Photoexcitation dynamics of poly(1,4-phenylene vinylene) (PPV) and poly(2-fluoro-1,4-phenylene vinylene) (PFPV) were investigated. The femtosecond transient absorption spectra were recorded by using a high power femtosecond Ti:sapphire laser system. The laser pulses with 150 fs and 300 μ J/pulse at 800 nm were used to generate white light continuum for a probe beam, and the second harmonic generation at 400nm 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 from 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 ring of PPV.
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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 intra-chain 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 Acknowledgment. This work was supportedb y a grant fYomt he Ministry of Science and Technology and the Center for Molecular Sciencet hrought he Korea Sciencea nd Engineering Foundation.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Condensed Matter Physics
- Mechanics of Materials
- Mechanical Engineering
- Metals and Alloys
- Materials Chemistry