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

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 rotors and associated strain in the folded cyclophane-like structures far outweighs the enthalpic gain of ∼350 mV. We also designed and synthesized a derivative of diarylpropane with a bulky alkyl group at the second carbon of three-methylene spacer, which undergoes quantitative folding due to a reduction in the entropic penalty by hindering the C–C bond rotors. Unlike diarylpropane cation radicals, diarylethane cation radicals undergo ready intermolecular self-association due to the favorable enthalpic gain (∼700 mV) from two pairs of sandwiched aryl groups from two molecules of diarylethane cation radical. This demonstration of the role of enthalpy and entropy in intramolecular folding of diarylpropane cation radicals will open new avenues for designing next-generation cofacially arrayed structures for modern photovoltaic applications.