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) PtII (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.

Hitherto, great strides have been made in the development of organic systems that exhibit triplet-triplet annihilation-induced photon-energy upconversion (TTA-UC). Yet, the exact role of intermolecular states in solid-state TTA-UC composites remains elusive. Here we perform a comprehensive spectroscopic study in a series of solution-processable solid-state TTA-UC organic composites with increasing segregated phase content for elucidating the impact of aggregate formation in their TTA-UC properties. Six different states of aggregation are reached in composites of the 9,10-diphenylanthracene (DPA) blue emitter mixed with the (2,3,7,8,12,13,17,18-octaethylporphyrinato)platinum(II) sensitizer (PtOEP) in a fixed nominal ratio (2 wt % PtOEP). Fine-tuning of the PtOEP and DPA phase segregation in these composites is achieved with a low-temperature solution-processing protocol when three different solvents of increasing boiling point are alternatively used and when the binary DPA:PtOEP system is dispersed in the optically inert polystyrene (PS) matrix (PS:DPA:PtOEP). Time-gated (in the nanosecond and microsecond time scales) photoluminescence measurements identify the upper level of PtOEP segregation at which the PtOEP aggregate-based networks favor PtOEP triplet exciton migration toward the PtOEP:DPA interfaces and triplet energy transfer to the DPA triplet manifold. The maximum DPA TTA-UC luminescence intensity is ensured when the bimolecular annihilation constant of PtOEP remains close to γ = 1.1 × 10 cm s. Beyond this PtOEP segregation level, the DPA TTA-UC luminescence intensity decreases because of losses caused by the generation of PtOEP delayed fluorescence and DPA phosphorescence in the nanosecond and microsecond time scales, respectively.

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