Singlet fission (SF) can enhance light-conversion efficiencies by splitting high-energy photons into multiple low-energy triplet excitons, preserving otherwise lost energy due to thermalization. This can potentially improve efficiencies by 30% relative to the Shockley-Queisser limit in single-junction solar cells, driving extensive research into suitable sensitizer materials. A bottleneck of research is that strong coupling promotes efficient SF but also hinders separation of generated triplets. Striking just the right balance is challenging, due to the multitude of factors that may affect the sensitizers’ SF capabilities. Moreover, the presence of multiple dark states in the initial stages of SF complicates its experimental evaluation in regards to those critical factors. This thesis aims to address these challenges by studying the impact of targeted system modifications on the SF mechanism under weak coupling conditions using time-resolved spectroscopic methods, such as transient absorption (TA) and two-dimensional electronic spectroscopy (2DES). For example, the feasibility of intermolecular SF is studied in a pentacene derivate, where acene faces are shielded with additional phenylene functional groups that restrict interaction between chromophores. While SF is not observed in solution, thin films of the materials dispersed in polymer matrices show SF rates (kSF) that follow a Förster resonance energy transfer (FRET) scaling with the average chromophore separation (R): kSF ∝ R-6 . In turn, the excitation-energy transfer (EET) mechanism acts as the rate-limiting step to SF and enhances it in a similar fashion to how natural light-harvesting networks improve energy flow in proteins: Singlet excitons are funneled by FRET to SF-reaction sites where it can proceed with near-unity quantum yields. More attention is directed towards intramolecular SF (i-SF) in spiro-linked dimers, consisting of two modifiable chromophores covalently bound to a spiro-carbon linker. Through spectral reconstruction and semi-classical simulations of 2DES signal responses, it is shown that the linker provides a framework of weak coupling that is still strong enough to enable efficient i-SF. Dimerization of two dissimilar chromophores into heterodimers finely and predictively tunes the energetic driving force of i-SF, which shown to have a great impact on its rate and efficiency. Hetero-oligomerization proves especially effective in furnishing i-SF sensitizers that show improved triplet yields (≈174% yield for separated triplets) compared to their homodimeric counterparts, facilitated also by downhill EET. Direct control over the excitonic coupling responsible for i-SF is achieved by additionally linking the individual chromophores via molecular bridges; by modulating the dihedral geometry of chromophores, the strength of their intramolecular coupling is directly affected. How this chemical modification translates to changes in the excitonic coupling, and in turn, the i-SF process, is evaluated experimentally II using transient 2DES, which was implemented during this work. Through this advanced spectroscopic method, a primarily direct i-SF mechanism is concluded for when chromophores are in closer proximity. As the chromophores are further separated by insertion of molecular bridges, direct i-SF becomes increasingly suppressed in favor of a super-exchange mediated i-SF mechanism and the overall rate and efficiency of the excited-state process decreases. Obtaining such high levels of mechanistic detail is usually reserved to theoretical studies, which has been made possible by utilizing the extraordinaire selectivity of transient 2DES towards excited state transitions, opening up the avenue to study SF or other excited-state phenomena with unprecedented detail. Altogether, this thesis lays out the impact of excitonic coupling (through dimer geometries), energetic driving force (through alterations of chemical compositions), and excitation energy transfer on the outcome of singlet fission, understood through the lens of multiple time-resolved spectroscopies. As demonstrated here, these properties of nature act synergistically and can be utilized to improve the efficiency of (i-) SF, providing insight into molecular design principles for organic materials intended for applications in solar energy technologies.
Oskar Kefer (Thu,) studied this question.