From valence trapped to valence delocalized by bridge state modification in bis(triarylamine) radical cations: Evaluation of coupling matrix elements in a three-level system

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 AMI-Cl 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 11 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.