The Sun is the main source of renewable energy on Earth. Our planet receives about 174 PW of solar power. At the same time, global energy consumption from all energy sources is orders of magnitude lower and is equal to approximately 16 TW. Clearly, solar energy has a tremendous potential, as well as numerous advantages over conventional sources of energy. In recent years the photovoltaic industry exhibited significant growth, however the main obstacles that are preventing widespread introduction of solar cells are their still low efficiency and high cost. The major factors contributing to the low efficiency are non-efficient conversion and lack of absorption of sunlight by the active layer material. Sunlight has a broad spectrum spanning over the UV, visible, and NIR optical ranges. Commonly, solar cells are constructed using a single junction, which leads to two fundamental loss mechanisms: i) for photons with a higher energy than the band gap the energy surplus is lost as heat and only the band gap energy harvested, ii) photons with an energy lower than the band gap are not harvested at all. Together, these losses constitute a limitation of the efficiency of a silicon solar cell by half. In the work described in this thesis we have explored ways to decrease these losses by using advanced materials that allow to efficiently down-convert high energy photons, and up-convert low energy photons to the energies close to the band gap energy. Both of these approaches can be achieved in certain classes of organic materials and are known as singlet exciton fission and photochemical upconversion. This thesis combines the results of experimental research of both singlet exciton fission and photochemical upconversion in organic materials by means of laser spectroscopy tools: transient absorption and time resolved luminescence. Singlet exciton fission is a process by which a singlet excited state is converted into a combination two triplet excited states with half the energy. The two triplet together constitute an overall singlet state and hence it is a spin-allowed process that can in principle be very efficient. In this thesis, singlet fission has been studied in a range of perylenediimide (PDI) derivatives in the crystalline state. Substitution at imide nitrogen position allows to obtain different crystal structures in the solid state. In this way the electronic coupling between neighboring molecules in a crystal can be varied without significantly changing the energetics of their singlet and triplet levels. Singlet exciton fission was experimentally detected for a variety of different crystal structures of PDI. The formation of triplet excited states was found to occur on a sub-picosecond time scale. The experimentally detected fission rates and triplet yields were significantly higher than predicted by earlier theoretical calculations and were found to depend only very weakly on the crystal structure. The latter can be explained by intermolecular vibrational modes that could significantly speed up fission in perylenediimides. Photochemical upconversion can be seen as the reverse process of singlet fission. In this case, two triplet excited states, formed through a triplet sensitizer, are combined into a single higher lying singet excited state by triplet-triplet annihilation. Using this approach it is possible to convert low-energy photons that are normally not absorbed in a solar cell into higher energy photons that can be converted efficiently. In this thesis, photochemical upconversion was studied in bi-component mixtures of triplet sensitizer and triplet acceptor in solutions. Metal based porphyrins were used as triplet sensizers, and diphenylanthracene was used as triplet acceptor. The triplet sensitizer produces triplets by fast intersystem crossing due to spin-orbit coupling. The triplets are subsequently transferred to the triplet acceptor by Dexter energy transfer. When two acceptor molecules in the triplet state encounter each other, triplet-triplet annihilation occurs which results in emission from a singlet state. The overall process can be described as conversion of two low energy photons into one high energy photon. In this way, photons that are normally not absorbed by an active material in a solar cell can be converted into photons that can be absorbed, leading to a significant potential enhancement of the overall efficiency. In this work the dependence of the photochemical upconversion process on the metal in triplet sensitizer (porphyrin) was studied. Efficient upconversion was observed for platinum and palladium based porphyrins ( 25-30%) while for the zinc porphyrin the efficiency was considerably less ( 12%). For free base porphyrin, no upconversion was detected. The upconversion process is the result of a series of individual steps outlined above where most steps are the same for all the combinations. The differences in efficiency are traced back to the dependence of energy transfer efficiency on core metal of the porphyrin. Finally, we have made an attempt to achieve upconversion from the near infrared region. For this purpose, we have used porphyrin oligomers consisting of two or four porphyrin rings that have a red-shifted absorption beyond 700 nm. It is shown that using the two-ring oligomer as a sensitizer for a perylene bisimid, it is possible to convert 700 nm light to the 500-600 nm range. For the four-ring oligomer, no noticeable upconversion was observed, possibly because the triplet of the porphyrin oligomer is just below that of the perylene bisimid in this case.@en

Photoinduced charge transfer in organic materials is a fundamental process in various biological and technological areas. Donor-bridge-acceptor (DBA) molecules are used as model systems in numerous theoretical and experimental work to systematically study and unravel the underlying mechanisms of charge transfer. Despite the respectable successes, especially experimental work tends to suffer to some extent from oversimplification or unverified assumptions. This thesis points out common pitfalls: the general assumption of localised initial and final states for charge transfer, the reduction of the electronic structure of the DBA molecule to the frontier orbitals of the donor, bridge, and acceptor fragments, and the consideration of the electronic structure of these fragments as an inherent property. I would like to stress, that this thesis does not claim substantially new theoretical insight. The aim rather is to make one step towards a more thorough interpretation and prediction of experimental results in a combined computational and experimental effort based on existing theoretical models. The level of theory is sufficiently high to capture the underlying principles of the experimental observations. This in turn leads to the rational design of functional organic materials. Photoinduced charge transfer is experimentally investigated by means of broadband femtosecond transient absorption spectroscopy. The photophysical processes upon light absorption are disentangled and marked with rate constants by careful analysisof the two-dimensional data. (Time-dependent) density functional theory is employed to interpret the results, particularly to assess the initial and final states, and the effective electronic coupling that dictates charge transfer. Chapter 2 emphasises the role of the initial state for charge transfer. In a series of linearly conjugated DBA systems containing n = 1-3 phenyl bridges, the distance dependence of electron and hole transfer are compared. Although both processes seemingly exhibit equally high energy barriers that were estimated on basis of isolated fragment, electron transfer features an almost distance independent charge transfer rate constant whereas a relatively strong distance dependence is observed for hole transfer. This discrepancy is traced back to modified energy barriers caused by connecting the donor, bridge, and acceptor fragments. The changed electronic structure of the fragments leads to a delocalisation of the initial excitation over the donor and bridge in case of electron transfer. The delocalisation was not expected based on the experimental steady-state and transient absorption spectra. This chapter demonstrates the issue of estimating charge transfer properties of a DBA molecule in terms of properties of the isolated fragments, and the necessity of computational chemistry to examine the initial excitation. Chapter 3 investigates the effect of cross-conjugation on photoinduced charge transfer that is usually treated in the context of quantum interference. Hole transfer in the DBA molecule from Chapter 2 containing the linearly conjugated biphenyl bridge is compared to DBA systems with a singly and a doubly cross-conjugated biphenyl bridge. Moreover, charge transfer in these DBA molecules is compared to charge transport in single molecule junctions containing the same biphenyl bridges in between electrodes. The experimental results on hole transfer are counter-intuitive since the linearly conjugated DBA system exhibits an equally small hole transfer rate constant as the singly cross-conjugated DBA system. Additionally, the rate constant is even lower than for the doubly cross-conjugated DBA molecule. By contrast, charge transport follows the general expectation of similarly low conductance for the two cross-conjugated bridges and significantly higher conductance for the linearly conjugated one. The peculiar behaviour of the DBA molecules is found to stem from the specific symmetry of the initial and final state with respect to the fragment orbitals of the bridge. This symmetry relation inhibits the direct electronic coupling of the initial state to a number of bridge orbitals in the linearly and the singly cross-conjugated DBA systems and leads to unusual quantum interference effects. This pathway exclusion for charge transfer by symmetry is further experimentally confirmed in Chapter 4. This chapter compares hole transfer in a linearly and cross-conjugated DBA molecule containing the same donor and acceptor as in Chapter 3 but different bridge moieties. Opposite to the previous bridges, the conjugation is varied by using chemically different bridges instead of the positions at which donor and acceptor are connected. While the pathway selection on grounds of symmetry is responsible for the difference between the investigated DBA systems and single molecule junctions, it might be exploited in a functional way. The results of Chapters 2 to 4 pointed out two rules for designing DBA systems exhibiting pronounced effects of quantum interference: initial and final states have to be localised on the donor and acceptor while still coupling to all fragment orbitals of the bridge. On basis of these conditions the computational design of appropriate DBA molecules is presented in Chapter 5. Using the symmetric linearly and singly cross-conjugated biphenyl bridges from Chapter 3 required the search for asymmetric donor and acceptor moieties. This chapter demonstrates the challenge to find the right balance between a too weak and a too strong electronic coupling and the importance of computational methods for the rational design of functional DBA systems. Chapters 2 to 5 deal with photoinduced charge transfer in DBA molecules where the bridge moiety acts as a tunnelling medium. Chapter 6 takes a step away from this tunnelling regime and investigates experimentally electron injection from an electron donor into base pairs in DNA hairpins. The dynamics of electron injection and recombination in various sequences of natural and halogenated bases, which constitute different energetic landscapes for charge transfer, are examined in view of efficient long range electron transfer. Expectedly, the rate constant of electron injection is enhanced with a stronger driving force originating from the reduction potential of the first base. Yet while the identity of the second base only slightly affects the rate constant of electron injection, the recombination dynamics are strongly altered in the presence of halogenated bases. This observation indicates that electrons are injected into localised states on the first base with subsequent electron migration. Equal efficiency of electron migration in base pair sequences with a down-hill and a up-hill energetic landscape leads to speculations that this migration occurs via delocalisation rather than hopping between consecutive bases.@en