Two classes of alongside charge-transfer interactions defined in one [2]catenane

A [2]catenane, which incorporates hydroquinone (HQ) and a sterically bulky tetrathiafulvalene (TTF) into a bismacrocycle, has been designed to probe the alongside charge-transfer (CT) interactions taking place between a TTF unit and one of the bipyridinium moieties in the tetracationic cyclophane cyclobis(paraquat-p-phenylene) (CBPQT(4+)). A template-directed strategy employs the HQ unit as the primary template for formation of the tetracationic cyclophane CBPQT(4+), affording the desired [2]catenane structure but as an uncharacteristic green solid. The X-ray crystal structure and detailed C-13 NMR assignments have identified a stereoselective preference for catenation about the cis isomer. The H-1 NMR spectroscopy, electrochemistry, and X-ray crystallography all confirm that the CBPQT(4+) cyclophane encircles the HQ unit, thereby defining a structure which would normally determine a red color. The visible-NIR region of the absorption spectrum displays a band at similar to 740 nm that is unambiguously assigned to a TTF -> CBPQT(4+) CT transition on the basis of resonance Raman spectroscopy using 785 nm excitation. The profile of the CT band changes depending on the ratio of the cis- to trans-TTF isomers in the [2]catenane for which the molar absorptivity of each isomer is estimated to be significantly different at epsilon(max) = 380 and 3690 M-1 cm(-1), respectively. Molecular modeling studies confirmed that the observed difference in the absorp-tion spectroscopic profile can be accounted for by both a better overlap of the HOMO(TTF) and LUMO+1(CBPQT(4+)) as well as a more stable face-to-face (pi center dot center dot center dot pi) conformation in the trans isomer compared to the edge-to-face cis isomer of the [2]catenane. The latter is arranged for pi-orbital overlap through the sulfur atoms of the TTF unit, thereby defining an [S pi center dot center dot center dot pi] interaction.