Luminescence and upconversion from thulium(iii) species in solutionElectronic supplementary information (ESI) available: Absorption spectra, TCSPC traces and fits and emission spectra. See DOI: 10.1039/c2cp42228j

Thulium salts and complexes are shown to be emissive from three states in the excited state manifold of Tm 3+ . Formation of the 1 D 2 state can result in luminescence, or in energy transfer to the lower energy 1 G 4 and 3 H 4 emissive states. Where chromophores are present in the ligand structure, emission is restricted to thulium centred emissive states that are lower in energy than the chromophore centred donor state. We have also observed direct multi-photon excitation of the thulium excited state manifold. Furthermore, additional transitions are observed in the multi-photon excitation spectra that are consistent with upconversion as a consequence of sequential single photon absorption and relaxation processes within the thulium excited state manifold. Solutions of thulium salts and complexes are emissive from three states and exhibit upconversion through the f-electron excited state manifold.