Ultrafast optical probes of excited states in halogen substituted PPV derivatives

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 PEPV was faster than that in PPV. This behavior is attributed to the large electronegativity of fluorine substituted at phenyl ring of PPV.