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Ultrafast spectroscopy of conjugated polymers
ELSEVIER
Synthetic
Ultrafast
Metals 71 (1995) 1663-1666
spectroscopy
of conjugated
polymers
Takayoshi Kobayashi Department of Physics, University of Tokyo, Hongo 7-3-1, Bunkyo. Tokyo 113 Japan Abstract The experimented data of ultrafast spectroscopy of conjugated plymers we obtained are summarized, and a model we proposed to explain the results is described, and key issues to be solved is mentioned.
1.
Introduction Ultrafast processes in the excited states in conjugated polymers have different features from those in low molecular weight systems and in organic semiconductors [ 11. They are ultrashort lifetime of excitons and strong coupling of the excitonic transition to the intramoleculer vibrations. Here in the present paper, I summarize the experimented data we obtained and describe a model we proposed to explain the results[2-81 and finally key issues to be solved will be mentioned.
(2) There are common features in the transient absorption spectra of both PDAs (polydiacetylenes) [bluephase PDA-3BCMU(poly[4,6-decadiyne-l,lO-diol - bis([(nbutoxycarbonyl)methyl]urethane)]) and PDA4BCMU(poly[5,7-dodecadiyne-1,12-diol bis([(nbutoxycarbonyl)nethyl]urethane)]) and red-phase PDA4BCMU] and PTs(polythiophenes) either in random films or in oriented films. One feature is very broad absorption spectra extending from about 1.4 eV to just below the excitonic transition energy region observed just at excitation (0 ps)[4]. (3) Triplet exciton cannot be formed directly from the singlet exciton. They are formed only at high density excitation of exciton transition or by charge carrier creation by the interband transition[9]. (4) The formation and decay times and photoluminescence properties of the ST excitons in several PDAs and PTs studied are summarized in Table 1[5].
2.
Summary of Experimented Results The decay dynamics of induced absorption due to the self-trapped (ST) excitons are summarized as follows. (1) Many of conjugated polymers are nonfluorescent or only weakly fluorescent except poly-parahenylene vinylene and its derivatives. Especially the quantum efficiency of the nonfluorescent blue form of polydiacetylenes is estimated to be lower than 10e5[2-4, 71 and that of the fluorescent red-phase polydiacetylenes is only of the order of 10-4.
Table 1: The formation time ($ and the decay time *1 (rd) of self-trapped (ST) excitons and fluorescence property of several polymers
MS)
polymer
(290K) PDA-3BCMU (blue phase,oriented film*3) 150?40 15ok50 PDA-3BCMU (blue phase,cast flm) look30 PDA-DFMP (blue phase,single crystal) 14ok50 PDA-IBCMU (blue phase,oriented film) 120&60 PDAdBCMU (red phase,oriented film) <200 PDA-4BCMU (red phase,cast) 7W50 P3MT (elecuochemicallypreparedfilm) look50 P3DT (electmchemicallypreparedfilm)
rd(Ps)
‘c&S)
flUox%ence
(290K) property*2 n 1.9kO.2 1.6kO.l n 2.OkO.2 lSf0.2 ___ 1.6M.l n 1.6+0.1 2.lfO.l 9 0.96fo.09*4 --f 0.88M.08’4 --f 0.62?10.07*~ --f 0.45+0.06*4 --(10K)
*l The formation of ST exciton corresponds to the emission process of strongly coupled phonon to the excitonic transition. *2 Polymers with higher and lower fluorescence quantum efficiency than 10-s is indicated by f (fluorescent) and n (nonfluorescent, respectively. *3 The sample is prepared by the polymerization of evaporated monomer film of PDA3BCMU on a KC1 crystal. The sample is composed of small crystals and amorphous regions. *4 This decay time is determined by the initial slope of the semilogarithmic plot in the early delay time.
0379-6779/95/$09.50 0 1995 Elsevier SSDI 0379-6779(94)02996-C
Science
S.A. All rights
reserved
T. Kobayashi I Synthetic Metals 71 (1995)
1664
Figure 1 shows the examples of the time dependence of the absorbance change due to the self-trapped excitons in several PDAs and PTs, after the component, which does not decay within 150 ps, being subtracted[5, lo]. As can clearly be seen in the figure, the decay of nonfluorescent PDA3BCMU in the blue phase and PDA4BCMU in the blue phase are approximately given by an exponential function except the early stage within 500 fs. The other weakly fluorescent polymers have nonexponential decay from very early delay time until 30 ps. The initial decay times defined by the slope of the semilogarithmic plot of the absorbance change against delay time just after excitation are determined as 890_+160 fs, 620160 fs, and 450?50 fs in PDA4BCMU, P3MT. and P3DT, respectively, at 10 K. The decay becomes slower and slower at longer delay times because of the less efficient tunneling due to the thicker barrier width through which the population of excitons must tunnel to relax to the ground state[4, 61.
1663-1666
3.
A Model of the Exciton Relaxation From the experimented data summarized in 2., we have proposed a model to exnlain the decav kinetics and fluorescence properties of ihe rr-conjugated polymers. As shown in Figures 2 and 3, the first process is the selftrapping of free exciton within one cycle of vibration i.e. lo-20 fs of C=C and C=C modes, which are coupled T.hen intrachain strongly to the excitonic transition. vibrational energy redistribution takes place with time constant of loo-150 fs. We call this process the quasithermalization process. Energy randomization among intermolecular lattice vibrational models takes place with l-l.5 ps time constant. This time constant was determined by the spectral change of the induced absorption due to the self-trapping exciton. Even before the thermalization the STEs re1a.x to the ground state by tunneling with the time constant of about l-2 ps in blue-chase PDAt of which auantum efficiency is lower than 1O‘5.’ In red-pha’se PDAs and PTs, the competition between the tunneling of the thermal STEs and relaxation to trap states takes place. Table 2 lists the values of time constants of these processes in various polymers at room (290K) and low (10) temperatures. Table 3 summarizes the ultrafast relaxation dynamics of excitons in conjugated polymers. They exhibit quite clear contrast to those in inorganic semiconductors. Figure 4 shows the whole relaxation processes including the triplet STE (3STE) and polarons p and bypolarons BP*. The time scales are obtained in our previous studies. Relaxation
Model of Exciton
1. Free
Exciton (FE1 { self-k& 10-20 f s 2,honthermal STE
4. thermal
STE
___~ .-.-- _ Ground State (C) lnnoeling Y I - 5,: ??
0
1 Delay
2
3
Time
(ps)
4
5
Fig. 1. Time dependence of the absorbance change due to the self-traooed excitons in several PDAs and PTs. after the verv long-li~e‘component being subtracted. The decay curves from the top to the bottom correspond to 1. P3MT (observed at 10K at the probe photon energy of 1.63 eV); 2. P3DT (lOK, 2.34 eV); 3. PDA-4BCMU (red-phase cast film, lOK, 1.77 eV); 4. PDA-4BCMU (blue-phase oriented film, lOK, 1.77eV); 5. PDA-3BCMU (blue-phase oriented film, 290K, 1.77 eV); 6. PDA3BCMU (blue-phase cast film, 290K, 1.77 eV): 7. PDA-SBCMU (oriented sinale crystals prepared by evaporation on a KC1 ‘single crystal surface (loo), 29OK. 1.77 eV), and 8, PDA-DFMP (297K, 1.98 eV). The excitation pulse width and energy are 1OOfs and 1.97 eV, respectively. The samples of PDA-3BCMU in red-phase are excited by the two-photon process.
Fia. 2. A model of the relaxation kinetics and the debendence on exciton energy shown in the polential surfaces of excitons in PDA. State 1, 2, 3, and 4 are free exciton (FE), nonthermal self-trapped exciton (STE), quasithermal STE, and thermal STE. respectively.
1665
T. Kobayashi / Synthetic MetaLr 71 (1995) 1663-1666
Relaxation
Model (Red PDAs)
energy
intrachain
distribution
V-V
pump1
I
+
trap
& & luminescence
G /
Fig. 3. Adiabatic potential curves of the ground state (G), free exciton (FE), and self-trapped exciton (STE) of the polymers. There is no barrier between the bottom of FE band and STE exciton potential curves. The curvature radius of the potential of STE is larger than those of the G and FE states, to take into account the discussion by Sumi et al. [H. Sumi, M. Georgiev, and A. Sumi, Rev. Solid State Sci., 4, 209 (1990)] and the small Stokes shift observed for the fluorescent polymers. In the fluorescent PDA4BCMU (red phase and P3MT. the crossing points are higher than in the nonfluorescent polymer excitons. After a few tens of picoseconds, the excitons in the fluorescent polymers are thermalized to the state (process 34) at which the temperature can be defined among the exchange modes connected to the bath mode. The temperature defined is still higher than the experimental The cooling down process of the exchange mode temperature to the temperature, which is equal to the bath temperature. experimental temperature by the vibrational energy flow to the bath mode (process 4-5) is not indicated in this figure. During this cooling process, the tunneling rate becomes smaller and smaller because of the increase in the barrier thickness and height[6]. Finally, the system is cooled to the bottom of the potential curve of STE and then relaxed either radiatively or nonradiatively to the ground state, either by slow tunneling to the ground state or by being trapped by the defect states.
Table 2. Ii e 1 :r x A t i 0 11 SampIps I’DA
T i
111 c
C 0
11 s
t
a
n t s
nonth. STE nonth. STE quasi-thermal & _m’ quasi. STE + G thermal STE+G
$1~)
3BCMU (1OK) 150i50fs (29OKI 150?50fs
4BCIW (IOK) 120QOfs (290K) 140!40fs
ops ops ops
2. o_+o.2ps 1. 51-o. Ips
(1. ops
2. Ito. 2ps 1. 6to. lps
0. Sto. 2ps 0. WI Ips
2. l?O. lps I. 5FO. ips
!12. 8l i1CK.i 1COi60fs 0.6?0. ips IESOKi 1201-50fs 0. 6?0. lps
I. II!0. Ip’ 1.3’0. lps
112, 81 !lOK) 130t50fs 1290Kl 90k60fs
IJ11 A :: cdj
4l!C!lL (IOK) <3OOfs (?$@bJ 120tGOfs
0. 910. lps O.?iO. ips
5. Ftl. lps 4. It_O.5ps
(12, 61 (IOii) 100i60fs I29OK) 130i60fs
I. oio. 2ps 0. 8_tO.2ps
6. 1t1. 2ps 3. 9tO. 5ps
Table 3. Ultrafast relaxation
dynamics
of exciton
Free exiton (FE) + Self-trapped exciton (STE) generation of FE l-2 fs generation of nonthermal STE lo-2Ofs (self-trapping process) exciton dephasing (T2)
nonthermal STE + quasi-thermal STE (intrachain vibrational relaxation)
I-2ps
quasi-thermal (intrachain
STE -_) thermal STE vibrational relaxation)
Self-trapped exciton (STE) -+ Ground state (G) relaxation from nonthermal STE to G -500fs tunneling from quasi-thermal STE to G 5OOfs-5ps tunneling from thermal STE to G -5ps Fluorescence <3ops >3Ops
fluorescence fluorescence
from FE and STE from trap
T. Kobayashi / Synthetic Metak 71 (1995) 1663-1666
1666
Fig. 4. 4.
Key Issues to be Solved In the above model, the 21Ag exciton state is not included. There are two possibilities of the explanation including 2l A, excitons. The energy relaxation The first one is as follows: from the l’B, exciton to 2lA, exciton does not take place efficiently because of the quite different electronic properties between them in terms of ionicity and covalency. The second possibility is as follows: The process of 100-150 fs corresponds to l’B, -+ 2’A and that of l-2 ps in the nonfluorescent (fluorescence efficiency @r<1 0e5) polymers and 4-6 ps in the weakly fluorescent polymers (@--10e4) to 2lA, to the ground state. In this case absorption must be assigned to the very weakly (radiationally) coupled 21A,8 exciton state. Since there is no data of the induced absorption from 2lA, state, it may be difficult to distinguish the above two Recently the experimented data of the explanations. lifetime of the weak fluorescence from 2l A, exciton state in several polymers are reported. The lifetimes are of the order of 100 fs. If the lifetime is corresponding to the decay of induced absorption, then the explanation (2) seems to be appropriate. More systematic studies are needed to finalize the problem.
References [l] T. Kobayashi, IEICE Trans. Fundamentals, E75-A(l), (1992) 38. [2] M. Yoshizawa, Y. Hattori, and T. Kobayashi. Phys. Rev. B 47 (1993) 3882. [3] M. Yoshizawa. M. Taiji, and T. Kobayashi, IEEE J. Quantum Electron. QE-25 (1989) 2532. [4] T. Kobayashi, M. Yoshizawa, U. Stamm. M. Taiji, and M. Hasegawa, J. Opt. Sot. Am. B 7(8), t.1990) 1558. [5] M. Yoshizawa, A. Yasuda, and T. Kobayashi, Appl. Phys. B 53 (1991) 296. [6] M. Yoshizawa. K. Nishiyama, M.. Fujihira. and T. Kobayashi, Chem. Phys. Lett. 270 (1993) 461. [7] A. Yasuda, M. Yoshizawa, and T. Kobayashi, Chem. Phys. Lett. 269 (1993) 281. [S] M. Yoshizawa, Y. Hattori, and 7’. Kobayashi, Phys. Rev. B 49 (1994) 13259. 191 K. Ichimura, M. Yoshizawa, H. Matsuda, S. Okada, M. M. Ohsugi, H. Nakanishi, and T. Kobayashi, J. Chem. Phys. 99 (1993) 7404. Ultrafast Responses in Various [lo] T. Kobayashi, Conjugate Polymers with Large Optical Nonlinearity, in Relaxation in Polymers, ed. by T. Kobayashi (World Scientific, Singapore, 1993) 1.
Synthetic
Ultrafast
Metals 71 (1995) 1663-1666
spectroscopy
of conjugated
polymers
Takayoshi Kobayashi Department of Physics, University of Tokyo, Hongo 7-3-1, Bunkyo. Tokyo 113 Japan Abstract The experimented data of ultrafast spectroscopy of conjugated plymers we obtained are summarized, and a model we proposed to explain the results is described, and key issues to be solved is mentioned.
1.
Introduction Ultrafast processes in the excited states in conjugated polymers have different features from those in low molecular weight systems and in organic semiconductors [ 11. They are ultrashort lifetime of excitons and strong coupling of the excitonic transition to the intramoleculer vibrations. Here in the present paper, I summarize the experimented data we obtained and describe a model we proposed to explain the results[2-81 and finally key issues to be solved will be mentioned.
(2) There are common features in the transient absorption spectra of both PDAs (polydiacetylenes) [bluephase PDA-3BCMU(poly[4,6-decadiyne-l,lO-diol - bis([(nbutoxycarbonyl)methyl]urethane)]) and PDA4BCMU(poly[5,7-dodecadiyne-1,12-diol bis([(nbutoxycarbonyl)nethyl]urethane)]) and red-phase PDA4BCMU] and PTs(polythiophenes) either in random films or in oriented films. One feature is very broad absorption spectra extending from about 1.4 eV to just below the excitonic transition energy region observed just at excitation (0 ps)[4]. (3) Triplet exciton cannot be formed directly from the singlet exciton. They are formed only at high density excitation of exciton transition or by charge carrier creation by the interband transition[9]. (4) The formation and decay times and photoluminescence properties of the ST excitons in several PDAs and PTs studied are summarized in Table 1[5].
2.
Summary of Experimented Results The decay dynamics of induced absorption due to the self-trapped (ST) excitons are summarized as follows. (1) Many of conjugated polymers are nonfluorescent or only weakly fluorescent except poly-parahenylene vinylene and its derivatives. Especially the quantum efficiency of the nonfluorescent blue form of polydiacetylenes is estimated to be lower than 10e5[2-4, 71 and that of the fluorescent red-phase polydiacetylenes is only of the order of 10-4.
Table 1: The formation time ($ and the decay time *1 (rd) of self-trapped (ST) excitons and fluorescence property of several polymers
MS)
polymer
(290K) PDA-3BCMU (blue phase,oriented film*3) 150?40 15ok50 PDA-3BCMU (blue phase,cast flm) look30 PDA-DFMP (blue phase,single crystal) 14ok50 PDA-IBCMU (blue phase,oriented film) 120&60 PDAdBCMU (red phase,oriented film) <200 PDA-4BCMU (red phase,cast) 7W50 P3MT (elecuochemicallypreparedfilm) look50 P3DT (electmchemicallypreparedfilm)
rd(Ps)
‘c&S)
flUox%ence
(290K) property*2 n 1.9kO.2 1.6kO.l n 2.OkO.2 lSf0.2 ___ 1.6M.l n 1.6+0.1 2.lfO.l 9 0.96fo.09*4 --f 0.88M.08’4 --f 0.62?10.07*~ --f 0.45+0.06*4 --(10K)
*l The formation of ST exciton corresponds to the emission process of strongly coupled phonon to the excitonic transition. *2 Polymers with higher and lower fluorescence quantum efficiency than 10-s is indicated by f (fluorescent) and n (nonfluorescent, respectively. *3 The sample is prepared by the polymerization of evaporated monomer film of PDA3BCMU on a KC1 crystal. The sample is composed of small crystals and amorphous regions. *4 This decay time is determined by the initial slope of the semilogarithmic plot in the early delay time.
0379-6779/95/$09.50 0 1995 Elsevier SSDI 0379-6779(94)02996-C
Science
S.A. All rights
reserved
T. Kobayashi I Synthetic Metals 71 (1995)
1664
Figure 1 shows the examples of the time dependence of the absorbance change due to the self-trapped excitons in several PDAs and PTs, after the component, which does not decay within 150 ps, being subtracted[5, lo]. As can clearly be seen in the figure, the decay of nonfluorescent PDA3BCMU in the blue phase and PDA4BCMU in the blue phase are approximately given by an exponential function except the early stage within 500 fs. The other weakly fluorescent polymers have nonexponential decay from very early delay time until 30 ps. The initial decay times defined by the slope of the semilogarithmic plot of the absorbance change against delay time just after excitation are determined as 890_+160 fs, 620160 fs, and 450?50 fs in PDA4BCMU, P3MT. and P3DT, respectively, at 10 K. The decay becomes slower and slower at longer delay times because of the less efficient tunneling due to the thicker barrier width through which the population of excitons must tunnel to relax to the ground state[4, 61.
1663-1666
3.
A Model of the Exciton Relaxation From the experimented data summarized in 2., we have proposed a model to exnlain the decav kinetics and fluorescence properties of ihe rr-conjugated polymers. As shown in Figures 2 and 3, the first process is the selftrapping of free exciton within one cycle of vibration i.e. lo-20 fs of C=C and C=C modes, which are coupled T.hen intrachain strongly to the excitonic transition. vibrational energy redistribution takes place with time constant of loo-150 fs. We call this process the quasithermalization process. Energy randomization among intermolecular lattice vibrational models takes place with l-l.5 ps time constant. This time constant was determined by the spectral change of the induced absorption due to the self-trapping exciton. Even before the thermalization the STEs re1a.x to the ground state by tunneling with the time constant of about l-2 ps in blue-chase PDAt of which auantum efficiency is lower than 1O‘5.’ In red-pha’se PDAs and PTs, the competition between the tunneling of the thermal STEs and relaxation to trap states takes place. Table 2 lists the values of time constants of these processes in various polymers at room (290K) and low (10) temperatures. Table 3 summarizes the ultrafast relaxation dynamics of excitons in conjugated polymers. They exhibit quite clear contrast to those in inorganic semiconductors. Figure 4 shows the whole relaxation processes including the triplet STE (3STE) and polarons p and bypolarons BP*. The time scales are obtained in our previous studies. Relaxation
Model of Exciton
1. Free
Exciton (FE1 { self-k& 10-20 f s 2,honthermal STE
4. thermal
STE
___~ .-.-- _ Ground State (C) lnnoeling Y I - 5,: ??
0
1 Delay
2
3
Time
(ps)
4
5
Fig. 1. Time dependence of the absorbance change due to the self-traooed excitons in several PDAs and PTs. after the verv long-li~e‘component being subtracted. The decay curves from the top to the bottom correspond to 1. P3MT (observed at 10K at the probe photon energy of 1.63 eV); 2. P3DT (lOK, 2.34 eV); 3. PDA-4BCMU (red-phase cast film, lOK, 1.77 eV); 4. PDA-4BCMU (blue-phase oriented film, lOK, 1.77eV); 5. PDA-3BCMU (blue-phase oriented film, 290K, 1.77 eV); 6. PDA3BCMU (blue-phase cast film, 290K, 1.77 eV): 7. PDA-SBCMU (oriented sinale crystals prepared by evaporation on a KC1 ‘single crystal surface (loo), 29OK. 1.77 eV), and 8, PDA-DFMP (297K, 1.98 eV). The excitation pulse width and energy are 1OOfs and 1.97 eV, respectively. The samples of PDA-3BCMU in red-phase are excited by the two-photon process.
Fia. 2. A model of the relaxation kinetics and the debendence on exciton energy shown in the polential surfaces of excitons in PDA. State 1, 2, 3, and 4 are free exciton (FE), nonthermal self-trapped exciton (STE), quasithermal STE, and thermal STE. respectively.
1665
T. Kobayashi / Synthetic MetaLr 71 (1995) 1663-1666
Relaxation
Model (Red PDAs)
energy
intrachain
distribution
V-V
pump1
I
+
trap
& & luminescence
G /
Fig. 3. Adiabatic potential curves of the ground state (G), free exciton (FE), and self-trapped exciton (STE) of the polymers. There is no barrier between the bottom of FE band and STE exciton potential curves. The curvature radius of the potential of STE is larger than those of the G and FE states, to take into account the discussion by Sumi et al. [H. Sumi, M. Georgiev, and A. Sumi, Rev. Solid State Sci., 4, 209 (1990)] and the small Stokes shift observed for the fluorescent polymers. In the fluorescent PDA4BCMU (red phase and P3MT. the crossing points are higher than in the nonfluorescent polymer excitons. After a few tens of picoseconds, the excitons in the fluorescent polymers are thermalized to the state (process 34) at which the temperature can be defined among the exchange modes connected to the bath mode. The temperature defined is still higher than the experimental The cooling down process of the exchange mode temperature to the temperature, which is equal to the bath temperature. experimental temperature by the vibrational energy flow to the bath mode (process 4-5) is not indicated in this figure. During this cooling process, the tunneling rate becomes smaller and smaller because of the increase in the barrier thickness and height[6]. Finally, the system is cooled to the bottom of the potential curve of STE and then relaxed either radiatively or nonradiatively to the ground state, either by slow tunneling to the ground state or by being trapped by the defect states.
Table 2. Ii e 1 :r x A t i 0 11 SampIps I’DA
T i
111 c
C 0
11 s
t
a
n t s
nonth. STE nonth. STE quasi-thermal & _m’ quasi. STE + G thermal STE+G
$1~)
3BCMU (1OK) 150i50fs (29OKI 150?50fs
4BCIW (IOK) 120QOfs (290K) 140!40fs
ops ops ops
2. o_+o.2ps 1. 51-o. Ips
(1. ops
2. Ito. 2ps 1. 6to. lps
0. Sto. 2ps 0. WI Ips
2. l?O. lps I. 5FO. ips
!12. 8l i1CK.i 1COi60fs 0.6?0. ips IESOKi 1201-50fs 0. 6?0. lps
I. II!0. Ip’ 1.3’0. lps
112, 81 !lOK) 130t50fs 1290Kl 90k60fs
IJ11 A :: cdj
4l!C!lL (IOK) <3OOfs (?$@bJ 120tGOfs
0. 910. lps O.?iO. ips
5. Ftl. lps 4. It_O.5ps
(12, 61 (IOii) 100i60fs I29OK) 130i60fs
I. oio. 2ps 0. 8_tO.2ps
6. 1t1. 2ps 3. 9tO. 5ps
Table 3. Ultrafast relaxation
dynamics
of exciton
Free exiton (FE) + Self-trapped exciton (STE) generation of FE l-2 fs generation of nonthermal STE lo-2Ofs (self-trapping process) exciton dephasing (T2)
nonthermal STE + quasi-thermal STE (intrachain vibrational relaxation)
I-2ps
quasi-thermal (intrachain
STE -_) thermal STE vibrational relaxation)
Self-trapped exciton (STE) -+ Ground state (G) relaxation from nonthermal STE to G -500fs tunneling from quasi-thermal STE to G 5OOfs-5ps tunneling from thermal STE to G -5ps Fluorescence <3ops >3Ops
fluorescence fluorescence
from FE and STE from trap
T. Kobayashi / Synthetic Metak 71 (1995) 1663-1666
1666
Fig. 4. 4.
Key Issues to be Solved In the above model, the 21Ag exciton state is not included. There are two possibilities of the explanation including 2l A, excitons. The energy relaxation The first one is as follows: from the l’B, exciton to 2lA, exciton does not take place efficiently because of the quite different electronic properties between them in terms of ionicity and covalency. The second possibility is as follows: The process of 100-150 fs corresponds to l’B, -+ 2’A and that of l-2 ps in the nonfluorescent (fluorescence efficiency @r<1 0e5) polymers and 4-6 ps in the weakly fluorescent polymers (@--10e4) to 2lA, to the ground state. In this case absorption must be assigned to the very weakly (radiationally) coupled 21A,8 exciton state. Since there is no data of the induced absorption from 2lA, state, it may be difficult to distinguish the above two Recently the experimented data of the explanations. lifetime of the weak fluorescence from 2l A, exciton state in several polymers are reported. The lifetimes are of the order of 100 fs. If the lifetime is corresponding to the decay of induced absorption, then the explanation (2) seems to be appropriate. More systematic studies are needed to finalize the problem.
References [l] T. Kobayashi, IEICE Trans. Fundamentals, E75-A(l), (1992) 38. [2] M. Yoshizawa, Y. Hattori, and T. Kobayashi. Phys. Rev. B 47 (1993) 3882. [3] M. Yoshizawa. M. Taiji, and T. Kobayashi, IEEE J. Quantum Electron. QE-25 (1989) 2532. [4] T. Kobayashi, M. Yoshizawa, U. Stamm. M. Taiji, and M. Hasegawa, J. Opt. Sot. Am. B 7(8), t.1990) 1558. [5] M. Yoshizawa, A. Yasuda, and T. Kobayashi, Appl. Phys. B 53 (1991) 296. [6] M. Yoshizawa. K. Nishiyama, M.. Fujihira. and T. Kobayashi, Chem. Phys. Lett. 270 (1993) 461. [7] A. Yasuda, M. Yoshizawa, and T. Kobayashi, Chem. Phys. Lett. 269 (1993) 281. [S] M. Yoshizawa, Y. Hattori, and 7’. Kobayashi, Phys. Rev. B 49 (1994) 13259. 191 K. Ichimura, M. Yoshizawa, H. Matsuda, S. Okada, M. M. Ohsugi, H. Nakanishi, and T. Kobayashi, J. Chem. Phys. 99 (1993) 7404. Ultrafast Responses in Various [lo] T. Kobayashi, Conjugate Polymers with Large Optical Nonlinearity, in Relaxation in Polymers, ed. by T. Kobayashi (World Scientific, Singapore, 1993) 1.