Mechanism and Dynamics of Charge Transfer in Donor-Bridge-Acceptor Systems

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.