title: All-Solution-Based Aggregation Control in Solid-State Photon Upconverting Organic Model Composites.
abstract: 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 γTTA-PtOEP= 1.1 × 10-13cm3s-1. 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.
authors.affiliation: Fondazione Istituto Italiano di Tecnologia, Centre for Nano Science and Technology @PoliMi , Via Pascoli 70/3, 20133 Milano, Italy.
authors.forename: Hossein
authors.initials: H
authors.lastname: Goudarzi
authors.affiliation: Department of Mechanical Engineering and Materials Science and Engineering, Cyprus University of Technology , 45 Kitiou Kyprianou Street, Limassol 3041, Cyprus.
authors.forename: Panagiotis E
authors.identifier: http://orcid.org/0000-0002-5336-249X
authors.initials: PE
authors.lastname: Keivanidis
keywords: energy migration; exciton hopping; phosphorescence; photon upconversion; sensitization; triplet fusion