Selective acceleration of the protonated merocyanine-spiropyran photochromic transformation by inclusion in cucurbit[7]uril.

The effect of cucurbit[7]uril (CB7) on the spiropyran-merocyanine photochromic interconversion was studied in acidic and alkaline aqueous solutions. The merocyanine (MC) isomer was found to be the thermodynamically most stable form both in water and in the presence of CB7. A preferential binding of the protonated merocyanine (MCH(+)) to CB7 was observed with an equilibrium constant of 7.4×10(4) m(-1), and the complex formation led to significant diminution of acidity of the guest. The photoinduced transformation of MCH(+) to the spiropyran isomer was accelerated 2.3-fold upon addition of CB7, whereas the rates of the other photochromic processes were not affected. The partial inclusion of MCH(+) in CB7 led to dual fluorescence due to the incomplete deprotonation in the singlet-excited state.