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Low power photon up-conversion using MLCT sensitizers

The proposed route to achieve short-wavelength (high energy) light production from low energy sources utilizes metal-ligand charge transfer (MLCT)-sensitized anti-Stokes up-converted fluorescence in non-covalent triplet energy transfer assemblies. The data is consistent with a reaction sequence where visible light produces the metal-ligand charge transfer (MLCT) excited state exclusively, which in turn forms 3An* or 3DPA*, where An = anthracene, and DPA = 9,10-diphenylanthracene. Two excited sensitized triplet molecules diffuse to form an encounter complex leading to triplet-triplet annihilation and yielding one ground and one excited state chromophore. The latter decays radiatively to the ground state by the emission of UV and blue light, whose spectra are indistinguishable from their respective singlet fluorescence profile.

Schematic diagram of the process, where dmb = 4,4'-dimethyl-2,2'-bipyride.

The Jablonski diagram of the sensitized anti-Stokes delayed fluorescence

In essence, the energy from two separately absorbed quanta on two different sensitized triplets can be transferred to one molecule via triplet-triplet annihilation, resulting in delayed singlet fluorescence with a higher frequency than that of the exciting light, thus called “up-converted” fluorescence.