Lactonization of dyes promoted by a coordination cage facilitates cavity-induced chromism as ion mobile detector

Dye molecules often change colors (so-called “chromism”) according to the environment variation. However, they are rarely induced by a catalytic amount of cavity. Through encapsulation in the cavity of a porous coordination cage (PCC-2), Rhodamine B (1) is transformed from red quinonoid form (1q) into colorless lactone form (1l) in aprotic polar solutions. The μ₄-OH groups in the cavity of PCC-2 are shown to stabilize the uncommon zwitterion intermediate (1z), followed by converting to 1l, thus accelerating the equilibrium. The chromism is catalyzed by 0.25 mol% of PCC-2, and the reaction rate is improved by 80,400 times. 1@PCC-2 can be further fabricated to a sol-gel that exhibits ion recognition properties. The resulting encapsulation and stabilization of an unconventional intermediate by a catalytic amount of the coordination cage provides fundamental insights into molecular isomerization and has potential use in chemical sensing.