Synthesis of Phototrappable Shape-Shifting Molecules for Adaptive Guest Binding

We have designed and synthesized oligosubstituted bullvalenes 1 and 2 as adaptive molecules that can change their shapes in order to bind tightly to a suitable guest. By incorporation of a photolabile o-nitroveratryloxycarbonate (NVOC) group into bullvalenes 1 and 2, tightly binding species can be selectively isolated from a population of hundreds of interconverting structural isomers. Spontaneous strain-assisted Cope rearrangements allow these shape-shifting molecules to exist in a dynamic equilibrium of configurationally distinct valence isomers, as revealed by dynamic NMR and HPLC studies. When NVOC bullvalenes 1 and 2 were exposed to UV light, the cleavage of the NVOC group resulted in a mixture of static isomers of the corresponding bullvalone. Binding studies of NVOC bisporphyrin bullvalene 1 demonstrated that the dynamic isomeric equilibrium shifted in the presence of C-60, favoring configurations with more favorable binding affinities. Irradiation of a mixture of 1 and C-60 with UV light and isolation of the major static isomer yielded an isomer of bisporphyrin bullvalone with a binding affinity for C-60 that was similar to 2 times larger than that of the nonadapted isomer bisporphyrin bullvalone 41.