A new molecular recognition motif between a water soluble pillar10]arene ( WP10 ) and 1,10‐phenanthrolinium guest ( G ) in water is established. Mainly driven by the cooperativity of multiple electrostatic interactions, hydrophobic interactions, and π–π stacking interactions between WP10 and G , this host–guest complex exhibits a high association constant in water, which is about 17 times higher than that between WP10 and paraquat ( PQ ). Furthermore, this size selective host–guest complexation is employed to tune the lower critical solution temperature behavior of a random copolymer with PQ derivative pendants.
