Dicyanomethyl Radical-Based Near-Infrared Thermochromic Dyes with High Transparency in the Visible Region

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.