Two-photon absorption spectrum of poly(fluorene)

We report here on the two-photon absorption (TPA) properties of poly(fluorene). The two-photon absorption spectrum has been characterized in the range 550–680 nm. A resonance at λ=625 nm is observed with a cross-section of 2×10−46 cm4 s/photon-molecule. This large value is interpreted as the result of dipolar coupling between 12 monomer units, i.e. the polymer coherence length. In the framework of a three-level model, the TPA transition is related to the second excited state 2Ag lying at 6200 cm−1 above the lowest excited state 1Bu.