Circumventing Spectrum Mismatch - Studies of Triplet-Triplet Annihilation Upconversion, Singlet Fission and Two-Photon Absorption in Photoactive Materials

Abstract: Solar energy stands out for its potential to supply the global energy demand by itself. Therefore, it is of great value to expand the use of processes that involve light such as conversion to electricity or fuels, or to drive high-energy reactions. However, the limited availability of photons with the desired energy required to induce a certain photophysical process poses a challenge. To circumvent this spectral mismatch, processes that up- and down-convert photon energy can be used. In this work the focus lies on photon upconversion through triplet-triplet annihilation (TTA-UC) and two-photon absorption (2PA) and downconversion through singlet fission (SF).  One key challenge for up- and downconversion processes is that for them to be useful they must be incorporated into practical devices. Therefore, one of the overall objectives of this work is to evaluate photoactive materials that both change the photon energy and have potential to be incorporated with a working device. This means moving away from diffusion control in liquid solution.     Self-assembling organogels, with chromophores covalently attached to the gelator backbone, were tested as platforms for TTA-UC and SF. The results show that chromophore-chromophore interactions can be tuned by choice of substitution position, and that it is possible to obtain photon energy conversion in the studied self-assembling gels, although efficiencies need to be improved for practical applications. The results indicate that there is potential for future development of gel-based self-assembled photoactive materials.  To demonstrate how TTA-UC can be applied to circumvent spectral mismatches in devices, it was used to sensitize the well-known catalyst titanium dioxide (TiO2). A TTA-UC solution was used to absorb visible light and the upconverted emission was in turn used to sensitize a TiO2 thin film. Despite needs for optimizing the setup, visible light photoexcitation could be confirmed and the number of holes in valence band could be quantified to mM concentrations, using a redox couple, which at the same time confirmed the reactivity of the sensitized TiO2.