The interaction region indicator (IRI) is a simple real space function that can clearly reveal both chemical bonds and weak interactions between atoms or molecular fragments . The isosurfaces and color-filled maps of IRI function for the π MOs, referred to as IRI-, are depicted in Fig. . It can be seen that the IRI-π isosurface of the C-C bonds in the ring except for those adjacent to the -CO groups is annular, indicating that these C-C bonds have dual π (i.e. π out and π in ) interaction features. However, the strength of π interaction of two types of C-C bonds is obviously different. The annular IRI-π isosurface of the short C-C bonds is broader and bluer than that of the long C-C bonds, providing intuitive evidence for the larger electron density in the π interaction region of the short ones, so the π interaction of the short bonds is stronger. In contrast, since the corresponding IRI-π isosurface is obviously not annular, only  out interactions exist on the C-C bonds adjacent to the -CO groups.