Four triphenylamines with extended π-systems were synthesized. Cyclic voltammetry (CV) measurements showed that they gave radical cations, which are stable in solution. Radical cations obtained upon one electron chemical oxidation showed strong absorption in the near-infrared region. The radical cations of the naphthalene-substituted derivatives show a maximum absorption wavelength above 1000 nm and are classified as NIR-II dyes. Molecular design rules of novel near-infrared absorbing dyes are described.

Five triphenyltriphenylamines with various substituents were investigated as precursors for near-infrared absorbing materials. Cyclic voltammetry (CV) studies showed that they all give stable radical cations in solution. The radical cations obtained by one-electron chemical oxidation of these compounds show strong absorption in the near-infrared region, and the position of the absorption is strongly influenced by the substituent. DFT (density functional theory) calculations suggest that the introduction of stronger electron-donating substituents would result in a smaller HOMO–SOMO energy gap and thus a larger long wavelength shift, which is consistent with the experimental results. On the other hand, strong electron-withdrawing substituents increase the HOMO–SOMO energy gap, resulting in a short wavelength shift. The position of the near-infrared absorption peak of the triphenylamine radical cation can be controlled to the longer or shorter wavelength direction depending on the substituent. A molecular design of near-infrared absorbing dyes utilizing the electronic effects of substituents is described.