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.