same transition, we find a strong delocalization of the exciton wavefunction over about 25 PDI molecules (see ESI, part B). It can thus be concluded that about half of the initially delocalized excitons localize into excimers within 1.6 ps. A small tail in the experimental TA dynamics (see ESI, part B), not rendered by the one step target model in Fig. 5c, might be caused by diffusive motion of sites.57 further excitons towards pre-dimer If the same experiment is performed on the PDI:PFO blends, radically different TA spectra are obtained even in the femtosecond time domain (compare Fig. 5b and 5a, respectively). Optical probes for PDI anions (C ) dominate the TA spectra at time scales as early as 600 fs after the A excitation pulse. The relatively sharp PA band at 2.05 eV, shown in Fig. 5b,d, does not arise from PDI.42 excimers but is also a feature of charge separated states on This shows the formation of charged states in the PDI:PFO blends on a femtosecond time scale. However, the corresponding optical probe for the PFO cations is hardly visible after 600 fs; a target analysis shows that it builds up from zero with a 3 ps rate constant with first order kinetics (compare red and black curves in Fig. 5d around 1.85 eV). Concomitantly, a build-up of the B band is observed with the same kinetics. Dg Although one cannot observe the PDI cation bands located at 2.3 eV, due to scattering of the pump pulse, the delayed build-up of PFO specific probes for charged states provides sufficient evidence to ascribe the first characteristic spectrum in Fig. 5d to a charge pair in the PDI phase. In a second step, holes are transferred from the PDI to the PFO within the intercalated PDI:PFO phase in a first order reaction of 3 ps lifetime. An alternative mechanism, namely dissociation of excimer states, was excluded due to the following reasons: excimer dissociation would lead to an increase of the PDI anion bands at 1.3 and 1.5 eV. However, when comparing the black and the red spectra in Fig. 5d, we find only a slight reduction of the background underneath the C bands, while these bands A themselves do not change their strength during the process. Therefore, the process under discussion is creating charges in the PFO phase, while not changing the density of anions in the PDI phase, leaving a hole transfer to PFO as the only consistent explanation. Finally, the observed one-step kinetics and the exclusive encounter of optical probes from charged states allows us to exclude singlet fission as a possible competing pathway after resonant excitation of PDI, in agreement with results.47 literature

$$Why does the presence of PFO induce such a two-step mechanism involving the intermediate formation of charge pairs in the PDI phase, rather than causing exciton dissociation at the D-A interface? To address this question, the first few hundred femtoseconds of photoexcitation dynamics were resolved. As shown in the ESI, part G, there is evidence for a delayed process with about 400 fs delay time in both the pristine PDI film and the blends. In pristine PDI, this delayed process produces a symmetric charge transfer state that later evolves into an excimer. In the blends however, the delayed process is described by the build-up of the anion specific features C . The presence of A the PFO therefore causes the formation of a symmetry breaking CT-state inside the PDI phase. It is$$