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The interaction between tetrathiafulvalene and tetracation cyclobis(paraquat-p-phenylene) fragments-the key elements of many rotaxane systems-was investigated theoretically by using ab-initio second-order perturbation methods. In addition to the inclusion complex observed in the solid state, a thermodynamically stable "exterior" complex was identified. Calculation of the UV/Vis spectra for the inclusion and the exterior complexes indicated that the charge-transfer band that is often used to predict the formation of the inclusion complexes in solution is, in reality, due to the exterior mode of complexation. These results suggest that UV/Vis spectroscopy is not a reliable method for assigning the complexation modes in TTF:BB(4+) rotaxanes and related systems., (Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)