possesses Z-configuration of the photoisomerizable double bond, an energetically favorable (P)helicity, and a (Sa)-configured biaryl axis. This stereo-configuration forces the PEG-linker chain to be in a tensed state (T) as its attachment points at the rotor and the biaryl fragments are residing in opposite half-spaces with respect to the thioindigo-fragment plane (see also Figure for definitions of molecular fragments and half-spaces). The CR-1 isomer has the same stereoconfiguration except for the axial chirality of the biaryl, which is (Ra)-configured in this case. As a result, the PEG-linker chain is in the relaxed state (R) in this isomer, which makes it in fact the expected global minimum. However, the calculated energy difference between CR-1 and CT-1 is only 2.2 kcal/mol, which lies within the typical error for DFT calculations. For this reason, it can be assumed that both isomers CR-1 and CT-1 are rather similar in energy and could both be They were therefore analyzed as mixtures in solution. However, we found that it is possible to strongly enrich each isomer CR-1 or CT-1 by choosing either polar or apolar solvents, respectively (Figure -e, Figure ). In polar CH2Cl2 solution CR-1 is enriched in up to 71% and crystals suitable for X-ray analysis unambiguously evidenced Z-(S)-(P)-(Ra) configuration of this isomer (Figure and). In apolar CS2 solution the C-isomeric mixture is strongly enriched in the CT-1 isomer in up to 76% (also see the Supporting Information chapter 4 for additional solvent screens).