Cyclo-biphenalenyl biradicaloid molecular materials: Conformation, tautomerization, magnetism, and thermochromism

Phenalenyl and its derivatives have recently attracted a great deal of interest as a result of a two-electron multicenter (2e/mc) ′- ′bonding between two ′-stacked phenalenyl units. The 2e/mc bonded ′-dimers are close in energy to the σ-dimers of phenalenyl and therefore fickle properties may emerge from bond fluctuation, yielding "smart" ′-functional materials. Here, we examine the valence tautomerization of two cyclo-biphenalenyl biradicaloid molecular materials with chair and boat conformations by spinrestricted (R) and unrestricted (U) DFT using the M06 and B3LYP functionals. We found that the chair conformation involves a 2e/4c ′-′ bonded structure, whereas the boat conformation involves a 2e/12c ′-′ bonded structure on their potential energy surfaces. The global minimum for the chair conformation is the σ-bonded structure, whereas it is the ′-′ bonded structure for the boat conformation. The chair conformation exhibits a stepwise [3,3]-sigmatropic rearrangement, and calculations predict a negligible paramagnetic susceptibility near room temperature. In comparison, the paramagnetism of the boat conformation should be observable by SQUID and ESR. According to the energy differences of the respective σ-and ′-dimers of the two conformations and the UV-vis calculations, the color of the chair conformation is expected to become darker, whereas that of the boat conformation should become lighter with increasing temperature.