Intramolecular Electronic Interactions in Photon Upconversion and Singlet Fission

Doctoral thesis, 2021

The sun provides our planet with an abundance of energy in the form of solar light – a continuous stream of photons. Increasing our direct utilization of solar energy will constitute a necessary part in our ongoing replacement of fossil fuels with energy sources free from greenhouse gas emissions. The sunlight contains a broad spectrum of photon energies, but only a fraction of the photons in this spectrum can be harvested by a solar energy device. In fact, the energy harvesting efficiency of solar energy technologies is mainly limited by the mismatch between the solar spectrum and the photon energies that the solar energy device can utilize efficiently. Photon energy conversion techniques provide a way to circumvent this mismatch by converting incoming photons of too high or too low energy to photons with energy that matches the absorption of the solar energy device, thus enabling utilization of a larger part of the solar spectrum.

Photon energy conversion includes both upconversion and downconversion. In this thesis, the photophysical processes of photon upconversion (PUC) by triplet-triplet annihilation and exciton downconversion by singlet fission (SF) have been investigated. The work presented in this thesis focuses on gaining knowledge and in-depth mechanistic understanding of the electronic interactions and excitation energy transfer events between chromophores that govern PUC and SF. More specifically, this thesis presents results from an investigation of intramolecular electronic interactions between chromophores within a molecular construct designed for PUC or SF. The mechanisms of intramolecular energy transfer between chromophores used for PUC have been investigated with respect to rate and efficiency. Intramolecular SF in a molecular dimer has been investigated in a detailed study of how the relative orientation of the chromophores and molecular conformational flexibility influence the kinetics of SF.

The results presented in this thesis show how the relative orientation of chromophores as well as the moiety connecting the chromophores, control the nature and magnitude of the intramolecular electronic coupling. This insight highlights the importance of controlling molecular orientations and conformation flexibility as a design parameter in the development of novel molecular systems for photon energy conversion. Further, it has been shown that the overall process of PUC or SF is faster in an intramolecular system compared to a corresponding intermolecular system. Finally, the work presented in this thesis has shown that careful design of molecular frameworks could enable efficient intramolecular PUC and SF materials, which have potential to increase the energy harvesting efficiency of solar energy technologies.