Triplet–Triplet Annihilation-Induced Up-Converted Delayed Luminescence in Solid-State Organic Composites: Monitoring Low-Energy Photon Up-Conversion at Low Temperatures

Hitherto, the role of the enhanced intermolecular interactions and the effect of lowering the temperature on the process of triplet–triplet annihilation-induced up-converted delayed luminescence in solid-state composites systems have remained controversial. Here we address these issues by performing temperature-dependent time-integrated and time-gated luminescence spectroscopic studies on the model photon up-converting solid composite comprising the (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) Pt^II (PtOEP) sensitizer, mixed with the blue-light emitting 9,10 diphenyl anthracene (DPA) activator. Atomic force microscopy imaging and photoluminescence (PL) spectra confirm that the strength of intermolecular interactions in the DPA:PtOEP system can be tuned by keeping the composite either in its binary or in its ternary form with the use of the optically inert matrix of polystyrene (PS). By diluting DPA:PtOEP in PS, the concentration of the DPA excimeric and the PtOEP triplet dimer quenching sites is reduced and the lifetime of the DPA up-converted PL signal is prolonged to the microsecond time scale. By lowering the temperature to 100 K, the DPA up-converted luminescence intensity increases by a factor of 3, and this is attributed to the increased energetic disorder of the DPA excited states in the PS:DPA:PtOEP ternary system. These findings provide useful guidelines for the fabrication of efficient solid-state photon up-converting organic layers