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 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...