To test effects of solvent polarity on the photochemistry of PAT 1, photoisomerization experiments were conducted additionally in more polar THF (see Supporting Information for details and data). A marked redshift of the E isomer absorption is observed with the lowest energy maximum now positioned at 724 nm. At the same time the absorption of the Z isomer is blueshifted with the corresponding maximum at 581 nm. This results in an extraordinary maximum separation of 143 nm. Extinctions are a bit weaker in THF but the photoswitching performance is fully retained. Quantum yields for photoisomerization are further improved in the more polar solvent reaching 12% for the E to Z and 31% for the Z to E photoisomerization.