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

Abstract

Herein, we demonstrate that the dimerization behavior of amine-substituted dicyanomethyl radicals can be switched from σ- to π-dimerization simply by varying the electron-donating substituents. For dicyanomethyl radicals with a 4,4'-ditolylamine (DT ) or a phenothiazine (PT ) substituent, the monomeric radical form and the corresponding dimer connected by a reversible C-C bond (σ-dimer) are in equilibrium in solution. On the other hand, the radical with the julolidine skeleton (JD ) does not undergo σ-dimerization and was isolated as a stable radical in spite of the absence of bulky protecting groups. X-ray single-crystal analysis revealed that JD forms the π-dimer in the crystalline state, and variable-temperature spectroscopy showed that JD is in equilibrium with the π-dimer in toluene solution. DFT calculations point to the importance of electrostatic interactions as a driving force for the π-dimerization of JD because of its polarized structure.

Sigma‐ versus Pi‐Dimerization Modes of Triangulene

“…A diradical quinonoid oligothiophene has been synthesized which showed π‐, σ‐dimerization and polymerization . Another example is provided by the dimerization behavior of amine‐substituted dicyanomethyl radicals which can be switched from σ‐ to π‐dimerization as a function of substituents . Further examples include diradical cethrenes in which a ring‐closing σ‐dimerization can occur .…”

“…[24] Another example is provided by the dimerization behavior of amine-substituted dicyanomethyl radicals which can be switched from s-t op-dimerization as af unction of substituents. [25] Further examples include diradical cethrenesi nw hicharing-closing s-dimerization can occur. [26] The cethrene case is analogous even though in that case the s-v s. p-dimerization occurs within the same helical molecule.…”

“…Monomerso fradicals and diradicals that can undergo s-and p-dimerizations. [21][22][23][24][25][26][27]…”

π‐Dimerization ability of conjugated oligomer dication diradicaloids composed of dithienylpyrrole and benzodithiophene units

“…In recent years, there is a growing interest in the self‐association of stable radicals, so‐called π‐dimer, and its application to supramolecular chemistry. In particular, the π‐dimerizations of radical cations have been utilized for the redox‐responsive control of intramolecular motion and construction of supramolecular structure .…”

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

“…Thepreference of the gauche conformation cannot be explained by a"gauche effect", [11] as the dominant interactions between cyano groups is dipole-dipole/ electrostatic repulsion, [12] and the CÀCN* orbital is weak for hyperconjugation. [6,14] It seems more reasonable that atwo-dimensional polymer adopts all-anti conformation should form for monomer 1 (Supporting Information, Scheme S1). Both phenyl and TPAg roups are used here as the Ar group in the dimer to build potential energy surfaces by scanning the Ar-C(sp 3 )-C(sp 3 )-Ar dihedral angle from 08 8 to 3608 8.S tep of 108 8 is applied and each fixed dihedral angle is optimized at the B3LYP/6-31G level.…”