The ππ * absorption band of the tetramer at ∼300 K possesses a tail in the low-energy range, at 3.3-4.0 eV (Fig. ), as mentioned above. We analyzed the exciton localization for the S 0 → S 5 transition, which is the lowest energy ππ * transition and is located at the ππ * absorption onset (see SI2.2). At the model tetramer geometry, this transition is predominantly delocalized over the central dimer (Fig. ). It also shows sizable CT excitations between the middle molecules. MD at ∼300 K again leads to a considerable single-geometry localization, whereas the excitons are delocalized at low temperature (< 26 K in our case), see SI2.2. In this respect, it is interesting to note that photoisomerization in the SAMs (if it occurs) is believed to be triggered by localized excitations. Further, we turn to the lower energy nπ * band. The S 0 → S 1 transition of the trans monomer is a dark transition (i.e., a transition with zero oscillator strength) located at ∼2.55