were observed by subpicosecond timeresolved Raman spectroscopy, a fact which indicated that vibrational relaxation took place in a time scale comparable to that of internal conversion [11]. In the present investigation, we have attempted to identify, by means of Kerr-gate fluorescence spectroscopy, the vibrational relaxation processes upon excitation to the higher vibrational levels of the 1Bþ u state in neurosporene and spheroidene. Since we determined, by up-conversion fluorescence spectroscopy upon exci-
tation specifically to the 1Bþ u ð0Þ vibronic level, the time constant of the 1Bþ u to 1Bu internal conversion to be 60 fs in neurosporene [12], the results would provide us a unique opportunity to compare the rate of vibrational relaxation to that of internal conversion in the extremely short-lived 1Bþ u state of the particular Cars. 2. Experimental The experimental set-up for time-resolved fluorescence spectroscopy, based on the optical Kerr gate, is similar to that described elsewhere [13]. An amplified mode-locked Ti:sapphire laser (Coherent, RegA9000) was operated at 800 nm with the repetition rate of 200 kHz. The light pulses from the amplified mode-locked Ti:sapphire laser were used as the optical Kerr-gating pulses, while their second harmonics were used to excite the sample in a flow cell, in which 0.3-mm thick sample solution was sandwiched between a pair of 0.7-mm thick fused-silica plates. A thin crystal of SrTiO3 (50-lm thickness) was used as the Kerr material. The time resolution, the spectral resolution, and the Kerr-gated transmittance were 180 fs, 5 nm, and 5%, respectively. The gated luminescence was colleted by a lens, and lead to an optical fiber attached to a polychromator (Acton, SpectraPro-275); the fluorescence signal was detected by a charge-coupled-device camera (Princeton Instruments, LN/CCD-1024/EEV) equipped with an image intensifier. To reduce the strong scattering of the gating and the excitation pulses, a UV-IR cut filter was inserted in front of the optical fiber. The arrival time of the gating pulse was varied by the use of an optical delay stage. At a delay position where there was no signal, we measured the background signal originating from the twophoton-excited luminescence of the Kerr material and slightly-leaking luminescence through the analyzer. The background was subtracted from the obtained signal at each delay time. The optical components between the sample and the analyzer, including the sample cell, the polarizer and the Kerr material, affect the arrival time of pulses emitted from the sample because of their group delay dispersion (GDD). We estimated the GDD of each component, and corrected for the wavelengthdependent time shift in the time-resolved spectra. All-trans-neurosporene and spheroidene were prepared as described elsewhere [14]. The concentration of the sample in n-hexane solution was 1.6 · 104 M (1.7 · 104 M) for neurosporene (spheroidene), and the purities of the sample before and after the spectral measurement were 100% and 99% (96% and 96%), the values of which were determined by HPLC detected at 450 nm. The procedures of singular-value-decomposition (SVD) and global fitting analysis were described previously [10].
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
classified by a set of Dv values depending on the starting vibrational level in the 1Bþ u state; (i) Dv = +2, +1, 0, 1, 2. . . starting from v = 2; (ii) Dv = +1, 0, 1, 2. . . starting from v = 1; and (iii) Dv = 0, 1, 2 starting from v = 0. Therefore, the narrowing of the fluorescence progression is naturally expected during the process of vibrational relaxation in the order, v = 2 ! v = 1 ! v = 0. Fig. 2 shows the time-resolved fluorescence spectra of neurosporene and spheroidene (1.6 and 1.7 · 104 M) recorded by Kerr-gate spectroscopy. (Sharp structures with a width smaller than the spectral resolution of 200 cm1 that appear on the top of broader profiles in the time-resolved spectra are due to an experimental artifact.) The stationary-state fluorescence spectra of neurosporene and spheroidene (both 1.0 · 105 M) [14] are also shown, for comparison, on the top of the time-resolved spectra. (The sharp and strong peaks at 22 120 cm1 in both spectra can be assigned to the C– H stretching Raman line due to the solvent, n-hexane; the Raman shift is 2880 cm1 in reference to the pumping pulse at 25 000 cm1). The three peaks at 20 960, 19 840 and 18 660 cm1 for neurosporene, and those at 19 920, 18 800 and 17 760 cm1 for spheroidene,
3. Results 3.1. Characterization of time-resolved fluorescence spectra Fig. 1 shows an energy diagram for the vibrational sublevels on the 1Bþ u and 1Ag (ground) electronic states of neurosporene and spheroidene in n-hexane. The energies of the 1Bþ u state of those Cars (21 300 and 20 600 cm1) were determined by stationary-state fluorescence spectroscopy; the spacing of vibrational progression in 1 the 1Bþ u state (1400 cm ) and that in the 1Ag state 1 (1100 cm ) were determined by fluorescence-excitation and fluorescence spectroscopy, respectively [14]. In the time-resolved Kerr-gate fluorescence spectroscopy, the Car molecules were excited at 400 nm, i.e., just below the 1Bþ u ð3Þ level in neurosporene, and slightly above it in spheroidene. Since the rate of vibrational relaxation from the v to the v 1 level is proportional to v [15], the v = 1 to v = 0 vibrational relaxation should be most easily time-resolved, but the v = 3 to v = 2 vibrational relaxation can be too fast to be detected by the present Kerr-gate spectroscopy of 180 fs time resolution. If we define Dv = v v0, the fluorescence transitions can be
25
20
v=3 v=2
v=3
v=1
v=2
v=0
v=1
+ 1B u
9
v=0 1B u
v=3
v=5
v=2
v=4
v=1
v=3
v=0
v=2
+ 1B u
v=1 v=0
15
3
Energy / 10 cm
—1
1B u
10
5
0
1A g
1A g
1A g
1A g
v0 = 4 v0 = 3 v0 = 2 v0 = 1 v0 = 0
v0 = 4 v0 = 3 v0 = 2 v0 = 1 v0 = 0
v0 = 4 v0 = 3 v0 = 2 v0 = 1 v0 = 0
v0 = 4 v0 = 3 v0 = 2 v0 = 1 v0 = 0
Neurosporene
Spheroidene
Fig. 1. Energy diagrams for neurosporene and spheroidene showing the vibrational sublevels on the 1Bþ u and 1Ag (ground) states, and fluorescence þ transitions from the v = 2, v = 1 and v = 0 vibrational levels in the 1Bu state down to various vibrational levels in the 1A g state. The vibrational levels 1 þ on the 1B for neurosporene, and 20 600 u state are also shown for comparison. The energies of the 1Bu and 1Bu states (21 300 and 19 800 cm and 17 800 cm1 for spheroidene) were determined by fluorescence spectroscopy. The spacing of the vibrational levels in the 1Bþ u and 1Bu states 1 (1400 cm1) and that in the 1A state (1100 cm ) were determined by fluorescence-excitation and fluorescence spectroscopy [14]. g
10
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
roidene. Those spectra exhibit no clear peaks at the posi tion of the 1Bþ u ð0Þ ! 1Ag ð0Þ transition that are clearly seen in the stationary-state spectra (use the vertical broken lines in Figs. 2 and 3 for comparison). This is a unique characteristic of the very initial component. (2) The time-resolved spectra at the last stage (0.55 ps) exhibit three peaks in both neurosporene and spheroidene cor þ responding to the 1Bþ u ð0Þ ! 1Ag ð0Þ, 1Bu ð0Þ ! 1Ag ð1Þ þ and 1Bu ð0Þ ! 1Ag ð2Þ peaks. Actually, the spectral patterns at this very last stage are very similar to those of the stationary-state spectra. (3) The time-resolved spectra of the two Cars immediately after excitation (0.00
Neurosporene
Spheroidene
Intensity (a.u.)
(a)
—0.10 ps
—0.10 ps
0.00 ps
0.00 ps
0.55 ps
0.55 ps
Fig. 2. Time-resolved fluorescence spectra of neurosporene and spheroidene recorded by means of Kerr-gate spectroscopy. The stationary-state fluorescence spectra were also shown for comparison on the top of them; the sharp and strong peak in the spectra are due to the C–H stretching Raman line of the solvent n-hexane. The vertical broken lines indicate the position of the 1Bþ u ð0Þ ! 1Ag ð0Þ transition. can be definitely assigned to the 1Bþ u ð0Þ ! 1Ag ð0Þ, þ þ 1Bu ð0Þ ! 1Ag ð1Þ and 1Bu ð0Þ ! 1Ag ð2Þ fluorescence transitions. Fig. 3a shows the time-resolved spectra at 0.10, 0.00 and 0.55 ps after normalization of their intensities. Since the spectral patterns of neurosporene and spheroidene are similar to one the other, they can be inclusively characterized as follows: (1) The time-resolved spectra at the very initial stage (0.10 ps) exhibit two peaks shifted to the lower-energy side in both neurosporene and sphe-
Intensity (a.u.)
(b)
16
v=2
v=2
v=1
v=1
v=0
v=0
22
Wavenumber / 103cm—1
16
22 Wavenumber / 103cm—1
Fig. 3. Normalized time-resolved spectra at delay times at 0.10, 0.00 and 0.55 ps (a) and simulated fluorescence from the v = 2, v = 1 and v = 0 levels of the 1Bþ u state (b) for neurosporene and spheroidene. The vertical broken lines indicate the 1Bþ u ð0Þ ! 1Ag ð0Þ transition.
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14 ps) exhibit some indication of the 1Bþ u ð0Þ ! 1Ag ð0Þ peak, and also a peak on the higher-energy side ascrib able to the 1Bþ u ð1Þ ! 1Ag ð0Þ transition. The latter peak is seen strongly in neurosporene but weakly in spheroidene; Fig. 2 shows that it is covered and enhanced by the Raman line in the case of neurosporene, but it appears on the lower-energy side of the Raman line in spheroidene. The above empirical characterization of the spectral changes suggests that the time-resolved fluorescence spectra may mainly reflect the process of vibrational relaxation from v = 1 to v = 0.
3.2. Simulation of fluorescence spectra from different vibrational levels of the 1Bþ u state The next step is to determine the origin of the spectral components in the time-resolved spectra. We tried to simulate the fluorescence spectral patterns originating from the v = 2, 1 and 0 levels in the 1Bþ u state by the calculation of the Franck–Condon factors. Fig. 4 shows the models used for the calculations. Here, we used an effec-
11
tive carbon–carbon stretching mode with a width of 900 cm1, the value of which was determined by the simulation of 1Bþ 1A transitions; u ðv ¼ 0; 1 and 2Þ g ð0Þ the details of calculation using both the C@C and the C–C stretching modes were described elsewhere [16]. We assumed harmonic potentials for both the 1Bþ u and 1A g states, whose curvatures were determined by the spacings of the vibrational levels, i.e., 1400 and 1100 cm1, respectively (see Fig. 1). The shift of the 1Bþ u potential minimum with respect to the 1Ag potential minimum was fit, by the use of the relative intensities of the 1Bþ 1A u ðv ¼ 0; 1 and 2Þ g ð0Þ absorption transitions, to be DQ = 1.4 for neurosporene and DQ = 1.5 for spheroidene. Fig. 3 compares the results of simulation (b) to the spectra at 0.10, 0.00 and 0.55 ps after excitation: The fluorescence spectra at 0.10 and 0.00 ps look similar to the simulated fluorescence progression from the v = 2 and v = 1 levels, respectively. In the latter, the peaks on the higher-energy side that were ascribed to the 1Bþ u ð1Þ ! 1Ag ð0Þ transition is reproduced for the simulated fluorescence from the v = 1 level. The fluorescence
35
30
25
3
Energy / 10 cm
-1
v=2 v=1 v=0
v=2 v=1 v=0
20
15
10
5 v0= 3 v0= 2 v0= 1 v0= 0
0 0 Neurosporene
Q
v0= 3 v0= 2 v0= 1 v0= 0
0
Q
Spheroidene
Fig. 4. Models for the simulation of fluorescence progression from the v = 2, v = 1 and v = 0 levels in the 1Bþ u state down to the v = 3, 2, 1, and 0 1 levels in the 1A and g state, based on the calculation of Franck–Condon factors. The spacings of the vibrational levels were assumed to be 1400 cm þ 1100 cm1 in the 1Bþ and 1A states, respectively, which determined the curvatures of the harmonic potentials. The shift of the 1B potential u u g minimum with respect to the 1A g potential minimum was determined to be DQ = 1.4 for neurosporene and DQ = 1.5 for spheroidene, by fitting the relative intensities of the 1Bþ 1A u ðv ¼ 0; 1 and 2Þ g ð0Þ absorption transitions.
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
spectrum at 0.55 ps is analogous to the simulated fluorescence progression from the v = 0 level. Thus, the results of simulation support the idea that the raw time-resolved spectra shown in Fig. 2 mainly reflect vibrational relaxation from the v = 1 to the v = 0 level in the 1Bþ u state; some contribution of the fluorescence from the v = 2 may be present at the very early stage after excitation.
Neurosporene
Spheroidene
(a)
I
v=1
I
v=1
II
v=0
II
v=0
SAFS
12
3.3. The SVD and global-fitting analyses of the time-resolved fluorescence spectra 16
18
20
22 3
-1
Wavenumber / 10 cm
16
18
20
22 3
-1
Wavenumber / 10 cm
(b)
II
0.6
II Population
In order to establish the above conclusion, it was necessary to extract major spectral patterns from the observed time-resolved fluorescence spectra; here, we used singular-value decomposition (SVD) followed by global fitting. The SVD was performed for neurosporene (spheroidene) on an intensity data matrix consisting of 293 · 89 (293 · 86) data points in the wavenumber region of 16 000–23 000 cm1 (16 000– 23 000 cm1) and in the time region of 0.65–0.80 ps (0.60–0.80 ps). The singular values obtained were V1 = 1, V2 = 0.136, V3 = 0.047 and V4 = 0.025 (V1 = 1, V2 = 0.139, V3 = 0.065 and V4 = 0.036) for neurosporene (spheroidene). In both Cars, clear basis spectra si and time profiles Viti were obtained for the 1st and 2nd major components. Since strong interference was seen between the 3rd and 4th components, we confined ourselves to including only the first two major components in the global-fitting analysis by the use of a sequential model. Fig. 5 shows the results: (a) shows species-associated fluorescence spectra (SAFS), and (b) shows timedependent changes in population for component I and component II (temporally the first and the second components). The apparent decay time constants of component I and component II are now determined to be 50 and 260 fs for neurosporene and 25 and 260 fs for spheroidene. The errors of the decay constants were estimated to be ±20 fs for component I and ±10 fs for component II in both neurosporene and spheroidene. Now, we are in the position to interpret the SAFS of component I and component II (compare the Fig. 5a to Fig. 3b): We find agreement between the SAFS of component I and the simulated fluorescence from the v = 1 level, and also between the SAFS of component II and the simulated fluorescence from the v = 0 level. The agreement is regarded as a strong support for the above conclusion that the time-resolved fluorescence spectra reflect the process of vibrational relaxation from the v = 1 to the v = 0 vibrational level in the 1Bþ state. Thus, it is concluded that the u þ 1Bþ ð1Þ ! 1B u u ð0Þ vibrational relaxation takes place in 50 fs in neurosporene and in 25 fs in spheroidene, and that the 1Bþ u ð0Þ state decays in 260 fs in both neurosporene and spheroidene.
v=0 260 fs
v=0 260 fs
0.4
0.2
v=1 50 fs
I
v=1 25 fs
I
0 0
0.5
Delay time / ps
0
0.5
Delay time / ps
Fig. 5. The results of the SVD and global-fitting analyses by the use of a sequential model; they include the SAFS of component I and component II that are assigned to fluorescence from the v = 1 and v = 0 levels on the 1Bþ u state (a), and time dependent changes in the population on those vibrational levels (b). The weak arrows in the (a) indicate possible contribution of the 1B u ð0Þ ! 1Ag ð0Þ fluorescence transitions.
4. Discussion 4.1. Vibrational relaxation and redistribution in the 1Bþ u state Akimoto et al. [17] first applied fluorescence upconversion spectroscopy to neurosporene in n-hexane upon excitation at 420 nm (approximately to the v = 2 level). They identified two kinetic components with decay time constants of 0.04 and 0.25 ps, and ascribed the former component to a process where the excess vibrational energy of 3000 cm1 was transferred from the Franck–Condon active modes to dark modes (vibrational redistribution), whereas the latter component to direct internal conversion from the 1Bþ u state down to the 2A state that is competing with vibrag tional relaxation. In the present investigation, we have shown that vibrational relaxation from the 1Bþ u ð1Þ to the 1Bþ ð0Þ level actually takes place in 0.05 ps. The u rough agreement of those time constants (0.04 vs.
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
0.05 ps) suggests that the first component is mainly due to vibrational relaxation. In general, the time-dependent red shift of the fluorescence spectrum (Ôdynamic Stokes shiftÕ) can be observed in the time-resolved fluorescence spectra, in the course of relaxation from the Franck–Condon state. It originates from the redistribution of the low-frequency intermolecular vibrational modes of the Car and solvent molecules. The red shift of fluorescence due to the vibrational redistribution has been estimated to be a few hundred cm1 in energy in the case of b-carotene [18]. In the present investigation, we neglected the effect of such vibrational redistribution in the analysis of the timeresolved fluorescence spectra. We could not clearly identify such time-dependent red shift most probably due to the facts that vibrational redistribution of the lowfrequency vibrational modes is similar, in time scale, to vibrational relaxation of the m1, m2 modes, and that the latter predominates in the time-resolved spectra. However, the dynamic Stokes shift can affect the spectral patterns of time-resolved spectra in the femtosecond time range. Therefore, this factor should be definitely incorporated into a more refined analysis of timeresolved fluorescence spectra.
13
resolved fluorescence spectra, although comparison of the SAFS of components I and II with the 1Bþ u fluorescence from v = 1 and v = 0 levels suggests the contribu tion of the 1B u ð0Þ ! 1Ag ð0Þ fluorescence (see the thin arrows in Fig. 5). At this stage, we have not established a picture to give the consistent interpretation of the experimental results which have been successfully obtained. In order to determine the excited-state dynamics including the internal conversion, vibrational relaxation and vibrational redistribution, further investigation is absolutely necessary. In this relation, femtosecond fluorescence spectroscopy of a Car with n = 8 whose 1Bþ u and 1Bu states are supposed to be completely overlapped with each other, and that of spirilloxanthin with n = 13 whose 1Bþ u ; 3Ag and 1Bu states are arranged in a completely different way (see [2]), may provide us with deeper insight into the above-mentioned unique observation in the present Cars with n = 9 and 10, whose vibronic levels are almost completely overlapped with each other except for the shift of the vibronic origin by one and two vibrational quanta, respectively (see Fig. 1).
Acknowledgements 4.2. Comparison with other spectroscopic studies Near-infrared, subpicosecond time-resolved absorption spectroscopy of a set of Cars showed that the 1Bþ u ! 1Bu and the 1Bu ! 2Ag internal conversions, respectively, took place in 0.10 and 0.24 ps in neurosporene, and 0.10 and 0.23 ps in spheroidene [6]. Further, fluorescence up-conversion spectroscopy of neurosporene determined the decay time constants of the 1Bþ u and 1Bu states to be 0.06 and 0.27 ps [12]. Surprisingly, the decay time constant of the 1Bþ u ð0Þ state which has been determined in the present investigation, i.e., 0.26 ps for both neurosporene and spheroidene, corresponds not to the 1Bþ u lifetime of both Cars, i.e., 0.10 ps, but rather to the 1B u lifetimes, i.e., 0.24 ps for neurosporene and 0.23 ps for spheroidene. In the above-mentioned absorption spectroscopy of neurosporene and spheroidene, there was little chance þ of misinterpretation of the 1B u state to be the 1Bu state, because the Car molecules were specifically excited to the 1Bþ u state as temporally the first component, and the transient-absorption patterns from those electronic states were completely different from each other (see Fig. 4 of [6]). In the up-conversion spectroscopy of neurosporene upon excitation specifically to the 1Bþ u ð0Þ state, there was little chance of vibrational relaxation in the 1Bþ u state manifold. In the present Kerr-gate fluorescence spectroscopy upon excitation to the v = 2 and v = 3 levels of the 1Bþ u state of those Cars, the vibrational relaxation from þ the 1Bþ u ð1Þ to the 1Bu ð0Þ state predominates in the time-
This work has been supported by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture (#15204036) and by a grant from JST-CREST given to Osaka University. Also, Grants from the Ministry of Education, Science, Sports and Culture (Open Research Center Project) and from NEDO (New Energy and Industrial Technology Development Organization, International Joint Research Grant) to Kwansei Gakuin University are gratefully acknowledged.
References [1] Y. Koyama, J. Photochem. Photobiol. B: Biol. 9 (1991) 265. [2] K. Furuichi, T. Sashima, Y. Koyama, Chem. Phys. Lett. 356 (2002) 547. [3] Y. Koyama, R. Fujii, in: H.A. Frank, A.J. Young, G. Britton, R.J. Cogdell (Eds.), Advances in Photosynthesis, The Photochemistry of Carotenoids, vol. 8, Kluwer Academic Publisher, London, 1999, p. 161. [4] C.C. Gradinaru, J.T.M. Kennis, E. Papaginannakis, I.H.M. van Stokkum, R.J. Cogdell, G.R. Fleming, R.A. Niederman, R. van Grondelle, Proc. Natl. Acad. Sci. USA 98 (2001) 2364. [5] E. Papagiannakis, J.T.M. Kennis, I.H.M. van Stokkum, R.J. Cogdell, R. van Grondelle, Proc. Natl. Acad. Sci. USA 99 (2002) 6017. [6] R. Fujii, T. Inaba, Y. Watanabe, Y. Koyama, J.-P. Zhang, Chem. Phys. Lett. 369 (2003) 165. [7] F.S. Rondonuwu, K. Yokoyama, R. Fujii, Y. Koyama, R.J. Cogdell, Y. Watanabe, Chem. Phys. Lett. 390 (2004) 314. [8] J. Akahane, F.S. Rondonuwu, L. Fiedor, Y. Watanabe, Y. Koyama, Chem. Phys. Lett. 393 (2004) 184.
14
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
[9] J.-P. Zhang, C.-H. Chen, Y. Koyama, H. Nagae, J. Phys. Chem. B 102 (1998) 1632. [10] J.-P. Zhang, T. Inaba, Y. Watanabe, Y. Koyama, Chem. Phys. Lett. 331 (2000) 154. [11] F.S. Rondonuwu, Y. Watanabe, J.-P. Zhang, K. Furuichi, Y. Koyama, Chem. Phys. Lett. 357 (2002) 376. [12] R. Fujii, T. Fujino, T. Inaba, H. Nagae, Y. Koyama, Chem. Phys. Lett. 384 (2004) 9. [13] R. Nakamura, Y. Kanematsu, Rev. Sci. Inst. 75 (2004) 636.
[14] R. Fujii, K. Onaka, M. Kuki, Y. Koyama, Y. Watanabe, Chem. Phys. Lett. 288 (1998) 847. [15] J.T. Fourkas, H. Kawashima, K.A. Nelson, J. Chem. Phys. 103 (1995) 4393. [16] R. Fujii, K. Onaka, H. Nagae, Y. Koyama, Y. Watanabe, J. Lumin. 92 (2001) 213. [17] S. Akimoto, I. Yamazaki, S. Takaichi, M. Mimuro, Chem. Phys. Lett. 313 (1999) 63. [18] R. Nakamura, S. Yamamoto, J. Nakahara, J. Chem. Phys. 117 (2002) 238.
Vibrational relaxation in the 1Bþ u state of carotenoids as determined by Kerr-gate fluorescence spectroscopy Ryosuke Nakamura a, Ritsuko Fujii b,1, Hiroyoshi Nagae c, Yasushi Koyama b,*, Yasuo Kanematsu a a
JST-CREST, Venture Business Laboratory, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan b Faculty of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda, Hyogo 669-1337, Japan c Kobe City University of Foreign Studies, Gakuen Higashi-machi, Kobe 651-2187, Japan Received 5 August 2004; in final form 7 October 2004 Available online 6 November 2004
Abstract Subpicosecond time-resolved fluorescence spectra of all-trans-neurosporene and spheroidene, upon excitation above the þ 1Bþ u (v = 2) and 1Bu (v = 3) vibronic levels, respectively, were recorded by means of Kerr-gate spectroscopy. The spectral data matrices were analyzed by singular-value decomposition followed by global-fitting by the use of a sequential model. A pair of speciesþ associated difference spectra thus obtained in each carotenoid was ascribed to fluorescence from the 1Bþ u (v = 1) and 1Bu (v = 0) þ þ vibronic levels down to the ground 1Ag (v = 0, 1 and 2) levels. The lifetimes of the 1Bu (v = 1) and 1Bu (v = 0) states were determined to be 50 and 260 fs in neurosporene and 25 and 260 fs in spheroidene. 2004 Elsevier B.V. All rights reserved.
1. Introduction All-trans-carotenoids (Cars) are selectively bound to the antenna (LH1 and LH2) complexes from purple photosynthetic bacteria [1], and play an important role of light harvesting, which includes absorption of photons by Car followed by singlet-energy transfer to bacteriochlorophyll (BChl). The C2h symmetry of the all-trans conjugated chain gives rise to low-lying singlet-excited states including the 1Bþ u ; 3Ag ; Bu and 2Ag states; the optically forbidden 3Ag and 1Bu states were recently identified by measurement of resonance-Raman excitation profiles (RREPs) for Cars having the number of conjugated double bonds, n = 9–13 [2]. The RREP measurement determined the state ordering of *
Corresponding author. Fax: +81 795 65 9077. E-mail address: [email protected] (Y. Koyama). 1 Present address: Department of Physics, Graduate School of Science, Osaka City University, 3-3-138, Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan. 0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.069
(ground) for Cars having 1Bþ u > 1Bu > 2Ag > 1Ag n = 9 and 10, and that of 1Bþ u > 3Ag > 1Bu > 2Ag > 1Ag for Cars having n = 11–13 (see Fig. 5a of [2]). The RREP measurement showed also that all the state energies decrease when n increases as functions of 1/(2n + 1). Those singlet-excited states that are located close-by facilitate efficient internal conversion in Car, and provide plural channels of singlet-energy transfer from Car to BChl [3–5]. This is most probably the reason for the natural selection of the all-trans configuration by Cars in the antenna complexes. Subpicosecond time-resolved, near-infrared absorption spectroscopy determined two different pathways of internal conversion in accord with the above state ordering: 1Bþ u ! 1Bu ! for Cars with n = 9 and 10, þ and 1Bu ! 3Ag ! for Cars with n = 11–13 [6]. The sudden drop in the efficiency of Car-to-BChl singlet-energy transfer on going from n = 10 to n = 11, which was determined by subpicosecond time-resolved absorption spectroscopy, was explained in terms of the relative energies between Car and BChl; the 1B u state becomes
8
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
lower than the Qx state and the 2A g state becomes too close to the Qy state. As a result, both the 1B u -to-Qx and the 2A g -to-Qy channels become closed, and the overall singlet energy-transfer efficiency drops from 84% down to 51% in the LH2 complex [7] and from 75% down to 46% in the LH1 complex [8]. In order to obtain a clearer picture of internal conversion in Cars and singlet-energy transfer from Car to BChl, it is absolutely necessary to determine the processes of vibrational relaxation. Here, we define the term Ôinternal conversionÕ more strictly as quasi-isothermal transformation from a vibrational level of one electronic state to a vibrational level of the other electronic state. (In the above description, we used this word in a broader sense, implying both processes of internal conversion and vibrational relaxation.) In many cases, vibrational relaxation takes place much faster than internal conversion. However, this is not necessarily the case in Cars, where internal conversion reactions take place very rapidly among the singlet-excited states that are located close-by. There have been some examples in which vibrational relaxation takes place not necessarily faster than, but in the same time scale or even slower than, internal conversion: In the 2A g state of all-trans-lycopene (n = 11), picosecond time-resolved absorption spectroscopy identified vibrational relaxation which took place in a time scale slower than that of internal conversion. The timeresolved spectra were explained in terms of vibrational structures originating from the C@C (m1) and C–C (m2) stretching modes, and the rates of the lowest vibrational relaxation ðk v1 and k v2 Þ relative to that of internal conversion (k) were determined to be k v1 =k and k v2 =k around 0.6 and 0.3 in quinoline and 0.2 and 1.6 in CS2 [9]. In the 1Bþ u state of neurosporene (n = 9), subpicosecond time-resolved absorption spectroscopy showed that the relative intensities of the vibrational progression peaks in stimulated emission changed depending on þ excitation either to the 1Bþ u ð0Þ level or to the 1Bu ð1Þ level, suggesting that vibrational relaxation did not take place faster than the 1Bþ u ! 1Ag internal conversion [10]. In the 1Bþ u and the 2Ag states of neurosporene, the high-frequency-shifts of the C@C stretching Raman line, reflecting vibrational relaxation in an anharmonic potential, were observed by subpicosecond timeresolved Raman spectroscopy, a fact which indicated that vibrational relaxation took place in a time scale comparable to that of internal conversion [11]. In the present investigation, we have attempted to identify, by means of Kerr-gate fluorescence spectroscopy, the vibrational relaxation processes upon excitation to the higher vibrational levels of the 1Bþ u state in neurosporene and spheroidene. Since we determined, by up-conversion fluorescence spectroscopy upon exci-
tation specifically to the 1Bþ u ð0Þ vibronic level, the time constant of the 1Bþ u to 1Bu internal conversion to be 60 fs in neurosporene [12], the results would provide us a unique opportunity to compare the rate of vibrational relaxation to that of internal conversion in the extremely short-lived 1Bþ u state of the particular Cars. 2. Experimental The experimental set-up for time-resolved fluorescence spectroscopy, based on the optical Kerr gate, is similar to that described elsewhere [13]. An amplified mode-locked Ti:sapphire laser (Coherent, RegA9000) was operated at 800 nm with the repetition rate of 200 kHz. The light pulses from the amplified mode-locked Ti:sapphire laser were used as the optical Kerr-gating pulses, while their second harmonics were used to excite the sample in a flow cell, in which 0.3-mm thick sample solution was sandwiched between a pair of 0.7-mm thick fused-silica plates. A thin crystal of SrTiO3 (50-lm thickness) was used as the Kerr material. The time resolution, the spectral resolution, and the Kerr-gated transmittance were 180 fs, 5 nm, and 5%, respectively. The gated luminescence was colleted by a lens, and lead to an optical fiber attached to a polychromator (Acton, SpectraPro-275); the fluorescence signal was detected by a charge-coupled-device camera (Princeton Instruments, LN/CCD-1024/EEV) equipped with an image intensifier. To reduce the strong scattering of the gating and the excitation pulses, a UV-IR cut filter was inserted in front of the optical fiber. The arrival time of the gating pulse was varied by the use of an optical delay stage. At a delay position where there was no signal, we measured the background signal originating from the twophoton-excited luminescence of the Kerr material and slightly-leaking luminescence through the analyzer. The background was subtracted from the obtained signal at each delay time. The optical components between the sample and the analyzer, including the sample cell, the polarizer and the Kerr material, affect the arrival time of pulses emitted from the sample because of their group delay dispersion (GDD). We estimated the GDD of each component, and corrected for the wavelengthdependent time shift in the time-resolved spectra. All-trans-neurosporene and spheroidene were prepared as described elsewhere [14]. The concentration of the sample in n-hexane solution was 1.6 · 104 M (1.7 · 104 M) for neurosporene (spheroidene), and the purities of the sample before and after the spectral measurement were 100% and 99% (96% and 96%), the values of which were determined by HPLC detected at 450 nm. The procedures of singular-value-decomposition (SVD) and global fitting analysis were described previously [10].
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
classified by a set of Dv values depending on the starting vibrational level in the 1Bþ u state; (i) Dv = +2, +1, 0, 1, 2. . . starting from v = 2; (ii) Dv = +1, 0, 1, 2. . . starting from v = 1; and (iii) Dv = 0, 1, 2 starting from v = 0. Therefore, the narrowing of the fluorescence progression is naturally expected during the process of vibrational relaxation in the order, v = 2 ! v = 1 ! v = 0. Fig. 2 shows the time-resolved fluorescence spectra of neurosporene and spheroidene (1.6 and 1.7 · 104 M) recorded by Kerr-gate spectroscopy. (Sharp structures with a width smaller than the spectral resolution of 200 cm1 that appear on the top of broader profiles in the time-resolved spectra are due to an experimental artifact.) The stationary-state fluorescence spectra of neurosporene and spheroidene (both 1.0 · 105 M) [14] are also shown, for comparison, on the top of the time-resolved spectra. (The sharp and strong peaks at 22 120 cm1 in both spectra can be assigned to the C– H stretching Raman line due to the solvent, n-hexane; the Raman shift is 2880 cm1 in reference to the pumping pulse at 25 000 cm1). The three peaks at 20 960, 19 840 and 18 660 cm1 for neurosporene, and those at 19 920, 18 800 and 17 760 cm1 for spheroidene,
3. Results 3.1. Characterization of time-resolved fluorescence spectra Fig. 1 shows an energy diagram for the vibrational sublevels on the 1Bþ u and 1Ag (ground) electronic states of neurosporene and spheroidene in n-hexane. The energies of the 1Bþ u state of those Cars (21 300 and 20 600 cm1) were determined by stationary-state fluorescence spectroscopy; the spacing of vibrational progression in 1 the 1Bþ u state (1400 cm ) and that in the 1Ag state 1 (1100 cm ) were determined by fluorescence-excitation and fluorescence spectroscopy, respectively [14]. In the time-resolved Kerr-gate fluorescence spectroscopy, the Car molecules were excited at 400 nm, i.e., just below the 1Bþ u ð3Þ level in neurosporene, and slightly above it in spheroidene. Since the rate of vibrational relaxation from the v to the v 1 level is proportional to v [15], the v = 1 to v = 0 vibrational relaxation should be most easily time-resolved, but the v = 3 to v = 2 vibrational relaxation can be too fast to be detected by the present Kerr-gate spectroscopy of 180 fs time resolution. If we define Dv = v v0, the fluorescence transitions can be
25
20
v=3 v=2
v=3
v=1
v=2
v=0
v=1
+ 1B u
9
v=0 1B u
v=3
v=5
v=2
v=4
v=1
v=3
v=0
v=2
+ 1B u
v=1 v=0
15
3
Energy / 10 cm
—1
1B u
10
5
0
1A g
1A g
1A g
1A g
v0 = 4 v0 = 3 v0 = 2 v0 = 1 v0 = 0
v0 = 4 v0 = 3 v0 = 2 v0 = 1 v0 = 0
v0 = 4 v0 = 3 v0 = 2 v0 = 1 v0 = 0
v0 = 4 v0 = 3 v0 = 2 v0 = 1 v0 = 0
Neurosporene
Spheroidene
Fig. 1. Energy diagrams for neurosporene and spheroidene showing the vibrational sublevels on the 1Bþ u and 1Ag (ground) states, and fluorescence þ transitions from the v = 2, v = 1 and v = 0 vibrational levels in the 1Bu state down to various vibrational levels in the 1A g state. The vibrational levels 1 þ on the 1B for neurosporene, and 20 600 u state are also shown for comparison. The energies of the 1Bu and 1Bu states (21 300 and 19 800 cm and 17 800 cm1 for spheroidene) were determined by fluorescence spectroscopy. The spacing of the vibrational levels in the 1Bþ u and 1Bu states 1 (1400 cm1) and that in the 1A state (1100 cm ) were determined by fluorescence-excitation and fluorescence spectroscopy [14]. g
10
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
roidene. Those spectra exhibit no clear peaks at the posi tion of the 1Bþ u ð0Þ ! 1Ag ð0Þ transition that are clearly seen in the stationary-state spectra (use the vertical broken lines in Figs. 2 and 3 for comparison). This is a unique characteristic of the very initial component. (2) The time-resolved spectra at the last stage (0.55 ps) exhibit three peaks in both neurosporene and spheroidene cor þ responding to the 1Bþ u ð0Þ ! 1Ag ð0Þ, 1Bu ð0Þ ! 1Ag ð1Þ þ and 1Bu ð0Þ ! 1Ag ð2Þ peaks. Actually, the spectral patterns at this very last stage are very similar to those of the stationary-state spectra. (3) The time-resolved spectra of the two Cars immediately after excitation (0.00
Neurosporene
Spheroidene
Intensity (a.u.)
(a)
—0.10 ps
—0.10 ps
0.00 ps
0.00 ps
0.55 ps
0.55 ps
Fig. 2. Time-resolved fluorescence spectra of neurosporene and spheroidene recorded by means of Kerr-gate spectroscopy. The stationary-state fluorescence spectra were also shown for comparison on the top of them; the sharp and strong peak in the spectra are due to the C–H stretching Raman line of the solvent n-hexane. The vertical broken lines indicate the position of the 1Bþ u ð0Þ ! 1Ag ð0Þ transition. can be definitely assigned to the 1Bþ u ð0Þ ! 1Ag ð0Þ, þ þ 1Bu ð0Þ ! 1Ag ð1Þ and 1Bu ð0Þ ! 1Ag ð2Þ fluorescence transitions. Fig. 3a shows the time-resolved spectra at 0.10, 0.00 and 0.55 ps after normalization of their intensities. Since the spectral patterns of neurosporene and spheroidene are similar to one the other, they can be inclusively characterized as follows: (1) The time-resolved spectra at the very initial stage (0.10 ps) exhibit two peaks shifted to the lower-energy side in both neurosporene and sphe-
Intensity (a.u.)
(b)
16
v=2
v=2
v=1
v=1
v=0
v=0
22
Wavenumber / 103cm—1
16
22 Wavenumber / 103cm—1
Fig. 3. Normalized time-resolved spectra at delay times at 0.10, 0.00 and 0.55 ps (a) and simulated fluorescence from the v = 2, v = 1 and v = 0 levels of the 1Bþ u state (b) for neurosporene and spheroidene. The vertical broken lines indicate the 1Bþ u ð0Þ ! 1Ag ð0Þ transition.
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14 ps) exhibit some indication of the 1Bþ u ð0Þ ! 1Ag ð0Þ peak, and also a peak on the higher-energy side ascrib able to the 1Bþ u ð1Þ ! 1Ag ð0Þ transition. The latter peak is seen strongly in neurosporene but weakly in spheroidene; Fig. 2 shows that it is covered and enhanced by the Raman line in the case of neurosporene, but it appears on the lower-energy side of the Raman line in spheroidene. The above empirical characterization of the spectral changes suggests that the time-resolved fluorescence spectra may mainly reflect the process of vibrational relaxation from v = 1 to v = 0.
3.2. Simulation of fluorescence spectra from different vibrational levels of the 1Bþ u state The next step is to determine the origin of the spectral components in the time-resolved spectra. We tried to simulate the fluorescence spectral patterns originating from the v = 2, 1 and 0 levels in the 1Bþ u state by the calculation of the Franck–Condon factors. Fig. 4 shows the models used for the calculations. Here, we used an effec-
11
tive carbon–carbon stretching mode with a width of 900 cm1, the value of which was determined by the simulation of 1Bþ 1A transitions; u ðv ¼ 0; 1 and 2Þ g ð0Þ the details of calculation using both the C@C and the C–C stretching modes were described elsewhere [16]. We assumed harmonic potentials for both the 1Bþ u and 1A g states, whose curvatures were determined by the spacings of the vibrational levels, i.e., 1400 and 1100 cm1, respectively (see Fig. 1). The shift of the 1Bþ u potential minimum with respect to the 1Ag potential minimum was fit, by the use of the relative intensities of the 1Bþ 1A u ðv ¼ 0; 1 and 2Þ g ð0Þ absorption transitions, to be DQ = 1.4 for neurosporene and DQ = 1.5 for spheroidene. Fig. 3 compares the results of simulation (b) to the spectra at 0.10, 0.00 and 0.55 ps after excitation: The fluorescence spectra at 0.10 and 0.00 ps look similar to the simulated fluorescence progression from the v = 2 and v = 1 levels, respectively. In the latter, the peaks on the higher-energy side that were ascribed to the 1Bþ u ð1Þ ! 1Ag ð0Þ transition is reproduced for the simulated fluorescence from the v = 1 level. The fluorescence
35
30
25
3
Energy / 10 cm
-1
v=2 v=1 v=0
v=2 v=1 v=0
20
15
10
5 v0= 3 v0= 2 v0= 1 v0= 0
0 0 Neurosporene
Q
v0= 3 v0= 2 v0= 1 v0= 0
0
Q
Spheroidene
Fig. 4. Models for the simulation of fluorescence progression from the v = 2, v = 1 and v = 0 levels in the 1Bþ u state down to the v = 3, 2, 1, and 0 1 levels in the 1A and g state, based on the calculation of Franck–Condon factors. The spacings of the vibrational levels were assumed to be 1400 cm þ 1100 cm1 in the 1Bþ and 1A states, respectively, which determined the curvatures of the harmonic potentials. The shift of the 1B potential u u g minimum with respect to the 1A g potential minimum was determined to be DQ = 1.4 for neurosporene and DQ = 1.5 for spheroidene, by fitting the relative intensities of the 1Bþ 1A u ðv ¼ 0; 1 and 2Þ g ð0Þ absorption transitions.
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
spectrum at 0.55 ps is analogous to the simulated fluorescence progression from the v = 0 level. Thus, the results of simulation support the idea that the raw time-resolved spectra shown in Fig. 2 mainly reflect vibrational relaxation from the v = 1 to the v = 0 level in the 1Bþ u state; some contribution of the fluorescence from the v = 2 may be present at the very early stage after excitation.
Neurosporene
Spheroidene
(a)
I
v=1
I
v=1
II
v=0
II
v=0
SAFS
12
3.3. The SVD and global-fitting analyses of the time-resolved fluorescence spectra 16
18
20
22 3
-1
Wavenumber / 10 cm
16
18
20
22 3
-1
Wavenumber / 10 cm
(b)
II
0.6
II Population
In order to establish the above conclusion, it was necessary to extract major spectral patterns from the observed time-resolved fluorescence spectra; here, we used singular-value decomposition (SVD) followed by global fitting. The SVD was performed for neurosporene (spheroidene) on an intensity data matrix consisting of 293 · 89 (293 · 86) data points in the wavenumber region of 16 000–23 000 cm1 (16 000– 23 000 cm1) and in the time region of 0.65–0.80 ps (0.60–0.80 ps). The singular values obtained were V1 = 1, V2 = 0.136, V3 = 0.047 and V4 = 0.025 (V1 = 1, V2 = 0.139, V3 = 0.065 and V4 = 0.036) for neurosporene (spheroidene). In both Cars, clear basis spectra si and time profiles Viti were obtained for the 1st and 2nd major components. Since strong interference was seen between the 3rd and 4th components, we confined ourselves to including only the first two major components in the global-fitting analysis by the use of a sequential model. Fig. 5 shows the results: (a) shows species-associated fluorescence spectra (SAFS), and (b) shows timedependent changes in population for component I and component II (temporally the first and the second components). The apparent decay time constants of component I and component II are now determined to be 50 and 260 fs for neurosporene and 25 and 260 fs for spheroidene. The errors of the decay constants were estimated to be ±20 fs for component I and ±10 fs for component II in both neurosporene and spheroidene. Now, we are in the position to interpret the SAFS of component I and component II (compare the Fig. 5a to Fig. 3b): We find agreement between the SAFS of component I and the simulated fluorescence from the v = 1 level, and also between the SAFS of component II and the simulated fluorescence from the v = 0 level. The agreement is regarded as a strong support for the above conclusion that the time-resolved fluorescence spectra reflect the process of vibrational relaxation from the v = 1 to the v = 0 vibrational level in the 1Bþ state. Thus, it is concluded that the u þ 1Bþ ð1Þ ! 1B u u ð0Þ vibrational relaxation takes place in 50 fs in neurosporene and in 25 fs in spheroidene, and that the 1Bþ u ð0Þ state decays in 260 fs in both neurosporene and spheroidene.
v=0 260 fs
v=0 260 fs
0.4
0.2
v=1 50 fs
I
v=1 25 fs
I
0 0
0.5
Delay time / ps
0
0.5
Delay time / ps
Fig. 5. The results of the SVD and global-fitting analyses by the use of a sequential model; they include the SAFS of component I and component II that are assigned to fluorescence from the v = 1 and v = 0 levels on the 1Bþ u state (a), and time dependent changes in the population on those vibrational levels (b). The weak arrows in the (a) indicate possible contribution of the 1B u ð0Þ ! 1Ag ð0Þ fluorescence transitions.
4. Discussion 4.1. Vibrational relaxation and redistribution in the 1Bþ u state Akimoto et al. [17] first applied fluorescence upconversion spectroscopy to neurosporene in n-hexane upon excitation at 420 nm (approximately to the v = 2 level). They identified two kinetic components with decay time constants of 0.04 and 0.25 ps, and ascribed the former component to a process where the excess vibrational energy of 3000 cm1 was transferred from the Franck–Condon active modes to dark modes (vibrational redistribution), whereas the latter component to direct internal conversion from the 1Bþ u state down to the 2A state that is competing with vibrag tional relaxation. In the present investigation, we have shown that vibrational relaxation from the 1Bþ u ð1Þ to the 1Bþ ð0Þ level actually takes place in 0.05 ps. The u rough agreement of those time constants (0.04 vs.
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
0.05 ps) suggests that the first component is mainly due to vibrational relaxation. In general, the time-dependent red shift of the fluorescence spectrum (Ôdynamic Stokes shiftÕ) can be observed in the time-resolved fluorescence spectra, in the course of relaxation from the Franck–Condon state. It originates from the redistribution of the low-frequency intermolecular vibrational modes of the Car and solvent molecules. The red shift of fluorescence due to the vibrational redistribution has been estimated to be a few hundred cm1 in energy in the case of b-carotene [18]. In the present investigation, we neglected the effect of such vibrational redistribution in the analysis of the timeresolved fluorescence spectra. We could not clearly identify such time-dependent red shift most probably due to the facts that vibrational redistribution of the lowfrequency vibrational modes is similar, in time scale, to vibrational relaxation of the m1, m2 modes, and that the latter predominates in the time-resolved spectra. However, the dynamic Stokes shift can affect the spectral patterns of time-resolved spectra in the femtosecond time range. Therefore, this factor should be definitely incorporated into a more refined analysis of timeresolved fluorescence spectra.
13
resolved fluorescence spectra, although comparison of the SAFS of components I and II with the 1Bþ u fluorescence from v = 1 and v = 0 levels suggests the contribu tion of the 1B u ð0Þ ! 1Ag ð0Þ fluorescence (see the thin arrows in Fig. 5). At this stage, we have not established a picture to give the consistent interpretation of the experimental results which have been successfully obtained. In order to determine the excited-state dynamics including the internal conversion, vibrational relaxation and vibrational redistribution, further investigation is absolutely necessary. In this relation, femtosecond fluorescence spectroscopy of a Car with n = 8 whose 1Bþ u and 1Bu states are supposed to be completely overlapped with each other, and that of spirilloxanthin with n = 13 whose 1Bþ u ; 3Ag and 1Bu states are arranged in a completely different way (see [2]), may provide us with deeper insight into the above-mentioned unique observation in the present Cars with n = 9 and 10, whose vibronic levels are almost completely overlapped with each other except for the shift of the vibronic origin by one and two vibrational quanta, respectively (see Fig. 1).
Acknowledgements 4.2. Comparison with other spectroscopic studies Near-infrared, subpicosecond time-resolved absorption spectroscopy of a set of Cars showed that the 1Bþ u ! 1Bu and the 1Bu ! 2Ag internal conversions, respectively, took place in 0.10 and 0.24 ps in neurosporene, and 0.10 and 0.23 ps in spheroidene [6]. Further, fluorescence up-conversion spectroscopy of neurosporene determined the decay time constants of the 1Bþ u and 1Bu states to be 0.06 and 0.27 ps [12]. Surprisingly, the decay time constant of the 1Bþ u ð0Þ state which has been determined in the present investigation, i.e., 0.26 ps for both neurosporene and spheroidene, corresponds not to the 1Bþ u lifetime of both Cars, i.e., 0.10 ps, but rather to the 1B u lifetimes, i.e., 0.24 ps for neurosporene and 0.23 ps for spheroidene. In the above-mentioned absorption spectroscopy of neurosporene and spheroidene, there was little chance þ of misinterpretation of the 1B u state to be the 1Bu state, because the Car molecules were specifically excited to the 1Bþ u state as temporally the first component, and the transient-absorption patterns from those electronic states were completely different from each other (see Fig. 4 of [6]). In the up-conversion spectroscopy of neurosporene upon excitation specifically to the 1Bþ u ð0Þ state, there was little chance of vibrational relaxation in the 1Bþ u state manifold. In the present Kerr-gate fluorescence spectroscopy upon excitation to the v = 2 and v = 3 levels of the 1Bþ u state of those Cars, the vibrational relaxation from þ the 1Bþ u ð1Þ to the 1Bu ð0Þ state predominates in the time-
This work has been supported by a Grant-in-Aid from the Ministry of Education, Science, Sports and Culture (#15204036) and by a grant from JST-CREST given to Osaka University. Also, Grants from the Ministry of Education, Science, Sports and Culture (Open Research Center Project) and from NEDO (New Energy and Industrial Technology Development Organization, International Joint Research Grant) to Kwansei Gakuin University are gratefully acknowledged.
References [1] Y. Koyama, J. Photochem. Photobiol. B: Biol. 9 (1991) 265. [2] K. Furuichi, T. Sashima, Y. Koyama, Chem. Phys. Lett. 356 (2002) 547. [3] Y. Koyama, R. Fujii, in: H.A. Frank, A.J. Young, G. Britton, R.J. Cogdell (Eds.), Advances in Photosynthesis, The Photochemistry of Carotenoids, vol. 8, Kluwer Academic Publisher, London, 1999, p. 161. [4] C.C. Gradinaru, J.T.M. Kennis, E. Papaginannakis, I.H.M. van Stokkum, R.J. Cogdell, G.R. Fleming, R.A. Niederman, R. van Grondelle, Proc. Natl. Acad. Sci. USA 98 (2001) 2364. [5] E. Papagiannakis, J.T.M. Kennis, I.H.M. van Stokkum, R.J. Cogdell, R. van Grondelle, Proc. Natl. Acad. Sci. USA 99 (2002) 6017. [6] R. Fujii, T. Inaba, Y. Watanabe, Y. Koyama, J.-P. Zhang, Chem. Phys. Lett. 369 (2003) 165. [7] F.S. Rondonuwu, K. Yokoyama, R. Fujii, Y. Koyama, R.J. Cogdell, Y. Watanabe, Chem. Phys. Lett. 390 (2004) 314. [8] J. Akahane, F.S. Rondonuwu, L. Fiedor, Y. Watanabe, Y. Koyama, Chem. Phys. Lett. 393 (2004) 184.
14
R. Nakamura et al. / Chemical Physics Letters 400 (2004) 7–14
[9] J.-P. Zhang, C.-H. Chen, Y. Koyama, H. Nagae, J. Phys. Chem. B 102 (1998) 1632. [10] J.-P. Zhang, T. Inaba, Y. Watanabe, Y. Koyama, Chem. Phys. Lett. 331 (2000) 154. [11] F.S. Rondonuwu, Y. Watanabe, J.-P. Zhang, K. Furuichi, Y. Koyama, Chem. Phys. Lett. 357 (2002) 376. [12] R. Fujii, T. Fujino, T. Inaba, H. Nagae, Y. Koyama, Chem. Phys. Lett. 384 (2004) 9. [13] R. Nakamura, Y. Kanematsu, Rev. Sci. Inst. 75 (2004) 636.
[14] R. Fujii, K. Onaka, M. Kuki, Y. Koyama, Y. Watanabe, Chem. Phys. Lett. 288 (1998) 847. [15] J.T. Fourkas, H. Kawashima, K.A. Nelson, J. Chem. Phys. 103 (1995) 4393. [16] R. Fujii, K. Onaka, H. Nagae, Y. Koyama, Y. Watanabe, J. Lumin. 92 (2001) 213. [17] S. Akimoto, I. Yamazaki, S. Takaichi, M. Mimuro, Chem. Phys. Lett. 313 (1999) 63. [18] R. Nakamura, S. Yamamoto, J. Nakahara, J. Chem. Phys. 117 (2002) 238.