contribution and a slight build-up of the β-phase signal. Since the pump pulse is centered at 3.25 eV, predominantly exciting the glassy phase, the observations are in agreement with excitation β-phase.38 energy migration from the glassy phase into the After spectral relaxation, there is no further spectral evolution, demonstrating that in pristine PFO, singlet excitons remain the dominant photoexcited states, and no further states, such as triplets or polarons, are produced on a picosecond time scale. Singlet exciton decay in PFO can be described by a dispersive law which is a typical scenario in a disordered polymer phase, in which singlet diffusion.39,40 excitons are quenched by defects encountered during exciton These findings are confirmed by a target analysis utilizing a two-state sequential model as shown in Fig. 4c, resulting in characteristic spectra for singlet excitons in PFO immediately after excitation and after relaxation and energy migration (“S ” and “S ” in Fig. 4c, respectively). 1g 1β The formation of the intercalated PDI:PFO phase significantly changes the characteristics of the optical probes, as well as the deactivation scenario (comparing Figures 4b and 4a). On a 5-10 ps time scale, both S and B are fully quenched, following a slightly stretched exponential decay law, D Dβ giving rise to a series of new bands in the NIR and visible spectral region. The sharp and structured C bands at 1.3 and 1.5 eV have been assigned to excited state absorptions specific for PDI(-) A anions,41,42 anions.43–45 while a further sharp band at 1.7 eV is seen both in PDI singlet states and In Fig. 4b, these features are denoted by C . By comparison with the PDI GSA spectrum, the negative A bands at 2.35 and 2.6 eV were assigned to bleach bands B . A The absence of the known PDI cationic PA bands at 2.2 and 2.4 eV gives evidence against a possible formation of charge pairs in the PDI phase and suggests that the counter charge resides in the PFO phase. This is confirmed by the presence of bleach B even after hundreds of nanoseconds when g all specific optical probes for singlet states in PFO have disappeared (see ESI, part H). The sharp PA band at 1.85 eV (C ) is unknown in the literature and cannot be assigned to pristine D PFO or PDI. Polarons in PFO are known to produce a broad PA band in the region around 2.3 eV, SE.36 strongly overlapping with The target analysis in Fig. 4d shows that this band is formed with the same kinetics as the PDI anionic features and the disappearance of the PFO singlet exciton (see Fig. 4d: the band at 1.85 eV is present only in the red spectrum but not in the black one). The target analysis in Fig. 4d also shows clear evolution of the bands in the PB region: while the singlet excited state shows a PB of both β and glassy phases, the charge separated state shows one single PB band that matches exactly the position of the second derivative of the glassy phase in Fig.1b, bottom, orange line. It is known that a second derivative of a band occurs at the same spectral position but bandwidth.46 with strongly reduced We can therefore conclude that the sharp PB at 3.1 eV should originate from a region with reduced energetic disorder as compared to the glassy phase but lacking β-phase.29 the highly anisotropic polarizability of the It is plausible to assign this region to the