- Email: [email protected]
Multiple pulse transient spectroscopy in luminescent π-conjugated polymers
Synthetic Metals 116 (2001) 5±7
Multiple pulse transient spectroscopy in luminescent p-conjugated polymers S.V. Frolova,*, Z. Baoa, M. Wohlgenanntb, Z.V. Vardenyb a
Bell Laboratories, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974, USA b Physics Department, University of Utah, Salt Lake City, UT 84112, USA
Abstract We closely examined relaxation processes between even and odd parity excitons in poly(p-phenylene vinylene) derivative polymers using a multiple femtosecond pulse transient spectroscopy. By tuning the energy of re-excitation and probe pulses across a wide spectral range, we were able to measure a complete spectrum of the 1Bu exciton and also probe its counterparts: higher lying mAg and kAg excitons. Dramatically different relaxation paths were found for two later states: whereas the majority of mAg excitons experience an ultrafast internal conversion back to their 1Bu state, most of kAg excitons transform into a non-emissive state, which we attribute to weakly bound polaron pairs. # 2001 Published by Elsevier Science B.V. Keywords: Optical absorption and emission spectroscopy; Time-resolved fast spectroscopy; Photoinduced absorption spectroscopy; Poly(phenylene vinylene) and derivatives
1. Introduction Ultrafast photophysics of luminescent p-conjugated polymers has been thoroughly investigated in the last several years [1±4]. It has been shown that intrachain excitons [5,6] determine the luminescent yield of a polymer and govern most of its optical transitions observed on a femto and picosecond time scales [7,8]. Particularly, in poly(p-phenylene vinylene) [PPV] polymers two broad infrared (IR) photoinduced absorption (PA) bands have been correlated with excitonic stimulated emission (SE) and attributed to exciton transitions to even parity (Ag) states [6]. We study ultrafast relaxation of Ag excitons in ®lms of PPV derivative polymers using a multiple femtosecond pulse spectroscopy. First, exciton absorption and emission spectra were measured continuously from 0.6 to 2.2 eV by means of a standard two-pulse pump-and-probe technique. Second, excitons were optically re-excited and resulting higher excited states were monitored using a three-pulse 1st pump-2nd pump-probe technique, which provided an insight into the nature of exciton optical transitions. Third, photoluminescence quenching (PLQ) was observed for a wide range of re-excitation energies and attributed to exciton dissociation. The resultant PLQ spectrum indicated that
* Corresponding author. E-mail address: [email protected] (S.V. Frolov).
0379-6779/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 0 3 - 8
optical exciton dissociation predominantly occurs for photon energies above 1 eV. 2. Experimental Thin polymer ®lms (thickness of 0.5±1 mm) were prepared by evaporation from chloroform solutions of two different PPV derivatives: dioctyloxy-PPV (DOO-PPV) [6] and dendritic side-chain substituted PPV (PPVD0) [9]. A pump-and-probe pulse correlation technique was used to obtain time-resolved photomodulation (PM) spectra. The excitation wavelength varied between 375 nm and 500 nm; the probe wavelength could be continuously changed from 560 nm to 2.3 mm and from 6 to 11 mm. In order to clarify the nature of two-photon states, we used a three-pulse transient PM technique. In this technique a population of 1Bu excitons is initially excited by a 1st pump-pulse (1) and then reexcited after delay time t2 by a 2nd pump-pulse (2) with a photon energy E2, tuned to a speci®c exciton transition (within PA1 or PA2 bands). The resulting exciton PM dynamics is monitored by a probe pulse (3) with a variable photon energy E3, and delay time t3 using either an absolute or relative measuring mode [10]. In the absolute mode total PM (DT) due to both pump pulses is measured, whereas in the relative mode only the PM change due to the 2nd pumppulse (dT) is detected using a double-frequency modulation (DM) technique [10]. In the DM technique the 1st pump and
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S.V. Frolov et al. / Synthetic Metals 116 (2001) 5±7
2nd pump beams were typically modulated at 0.9 MHz and 0.9 kHz, respectively; the PM signal was ®rst pre-ampli®ed by a high frequency lock-in ampli®er referenced at 0.9 MHz and then measured with a low frequency lock-in ampli®er referenced at 0.9 kHz. 3. Results Fig. 1 shows the transient PM spectra for DOO-PPV and PPVD0 ®lms measured at time delay t 2 ps after the excitation pulse. At such short time delays all prominent features in the PM spectra can be assigned to optical transitions of intrachain 1Bu excitons: a SE band above 1.7 eV and two PA bands in the IR spectral rangeÐPA1 and PA2, which were previously attributed to transitions from 1Bu to two Ag states denoted mAg and kAg, respectively [6]. The average exciton lifetime t measured from the SE decay was found to be about 200 ps for DOO-PPV ®lms and 300 ps for PPVD0 ®lms. The SE and PA decays are well correlated in the picosecond time range (Fig. 2), except for the 1.3± 1.7 eV range where at t > 300 ps contributions from longer lived excitations, such as triplet excitons and polaron pairs (which have overlapping PA bands), become evident [3±6]. The dynamics discussed below are identical for DOO-PPV and PPVD0. Exciton dynamics after re-excitation with E2 1:0 eV (PA1 band) was monitored by a probe pulse with E3 2:0 eV (SE band). Fig. 3(a) shows that re-excitation
Fig. 1. Transient PM spectra in DOO-PPV (a) and PPVD0 (b) films at t 2 ps. Inset shows the exciton energy level diagram in PPV derivatives.
Fig. 2. DT/T decays in PPVD0 probed at 2.1, 1.6 and 1.0 eV.
to mAg is followed by fast internal conversion from mAg back to 1Bu. Upon increasing E2 to 1.6 eV (PA2 range) we found a completely different dT dynamics: Fig. 2(b) indicates that kAg does not relax back to 1Bu. An intermediate behavior (both fast and long-lived dT components) was observed for 1.1 eV < E2 < 1:6 eV. It is possible that kAg relaxation occurs into a pair of triplets (singlet ®ssion), however, we ®nd that the long-lived triplet band at 1.4± 1.5 eV is quenched as a results of 1Bu depletion. Therefore,
Fig. 3. (a) dT decay in PPVD0 at E2 1:05 eV, E3 2:1 eV, t2 25 ps. (b) Normalized dT/DT decay in DOO-PPV at E2 1:6 eV, E3 0:6 eV, t2 50 ps.
S.V. Frolov et al. / Synthetic Metals 116 (2001) 5±7
singlet ®ssion cannot be the primary kAg relaxation channel; instead, exciton dissociation seems to be the only remaining alternative. Exciton dissociation by the 2nd pump-pulse resulted in well pronounced PLQ. The modulation of integrated PL (dPL) was measured using the DM technique. The onset of ef®cient PLQ corresponds to EQ 1:1 eV, which together with the 1Bu energy of 2.2 eV adds up to 3.2 eV. This energy is close to the onset of intrinsic photocarrier generation previously measured in PPV derivative ®lms, [11] which con®rms our assignment of PLQ to all-optical exciton dissociation. In summary, we studied ultrafast relaxation processes of mAg and kAg exciton states in PPV derivative ®lms and found that while the former remains a tightly bound exciton, the later is strongly mixed with the electron-hole continuum and auto-ionizes with high probability. For the ®rst time these states were isolated and their unique dynamics was measured. We demonstrated all-optical exciton dissociation into long-lived polaron pairs.
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Acknowledgements The work at Utah was supported in part by the NSF, DMR-9732820.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
R. Kersting et al., Phys. Rev. Lett. 70 (1993) 3820. R. Kersting et al., Phys. Rev. Lett. 73 (1994) 1440. M. Yan et al., Phys. Rev. Lett. 72 (1994) 1104. M. Yan et al., Phys. Rev. Lett. 75 (1995) 1992. N.T. Harison et al., Phys. Rev. Lett. 77 (1996) 1881. S.V. Frolov et al., Phys. Rev. Lett. 78 (1997) 4285. V.A. Shakin, S. Abe, Y. Shimoi, Phys. Rev. B 57 (1998) 6386. A. Chakrabarti, S. Mazumdar, Phys. Rev. B 59 (1999) 4839. Z. Bao, K.R. Amundson, A.J. Lovinger, Macromolecules 31 (1998) 8647. [10] S.V. Frolov, Z.V. Vardeny, Rev. Sci. Instrum. 69 (1998) 1257. [11] S. Barth, H. BaÈssler, Phys. Rev. Lett. 79 (1997) 4445.
Multiple pulse transient spectroscopy in luminescent p-conjugated polymers S.V. Frolova,*, Z. Baoa, M. Wohlgenanntb, Z.V. Vardenyb a
Bell Laboratories, Lucent Technologies, 600 Mountain Ave., Murray Hill, NJ 07974, USA b Physics Department, University of Utah, Salt Lake City, UT 84112, USA
Abstract We closely examined relaxation processes between even and odd parity excitons in poly(p-phenylene vinylene) derivative polymers using a multiple femtosecond pulse transient spectroscopy. By tuning the energy of re-excitation and probe pulses across a wide spectral range, we were able to measure a complete spectrum of the 1Bu exciton and also probe its counterparts: higher lying mAg and kAg excitons. Dramatically different relaxation paths were found for two later states: whereas the majority of mAg excitons experience an ultrafast internal conversion back to their 1Bu state, most of kAg excitons transform into a non-emissive state, which we attribute to weakly bound polaron pairs. # 2001 Published by Elsevier Science B.V. Keywords: Optical absorption and emission spectroscopy; Time-resolved fast spectroscopy; Photoinduced absorption spectroscopy; Poly(phenylene vinylene) and derivatives
1. Introduction Ultrafast photophysics of luminescent p-conjugated polymers has been thoroughly investigated in the last several years [1±4]. It has been shown that intrachain excitons [5,6] determine the luminescent yield of a polymer and govern most of its optical transitions observed on a femto and picosecond time scales [7,8]. Particularly, in poly(p-phenylene vinylene) [PPV] polymers two broad infrared (IR) photoinduced absorption (PA) bands have been correlated with excitonic stimulated emission (SE) and attributed to exciton transitions to even parity (Ag) states [6]. We study ultrafast relaxation of Ag excitons in ®lms of PPV derivative polymers using a multiple femtosecond pulse spectroscopy. First, exciton absorption and emission spectra were measured continuously from 0.6 to 2.2 eV by means of a standard two-pulse pump-and-probe technique. Second, excitons were optically re-excited and resulting higher excited states were monitored using a three-pulse 1st pump-2nd pump-probe technique, which provided an insight into the nature of exciton optical transitions. Third, photoluminescence quenching (PLQ) was observed for a wide range of re-excitation energies and attributed to exciton dissociation. The resultant PLQ spectrum indicated that
* Corresponding author. E-mail address: [email protected] (S.V. Frolov).
0379-6779/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 3 7 9 - 6 7 7 9 ( 0 0 ) 0 0 5 0 3 - 8
optical exciton dissociation predominantly occurs for photon energies above 1 eV. 2. Experimental Thin polymer ®lms (thickness of 0.5±1 mm) were prepared by evaporation from chloroform solutions of two different PPV derivatives: dioctyloxy-PPV (DOO-PPV) [6] and dendritic side-chain substituted PPV (PPVD0) [9]. A pump-and-probe pulse correlation technique was used to obtain time-resolved photomodulation (PM) spectra. The excitation wavelength varied between 375 nm and 500 nm; the probe wavelength could be continuously changed from 560 nm to 2.3 mm and from 6 to 11 mm. In order to clarify the nature of two-photon states, we used a three-pulse transient PM technique. In this technique a population of 1Bu excitons is initially excited by a 1st pump-pulse (1) and then reexcited after delay time t2 by a 2nd pump-pulse (2) with a photon energy E2, tuned to a speci®c exciton transition (within PA1 or PA2 bands). The resulting exciton PM dynamics is monitored by a probe pulse (3) with a variable photon energy E3, and delay time t3 using either an absolute or relative measuring mode [10]. In the absolute mode total PM (DT) due to both pump pulses is measured, whereas in the relative mode only the PM change due to the 2nd pumppulse (dT) is detected using a double-frequency modulation (DM) technique [10]. In the DM technique the 1st pump and
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S.V. Frolov et al. / Synthetic Metals 116 (2001) 5±7
2nd pump beams were typically modulated at 0.9 MHz and 0.9 kHz, respectively; the PM signal was ®rst pre-ampli®ed by a high frequency lock-in ampli®er referenced at 0.9 MHz and then measured with a low frequency lock-in ampli®er referenced at 0.9 kHz. 3. Results Fig. 1 shows the transient PM spectra for DOO-PPV and PPVD0 ®lms measured at time delay t 2 ps after the excitation pulse. At such short time delays all prominent features in the PM spectra can be assigned to optical transitions of intrachain 1Bu excitons: a SE band above 1.7 eV and two PA bands in the IR spectral rangeÐPA1 and PA2, which were previously attributed to transitions from 1Bu to two Ag states denoted mAg and kAg, respectively [6]. The average exciton lifetime t measured from the SE decay was found to be about 200 ps for DOO-PPV ®lms and 300 ps for PPVD0 ®lms. The SE and PA decays are well correlated in the picosecond time range (Fig. 2), except for the 1.3± 1.7 eV range where at t > 300 ps contributions from longer lived excitations, such as triplet excitons and polaron pairs (which have overlapping PA bands), become evident [3±6]. The dynamics discussed below are identical for DOO-PPV and PPVD0. Exciton dynamics after re-excitation with E2 1:0 eV (PA1 band) was monitored by a probe pulse with E3 2:0 eV (SE band). Fig. 3(a) shows that re-excitation
Fig. 1. Transient PM spectra in DOO-PPV (a) and PPVD0 (b) films at t 2 ps. Inset shows the exciton energy level diagram in PPV derivatives.
Fig. 2. DT/T decays in PPVD0 probed at 2.1, 1.6 and 1.0 eV.
to mAg is followed by fast internal conversion from mAg back to 1Bu. Upon increasing E2 to 1.6 eV (PA2 range) we found a completely different dT dynamics: Fig. 2(b) indicates that kAg does not relax back to 1Bu. An intermediate behavior (both fast and long-lived dT components) was observed for 1.1 eV < E2 < 1:6 eV. It is possible that kAg relaxation occurs into a pair of triplets (singlet ®ssion), however, we ®nd that the long-lived triplet band at 1.4± 1.5 eV is quenched as a results of 1Bu depletion. Therefore,
Fig. 3. (a) dT decay in PPVD0 at E2 1:05 eV, E3 2:1 eV, t2 25 ps. (b) Normalized dT/DT decay in DOO-PPV at E2 1:6 eV, E3 0:6 eV, t2 50 ps.
S.V. Frolov et al. / Synthetic Metals 116 (2001) 5±7
singlet ®ssion cannot be the primary kAg relaxation channel; instead, exciton dissociation seems to be the only remaining alternative. Exciton dissociation by the 2nd pump-pulse resulted in well pronounced PLQ. The modulation of integrated PL (dPL) was measured using the DM technique. The onset of ef®cient PLQ corresponds to EQ 1:1 eV, which together with the 1Bu energy of 2.2 eV adds up to 3.2 eV. This energy is close to the onset of intrinsic photocarrier generation previously measured in PPV derivative ®lms, [11] which con®rms our assignment of PLQ to all-optical exciton dissociation. In summary, we studied ultrafast relaxation processes of mAg and kAg exciton states in PPV derivative ®lms and found that while the former remains a tightly bound exciton, the later is strongly mixed with the electron-hole continuum and auto-ionizes with high probability. For the ®rst time these states were isolated and their unique dynamics was measured. We demonstrated all-optical exciton dissociation into long-lived polaron pairs.
7
Acknowledgements The work at Utah was supported in part by the NSF, DMR-9732820.
References [1] [2] [3] [4] [5] [6] [7] [8] [9]
R. Kersting et al., Phys. Rev. Lett. 70 (1993) 3820. R. Kersting et al., Phys. Rev. Lett. 73 (1994) 1440. M. Yan et al., Phys. Rev. Lett. 72 (1994) 1104. M. Yan et al., Phys. Rev. Lett. 75 (1995) 1992. N.T. Harison et al., Phys. Rev. Lett. 77 (1996) 1881. S.V. Frolov et al., Phys. Rev. Lett. 78 (1997) 4285. V.A. Shakin, S. Abe, Y. Shimoi, Phys. Rev. B 57 (1998) 6386. A. Chakrabarti, S. Mazumdar, Phys. Rev. B 59 (1999) 4839. Z. Bao, K.R. Amundson, A.J. Lovinger, Macromolecules 31 (1998) 8647. [10] S.V. Frolov, Z.V. Vardeny, Rev. Sci. Instrum. 69 (1998) 1257. [11] S. Barth, H. BaÈssler, Phys. Rev. Lett. 79 (1997) 4445.