Interplay between Entropy and Enthalpy in (Intramolecular) Cyclophane-Like Folding versus (Intermolecular) Dimerization of Diarylalkane Cation Radicals

Abstract

Diarylpropane cation radicals are known to exist as folded cyclophane-like structures, as evidenced by the appearance of intervalence transitions in their optical spectra. Despite the expected enthalpic stabilization of cyclophane-like cation radicals of diarylpropanes by ∼350 mV, we demonstrate that only partial folding (∼50%) occurs due to the entropic penalty associated with restriction of conformational flexibility via the freezing of multiple free C–C bond rotors together with the strain in the folded cyclophane-like structure. This important demonstration of the interplay between enthalpy and entropy is deduced via a systematic study of various diarylalkane cation radicals with two- to five-methylene spacers using electrochemistry, optical spectroscopy, X-ray crystallography, and DFT calculations. We also show that diarylalkane cation radicals with greater than three methylene spacers cannot fold into cyclophane-like structures, as the entropic penalty for freezing increasing number of C–C bond roto...