An ultrafast pre-organization of the [2 + 2] photocycloaddition of styryl dyes in 1:2 host-guest complexes with cucurbit[8]urils

Abstract The photocycloaddition of styryl dyes 1, mediated by 1:2 hostguest complexes with cucurbit[8]urils (CB[8]), was studied by fluorescence upconversion techniques. The lifetime of 14.5 ps for photoexcited 1 in aqueous solution and 3.8 ps for that inside the complexes were gained from the fluorescence decay, the rate constant of quenching being obtained within the diffusion control limit. Calculations confirmed that unexcited pairs of 1 inside CB[8] does not fit the topochemical principles. So a translational movement is required to produce a reaction-ready structure that is in agreement with the timeresolved fluorescence anisotropy measured in the range of 5 ps.