Excited state absorption in conjugated polymers: Photoinduced transparency

The understanding of excited states of conjugated polymers is of great interest for photonics, especially for fabricating electroluminescent and optical devices. This work concerns the excited state absorption of three conjugated polymers: poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), poly[2-[ethyl-[4-(4-nitro-phenylazo)-phenyl]-amino]-ethane-(3-thienyl)-ethanoate] (PAzT), and poly(3-octyl-thiophene) (POT). The nonlinear spectra from 450 nm up to 700 nm for the three polymers were obtained by means of the white-light continuum Z-scan technique. Saturation of absorption was observed due to the ground state depletion for the three polymers. In addition to this effect, we also observed a small excited state absorption for POT. The absorption from the first to a higher excited state is negligible for MEH-PPV and PAzT. In contrast, POT presented absorption for a higher excited state, which allowed the determination of its excited state absorption cross-section.