Intermeshed Diphenylacetylenes with Axial Rotation Restricted by Intramolecular Donor–Acceptor Interactions

Diphenylacetylene (DPA) rotates almost freely around the acetylene axis with rotation mainly restricted by steric repulsion or covalent tethers. We report intermeshed diphenylacetylenes, in which bent π-conjugated units attached to DPA termini cause two donor–acceptor pairs to overlap with each other on opposite sides of the axis. Density functional theory calculations showed that a twisted, stacked conformation with a 36–40° dihedral angle was the most stable for all four molecules. The estimated P/M interconversion barriers (12.8–21.4 kcal/mol) increased with donor and acceptor strength. Upfield shifts in 1H NMR spectra and long-wavelength absorption bands assigned to intramolecular charge-transfer transitions indicated that the stacked conformation predominated in solution. Because rotation was restricted rather than fixed, the molecules exhibited dynamic axial chirality. In a derivative bearing pyridyl groups as acceptors, acid–base interaction with (+)- or (–)-camphorsulfonic acid biased the P/M equilibrium and gave mirror-image induced circular dichroism spectra.