In contrast to the theoretical models, which discuss the photophysics in vacuum, multichromophoric systems can be surrounded by symmetric or asymmetric en-vironments such as solvent molecules or protein side chains in biological systems.
This environment is crucial for the first of the two processes that we focus on in this thesis, which is symmetry-breaking charge-separation (SB-CS).
Before discussing the SB-CS in molecular aggregates, we will make an excur-sion to SB-CS in Donor-Acceptor-Donor (DAD) and Acceptor-Donor-Acceptor (ADA) dyes, in which this process is simplified due to a smaller number of diabatic states which are mixing.36–38 The DAD and ADA systems consist of two Donor-Acceptor branches with the lowest energy transition being a CT-band, where the electron density is shifted from the donor to the acceptor. Upon combining the two branches, the degenerate S1 states are split into two excitonic bands which, in a symmetric environment, do not possess a permanent dipole moment and the
1.4 Symmetry-breaking charge-separation 7
A D A A D D A
A D A S1
S2 S3 S4
LES CSS1
FES CT
CSS2 S1
S2
symmetric symmetry-broken
a) b) symmetry-broken
Figure 1.3: a) Excitonic splitting in acceptor-donor-acceptor (A-D-A) systems in a sym-metric environment gives two states with negligible dipole moment. Upon solvent fluctu-ation the unsymmetrical environment can trigger symmetry-breaking charge-separfluctu-ation (SB-CS), in which the excitation is localized on one arm. (The same is true for DAD systems) b) SB-CS is a bichromophore is similarly triggered by the solvent, which breaks the degeneracy of the two charge separated states (CSS). However, the increased number of states which can still couple render the system more complex.
electronic distribution is completely symmetric on the two branches (Figure 1.3a).
The two excitonic states can be probed selectively due to different selection rules for one-photon and two-photon absorption illustrating the symmetric character of the quadrupolar S0 and FC state.39 However, the fluorescence exhibits a strong solvatochromism, a behaviour typical of a dipolar state indicating a larger CT character on one of the two branches in the relaxed S1 state. Using time-resolved infrared spectroscopy, our group could show that SB-CS is triggered by thermal solvent fluctuations, which lead to a different solvation of the two branches.38 The degeneracy of the two diabatic CT states therefore breaks down, the cou-pling is decreased and the excitation is asymmetrically distributed towards the better solvated branch. As solvent relaxation proceeds, first through ultrafast inertial motion and later by diffusional motion, this branch is even more stabi-lized and if the solvent field is strong enough, the excitation fully localizes (Figure 1.3a). In contrast to the symmetric FC state, the relaxed state therefore possesses a significant permanent dipole moment explaining the strong fluorescence solva-tochromism. The timescale of SB-CS could be related to the solvent relaxation times and the extent of localisation depends on the solvent polarity as well as the coupling between the two branches.38,40
Now we return to the discussion of homo aggregates, and concentrate on the de-scription of a dimer. Similarly to the DAD and ADA systems, purely symmetric adiabatic states of a homo-dimer, illustrated in Figure1.2b, exist only in a symmet-ric environment and strong interchromophore coupling.41However, in solution or in a protein environment the non-symmetric local environment can lift the degen-eracy of the diabatic monomer states, leading to an uneven distribution of electron density on the two chromophores. In contrast to the LES, the higher permanent dipole moment of the CSS makes them more susceptible to a different environment around the two chromophores.
The most famous biological example for SB-CS is the bacteriochlorophyll dimer (BC) in the reaction center of photosynthetic purple bacteria. Even though the
bi-O
Pe2 C
3-Pe
2 Xan-Pe
BA 2
Figure 1.4: Chemical structures of bichromophores that are studied as model systems for symmetry breaking charge separation and excimer formation.
ological bichromophore is surrounded by two almost identical branches of protein-bound cofactors, the CS happens almost exclusively along one branch on the ps timescale.40,42 Similarly to the thermal fluctuations for the DAD and ADA sys-tems, this has been explained by slightly asymmetric local environments around BC, lifting the degeneracy and therefore resulting in a net CS character of S1. CS in BC is highly efficient due to (i) the fast (ps time scale) CS which outcompetes radiative and non-radiative decay channels and (ii) due to the minimal energy loss between the FC exciton state and the relaxed CS state.
In addition to the solvent coordinate that controls the process in ADA and DAD systems, the dynamics along the structural coordinate between the chromophores plays a decisive role in molecular aggregates. SB-CS in covalently linked chro-mophores were studied extensively in model systems based on e.g. perylenes,43,44 anthracenes,45,46or PDIs,47–49and indicate that, in line with the DAD and ADA systems, the amount of CS depends on the coupling between the two chromophores and the solvent polarity. The fluorescence spectrum of the directly liked perylene dimer Pe2 substantially depends on the solvent polarity, indicating a transition from a LE to a CT character of the relaxed S1 state. In non-polar solvents, the emission originates from a LES, whereas in polar solvents it is rather a symmetry broken CT state.44 The results are similar to the extensively studied bianthryl (BA) for which, additionally to the different character of the emission state, the radical anion and cation feature in the transient absorption spectra confirmed the SB-CS.45However, due to the strong coupling in those systems, the CS is not com-plete, as indicated by a non-vanishing radiative rate constant of the CT emission.
Full CS, on the other hand, could be observed for the weakly coupled biperylenyl-propane (C3Pe2) in which the two heads are linked by an innocent propane linker in acetonitrile. The radical anion and cation features were clearly observed in polar solvents by transient absorption (TA) spectroscopy, which together with the ab-sence of a CT emission suggested a full CS. Even though excimer formation should be structurally possible, neither a red shifted emission nor an additional signal in the TA was observed, irrespective of the solvent.43 The dynamics become even more complicated if the linker guarantees close face-to-face contact of the chro-mophores that models the packing arrangement in a bulk material. Wasielewskiet
1.5 Singlet fission 9