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
