Structures and Properties of Molecular Torsion Balances to Decipher the Nature of Substituent Effects on the Aromatic Edge-to-Face Interaction

Various recent computational studies initiated this systematic re‐investigation of substituent effects on aromatic edge‐to‐face interactions. Five series of Tröger base derived molecular torsion balances (MTBs), initially introduced by Wilcox and co‐workers, showing an aromatic edge‐to‐face interaction in the folded, but not in the unfolded form, were synthesized. A fluorine atom or a trifluoromethyl group was introduced onto the edge ring in ortho‐, meta‐, and para‐positions to the CH group interacting with the face component. The substituents on the face component were varied from electron‐donating to electron‐withdrawing. Extensive X‐ray crystallographic data allowed for a discussion on the conformational behavior of the torsional balances in the solid state. While most systems adopt the folded conformation, some were found to form supramolecular intercalative dimers, lacking the intramolecular edge‐to‐face interaction, which is compensated by the gain of aromatic π‐stacking interactions between four aryl rings of the two molecular components. This dimerization does not take place in solution. The folding free enthalpy ΔGfold of all torsion balances was determined by 1H NMR measurements by using 10 mM solutions of samples in CDCl3 and C6D6. Only the ΔGfold values of balances bearing an edge‐ring substituent in ortho‐position to the interacting CH show a steep linear correlation with the Hammett parameter (σmeta) of the face‐component substituent. Thermodynamic analysis using van′t Hoff plots revealed that the interaction is enthalpy‐driven. The ΔGfold values of the balances, in addition to partial charge calculations, suggest that increasing the polarization of the interacting CH group makes a favorable contribution to the edge‐to‐face interaction. The largest contribution, however, seems to originate from local direct interactions between the substituent in ortho‐position to the edge‐ring CH and the substituted face ring. Direct interactions dominate: The importance of local direct interactions in the edge‐to‐face interaction was examined with a large library of Tröger base derived molecular torsion balances. Modulation of the folding free enthalpy is only observed when edge‐ring substituents are capable of directly interacting with the face‐ring π system (see figure). Experimental results and theoretical calculations indicate that local direct interactions make the largest contribution to substituent effects in the edge‐to‐face aromatic interactio