- Email: [email protected]
Energy transfer dynamics in light-harvesting small dendrimers studied by time-frequency two-dimensional imaging spectroscopy
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1898–1900
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Energy transfer dynamics in light-harvesting small dendrimers studied by time-frequency two-dimensional imaging spectroscopy A. Yamada a, A. Ishida a, I. Akai b, M. Kimura c, I. Katayama d, J. Takeda a, a
Department of Physics, Yokohama National University, Yokohama 240-8501, Japan Shock Wave and Condensed Matter Research Center, Kumamoto University, Kumamoto 860-8555, Japan c Department of Functional Polymer Science, Shinshu University, Ueda 386-8567, Japan d Interdisciplinary Research Center, Yokohama National University, Yokohama 240-8501, Japan b
a r t i c l e in f o
a b s t r a c t
Available online 8 May 2009
Rapid energy transfer dynamics in light-harvesting small dendrimers (star-shaped stilbenoid phthalocyanine: SSS2Pc-m, m ¼ 1,2) having different numbers of oligo(p-phenylenevinylene) periphery antennas was studied by time-frequency two-dimensional (2D) real-time pump-probe imaging spectroscopy; the dendrimers having one light-harvesting periphery antenna for each aromatic ring (SSS2Pc-1) are expected to maintain a planer structure, leading to good p-conjugation, while those having two periphery antennas for each aromatic ring (SSS2Pc-2) have a large steric hindrance between the periphery antennas. Under the selective excitation of the periphery antennas (400 nm), the timefrequency 2D image of the absorbance changes in SSS2Pc-1 shows the transient absorption due to the core around 500–600 nm and the ground-state bleaching of the Q-band around 700 nm with a rise time of 0.5 ps. The transient absorption also shows clear oscillatory components of 0.6 Thz, which might come from the torsional motion of the peripheries. These results show that rapid and highly efficient energy transfer occurs from the periphery antennas to the core, and that the torsional vibration plays an important role for the energy transfer. The experimental results obtained in SSS2Pc-1 are compared with those in SSS2Pc-2 having a large steric hindrance. & 2009 Elsevier B.V. All rights reserved.
PACS: 78.47.J 82.20.Rp 82.53.k Keywords: Femtosecond Imaging spectroscopy Dendrimer Rapid energy transfer
1. Introduction Light-harvesting dendrimers have attracted much interest since optical energy absorbed in light-harvesting antennas transfers to central core with high efficiency [1,2]. The lightharvesting dendrimers are therefore considered to be promising candidates for molecular-based devices and/or biomimetic systems for photosynthesis in future [3,4]. Recently, star-shaped small dendrimers having p-conjugated light-harvesting antennas were synthesized as a new class of light-harvesting materials [5], and rapid and highly efficient energy transfer from the periphery antennas to the core was observed [6,7]. In these small dendrimers, it is pointed out that not only the p-conjugation between the periphery antennas and core but also molecular vibrations such as torsional motions play an important role for the energy transfer both in experiments and theoretical calculation— recently reported [6–8]. In this study, in order to clarify importance of the pconjugation between the antennas and core on the energy transfer, ultrafast excited-state dynamics in light-harvesting small Corresponding author. Tel./fax: +81 45 339 3953.
E-mail address: [email protected] (J. Takeda). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.070
dendrimers having different numbers of the light-harvesting periphery antennas has been studied by time-frequency twodimensional (2D) real-time pump-probe imaging spectroscopy; the dendrimers having one light-harvesting periphery antenna for each aromatic ring are expected to maintain a planer structure, leading to good p-conjugation, while those having two periphery antennas for each aromatic ring has a larger steric hindrance between the periphery antennas.
2. Experimental Fig. 1 shows the molecular structures of light-harvesting small dendrimers (star-shaped stilbenoid phthalocyanine: SSS2Pc-m, m ¼ 1,2) and their component molecules, zinc phthalocyanine (Zn-Pc) and oligo(p-phenylenevinylene) (OPV2-m, m ¼ 1,2) [5]. Here, m indicates number of the light-harvesting periphery antennas per each aromatic ring of the core component. The abbreviations of the sample names are shown in the caption of Fig. 1. They were diluted in anhydrous tetrahydrofuran (THF) with a concentration of 104103 M. Each diluted solution of 0.5 cc was contained in a disk-shaped quartz cell with a thickness of 1 mm, and the cell was rotated during experiments to avoid
ARTICLE IN PRESS A. Yamada et al. / Journal of Luminescence 129 (2009) 1898–1900
A
1899
B ZnPc SSS2PC-m
N
N
N
B N
A
Zn
A
OR1 B
N
N
R1O
N N
OR1 R1O
B
A
NC
A
NC
B
OR1 OPV2-m
R1=C3H7
[L]=
Fig. 1. Molecular structures of small dendrimers (SSS2Pc-m), core (Zn-Pc) and peripheries (OPV2-m) (m ¼ 1,2). The abbreviations of the samples are listed as follows. Zn-Pc: A ¼ B ¼ H, SSS2Pc-2: A ¼ B ¼ [L], SSS2Pc-1: one of two branches for each aromatic ring is [L] while the other is H, OPV2-2: A ¼ B ¼ [L], OPV2-1: A ¼ H, B ¼ [L] or vice versa.
SSS2Pc-2 0.2
0.0
-0.15
Delay Time (ps)
Absorbance Change
10 0.15
SSS2Pc-1 Q-band 0.3 2.0x10-4 M
Q-band 1.2x10-4M x0.1
10
x0.09
ZnPc Q-band 1.2x10-3M 2 10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
0
-2
-2 500 600 700 Wavelength (nm)
x0.07
-2 500 600 700 Wavelength (nm)
500 600 700 Wavelength (nm)
Fig. 2. Time-frequency 2D images of transient absorbance changes of SSS2Pc-2, SSS2Pc-1 and Zn-Pc in THF solutions.
photodegradation and thermal heating of samples. Since quantities of SSS2Pc-m and OPV2-m were limited to less than 1 cc, conventional pump-probe technique, in which many repetitions of pump-probe sequence are necessary, is not feasible. For excited-state dynamics measurements in these samples, therefore, we employed real-time pump-probe imaging spectroscopy based on a single shot detection recently developed [9,10]. A second harmonic (400 nm) from a regenerative Ti:sapphire laser system with a 100 fs pulse duration and 1 kHz repetition rate was used as the excitation laser pulse, whose energy corresponds to the selective excitation of the periphery antennas. Using this spectroscopy, without many repetitions of pump-probe sequence, we could simultaneously map timefrequency 2D absorbance changes of materials in real-time with a wide spectral and temporal ranges of 300 nm and 6 ps, respectively. Since the accumulation time to measure multiple pump-probe data in region of interest is extremely reduced in comparison with that by the conventional pump-probe technique, this method is a powerful spectroscopic tool for observation of ultrafast excited-state dynamics in organic and biological materials, for which large quantities of samples are not readily available and/or photodegradation readily takes place. The detailed experimental setup is reported elsewhere [9,10].
3. Results and discussion Fig. 2 shows time-frequency 2D images of transient absorbance changes of SSS2Pc-2, SSS2Pc-1 and Zn-Pc dissolved in THF solutions with an accumulation of 1000 laser shots per unit frame. The 2D images were obtained by adding a few frames with different delay times and center-wavelengths in order to cover whole transient processes. As comparison, the steady-state absorption spectra are also shown in the upper figures. The dotted lines are visual guides for eyes to show the rise behavior of the transient signals. In SSS2Pc-m, from the steady-state absorption measurements, the sharp absorption band corresponding to Q-band of the core is located at 720–740 nm, while the broad absorption bands which are attributed to sum of the pp* transitions of the peripheries and Soret band (B-band) of the Zn-Pc core lie at 400 nm. Because the absorption intensity due to the peripheries is much higher than that due to the core around 400–500 nm, the selective excitation of the periphery antennas was achieved by the excitation of 400 nm (see hatched areas). In Zn-Pc, not only ground-state bleaching of the Q-band but also the transient absorption appears instantaneously at 670 and 420–520 nm, respectively, under the excitation of the B-band. In SSS2Pc-2 and SSS2Pc-1 dendrimers, on the other hand, even
ARTICLE IN PRESS 1900
A. Yamada et al. / Journal of Luminescence 129 (2009) 1898–1900
Absorbance Change
0.04
0.04
SSS2Pc-2 (600 nm) rise = 0.6 ± 0.05 ps
0.05 SSS2Pc-1 (600 nm) rise = 0.55 ± 0.05 ps
Zn-Pc (460nm) rise < 0.1 ps
0 0
0 0
0 0
SSS2Pc-2 (740 nm) rise = 0.6 ± 0.05 ps
SSS2Pc-1 (700 nm) rise = 0.55 ± 0.05 ps
0.4
Zn-Pc (660nm) rise < 0.1 ps
0.2 -2
0
0.5 2 4 6 Delay Time (ps)
8
10
-2
0
2 4 6 Delay Time (ps)
8
10
-2
0
2 4 6 Delay Time (ps)
8
10
Fig. 3. Time evolution of the absorbance changes in SSS2Pc-2, SSS2Pc-1 and Zn-Pc at the specific wavelengths. An instrumental response function of the system is shown by a thin dotted curve in the right-top figure. Fitted results for rising components are indicated by thick dotted lines. To compare the rise component of SSS2Pc-2 and SSS2Pc-1 dendrimers with that of Zn-Pc, the fitted line for Zn-Pc are plotted in every bottom figure.
under the selective excitation of the peripheries, ground-state bleaching of the Q-band as well as the transient absorption due to the core is clearly observed having a fast rise time, suggesting that a rapid energy transfer takes place from the light-harvesting periphery antennas to the central core. In order to clarify the rapid energy transfer, time evolution of the absorbance changes at the specific wavelengths in SSS2Pc-2, SSS2Pc-1 and Zn-Pc is shown in Fig. 3. The data are obtained from the 2D map in Fig. 2. The instrumental response function is shown by a thin dotted curve in the right-top figure. The rise times of the transient signals were evaluated from a fitting procedure by a single exponential rise convoluted with the instrumental response function as shown by thick broken lines. In Zn-Pc, the rise time of the transient absorption is less than 0.1 ps and is comparable with the pulse duration of the laser. This result shows that the internal relaxation from the B-band to the Q-band and the subsequent transient absorption from the Q-band to higher excited-states take place within the pulse duration [11]. In SSS2Pc-2 and SSS2Pc1 dendrimers, on the other hand, the rise time of the transient absorption as well as that of the ground-state bleaching due to the core is estimated to be 0.5–0.6 ps. Because the light-harvesting periphery antennas are selectively excited, the observed rise time of 0.5–0.6 ps presents a characteristic time for the rapid energy transfer from the periphery antennas to the core. In previous experimental and theoretical studies, not Fo¨rster mechanisms but short-range interactions caused by the wave function overlap between the excited-states of the peripheries and the core should be responsible for the energy transfer in light-harvesting small dendrimers [6,8]. The observed rapid energy transfer of 0.5–0.6 ps is in good agreement with the previous results. Most striking features of the transient signals in SSS2Pc-2 and SSS2Pc-1 dendrimers are as follows; the SSS2Pc-2 dendrimer having two periphery antennas for each aromatic ring does not show apparent oscillatory components, while the SSS2Pc-1 dendrimer having one periphery antenna has clear oscillatory components as shown by closed circles in Fig. 3. The oscillation observed is about 0.6 Thz (20 cm1), whose value is comparable with those due to the low-frequency torsional vibrations of the periphery antennas at the connecting point of the vinylene group: 8 cm1 for the ground state and 48 cm1 for the first excited state [12,13]. Akai et al. found that freezing of molecular vibrations quenches the energy transfer efficiency, and that temperature dependence of the energy transfer efficiency is well explained by an energy diagram considering the torsional vibration with tilt angle Y [6,14]. Moreover, theoretical calculation base on the timedependent Kohn-Sham equation shows that one-way electron and hole transfer from the periphery antennas to the core takes
place more easily in dendrimers with a planer structure than in those with steric hindrance since p-conjugation is well maintained in the planer structure [8]. These results strongly suggest that the torsional vibrations of the periphery antennas play an important role for the energy transfer. In summary, we investigated the rapid energy transfer dynamics in light-harvesting small dendrimers having different numbers of the light-harvesting periphery antennas (SSS2Pc-1 and SSS2Pc-2) by utilizing time-frequency 2D real-time pumpprobe imaging spectroscopy. Under the selective excitation of the periphery antennas, the time-frequency 2D image of the transient absorbance changes in SSS2Pc-1 shows the transient absorption due to the core around 500–600 nm and the ground-state bleaching of the Q-band around 700 nm with a rise time of 0.5 ps. The transient absorption also shows clear oscillatory components of 0.6 Thz. These results show that rapid and highly efficient energy transfer occurs from the periphery antennas to the core, and that the torsional vibration plays an important role for the energy transfer.
Acknowledgements This work was partly supported by the Grant-in-Aids for Scientific Research (A) from The Japan Society for the Promotion of Science (JSPS), and for Scientific Research on Priority Areas ‘‘Development of New Quantum Simulator and Quantum Design’’ from The Ministry of Education, Culture, Sports, Science and Technology (MEXT). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C. Devadoss, P. Bharathi, J.S. Moore, J. Am. Chem. Soc. 118 (1996) 9635. M. Cotlet, et al., J. Am. Chem. Soc. 125 (2003) 13609. S.M. Milas, et al., Appl. Phys. Lett. 82 (2003) 994. S. Raychaudhuri, et al., Phys. Rev. Lett. 85 (2000) 282. M. Kimura, et al., Chem. Mater. 14 (2002) 2711. I. Akai, et al., J. Lumin. 119–120 (2006) 283. A. Ishida, Y. Makishima, A. Okada, I. Akai, K. Kanemoto, T. Karasawa, M. Kimura, J. Takeda, J. Lumin. 128 (2008) 771. Y. Kodama, S. Ishii, K. Ohno, J. Phys.: Condens. Matter 19 (2007) 365242. N. Furukawa, C.E. Mair, V.D. Kleiman, J. Takeda, Appl. Phys. Lett. 85 (2004) 4645. Y. Makishima, N. Furukawa, A. Ishida, J. Takeda, Jpn. J. Appl. Phys. 45 (2006) 5986. A.V. Nikolaitchik, O. Korth, M.A.J. Rodgers, J. Phys. Chem. A 103 (1999) 7587. J. Gierschner, et al., J. Chem. Phys. 116 (2002) 8596. T. Suzuki, N. Mikami, M. Ito, J. Phys. Chem. 90 (1986) 6431. I. Akai, K. Miyanari, T. Shimamoto, A. Fujii, H. Nakao, A. Okada, K. Kanemoto, T. Karasawa, A. Ishida, A. Yamada, I. Katayama, J. Takeda, M. Kimura, New J. Phys. 10 (2008) 125024.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Energy transfer dynamics in light-harvesting small dendrimers studied by time-frequency two-dimensional imaging spectroscopy A. Yamada a, A. Ishida a, I. Akai b, M. Kimura c, I. Katayama d, J. Takeda a, a
Department of Physics, Yokohama National University, Yokohama 240-8501, Japan Shock Wave and Condensed Matter Research Center, Kumamoto University, Kumamoto 860-8555, Japan c Department of Functional Polymer Science, Shinshu University, Ueda 386-8567, Japan d Interdisciplinary Research Center, Yokohama National University, Yokohama 240-8501, Japan b
a r t i c l e in f o
a b s t r a c t
Available online 8 May 2009
Rapid energy transfer dynamics in light-harvesting small dendrimers (star-shaped stilbenoid phthalocyanine: SSS2Pc-m, m ¼ 1,2) having different numbers of oligo(p-phenylenevinylene) periphery antennas was studied by time-frequency two-dimensional (2D) real-time pump-probe imaging spectroscopy; the dendrimers having one light-harvesting periphery antenna for each aromatic ring (SSS2Pc-1) are expected to maintain a planer structure, leading to good p-conjugation, while those having two periphery antennas for each aromatic ring (SSS2Pc-2) have a large steric hindrance between the periphery antennas. Under the selective excitation of the periphery antennas (400 nm), the timefrequency 2D image of the absorbance changes in SSS2Pc-1 shows the transient absorption due to the core around 500–600 nm and the ground-state bleaching of the Q-band around 700 nm with a rise time of 0.5 ps. The transient absorption also shows clear oscillatory components of 0.6 Thz, which might come from the torsional motion of the peripheries. These results show that rapid and highly efficient energy transfer occurs from the periphery antennas to the core, and that the torsional vibration plays an important role for the energy transfer. The experimental results obtained in SSS2Pc-1 are compared with those in SSS2Pc-2 having a large steric hindrance. & 2009 Elsevier B.V. All rights reserved.
PACS: 78.47.J 82.20.Rp 82.53.k Keywords: Femtosecond Imaging spectroscopy Dendrimer Rapid energy transfer
1. Introduction Light-harvesting dendrimers have attracted much interest since optical energy absorbed in light-harvesting antennas transfers to central core with high efficiency [1,2]. The lightharvesting dendrimers are therefore considered to be promising candidates for molecular-based devices and/or biomimetic systems for photosynthesis in future [3,4]. Recently, star-shaped small dendrimers having p-conjugated light-harvesting antennas were synthesized as a new class of light-harvesting materials [5], and rapid and highly efficient energy transfer from the periphery antennas to the core was observed [6,7]. In these small dendrimers, it is pointed out that not only the p-conjugation between the periphery antennas and core but also molecular vibrations such as torsional motions play an important role for the energy transfer both in experiments and theoretical calculation— recently reported [6–8]. In this study, in order to clarify importance of the pconjugation between the antennas and core on the energy transfer, ultrafast excited-state dynamics in light-harvesting small Corresponding author. Tel./fax: +81 45 339 3953.
E-mail address: [email protected] (J. Takeda). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.04.070
dendrimers having different numbers of the light-harvesting periphery antennas has been studied by time-frequency twodimensional (2D) real-time pump-probe imaging spectroscopy; the dendrimers having one light-harvesting periphery antenna for each aromatic ring are expected to maintain a planer structure, leading to good p-conjugation, while those having two periphery antennas for each aromatic ring has a larger steric hindrance between the periphery antennas.
2. Experimental Fig. 1 shows the molecular structures of light-harvesting small dendrimers (star-shaped stilbenoid phthalocyanine: SSS2Pc-m, m ¼ 1,2) and their component molecules, zinc phthalocyanine (Zn-Pc) and oligo(p-phenylenevinylene) (OPV2-m, m ¼ 1,2) [5]. Here, m indicates number of the light-harvesting periphery antennas per each aromatic ring of the core component. The abbreviations of the sample names are shown in the caption of Fig. 1. They were diluted in anhydrous tetrahydrofuran (THF) with a concentration of 104103 M. Each diluted solution of 0.5 cc was contained in a disk-shaped quartz cell with a thickness of 1 mm, and the cell was rotated during experiments to avoid
ARTICLE IN PRESS A. Yamada et al. / Journal of Luminescence 129 (2009) 1898–1900
A
1899
B ZnPc SSS2PC-m
N
N
N
B N
A
Zn
A
OR1 B
N
N
R1O
N N
OR1 R1O
B
A
NC
A
NC
B
OR1 OPV2-m
R1=C3H7
[L]=
Fig. 1. Molecular structures of small dendrimers (SSS2Pc-m), core (Zn-Pc) and peripheries (OPV2-m) (m ¼ 1,2). The abbreviations of the samples are listed as follows. Zn-Pc: A ¼ B ¼ H, SSS2Pc-2: A ¼ B ¼ [L], SSS2Pc-1: one of two branches for each aromatic ring is [L] while the other is H, OPV2-2: A ¼ B ¼ [L], OPV2-1: A ¼ H, B ¼ [L] or vice versa.
SSS2Pc-2 0.2
0.0
-0.15
Delay Time (ps)
Absorbance Change
10 0.15
SSS2Pc-1 Q-band 0.3 2.0x10-4 M
Q-band 1.2x10-4M x0.1
10
x0.09
ZnPc Q-band 1.2x10-3M 2 10
8
8
8
6
6
6
4
4
4
2
2
2
0
0
0
-2
-2 500 600 700 Wavelength (nm)
x0.07
-2 500 600 700 Wavelength (nm)
500 600 700 Wavelength (nm)
Fig. 2. Time-frequency 2D images of transient absorbance changes of SSS2Pc-2, SSS2Pc-1 and Zn-Pc in THF solutions.
photodegradation and thermal heating of samples. Since quantities of SSS2Pc-m and OPV2-m were limited to less than 1 cc, conventional pump-probe technique, in which many repetitions of pump-probe sequence are necessary, is not feasible. For excited-state dynamics measurements in these samples, therefore, we employed real-time pump-probe imaging spectroscopy based on a single shot detection recently developed [9,10]. A second harmonic (400 nm) from a regenerative Ti:sapphire laser system with a 100 fs pulse duration and 1 kHz repetition rate was used as the excitation laser pulse, whose energy corresponds to the selective excitation of the periphery antennas. Using this spectroscopy, without many repetitions of pump-probe sequence, we could simultaneously map timefrequency 2D absorbance changes of materials in real-time with a wide spectral and temporal ranges of 300 nm and 6 ps, respectively. Since the accumulation time to measure multiple pump-probe data in region of interest is extremely reduced in comparison with that by the conventional pump-probe technique, this method is a powerful spectroscopic tool for observation of ultrafast excited-state dynamics in organic and biological materials, for which large quantities of samples are not readily available and/or photodegradation readily takes place. The detailed experimental setup is reported elsewhere [9,10].
3. Results and discussion Fig. 2 shows time-frequency 2D images of transient absorbance changes of SSS2Pc-2, SSS2Pc-1 and Zn-Pc dissolved in THF solutions with an accumulation of 1000 laser shots per unit frame. The 2D images were obtained by adding a few frames with different delay times and center-wavelengths in order to cover whole transient processes. As comparison, the steady-state absorption spectra are also shown in the upper figures. The dotted lines are visual guides for eyes to show the rise behavior of the transient signals. In SSS2Pc-m, from the steady-state absorption measurements, the sharp absorption band corresponding to Q-band of the core is located at 720–740 nm, while the broad absorption bands which are attributed to sum of the pp* transitions of the peripheries and Soret band (B-band) of the Zn-Pc core lie at 400 nm. Because the absorption intensity due to the peripheries is much higher than that due to the core around 400–500 nm, the selective excitation of the periphery antennas was achieved by the excitation of 400 nm (see hatched areas). In Zn-Pc, not only ground-state bleaching of the Q-band but also the transient absorption appears instantaneously at 670 and 420–520 nm, respectively, under the excitation of the B-band. In SSS2Pc-2 and SSS2Pc-1 dendrimers, on the other hand, even
ARTICLE IN PRESS 1900
A. Yamada et al. / Journal of Luminescence 129 (2009) 1898–1900
Absorbance Change
0.04
0.04
SSS2Pc-2 (600 nm) rise = 0.6 ± 0.05 ps
0.05 SSS2Pc-1 (600 nm) rise = 0.55 ± 0.05 ps
Zn-Pc (460nm) rise < 0.1 ps
0 0
0 0
0 0
SSS2Pc-2 (740 nm) rise = 0.6 ± 0.05 ps
SSS2Pc-1 (700 nm) rise = 0.55 ± 0.05 ps
0.4
Zn-Pc (660nm) rise < 0.1 ps
0.2 -2
0
0.5 2 4 6 Delay Time (ps)
8
10
-2
0
2 4 6 Delay Time (ps)
8
10
-2
0
2 4 6 Delay Time (ps)
8
10
Fig. 3. Time evolution of the absorbance changes in SSS2Pc-2, SSS2Pc-1 and Zn-Pc at the specific wavelengths. An instrumental response function of the system is shown by a thin dotted curve in the right-top figure. Fitted results for rising components are indicated by thick dotted lines. To compare the rise component of SSS2Pc-2 and SSS2Pc-1 dendrimers with that of Zn-Pc, the fitted line for Zn-Pc are plotted in every bottom figure.
under the selective excitation of the peripheries, ground-state bleaching of the Q-band as well as the transient absorption due to the core is clearly observed having a fast rise time, suggesting that a rapid energy transfer takes place from the light-harvesting periphery antennas to the central core. In order to clarify the rapid energy transfer, time evolution of the absorbance changes at the specific wavelengths in SSS2Pc-2, SSS2Pc-1 and Zn-Pc is shown in Fig. 3. The data are obtained from the 2D map in Fig. 2. The instrumental response function is shown by a thin dotted curve in the right-top figure. The rise times of the transient signals were evaluated from a fitting procedure by a single exponential rise convoluted with the instrumental response function as shown by thick broken lines. In Zn-Pc, the rise time of the transient absorption is less than 0.1 ps and is comparable with the pulse duration of the laser. This result shows that the internal relaxation from the B-band to the Q-band and the subsequent transient absorption from the Q-band to higher excited-states take place within the pulse duration [11]. In SSS2Pc-2 and SSS2Pc1 dendrimers, on the other hand, the rise time of the transient absorption as well as that of the ground-state bleaching due to the core is estimated to be 0.5–0.6 ps. Because the light-harvesting periphery antennas are selectively excited, the observed rise time of 0.5–0.6 ps presents a characteristic time for the rapid energy transfer from the periphery antennas to the core. In previous experimental and theoretical studies, not Fo¨rster mechanisms but short-range interactions caused by the wave function overlap between the excited-states of the peripheries and the core should be responsible for the energy transfer in light-harvesting small dendrimers [6,8]. The observed rapid energy transfer of 0.5–0.6 ps is in good agreement with the previous results. Most striking features of the transient signals in SSS2Pc-2 and SSS2Pc-1 dendrimers are as follows; the SSS2Pc-2 dendrimer having two periphery antennas for each aromatic ring does not show apparent oscillatory components, while the SSS2Pc-1 dendrimer having one periphery antenna has clear oscillatory components as shown by closed circles in Fig. 3. The oscillation observed is about 0.6 Thz (20 cm1), whose value is comparable with those due to the low-frequency torsional vibrations of the periphery antennas at the connecting point of the vinylene group: 8 cm1 for the ground state and 48 cm1 for the first excited state [12,13]. Akai et al. found that freezing of molecular vibrations quenches the energy transfer efficiency, and that temperature dependence of the energy transfer efficiency is well explained by an energy diagram considering the torsional vibration with tilt angle Y [6,14]. Moreover, theoretical calculation base on the timedependent Kohn-Sham equation shows that one-way electron and hole transfer from the periphery antennas to the core takes
place more easily in dendrimers with a planer structure than in those with steric hindrance since p-conjugation is well maintained in the planer structure [8]. These results strongly suggest that the torsional vibrations of the periphery antennas play an important role for the energy transfer. In summary, we investigated the rapid energy transfer dynamics in light-harvesting small dendrimers having different numbers of the light-harvesting periphery antennas (SSS2Pc-1 and SSS2Pc-2) by utilizing time-frequency 2D real-time pumpprobe imaging spectroscopy. Under the selective excitation of the periphery antennas, the time-frequency 2D image of the transient absorbance changes in SSS2Pc-1 shows the transient absorption due to the core around 500–600 nm and the ground-state bleaching of the Q-band around 700 nm with a rise time of 0.5 ps. The transient absorption also shows clear oscillatory components of 0.6 Thz. These results show that rapid and highly efficient energy transfer occurs from the periphery antennas to the core, and that the torsional vibration plays an important role for the energy transfer.
Acknowledgements This work was partly supported by the Grant-in-Aids for Scientific Research (A) from The Japan Society for the Promotion of Science (JSPS), and for Scientific Research on Priority Areas ‘‘Development of New Quantum Simulator and Quantum Design’’ from The Ministry of Education, Culture, Sports, Science and Technology (MEXT). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
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