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) (CBPQT4+). A template-directed strategy employs the HQ unit as the primary
template for formation of the tetracationic cyclophane CBPQT4+, affording the desired [2]catenane structure
but as an uncharacteristic green solid. The X-ray crystal structure and detailed 13C NMR assignments
have identified a stereoselective preference for catenation about the cis isomer. The 1H NMR spectroscopy,
electrochemistry, and X-ray crystallography all confirm that the CBPQT4+ 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 ∼740 nm that is unambiguously assigned to a TTF → CBPQT4+
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 εmax = 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(CBPQT4+) as well as a more stable face-to-face (π···π) conformation in the trans isomer compared
to the edge-to-face cis isomer of the [2]catenane. The latter is arranged for π-orbital overlap through the
sulfur atoms of the TTF unit, thereby defining an [Sπ···π] interaction.