Nonlinear absorption processes in triphenylamine-dithienothiophene- triphenylamine

We investigated the nonlinear absorption mechanism in the triphenylamlne-dithienothiophene-triphenylamine molecule by using laser pulses with two different pulse widths, 7 ns and 35 ps. The single-beam transmission measurement with a 7-ns pulse resulted in a larger nonlinear absorption than that with a 35-ps pulse and saturated at high input intensities due to the excited state absorption (ESA) initiated by two-photon absorption (TPA). For a quantitative characterization of the TPA and ESA contributions to the nonlinear absorption, we estimated the ESA spectrum and lifetime by using a pump-probe transmission measurement with a 35-ps pulse. A 5-level model was used to explain both the nonlinear absorption and the one-photon and the two-photon pumped fluorescence.