Photon Upconversion through Triplet-Triplet Annihilation: Towards Higher Efficiency and Solid State Applications

Doctoral thesis, 2016

The sun is the only renewable energy source that can accommodate humanity’s energy needs today and in the foreseeable future. The sunlight reaching the planet’s surface is filtrated through the atmosphere, reducing its UV-light intensity in the 300-400 nm range, which indeed can be harmful to life as we know it in too large doses. However, many useful photoreactions require in practice such high-energy UV-photons, like the catalytic splitting of water to oxygen and hydrogen gas or molecular in-bond energy storage through isomerization to produce heat when reverting to the initial state. The efficiency of these applications could be improved with efficient conversion of low-energy visible to high-energy UV-light. One way to achieve this type of photon upconversion (UC) is through the process called Triplet-Triplet Annihilation (TTA) relying on the interaction between two molecules; a sensitizer and an annihilator. The sensitizer absorbs low energy visible photons as input and transfers that energy through Triplet Energy Transfer (TET) to an annihilator. Two triplet-excited annihilators can thereafter perform TTA, merging the energy equivalents of the two low-energy photons to produce one high-energy photon as output. This Thesis is focused on improving the known bimolecular UC system in fluid environment and approaching the ultimate goal of high efficiency UC in solid materials. In the fluid system I demonstrate the employment of thiol- and thioether-based compounds as scavengers for singlet excited oxygen with positive effect on UC efficiency and stability. In an attempt to aid future design and optimization of upconversion system components the anthracene is 9,10-substituted with electron withdrawing or donating groups while its TTA-UC function is evaluated revealing that substitution at para-positions leads to least perturbation of its spectroscopic properties. The ultimate goal is to achieve supramolecular TTA-UC systems capable of efficient intra-molecular TET and TTA processes and unhindered emission of the upconverted photons. As a first step, we focus on the intra-molecular TTA process with oligomers and dendrimers of 9,10-diphenylanthracene (DPA) which display positive effects on UC efficiency in solid matrix. In the second step the focus is on the intra-molecular TET process where the sensitizer-annihilator complexes are explored through Lewis base-acid coupling with orthogonal transition moments for minimum excited state short circuit effect. Additionally, kinetic simulations of the TTA-UC processes are conducted to aid understanding and optimization. Finally, one TTA-UC system is also applied to chemical in-bond energy storage.