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 d₁₂-coronene as a guest and β-estradiol as a rigid amorphous host. The large highly symmetric two-dimensional (2D) aromatic structure of d₁₂-coronene doped into the rigid host causes ultralong triplet lifetime as well as high triplet yield, serving efficient accumulation of the triplet excitons of d₁₂-coronene under weak excitation light. The high absorption coefficient of the triplet state (σ₃₄) compared with that of the ground state (σ₀₁) for d₁₂-coronene is also caused by the highly symmetric 2D aromatic structure. The efficient accumulation of the triplet excitons and high σ₃₄ - σ₀₁ 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.