Unusual interchromophoric interactions in beta,beta' directly and doubly linked corrole dimers: prohibited electronic communication and abnormal singlet ground states.

Directly and doubly beta,beta'-linked corrole dimers (DH(3)CD, DH(2)CD, 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 H(3)CM and the singly beta,beta'-linked corrole dimer SH(3)CD, 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 DH(3)CD and the broad NIR absorption bands and fast excitation-energy relaxation processes in DH(2)CD 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 pi-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(2)CD 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 DH(2)CD and DZnCD with singlet biradical character have been confirmed to be significantly enhanced compared with those of closed-shell DH(3)CD. Collectively, double beta,beta'-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.