Molecularly Dispersed Donors in Acceptor Molecular Crystals for Photon Upconversion under Low Excitation Intensity

For real‐world applications of photon upconversion based on the triplet–triplet annihilation (TTA‐UC), it is imperative to develop solid‐state TTA‐UC systems that work effectively under low excitation power comparable to solar irradiance. As an approach in this direction, aromatic crystals showing high triplet diffusivity are expected to serve as a useful platform. However, donor molecules inevitably tend to segregate from the host acceptor crystals, and this inhomogeneity results in the disappointing performance of crystalline state TTA‐UC. In this work, a series of cast‐film‐forming acceptors was developed, which provide both regular acceptor alignment and soft domains of alkyl chains that accommodate donor molecules without segregation. A typical triplet sensitizer, PtII octaethylporphyrin (PtOEP), was dispersed in these acceptor crystals without aggregation. As a result, efficient triplet energy transfer from the donor to the acceptor and diffusion of triplet excitons among regularly aligned anthracene chromophores occurred. It resulted in TTA‐UC emission at low excitation intensities, comparable to solar irradiance. Overcoming donor aggregation: A long‐standing problem of aromatic‐crystal‐based photon upconversion (UC), donor aggregation in the acceptor crystal, is solved herein by modifying the acceptor with multiple alkyl chains. This improved the donor‐to‐acceptor energy‐transfer efficiency, leading to effective UC processes even at low excitation intensities comparable to solar irradiance (see figure).