Ultrafast intrachain photoexcitation of polymeric semiconductors

We report on excited state dynamics in isolated poly(9,9-dioctylfluorene) chains obtained by embeddingthe polymer in an inert plastic matrix. Early events (<300 fs) of intrachain photophysics are detectedby pump-probe spectroscopy using tunable UV 25-fs pump pulses and sub-10-fs visible probe pulses. Weshow that higher-lying optical states, reached by multiphoton transitions, give rise to on-chain chargeseparation on the ultrafast time scale. The intrachain charge pair decays geminately within 500 fs to thelowest singlet state. Characteristic time scales for internal conversion and intramolecular vibrationalredistribution are also determined.