Unlocking New Principles of Singlet Fission: Characterization, Optimization, and Identification of Novel Singlet Fission Materials

Abstract

Humankind undeniably has left its marks on the planet. The ever-growing world population necessitates the demand for more living space, food resources, and energy. These demands all in all lead to a deterioration of the global environment and climate, thus highlighting the overall negative impact humankind has on earth. Over the last years the outcry of society, especially from the younger generations, steadily grew, demanding from politicians around the world to fulfill the promises on working towards a sustainable, environmental friendly, and climate neutral energy production, in order to counter global warming and decrease the toll we create on the planet. The arguable best alternatives are renewable energies, with solar energy conversion through photovoltaic cells exhibiting the largest potential. However, single junction solar cells are starting to reach their maximum possible thermodynamic efficiency (SHOCKLEY QUEISSER limit) of about 33.7%. Therefore, new and innovative concepts are pivotal to elevate the efficiency of solar cells beyond their present limit. They furthermore need to be affordable and scalable to an industrial level to compete with currently fossil fuel based forms of energy generation. One concept is given by singlet fission (SF). It evolves around the down conversion of a singlet excited state (S1) into two triplet excited states (T1), resulting in multiple exciton generation (MEG). By implementing SF chromophores into the solar cells and exploiting MEG, it is theoretical possible to go beyond the SHOCKLEY QUEISSER limit. To achieve this goal, it is imperative to obtain a profound understanding on the fundamentals of SF. Numerous scientists dedicate their research towards the investigation of SF chromophores and the elucidation of its mechanism. The present thesis describes my input to this overarching area of investigation. To this end, this thesis provides a brief summary on the photophysical fundamentals, the most current insights into SF and its relevant intricacies. Moreover, a report on the widely-used SF chromophores gives the reader the necessary insight to achieve a general understanding of this topic and, by extension, of my research. My research, in particular, encompassed three main parts, namely the characterization of pentacene derived SF chromophores, the optimization of their SF efficiencies, and the identification of novel and superior SF chromophores beyond pentacene. The investigations were performed by applying a wide range of photophysical techniques including steady-state absorption and fluorescence, time correlated single photon counting (TCSPC), time-resolved transient absorption (TA), time-resolved infra-red (fsIR), and time-resolved electron paramagnetic resonance (TrEPR) spectroscopy. In addition, electrochemical and spectroelectrochemical (SEC) measurements further assisted in the investigations. Elaborating on the individual parts, the first one focused on the characterization of pentacene dimers, utilizing a variety of linkers and exploring the different electronic coupling regimes as they impact the SF mechanism and efficiency. The bulk of this thesis was dedicated to this part. In the first project, cross conjugation was chosen to ensure strong electronic through-bond coupling, resulting in intramolecular SF and triplet quantum yields (TQYs) as high as 162%. This study highlighted the importance of the solvent polarity and, thus, significance of the charge transfer (CT) state as part of SF, serving as an intermediator to the singlet correlated triplet pair state 1(T1T1). 1(T1T1) did not yield any uncorrelated triplet excited states (T1+T1) and exclusively decayed back to the ground state (S0S0) via triplet-triplet annihilation (TTA). This, again, showcased the strong electronic coupling between the pentacene moieties due to the selected binding motif. The second project went the opposite direction. By breaking the conjugation between the pentacenes via non-conjugated spacers, the weak coupling regime was scrutinized. This purely led to through-space interactions, upon which their ramifications on SF were explored. The utilization of flexible, alkyl derived spacers added a certain flexibility to the system. Only a gauche conformer allowed for sufficient interpentacene through space interactions. The damping in electronic coupling not only led to the additional population of a quintet correlated triplet pair 5(T1T1), but also allowed for its separation into free triplets (T1+T1). The investigated chromophores also highlighted the fine balance between exhibiting strong coupling and, thus, yielding high TQYs at the expense of separating into the (T1+T1), and weak coupling trading off high amounts of (T1+T1) with low TQYs. After dedicating a respective project to the strong and weak coupling regimes, the goal of the third project was to go beyond the classical description of coupling regimes. This was achieved by inserting platinum into the linker and investigate the ensuing impact of the internal heavy-atom effect on the SF mechanism. Platinum led to a unique perturbation of the ground, singlet, and triplet excited state potential energy surfaces and influenced the spin dynamics of SF. The fast and efficient SF led to a strong population of 1(T1T1), which subsequently, by virtue of the induced spin-orbit coupling of platinum, parallel transitioned into 3(T1T1) and 5(T1T1). These states ultimately led to a major population of (T1S0) in the former case, and a minor population of (T1+T1) in the latter case. The fourth project was a direct follow-up and involved the modification of the bridge by including chiral substituents and obtaining enantiomerically pure platinum dimers. This study featured the first investigations on the possible effects of chirality on the SF dynamics. However, the observed mechanism and yields were rather similar to the achiral counterpart. Only minor differences in rate constant and yields were observed, ultimately exposing the small impact chirality imparts upon the overall system. The second part involved the optimization of an already established SF chromophore. The optimization was realized by covalently connecting an energy donor, with complementary absorptions, to the linker of the pentacene dimer. This introduced panchromatic light harvesting to the SF chromophore. Following this design concept, the fifth project dealt with the optimization of an already published phenylene linked pentacene dimer (Pnc2) by applying zinc porphyrazine (ZnPz) based energy donors. The conjugates featured an efficient and unidirectional FÖRSTER RESONANCE ENERGY TRANSFER (FRET) from ZnPz to Pnc2, which subsequently underwent SF. This concept led to two follow-up projects, in which the first one replaced zinc porphyrazine with subporphyrazine as energy donor, as part of the sixth project. The goal was to expand the variety of applicable energy donors in the context of SF optimization. Subporphyrazine resulted in an improved panchromatic light harvesting, due to its broader and more intense absorptions in the 400 to 600 nm range, usually devoid of strong pentacene dimer related absorptions. It also introduced another layer of control regarding FRET, owing to its solvent polarity dependent fluorescence. The fluorescence of subporphyrazine was found to blue-shift after increasing the solvent polarity. This changed the spectral overlap between donor fluorescence and acceptor absorption, ultimately impacting the efficiency of FRET. The second follow-up and overall seventh project utilized subphthalocyanines as an energy donor and more specifically explored the role and nature of the molecular spacer on the intramolecular FRET. This was realized by controlling the distance between energy donor and acceptor, through the number of aryl units in between, and its flexibility, introduced via a CH2 group. Overall increasing the distance negatively impacted FRET. Introducing flexibility to the system circumvented the negative impact, as it brought the two interacting moieties closer together. Notably, the inclusion of aryl rings into the spacer negatively impacted intramolecular SF. The quinone-like conjugation led to an additional acceptor orbital, which was delocalized over both pentacenes and the bridging phenyl. This, in turn, disrupted the interactions between the pentacenes and restrained SF. The goal of the third part was to identify novel and modified SF chromophores, going beyond their pentacene derived counterparts and featuring superior properties. One such type of chromophore was explored in the eighth project with a nitrogen doped hexacene and two added off-linear benzene rings. The inserted nitrogens induced a weak decoupling resulting in a unique electronic structure and compartmentalized the acene backbone into a pyrene and an aza-tetracene part. These chromophores offered a superior stability and expanded the range of absorption compared to pentacenes. In terms of coupling strength, these chromophores are placed in the medium coupling regime, featuring a good balance between efficient SF and 1(T1T1) TQYs, while still allowing decoupling into (T1+T1). As a direct follow-up, the ninth project combined the ortho-dibenzodiazahexacene dimer from the previous study together with carbon nanotubes. The superior stability of dibenzodiazahexacene compared to pentacenes certainly stood out. In contrast to pentacene, they were able to sustain the required sonication during hybrid preparation. The carbon nanotubes served as an energy sink, which still allowed the initial SF, while collecting energy through an unidirectional energy transfer from the various SF related species during the process. This resulted altogether in an energy transfer efficiency beyond 100%. Another known class of SF chromophores includes the family of rylenes, upon which a perylene-monoimide dimer was investigated in greater detail as the tenth project. The perylene-monoimide dimer revealed to be subject to a geminate triplet-triplet annihilation up conversion (TTA-UC) channel at room temperature. This competed with the disentanglement of the formed correlated triplet pair from SF. Furthermore, the strong involvement of an intermediate CT state manifested itself by imposing a CT character onto the singlet (S1S0)CT and triplet 1(T1T1)CT excited states. Either lowering the temperature or increasing the solvent polarity suppressed TTA UC and stabilized 1(T1T1)CT. This highlighted the fine intricacies of adjusting couplings and thereupon manipulating the fate of the correlated triplet pair within SF. Diketopyrrolopyrroles are yet another class of SF chromophores and were picked up in the eleventh project. The utilization of a dithienylphenylene spacer and led to the construction of an ortho-, meta-, and para-regioisomer. This study highlighted the deep interconnectivity and participation of a CT state and its impact on SF in DPPs. Only a polar solvent allowed the population of 1(T1T1) and (T1+T1) by stabilizing the intermediate (S1S0)CT. But, the para-dimer also revealed that too strong of a stabilization and spatial separation of the diketopyrrolopyrroles can lead to a trap state and prevent SF from occurring by undergoing a symmetry breaking charge separation. This emphasized the fine balance when a CT state is involved as the extent of stabilization either mediates or blocks intramolecular SF in diketopyrrolopyrroles-based dimers. Besides the three parts, additional supplementary studies on a set of DPP monomers were performed in the twelfth project. This study aimed at obtaining a fundamental understanding on the photophysical behavior of DPPs and was not focused on SF. The measurements revealed a strong involvement of CT states during the excited state decay of DPPs, valuable knowledge explored in the abovementioned study. The accumulated knowledge of SF during my research additionally led to the creation of a review article. The main goal of the article was to provide the essential tools and knowledge required for studies based on SF energy conversion devices and pave the way towards their realization. In particular, the review article included a summary on the fundamental processes of inter- and intramolecular SF, gave an overview on the essential parameters of solar cells and the first achievements implementing SF into their device architecture. Most importantly it contained a collection of the most prevalent SF chromophores, with a listing of their photophysical and electrochemical properties, as found in the literature on their respective publications. These are crucial for choosing the appropriate dye and designing the respective solar cell. In essence, the three main parts presented in this thesis aimed at achieving a similar conclusion.