Bimolecular processes involving exciton spin-state interactions gain attention for their deployment as wavelength-shifting tools. Particularly triplet–triplet annihilation induced photon energy up-conversion (TTA-UC) holds promise to enhance the performance of solar cell and photodetection technologies. Despite the progress noted, a correlation between the solid-state microstructure of photoactuating TTA-UC organic composites and their photophysical properties is missing. This lack of knowledge impedes the effective integration of functional TTA-UC interlayers as ancillary components in operating devices. We here investigate a solution-processed model green-to-blue TTA-UC binary composite. Solid-state films of a 9,10 diphenyl anthracene (DPA) blue-emitting activator blended with a (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) green-absorbing sensitizer are prepared with a range of compositions and examined by a set of complementary characterization techniques. Grazing incidence X-ray diffractometry (GIXRD) measurements identify three PtOEP composition regions wherein the DPA:PtOEP composite microstructure varies due to changes in the packing motifs of the DPA and PtOEP phases. In Region 1 (≤2 wt%) DPA is semicrystalline and PtOEP is amorphous, in Region 2 (between 2 and 10 wt%) both DPA and PtOEP phases are amorphous, and in Region 3 (≥10 wt%) DPA remains amorphous and PtOEP is semicrystalline. GIXRD further reveals the metastable DPA-β polymorph species as the dominant DPA phase in Region 1. Composition dependent UV-vis and FT-IR measurements identify physical PtOEP dimers, irrespective of the structural order in the PtOEP phase. Time-gated photoluminescence (PL) spectroscopy and scanning electron microscopy imaging confirm the presence of PtOEP aggregates, even after dispersing DPA:PtOEP in amorphous poly(styrene). When arrested in Regions 1 and 2, DPA:PtOEP exhibits delayed PtOEP fluorescence at 580 nm that follows a power-law decay on the ns time scale. The origin of PtOEP delayed fluorescence is unraveled by temperature- and fluence-dependent PL experiments. Triplet PtOEP excitations undergo dispersive diffusion and enable TTA reactions that activate the first singlet-excited (S1) PtOEP state. The effect is reproduced when PtOEP is mixed with a poly(fluorene-2-octyl) (PFO) derivative. Transient absorption measurements on PFO:PtOEP films find that selective PtOEP photoexcitation activates the S1 of PFO within ∼100 fs through an up-converted 3(d, d*) PtII-centered state.
KAUST Repository Item: Exported on 2023-03-06
Acknowledged KAUST grant number(s): OSR-CARF/CCF-3079
Acknowledgements: We thank Mr Marios Papageorgiou for his assistance during the sample preparation and acquisition of the composition-dependent FT-IR spectra. This work was co-funded by the European Regional Development Fund and the Republic of Cyprus through project EXCELLENCE/1216/0010 of the Cyprus Research and Innovation Foundation. This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) under Award No: OSR-CARF/CCF-3079. S. A. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – through the project “MARS” (project number 446281755) and the Spanish Ministry of Science, Innovation and Universities (MICINN) through project RTI2018-101020-B-100. J. C.-G. and C. S. are grateful to the China Scholarship Council (201608390023), to the Spanish Ministry of Science and Innovation (RTI2018-097508-B-I00, PID2021-128313OB-I00), the Regional Government of Madrid through projects NMAT2D-CM (S2018/NMT-4511), the program Proyectos Sinérgicos de I + D (Grant Y2018/NMT-5028 FULMATEN-CM), NANOCOV-CM (REACT-UE), and to the Campus of International Excellence (CEI). IMDEA Nanociencia acknowledges support from the Severo Ochoa Programme for Centres of Excellence in R&D (MINECO, grant CEX2020-001039-S).