Solution to the Host–Guest Compatibility Problem of Solid Triplet–Triplet Annihilation Photon Upconversion by a Molecular-Anchor Sensitizer Approach

Efficient and uniform solid-state triplet–triplet annihilation photon upconversion (TTA-UC) is demonstrated by the solution cast film doped with the emitter-tethered sensitizer, in which the emitter part acts as a molecular anchor to control segregation of guest sensitizer molecules from the host matrix of emitter molecules on solidification. The tethered sensitizer, named as molecular-anchor sensitizer (MAS), was implemented as the supramolecules consisting of Pd-tetraphenylporphyrin (PdTPP) unit as the core and 4 or 8 units of 9,10-diphenylanthracene (DPA) tethered to the core with a flexible molecular chain. Saturated solution of DPA doped with a tiny amount of MAS (1:1000 or less in a molar ratio) was used for casting. The solid films fabricated from this solution showed good uniformity of upconversion (UC) emission. Most crystalline grains showed intense UC emission by 532 nm excitation, while a small number of crystalline grains showed UC emission for the film with a conventional (nontethered) sensitizer. The UC quantum yield (UC-QY, 50% basis) of the solid film with MAS was found to be 0.01–0.02 at a molar ratio of sensitizer to emitter of 1:1000. It was found that a decrease of the sensitizer concentration suppressed the fluorescence quenching of DPA by the sensitizer and the UC-QY increased to 0.13. The internal quantum yield, which is the quotient of doubled UC-QY divided by the fluorescence quantum yield, was found to depend on the number and length of the tether of MAS and to be close to unity for the sensitizers with fewer (4) and longer (C6) tethers regardless of the kind. The concept of MAS opens a new way for further development of efficient solid TTA-UC systems by providing better compatibility between the sensitizer and the emitter in the solid matrix.