High-Spin S = 3/2 Ground-State Aminyl Triradicals: Toward High-Spin Oligo-Aza Nanographenes

Abstract

We report high-spin aminyl triradicals with near-planar triphenylene backbones. Near-planarity of the fused aminyl radicals and the 2,6,10-triphenylene ferromagnetic coupling unit (FCU), magnetically equivalent to three fused 3,4′-biphenyl FCUs, assures an effective 2pπ–2pπ overlap within the cross-conjugated π-system, leading to an S = 3/2 (quartet) ground state that is well separated from low-spin excited doublet states. Thermal populations of the low-spin (S = 1/2) excited states are detectable both by SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy, providing doublet–quartet energy gaps, ΔE DQ, corresponding to >85% population of the quartet ground states at room temperature. Notably, EPR-based determination of ΔE DQ relies on direct detection of the quartet ground state and doublet excited states. The ΔE DQ values are 1.0–1.1 kcal mol–1, with the more sterically shielded triradical having the larger value. The half-life of the more sterically shielded triradical in 2-methyltetrahydrofuran (2-MeTHF) is about 6 h at room temperature. The less sterically shielded triradical in 2-MeTHF decomposes at 158 K with a half-life of about 4 h, while at 195 K, the half-life is still about 2 h. The dominant products of the decay of triradicals are the corresponding triamines, suggesting hydrogen atom abstraction from the solvent as the primary mechanism. This study expands the frontier of the open-shell PAHs/nanographenes, of which the unique electronic, nonlinear optical, and magnetic properties could be useful in the development of novel organic electronics, photonics, and spintronics.

Triply-Reduced Hexa-peri-hexabenzocoronenes: Solid-State Structures and Magnetic Properties

“…Indeed, in any of the resonance forms depicted in Figure , each pair of radicals is separated by an odd number of π-conjugated carbon atoms, which should result in a parallel arrangement of the spin centers. Similar behavior has been reported recently for aminyl triradicals with a cross-conjugated π-system . In contrast, the loss of conjugation leads to weakening of the intramolecular spin correlations, as was shown for several systems with variable strength and sign of intramolecular magnetic exchange.…”

“…Similar behavior has been reported recently for aminyl triradicals with a cross-conjugated π-system. 25 In contrast, the loss of conjugation leads to weakening of the intramolecular spin correlations, as was shown for several systems with variable strength and sign of intramolecular magnetic exchange. For example, a pronounced temperature dependence of χT was observed in a series of nitroxide triradicals, due to weaker ferromagnetic exchange that led to the room-temperature χT value being much closer to 1.125 emu K mol −1 , expected for three noninteracting S = 1/2 centers.…”

Design and Synthesis of Kekulè and Non-Kekulè Diradicaloids via the Radical Periannulation Strategy: The Power of Seven Clar’s Sextets

“…So, the search for stable non-Kekulé diradicaloids continues (Figure ). Very recently, Juríček and co-workers reported that trimesityltriangulene is persistent, while the groups of Wu and Rajca reported high-spin nitrogen-doped nanographenes.…”

Thermally Ultrarobust S = 1/2 Tetrazolinyl Radicals: Synthesis, Electronic Structure, Magnetism, and Nanoneedle Assemblies on Silicon Surface

“…2,2′-Biphenylene-fused tetrazolinyl (BFTZ) monoradicals, or so-called phototetrazolinyl radicals, have attracted our attention because of their exceptional stability and, most importantly, the potential to achieve relatively large spin densities at the para -positions (and ortho -positions) with respect to the nitrogens of the tetrazolinyl moiety (Figure ). Large spin densities at the para / ortho positions are the essential prerequisite in the design of high-spin polyradicals with large energy gaps between the high-spin ground state and low-spin excited state. − …”