Unusual interchromophoric interactions in β,β' directly and doubly linked corrole dimers: Prohibited electronic communication and abnormal singlet ground states

Directly and doubly β,β'-linked corrole dimers (DH₃CD, DH₂CD, and DZnCD) are excellent platforms for the investigation of intercorrole interactions because of their enforced coplanar geometries and short edge-to-edge distances. Through the use of these molecules along with the reference monomer H₃CM and the singly β,β'-linked corrole dimer SH₃CD, the intercorrole interactions have been systematically studied by density functional theory calculations, ultrafast photophysical measurements, and two-photon absorption measurements. A particular focus was placed on revealing factors that are important for the induced photophysical properties of the doubly linked corrole dimers compared with corrole monomer. In the doubly linked corrole dimers, strong molecular orbital interactions caused by the coplanar geometry and the short interchromophoric distance give rise to perturbations of the electronic states that are responsible for the red-shifted and intensified Q-like band in DH₃CD and the broad NIR absorption bands and fast excitation-energy relaxation processes in DH₂CD and DZnCD. On the other hand, electronic communication between corrole units is prohibited by the structurally constrained octagonal core in the center, so each constituent corrole unit in the doubly linked corrole dimers maintains an intrinsic π-conjugation system. Consequently, the overall aromaticity of the directly linked corrole dimers can be explained in terms of a linear sum of two constituent corrole monomers, and the singlet biradical character of DH ₂CD and DZnCD can be understood in terms of two unpaired electrons (one from each constituent oxidized corrole monomer) and their appropriate interaction. In addition, the nonlinear optical properties of DH₂CD and DZnCD with singlet biradical character have been confirmed to be significantly enhanced compared with those of closed-shell DH₃CD. Collectively, double β,β'-linkages of corroles provide the coplanar geometry with a short interchromophoric distance and the strained octagonal core that play key roles in allowing the strong molecular orbital interactions and restricting the electronic communication between the two corroles, respectively.