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Ultrafast optical probes of excited states in halogen substituted PPV derivatives
Synthetic
Metals
84 (1997)
5 15-5 16
Ultrafast Optical Probes of Excited States in Halogen Substituted PPV Derivatives H. S. Eom”, S. C. Jeoung”, D. Kim”*, J. I. Leeb, H. K. Shimb*, C. M. Kim”, C. S. Yoon’* Y$ectroscopy laboratory, Korea Research Institute of Standards and Science, Taejon 305606, South Korea department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea %artment of Physics, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea Abstract Photoexcitation dynamics of poly( 1,4-phenylene vinylene) (PPV) and poly(2-fluoro-I ,4-phenylene vinylene) (PFPV) were investigated. The femtosecond transient absorption spectra were recorded by using a high power fwtosecond Tisapphire laser system. The laser pulses with 150 fs and 300 p J/pulse at 800 run were used to generate white light continuum for a probe heam, and the second harmonic generation at 4OOnm was employed as a pump beam. Transient absorption spectra and their decay profiles were measured by a dualdiode array detector and two photodiodes, respectively. We assign that photoinduced absorption(PA), which has -1 ps lifetime, arises Tom inter-chain polaron pairs. The decay of PA in PFPV was faster than that in PPV. This behavior is attributed to the large electronegativity of fluorine substituted at phenyl ringlof PPV. K~YIVCJ~~S : Laser Spectroscopy, Optical absorption and emission spectroscopy, Time-Resolved fast spectroscopy, Photoinduced absorption spectroscopy, Poly(phenylene vmylene) and derivatives 1. Introduction Conjugated polymers have attracted a considerable attention as quasi-one dimensional semiconductors owing to easily tunable optical and electronic properties. One of the exciting examples is the development of electroluminescent devices based on PPV(poly 1,4-phenylenevinylene). A thorough understanding of photogeneration of carriers and photoexcitation dynamics, however, is needed to have further information on the mechanism of electroluminescence and enhance the performance of LED devices. Because of the luminescent characteristics of PPVs, numerous experiments have been performed to investigate the photoluminescence dynamics.’ However, these mesurements probe only the luminescent excited species, which are usually a small tiaction of all the excited species created by photoexcitation. Studying photoinduced absorption (PA) in picosecond regime, a complementary experiment to PL, enables us to understand the nature of photoexcited species immediately generated upon photoexcitation and subsequent relaxation dynamics. Recently, many studies on the characteristics of PA in PPV and other conducting polymers have been carried out to clarify the electronic structure of excited species and their dynamics.2 In this work, we investigated photoexcited species and their relaxation dynamics of PPV and fluorine, a strong electronegative atom, substituted PPV derivative, poly(2-fluoro1,4-phenylene vinylene) (PFPV) by a pump/probe transient absorption measurement. 2. Experimental PPV and PFPV were synthesized through water-soluble precursor route reported previously by us3 The viscous * Corresponding
authors.
0379-6779/97/%17.00 PII SO379-6779(96)LMo33-7
Q 1997
Else&r
Science
S.A. AlI rights
resewed
I)recursor nolvmer solutions were subject to snincoating on a quartz plate. The desired optical density was obtained by controlling the spinning rate. The fuLa conjugated polymer films were obtained by thermal elimination under vacuum The femtosecond time-resolved transient absorption spectra were recorded by using a high power Tisapphire laser system. The laser pulses with 150 fs and 300 N/pulse at 800 MI and 1lcHz repetition rate were used to generate white light continuum for probe and reference beam, and the second harmonic generation at 4OOnm was employed as a pump beam. The transient absorption signals were detected by a dual 512 channel photodiode array detector at the desired time delay between pump and probe pulses. For precise measurement of decay profiles of transients absorption signals, the pump beam was modulated at 48 Hz by a chopper and the desired wavelength of the prohe and reference heam was obtained with 10 nm fwhm band pass filters. The output current detected by a photodiode was amplified and the resultant voltages of probe and reference beams were normalized by a boxcar averager by using pulse-tG pulse contiguration. The normalized signal was modulated by a lock-in-amplifier and then fed into a personal computer for further signal processing. 3. Results and Discussion Fig. 1 displays the room-temperature absorption and photoluminescencc spectra of PPV and PFPV. The absorption maximum wavelengths for X--SC* transitions were found to bc 415 mn for PPV and 4 10 nm for PFPV. The blue shift of PFPV compared with PPV is ascribed to the electronegativity of the fluorine substituent attached to the phenylene ring which reduces the delocalized electron density along the polymer backbone. The structureless PL spectrum of PFPV indicates that PFPV does not have well ordered structure in comparison with unsubstitted PPV.
H.S. Eom et al. /SyntheticMetals
516
The observation of much better resolved structure in the PL spectrum of PPV than in the absorption spectrum indicates that spectral diffusion occurs following photoexcitation prior to emission.
-
84 (1997) 51.5-516
state population and the stimulated emission by the singlet excitons. Thus, the singlet excitons in PFPV seem to contribute less to the bleaching signal
PPPV 0.6 2
0.6 -
ti 8 4
0.4 0.2 ^A
___L
Y’V r=I
300
I 6W
4x
wavelength
I 600
-0.10
---OfS . . . . . . mf5 -.-.- 8lOfs ----. 12 ps
8
’ 440
c
-----W@ . . .
’ 460
’ 480
St.\.. :,a ..T,
Of-3
-.-.-
270b
-_.-...
mfs
’ 5!30
’
/f., !: :
’
’ 520
540
k.. “‘.zyY. %
-Q.l
II
s 460
1 480
c
1 500 Wavelength
’
1 520
”
f 540
.
A
A. I
0
1
2
3
time ( ps )
(nm)
---180,s
0.05
-0.2
TC4
Fig 1. Absorption and photoluminescence spectra of PPV and PFPV
-
aP
I 560
Fig. 2 Transient absorption spectra Fig of PFPV(top) and PPV(bottom) Fig. 2 shows the series of transient absorption spectra of PPV and PFPV at various time delays with 400 mn excitation at 13 K In PPV, the bleaching band has a structure which is absent in PFPV. This structure originates from the depletion of the ground
Fig. 3 Transient absorption spectra of PPV and PFPV Fig. 3 shows the decay profiles of PA and bleaching signals of PPV and PFPV at 520 and 460 nm, respectively. The PA signal appears very quickly without detectable rise time and its decay is remarkably faster than that of bleaching indicating that this signal is due to the excited species instaneously generated by photoexcitation. For PA signal, PFPV has a shorter lifetime than PPV, which is caused by nonradiatively geminate recombination of polaron pairs. This behavior is due to the large electronegativity of fluorine attached to phenylene ring. Fluorine makes the conjugation of polymer backbone weak by reducing the density of delocalized electrons along the backbone and consequently i&a-chain interactions become weak. It means that the photoexcitation of PFPV generates interchain polaron pairs more easily than PPV, therefore the density of bound polaron pairs is high which makes decay faster. Bleaching signal decay exhibits double exponential decay. The lifetime of short component is about 1 ps and that of long component is about 800 ps. The short component comes from polaron pair recombination (-1 ps) and the long component comes from singlet exciton lifetim<-800 ps). The lifetime of short component of PFPV is faster than that of PPV, which is consistent with the result of PA decay profiles. Since the interchain polaron pairs generation in PFPV is superior to the intrachain exciton, which is responsible for the photoluminescence, the density of singlet excitons is lower and consquently the long component becomes longer in PFPV than that in PPV. This result is consistent with our time-resolved PL experiment3 This work wassupportedby a grantfYomthe Ministry of Science and Technology and the Center for Molecular Sciencethroughthe Korea Scienceand Engineering Foundation. Acknowledgment.
References
[l] K. S.Wong et al., J. Phys.C 20 (1987)L187. [2] J. W. P. Hsu, Appl. Rev.B, 49 (1994-1)712. [3] J. I. Leeet al., (unpublished).
Metals
84 (1997)
5 15-5 16
Ultrafast Optical Probes of Excited States in Halogen Substituted PPV Derivatives H. S. Eom”, S. C. Jeoung”, D. Kim”*, J. I. Leeb, H. K. Shimb*, C. M. Kim”, C. S. Yoon’* Y$ectroscopy laboratory, Korea Research Institute of Standards and Science, Taejon 305606, South Korea department of Chemistry, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea %artment of Physics, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea Abstract Photoexcitation dynamics of poly( 1,4-phenylene vinylene) (PPV) and poly(2-fluoro-I ,4-phenylene vinylene) (PFPV) were investigated. The femtosecond transient absorption spectra were recorded by using a high power fwtosecond Tisapphire laser system. The laser pulses with 150 fs and 300 p J/pulse at 800 run were used to generate white light continuum for a probe heam, and the second harmonic generation at 4OOnm was employed as a pump beam. Transient absorption spectra and their decay profiles were measured by a dualdiode array detector and two photodiodes, respectively. We assign that photoinduced absorption(PA), which has -1 ps lifetime, arises Tom inter-chain polaron pairs. The decay of PA in PFPV was faster than that in PPV. This behavior is attributed to the large electronegativity of fluorine substituted at phenyl ringlof PPV. K~YIVCJ~~S : Laser Spectroscopy, Optical absorption and emission spectroscopy, Time-Resolved fast spectroscopy, Photoinduced absorption spectroscopy, Poly(phenylene vmylene) and derivatives 1. Introduction Conjugated polymers have attracted a considerable attention as quasi-one dimensional semiconductors owing to easily tunable optical and electronic properties. One of the exciting examples is the development of electroluminescent devices based on PPV(poly 1,4-phenylenevinylene). A thorough understanding of photogeneration of carriers and photoexcitation dynamics, however, is needed to have further information on the mechanism of electroluminescence and enhance the performance of LED devices. Because of the luminescent characteristics of PPVs, numerous experiments have been performed to investigate the photoluminescence dynamics.’ However, these mesurements probe only the luminescent excited species, which are usually a small tiaction of all the excited species created by photoexcitation. Studying photoinduced absorption (PA) in picosecond regime, a complementary experiment to PL, enables us to understand the nature of photoexcited species immediately generated upon photoexcitation and subsequent relaxation dynamics. Recently, many studies on the characteristics of PA in PPV and other conducting polymers have been carried out to clarify the electronic structure of excited species and their dynamics.2 In this work, we investigated photoexcited species and their relaxation dynamics of PPV and fluorine, a strong electronegative atom, substituted PPV derivative, poly(2-fluoro1,4-phenylene vinylene) (PFPV) by a pump/probe transient absorption measurement. 2. Experimental PPV and PFPV were synthesized through water-soluble precursor route reported previously by us3 The viscous * Corresponding
authors.
0379-6779/97/%17.00 PII SO379-6779(96)LMo33-7
Q 1997
Else&r
Science
S.A. AlI rights
resewed
I)recursor nolvmer solutions were subject to snincoating on a quartz plate. The desired optical density was obtained by controlling the spinning rate. The fuLa conjugated polymer films were obtained by thermal elimination under vacuum The femtosecond time-resolved transient absorption spectra were recorded by using a high power Tisapphire laser system. The laser pulses with 150 fs and 300 N/pulse at 800 MI and 1lcHz repetition rate were used to generate white light continuum for probe and reference beam, and the second harmonic generation at 4OOnm was employed as a pump beam. The transient absorption signals were detected by a dual 512 channel photodiode array detector at the desired time delay between pump and probe pulses. For precise measurement of decay profiles of transients absorption signals, the pump beam was modulated at 48 Hz by a chopper and the desired wavelength of the prohe and reference heam was obtained with 10 nm fwhm band pass filters. The output current detected by a photodiode was amplified and the resultant voltages of probe and reference beams were normalized by a boxcar averager by using pulse-tG pulse contiguration. The normalized signal was modulated by a lock-in-amplifier and then fed into a personal computer for further signal processing. 3. Results and Discussion Fig. 1 displays the room-temperature absorption and photoluminescencc spectra of PPV and PFPV. The absorption maximum wavelengths for X--SC* transitions were found to bc 415 mn for PPV and 4 10 nm for PFPV. The blue shift of PFPV compared with PPV is ascribed to the electronegativity of the fluorine substituent attached to the phenylene ring which reduces the delocalized electron density along the polymer backbone. The structureless PL spectrum of PFPV indicates that PFPV does not have well ordered structure in comparison with unsubstitted PPV.
H.S. Eom et al. /SyntheticMetals
516
The observation of much better resolved structure in the PL spectrum of PPV than in the absorption spectrum indicates that spectral diffusion occurs following photoexcitation prior to emission.
-
84 (1997) 51.5-516
state population and the stimulated emission by the singlet excitons. Thus, the singlet excitons in PFPV seem to contribute less to the bleaching signal
PPPV 0.6 2
0.6 -
ti 8 4
0.4 0.2 ^A
___L
Y’V r=I
300
I 6W
4x
wavelength
I 600
-0.10
---OfS . . . . . . mf5 -.-.- 8lOfs ----. 12 ps
8
’ 440
c
-----W@ . . .
’ 460
’ 480
St.\.. :,a ..T,
Of-3
-.-.-
270b
-_.-...
mfs
’ 5!30
’
/f., !: :
’
’ 520
540
k.. “‘.zyY. %
-Q.l
II
s 460
1 480
c
1 500 Wavelength
’
1 520
”
f 540
.
A
A. I
0
1
2
3
time ( ps )
(nm)
---180,s
0.05
-0.2
TC4
Fig 1. Absorption and photoluminescence spectra of PPV and PFPV
-
aP
I 560
Fig. 2 Transient absorption spectra Fig of PFPV(top) and PPV(bottom) Fig. 2 shows the series of transient absorption spectra of PPV and PFPV at various time delays with 400 mn excitation at 13 K In PPV, the bleaching band has a structure which is absent in PFPV. This structure originates from the depletion of the ground
Fig. 3 Transient absorption spectra of PPV and PFPV Fig. 3 shows the decay profiles of PA and bleaching signals of PPV and PFPV at 520 and 460 nm, respectively. The PA signal appears very quickly without detectable rise time and its decay is remarkably faster than that of bleaching indicating that this signal is due to the excited species instaneously generated by photoexcitation. For PA signal, PFPV has a shorter lifetime than PPV, which is caused by nonradiatively geminate recombination of polaron pairs. This behavior is due to the large electronegativity of fluorine attached to phenylene ring. Fluorine makes the conjugation of polymer backbone weak by reducing the density of delocalized electrons along the backbone and consequently i&a-chain interactions become weak. It means that the photoexcitation of PFPV generates interchain polaron pairs more easily than PPV, therefore the density of bound polaron pairs is high which makes decay faster. Bleaching signal decay exhibits double exponential decay. The lifetime of short component is about 1 ps and that of long component is about 800 ps. The short component comes from polaron pair recombination (-1 ps) and the long component comes from singlet exciton lifetim<-800 ps). The lifetime of short component of PFPV is faster than that of PPV, which is consistent with the result of PA decay profiles. Since the interchain polaron pairs generation in PFPV is superior to the intrachain exciton, which is responsible for the photoluminescence, the density of singlet excitons is lower and consquently the long component becomes longer in PFPV than that in PPV. This result is consistent with our time-resolved PL experiment3 This work wassupportedby a grantfYomthe Ministry of Science and Technology and the Center for Molecular Sciencethroughthe Korea Scienceand Engineering Foundation. Acknowledgment.
References
[l] K. S.Wong et al., J. Phys.C 20 (1987)L187. [2] J. W. P. Hsu, Appl. Rev.B, 49 (1994-1)712. [3] J. I. Leeet al., (unpublished).