Sensitizer-Free Photon Upconversion in Single-Component Brominated Aromatic Crystals

A new methodology of single-component photon upconversion (UC) based on S₀-to-T₁ (S−T) absorption is demonstrated. Although the S−T absorption is spin-forbidden, modification of anthracene chromophores with heavy bromine atoms allows the direct population of the excited triplets in crystals upon near-infrared (NIR) excitation with incident light intensities of ∼W cm⁻². The consequent triplet energy migration (TEM) and triplet-triplet annihilation (TTA) events in crystals lead to blue UC emission with a large anti-Stokes shift over 0.9 eV. The generality of the S−T absorption-based UC is further confirmed with crystals of brominated perylene-derivatives that show a green UC emission under a longer-wavelength NIR excitation (λ_ex=856 nm). The single-component UC with large anti-Stokes shifts observed for brominated aromatic crystals compensates for the shortcomings of conventional two-component solid UC system, i. e., energy losses intrinsic to the intersystem crossing of the sensitizers and the triplet energy transfer from sensitizer to emitter. It offers a simple and complementary UC methodology to the conventional two-photon absorption (TPA) and TTA-UC.