Effect of Substituents on the Bond Strength of Air-Stable Dicyanomethyl Radical Thermochromes

Abstract

A series of substituted aryl dicyanomethyl radicals were synthesized, and the bonding thermodynamic parameters for self-dimerization were determined from van't Hoff plots obtained from variable-temperature electron paramagnetic resonance and ultraviolet−visible spectroscopy. At low temperatures, the radicals dimerize, but the colored, air-stable free radicals return upon heating. Heating and cooling cycles (5−95 °C) can be repeated without radical degradation and with striking thermochromic behavior. We find a linear free energy relationship between the Hammett para substituent parameter and the dimerization equilibrium constant, with para electron-donating substituents leading to a weaker bond and electron-withdrawing substituents leading to stronger bonds, following a captodative effect. Density functional theory investigations [B98D/6-31+G(d,p)] reveal that the dimers prefer a slip-stacked geometry and feature elongated bonds.

The Divergent Dimerization Behavior of N‐Substituted Dicyanomethyl Radicals: Dynamically Stabilized versus Stable Radicals

“…Dynamic covalent chemistry (DCC) has been attracting increasing interest in the field of molecular self‐assembly . In particular, the reversible formation of covalent bonds based on dimerization–dissociation reactions of organic radicals is the simplest process of dynamic covalent chemistry and does not involve any catalysts or byproducts. Recently, we demonstrated that para ‐amino‐substituted α,α‐dicyanobenzyl radicals are useful motifs for DCC owing to the dynamic equilibrium between the radical form and the dimer formed via a reversible C−C bond (σ‐dimer) .…”

Formation of a Macrocycles‐in‐a‐Macrocycle Superstructure with All‐gauche Conformation by Reversible Radical Association

“…Para electron-donating substitution was found to be crucial for stabilizing aryldicyanomethyl radicals and weakening C(sp 3 )ÀC(sp 3 )bonds formed between them. [6,9] Thus the TPAc ore is used here to facilitate reversible association and dissociation of radicals.Alow dissociation enthalpy of about 55 kJ mol À1 is expected for the corresponding C(sp 3 ) À C(sp 3 )b onds,a sd etermined from ad imer model compound, [6] enabling reversible conversion of the C(sp 3 )ÀC(sp 3 )b ond to radicals at room temperature.…”

The Divergent Dimerization Behavior of N‐Substituted Dicyanomethyl Radicals: Dynamically Stabilized versus Stable Radicals

“…[1] In particular,t he reversible formation of covalent bonds based on dimerization-dissociationreactions of organic radicals [2][3][4][5][6][7][8][9][10] is the simplest process of dynamic covalent chemistry and does not involve any catalysts or byproducts.R ecently,w e demonstrated that para-amino-substituted a,a-dicyanobenzyl radicals [2] are useful motifs for DCC owing to the dynamic equilibrium between the radical form and the dimer formed via ar eversible C À Cb ond (s-dimer). [1] In particular,t he reversible formation of covalent bonds based on dimerization-dissociationreactions of organic radicals [2][3][4][5][6][7][8][9][10] is the simplest process of dynamic covalent chemistry and does not involve any catalysts or byproducts.R ecently,w e demonstrated that para-amino-substituted a,a-dicyanobenzyl radicals [2] are useful motifs for DCC owing to the dynamic equilibrium between the radical form and the dimer formed via ar eversible C À Cb ond (s-dimer).…”