Constructing Stable π‐Dimers: Two Parallel Pancake π–π Bonds

Stable phenalenyl (PLY) radical π‐dimers still play an important role for new multifunctional materials owing to their intriguing molecular structure and unusual pancake π–π bonding mode. Herein, we design a new biphenalenyl biradicaloid ( 1 ) consisting of two PLYs and one benzene ring linkage tethered by single bonds, which presents an open‐shell character. Further, three π‐dimers ( a1 , b1 , and c1 ) combined with 1 and conventional biphenalenyl biradicaloid ( a , b , and c ), which are capable of forming two staggered PLY dimers, exhibiting a short interlayer distance between the monomers. Interestingly, the analysis of the frontier molecular orbital reveals that two bonding orbitals, namely, the two highest occupied molecular orbitals (HOMO and HOMO−1) are doubly occupied. The results reveal that three π‐dimers display two parallel pancake bonds. Moreover, they have small diradical and tetraradical characters, large interaction energies, and strong aromaticity, which indicate the stability of these π‐dimers. The present work creates a new class of strong pancake bonding and might be utilized in devising an array of materials.