We demonstrate up-conversion of noncoherent sunlight realized by ultralow excitation intensity. The bimolecular up-conversion process in our systems relies on the presence of a metastable triplet excited state, and thus has dramatically different photophysical characteristics relative to the other known methods for photon up-conversion (two-photon absorption, parametric processes, second harmonic generation, sequential multiphoton absorption, etc.).

An emitter/sensitizer couple (see picture; C red/green, H white, N blue, Pd gray) was specially designed for the process of noncoherently excited photon up‐conversion. The hypsochromic shift between the energy of the excitation photons and the emitted photons is about 0.7 eV, and an external quantum yield of 0.04 is achieved. As noncoherent excitation source, the near‐infrared part of the solar spectrum was used.

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The up‐conversion photoluminescence (PL) in films of polyfluorene (PF) doped with metallated porphyrins is reported for the first time. The dependence of the up‐conversion process on the pump laser intensity and wavelength, the central metal moiety of the dopants, and the temperature is presented. Up‐conversion emission is observed at pump intensities as low as 2 kW cm–2. Comparison of the PF integral PL intensities after laser excitation by 532, 543, and 405 nm enables the discussion of the energy‐transfer mechanism and the efficiency of the process.