| Abstract
| - 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) interactionstaking 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 primarytemplate for formation of the tetracationic cyclophane CBPQT4+, affording the desired [2]catenane structurebut as an uncharacteristic green solid. The X-ray crystal structure and detailed 13C NMR assignmentshave 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 HQunit, thereby defining a structure which would normally determine a red color. The visible−NIR region ofthe 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 theCT band changes depending on the ratio of the cis- to trans-TTF isomers in the [2]catenane for which themolar absorptivity of each isomer is estimated to be significantly different at εmax = 380 and 3690 M-1cm-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) andLUMO+1(CBPQT4+) as well as a more stable face-to-face (π···π) conformation in the trans isomer comparedto the edge-to-face cis isomer of the [2]catenane. The latter is arranged for π-orbital overlap through thesulfur atoms of the TTF unit, thereby defining an [Sπ···π] interaction.
|
