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.

In this work, we prepared three novel dicyanomethyl radicals having a triphenylamine skeleton, which show thermochromism in the near-infrared (NIR) region based on the reversible dimerization–dissociation reactions. We demonstrated that the coplanar arrangement of the amino group with respect to the benzene ring bearing the dicyanomethyl radical by O or S bridges causes the drastic bathochromic shift of the radical absorption bands toward the NIR region. In particular, the radical with two bridging O atoms has the most red-shifted radical band of λmax = 1059 nm compared to that of the radical with no bridging O atoms (λmax = 712 nm). Due to the significant bathochromic shift of the radical bands, these radicals have a wide optical window in the visible region. In particular, the radical with the two-O bridge shows almost no perceptible color change to the naked eye and remains highly transparent over all temperature ranges, whereas the intensity of the NIR absorption drastically changes. These radicals maintained their absorption properties in a polycarbonate matrix and high stability.

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.