Quantum Mechanical and Experimental Validation that Cyclobis(paraquat-p-phenylene) Forms a 1:1 Inclusion Complex with Tetrathiafulvalene

The promiscuous encapsulation of π‐electron‐rich guests by the π‐electron‐deficient host, cyclobis(paraquat‐p‐phenylene) (CBPQT4+), involves the formation of 1:1 inclusion complexes. One of the most intensely investigated charge‐transfer (CT) bands, assumed to result from inclusion of a guest molecule inside the cavity of CBPQT4+, is an emerald‐green band associated with the complexation of tetrathiafulvalene (TTF) and its derivatives. This interpretation was called into question recently in this journal based on theoretical gas‐phase calculations that reinterpreted this CT band in terms of an intermolecular side‐on interaction of TTF with one of the bipyridinium (BIPY2+) units of CBPQT4+, rather than the encapsulation of TTF inside the cavity of CBPQT4+. We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT4+ arises from the formation of a 1:1 inclusion complex. In support of this conclusion, we have performed additional experiments on a [2]rotaxane in which a TTF unit, located in the middle of its short dumbbell, is prevented sterically from interacting with either one of the two BIPY2+ units of a CBPQT4+ ring residing on a separate [2]rotaxane in a side‐on fashion. This [2]rotaxane has similar UV/Vis and 1H NMR spectroscopic properties with those of 1:1 inclusion complexes of TTF and its derivatives with CBPQT4+. The [2]rotaxane exists as an equimolar mixture of cis‐ and trans‐isomers associated with the disubstituted TTF unit in its dumbbell component. Solid‐state structures were obtained for both isomers, validating the conclusion that the TTF unit, which gives rise to the CT band, resides inside CBPQT4+. In, out, shake it all about: DFT calculations including solvation and the synthesis of a single station [2]rotaxane have been used to show that the formation of the charge‐transfer band generated upon mixing tetrathiafulvalene and cyclobis(paraquat‐p‐phenylene) is a consequence of the formation of a 1:1 inclusion complex (see figure).