How Delocalized Is <i>N</i>,<i>N</i>,<i>N</i><i>‘</i>,<i>N</i><i>‘</i>-Tetraphenylphenylenediamine Radical Cation? An Experimental and Theoretical Study on the Electronic and Molecular Structure

The electronic and molecular structure of <i>N</i>,<i>N</i>,<i>N</i>‘,<i>N</i><i>‘</i>-tetraphenylphenylenediamine radical cation <b>1</b><b><sup>+</sup></b><sup></sup> is in focus of this study. Resonance Raman experiments showed that at least eight vibrational modes are strongly coupled to the optical charge resonance band which is seen in the NIR. With the help of a DFT-based vibrational analysis, these eight modes were assigned to symmetric vibrations. The contribution of these symmetric modes to the total vibrational reorganization energy is dominant. These findings are in agreement with the conclusions from a simple two-state two-mode Marcus−Hush analysis which yields a tiny electron-transfer barrier. The excellent agreement of the X-ray crystal structure analysis and the DFT computed molecular structure of <b>1</b><b><sup>+</sup></b><sup></sup> on one hand as well as the solvent and solid-state IR spectra and the DFT-calculated IR active vibrations on the other hand prove <b>1</b><b><sup>+</sup></b><sup></sup> adopts a symmetrical delocalized Robin−Day class III structure both in the solid state and in solution.