What does it take to induce equilibrium in bidirectional energy transfers?

Abstract

Two dyads built with a co-facial slipped bis(zinc(II)porphyrin), a free base and a bridge, [Zn₂]–bridge–[Fb] (bridge = C₆H₄CC, 1 and C₆H₄CCC₆H₄, 2), exhibit S₁ energy equilibrium [Zn₂]* ↔ [Fb]* at 298 K, an extremely rare situation, which depends on the degree of MO coupling between the units. At 77 K, 2 becomes bi-directional due to the two large C₆H₄–[Zn₂] and C₆H₄–[Fb] dihedral angles.