Photon Up-Conversion Process to Test Media Ordering

In this work, triplet-triplet annihilation upconversion (TTA-UC) was employed as a sensitive tool to study the influence of media organization on the efficiency of bimolecular processes. For this purpose, an archetypal sensitizer/emitter pair of platinum octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA) was probed in oleic (OA) and elaidic acid (EA), chosen as isomeric phase change materials (PCMs). TTA-UC was monitored below and above the melting points of the PCMs. The results point out that the arrangement of solid- and liquid-phase media imposes constraints able to assist TTA-UC, resulting in higher efficiency in the solid phase of OA. A green-to-blue UC quantum yield of up to 3.6% was observed under ambient conditions with incoherent and low power excitation. The role and the effects of media constraints were monitored through steady-state and time-resolved luminescence, small-angle X-ray scattering (SAXS), Raman and UV-vis spectroscopy and were rationalized in terms of PtOEP aggregation.