Upconversion (UC) has attracted much attention due to its potential applications for photovoltaics, photocatalysis, nonlinear photonics, and so forth. [1][2][3][4][5][6][7][8][9][10][11][12] In principle, two techniques are available for upconversion from a chemist's perspective. The first one is to use two-photon absorption (TPA) fluorescent dyes. [4] However, this approach suffers from fundamental drawbacks, for example, coherent light with high power density (typically MW cm À2 , that is, 10 6 W cm À2 ) is required for excitation, which is well beyond the energy level of normal light sources (the terrestrial solar radiation is ca. 0.10 W cm À2 ). [4] Furthermore, it is difficult to modify the molecular structures of TPA dyes to achieve a specific upconversion wavelength and at the same time to keep a high TPA cross section.
A new approach for upconversion is based on triplettriplet annihilation (TTA), which is promising for practical applications, such as photovoltaics (e.g. dye-sensitized solar cells). [4][5][6][7][8][9][10][11][12][13][14][15][16][17] In this approach, a triplet sensitizer, normally a transition-metal complex with triplet excited states that are accessible upon photoexcitation (e.g. platinum(II) or palladium(II) porphyrin complexes), is used to harvest the excitation energy and transfer it to the triplet acceptor (annihilator/emitter, such as anthracene, perylene, etc.) via triplet-triplet energy transfer (TTET; see the Jablonski diagram in the Supporting Information). The excitation and emission wavelengths of TTA upconversion can be readily changed by independent selection of the triplet sensitizers and triplet acceptors, and the excitation power can be as low as a few mW cm À2 (lower than solar light). [1,2,5] Recently we showed that long-lived 3 IL (intraligand) excited states are more efficient to sensitize TTA upconversion, [12] than the normal short-lived 3 MLCT excited states Scheme 1. Chemical structures of the sensitizers and the triplet acceptor 9,10-diphenylanthracene (DPA). Note that the complexes are dications and the PF 6 À anions were omitted for clarity.