"Almost Delocalized" Intervalence Compounds

Interest is high in +M‐bridge‐M systems that have very low barriers to intramolecular electron transfer giving M‐bridge‐M+ (“almost delocalized” systems). Hush showed how to evaluate the electronic coupling across the bridge (H) from the M‐to‐M charge‐transfer optical absorption band, but did not point out that his classical model causes the extinction coefficient to suddenly drop to zero at a photon energy of 2 H. Ignoring this band cutoff leads to a low estimation of H. Charge‐localized intervalence compounds have extra charge on one of two charge‐bearing units and are the simplest electron‐transfer systems (see diagram). Hush showed how to evaluate the electronic coupling (H) across the bridge from the lowest energy charge‐transfer optical band of intervalence compounds. He did not explicitly point out that the extinction coefficient for this band drops to zero at a photon energy of 2H. Ignoring this band cutoff leads to estimations of H that are too small for “almost delocalized” compounds.