Cyclo-biphenalenyl Biradicaloid Molecular Materials: Conformation, Tautomerization, Magnetism, and Thermochromism

Abstract

Phenalenyl and its derivatives have recently attracted a great deal of interest as a result of a two-electron multicenter (2e/mc) π−π bonding between two π-stacked phenalenyl units. The 2e/mc bonded π-dimers are close in energy to the σ-dimers of phenalenyl and therefore fickle properties may emerge from bond fluctuation, yielding “smart” π-functional materials. Here, we examine the valence tautomerization of two cyclo-biphenalenyl biradicaloid molecular materials with chair and boat conformations by spin-restricted (R) and unrestricted (U) DFT using the M06 and B3LYP functionals. We found that the chair conformation involves a 2e/4c π−π bonded structure, whereas the boat conformation involves a 2e/12c π−π bonded structure on their potential energy surfaces. The global minimum for the chair conformation is the σ-bonded structure, whereas it is the π−π 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 π-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.