Although MEP can be used to readily identify surfaces suitable for anion-π catalysis, it is typically not sufficient to fully explain trends observed as catalyst polarizability can lead to enhanced stabilisation of the substrate-catalyst complex. 12,13 Thus we calculated the polarizability of 4aH + , 4bH + , 6H + and 1H + perpendicular to the π-surface. This revealed that, as expected, the most selective catalyst is also the most polarisable structure (4bH + ), which suggests that the -stacking interaction between the bipyridinium and NDI unit, regardless of which surface is in direct contact with the substrate, is responsible for the high activity observed, as previously observed in oligo-NDI and NDI-fullerene conjugate-based catalysts. Similarly, the polarizability of 6H + perpendicular to the bipyridine unit was calculated to be larger than 1H + thanks to the stacking interaction with the piperidine-2,6-dione unit (Figure ), which, alongside the competition between threaded and stacked complexes for 1, explains the higher selectivity demonstrated by 6. These results clearly emphasize that both electrostatic and polarizability effects are important rationalizing anion-π catalysis.