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Published February 18, 2016 | Supplemental Material
Journal Article Open

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) (CBPQT^(4+)), 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 CBPQT^(4+), 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 (BIPY^(2+)) units of CBPQT^(4+), rather than the encapsulation of TTF inside the cavity of CBPQT^(4+). We carried out DFT calculations, including solvation, that reveal conclusively that the CT band emerging upon mixing TTF with CBPQT^(4+) 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 BIPY^(2+) units of a CBPQT^(4+) 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 CBPQT^(4+). 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 CBPQT^(4+).

Additional Information

© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Received: June 2, 2015; First published: 19 January 2016. This research is part (project 34-945) of the Joint Center of Excellence in Integrated Nano-Systems (JCIN) at the King Abdulaziz City of Science and Technology (KACST) and Northwestern University (NU). The authors would like to thank both KACST and NU for their continued support of this research. W.G.L. and W.A.G. acknowledge support from the US National Science Foundation (EFRI-1332411 and DMR-1436985). A.C.F. acknowledges support from a National Science Foundation (NSF) Graduate Research Fellowship. A.K.B. acknowledges Fulbright New Zealand for a Fulbright Graduate Award and the New Zealand Federation of Graduate Women for a Postgraduate Fellowship award.

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