Synthesis and Photophysical Characterization of Annihilator-Sensitizer Pairs for Triplet-Triplet Annihilation Based Photon Upconversion
Doctoral thesis, 2017
Transitioning to a future society, independent of fossil fuels, will denitely require the use of solar radiation for power and fuel production. Due to energy mismatch between
device absorption and the broad band irradiation provided by the sun there are severe limitations to how efficient devices for direct conversion of solar radiation can be. One
way to better use the solar radiation would be to use transmitted low energy photons and convert them to higher energy photons that can be used in the device. Triplet-triplet
annihilation (TTA) based photon upconversion (UC) is one viable way of utilizing these transmitted low energy photons. In TTA-UC two low energy photons are fused into one photon of higher energy. To achieve TTA-UC two components are required. The first type, the sensitizer, absorbs the photon energy and transfers it through triplet energy transfer (TET) to the second type, the annihilator. Photon absorption and energy transfer must occur at least twice for two distinct annihilators to be able to fuse the energy of two photons through triplet-triplet annihilation, forming one annihilator in its first excited singlet state. The singlet excited annihilator can then emit a photon of high energy.
This Thesis covers the design, synthesis and characterization of new annihilators and sensitizers with the overall aim to develop design parameters to rationally design efficient annihilator-sensitizer pairs capable of forming supra-molecular structures with intra-molecular TET and TTA. First, semiconductor nanocrystals (NCs) based on CdS are explored as triplet sensitizers for visible to UV upconversion using 2,5-diphenyloxazole (PPO) as the annihilator. With the NC based sensitizers a 5 times improvement of upconversion quantum yield (ΦUC) is realized. Further improvements of the ΦUC would require better annihilators. Looking closer at the popular blue-emitting annihilator 9,10- diphenylanthracene (DPA) I show that substitution on the phenyl rings does not affect the energy levels, resulting in similar UC properties for all studied 9,10-phenylsubstituted anthracenes. In the conformationally flexible anthracene 9,10-bis(phenylethynyl)anthracene (BPEA), however, a more than 7 times reduction in the ΦUC is observed. A new loss mechanism based on the shapes of the singlet and triplet energy surfaces is introduced to explain the lower ΦUC.
Finally, anthracene annihilators are connected axially to metal porphyrin sensitizers through coordination. Both the desired triplet energy transfer from sensitizer to annihilator and the undesired singlet energy transfer from annihilator to sensitizer are studied in these systems by time resolved emission and absorption techniques. Based on the results presented herein we are now moving closer to developing supra-molecular structures suitable for intra-molecular TTA-UC.