No abstract

This paper presents an analysis of the visible/near-infrared (vis/NIR) spectra of four bis(triarylamine) radical cation mixed valence systems with varying bridge units in the framework of the generalized Mulliken−Hush theory. We outline how to apply a three-level model by using both computational AM1-CI derived as well as experimental transition moments and energies in order to extract electronic coupling matrix elements. The most important outcome is that the much simpler two-level model is a good approximation only if the adiabatic dipole moment difference between the terminal states is large compared to the transition moments associated with the bridge state. This implies that the two-level model is only applicable to mixed valence compounds in the Robin−Day class II with strongly localized redox states if qualitative correct values are desired. We demonstrate that both the spectral features and the potential energy surface of the mixed valence compounds can solely be tuned by bridge state modification reaching from asymmetrically localized to symmetrically localized and from a single minimum potential to a triple minimum potential. For the particular case of an anthracene bridge, we show that solvent induced symmetry breaking has a dramatic influence on the spectral characteristics.

The electronic and molecular structure of N,N,N',N'-tetraphenylphenylenediamine radical cation 1(+) 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 1(+) 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 1(+) adopts a symmetrical delocalized Robin-Day class III structure both in the solid state and in solution.