Large Reverse Saturable Absorption at the Sunlight Power Level Using the Ultralong Lifetime of Triplet Excitons

Large reverse saturable absorption (RSA) at the sunlight power level was observed from a host–guest amorphous material composed of d 12 -coronene as a guest and β-estradiol as a rigid amorphous host. The large highly symmetric two-dimensional (2D) aromatic structure of d 12 -coronene doped into the rigid host causes ultralong triplet lifetime as well as high triplet yield, serving efficient accumulation of the triplet excitons of d 12 -coronene under weak excitation light. The high absorption coefficient of the triplet state (σ 34 ) compared with that of the ground state (σ 01 ) for d 12 -coronene is also caused by the highly symmetric 2D aromatic structure. The efficient accumulation of the triplet excitons and high σ 34 – σ 01 led to an onset of the RSA characteristics under irradiance below 10 –1 mW cm –2 . A film of the amorphous material showed a large decrease of transmittance from 66% at 0.02 mW cm –2 to 24% at 50 mW cm –2 for continuous light at 405 nm.