Two-photon absorption spectra of luminescent conducting polymers

We report the two-photon absorption (TPA) spectra of poly(2,5-dibutoxy-p-phenylene acetylene) (PPA-DBO), poly(2,5-dioctyloxy-p-phenylene vinylene) (PPV-DOO), and poly(3-hexylthiophene) (PAT6) in the spectral range extending from 576 nm to 846 nm. Using the Z-scan technique on the polymers in solution, we measured a strong two-photon allowed transition in all three materials which we attribute to the mA(g) essential state. The mA(g) in all three polymers occurs at, approximately, 1.2 times the optical bandgap, E-g [E(1B(u))]. The excitonic nature of the A(g)-character states is indicated by the lack of a TPA band at or near E-g. Calculated chi((3)) spectra, using the mAg state, two additional states above the 1B(u) exciton, and a lineshape function to account for chain length distribution, exhibit the same spectral features observed in the TPA spectra.