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.

11,11-Dimethyl-1,6-methano[10]annulene—An Annulene with an Ultralong CC Bond or a Fluxional Molecule?

“…Representation of energy, enthapy, and free energy changes as a function of the 1,6-distance R of (a) 11,11-dimethyl-1,6-methano [10]annulene (1a), (b) 1,6-methano [10]annulene (2a), and (c) 1,3,5-cycloheptatriene (3a). Potential energy curves in (a) are obtained at the CASPT2 (14,14) and CCSD(T) levels of theory. For estimated (est) curves, see text.…”

“…For estimated (est) curves, see text. Potential energy curves in (b) and (c) are determined at CASPT2 (14,14) and CASPT2(10,10) levels of theory. See text.…”

“…91 The potential curve obtained for ΔH(298) is shown in Figure 3c and probably presents the most accurate description of the energetics of the valence tautomeric system 3 obtained so far. Based on the experience earned for 3, we extended the (10,10) active spaces of 1 and 2 to (14,14) spaces, which include the C1C11 and C1C6 σ and σ*-orbitals (Walsh orbitals). Here we discuss first the results for 2.…”

“…For 1b, the following orbitals established the (14,14) space: 14a 1 , 16a 1 , 9b 1 , 12b 1 , 17b 1 , 15b 2 (Walsh orbital set); 15a 1 , 17a 1 , 10b 1 , 11b 1 , 12b 2 , 13b 2 , 8a 2 , 10a 2 (π-orbital set). In the case of 2b, the (14,14) space comprised the following: 10a 1 , 13a 1 , 17a 1 , 8b 2 , 11b 2 , 14b 2 (Walsh orbital set); 12a 1 , 15a 1 , 9b 1 , 10b 1 , 9b 2 , 10b 2 , 7a 2 , 9a 2 (π-orbital set).…”

“…This result is obtained by using CASPT2 (14,14) and CCSD(T) together with a VTZ basis set. The average R(C1C6) distance of 1 is close to 1.8 Å, in agreement with X-ray diffraction measurements.…”