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 beta-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 (sigma(34)) compared with that of the ground state (sigma(01)) for d(12)-coronene is also caused by the highly symmetric 2D aromatic structure. The efficient accumulation of the triplet excitons and high sigma(34) - sigma(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.