Excellent ultrafast broadband optical limiting of functionalized twistacenes based on two-photon absorption-induced excited state absorption

Twistacene is an outstanding building block due to the large conjugated system. Herein, a series of twistacenes have been designed and synthesized with the assistant from a theoretical analysis based on the morphology of the natural transition orbitals of first excited state. These twistacenes exhibit two photon absorption and unique broadband excited singlet state absorption features, which is systematically confirmed by transient absorption spectra, two-photon excited fluorescence and z-scan experiments. The overlap of two-photon absorption band with excited state absorption band can improve the OL performance. The experimental results are consistent with theoretical prediction, which confirm the rationalization of molecular design strategy. The screened twistacene 5 exhibits excellent optical limiting behavior with extremely high transmittance and low threshold, where the threshold is 1.3–3.1 mJ/cm2 in a wide optical window from 532 nm to 900 nm in femtosecond regime and 69 mJ/cm2 in picosecond regime. The experimental results suggest twistacene is a promising candidate for nonlinear optical applications, especially for protection against ultrashort laser pulses.