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Energies and excited-state dynamics of 1Bu+ , 1Bu- and 3Ag- states of carotenoids bound to LH2 antenna complexes from purple photosynthetic bacteria
Chemical Physics Letters 480 (2009) 289–295
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Energies and excited-state dynamics of 1Bþ u , 1Bu and 3Ag states of carotenoids bound to LH2 antenna complexes from purple photosynthetic bacteria
Rebecca Christiana a,b, Takeshi Miki a, Yoshinori Kakitani a, Shiho Aoyagi a, Yasushi Koyama a,*, Leenawaty Limantara b a b
Faculty of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda 669-1337, Japan Research Center for Photosynthetic Pigments, Ma Chung University, Villa Puncak Tidar, Malang 65151, Indonesia
a r t i c l e
i n f o
Article history: Received 2 August 2009 In final form 29 August 2009 Available online 1 September 2009
a b s t r a c t Time-resolved pump–probe stimulated-emission and transient-absorption spectra were recorded after excitation with 30 fs pulses to the 1Bþ u ð0Þ and optically-forbidden diabatic levels of carotenoids, neurosporene, spheroidene and lycopene having n = 9–11 double bonds, bound to LH2 antenna complexes from Rhodobacter sphaeroides G1C, 2.4.1 and Rhodospirillum molischianum. The low-energy shift of stimulated þ emission from the covalent 1B u ð0Þ and 3Ag ð0Þ levels slightly larger than that from the ionic 1Bu ð0Þ state suggests the polarization, whereas more efficient triplet generation suggests the twisting of the conjugated chain in Cars bound to the LH2 complexes, when compared to Cars free in solution. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the first identification of the optically-forbidden 1B u and 3A g states of carotenoids (Cars), in addition to the optically allowed 1Bþ u and the optically-forbidden 2Ag states, with respect to transitions from/to the ground 1Ag state (Fig. 1a), the excitedstate dynamics and the roles of these low-lying singlet states in the light-harvesting function have been studied extensively (see reviews, Refs. [1,2]). The 1B u forbidden state of Cars, having the number of conjugated double bonds n = 9–13, were systematically determined by measurement of resonance-Raman excitation profiles (RREPs) [3]. The conjugation-length (n) dependence of the 1B u energy was confirmed by fluorescence spectroscopy [3] and by electronic-absorption spectroscopy [4]. The singlet internal-conversion and the singlet to triplet fission processes from the 1B u state were identified, and the role of this particular state in the light-harvesting function has been determined. The Car-to-bacteriochlorophyll (BChl) singlet-energy transfer ð1B u ! Q x and 2Ag ! Q y Þ has turned out to play a crucial role in determining the singlet-energy transfer efficiency [1]. The 1B u state is formerly optically-forbidden, but it exhibits fluorescence due to its strong electronic coupling with the optically-allowed 1Bþ u state that is located close by. The 3A g state of Cars (n = 9–13) were also determined by the RREP measurement [3], and their internal-conversion processes were identified by subpicosecond time-resolved Raman spectroscopy of Cars (n = 9–11) [2,5] and by 5-fs pump–probe spectroscopy
* Corresponding author. Fax: +81 79 565 9077. E-mail address: [email protected] (Y. Koyama). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.08.074
of Cars (n = 11–13) [2,6]. The light-harvesting function of this state still remains to be revealed. Recently, the unique excited-state properties of the pairs of optically-allowed 1Bþ u and optically-forbidden 1Bu or 3Ag vibronic levels, energetically overlapped with each other, have been found (Fig. 1b). The pair of vibronic levels are called ‘the diabatic pair’, because the diabatic, rather than adiabatic, basis set needs to be used to describe its excited-state properties (see Ref. [7] for a summary of experimented results followed by theoretical analyses). Upon incoherent excitation, using 100 fs pulses, to the 1Bþ u ð0Þ level of neurosporene (n = 9) and spheroidene (n = 10), stimulated emis þ sion took place from the 1Bþ u ð0Þ þ 1Bu ð1Þ and 1Bu ð0Þ þ 1Bu ð2Þ diabatic vibronic levels, respectively. On the other hand, stimulated emission took place from the 1Bþ u ð0Þ counterpart, but no stimulated emission was observed from the diabatic counterpart, 3A g ð1Þ, 3Ag ð2Þ and 3Ag ð3Þ levels of lycopene (n = 11), anhydrorhodovibrin (n = 12) and spirilloxanthin (n = 13), respectively. Thus, the following selection rule for the diabatic electronic mixing þ has been determined: ‘1Bþ u -to-1Bu is allowed, but 1Bu -to-3Ag is forbidden’. This selection rule holds true in the internal-conversion processes, as well. Here, Cars dissolved in non-polar solvents were incoherently excited with 100 fs pulses. Most recently, completely-different excited-state dynamics have been found when the diabatic pairs of the optically-allowed 1Bþ u ð0Þ and the optically-forbidden 1Bu or 3Ag counterparts are coherently excited by the use of 30 fs pulses [7,8]. This timelyshort but spectrally-broad pulses can excite simultaneously and coherently the pair of diabatic levels located close by. Here, Cars were dissolved in a polar solvent, THF, to induce the polarization of the conjugated chain. We found that the stimulated emission
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n
a
13
12
11
10
9
20
Qx B800 B850
3A g–(0)
3
–1 Energy / 10 cm
1Bu+(0)
16 1Bu–(0)
2A g–(0)
Qy
12
0.04
B800 B850
0.05
2. Experimental
1 / (2n + 1) n
b
13
12
11
20
3
–1 Energy / 10 cm
1B 4
13
1 2
3
02
0 1
2
1
0
10
+ u
9
3
1
2
0
1
2
1
1
0
0
0 1 0
1Bu–
3A g–
16
1
0
0
0
0
LH2 complexes from Rba. sphaeroides G1C, Rba. sphaeroides 2.4.1 and Rsp. molischianum have been prepared as described previously [9] with some modifications. The set-up and the method of femtosecond time-resolved absorption spectroscopy are as described previously (see Supporting Information of Ref. [10]) except for some modification for white-continuum generation: The strongest 550-nm laser pulse from a TOPAS-White (Quantronix) of lower energy was focused onto a 1-mm glass plate to generate a white-continuum probe pulse by the use of a pair of concave reflection mirrors with a focal length of 5 cm. As a result, the full width at half-maximum (FWHM) of the cross-correlation traces between the pump and probe pulses (30 fs) was determined to be 60 fs by the use of optical Kerr-effect signal. 3. Results and discussion
0
12
intensity in lycopene (see Fig. 4 of Ref. [8]). In addition to the rap idly-decaying 1B u ð0Þ and 3Ag ð0Þ emission, the long-lived emission þ þ 1B or 1B þ 3A from the 1Bþ u u u g diabatic levels may also contribute to the Car-to-BChl singlet-energy transfer. Importantly, not only the 1B u state but also the 3Ag state can play a role in the light-harvesting function, when coherently excited. In the present investigation, we have tried to examine if the same type of observation can be obtained in neurosporene, spheroidene and lycopene (n = 9–11) that are bound to the real LH2 antenna complexes from purple photosynthetic bacteria, Rhodobacter sphaeroides G1C, 2.4.1 and Rhodospirillum molischianum. We have addressed the following three specific questions: (1) Does the same type of coherent excitation of the diabatic vibronic levels take place in the set of Cars bound to the LH2 antenna complexes? (2) Does stimulated emission from the covalent 1B u and 3Ag states state due shift to the lower energies, as that from the ionic 1Bþ u to the Car to apo-peptide intermolecular interaction? (3) Is the excited-state dynamics of Cars bound to the antenna complexes in a way similar to, or different from, that of Cars free in solution [8]?
3.1. Characterization of deca-femtosecond time-resolved, stimulatedemission and transient-absorption spectra
0.04
0.05 1 / (2n + 1)
Fig. 1. (a) Energies of the 1Bþ u ð0Þ, 3Ag ð0Þ, 1Bu ð0Þ and 2Ag ð0Þ levels determined by measurement of resonance-Raman excitation profiles of crystalline Cars (n = 9–13). The energies of the Qx and Qy levels of B800 and B850 BChl a in the LH2 antenna complexes are shown for comparison. On going from n = 10 to 11, as expected by the relative energies, the 1B u ! Q x and 2Ag ! Q y Car-to-BChl singlet energytransfer channels become closed [1,2]. (b) Overlap of vibrational ladders (a spacing of 1400 cm1 is assumed) starting from the 1Bþ u ð0Þ, 3Ag ð0Þ and 1Bu ð0Þ origins determined as linear functions of 1/(2n + 1) in (a). The 1Bþ u ð0Þ level is overlapped completely or approximately, with the 1B u ð1Þ and 1Bu ð2Þ levels in Cars (n = 9 and 10), and with the 3A g ð1Þ, 3Ag ð2Þ and 3Ag ð3Þ levels in Cars (n = 11–13), respectively.
composed of three components that are described in terms of the quantum-beat equation: (i) rapidly-decaying 1Bþ u ð0Þ emission (the optically-allowed counterpart), (ii) rapidly-decaying 1B u ð0Þ or 3A g ð0Þ emission (the optically-forbidden counterpart after instaneous vibrational relaxation), and (iii) long-standing stimulated emission from the diabatic pair, i.e., the 1Bþ u ð0Þ optically-allowed and the iso-energetic optically-forbidden 1B u or 3Ag counterparts overlapped with each other. The three components correspond to a pair of incoherent, independent terms and the coherent cross-term in the quantum-beat expression (see Eq. (48) of Ref. [7], for example). Actually, we detected an oscillatory change in fluorescence
Fig. 2 shows the 1Bþ u ð0Þ pump white-continuum probe time-resolved stimulated-emission and transient-absorption spectra of neurosporene, spheroidene and lycopene (n = 9–11) bound to the LH2 antenna complexes from Rba. sphaeroides G1C, 2.4.1 and Rsp. molischianum, respectively. (See the caption for the specification of observed peaks. In particular, the negative signals with energies around the 1Bþ u level are classified into three different categories depending on the delay times.) The time-resolved spectra can be characterized in terms of different phases according to the delay time, after coherent excitation to the 1Bþ u ð0Þ plus iso-energetic 1B u or 3Ag diabatic pair: The 1Bþ u ð0Þ þ 1Bu ð1Þ pair of neurosporene (Rba. sphaeroides G1C). Phase I (0.04 to 0.00 ps): A sharp and weak 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission (labeled ‘se’) mainly ap and pears. Phase II (0.00–0.12 ps): The 1Bþ u ð0Þ ! 1Ag ð1Þ 1Bu ð0Þ ! 1Ag ð0Þ stimulated emissions appear and increase in intensity. The sharp and stronger 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission suddenly increases in intensity and bandwidth trans forming into stimulated emission from the 1Bþ u ð0Þ þ 1Bu ð1Þ diabatic pair (‘sed’). All of these three peaks reach maxima and start to decay. Phase III (0.12–0.18 ps): Extremely-rapid and drastic changes in the spectral pattern take place in this time region: the stimulated emission is replaced by the 1B u ð0Þ ! 1Ag ð0Þ
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Rba. sphaeroides 2.4.1 spheroidene (n = 10)
Rba. sphaeroides G1C neurosporene (n = 9)
ps
1Bu+
ΔOD = 0.02
ΔOD = 0.05
se
(0
(0
I
1)
Rsp. molischianum lycopene (n = 11)
ΔOD = 0.05
1)
1Bu+
1Bu- 1Bu+
I
–0.04
I
–0.02
1Bu- 1Bu+
(1 0)
se
1Bu+ 1Bu
II II
+
3Agsed
sed
1Bu-
0.06 0.08
1Bu-
0.10
III
0.12
T1
T1
0.14
1Bu-
III
T1
0.02 0.04
II sed
III
0.00
0.16
IV
IV 2Ag-
0.18
2Ag-
0.20 2Ag
0.22
-
0.24
IV
0.26 0.28 0.30
V
V
0.45 0.60
V
0.90 bga
bga
T1
bga
bga
T1
T1
1.20 1.50 3.00
VI
VI
VI
4.00 5.00
16
18 20 Energy / 103 cm–1
16
18 20 Energy / 103 cm–1
16 18 20 Energy / 103 cm–1
Fig. 2. The 1Bþ u ð0Þ pump and the visible white-continuum probe time-resolved stimulated-emission and transient-absorption spectra of LH2 antenna complexes from Rba. sphaeroides G1C, 2.4.1 and Rsp. molischianum, binding Cars (n = 9–11). On the top of each panel, the ground-state absorption spectrum is shown for comparison. Different phases of relaxation processes, classified by the spectral patterns, are shown on the left-hand side of each panel. Stimulated emission is shown in this figure by downward arrows with labels that assign the starting electronic state, where the initial and final vibrational levels are specified when necessary; the transition without arrows should be considered to be the vibrationally t = 0 ? t = 0 transition. The negative signal with a 1Bþ u ð0Þ energy is assigned, depending on the delay time, as stimulated emission from the þ þ þ 1A 1A 1Bþ g ð0Þ or 1Bu ð1Þ g ð0Þ u ð0Þ level (se) ? stimulated emission from the 1Bu ð0Þ þ 1Bu or 3Ag diabatic levels (sed) ? the bleaching of the ground-state 1Bu ð0Þ absorptions (bga); here, ‘se’ stands for stimulated emission, ‘sed’, stimulated emission from diabatic levels, and ‘bga’, bleaching of the ground-state absorption. On the other hand, transient absorption is shown by upward arrows with labels that assign the starting electronic state. Presumably, the ‘1B u ’ and ‘2Ag ’ transient absorptions are to be þ 1B 2A assigned to the mAþ g g transitions; the two transitions are similar in energy but the peak is broad in the former and sharp in the latter. The ‘T1’ transient u and nBu T1 transition. absorption should be due to the Tn
nAþ 1B u transient absorption (hereafter, simply abbreviated as g ‘1Bu transient absorption’). The 1Bþ u ð0Þ ! 1Ag ð1Þ stimulated emission decreases in intensity and disappears. Most importantly, a new profile appears in-between the 1B u ð0Þ transient absorption and the persistent stimulated emission from the 1Bþ u ð0Þ þ 1Bu ð1Þ T1 absorption diabatic pair (sed), which is ascribable to the Tn 3 reflecting the 1B u ð0Þ ! Bu ðT 1 Þ singlet heterofission reaction [11]. Phase IV (0.18–0.30 ps): Gradual transformation takes place from the broad and round peak of the 1B u transient absorption to the sharper and clear peak of the 2A g transient absorption. T1 absorption increases in intensity and bandwidth, while The Tn the 1Bþ u ð0Þ þ 1Bu ð1Þ diabatic stimulated emission (sed) decreases
in intensity and gradually sharpens up. Phase V (0.30–1.50 ps): T1 transient-absorpThe integrated intensity of the 2A g and Tn tion profiles reach maxima and start to decay. The stimulated emission from the diabatic pair transforms into pure bleaching of 1A the 1Bþ u ð0Þ g ð0Þ ground-state absorption (‘bga’) decreasing in intensity. Phase VI (1.50–5.00 ps): The 2A g transient absorption T1 trandecreases in intensity and disappears, and only the Tn sient absorption remains together with negligible bleaching of 1A the 1Bþ u ð0Þ g ð0Þ absorption (bga). þ The 1Bu ð0Þ þ 1B u ð2Þ pair of spheroidene (Rba. sphaeroides 2.4.1). Phase I (0.04 to 0.00 ps): The strong and sharp 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission (se) and the weak and broad
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1Bþ u ð0Þ ! 1Ag ð1Þ stimulated emission appear together with the 1Bu ð0Þ ! 1A g ð0Þ stimulated emission. Phase II (0.00–0.06 ps): The 1Bþ u ð0Þ ! 1Ag ð0Þ emission (se) substantially broadens transferring into stimulated emission from the 1Bþ u ð0Þ þ 1Bu ð2Þ diabatic pair ð0Þ ! 1A ð1Þ emission and the 1B (sed). The 1Bþ u u ð0Þ ! 1Ag ð0Þ g emission increase in intensity, become saturated, and start to de crease. Phase III (0.06–0.10 ps): The 1B u ð0Þ ! 1Ag ð0Þ stimulated emission decreases in intensity and disappears, while the 1Bþ u ð0Þ ! 1Ag ð1Þ stimulated emission is replaced by the 1Bu ð0Þ transient absorption. Stimulated emission from the ð0Þ þ 1B ð2Þ diabatic pair (sed) remains as a broad profile. Phase 1Bþ u u IV (0.10–0.24 ps): The round 1B u transient absorption transforms into the clearly-peaked 2A g transient absorption. Slightly-sharp ened 1Bþ u ð0Þ þ 1Bu ð2Þ diabatic emission (sed) persists in this time T1 absorption appears in-between these region. Again, the Tn two peaks and increases in intensity. Phase V (0.24–1.50 ps): The 2A g and T1 transient absorptions increase in the integrated intensity and become saturated. The diabatic stimulated emission from the 1Bþ u ð0Þ þ 1Bu ð2Þ pair (sed) transforms into the weaker bleaching þ 1A of the 1Bu ð0Þ g ð0Þ ground-state absorption (bga). Phase VI (1.50–5.00 ps): The 2A g transient absorption decreases in intensity T1 absorption decays but stays with the to disappear, and the Tn 1A bleaching of the 1Bþ u ð0Þ g ð0Þ ground-state absorption (bga). ð0Þ þ 3A ð1Þ pair of lycopene (Rsp. molischianum). Phase The 1Bþ u g I (0.04 to 0.02 ps): Broad stimulated emission due to the þ 1Bþ u ð0Þ ! 1Ag ð0Þ and 1Bu ð1Þ ! 1Ag ð0Þ transitions (se) mainly appear together with a stimulated emission in the lower-energy region (possibly due to the stimulated emission from the Qx level of BChl). Phase II (0.02–0.08 ps): The 3A g ð0Þ stimulated emission emerges and a pair of broad and strong profiles assignable to the þ 1Bþ u ð0Þ ! 1Ag ð0Þ and 1Bu ð1Þ ! 1Ag ð0Þ stimulated emission appear and grow in intensity. Phase III (0.08–0.12 ps): The 3A g ð0Þ ! 1Ag ð0Þ stimulated emission becomes replaced by the transient absorption and the Tn T1 absorption in a simbroad 1B u ilar spectral region. The pair of extremely-broad profiles ascribable þ to the 1Bþ u ð0Þ þ 3Ag ð1Þ and 1Bu ð1Þ þ 3Ag ð2Þ diabatic pairs (sed’s) grow and still remains in high intensity. Phase IV (0.12–0.22 ps): The broad 1B u transient absorption transforms into the more T1 transient absorptions and inclearly-peaked 2A g and the Tn crease in intensity. Phase V (0.22–1.50 ps): The 2A g transient T1 transient absorption become saturated absorption and the Tn and start to decrease in intensity. Also, a pair of stimulated emission þ from the 1Bþ u ð0Þ þ 3Ag ð1Þ and 1Bu ð1Þ þ 3Ag ð2Þ diabatic pairs (sed’s) decrease in intensity and transform into the weak bleaching of the þ 1A 1A 1Bþ u ð0Þ g ð0Þ and the 1Bu ð1Þ g ð0Þ ground-state absorptions (bga’s). Phase VI (1.50–5.00 ps): Finally, the 2A g transient T1 absorption remains with a vibraabsorption decays and the Tn 1A tional structure together with the bleaching of the 1Bþ u ð0Þ g ð0Þ ð1Þ 1A ð0Þ ground-state absorption (bga’s). and 1Bþ u g
1Bþ u ð1Þ þ 3Ag ð2Þ, appears and decays. Second, the 1Bu transient absorption appears and transforms into the 2Ag transient absorpT1 absorption. Finally, the 2A tion and the Tn g transient absorpT1 absorption remains with the weak tion decays and the Tn þ 1A 1A bleaching of the 1Bþ u ð0Þ g ð0Þ and 1Bu ð1Þ g ð0Þ groundstate absorptions. The above spectral changes lead us to propose the relaxation scheme shown in Scheme 1, which includes three steps: The first step is the coherent excitation of the diabatic pair, i.e., þ 1Bþ u ð0Þ þ 1Bu ð1Þ in neurosporene (n = 9), 1Bu ð0Þ þ 1Bu ð2Þ in sphe ð0Þ þ 3A ð1Þ in lycopene (n = 11). This roidene (n = 10) and 1Bþ u g coherent excitation of the optically-allowed 1Bþ u ð0Þ and the isoenergetic optically-forbidden counterparts gives rise to the longlived stimulated emission (coherent cross-term) as well as the short-lived pair of stimulated emission (incoherent split terms). Stimulated emission from the optically-forbidden counterpart takes place after instantaneous vibrational relaxation (VR). In this scheme, the three different stimulated emissions are shown by wavy downward arrows with hm. This step has been already shown in the same set of Cars in THF solution (see Scheme 1 of Ref. [8] as well as Eqs. (48) and (53) of Ref. [7]). The second step is internal conversion from 1Bþ u ð0Þ to 1Bu ð0Þ; ð0Þ ! strictly speaking, this includes quasi-iso-energetic 1Bþ u ðnÞ internal conversion followed by 1B ðnÞ ! 1B ð0Þ vibra1B u u u tional relaxation (see the energy diagram in Fig. S1 of Ref. [12]). In the present scheme, the processes of internal conversion followed by vibrational relaxation are inclusively termed as ‘internal conversion’ (IC with a broken arrow). The last step is a parallel transformation from 1B u ð0Þ to 2Ag ð0Þ and T1(0) through ‘internal conversion’ (IC) and singlet heterofission (SF), respectively.
3.2. Relaxation schemes revealed by the characterization and SVD and global-fitting analysis of time-resolved spectra The spectral changes described in Section 3.1 can be briefly summarized as follows: Neurosporene and spheroidene. First, a set of three components, including a pair of independent 1Bþ u ð0Þ and 1Bu ð0Þ stimulated emission and stimulated emission from the diabatic pair, i.e., þ 1Bþ u ð0Þ þ 1Bu ð1Þ or 1Bu ð0Þ þ 1Bu ð2Þ, appears and decays. Second, transient absorption appears and then transforms into the 1B u T1 absorption. Finally, the 2A g transient absorption and the Tn T1 transient the 2A g transient absorption disappears, and the Tn absorption remains with bleaching of the ground-state absorption. Lycopene. First, a set of three components, including a pair of independent 1Bþ u ð0Þ and 3Ag ð0Þ stimulated emission and stimu lated emission from the diabatic pairs, i.e., 1Bþ u ð0Þ þ 3Ag ð1Þ and
Scheme 1.
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n 13
1Bu− 1Bu+
10
9
18
3A g–
16
1Bu–
14
12
0.04
0.05
1 / (2n + 1) Fig. 4. Comparison of the 1Bþ u ð0Þ, 1Bu ð0Þ and 3Ag ð0Þ energies determined for Cars bound to the LH2 complexes from Rba. sphaeroides G1C, 2.4.1 and Rsp. molischianum (crossed open symbols) and those determined for Cars free in THF solution (open symbols). The former data points are transferred from Fig. 2, whereas the latter data points from Fig. 5.
Rsp. molischianum lycopene (n = 11) 1Bu+ + 3Ag−
3Ag− 1
1
1Bu−
SADS
−
SADS
11
20
Rba. sphaeroides 2.4.1 spheroidene (n = 10) 1Bu+ + 1Bu−
12
1Bu+
Energy / 103 cm–1
It is to be noted that the 1B u state appears twice in the time-resolved spectra of neurosporene and spheroidene, i.e., as stimulated emission in the first step and as transient absorption in the second step (see Figs. 2, 3 and 5 and Scheme 1). They have different ori gins: the former originates from the 1Bþ u ð0Þ þ 1Bu ð1Þ and þ 1Bu ð0Þ þ 1Bu ð2Þ diabatic levels, whereas the latter, from the 1Bþ u ð0Þ level. In lycopene, only the transient absorption appears originating from the 1Bþ u ð0Þ level through internal conversion. We have tried to prove the above three steps by means of singular-value-decomposition (SVD) followed by global-fitting: Fig. 3 (left-hand side) shows the results of SVD and global-fitting analyses for spheroidene bound to the LH2 complex from Rba. sphaeroides 2.4.1. The species-associated difference spectra (SADS) show the following spectral components in a sequence. Compo þ nent 1: the incoherent 1B u ð0Þ ! 1Ag ð0Þ and the 1Bu ð0Þ ! 1Ag ð1Þ ð0Þ þ 1B ð2Þ and stimulated emission and the coherent 1Bþ u u 1Bþ u ð1Þ þ 1Bu ð3Þ diabatic stimulated emission (the latter is partly seen). The coherent 1Bþ u ð0Þ þ 1Bu ð2Þ diabatic stimulated emission is most probably overlapped with the incoherent 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission. Component 2: The 1Bu transient absorption and the remaining coherent stimulated þ emission from the 1Bþ u ð0Þ þ 1Bu ð2Þ and 1Bu ð1Þ þ 1Bu ð3Þ diabatic ð0Þ and the T T transient absorppairs. Component 3: The 2A n 1 g tions accompanying the bleaching of the ground-state þ 1A 1A 1Bþ u ð0Þ g ð0Þ and 1Bu ð1Þ g ð0Þ absorptions.
1Bu
2
2
2Ag−
2Ag−
T1
T1 3
3
16 18 20 Energy / 103 cm-1
16 18 20 Energy / 103 cm-1 1.0
1 57 fs 2 425 fs 3 2.3 ps
Population
Population
1.0
22
0.5
0.0
22
1 29 fs 2 94 fs 3 1.5 ps
0.5
0.0 0 Delay time / ps
0.5
0 Delay time / ps
0.5
Fig. 3. The results of singular-value-decomposition (SVD) followed by global-fitting for the LH2 antenna complexes from Rba. sphaeroides 2.4.1 and Rsp. molischianum, þ containing Cars (n = 10 and 11); two typical examples representing the coherent excitation of the 1Bþ u þ 1Bu and 1Bu þ 3Ag diabatic pair are shown.
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Fig. 3 (right-hand side) shows the SVD and global-fitting results for lycopene bound to the LH2 complex from Rsp. molischianum. The SADS show the following spectral components in a sequence. Component 1: The incoherent 3A g ð0Þ ! 1Ag ð0Þ stimulated emission and the coherent emission from the 1Bþ u ð0Þ + ð1Þ diabatic pair overlapped with the incoherent 3A g 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission. Component 2: The 1Bu transient absorption and the coherent stimulated emission from þ the 1Bþ u ð0Þ þ 3Ag ð1Þ and 1Bu ð1Þ þ 3Ag ð2Þ diabatic pairs. Compoand T T transient absorptions accompanying nent 3: The 2A n 1 g 1A and the bleaching of the ground-state 1Bþ u ð0Þ g ð0Þ þ 1Ag ð0Þ absorptions. 1Bu ð1Þ
3.3. Energies and excited-state dynamics of the 1B u and 3Ag states: relevance to the light-harvesting function
Let us now compare the energies and the excited-state dynamics of Cars (n = 9–11) between two different cases, i.e., one free in solution and the other bound to the LH2 antenna complexes:
Neurosporene (n = 9)
Fig. 4 compares the 1Bþ u ð0Þ, 1Bu ð0Þ and 3Ag ð0Þ energies determined in THF solution (open symbols) and those determined when bound to the LH2 antennae (crossed open symbols). It is totally unexpected for us that the low-energy shift upon binding of the Cars is slightly larger in the covalent 1B u ð0Þ or 3Ag ð0Þ levels than þ that in the ionic 1Bu ð0Þ level. The results suggest strong polarization of the Car conjugated chain due to direct contact to an ionic amino-acid sidechain of the apo-peptide, although the twisting of the conjugated chain due to van der Waals interaction is also a possible reason for the low-energy shifts. Fig. 5 shows the 1Bþ u ð0Þ pump (30 fs) white-continuum probe time-resolved stimulated-emission and transient-absorption spectra of the same set of Cars (n = 9–11) free in THF solution (reproduced from Fig. 2 of Ref. [8]). Spectral comparison with Fig. 2 reveals the following unique characteristics of the excited-state dynamics when bound to the LH2 antenna complexes: (i) The initial coherent excitation of the diabatic pair took place as in the free case in solution. The uniqueness of LH2 bound Cars, however, is the substantially higher intensity of stimulated emis sion from the optically-forbidden 1B u ð0Þ and 3Ag ð0Þ levels and
Spheroidene (n = 10)
ΔOD = 0.03
Lycopene (n = 11)
ΔOD = 0.18
1B u+
ps –0.04
ΔOD = 0.03
1B u+
1B u+
–0.02 0.00 0.02
1B u+
0.04
1B u−
0.06 3A g−
0.08
1B u− 1B u+
0.10 0.12 0.14 0.16 0.18 0.20
1B u−
0.22 0.24 0.26
3A g−
0.28
1B u−
0.30
1B u−
0.45 0.60 0.90 1.20 1.50 3.00 4.00
2A g−
18
2A g−
20
Energy / 103 cm–1
22
18
5.00
2A g−
20
Energy / 103 cm–1
16
18
20
Energy / 103 cm–1
Fig. 5. The 1Bþ u ð0Þ pump and visible probe (both using 30 fs pulses) time-resolved stimulated-emission and transient-absorption spectra of Cars (n = 9–11) free in THF þ solution. (This set of spectra have been reproduced from Fig. 2 of Ref. [8] after correction of the assignment of the 1B u and 1Bu stimulated-emission peaks in neurosporene).
R. Christiana et al. / Chemical Physics Letters 480 (2009) 289–295
the much larger bandwidths (or even splitting) of stimulated emission from the counterparts of the diabatic pair, i.e., the opti cally-allowed 1Bþ u ð0Þ and the optically-forbidden 1Bu or 3Ag counterparts. The pair of observations strongly supports the above ideas of the polarization of the conjugated chain and the resultant stronger diabatic electronic mixing. (ii) The overall relaxation dynamics, in particular the decay of the initial stimulated emission due to the coherent diabatic excitation, is much faster in the Cars bound to LH2 than in solution. Obviously, this is the result of the branching of the relaxation pathway into the efficient Car-to-BChl singlet-energy transfer. (iii) The strong and broad stimulated emission from the diabatic pairs decays faster when bound to the LH2 antennae; this fact strongly suggests that the diabatic pairs also contribute the Car-to-BChl singlet-energy transfer. (iv) The most conspicuous change in the excited-state dynamics, upon binding of the Cars, is the efficient triplet generation. The time-resolved spectra show that this is due to an intramolecular reaction, i.e., the singlet heterofission reaction of the 1B u singlet state [11]. The high efficiency of triplet generation suggests a substantial twisting around the C@C bond(s) in the Car conjugated chain. 4. Conclusion The following answers to the questions, addressed in Introduction (Section 1), have been obtained: (1) Yes, it does. When bound to antennae, stimulated emission from the optically-forbidden 1B u and 3Ag counterparts is stronger, and stimulated emission from the 1Bþ u + 1Bu or 3Ag diabatic pair is broader in comparison to the free case in solution. The observed facts suggest the polarization of the Car conjugated chain and the resultant enhancement of diabatic coupling. (2) Yes, it does. Surprisingly, the low-energy shifts in the covalent states are slightly larger than in the ionic state. This result supports the above idea of the polarization of the Car conjugated chain.
295
(3) It is different. The relaxation dynamics is much faster when bound, reflecting the efficient Car-to-BChl singlet-energy transfer. The most striking difference, however, is the highly efficient triplet generation from the 1B u state through singlet heterofission. This finding strongly suggests the twisting of the Car conjugated chain due to strong intermolecular interaction between Car and apo-peptide. Acknowledgements The authors are grateful to Mr. Kuniaki Kobayashi of Excel Technology, Japan for supporting the construction of a system of whitecontinuum generation. This work has been supported by a grant ‘Open Research Center Project’ from the Ministry of Education, Culture, Sports, Science and Technology, Japan (to Y. Koyama); a Beasiswa Unggulan Scholarship from the Ministry of National Education, Republic of Indonesia (to R. Christiana); and Fundamental Research Project from Ministry of Research and Technology, Indonesia No.: 27/RD/Insentif/PPK/II/2008 (to L. Limantara). References [1] Y. Koyama, Y. Kakitani, in: B. Grimm, R.J. Porra, W. Rüdiger, H. Scheer (Eds.), Chlorophyll and Bacteriochlorophylls: Biochemistry, Biophysics, Functions and Applications, Springer, The Netherlands, 2006 (Chapter 30). [2] Y. Koyama, Y. Kakitani, Y. Watanabe, in: G. Renger (Ed.), Primary Processes of Photosynthesis – Part 1, RCS Publishing, Cambridge, 2008 (Chapter 5). [3] K. Furuichi, T. Sashima, Y. Koyama, Chem. Phys. Lett. 356 (2002) 547. [4] P. Wang et al., Chem. Phys. Lett. 410 (2005) 108. [5] F.S. Rondonuwu, Y. Kakitani, H. Tamura, Y. Koyama, Chem. Phys. Lett. 429 (2006) 234. [6] M. Ikuta, A. Yabushita, F.S. Rondonuwu, J. Akahane, Y. Koyama, T. Kobayashi, Chem. Phys. Lett. 422 (2006) 95. [7] H. Nagae, Y. Kakitani, Y. Koyama, Chem. Phys. Lett. 474 (2009) 342. [8] T. Miki, Y. Kakitani, Y. Koyama, H. Nagae, Chem. Phys. Lett. 457 (2008) 222. [9] R.J. Cogdell, I. Durant, J. Valentine, J.G. Lindsay, K. Schmidt, Biochim. Biophys. Acta 722 (1983) 427. [10] C. Li, T. Miki, Y. Kakitani, Y. Koyama, H. Nagae, Chem. Phys. Lett. 450 (2007) 112. [11] F.S. Rondonuwu, Y. Watanabe, R. Fujii, Y. Koyama, Chem. Phys. Lett. 376 (2003) 292. [12] Y. Kakitani, T. Miki, Y. Koyama, H. Nagae, R. Nakamura, Y. Kanematsu, Chem. Phys. Lett. 447 (2009) 194.
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Energies and excited-state dynamics of 1Bþ u , 1Bu and 3Ag states of carotenoids bound to LH2 antenna complexes from purple photosynthetic bacteria
Rebecca Christiana a,b, Takeshi Miki a, Yoshinori Kakitani a, Shiho Aoyagi a, Yasushi Koyama a,*, Leenawaty Limantara b a b
Faculty of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda 669-1337, Japan Research Center for Photosynthetic Pigments, Ma Chung University, Villa Puncak Tidar, Malang 65151, Indonesia
a r t i c l e
i n f o
Article history: Received 2 August 2009 In final form 29 August 2009 Available online 1 September 2009
a b s t r a c t Time-resolved pump–probe stimulated-emission and transient-absorption spectra were recorded after excitation with 30 fs pulses to the 1Bþ u ð0Þ and optically-forbidden diabatic levels of carotenoids, neurosporene, spheroidene and lycopene having n = 9–11 double bonds, bound to LH2 antenna complexes from Rhodobacter sphaeroides G1C, 2.4.1 and Rhodospirillum molischianum. The low-energy shift of stimulated þ emission from the covalent 1B u ð0Þ and 3Ag ð0Þ levels slightly larger than that from the ionic 1Bu ð0Þ state suggests the polarization, whereas more efficient triplet generation suggests the twisting of the conjugated chain in Cars bound to the LH2 complexes, when compared to Cars free in solution. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the first identification of the optically-forbidden 1B u and 3A g states of carotenoids (Cars), in addition to the optically allowed 1Bþ u and the optically-forbidden 2Ag states, with respect to transitions from/to the ground 1Ag state (Fig. 1a), the excitedstate dynamics and the roles of these low-lying singlet states in the light-harvesting function have been studied extensively (see reviews, Refs. [1,2]). The 1B u forbidden state of Cars, having the number of conjugated double bonds n = 9–13, were systematically determined by measurement of resonance-Raman excitation profiles (RREPs) [3]. The conjugation-length (n) dependence of the 1B u energy was confirmed by fluorescence spectroscopy [3] and by electronic-absorption spectroscopy [4]. The singlet internal-conversion and the singlet to triplet fission processes from the 1B u state were identified, and the role of this particular state in the light-harvesting function has been determined. The Car-to-bacteriochlorophyll (BChl) singlet-energy transfer ð1B u ! Q x and 2Ag ! Q y Þ has turned out to play a crucial role in determining the singlet-energy transfer efficiency [1]. The 1B u state is formerly optically-forbidden, but it exhibits fluorescence due to its strong electronic coupling with the optically-allowed 1Bþ u state that is located close by. The 3A g state of Cars (n = 9–13) were also determined by the RREP measurement [3], and their internal-conversion processes were identified by subpicosecond time-resolved Raman spectroscopy of Cars (n = 9–11) [2,5] and by 5-fs pump–probe spectroscopy
* Corresponding author. Fax: +81 79 565 9077. E-mail address: [email protected] (Y. Koyama). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.08.074
of Cars (n = 11–13) [2,6]. The light-harvesting function of this state still remains to be revealed. Recently, the unique excited-state properties of the pairs of optically-allowed 1Bþ u and optically-forbidden 1Bu or 3Ag vibronic levels, energetically overlapped with each other, have been found (Fig. 1b). The pair of vibronic levels are called ‘the diabatic pair’, because the diabatic, rather than adiabatic, basis set needs to be used to describe its excited-state properties (see Ref. [7] for a summary of experimented results followed by theoretical analyses). Upon incoherent excitation, using 100 fs pulses, to the 1Bþ u ð0Þ level of neurosporene (n = 9) and spheroidene (n = 10), stimulated emis þ sion took place from the 1Bþ u ð0Þ þ 1Bu ð1Þ and 1Bu ð0Þ þ 1Bu ð2Þ diabatic vibronic levels, respectively. On the other hand, stimulated emission took place from the 1Bþ u ð0Þ counterpart, but no stimulated emission was observed from the diabatic counterpart, 3A g ð1Þ, 3Ag ð2Þ and 3Ag ð3Þ levels of lycopene (n = 11), anhydrorhodovibrin (n = 12) and spirilloxanthin (n = 13), respectively. Thus, the following selection rule for the diabatic electronic mixing þ has been determined: ‘1Bþ u -to-1Bu is allowed, but 1Bu -to-3Ag is forbidden’. This selection rule holds true in the internal-conversion processes, as well. Here, Cars dissolved in non-polar solvents were incoherently excited with 100 fs pulses. Most recently, completely-different excited-state dynamics have been found when the diabatic pairs of the optically-allowed 1Bþ u ð0Þ and the optically-forbidden 1Bu or 3Ag counterparts are coherently excited by the use of 30 fs pulses [7,8]. This timelyshort but spectrally-broad pulses can excite simultaneously and coherently the pair of diabatic levels located close by. Here, Cars were dissolved in a polar solvent, THF, to induce the polarization of the conjugated chain. We found that the stimulated emission
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n
a
13
12
11
10
9
20
Qx B800 B850
3A g–(0)
3
–1 Energy / 10 cm
1Bu+(0)
16 1Bu–(0)
2A g–(0)
Qy
12
0.04
B800 B850
0.05
2. Experimental
1 / (2n + 1) n
b
13
12
11
20
3
–1 Energy / 10 cm
1B 4
13
1 2
3
02
0 1
2
1
0
10
+ u
9
3
1
2
0
1
2
1
1
0
0
0 1 0
1Bu–
3A g–
16
1
0
0
0
0
LH2 complexes from Rba. sphaeroides G1C, Rba. sphaeroides 2.4.1 and Rsp. molischianum have been prepared as described previously [9] with some modifications. The set-up and the method of femtosecond time-resolved absorption spectroscopy are as described previously (see Supporting Information of Ref. [10]) except for some modification for white-continuum generation: The strongest 550-nm laser pulse from a TOPAS-White (Quantronix) of lower energy was focused onto a 1-mm glass plate to generate a white-continuum probe pulse by the use of a pair of concave reflection mirrors with a focal length of 5 cm. As a result, the full width at half-maximum (FWHM) of the cross-correlation traces between the pump and probe pulses (30 fs) was determined to be 60 fs by the use of optical Kerr-effect signal. 3. Results and discussion
0
12
intensity in lycopene (see Fig. 4 of Ref. [8]). In addition to the rap idly-decaying 1B u ð0Þ and 3Ag ð0Þ emission, the long-lived emission þ þ 1B or 1B þ 3A from the 1Bþ u u u g diabatic levels may also contribute to the Car-to-BChl singlet-energy transfer. Importantly, not only the 1B u state but also the 3Ag state can play a role in the light-harvesting function, when coherently excited. In the present investigation, we have tried to examine if the same type of observation can be obtained in neurosporene, spheroidene and lycopene (n = 9–11) that are bound to the real LH2 antenna complexes from purple photosynthetic bacteria, Rhodobacter sphaeroides G1C, 2.4.1 and Rhodospirillum molischianum. We have addressed the following three specific questions: (1) Does the same type of coherent excitation of the diabatic vibronic levels take place in the set of Cars bound to the LH2 antenna complexes? (2) Does stimulated emission from the covalent 1B u and 3Ag states state due shift to the lower energies, as that from the ionic 1Bþ u to the Car to apo-peptide intermolecular interaction? (3) Is the excited-state dynamics of Cars bound to the antenna complexes in a way similar to, or different from, that of Cars free in solution [8]?
3.1. Characterization of deca-femtosecond time-resolved, stimulatedemission and transient-absorption spectra
0.04
0.05 1 / (2n + 1)
Fig. 1. (a) Energies of the 1Bþ u ð0Þ, 3Ag ð0Þ, 1Bu ð0Þ and 2Ag ð0Þ levels determined by measurement of resonance-Raman excitation profiles of crystalline Cars (n = 9–13). The energies of the Qx and Qy levels of B800 and B850 BChl a in the LH2 antenna complexes are shown for comparison. On going from n = 10 to 11, as expected by the relative energies, the 1B u ! Q x and 2Ag ! Q y Car-to-BChl singlet energytransfer channels become closed [1,2]. (b) Overlap of vibrational ladders (a spacing of 1400 cm1 is assumed) starting from the 1Bþ u ð0Þ, 3Ag ð0Þ and 1Bu ð0Þ origins determined as linear functions of 1/(2n + 1) in (a). The 1Bþ u ð0Þ level is overlapped completely or approximately, with the 1B u ð1Þ and 1Bu ð2Þ levels in Cars (n = 9 and 10), and with the 3A g ð1Þ, 3Ag ð2Þ and 3Ag ð3Þ levels in Cars (n = 11–13), respectively.
composed of three components that are described in terms of the quantum-beat equation: (i) rapidly-decaying 1Bþ u ð0Þ emission (the optically-allowed counterpart), (ii) rapidly-decaying 1B u ð0Þ or 3A g ð0Þ emission (the optically-forbidden counterpart after instaneous vibrational relaxation), and (iii) long-standing stimulated emission from the diabatic pair, i.e., the 1Bþ u ð0Þ optically-allowed and the iso-energetic optically-forbidden 1B u or 3Ag counterparts overlapped with each other. The three components correspond to a pair of incoherent, independent terms and the coherent cross-term in the quantum-beat expression (see Eq. (48) of Ref. [7], for example). Actually, we detected an oscillatory change in fluorescence
Fig. 2 shows the 1Bþ u ð0Þ pump white-continuum probe time-resolved stimulated-emission and transient-absorption spectra of neurosporene, spheroidene and lycopene (n = 9–11) bound to the LH2 antenna complexes from Rba. sphaeroides G1C, 2.4.1 and Rsp. molischianum, respectively. (See the caption for the specification of observed peaks. In particular, the negative signals with energies around the 1Bþ u level are classified into three different categories depending on the delay times.) The time-resolved spectra can be characterized in terms of different phases according to the delay time, after coherent excitation to the 1Bþ u ð0Þ plus iso-energetic 1B u or 3Ag diabatic pair: The 1Bþ u ð0Þ þ 1Bu ð1Þ pair of neurosporene (Rba. sphaeroides G1C). Phase I (0.04 to 0.00 ps): A sharp and weak 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission (labeled ‘se’) mainly ap and pears. Phase II (0.00–0.12 ps): The 1Bþ u ð0Þ ! 1Ag ð1Þ 1Bu ð0Þ ! 1Ag ð0Þ stimulated emissions appear and increase in intensity. The sharp and stronger 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission suddenly increases in intensity and bandwidth trans forming into stimulated emission from the 1Bþ u ð0Þ þ 1Bu ð1Þ diabatic pair (‘sed’). All of these three peaks reach maxima and start to decay. Phase III (0.12–0.18 ps): Extremely-rapid and drastic changes in the spectral pattern take place in this time region: the stimulated emission is replaced by the 1B u ð0Þ ! 1Ag ð0Þ
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Rba. sphaeroides 2.4.1 spheroidene (n = 10)
Rba. sphaeroides G1C neurosporene (n = 9)
ps
1Bu+
ΔOD = 0.02
ΔOD = 0.05
se
(0
(0
I
1)
Rsp. molischianum lycopene (n = 11)
ΔOD = 0.05
1)
1Bu+
1Bu- 1Bu+
I
–0.04
I
–0.02
1Bu- 1Bu+
(1 0)
se
1Bu+ 1Bu
II II
+
3Agsed
sed
1Bu-
0.06 0.08
1Bu-
0.10
III
0.12
T1
T1
0.14
1Bu-
III
T1
0.02 0.04
II sed
III
0.00
0.16
IV
IV 2Ag-
0.18
2Ag-
0.20 2Ag
0.22
-
0.24
IV
0.26 0.28 0.30
V
V
0.45 0.60
V
0.90 bga
bga
T1
bga
bga
T1
T1
1.20 1.50 3.00
VI
VI
VI
4.00 5.00
16
18 20 Energy / 103 cm–1
16
18 20 Energy / 103 cm–1
16 18 20 Energy / 103 cm–1
Fig. 2. The 1Bþ u ð0Þ pump and the visible white-continuum probe time-resolved stimulated-emission and transient-absorption spectra of LH2 antenna complexes from Rba. sphaeroides G1C, 2.4.1 and Rsp. molischianum, binding Cars (n = 9–11). On the top of each panel, the ground-state absorption spectrum is shown for comparison. Different phases of relaxation processes, classified by the spectral patterns, are shown on the left-hand side of each panel. Stimulated emission is shown in this figure by downward arrows with labels that assign the starting electronic state, where the initial and final vibrational levels are specified when necessary; the transition without arrows should be considered to be the vibrationally t = 0 ? t = 0 transition. The negative signal with a 1Bþ u ð0Þ energy is assigned, depending on the delay time, as stimulated emission from the þ þ þ 1A 1A 1Bþ g ð0Þ or 1Bu ð1Þ g ð0Þ u ð0Þ level (se) ? stimulated emission from the 1Bu ð0Þ þ 1Bu or 3Ag diabatic levels (sed) ? the bleaching of the ground-state 1Bu ð0Þ absorptions (bga); here, ‘se’ stands for stimulated emission, ‘sed’, stimulated emission from diabatic levels, and ‘bga’, bleaching of the ground-state absorption. On the other hand, transient absorption is shown by upward arrows with labels that assign the starting electronic state. Presumably, the ‘1B u ’ and ‘2Ag ’ transient absorptions are to be þ 1B 2A assigned to the mAþ g g transitions; the two transitions are similar in energy but the peak is broad in the former and sharp in the latter. The ‘T1’ transient u and nBu T1 transition. absorption should be due to the Tn
nAþ 1B u transient absorption (hereafter, simply abbreviated as g ‘1Bu transient absorption’). The 1Bþ u ð0Þ ! 1Ag ð1Þ stimulated emission decreases in intensity and disappears. Most importantly, a new profile appears in-between the 1B u ð0Þ transient absorption and the persistent stimulated emission from the 1Bþ u ð0Þ þ 1Bu ð1Þ T1 absorption diabatic pair (sed), which is ascribable to the Tn 3 reflecting the 1B u ð0Þ ! Bu ðT 1 Þ singlet heterofission reaction [11]. Phase IV (0.18–0.30 ps): Gradual transformation takes place from the broad and round peak of the 1B u transient absorption to the sharper and clear peak of the 2A g transient absorption. T1 absorption increases in intensity and bandwidth, while The Tn the 1Bþ u ð0Þ þ 1Bu ð1Þ diabatic stimulated emission (sed) decreases
in intensity and gradually sharpens up. Phase V (0.30–1.50 ps): T1 transient-absorpThe integrated intensity of the 2A g and Tn tion profiles reach maxima and start to decay. The stimulated emission from the diabatic pair transforms into pure bleaching of 1A the 1Bþ u ð0Þ g ð0Þ ground-state absorption (‘bga’) decreasing in intensity. Phase VI (1.50–5.00 ps): The 2A g transient absorption T1 trandecreases in intensity and disappears, and only the Tn sient absorption remains together with negligible bleaching of 1A the 1Bþ u ð0Þ g ð0Þ absorption (bga). þ The 1Bu ð0Þ þ 1B u ð2Þ pair of spheroidene (Rba. sphaeroides 2.4.1). Phase I (0.04 to 0.00 ps): The strong and sharp 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission (se) and the weak and broad
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1Bþ u ð0Þ ! 1Ag ð1Þ stimulated emission appear together with the 1Bu ð0Þ ! 1A g ð0Þ stimulated emission. Phase II (0.00–0.06 ps): The 1Bþ u ð0Þ ! 1Ag ð0Þ emission (se) substantially broadens transferring into stimulated emission from the 1Bþ u ð0Þ þ 1Bu ð2Þ diabatic pair ð0Þ ! 1A ð1Þ emission and the 1B (sed). The 1Bþ u u ð0Þ ! 1Ag ð0Þ g emission increase in intensity, become saturated, and start to de crease. Phase III (0.06–0.10 ps): The 1B u ð0Þ ! 1Ag ð0Þ stimulated emission decreases in intensity and disappears, while the 1Bþ u ð0Þ ! 1Ag ð1Þ stimulated emission is replaced by the 1Bu ð0Þ transient absorption. Stimulated emission from the ð0Þ þ 1B ð2Þ diabatic pair (sed) remains as a broad profile. Phase 1Bþ u u IV (0.10–0.24 ps): The round 1B u transient absorption transforms into the clearly-peaked 2A g transient absorption. Slightly-sharp ened 1Bþ u ð0Þ þ 1Bu ð2Þ diabatic emission (sed) persists in this time T1 absorption appears in-between these region. Again, the Tn two peaks and increases in intensity. Phase V (0.24–1.50 ps): The 2A g and T1 transient absorptions increase in the integrated intensity and become saturated. The diabatic stimulated emission from the 1Bþ u ð0Þ þ 1Bu ð2Þ pair (sed) transforms into the weaker bleaching þ 1A of the 1Bu ð0Þ g ð0Þ ground-state absorption (bga). Phase VI (1.50–5.00 ps): The 2A g transient absorption decreases in intensity T1 absorption decays but stays with the to disappear, and the Tn 1A bleaching of the 1Bþ u ð0Þ g ð0Þ ground-state absorption (bga). ð0Þ þ 3A ð1Þ pair of lycopene (Rsp. molischianum). Phase The 1Bþ u g I (0.04 to 0.02 ps): Broad stimulated emission due to the þ 1Bþ u ð0Þ ! 1Ag ð0Þ and 1Bu ð1Þ ! 1Ag ð0Þ transitions (se) mainly appear together with a stimulated emission in the lower-energy region (possibly due to the stimulated emission from the Qx level of BChl). Phase II (0.02–0.08 ps): The 3A g ð0Þ stimulated emission emerges and a pair of broad and strong profiles assignable to the þ 1Bþ u ð0Þ ! 1Ag ð0Þ and 1Bu ð1Þ ! 1Ag ð0Þ stimulated emission appear and grow in intensity. Phase III (0.08–0.12 ps): The 3A g ð0Þ ! 1Ag ð0Þ stimulated emission becomes replaced by the transient absorption and the Tn T1 absorption in a simbroad 1B u ilar spectral region. The pair of extremely-broad profiles ascribable þ to the 1Bþ u ð0Þ þ 3Ag ð1Þ and 1Bu ð1Þ þ 3Ag ð2Þ diabatic pairs (sed’s) grow and still remains in high intensity. Phase IV (0.12–0.22 ps): The broad 1B u transient absorption transforms into the more T1 transient absorptions and inclearly-peaked 2A g and the Tn crease in intensity. Phase V (0.22–1.50 ps): The 2A g transient T1 transient absorption become saturated absorption and the Tn and start to decrease in intensity. Also, a pair of stimulated emission þ from the 1Bþ u ð0Þ þ 3Ag ð1Þ and 1Bu ð1Þ þ 3Ag ð2Þ diabatic pairs (sed’s) decrease in intensity and transform into the weak bleaching of the þ 1A 1A 1Bþ u ð0Þ g ð0Þ and the 1Bu ð1Þ g ð0Þ ground-state absorptions (bga’s). Phase VI (1.50–5.00 ps): Finally, the 2A g transient T1 absorption remains with a vibraabsorption decays and the Tn 1A tional structure together with the bleaching of the 1Bþ u ð0Þ g ð0Þ ð1Þ 1A ð0Þ ground-state absorption (bga’s). and 1Bþ u g
1Bþ u ð1Þ þ 3Ag ð2Þ, appears and decays. Second, the 1Bu transient absorption appears and transforms into the 2Ag transient absorpT1 absorption. Finally, the 2A tion and the Tn g transient absorpT1 absorption remains with the weak tion decays and the Tn þ 1A 1A bleaching of the 1Bþ u ð0Þ g ð0Þ and 1Bu ð1Þ g ð0Þ groundstate absorptions. The above spectral changes lead us to propose the relaxation scheme shown in Scheme 1, which includes three steps: The first step is the coherent excitation of the diabatic pair, i.e., þ 1Bþ u ð0Þ þ 1Bu ð1Þ in neurosporene (n = 9), 1Bu ð0Þ þ 1Bu ð2Þ in sphe ð0Þ þ 3A ð1Þ in lycopene (n = 11). This roidene (n = 10) and 1Bþ u g coherent excitation of the optically-allowed 1Bþ u ð0Þ and the isoenergetic optically-forbidden counterparts gives rise to the longlived stimulated emission (coherent cross-term) as well as the short-lived pair of stimulated emission (incoherent split terms). Stimulated emission from the optically-forbidden counterpart takes place after instantaneous vibrational relaxation (VR). In this scheme, the three different stimulated emissions are shown by wavy downward arrows with hm. This step has been already shown in the same set of Cars in THF solution (see Scheme 1 of Ref. [8] as well as Eqs. (48) and (53) of Ref. [7]). The second step is internal conversion from 1Bþ u ð0Þ to 1Bu ð0Þ; ð0Þ ! strictly speaking, this includes quasi-iso-energetic 1Bþ u ðnÞ internal conversion followed by 1B ðnÞ ! 1B ð0Þ vibra1B u u u tional relaxation (see the energy diagram in Fig. S1 of Ref. [12]). In the present scheme, the processes of internal conversion followed by vibrational relaxation are inclusively termed as ‘internal conversion’ (IC with a broken arrow). The last step is a parallel transformation from 1B u ð0Þ to 2Ag ð0Þ and T1(0) through ‘internal conversion’ (IC) and singlet heterofission (SF), respectively.
3.2. Relaxation schemes revealed by the characterization and SVD and global-fitting analysis of time-resolved spectra The spectral changes described in Section 3.1 can be briefly summarized as follows: Neurosporene and spheroidene. First, a set of three components, including a pair of independent 1Bþ u ð0Þ and 1Bu ð0Þ stimulated emission and stimulated emission from the diabatic pair, i.e., þ 1Bþ u ð0Þ þ 1Bu ð1Þ or 1Bu ð0Þ þ 1Bu ð2Þ, appears and decays. Second, transient absorption appears and then transforms into the 1B u T1 absorption. Finally, the 2A g transient absorption and the Tn T1 transient the 2A g transient absorption disappears, and the Tn absorption remains with bleaching of the ground-state absorption. Lycopene. First, a set of three components, including a pair of independent 1Bþ u ð0Þ and 3Ag ð0Þ stimulated emission and stimu lated emission from the diabatic pairs, i.e., 1Bþ u ð0Þ þ 3Ag ð1Þ and
Scheme 1.
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R. Christiana et al. / Chemical Physics Letters 480 (2009) 289–295
n 13
1Bu− 1Bu+
10
9
18
3A g–
16
1Bu–
14
12
0.04
0.05
1 / (2n + 1) Fig. 4. Comparison of the 1Bþ u ð0Þ, 1Bu ð0Þ and 3Ag ð0Þ energies determined for Cars bound to the LH2 complexes from Rba. sphaeroides G1C, 2.4.1 and Rsp. molischianum (crossed open symbols) and those determined for Cars free in THF solution (open symbols). The former data points are transferred from Fig. 2, whereas the latter data points from Fig. 5.
Rsp. molischianum lycopene (n = 11) 1Bu+ + 3Ag−
3Ag− 1
1
1Bu−
SADS
−
SADS
11
20
Rba. sphaeroides 2.4.1 spheroidene (n = 10) 1Bu+ + 1Bu−
12
1Bu+
Energy / 103 cm–1
It is to be noted that the 1B u state appears twice in the time-resolved spectra of neurosporene and spheroidene, i.e., as stimulated emission in the first step and as transient absorption in the second step (see Figs. 2, 3 and 5 and Scheme 1). They have different ori gins: the former originates from the 1Bþ u ð0Þ þ 1Bu ð1Þ and þ 1Bu ð0Þ þ 1Bu ð2Þ diabatic levels, whereas the latter, from the 1Bþ u ð0Þ level. In lycopene, only the transient absorption appears originating from the 1Bþ u ð0Þ level through internal conversion. We have tried to prove the above three steps by means of singular-value-decomposition (SVD) followed by global-fitting: Fig. 3 (left-hand side) shows the results of SVD and global-fitting analyses for spheroidene bound to the LH2 complex from Rba. sphaeroides 2.4.1. The species-associated difference spectra (SADS) show the following spectral components in a sequence. Compo þ nent 1: the incoherent 1B u ð0Þ ! 1Ag ð0Þ and the 1Bu ð0Þ ! 1Ag ð1Þ ð0Þ þ 1B ð2Þ and stimulated emission and the coherent 1Bþ u u 1Bþ u ð1Þ þ 1Bu ð3Þ diabatic stimulated emission (the latter is partly seen). The coherent 1Bþ u ð0Þ þ 1Bu ð2Þ diabatic stimulated emission is most probably overlapped with the incoherent 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission. Component 2: The 1Bu transient absorption and the remaining coherent stimulated þ emission from the 1Bþ u ð0Þ þ 1Bu ð2Þ and 1Bu ð1Þ þ 1Bu ð3Þ diabatic ð0Þ and the T T transient absorppairs. Component 3: The 2A n 1 g tions accompanying the bleaching of the ground-state þ 1A 1A 1Bþ u ð0Þ g ð0Þ and 1Bu ð1Þ g ð0Þ absorptions.
1Bu
2
2
2Ag−
2Ag−
T1
T1 3
3
16 18 20 Energy / 103 cm-1
16 18 20 Energy / 103 cm-1 1.0
1 57 fs 2 425 fs 3 2.3 ps
Population
Population
1.0
22
0.5
0.0
22
1 29 fs 2 94 fs 3 1.5 ps
0.5
0.0 0 Delay time / ps
0.5
0 Delay time / ps
0.5
Fig. 3. The results of singular-value-decomposition (SVD) followed by global-fitting for the LH2 antenna complexes from Rba. sphaeroides 2.4.1 and Rsp. molischianum, þ containing Cars (n = 10 and 11); two typical examples representing the coherent excitation of the 1Bþ u þ 1Bu and 1Bu þ 3Ag diabatic pair are shown.
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Fig. 3 (right-hand side) shows the SVD and global-fitting results for lycopene bound to the LH2 complex from Rsp. molischianum. The SADS show the following spectral components in a sequence. Component 1: The incoherent 3A g ð0Þ ! 1Ag ð0Þ stimulated emission and the coherent emission from the 1Bþ u ð0Þ + ð1Þ diabatic pair overlapped with the incoherent 3A g 1Bþ u ð0Þ ! 1Ag ð0Þ stimulated emission. Component 2: The 1Bu transient absorption and the coherent stimulated emission from þ the 1Bþ u ð0Þ þ 3Ag ð1Þ and 1Bu ð1Þ þ 3Ag ð2Þ diabatic pairs. Compoand T T transient absorptions accompanying nent 3: The 2A n 1 g 1A and the bleaching of the ground-state 1Bþ u ð0Þ g ð0Þ þ 1Ag ð0Þ absorptions. 1Bu ð1Þ
3.3. Energies and excited-state dynamics of the 1B u and 3Ag states: relevance to the light-harvesting function
Let us now compare the energies and the excited-state dynamics of Cars (n = 9–11) between two different cases, i.e., one free in solution and the other bound to the LH2 antenna complexes:
Neurosporene (n = 9)
Fig. 4 compares the 1Bþ u ð0Þ, 1Bu ð0Þ and 3Ag ð0Þ energies determined in THF solution (open symbols) and those determined when bound to the LH2 antennae (crossed open symbols). It is totally unexpected for us that the low-energy shift upon binding of the Cars is slightly larger in the covalent 1B u ð0Þ or 3Ag ð0Þ levels than þ that in the ionic 1Bu ð0Þ level. The results suggest strong polarization of the Car conjugated chain due to direct contact to an ionic amino-acid sidechain of the apo-peptide, although the twisting of the conjugated chain due to van der Waals interaction is also a possible reason for the low-energy shifts. Fig. 5 shows the 1Bþ u ð0Þ pump (30 fs) white-continuum probe time-resolved stimulated-emission and transient-absorption spectra of the same set of Cars (n = 9–11) free in THF solution (reproduced from Fig. 2 of Ref. [8]). Spectral comparison with Fig. 2 reveals the following unique characteristics of the excited-state dynamics when bound to the LH2 antenna complexes: (i) The initial coherent excitation of the diabatic pair took place as in the free case in solution. The uniqueness of LH2 bound Cars, however, is the substantially higher intensity of stimulated emis sion from the optically-forbidden 1B u ð0Þ and 3Ag ð0Þ levels and
Spheroidene (n = 10)
ΔOD = 0.03
Lycopene (n = 11)
ΔOD = 0.18
1B u+
ps –0.04
ΔOD = 0.03
1B u+
1B u+
–0.02 0.00 0.02
1B u+
0.04
1B u−
0.06 3A g−
0.08
1B u− 1B u+
0.10 0.12 0.14 0.16 0.18 0.20
1B u−
0.22 0.24 0.26
3A g−
0.28
1B u−
0.30
1B u−
0.45 0.60 0.90 1.20 1.50 3.00 4.00
2A g−
18
2A g−
20
Energy / 103 cm–1
22
18
5.00
2A g−
20
Energy / 103 cm–1
16
18
20
Energy / 103 cm–1
Fig. 5. The 1Bþ u ð0Þ pump and visible probe (both using 30 fs pulses) time-resolved stimulated-emission and transient-absorption spectra of Cars (n = 9–11) free in THF þ solution. (This set of spectra have been reproduced from Fig. 2 of Ref. [8] after correction of the assignment of the 1B u and 1Bu stimulated-emission peaks in neurosporene).
R. Christiana et al. / Chemical Physics Letters 480 (2009) 289–295
the much larger bandwidths (or even splitting) of stimulated emission from the counterparts of the diabatic pair, i.e., the opti cally-allowed 1Bþ u ð0Þ and the optically-forbidden 1Bu or 3Ag counterparts. The pair of observations strongly supports the above ideas of the polarization of the conjugated chain and the resultant stronger diabatic electronic mixing. (ii) The overall relaxation dynamics, in particular the decay of the initial stimulated emission due to the coherent diabatic excitation, is much faster in the Cars bound to LH2 than in solution. Obviously, this is the result of the branching of the relaxation pathway into the efficient Car-to-BChl singlet-energy transfer. (iii) The strong and broad stimulated emission from the diabatic pairs decays faster when bound to the LH2 antennae; this fact strongly suggests that the diabatic pairs also contribute the Car-to-BChl singlet-energy transfer. (iv) The most conspicuous change in the excited-state dynamics, upon binding of the Cars, is the efficient triplet generation. The time-resolved spectra show that this is due to an intramolecular reaction, i.e., the singlet heterofission reaction of the 1B u singlet state [11]. The high efficiency of triplet generation suggests a substantial twisting around the C@C bond(s) in the Car conjugated chain. 4. Conclusion The following answers to the questions, addressed in Introduction (Section 1), have been obtained: (1) Yes, it does. When bound to antennae, stimulated emission from the optically-forbidden 1B u and 3Ag counterparts is stronger, and stimulated emission from the 1Bþ u + 1Bu or 3Ag diabatic pair is broader in comparison to the free case in solution. The observed facts suggest the polarization of the Car conjugated chain and the resultant enhancement of diabatic coupling. (2) Yes, it does. Surprisingly, the low-energy shifts in the covalent states are slightly larger than in the ionic state. This result supports the above idea of the polarization of the Car conjugated chain.
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(3) It is different. The relaxation dynamics is much faster when bound, reflecting the efficient Car-to-BChl singlet-energy transfer. The most striking difference, however, is the highly efficient triplet generation from the 1B u state through singlet heterofission. This finding strongly suggests the twisting of the Car conjugated chain due to strong intermolecular interaction between Car and apo-peptide. Acknowledgements The authors are grateful to Mr. Kuniaki Kobayashi of Excel Technology, Japan for supporting the construction of a system of whitecontinuum generation. This work has been supported by a grant ‘Open Research Center Project’ from the Ministry of Education, Culture, Sports, Science and Technology, Japan (to Y. Koyama); a Beasiswa Unggulan Scholarship from the Ministry of National Education, Republic of Indonesia (to R. Christiana); and Fundamental Research Project from Ministry of Research and Technology, Indonesia No.: 27/RD/Insentif/PPK/II/2008 (to L. Limantara). References [1] Y. Koyama, Y. Kakitani, in: B. Grimm, R.J. Porra, W. Rüdiger, H. Scheer (Eds.), Chlorophyll and Bacteriochlorophylls: Biochemistry, Biophysics, Functions and Applications, Springer, The Netherlands, 2006 (Chapter 30). [2] Y. Koyama, Y. Kakitani, Y. Watanabe, in: G. Renger (Ed.), Primary Processes of Photosynthesis – Part 1, RCS Publishing, Cambridge, 2008 (Chapter 5). [3] K. Furuichi, T. Sashima, Y. Koyama, Chem. Phys. Lett. 356 (2002) 547. [4] P. Wang et al., Chem. Phys. Lett. 410 (2005) 108. [5] F.S. Rondonuwu, Y. Kakitani, H. Tamura, Y. Koyama, Chem. Phys. Lett. 429 (2006) 234. [6] M. Ikuta, A. Yabushita, F.S. Rondonuwu, J. Akahane, Y. Koyama, T. Kobayashi, Chem. Phys. Lett. 422 (2006) 95. [7] H. Nagae, Y. Kakitani, Y. Koyama, Chem. Phys. Lett. 474 (2009) 342. [8] T. Miki, Y. Kakitani, Y. Koyama, H. Nagae, Chem. Phys. Lett. 457 (2008) 222. [9] R.J. Cogdell, I. Durant, J. Valentine, J.G. Lindsay, K. Schmidt, Biochim. Biophys. Acta 722 (1983) 427. [10] C. Li, T. Miki, Y. Kakitani, Y. Koyama, H. Nagae, Chem. Phys. Lett. 450 (2007) 112. [11] F.S. Rondonuwu, Y. Watanabe, R. Fujii, Y. Koyama, Chem. Phys. Lett. 376 (2003) 292. [12] Y. Kakitani, T. Miki, Y. Koyama, H. Nagae, R. Nakamura, Y. Kanematsu, Chem. Phys. Lett. 447 (2009) 194.