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Conjugation length dependence of relaxation kinetics in β-carotene homologs probed by femtosecond Kerr-gate fluorescence spectroscopy
Chemical Physics Letters 425 (2006) 66–70 www.elsevier.com/locate/cplett
Conjugation length dependence of relaxation kinetics in b-carotene homologs probed by femtosecond Kerr-gate fluorescence spectroscopy Daisuke Kosumi a
a,1
, Kazuhiro Yanagi b,2, Ritsuko Fujii b, Hideki Hashimoto b, Masayuki Yoshizawa a,*
Department of Physics, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan b ‘Light and Control’, PRESTO/JST, Department of Physics, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Received 5 April 2006; in final form 8 May 2006 Available online 12 May 2006
Abstract Ultrafast dynamics of the optically allowed S2 ð1 1 Bþ u Þ state in all-trans-carotenoids with a number of conjugated double bonds, n = 7– 15, has been investigated by femtosecond Kerr-gate fluorescence spectroscopy. The lifetime of the S2 state is determined eliminating effect of excess vibrational energy. The lifetime has a maximum for n = 9 and shows anomalous dependence on the energy gap between the S2 and S1 states. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Linear polyene structure of carotenoids has some unique optical properties due to its one dimensional delocalized p electron system; e.g., ultrafast dynamics in excited states and large third-order nonlinear optical susceptibility [1]. Furthermore, carotenoids play dual essential roles of light-harvesting and photoprotection in bacterial photosynthesis [2–4]. The S0 ground state of carotenoids has A g symmetry assuming C2h symmetry of their linear polyene backbone (the 1 1 A g state). The lowest optically allowed excited state is the S2 ð1 1 Bþ u Þ state and the optically forbidden S1 ð2 1 A Þ state is lying below the S2 state. It has g been proposed that the excitation energy transfer to bacteriochlorophyll in the photosynthesis occurs from both the
*
Corresponding author. Fax: +81 22 795 6362. E-mail addresses: [email protected] (D. Kosumi), [email protected] (M. Yoshizawa). 1 JSPS Research Fellow. 2 Present address: Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8562, Japan. 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.05.023
S2 and S1 states of carotenoids [2–4]. However, theoretical calculation has indicated that there are more than two dark states (with B u and Ag symmetry) below the S2 state depending on a number of conjugated double bonds of the polyene backbone (n) [5]. Resonant Raman excitation 1 profiles have shown the presence of the 1 1 B u and 3 Ag states between the S2 and S1 states in longer carotenoids (n > 9) [6,7]. The ultrafast dynamics of the carotenoids has been investigated in depth and the role of the intermediate states has been discussed, but the initial ultrafast relaxation kinetics has not been well understood yet [4,8– 17]. The relaxation kinetics of the carotenoids depends on the conjugation length, because the energy of the excited states in linear polyenes are given as a linear function of 1/(2n + 1) [5]. The S1 ! S0 internal conversion rate of carotenoids increases with n. It is well explained by the energy gap law [18]. On the contrary, the S2 ! S1 internal conversion mechanism has not been clarified because of its extremely fast relaxation. The conjugation length dependence of the S2 ! S1 relaxation has been investigated mainly in open chain carotenoids [8,9,19]. The internal conversion rate has a minimum at n = 9, but this anoma-
D. Kosumi et al. / Chemical Physics Letters 425 (2006) 66–70
lous dependence has not been understood. One possible interpretation is consideration of the intermediate state below the S2 state. The presence of the intermediate state may show difference between the S2 decay and the S1 formation kinetics. However, the previous reports have not precisely determined the S2 decay kinetics, because the observed signals contain the influence of other effects. The conjugation length dependence of the S2 ! S1 internal conversion rate has been investigated by stationary fluorescence quantum yields [19], time-resolved fluorescence [8], and absorption spectroscopies [9]. The determination of the lifetime from the fluorescence quantum yield needs several assumptions such as the radiative relaxation rate. The time-resolved absorption spectroscopy is widely used to investigate the ultrafast phenomena. However, the signal is complicated because of many contributions such as depletion of the ground state, absorption and stimulated emission of the excited states, and coherent nonlinear optical effects. Our previous study using resonant and non-resonant excitations shows that the non-linear optical effect is miss-assigned to the excited state in b-carotene [20]. The time-resolved fluorescence spectroscopy is a powerful method to monitor directly the dynamics of the S2 state in carotenoids, because the S2 state is the only one emissive excited state due to the symmetry reason [8,21–24]. However, the conjugation length dependence of the fluorescence kinetics has been investigated using the excitation pulse with large amount of excess energy [8]. The excess vibrational energy affects the relaxation kinetics and the transient spectra [22,24]. To avoid contribution of the vibrational relaxation, the excitation without affording the excess vibrational energy and the detailed measurement of the time-resolved fluorescence spectra are required. In this study, the ultrafast fluorescence of the S2 state in homologs of all-trans-b-carotene with different conjugation lengths and all-trans-lycopene (an open chain carotenoid) has been measured using tunable excitation pulses. The excitation with small amount of vibrational energy allows us to neglect the influence of the vibrational relaxation and to determine the S2 lifetime properly. The conjugation length dependence of the S2 state dynamics is discussed considering the energy gap between the S2 and S1 states.
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the equipments. The instrumental response function determined by cross-correlation between the excitation and gate pulses was 300 fs for the SH excitation and 200 fs for the OPA excitation. After chirp compensation and fitting considering the system response, an experimental error of the obtained time constant was better than 30 fs. Sample preparation protocols were described in detail elsewhere [18]. Chemical structures of the carotenoids are shown in the insets of Figs. 1 and 2. b-Carotene and lycopene have 11 conjugated double bonds. The b-carotene homologs are called after the number of the conjugated 0 double bonds as m7 (n = 7), m9 (n = 9), m9 (n = 9), 0 M13 (n = 13), and M15 (n = 15). m and M stand for mini0 and macro-b-carotene, respectively. Prime symbols of m9 0 and M13 denote the absence of methyl groups at the central part of the polyene chain. The carotenoids were dissolved in cyclohexane and circulated in a 0.5 mm optical path length flow cell. Optical density of the sample was prepared to be 1.0 at the absorption maximum photon energy. 3. Results and discussion Fig. 1 shows fluorescence and absorption spectra of m7. The 3.12 eV pulse excites lower energy side of the S2 absorption. The stationary fluorescence shown in Fig. 1a has dual broad fluorescence around 2.2–2.8 eV. A sharp peak at 2.76 eV is due to spontaneous Raman signal of C–H stretching mode of the solvent (cyclohexane). The fluorescence around 2.8 eV is assigned to the S2 ! S0 emission, while the 2.2 eV fluorescence is assigned to the S1 ! S0 emission [25,26]. The S1 ð2 1 A g Þ state in the exact
2. Experimental Experimental setup of the femtosecond time-resolved fluorescence spectroscopy was based on the optical Kerrgate method [24]. Output pulses of a 1 kHz regenerative Ti:sapphire amplifier (Spitfire, Spectra Physics) were separated to excitation and gate pulses. The excitation pulses were generated by the second harmonic (SH) generation (3.12 eV) or a non-collinear optical parametric amplifier (OPA) (2.41–2.55 eV). A quartz plate with 1.0 mm thickness was used as the Kerr medium gated by the fundamental pulse. The polarization of the gated fluorescence was parallel to the excitation polarization. The fluorescence spectra were corrected for wavelength dependencies of
Fig. 1. (a) Stationary fluorescence (solid line) and absorption (dashed line) spectra and time-resolved fluorescence spectra at (b) 0.1 ps, (c) 0.3 ps, and (d) 0.5 ps of m7. An inset shows chemical structure. An arrow in (a) indicates the excitation photon energy and another arrow in (b) indicates the probe photon energy used for analysis of the temporal response.
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Fig. 2. Time-resolved fluorescence at 0.1 ps (solid lines) and stationary absorption (dashed lines) spectra of carotenoids. Insets show chemical structures. Arrows indicate the excitation photon energies.
C2h symmetry is optically forbidden, but the symmetry of the carotenoid is slightly broken in solution because of distortion around the terminal b-ring. The radiation rate of the S1 state is much smaller than that of the S2 state, but very long lifetime of the S1 state in short carotenoids causes the dual fluorescence from the S2 and S1 states [25,26].
The time-resolved fluorescence spectra of m7 shown in Fig. 1b–d consist of the Raman line at 2.76 eV and the broad S2 fluorescence around 2.8 eV. The S1 fluorescence is not observed because of the small radiation rate. The Raman signal has equal temporal response to the crosscorrelation between the excitation and gate pulses. It disappears at a delay time of 0.5 ps. The broad S2 fluorescence remains longer than the Raman signal, but it does not show the time dependent spectral change. The vibrational relaxation process is negligible in the S2 fluorescence of m7, because the excitation generates the vibrational ground level of the S2 state. The time-resolved fluorescence and stationary absorption spectra of the other carotenoids are shown in Fig. 2 in the order of n. The S2 absorption shifts to lower energy with increasing the conjugation length. The time-resolved fluorescence is measured using the excitation with the photon energies indicated by the arrows in Fig. 2. A sharp peak 0 at 2.76 eV in m9 and m9 is due to the spontaneous Raman signal. The time-resolved fluorescence of all the carotenoids is assigned to that from the S2 state, because it appears just below the S2 absorption. The S2 fluorescence shifts to lower energy with increasing the conjugation length in the same way with the S2 absorption. The relaxation kinetics of the carotenoids has been reported to depend on the excitation photon energy. The slower internal conversions following the excitation with the larger excess energy are observed in b-carotene and lycopene [24]. The S2 fluorescence following the excitation with the large excess energy decays faster at higher photon energy [22]. The excitation pulses in this study are selected to excite the vibrational ground level (0–0 transition) or low vibrational level (0–1 or 0–2 transition) of the S2 state. The temporal spectral change of the fluorescence is very small as shown in Fig. 3. Furthermore, the signals used for analysis of the temporal responses are selected to minimize the effect of the vibrational relaxation. In general, the
Fig. 3. Normalized time-resolved fluorescence spectra of carotenoids at delay times of 0.1 ps (dashed lines) and 0.3 ps (thin solid lines). Arrows indicate the probe photon energies used for analysis of the temporal responses.
D. Kosumi et al. / Chemical Physics Letters 425 (2006) 66–70
transient state with large excess vibrational energy has broader spectrum than that of the thermal equilibrium condition. The peak of the band decays slower than the edge, because the vibrational relaxation is involved with the population decay. However, a quasi-isosbestic energy exists near the shoulder where the vibrational relaxation has a minimal effect on the transient response [24]. Fig. 4 shows temporal responses of the S2 fluorescence. The probe photon energies are selected at the shoulder of the signal (arrows in Figs. 1b and 3) to minimize the influence of the vibrational relaxation. The transient signal is proportional to the transient population of the S2 state. The obtained S2 state lifetimes are denoted in Fig. 4 together. The lifetimes for the longer carotenoids are slightly longer than those reported in the open chain carotenoids [8,19] and in the b-carotene homologs observed by the time-resolved absorption spectroscopy [9], but the conjugation length dependence is equal to the
Fig. 4. Temporal responses of the S2 fluorescence of the carotenoids (open circles) probed at the photon energies indicated by the arrows in Figs. 1b and 3. Solid lines are the best fitted function of an exponential decay convoluted with the instrumental response function assuming a Gaussian temporal profile.
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previous reports. For n > 9, the S2 state lifetime decreases with the increase n, but the lifetime for n = 7 is shorter than that for n = 9. The lifetime has a maximum at n = 9. The S1 ! S0 internal conversion rate of the carotenoids is well explained by the energy gap law [4,9,15,17,19,25]. Therefore, the internal conversion rate from the S2 state is tentatively plotted as a function of the S2–S1 energy gap in Fig. 5. The S2–S1 energy gap is determined using the previously reported relation between the excited state energy and n for the open chain carotenoids [6]. First, we determine an effective number of conjugated double bonds neff [27] for the b-carotene homologs from the S2 absorption energy. Then, the S2–S1 energy gap is determined using the S1 energy calculated from neff. The S2–S1 energy gap spreads with increasing n in contrast to the S1–S0 energy gap, because the S1 state energy decreases more rapidly than the S2 state energy. The S2 lifetime of m5 (n = 5) plotted in Fig. 5 has been reported to be 45 fs by the transient absorption spectroscopy [9]. It can be treated reliably with the lifetimes obtained in this study, because m5 is excited by the pulse with small amount of excess energy and the transient absorption is assigned to the S2 state from the relation to the signals of the other longer carotenoids (n = 7–15) [17]. The internal conversion rate from the S2 state in the 0 short carotenoids (m5, m7, and m9 ) shows consistent behavior with the energy gap law. It decreases with increasing the S2–S1 energy gap. Assuming the energy gap law in weak vibronic coupling case, the energy of the vibrational accepting mode is determined as 0.25 eV (2000 cm1, a dashed line in Fig. 5). It is larger than the energy gap of m5. The energy gap law cannot apply to the S2 ! S1 internal conversion because the energy gap law should apply in case of the large energy gap compared to the vibrational accepting mode [18]. Fuß et al. have proposed that conical
Fig. 5. The internal conversion rates from the S2 state as a function of the S2–S1 energy gap. A dashed line is the best fit using the energy gap law for 0 the shorter carotenoids (m5, m7, and m9 ).
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intersection of the electronic states can explain the energy gap dependence of the internal conversion rate [28]. The activation energy needed for crossing the potential energy surfaces depends on the energy gap and the horizontal displacement of the surfaces. The horizontal displacement between the S2 and S1 states in carotenoid determined by the time-resolved absorption spectroscopy [29] is consistent with the decrease of the relaxation rate with increasing the energy gap as well as the energy gap law. The conjugation length dependence of the shorter carotenoids suggests the direct S2 ! S1 internal conversion. On the contrary, the S2 internal conversion rate of the longer carotenoids (n > 9) shows the reverse trend. As a natural consequence, it is well expected that the internal conversion from the S2 state should be assisted by some other mechanisms in the longer carotenoids. Frank et al. have proposed that increase in the density of the vibrational accepting modes in the longer polyene contributes to increase in the relaxation rate [19,30]. Akimoto et al. have suggested that uncertain parameter such as the displacement of the potential surfaces of the S2 and S1 states depends on n and affects the dynamics of the S2 state [8]. Another possible interpretation is the intervening of the intermediate states lying below the S2 state for n > 9 [6,7,31]. The intermediate states below the S2 state are expected to make a new internal conversion pathway with small energy gap. If the intermediate state has the reverse displacement from the S2 state compared to the S1 state, the reverse trend on the energy gap law can be interpreted in terms of the conical intersection model [28]. The presence of the intermediate states can explain the minimum S2 internal conversion rate at n = 9. 4. Conclusion The initial relaxation kinetics in all-trans-carotenoids with various conjugation length has been investigated by the time-resolved Kerr-gate fluorescence spectroscopy. The tunable excitation pulses allow us to excite the S2 state without large amount of excess vibrational energy. The time-resolved fluorescence spectra show that the vibrational relaxation is eliminated from the S2 dynamics. The internal conversion rate from the S2 state has a minimum at n = 9. In the shorter carotenoids, the internal conversion rate decreases with increasing the S2–S1 energy gap as observed in the internal conversion from the S1 state. This indicates that the S2 state directly relaxes to the S1 state. In the longer carotenoids, the internal conversion rate increases with increasing the energy gap. This suggests the presence of the intermediate state lying between the S2 and S1 states. The intermediate state may have a role to facilitate the S2 state relaxation faster. Acknowledgements This work was partly supported by the Ministry of Education, Culture, Sports, Science, and Technology through a
21st Century COE Program ‘Exploring New Science by Bridging Particle-Matter Hierarchy’. H.H thanks Grantin-aid from JSPS and MEXT (Grants Nos. 17204026 and 17654083) and the Grant-in-aid from SICP/JST. References [1] B.S. Hudson, B.E. Kohler, K. Schulten, in: E.C. Lim (Ed.), Excited States, vol. 6, Academic Press, New York, 1982, pp. 1–95. [2] V. Sundstro¨m, T. Pullerits, R. van Groundelle, J. Phys. Chem. B 103 (1999) 2327. [3] H.A. Frank, R.J. Cogdell, Photochem. Photobiol. 63 (1996) 257. [4] T. Polı´vka, V. Sundstro¨m, Chem. Rev. 104 (2004) 2021. [5] P. Tavan, K. Schulten, Phys. Rev. B 36 (1986) 4337. [6] K. Furuuchi, T. Sashima, Y. Koyama, Chem. Phys. Lett. 356 (2002) 547. [7] T. Sashima, Y. Koyama, T. Yamada, H. Hashimoto, J. Phys. Chem. B 104 (2000) 5011. [8] S. Akimoto, I. Yamazaki, S. Takaichi, M. Mimuro, J. Lum. 87–89 (2000) 797. [9] D. Polli, G. Cerullo, G. Lanzani, S. De Silvestri, K. Yanagi, H. Hashimoto, R.J. Cogdell, Phys. Rev. Lett. 93 (2004) 163002. [10] G. Cerullo, D. Polli, G. Lanzani, S. De Silvestri, H. Hashimoto, R.J. Cogdell, Science 298 (2002) 2395. [11] K. Nishimura, F.S. Rondonuwu, R. Fujii, J. Akahane, Y. Koyama, T. Kobayashi, Chem. Phys. Lett. 392 (2004) 68. [12] W. Wohlleben, T. Buckup, H. Hashimoto, R.J. Cogdell, J.L. Herek, M. Motzkus, J. Phys. Chem. B 108 (2004) 3320. [13] 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. [14] D.S. Larsen, E. Papagiannakis, I.H.M. Stokkum, M. Vengris, J.T.M. Kennis, R. van Grondelle, Chem. Phys. Lett. 381 (2003) 733. [15] H. Hashimoto, K. Yanagi, M. Yoshizawa, D. Polli, G. Cerullo, G. Lanzani, S. De Silvestri, A.T. Gardiner, R.J. Cogdell, Arch. Biochem. Biophys. 430 (2004) 61. [16] M. Yoshizawa, H. Aoki, H. Hashimoto, Phys. Rev. B 63 (2001) 180301. [17] M. Yoshizawa, H. Aoki, M. Ue, H. Hashimoto, Phys. Rev. B 67 (2003) 174302. [18] R. Englman, J. Jortner, J. Mol. Phys. 18 (1970) 145. [19] H.A. Frank, R.Z.B. Desamero, V. Chynwat, R. Gebhard, I. van der Hoef, F.J. Jansen, J. Lugtenburg, D. Gosztola, M.R. Wasielewski, J. Phys. Chem. A 101 (1997) 149. [20] D. Kosumi, M. Komukai, H. Hashimoto, M. Yoshizawa, Phys. Rev. Lett. 95 (2005) 213601. [21] R. Nakamura, R. Fujii, H. Nagae, Y. Koyama, Y. Kanematsu, Chem. Phys. Lett. 400 (2004) 7. [22] S. Akimoto, I. Yamazaki, S. Takaichi, M. Mimuro, Chem. Phys. Lett. 313 (1999) 63. [23] A.N. Macpherson, T. Gillbro, J. Phys. Chem. A 102 (1998) 5049. [24] D. Kosumi, K. Yanagi, T. Nishio, H. Hashimoto, M. Yoshizawa, Chem. Phys. Lett. 408 (2005) 89. [25] P.O. Andersson, S.M. Bachilo, R.L. Chen, T. Gillbro, J. Phys. Chem. 99 (1995) 16199. [26] R.L. Christensen, M. Goyette, L. Gallagher, J. Duncan, B. DeCoster, J. Lugtenburg, F.J. Jansen, I. van der Hoef, J. Phys. Chem. A 103 (1999) 2399. [27] R. Hemly, B.E. Kohler, Biophys. J. 20 (1977) 377. [28] W. Fuß, Y. Haas, S. Zilverg, Chem. Phys. 259 (2000) 273. [29] T. Polı´vka, D. Zigmantas, H.A. Frank, J.A. Bautista, J.L. Herek, Y. Koyama, R. Fujii, V. Sundstro¨m, J. Phys. Chem. B 105 (2001) 1072. [30] H.A. Frank, J.S. Josue, J.A. Bautista, I. van der Hoef, F.J. Jansen, J. Lugtenburg, G. Wiederrecht, R.L. Chirstensen, J. Phys. Chem. B 106 (2002) 2083. [31] K. Yanagi, A.T. Gardiner, R.J. Cogdell, H. Hashimoto, Phys. Rev. B 71 (2005) 195118.
Conjugation length dependence of relaxation kinetics in b-carotene homologs probed by femtosecond Kerr-gate fluorescence spectroscopy Daisuke Kosumi a
a,1
, Kazuhiro Yanagi b,2, Ritsuko Fujii b, Hideki Hashimoto b, Masayuki Yoshizawa a,*
Department of Physics, Graduate School of Science, Tohoku University, 6-3 Aramaki-Aza-Aoba, Aoba-ku, Sendai 980-8578, Japan b ‘Light and Control’, PRESTO/JST, Department of Physics, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan Received 5 April 2006; in final form 8 May 2006 Available online 12 May 2006
Abstract Ultrafast dynamics of the optically allowed S2 ð1 1 Bþ u Þ state in all-trans-carotenoids with a number of conjugated double bonds, n = 7– 15, has been investigated by femtosecond Kerr-gate fluorescence spectroscopy. The lifetime of the S2 state is determined eliminating effect of excess vibrational energy. The lifetime has a maximum for n = 9 and shows anomalous dependence on the energy gap between the S2 and S1 states. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction Linear polyene structure of carotenoids has some unique optical properties due to its one dimensional delocalized p electron system; e.g., ultrafast dynamics in excited states and large third-order nonlinear optical susceptibility [1]. Furthermore, carotenoids play dual essential roles of light-harvesting and photoprotection in bacterial photosynthesis [2–4]. The S0 ground state of carotenoids has A g symmetry assuming C2h symmetry of their linear polyene backbone (the 1 1 A g state). The lowest optically allowed excited state is the S2 ð1 1 Bþ u Þ state and the optically forbidden S1 ð2 1 A Þ state is lying below the S2 state. It has g been proposed that the excitation energy transfer to bacteriochlorophyll in the photosynthesis occurs from both the
*
Corresponding author. Fax: +81 22 795 6362. E-mail addresses: [email protected] (D. Kosumi), [email protected] (M. Yoshizawa). 1 JSPS Research Fellow. 2 Present address: Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8562, Japan. 0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.05.023
S2 and S1 states of carotenoids [2–4]. However, theoretical calculation has indicated that there are more than two dark states (with B u and Ag symmetry) below the S2 state depending on a number of conjugated double bonds of the polyene backbone (n) [5]. Resonant Raman excitation 1 profiles have shown the presence of the 1 1 B u and 3 Ag states between the S2 and S1 states in longer carotenoids (n > 9) [6,7]. The ultrafast dynamics of the carotenoids has been investigated in depth and the role of the intermediate states has been discussed, but the initial ultrafast relaxation kinetics has not been well understood yet [4,8– 17]. The relaxation kinetics of the carotenoids depends on the conjugation length, because the energy of the excited states in linear polyenes are given as a linear function of 1/(2n + 1) [5]. The S1 ! S0 internal conversion rate of carotenoids increases with n. It is well explained by the energy gap law [18]. On the contrary, the S2 ! S1 internal conversion mechanism has not been clarified because of its extremely fast relaxation. The conjugation length dependence of the S2 ! S1 relaxation has been investigated mainly in open chain carotenoids [8,9,19]. The internal conversion rate has a minimum at n = 9, but this anoma-
D. Kosumi et al. / Chemical Physics Letters 425 (2006) 66–70
lous dependence has not been understood. One possible interpretation is consideration of the intermediate state below the S2 state. The presence of the intermediate state may show difference between the S2 decay and the S1 formation kinetics. However, the previous reports have not precisely determined the S2 decay kinetics, because the observed signals contain the influence of other effects. The conjugation length dependence of the S2 ! S1 internal conversion rate has been investigated by stationary fluorescence quantum yields [19], time-resolved fluorescence [8], and absorption spectroscopies [9]. The determination of the lifetime from the fluorescence quantum yield needs several assumptions such as the radiative relaxation rate. The time-resolved absorption spectroscopy is widely used to investigate the ultrafast phenomena. However, the signal is complicated because of many contributions such as depletion of the ground state, absorption and stimulated emission of the excited states, and coherent nonlinear optical effects. Our previous study using resonant and non-resonant excitations shows that the non-linear optical effect is miss-assigned to the excited state in b-carotene [20]. The time-resolved fluorescence spectroscopy is a powerful method to monitor directly the dynamics of the S2 state in carotenoids, because the S2 state is the only one emissive excited state due to the symmetry reason [8,21–24]. However, the conjugation length dependence of the fluorescence kinetics has been investigated using the excitation pulse with large amount of excess energy [8]. The excess vibrational energy affects the relaxation kinetics and the transient spectra [22,24]. To avoid contribution of the vibrational relaxation, the excitation without affording the excess vibrational energy and the detailed measurement of the time-resolved fluorescence spectra are required. In this study, the ultrafast fluorescence of the S2 state in homologs of all-trans-b-carotene with different conjugation lengths and all-trans-lycopene (an open chain carotenoid) has been measured using tunable excitation pulses. The excitation with small amount of vibrational energy allows us to neglect the influence of the vibrational relaxation and to determine the S2 lifetime properly. The conjugation length dependence of the S2 state dynamics is discussed considering the energy gap between the S2 and S1 states.
67
the equipments. The instrumental response function determined by cross-correlation between the excitation and gate pulses was 300 fs for the SH excitation and 200 fs for the OPA excitation. After chirp compensation and fitting considering the system response, an experimental error of the obtained time constant was better than 30 fs. Sample preparation protocols were described in detail elsewhere [18]. Chemical structures of the carotenoids are shown in the insets of Figs. 1 and 2. b-Carotene and lycopene have 11 conjugated double bonds. The b-carotene homologs are called after the number of the conjugated 0 double bonds as m7 (n = 7), m9 (n = 9), m9 (n = 9), 0 M13 (n = 13), and M15 (n = 15). m and M stand for mini0 and macro-b-carotene, respectively. Prime symbols of m9 0 and M13 denote the absence of methyl groups at the central part of the polyene chain. The carotenoids were dissolved in cyclohexane and circulated in a 0.5 mm optical path length flow cell. Optical density of the sample was prepared to be 1.0 at the absorption maximum photon energy. 3. Results and discussion Fig. 1 shows fluorescence and absorption spectra of m7. The 3.12 eV pulse excites lower energy side of the S2 absorption. The stationary fluorescence shown in Fig. 1a has dual broad fluorescence around 2.2–2.8 eV. A sharp peak at 2.76 eV is due to spontaneous Raman signal of C–H stretching mode of the solvent (cyclohexane). The fluorescence around 2.8 eV is assigned to the S2 ! S0 emission, while the 2.2 eV fluorescence is assigned to the S1 ! S0 emission [25,26]. The S1 ð2 1 A g Þ state in the exact
2. Experimental Experimental setup of the femtosecond time-resolved fluorescence spectroscopy was based on the optical Kerrgate method [24]. Output pulses of a 1 kHz regenerative Ti:sapphire amplifier (Spitfire, Spectra Physics) were separated to excitation and gate pulses. The excitation pulses were generated by the second harmonic (SH) generation (3.12 eV) or a non-collinear optical parametric amplifier (OPA) (2.41–2.55 eV). A quartz plate with 1.0 mm thickness was used as the Kerr medium gated by the fundamental pulse. The polarization of the gated fluorescence was parallel to the excitation polarization. The fluorescence spectra were corrected for wavelength dependencies of
Fig. 1. (a) Stationary fluorescence (solid line) and absorption (dashed line) spectra and time-resolved fluorescence spectra at (b) 0.1 ps, (c) 0.3 ps, and (d) 0.5 ps of m7. An inset shows chemical structure. An arrow in (a) indicates the excitation photon energy and another arrow in (b) indicates the probe photon energy used for analysis of the temporal response.
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Fig. 2. Time-resolved fluorescence at 0.1 ps (solid lines) and stationary absorption (dashed lines) spectra of carotenoids. Insets show chemical structures. Arrows indicate the excitation photon energies.
C2h symmetry is optically forbidden, but the symmetry of the carotenoid is slightly broken in solution because of distortion around the terminal b-ring. The radiation rate of the S1 state is much smaller than that of the S2 state, but very long lifetime of the S1 state in short carotenoids causes the dual fluorescence from the S2 and S1 states [25,26].
The time-resolved fluorescence spectra of m7 shown in Fig. 1b–d consist of the Raman line at 2.76 eV and the broad S2 fluorescence around 2.8 eV. The S1 fluorescence is not observed because of the small radiation rate. The Raman signal has equal temporal response to the crosscorrelation between the excitation and gate pulses. It disappears at a delay time of 0.5 ps. The broad S2 fluorescence remains longer than the Raman signal, but it does not show the time dependent spectral change. The vibrational relaxation process is negligible in the S2 fluorescence of m7, because the excitation generates the vibrational ground level of the S2 state. The time-resolved fluorescence and stationary absorption spectra of the other carotenoids are shown in Fig. 2 in the order of n. The S2 absorption shifts to lower energy with increasing the conjugation length. The time-resolved fluorescence is measured using the excitation with the photon energies indicated by the arrows in Fig. 2. A sharp peak 0 at 2.76 eV in m9 and m9 is due to the spontaneous Raman signal. The time-resolved fluorescence of all the carotenoids is assigned to that from the S2 state, because it appears just below the S2 absorption. The S2 fluorescence shifts to lower energy with increasing the conjugation length in the same way with the S2 absorption. The relaxation kinetics of the carotenoids has been reported to depend on the excitation photon energy. The slower internal conversions following the excitation with the larger excess energy are observed in b-carotene and lycopene [24]. The S2 fluorescence following the excitation with the large excess energy decays faster at higher photon energy [22]. The excitation pulses in this study are selected to excite the vibrational ground level (0–0 transition) or low vibrational level (0–1 or 0–2 transition) of the S2 state. The temporal spectral change of the fluorescence is very small as shown in Fig. 3. Furthermore, the signals used for analysis of the temporal responses are selected to minimize the effect of the vibrational relaxation. In general, the
Fig. 3. Normalized time-resolved fluorescence spectra of carotenoids at delay times of 0.1 ps (dashed lines) and 0.3 ps (thin solid lines). Arrows indicate the probe photon energies used for analysis of the temporal responses.
D. Kosumi et al. / Chemical Physics Letters 425 (2006) 66–70
transient state with large excess vibrational energy has broader spectrum than that of the thermal equilibrium condition. The peak of the band decays slower than the edge, because the vibrational relaxation is involved with the population decay. However, a quasi-isosbestic energy exists near the shoulder where the vibrational relaxation has a minimal effect on the transient response [24]. Fig. 4 shows temporal responses of the S2 fluorescence. The probe photon energies are selected at the shoulder of the signal (arrows in Figs. 1b and 3) to minimize the influence of the vibrational relaxation. The transient signal is proportional to the transient population of the S2 state. The obtained S2 state lifetimes are denoted in Fig. 4 together. The lifetimes for the longer carotenoids are slightly longer than those reported in the open chain carotenoids [8,19] and in the b-carotene homologs observed by the time-resolved absorption spectroscopy [9], but the conjugation length dependence is equal to the
Fig. 4. Temporal responses of the S2 fluorescence of the carotenoids (open circles) probed at the photon energies indicated by the arrows in Figs. 1b and 3. Solid lines are the best fitted function of an exponential decay convoluted with the instrumental response function assuming a Gaussian temporal profile.
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previous reports. For n > 9, the S2 state lifetime decreases with the increase n, but the lifetime for n = 7 is shorter than that for n = 9. The lifetime has a maximum at n = 9. The S1 ! S0 internal conversion rate of the carotenoids is well explained by the energy gap law [4,9,15,17,19,25]. Therefore, the internal conversion rate from the S2 state is tentatively plotted as a function of the S2–S1 energy gap in Fig. 5. The S2–S1 energy gap is determined using the previously reported relation between the excited state energy and n for the open chain carotenoids [6]. First, we determine an effective number of conjugated double bonds neff [27] for the b-carotene homologs from the S2 absorption energy. Then, the S2–S1 energy gap is determined using the S1 energy calculated from neff. The S2–S1 energy gap spreads with increasing n in contrast to the S1–S0 energy gap, because the S1 state energy decreases more rapidly than the S2 state energy. The S2 lifetime of m5 (n = 5) plotted in Fig. 5 has been reported to be 45 fs by the transient absorption spectroscopy [9]. It can be treated reliably with the lifetimes obtained in this study, because m5 is excited by the pulse with small amount of excess energy and the transient absorption is assigned to the S2 state from the relation to the signals of the other longer carotenoids (n = 7–15) [17]. The internal conversion rate from the S2 state in the 0 short carotenoids (m5, m7, and m9 ) shows consistent behavior with the energy gap law. It decreases with increasing the S2–S1 energy gap. Assuming the energy gap law in weak vibronic coupling case, the energy of the vibrational accepting mode is determined as 0.25 eV (2000 cm1, a dashed line in Fig. 5). It is larger than the energy gap of m5. The energy gap law cannot apply to the S2 ! S1 internal conversion because the energy gap law should apply in case of the large energy gap compared to the vibrational accepting mode [18]. Fuß et al. have proposed that conical
Fig. 5. The internal conversion rates from the S2 state as a function of the S2–S1 energy gap. A dashed line is the best fit using the energy gap law for 0 the shorter carotenoids (m5, m7, and m9 ).
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intersection of the electronic states can explain the energy gap dependence of the internal conversion rate [28]. The activation energy needed for crossing the potential energy surfaces depends on the energy gap and the horizontal displacement of the surfaces. The horizontal displacement between the S2 and S1 states in carotenoid determined by the time-resolved absorption spectroscopy [29] is consistent with the decrease of the relaxation rate with increasing the energy gap as well as the energy gap law. The conjugation length dependence of the shorter carotenoids suggests the direct S2 ! S1 internal conversion. On the contrary, the S2 internal conversion rate of the longer carotenoids (n > 9) shows the reverse trend. As a natural consequence, it is well expected that the internal conversion from the S2 state should be assisted by some other mechanisms in the longer carotenoids. Frank et al. have proposed that increase in the density of the vibrational accepting modes in the longer polyene contributes to increase in the relaxation rate [19,30]. Akimoto et al. have suggested that uncertain parameter such as the displacement of the potential surfaces of the S2 and S1 states depends on n and affects the dynamics of the S2 state [8]. Another possible interpretation is the intervening of the intermediate states lying below the S2 state for n > 9 [6,7,31]. The intermediate states below the S2 state are expected to make a new internal conversion pathway with small energy gap. If the intermediate state has the reverse displacement from the S2 state compared to the S1 state, the reverse trend on the energy gap law can be interpreted in terms of the conical intersection model [28]. The presence of the intermediate states can explain the minimum S2 internal conversion rate at n = 9. 4. Conclusion The initial relaxation kinetics in all-trans-carotenoids with various conjugation length has been investigated by the time-resolved Kerr-gate fluorescence spectroscopy. The tunable excitation pulses allow us to excite the S2 state without large amount of excess vibrational energy. The time-resolved fluorescence spectra show that the vibrational relaxation is eliminated from the S2 dynamics. The internal conversion rate from the S2 state has a minimum at n = 9. In the shorter carotenoids, the internal conversion rate decreases with increasing the S2–S1 energy gap as observed in the internal conversion from the S1 state. This indicates that the S2 state directly relaxes to the S1 state. In the longer carotenoids, the internal conversion rate increases with increasing the energy gap. This suggests the presence of the intermediate state lying between the S2 and S1 states. The intermediate state may have a role to facilitate the S2 state relaxation faster. Acknowledgements This work was partly supported by the Ministry of Education, Culture, Sports, Science, and Technology through a
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