A Tunable Cyclic Container: Guest-Induced Conformational Switching, Efficient Guest Exchange, and Selective Isolation of C70 from a Fullerene Mixture

Abstract

An adaptable cyclic porphyrin dimer with highly flexible linkers has been used as an artificial molecular container that can efficiently encapsulate various aromatic guests (TCNQ/C60/C70) through strong π–π interactions by adjusting its cavity size and conformation. The planar aromatic guest (TCNQ) can be easily and selectively exchanged with larger aromatic guests (C60/C70). During the guest-exchange process, the two porphyrin rings switch their relative orientation according to the size and shape of the guests. This behavior of the cyclic container has been thoroughly investigated by using UV/Vis spectroscopy, NMR spectroscopy, and X-ray crystal structure determination of the host–guest assemblies. The electrochemical and photophysical studies demonstrated the occurrence of photoinduced electron transfer from bisporphyrin to TCNQ/C60/C70 in the respective host–guest assemblies. The cyclic host can form complexes with C60 and C70 with association constants of (2.8±0.2)×105 and (1.9±0.3)×108 m−1, respectively; the latter value represents the highest binding affinity for C70 reported so far for zinc(II) bisporphyrinic receptors. This high selectivity for the binding of C70 versus C60 allows the easy extraction and efficient isolation of C70 from a C60/C70 fullerene mixture. Experimental evidence was substantiated by DFT calculations.