Large Transmittance Change Induced by Exciton Accumulation under Weak Continuous Photoexcitation

Excited‐state absorption of materials is caused by instantaneous accumulation of excitons when irradiated with a high‐intensity laser pulse. If excitons can be accumulated by irradiation with weak continuous incoherent light at room temperature (RT) in air, then excited‐state absorption can be obtained over a large area, which is potentially useful for a variety of optical applications. Here, photoresponsive transmittance control using long‐lived RT triplet excitons is demonstrated. Host–guest glass films composed of a secondary amino‐substituted deuterated aromatic carbon material as a guest and an amorphous hydroxyl steroidal compound as a host form guest‐based triplet excitons with an RT lifetime of longer than 1 s upon photoexcitation and the triplet excitons of the guest exhibit a large absorption coefficient over the whole visible region. Efficient accumulation of the triplet excitons of the guest triggered by photoexcitation causes a large decrease in the transmittance of the host–guest films. The transmittance of one of the host–guest films decreases from 98% to 30% over the whole visible range and from 92% to 13% at the wavelength of maximum triplet–triplet absorption of the guest as the power of UV photoexcitation is increased from 10⁻¹ to 10³ mW cm⁻².